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![Chapter 1
Scenarios for Solar Electricity
at the TeraWatt Scale
A world shock has occurred in 2020, and it has strongly affected the energy sector.
According to the preliminary estimations included in the most recent report from the
International Energy Agency (World Energy Outlook 2020, [9]), the global energy
demand dropped by 5% in 2020, and energy-related CO2eq emissions dropped by
7%. This shock in the demand side, concentrated in a single year, is higher in terms
of energy demand reduction than the shock in the supply side that started in October
1973 due to an oil export embargo proclaimed by the Organization of the Petroleum
Exporting Countries (OPEC) that lead to a sudden rise in oil prices. The impact of the
oil crisis was long lasting, it reshaped the energy landscape worldwide and triggered
the first steps to unlock the “carbon lock-in” and initiate an energy transition that is
now fully fledged [1].
After a sudden shock, a well-established paradigm can be shifted if the policy
response is clearly defined and enough investment is provided, initially in research
activities and later in demonstration projects. The initial efforts triggered by the oil
crisis put in place technological advancements that supported the early stages of
the energy transition a few decades ago. In 2020, an external shock, the catastrophic
COVID-19 pandemia led to public policies designed with strong investment efforts to
reactivate the economy, and this “new deal” has created the opportunity to accelerate
the energy transition with renewable mature technologies that are cost-competitive.
Wind and photovoltaic technologies are already the cheapest source of electricity
in many parts of the world. This combination of shock, new investment and tech-
nological readiness could definitely move the world from the carbon lock-in to a
renewables lock-in. Large investments have been announced worldwide to reacti-
vate the economy, and a good share of this investment is oriented to reinforce the
energy transition and to mitigate climate change. It is a great opportunity that will
require new ambitious policies and a worldwide coordination of a good regulatory
framework to support this move towards a more sustainable energy landscape.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022
A. Urbina, Sustainable Solar Electricity, Green Energy and Technology,
https://doi.org/10.1007/978-3-030-91771-5_1
3](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-22-2048.jpg)
![4 1 Scenarios for Solar Electricity at the TeraWatt Scale
But despite the brilliant perspective, this transition is still in its early stages. In
2019, total primary energy consumption in the world was 583.9 Exajoules,1
with
an annual growth rate of 1.6% averaged for the past ten years. The renewables
contribution to the total primary energy consumption was 66.64 Exajoules (11.3%, of
which 6.4% from hydropower), while oil continues to hold the highest share (33.1%),
followed by coal (27.0%), natural gas (24.2%) and then renewables (11.3%) that have
already surpassed nuclear (4.3%). Electricity generation in 2019 was 27004.7 TWh2
(average annual growth in the past ten years was 2.7%, almost doubling the primary
energy average annual growth, a clear indicator of the “electrification” of the global
energy consumption), and renewable electricity generation was 7027.7 TWh (26.1%,
including hydropower, its main contributor, with 4222.2 TWh equivalent to 15.6%
followed by wind 1429.6 TWh, 5.2% and solar photovoltaic with 724.1TWh, 2.7%)
[2]. According to the International Energy Agency Photovoltaic Power Systems
Programme, world final electricity consumption was 24,700 TWh in 2019, with a
share of renewable energy in the global electricity production of 28%, including 810
TWh produced from solar photovoltaic systems; thus, the solar electricity production
share was 3.3% [11].
In 2020, due to the world reduced energy demand and the increment in photo-
voltaic power installed capacity, around 3.7% of world electricity production has
been supplied by photovoltaic systems and the avoided emissions have been 875
Mt of CO2eq (a calculation by the IEA-PVPS based on the emissions that would
have been generated from the same amount of electricity produced by the different
grid mixes in all countries and taking into consideration life cycle emissions of PV
systems). This world average hides a large variation among countries, where a group
of seven countries are in the range of 10% and another seven have already surpassed
5%. In this group, it is important to emphasize that the two most populated coun-
tries in the world have already reached 6.5% (India) and 6.2% (China) share of its
electricity supply from photovoltaic systems [10].
Despite the progress in rural electrification, still 733 million people are lacking
access to electricity, three quarters in sub-Saharan Africa (580 million), and another
100 million people cannot afford electricity although they have access to the grid [5].
Either to substitute electricity from non-renewable sources or to supply new demand,
the contribution of photovoltaic systems has been growing steadily since many years
ago and has now become the fastest growing technology in terms of annual installed
capacity. The share of world electricity supply from photovoltaics is going to increase
significantly in the coming decades in all scenarios that are proposed by different
institutions. The rate of growth and the cumulative capacity depend strongly on the
assumptions for these scenarios, and in all of them, photovoltaic technology share
is very high, in some cases the top of the list of annual installed capacity during
several years. This fact emphasizes the urgent need of a detailed evaluation of the
1 1 Exajoule (EJ) = 1018 Joules; another broadly used unit for primary energy is tonnes of oil
equivalent (toe), 1 toe = 4.1868×1010 Joules.
2 1 TWh = 1012 Wh = 3.6 × 1015 Joules.](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-23-2048.jpg)
![6 1 Scenarios for Solar Electricity at the TeraWatt Scale
second stage focuses on the use phase and the end-of-life phase (including recy-
cling and landfilling) and requires additional tools to calculate the electricity
produced during the operational phase. Part II starts with a detailed description of
the manufacturing process of all PV technologies, either commercial or emerging
(Chap. 4), and the requirements of raw materials (Chap. 5); the energy balance of
the PV system life cycle (Chap. 6) will be presented and, together, they comprise
a life cycle inventory of the PV technologies. Beyond the standard LCA approach,
an analysis of the energy payback time (EPBT) has been included; it is a param-
eter broadly used to assess the sustainability of electricity production but which
is strongly dependant on the operational phase of the PV system life, including
the geographical location where it is operated, and some authors consider that
it is not a reliable parameter. The impact assessment in several LCA categories
of the whole inventory (materials and energy) will be presented in Chap. 7 with
a special focus on commercial technologies and a section devoted to emerging
technologies. The focus will be shifted to end-of-life and recycling issues in Chap.
8 and the final chapter of Part II is devoted to Balance of System components with
a more detailed analysis of the use of batteries for energy storage.
Part III. Beyond Life Cycle Assessment: socioeconomics and geopolitics of solar
electricity. Finally, Part III goes beyond the standard approach to LCA and
includes economic and social assessment of impacts. Economic evaluation of
the economic cost of installed capacity and produced electricity is accomplished
in this part. Comments on the geopolitics of photovoltaics provide the closing
remarks of the whole book. In Chapter 10, the definition of economic parame-
ters used to evaluate the impact of PV systems is provided. Those comprise the
levelized cost of electricity (also with the modern definition of IEA, called the
“value-adjusted” LCOE). Employment opportunities by sector and by country
are analysed, including investigation on socioeconomic networks that range from
NGOs or other associations to small, medium or large companies linked to solar
electricity. Chapter 11 provides a list of the regulatory framework worldwide,
with a presentation of technical standards and regulatory policies, including a
comparison between countries and a comment about its evolution. The book ends
with Chap. 12 in which solar electricity will be put into the context of global-
ization, when on the one hand still a large amount of population lacks access to
electricity while on the other hand solar electricity is now subject of speculation
by investment funds and big multinationals. Climate change mitigation and the
related international agreements are the closing subjects of the book.
1.1 Evolution of Installed Photovoltaic Capacity
At the end of 2020, the cumulative installed photovoltaic capacity in the world
reached 760.4 GWDC , steadily approaching the landmark of 1 TW that could be
reached in two years if annual installed capacity follows the growing trends of the
past few years (see Fig. 1.1, reproduced from [8]). Despite the COVID-19 pandemic,](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-25-2048.jpg)
![1.1 Evolution of Installed Photovoltaic Capacity 7
Fig. 1.1 Evolution of cumulative installed capacity (GWp). Source IEA-PVPS (Reproduced with
permission from [8])
the annual installed capacity in 2020 was 139.4 GWDC , with at least 20 countries
installing more than 1 GW, indicating a sustained annual capacity installation of
more than 100 GW/year since 2017 that seems to be accelerating (see Fig. 1.2).
China alone represented 253.4 GW on cumulative installed capacity followed
by the European Union (as EU27, 151.3 GW), the USA (93.2 GW), Japan (71.4
GW) and India (47.4 GW). Considering that China installed a third of global new
capacity in 2020 and that Vietnam and Korea have seen their highest growth in
one year, the trend is clear: Asia is going to be the leading photovoltaic region in
the next decade, with Australia also becoming an important actor and reaching the
first position in the ranking of PV installed per capita (749 W/capita), surpassing
Germany which had been the leader in per capita PV capacity until 2019. The Asia-
Pacific region installed 61% of new global PV capacity in 2020. The European
Union have been leader for many years, but it seems that the trend is slowing down,
with only a few European countries keeping a strong growth (Germany still clearly
at the head of installed cumulative capacity with 53.9 GW, followed by Italy and
the United Kingdom at some distance). In America, the new USA administration
announced a strong investment in new renewable energy infrastructure that could
reinforce its already strong position in the photovoltaic market; two countries in
Latin America installed more than 1GW (Mexico and Brazil), but others presented a
contraction in annual installations (Argentina) or very limited growth (Perú, Chile).
Africa and the Middle East, with a large potential for PV (due to its very high
annual irradiation), showed a limited growth with new installed capacity in 2020
of only 3% of world total. Still both annually installed and cumulative capacity are
mostly concentrated in a few countries, with the rest of the world (ROW in Table
1.1) contributing only 6.8% and 0.3%, respectively. Details of world data for PV
annual and cumulative capacity and energy generation can be found in the regular](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-26-2048.jpg)
![8 1 Scenarios for Solar Electricity at the TeraWatt Scale
Fig. 1.2 Evolution of annual installed capacity (GWp) (Reproduced with permission from [10])
reports from the International Energy Agency (IEA) “World Energy Outlook”, the
IEA-International Renewable Energy (IRENA) “Capacity Statistics and Highlights”,
the IEA-Photovoltaic Power Systems Programme (PVPS) “Trends in Photovoltaic
Applications”, the IEA-PVPS “Snapshot of Global PV Markets” and the reports from
the World Bank initiatives for off-grid electrification programmes “Energy Sector
Management Assistance Programme (ESMAP)” and “Lighting Global”.
There are two main categories of photovoltaic system size classification: roof-top
or utility scales. Until 2014, the roof-top scale was predominant with more than 50%
of annual installed capacity, which kept the cumulative capacity also above 50% for
this kind of system; since 2015, the annual installations have been clearly dominated
by utility scale (grid-connected PV plants at MW scale), although roof-top systems
continued to grow and this application sector has seen an unexpected increase in 2020
due to the very large programme for roof-top systems in Vietnam (and a continuation
in Germany and United States were it was already strong): in 2020, around 55GW
of new PV systems were roof-top; regarding off-grid systems, further 180 million
of roof-top solar home systems have been installed to date providing electricity to
420 million people, and 47 million people are connected to 19,000 photovoltaic
powered minigrids in the world (mainly in Asia, with 85% of minigrids, while the
future planning is centred in Africa) [3, 4]. Nevertheless, the trend seems to point
to a future domination of medium to large size plants. On the other hand, the two
broad categories need to be extended to incorporate variations: building integrated
photovoltaics (BIPV) complementing the first group of “building attached” (BAPV)
roof-top systems (small to medium power systems), or floating systems, agrivoltaics
or other utility scale but with very flexible plant design adapted to multiple func-
tionalities of medium to large size plants. Other small groups of applications are
still not significant in terms of capacity, but represent targeted markets that could
grow significantly in the future: vehicle integrated systems, indoor systems adapted](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-27-2048.jpg)
![1.1 Evolution of Installed Photovoltaic Capacity 9
Table 1.1 Annual installed and cumulative photovoltaic capacity in 2020, with data from IEA-
PVPS “Snapshot of Global PV Markets 2021” [10]; the European Union grouped 27 countries in
2020; power is expressed in GWDC and when data are available in GWAC they have been converted
for better cross country comparison of data
Annual installed capacity Cumulative capacity
GWDC % GWDC %
1 China 48.2 34.6 1 China 253.4 33.3
(2) European
Union
19.6 14.1 (2) European
Union
151.3 19.9
2 United
States
19.2 13.8 2 United
States
93.2 12.3
3 Vietnam 11.1 8.0 3 Japan 71.4 9.4
4 Japan 8.2 5.9 4 Germany 53.9 7.1
5 Germany 4.9 3.5 5 India 47.4 6.2
6 India 4.4 3.2 6 Italy 21.7 2.9
7 Australia 4.1 2.9 7 Australia 20.2 2.7
8 Korea 4.1 2.9 8 Vietnam 16.4 2.2
9 Brazil 3.1 2.2 9 Korea 15.9 2.1
10 Netherlands 3.0 2.2 10 UK 13.5 1.8
ROW 9.5 6.8 ROW 2.1 0.3
Total: 139.4 Total: 760.4
to indoor light, portable flexible and low weight systems, cladding systems inte-
grated in paths or roads and a broad range of new system designs in an already old
application class dedicated to supply power to signals, lighting or electronic devices.
Off-grid systems, mainly for rural electrification in developing countries, represented
an important market at the beginning of PV system deployment (80s and 90s), and
now its share market is strongly reduced, although in terms of installed capacity,
it is still a significant application and have a very large impact in human develop-
ment in rural livelihoods without previous access to electricity; in 2030, the off-grid
PV systems should be extended to provide electricity to 1.2 billion people [3]. The
evolution of the broad classes of PV applications can be seen in Fig. 1.3.
The massive deployment of PV capacity is already producing electricity from a
renewable source at a lower price than grid electricity in some countries at some time
intervals. The produced solar photovoltaic electricity has been growing steadily at a
similar pace of installed capacity, in Fig. 1.4, and overview of the aggregated data for
different world regions is presented, the data can be downloaded from the IRENA
website, and it is updated regularly. Asia is now the leading country in solar electricity
production followed by Europe which was surpassed in 2016, North America comes
in third position and the rest of the regions are clearly lagging behind, but they are
expected to grow significantly in the coming years due to strong cost reductions of
PV systems.](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-28-2048.jpg)
![10 1 Scenarios for Solar Electricity at the TeraWatt Scale
Fig. 1.3 Annual share of centralized, distributed, off-grid and floating installations (GW). Source
IEA-PVPS Trends in PV Applications 2020 (Reproduced with permission from [11])
Fig. 1.4 Solar photovoltaic electricity production (TWh) per region during the past eleven years
(with most recent real production data from IRENA Renewable Energy Statistics website (last
update April 5, 2021, www.irena.org/Statistics/Download-Data)
The availability of cheap electricity from photovoltaics will also contribute to
enhance the penetration of other technologies that are energy consumers required in
an energy transition aimed at a 100% green electricity. These sectors are hydrogen
production and electric vehicles. The developments of PV technologies are acting as a
strong driver for the development of other technologies linked to the energy sector and](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-29-2048.jpg)
![1.1 Evolution of Installed Photovoltaic Capacity 11
it has created a synergy between the need for efficient storage of electricity produced
from photovoltaics at time intervals where demand is lower than supply (similarly
for other intermittent sources like wind) and the need for higher electrification of the
transport sector. The use of hydrogen as a fuel “vector” and the charging of batteries
in electrical vehicles require electricity produced from renewable sources. The link
between this renewable intermittent electricity production and the transport sector is
pushing the development of technologies for efficient charge storage and green fuel
production. This link is still not clear and a strong effort in research and development
is currently being carried out.
1.2 Photovoltaics in the Scenarios of the International
Energy Agency
The International Energy Agency scenarios are the basis for projections shown in the
World Energy Outlook reports, and they are linked to socioeconomic scenarios set
up by the United Nations and in particular, the Sustainable Development Goals now
used by most countries to set up their own sustainable objectives and to contribute
to international cooperation policies [5].
The Stated Policies Scenario (STEPS) is a baseline scenario that is built upon
the policies announced by each country; the targets related to new renewable energy
capacity installations or emission reductions are backed up by detailed technical
and economical measures needed for their realization. In particular, the Nationally
Determined Contributions (NDC) for emissions reductions that the countries are
announcing as part of their commitment with the Paris Agreement are considered
in the STEPS scenario only if they are backed by a clear plan of implementation.
In contrast, many policies that have been announced with net zero pledges already
reaching 70% of global GDP and CO2 emissions, but still with high level of uncer-
tainty or no technical backing in its energetic policies, are not considered; in general,
those lousy undefined pledges are not considered in the STEPS scenario. On the
other hand, the STEPS scenario already includes the impact of COVID-19 pandemic
in the economic activity of 2020 but considers that the pandemic is brought under
control and the economy will recover its pre-crisis levels before the end of 2021.
Prior to the crisis, energy demand was projected to grow by 12% between 2019 and
2030, and growth over this period is now estimated at 9% in the STEPS scenario.
Additionally, economic policies have already been modified by recovery policies and
stimulus packages including additional investments in the energy transition infras-
tructure towards a low-carbon energy sector. Nevertheless, commitments declared so
far, even if successfully fulfilled, will keep global annual emissions in the range of 34–
36 Gt CO2eq between 2020 and 2030, followed by a reduction that would still leave
around 22 billion tonnes of CO2 emissions worldwide in 2050; the continuation of
that trend is consistent with a temperature rise in 2100 of around 2.7 ◦
C (with a prob-
ability of 50%), well beyond the limits set in the Paris agreement. Furthermore, the](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-30-2048.jpg)
![12 1 Scenarios for Solar Electricity at the TeraWatt Scale
United Nations Framework Convention on Climate Change (UNFCCC) was even
more pessimistic and considered that the initial nationally declared commitments
(NDC) for greenhouse gases (GHG) emission reductions of 119 countries could lead
to a temperature increase in the range of 2.7–3.7 ◦
C, indicating that much greater
emission reduction efforts than those associated with the NDCs will be required in
the period after 2025 and 2030 to hold the temperature rise below 2 ◦
C above pre-
industrial levels [12]. The updates of NDCs by 75 parties (representing about 30%
of global GHG emissions) were recently assessed by the UNFCCC, but still are not
on track to meet the Paris Agreement; the reality is that far from a reduction, the
figures contained in the NDCs will lead in 2025 to GHG emissions around 14.04
Gt CO2eq, that is, 2.0% higher than the 1990 level (13.77 Gt CO2eq), 2.2% higher
than the 2010 level (13.74 Gt CO2eq) and 0.5% higher than the 2017 level (13.97
Gt CO2eq). Nevertheless, the long-term mitigation measures announced by many
countries for 2050 (still without detailed roadmaps for its fulfilment) are ambitious,
and the UNFCCC considers that if implemented, the per-capita emissions by 2050
could be reduced by 87–93% compared to 2017 levels and this is consistent with the
objective of a temperature rise in the range of 1.5–2 ◦
C with low overshoot scenarios
(with the IPCC models for scenario SR1.5) [13]. In the STEPS, renewables meet
80% of the growth in global electricity demand to 2030, hydropower remains the
largest renewable source of electricity, but solar is the main driver of growth as it sets
new records for deployment each year after 2022, almost tripling from today’s levels
and followed by onshore and offshore wind. The modelled change in global energy
generation from 2019 to 2040 is expected to be 4813 TWh for photovoltaics in the
STEPS scenario, a change in twenty years that is seven times larger than the change
occurred in the previous twenty years (664 TWh from 2000 to 2019). This deploy-
ment of PV capacity will require a fast development of smart, digital and flexible
electricity networks and the requirement of new transmission and distribution lines
is 80% larger for the next decade compared to the extension paths seen during the
past ten years. Data about population growth in the STEPS is taken from the United
Nations and considered that the total population rises from 7.7 billion in 2019 to 10.4
billion in 2070, an average growth of 0.6% per year, with almost three quarters of
global increase up to 2070 occurring in Africa, and India accounting for a 10% share
in the growth and becoming the most populous country in 2024.
The DelayedRecoveryScenario(DRS) isdesignedwiththesamepolicyassump-
tions as in the STEPS, but considering that a prolonged pandemic causes lasting dam-
age to economic prospects. The global economy returns to its pre-crisis size only in
2023, and the pandemic ushers in a decade with the lowest rate of energy demand
growth since the 1930s. Prior to the crisis, energy demand was projected to grow
by 12% between 2019 and 2030. Growth over this period is now 9% in the STEPS,
and only 4% in the DRS with the consequent slowdown of the economic activity
in all end-user sectors and, therefore, in energy demand (with important impacts on
transport, for example, where the number of cars in the DRS is 50 million lower than
in the STEPS).
The Sustainable Development Scenario (SDS), where a surge in clean energy
policies and investment puts the energy system on track to achieve sustainable energy](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-31-2048.jpg)
![1.2 Photovoltaics in the Scenarios of the International Energy Agency 13
objectives in full, including the Paris Agreement, energy access and air quality goals.
The assumptions on population growth, GDP and other socioeconomic parameters
are the same as in the STEPS. The SDS scenario is based on a stronger technological
development of the energy sector that is modelled by using the Energy Technology
Perspectives 2020 Model (ETP) of the International Energy Agency, which explores
the evolution in energy supply (using an energy conversion model from primary
energy, grouped in fossil, nuclear and renewables to final energy such as electricity,
heat, gasoline and diesel) and in the three end-user sectors with the highest energy
demand and largest greenhouse gas emissions (using models for industry, transport
and buildings). The energy conversion step considers 400 technological options,
described in terms of detailed technical and economical parameters including learn-
ing curves, thus providing a broad range of possible combinations. Interestingly,
the model also considers hydrogen-based fuels (synthetic hydrocarbon fuels from
hydrogen and CO2 or ammonia) and direct air capture of CO2 from the atmosphere,
though a cross-cutting technology option; but although these technological options
have been demonstrated at small or medium scale, they are still not deployed com-
mercially, and, therefore, some uncertainty is introduced in the model. The modelled
change in global energy generation from 2019 to 2040 is expected to be 8135 TWh
for photovoltaics in the SDS scenario. Details of the model can be found in the IEA
report “Energy Technologies Perspective 2020 Model” (updated in 2021 from its
previous 2016 version, [6]).
The new Net Zero Emissions by 2050 case (NZE2050) extends the SDS analysis.
The NZE2050 scenario is consistent with around a 50% chance of limiting the long-
term average global temperature rise to 1.5 ◦
C, as stated in the Paris Agreement. A
rising number of countries and companies are targeting net zero emissions, and all
stated policies are considered to come into force although there is still not a clear
commitment or detailed plans from governments to do so. The NZE2050 includes
the first detailed IEA modelling of what would be needed in the next ten years to
put global CO2 emissions on track for net zero by 2050. Reaching net zero globally
by 2050 would demand a set of dramatic additional policies and actions over the
next ten years, starting already in 2021 with no new oil and gas fields approved for
development and no new coal mines or mine extensions; only new coal plants with
carbon capture and storage could be approved beyond 2021.
The NZE2050 scenario considers that total energy supply falls by 7% between
2020 and 2030, reaching a total of 550 exajoules (EJ) and remains at around this
level until 2050, this reduction achievement occurs by reducing the energy intensity
of GDP growth by 2% annually. Renewable sources will supply 80% of total energy
supply by 2050, growing from 20% in 2020. Electrification is one of the key drivers
towards a de-carbonization of the energy sector with global electricity demand more
than doubling from 2020 to 2050.
Bringing about a 40% reduction in emissions by 2030 requires that low-emission
sources provide nearly 75% of global electricity generation in 2030 (up from less
than 40% in 2019). Again, hydrogen and CO2 capture are essential for this horizon;
150 million tonnes of hydrogen should be produced with 650 GW installed capacity
of electrolyzers by 2030 (rising to 435 million tonnes and 3000 GW, respectively, in](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-32-2048.jpg)
![14 1 Scenarios for Solar Electricity at the TeraWatt Scale
2045);4GtofCO2 shouldbecapturedby2035(risingto7.6Gtby2050).Importantly,
the NZE2050 model considers that by 2030 all world population will have access to
electricity and clean cooking (at an estimated cost of 40 USD billion) and the cost
of energy services for households will be affordable and stable even if an increase
in energy consumption is produced. The achievement of the NZE2050 scenario will
require of strong policy impulse for emission cuts already in 2030 and a constant
technological development (most of the reductions beyond 2030 rely on technologies
yet to come); only new international standards, regulations and intense cross-border
cooperation could guarantee the needed framework for this ambitious objective.
A large investment is required in the electricity generation, energy infrastructure
for distribution and end-user sectors. In electricity generation, an initial surge from
annual investment of about USD 0.5 trillion (average over the past five years) to
USD 1.6 trillion in 2030 should be achieved, then annual investment in renewables
in the electricity sector should be around USD 1.3 trillion (slightly more than the
highest level ever spent on fossil fuel supply which was USD 1.2 trillion in 2014);
after this peak in 2030, investment can be reduced to around 30% by 2050. Similarly,
investment in energy infrastructure for distribution (electric vehicle charging stations,
hydrogen) and carbon capture, transport and storage should increase from USD 290
billion over the past five years to about USD 880 billion in 2030 and for low-carbon
technologies in end-user sectors should rise from USD 530 billion in recent years to
USD 1.7 trillion in 2030.
The NZE2050 scenario can be considered as an optimistic path for a more sus-
tainable energy generation, and in particular electricity generation as indicated in
Table 1.2; therefore, it is an scenario where photovoltaic electricity will play a sub-
stantial role with a large increase both in installed capacity and electricity generation
in the coming decades. In this scenario, the TeraWatt scale for PV capacity will be
surpassed within two or three years, reaching almost 5 TW in 2030 and surpassing
10 TW in 2040. Beyond this point, new installed capacity will coincide with the
decommissioning of several GW of previously installed capacity that would have
reached its end of life and recycling could become an important industrial activity.
The contribution of renewable electricity generation is key to achieve the ambi-
tious objective of net zero emissions by 2050. The evolution of total CO2 emissions
in Table 1.2 includes carbon dioxide emissions from the combustion of fossil fuels
and non-renewable wastes, from industrial and fuel transformation processes (pro-
cess emissions) as well as CO2 removals. The energy transition becomes evident in
the evolution of the CO2 intensity (elec.) shown in Table 1.2, that refers to the CO2
emissions per each kWh of electricity generation; it will achieve a net zero balance
before 2040 and become negative afterwards, with the electricity sector acting as
a carbon sink for other sectors. Details of the scenario are provided in the Interna-
tional Energy Agency report “Net Zero by 2050—A Roadmap for the Global Energy
Sector” [7].](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-33-2048.jpg)
![1.3 The TeraWatt Scale of Photovoltaic Deployment: Is There Any Limit? 15
Table 1.2 Electricity capacity (GW) and generation (TWh) (total, renewables and solar PV) and
energy-related CO2 emissions evolution for the NZE2050 scenario of the International Energy
Agency. Data from the International Energy Agency report “Net Zero Emissions by 2050. A
Roadmap for the Global Energy Sector” [7]
Share (%) CAAGR∗ (%)
Electricity 2020 2030 2040 2050 2020 2030 2050 2020–
2030
2030–
2050
Total capacity GW 7795 14933 26384 33415 100 100 100 6.7 5
Renewables capacity GW 2994 10293 20732 26568 38 69 80 13 7.5
Solar PV capacity GW 737 4956 10980 14458 9 33 43 21 10
Total generation TWh 26778 37316 56553 71164 100 100 100 3.4 3.3
Renewables generation TWh 7660 22817 47521 62333 29 61 88 12 7.2
Solar PV generation TWh 821 6970 17031 23469 3 19 33 24 12
Total CO2 Mt CO2 33903 21147 6316 0 –4.6 –55.4
CO2 (electricity + heat) Mt CO2 13504 5816 –81 –369 –8.1 n.a.
CO2 intensity (elec.) kg
CO2/kWh
0.438 0.138 −0.001 −0.005 –11 n.a.
aCAAGR = compound average annual growth rate
1.3 The TeraWatt Scale of Photovoltaic Deployment: Is
There Any Limit?
The energy transition that slowly started after the oil crisis in 1973 has gained momen-
tum and it will change the energy landscape in the coming years. The main driver for
this change has been shifted from the fear of a supply risk of fossil fuels, sometimes
linked to the frequent claim that fossil fuels were achieving their peak production
and become more scarce and more expensive every year. This was not the case so far
(although some oil fields have indeed reached their peak). But the main driver now
is the need to reduce the demand and consumption of fossil fuels, due to the urgent
need to reduce CO2 emissions and mitigate climate change, the biggest challenge for
the twenty-first century. The contribution of renewable energies to the electricity mix
and the increasing electrification of the global energy production and consumption
for all end-user sectors create a synergy path where photovoltaic could become the
main electricity supplier and perhaps the main global primary energy supplier.
In all the International Energy Agency scenarios presented in Sect. 1.2, photo-
voltaic deployment is going to reach the TeraWatt scale in the coming years, with
the most optimistic NZE2050 scenario pointing to 2030 to nearly reach the 5 TW
milestone. It seems that at the initial stages of the TeraWatt scale, no insurmountable
limiting factor has been pointed out in the reports, although some barriers have been
identified and policies have been recommended to overcome them, but:
Is there any limit?](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-34-2048.jpg)
![20 2 Photovoltaic Technology
technology. Within the lifetime of the devices, solar photovoltaic technology will
provide much more energy than the one required to its manufacture: the balance is
positive from the point of view of generated versus embedded energy, and it is also
positive when the GHG emissions associated with electricity production is compared
to any other means to produce the same amount of electricity by non-renewable
sources. A quantification of this balance is one main conclusion of this book.
An important characteristic of photovoltaic technology is its modularity, that is,
its capability to work as an efficient power converter at all scales. A small solar cell is
as efficient as a module, or as a generator, or even as a very large plant; in fact, a small
laboratory cell is more efficient than the larger devices or systems. A device with
100 cm2
active area is more efficient than a 100 Ha PV plant. Of course, the small
cell will provide a few Watts of power, while a solar plant may reach hundreds of
Mega Watts (MW, even nowadays a few Giga Watts, GW), but the power conversion
efficiency (PCE) when converting the Sun light power into electrical power is better
in the small cell. In this chapter, an introduction of the working principles of the
solar cell is presented, followed by the “scaling-up” from cell to module with a focus
on its material components. Power management once it is converted from light into
electricity requires additional elements of the photovoltaic system which are grouped
in the so-called “balance of system” components, including electricity storage means.
2.1.1 A Brief History of the Development of the Solar Cell
Three main stages can be proposed to summarize the development of photovoltaic
technology.Oneearlystagecharacterizedbyslowexperimentalprogressduringnine-
teenth century since the discovery of the photovoltaic effect by Edmund Becquerel
in 1839 and culminating with the discovery of the electron by J. J. Thomson in 1897,
followed by a second stage coincident with the quantum revolution, from Planck’s
proposal of the Light Quanta in 1900 to the development of quantum solid-state
physics, where theory and experiment progressed steadily with two interruptions
caused by the First and Second World Wars. Two technological advances culminate
this second stage: the discovery of the transistor in 1947 and the first solar cell with
power conversion efficiency higher than 5% in 1954. These two stages are summa-
rized in this subsection. The third one is a stage of technological development in
pursuit of higher power conversion efficiencies, when experimental advancements
in small laboratory solar cells have been quickly applied to commercial photovoltaic
modules during fifty years and accelerated since the early 2000s with the onset of
organic and hybrid technologies and the massive deployment of installed power
capacity of inorganic technologies. This third stage of technological development
during the past seventy years is summarized in the final subsection of this chapter.
The first scientifically reported effect of the light on the electrical transport prop-
erties of a material was presented by Edmund Becquerel in 1839 [3, 4]. These reports
are considered the discovery of the photovoltaic effect. He observed an electrical cur-
rent passing through a liquid electrolyte (aqueous alkaline, neutral or acidic) when](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-38-2048.jpg)
![2.1 Introduction to the Physics of Solar Cells: Power Conversion from Sun to Electricity 21
the Sun light illuminated a silver chloride or silver bromide coated platinum electrode
and analysed the chemical reactions triggered by the action of light. It took almost
forty years for a new report of a photovoltaic effect, in this case on a solid-state
selenium sample; in 1876, Adams and Day were studying the photoconductivity of
selenium and they observed an increase in photocurrent when the sample was illu-
minated, but intriguingly, the current was also produced in the absence of a driving
voltage: the current was produced by the action of light and not by an applied voltage
[1]. They had invented the first solid-state photovoltaic cell: by using two platinum
electrodes in the selenium sample, a metal-semiconductor rectifying Schottky bar-
rier contact had been created, although those concepts were not known at that time.
The same structures (a metal pressed on a piece of semiconducting material) was
used by several scientists with the aim to develop a device which could work as a
reliable, calibrated, light sensor: Charles Fritts, by coating the selenium with gold,
created the first working solar cell in 1883 with 1% power conversion efficiency [7]
which was pushed up to 2% shortly afterwards by Heinrich Hertz with more focus
on the photodetector research that he was carrying out and which ultimately lead
to the discovery of the photoelectric effect when ultraviolet light was illuminating a
metallic plate and produced the effect of discharging the plate [10]. A decade later,
and also by illuminating with ultraviolet light, J. J. Thomson discovered that the
“cathodic rays” emitted by the metallic plate could be composed of tiny particles,
that he called “corpuscles” and were later named electrons [25]. All experimental
ingredients of the photovoltaic and photoelectric effects had been discovered by the
end of nineteenth century, but the theoretical explanation and the full understanding
of the difference between them was only possible after the full development of the
quantum theory, which started in the first year of twentieth century (Fig. 2.1).
The discovery of the electron by J. J. Thomson was followed by the revolution-
ary proposal of Max Planck in 1900, the equation which describes the blackbody
radiation in terms of Light Quanta [20]. The equation was successful in explaining
experimental data about the radiation emitted by a body at temperature T and which
had been elusive so far. Planck’s equation, written in terms of the light frequency, is
B(ν, T ) =
2hν3
c2
1
e
hν
kB T
− 1
. (2.1)
Equation 2.1 is the spectral distribution of the radiation emitted by the blackbody, that
is, the number of light quanta at each frequency interval from ν to ν + δν. Planck was
awarethathisempiricalequationwascorrectsincehehadfirst-handinformationfrom
experimental colleagues. He then tried to deduce the equation from first principles,
which he did in a second article where the revolutionary proposal of light quanta was
made in order to be able to deduce the equation proposed in his first 1900 paper. The
light came in packages of energy, each light quanta with an energy proportional to
its frequency ν [21]:
E = hν =
hc
λ
, (2.2)](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-39-2048.jpg)
![22 2 Photovoltaic Technology
Fig. 2.1 Time frame of the theoretical and experimental developments during the first half of the
twentieth century which led from the discovery of the electron and Planck’s quantum theory of light
to the fabrication of the first solar cell with power conversion efficiency higher than 5%
where h is Planck’s constant, h = 6.62607015 × 10−34
Js, the quantum of “action”
(energy×time), c is the speed of light, c = 299792458 ms−1
and λ is its wavelength.
Planck’s 1900 articles did not have a very strong impact in the first years of the twen-
tieth century. Planck was always trying to keep a connection to classical thermody-
namic theory via the concept of entropy and the inclusion of Boltzmann’s constant
in his equation (kB = 1.380649 × 10−23
JK−1
). It was only after Albert Einstein
applied the light quanta revolutionary concept to his successful explanation of the
photoelectric effect when the old quantum theory started to be broadly accepted [5].
The origin of the old quantum theory is, therefore, linked to photovoltaic technology
by two fundamental concepts: first, the blackbody radiation describes the resource
which is coming from the Sun, that is, the light and its spectral distribution in terms
of the number of photons with given energies at each frequency (or wavelength inter-
val), and second, the light quanta and Einstein explanation of the photoelectric effect
that describes how ultraviolet light interacts with matter; it explains how the light
quanta are absorbed by the material: in packages of well-defined energy, later called
photons [5].
Nevertheless, the photoelectric effect should not be confounded with the photo-
voltaic effect. In the photoelectric effect, high-energy photons (blue or ultraviolet)
are absorbed by a metallic material and electrons are expelled from the material (in
air or preferably in a vacuum chamber); its main application are in photomultiplier
detectors or photoelectron spectroscopy (UPS, XPS). In the photovoltaic effect, the](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-40-2048.jpg)
![24 2 Photovoltaic Technology
where Ee is the energy of the electron, and EF is the Fermi energy that indicates the
energy level below which all states are fully occupied at T = 0. If T 0, a small amount
of electrons is excited across this Fermi energy and occupy states with E EF . In
intrinsic semiconductors, with well-defined conduction and valence bands, the Fermi
level is given by
EF =
Ec − Ev
2
, (2.5)
where Ec is the minimum energy level within the conduction band and Ev is the
maximum energy level within the valence band. Both the Bose–Einstein and the
Fermi–Dirac statistics recover at high temperatures (and low concentrations of par-
ticles) the classical thermodynamic Maxwell–Boltzmann distribution function.
With those statistical ingredients, the development of solid-state physics pro-
gressed rapidly. Bloch’s theorem (1928) enabled the possibility of solving Schrö-
dinger’s equation in crystalline solids and obtaining the wavefunction and eigenen-
ergies of electrons within a solid. When solved for a large number of atoms, the
atomic orbitals are very closely spaced in energy (around 1022
available states per
eV1
), thus creating some ranges of quasicontinuum energy called “bands”; these
bands are separated by ranges of forbidden energy, commonly known as the “energy
gap”, Eg, for which there is no solution of the wave equation, i.e. there is no
wavefunction at this energy, and therefore, there is no available state to accom-
modate any electron. The combination of the Bloch theorem and the progress in
experimental solid-state physics enabled a very rapid progress in the understand-
ing of the behaviour of electrons within solids, with the works of Eugene Wigner
and León Brillouin on the atomic structure of materials and Arnold Sommerfeld
which developed the first models of electrons in solids (Drude–Sommerfeld model,
1927) and later by Nevill Mott who proposed a full quantum theory for elec-
trons within solids, including metal–insulator transitions and electrons in disordered
semiconductors [15, 16].
A detailed description of band calculations and quantum electronic transport in
solids is out of the scope of this book and can be found in very good solid-state
physics books, like the classical Ashcroft and Mermin book [2] and with more focus
on photovoltaic technology, in the excellent books by Jenny Nelson and Peter and
Uli Würfel [18, 27]. Nevertheless, the concepts of Fermi energy and energy gap
are at the core of semiconducting physics, and an understanding of the underlying
physics of photogeneration requires at least a grasp of its physical meaning which is
presented in the following subsections.
The final steps of the second stage of the evolution of the solar cell are provided
by two inventions. The first one is the fabrication of the first solid-state transistor
by John Bardeen, William Shockley and Walter Brattain at Bell Labs in 1947 on a
piece of germanium with metallic gold contacts; this experimental device opened the
door to solid-state electronics based on semiconducting materials which was rapidly
1 The electron-volt, eV, is a very convenient energy unit in solid-state physics, it is defined as the
energy that an electron acquires when accelerated in an electric field of 1V and, by definition, is
equal to 1.602 × 10−19 J.](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-42-2048.jpg)
![26 2 Photovoltaic Technology
Fig. 2.2 Spectral irradiance outside the Earth’s atmosphere (AM 0), on the Earth’s surface for direct
sunlight (AM 1.5D) and the direct sunlight together with the scattered contribution from atmosphere
integrated over a hemisphere (AM 1.5 G) (according to ASTM G173-03 and in comparison to the
spectrum used by Shockley and Queisser of a blackbody with a surface temperature of 6000 K (BB
6000 K). Reproduced with permission from reference [23]
have been constructed and are available. Diffuse radiation is calculated by using
isotropic and anisotropic models, where one circumsolar anisotropy component is
considered, or an additional horizon-dependant second anisotropy is also included [9,
17, 19]. Albedo contributions are strongly dependant on geographical location and
surrounding topography or structures; therefore, the best estimations are provided
by empirical databases, like the Copernicus Global Land Service3
of the European
Union, which is based on satellite observations. The most important solar radiation
database is PVGIS,4
the Photovoltaic Geographical Information System of the Joint
Research Centre of the European Commision, which provides free and open access
to its irradiation and meteorological database including the following, among other
data:
• Solar radiation and temperature, as monthly averages or daily profiles (database
and maps).
• Typical Meteorological Year data for nine climatic variables.
• Full-time series of hourly values of solar irradiance.
Other databases, such as those of the National Renewable Energy Laboratory (NREL)
and the National Aeronautics and Space Administration (NASA) (in the United States
of America) or other national meteorological organizations are also available. With
3 Copernicus Global Land Service, https://land.copernicus.eu/global/products/sa.
4 PVGIS-JRC(EU), https://ec.europa.eu/jrc/en/pvgis.](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-44-2048.jpg)
![2.1 Introduction to the Physics of Solar Cells: Power Conversion from Sun to Electricity 35
Table 2.2 Empirically determined coefficients used to predict module back surface temperature as
a function of irradiance, ambient temperature and wind speed with the SANDIA model [12]
Module type Mount a b
Glass/cell/glass Open rack –3.47 –0.0594
Glass/cell/glass Close roof mount –2.98 –0.0471
Glass/cell/polymer
sheet
Open rack –3.56 –0.0750
Glass/cell/polymer
sheet
Insulated back –2.81 –0.0455
Polymer/thin film/steel Open rack –3.58 –0.113
22× Linear
Concentrator
Tracker –3.23 –0.130
the operating temperature of the cell at any other ambient conditions by using the
linear Ross model given in Eq. 2.18, and the thermal coefficients of losses are fitted
experimentally [22]
Tm = Ta +
N OCT − 20
800
G = Ta + K G, (2.18)
where K is the Ross coefficient, it is expressed in units ◦
Cm2
/W and can be defined
as K = (N OCT − 20)/800 when G is expressed in units W/m2
. The first value
reported by Ross was 0.03 ◦
Cm2
/W for crystalline silicon and wind speed lower
than 1 m/s2
(which delivers a NOCT around 47 ◦
C) [22]. For other thin film PV
technologies such as a-Si, CIGS, CdTe in different orientations and even in BIPV
applications, the obtained Ross coefficient is always around 0.03 ◦
Cm2
/W with small
deviations; only organic technologies deliver lower values (around 0.02 ◦
Cm2
/W)
but in this case, the value seems to be more dependant on the encapsulation and fram-
ing material than the organic photovoltaic cell material [26]. Manufacturers always
provide the empirical NOCT for the PV modules as the main thermal parameter.
A more sophisticated thermal model for the solar cell includes the influence
of wind and an exponential behaviour was proposed by researchers from Sandia
National Laboratory (USA) in reference [12] and it is presented in Eq. 2.19; the two
parameters a and b to be used are obtained empirically for different combinations
of materials in cell, encapsulants, cover, backsheet and frames; they are found in
scientific references, but very rarely reported by the manufacturers of modules; a
summary is presented in Table 2.2.
Tm = Ta + e(a+bWs )
G. (2.19)
Outdoor tests carried out in different climatic regions have lead to more detailed
models for NOCT in real operating conditions according to the international stan-
dards IEC 61215 and IEC 61646, showing that natural convection can be neglected](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-53-2048.jpg)
![36 2 Photovoltaic Technology
Table 2.3 Empirically determined coefficients used to predict cell temperature [13]
U0 U1
c-Si 30.02 6.28
c-Sia 26.9 6.2
a-Si 25.73 6.24
CIS 22.64 3.61
CdTe 23.37 5.44
aUsed in reference [11] and calculated as an average of values reported in [13]
Table 2.4 Summary of thermal parameters used to characterize solar cells
Parameter Symbol Units
Nominal operating cell
temperature
NOCT ◦C
Power (Pmpp) thermal
coefficient
γ %/◦C (negative)
Current (Isc) thermal
coefficient
αI %/◦C (positive)
Voltage (Voc) thermal
coefficient
βV %/◦C (negative)
for wind speeds above 2 m/s, that the main effect of radiation cooling can be found
during night time which is not relevant for the solar energy gain and that the effect of
wind gusts and fast temperature changes is low [13]. Therefore, yet another method
was proposed to calculate module temperature in different ambient conditions; it is
used by the PVGIS model (European Commission Joint Research Centre, [6]):
Tm = Ta +
G
U0 + U1W
, (2.20)
where Ta is the ambient temperature and W is the wind speed. The coefficients
U0 and U1 used in PVGIS have been obtained by fitting experimental data and are
summarized in Table 2.3 by providing the average value for each PV technology
[13].
Once the module temperature is calculated by using any of those simple models
or others which include additional environmental variables such as wind direction
and relative humidity, the temperature losses present a linear dependence such as
thermal coefficient × T where the thermal coefficient is given as a relative loss in
% (with respect to nominal STC values) per temperature degree (older PV module
datasheets used to provide absolute thermal losses, but it is no longer the case).
Typical values are around -0.3%/◦
C for power and voltage losses and +0.05%/◦
C for
current gains when the temperature of the operating module is above 25◦
C (opposite
effect when temperature is below 25 ◦
C) (Table 2.4).](https://image.slidesharecdn.com/23351945-250521174757-9d6aa812/75/Sustainable-Solar-Electricity-Antonio-Urbina-54-2048.jpg)
Sustainable Solar Electricity Antonio Urbina
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- 7. Climate change, environmentalimpact and the limited natural resources urge scientific research and novel technical solutions. The monograph series Green Energy and Technology serves as a publishing platform for scientific and technological approaches to “green”—i.e. environmentally friendly and sustainable—technolo- gies. While a focus lies on energy and power supply, it also covers “green” solu- tions in industrial engineering and engineering design. Green Energy and Tech- nology addresses researchers, advanced students, technical consultants as well as decision makers in industries and politics. Hence, the level of presentation spans from instructional to highly technical. **Indexed in Scopus**. **Indexed in Ei Compendex**. More information about this series at https://link.springer.com/bookseries/8059
- 8. Antonio Urbina Sustainable SolarElectricity
- 9. Antonio Urbina Institute forAdvanced Materials and Mathematics (INAMAT2) and Department of Sciences Public University of Navarra (UPNA) Pamplona, Spain ISSN 1865-3529 ISSN 1865-3537 (electronic) Green Energy and Technology ISBN 978-3-030-91770-8 ISBN 978-3-030-91771-5 (eBook) https://doi.org/10.1007/978-3-030-91771-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland
- 10. To Marijose, for everything, includingthe little thing(s).
- 11. Foreword Over the lastthirty years (my time as a researcher of photovoltaics), the solar photo- voltaic (PV) industry has grown at an astonishing rate from an installed global capacity of less than 100 to over 800,000 MWp. Solar PV has become the fastest- growing energy technology and the primary future source of electricity in most scenarios for low-carbon development. The “coming of age” of renewable electricity has been one of few good news stories in our efforts to mitigate climate change. Thanks to solar power and wind, the decarbonisation of electricity is no longer seen as a major challenge and has become a tool that can assist the decarbonisation of other sectors (transport, buildings, industry). The question of how to harness solar electricity affordably is more or less solved. But we still need to establish how to achieve the energy transition sustainably. In this book, Antonio Urbina presents a lucid account of the principles and tech- nology of solar photovoltaics, alongside an introduction to the concept of sustain- ability and to the metrics that quantify sustainability. He shows how, starting from the production process, the environmental impacts, resource requirements and energy balance can be quantified, making these impact assessments a natural extension to the design of PV technology. He also places photovoltaic technology in its larger global context by addressing economic aspects and the international regulatory and policy framework, in a detailed, up to date and informative manner. To my knowledge, this is the first book of its kind and I find it timely for three reasons. First, while energy technologies are commonly compared in terms of cost and performance metrics (such as power conversion efficiency for a solar cell), cost is not enough to distinguish options in terms of their effectiveness at reducing carbon emissions. A solar module with a shorter energy payback time and a smaller life-cycle averaged emissions intensity will be more effective than an alternative at mitigating CO2 emissions when it replaces a higher carbon technology. Life Cycle Assessment (LCA) techniques allow PV developers to evaluate the module designs and produc- tion processes that optimise those metrics. The best options may not be those of the lowest cost or the highest conversion efficiency. Identifying these priorities at the design stage allows more efficient use of resources. vii
- 12. viii Foreword Second, asthe energy transition advances, it becomes more necessary to consider solutions from a system-level perspective. For solar PV, that means not only consid- ering the output of a module but also the effectiveness of integrated systems, such as solar PV integrated with hydrogen generation for fuel supply or solar PV with desalination for clean water supply. To evaluate different technical solutions to the same demand in terms of their energy balance, resource costs or emissions impact, a means of comparing quite different technologies is needed. Life Cycle Assessment provides that and can be applied as part of the selection of technologies, avoiding lock-in to solutions that are less effective in terms of energy or emissions balance. Third, the technological revolution that lies ahead of us (if we are to avert the worst consequences of climate change) will be as great as the last industrial revolution, but much more rapid. Rapid change brings risks of social, economic, environmental and geopolitical impacts as well as emissions impacts. Before choosing pathways, it would be wise to evaluate them in terms of their overall sustainability. This book provides the basic knowledge to formulate and evaluate these questions. Antonio Urbina is well qualified to write this work, having researched the science of PV materials, evaluated solar PV systems and pioneered the application of LCA and sustainability assessment to emerging PV technologies. From this experience, base he shows how to make sustainability a central part of technology evalua- tion. Although the book presents LCA and sustainability analysis in the context of solar electricity, the methodologies are very readily transferrable, and increasingly relevant, to other energy, and non-energy, technologies. London, UK November 2021 Jenny Nelson
- 13. Acknowledgements I started workingon photovoltaics at two levels in the mid 90s during the final years of my Ph.D. (which was focussed on the Quantum Hall Effect, a very different issue, but which shares with photovoltaics the use of advanced semiconducting devices). The first level was a very practical approach: the use of small photovoltaic solar home systems for rural electrification in developing countries, an interest which started with a course delivered by the Instituto de Energía Solar (Madrid) and I must acknowl- edge the enthusiasm on the subject put by the researchers that delivered the course: Dr. Pablo Díaz, Dr. Estefanía Caamaño and Dr. Miguel A. Egido, which taught me the fundamentals of practical PV system design. The second level was the deepening of the theoretical understanding provided by the books of Prof. Jenny Nelson (Imperial College London) and Prof. Eduardo Lorenzo (Instituto de Energía Solar, Madrid), and I must acknowledge the authors not only for writing the books, but also for facil- itating always friendly communications and discussions on photovoltaic technology and its practical deployment. The acknowledgement to Prof. Jenny Nelson must be extended to her invitation for a research stay at Imperial College, and the subsequent research collaboration that we have kept since then and which continues to this day, also including other colleagues at Imperial College which I acknowledge: Prof. Ji- Seon Kim, Prof. James Durrant, Dr. Sachetan Tudhalar, Dr. Christopher Emmott and Dr. Wing Chung Tsoi (now at Swansea University). Regarding my research work in organic and hybrid photovoltaic technologies with a special focus on stability studies, I acknowledge Prof. Frederik Krebs (CEO of Infinity PV, Denmark) and Prof. Mónica Lira-Cantú (Institut Catalá de Nanociència i Nanotecnolog a) for his and her constant support and fruitful collaboration, and Prof. Ana Rosa Lagunas (Centro Nacional de Energas Renovables, CENER, Spain) for helping me to bridge the gap between academic research and the complex world of standardization, certification and industrial applications of photovoltaic technology. It has also been very important the work of Dr. Lucía Serrano (Universidad Rey Juan Carlos, Madrid), Dr. Nieves Espinosa (Joint Research Centre, European Commis- sion), Dr. Rafael García-Valverde (Infinity PV, Denmark), Dr. Carlos Toledo (ENEA, Italy) and Dr. Rodolfo García (Universidad Politécnica de Cartagena, Spain), who ix
- 14. x Acknowledgements havebeenfundamentalcontributorstotheresearchofourgrouponLifeCycleAssess- ment ofphotovoltaic technologies, during and after their respective Ph.D. thesis work, which was completed under my supervision a few years ago. This research work was carried out in the context of projects in collaboration with Dr. José Abad, Dr. Antonio J. Fernández-Romero, Dr. Javier Padilla (UPCT), Prof. Jaime Colchero (Universidad de Murcia), Prof. Ana Cros and Prof. Nuria Garro (both at Univer- sidad de Valencia), Prof. Wolfgang Maser and Prof. Ana Benito (both at Instituto de Carboquímica ICB-CSIC, Zaragoza); to all of them I acknowledge their support with access to instruments and materials that have been used to fabricate and characterize organic and hybrid solar cells in the context of several collaborative projects and the discussions during seminars (and coffee breaks) during many fruitful years. Financial support must be acknowledged to Agencia Estatal de Investigación (Ministerio de Ciencia e Innovación, Spain), grant PID2019-104272RB-C55, and to Fundación Séneca (Spain), grant 19882-GERM-15, both including European Commission FEDER funds.
- 15. Contents Part I Introduction 1Scenarios for Solar Electricity at the TeraWatt Scale . . . . . . . . . . . . . 3 1.1 Evolution of Installed Photovoltaic Capacity . . . . . . . . . . . . . . . . . 6 1.2 Photovoltaics in the Scenarios of the International Energy Agency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.3 The TeraWatt Scale of Photovoltaic Deployment: Is There Any Limit? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2 Photovoltaic Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.1 Introduction to the Physics of Solar Cells: Power Conversion from Sun to Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.1.1 A Brief History of the Development of the Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.1.2 Solar Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.1.3 Metals and Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . 27 2.1.4 Equivalent Circuit and Parameters of the Solar Cell . . . . 30 2.2 The Basic Structure of a Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.2.1 Active Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.2.2 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.2.3 Transporting Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 2.3 Classification of PV Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . 45 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3 Assessment of Sustainability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.1 Environmental Sustainability: Life Cycle Assessment Applied to Energy Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.1.1 Goal and Scope of the LCA . . . . . . . . . . . . . . . . . . . . . . . . 55 3.1.2 Life Cycle Inventory Analysis (LCI) Phase . . . . . . . . . . . 57 3.1.3 Life Cycle Impact Assessment (LCIA) Phase . . . . . . . . . 59 3.1.4 Life Cycle Interpretation Phase . . . . . . . . . . . . . . . . . . . . . 64 xi
- 16. xii Contents 3.2 SocioeconomicSustainability: Energy and Sustainable Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 3.2.1 Life Cycle Costing and Total Cost of Ownership . . . . . . 66 3.2.2 Levelized Cost of Energy (LCOE) . . . . . . . . . . . . . . . . . . . 68 3.2.3 Value-Adjusted Levelized Cost of Electricity (VALCOE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 3.2.4 Circular Economy, Environmental Footprints and Sustainable Development . . . . . . . . . . . . . . . . . . . . . . . 72 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Part II Life Cycle Assessment of Solar Electricity 4 Production of PV Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 4.1 Crystalline Silicon Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 4.1.1 Silicon Processing: From Raw Material to Solar Grade Ingots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 4.1.2 Crystalline Solar Cell Manufacture . . . . . . . . . . . . . . . . . . 92 4.2 Thin Film Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 4.2.1 Amorphous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 4.2.2 Cadmium Telluride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.2.3 Chalcopyrites and Kesterites . . . . . . . . . . . . . . . . . . . . . . . 102 4.3 III-V Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 4.4 Organic and Hybrid Emerging Technologies . . . . . . . . . . . . . . . . . 107 4.4.1 Organic Bulk Heterojunctions . . . . . . . . . . . . . . . . . . . . . . 108 4.4.2 Dye Sensitized . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 4.4.3 Perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 4.5 From Cells to Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 5 The Limits of Raw Materials Embedded in PV Modules . . . . . . . . . . 131 5.1 Silicon Feedstock and Other Raw Materials Embedded in the PV Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 5.2 Glass, Plastics and Frames for the PV Modules . . . . . . . . . . . . . . . 139 5.3 Strategic and Scarce Materials Embedded in PV Modules . . . . . . 142 5.4 Polluting and Toxic Materials Embedded in PV Modules and Used in Its Manufacturing Process . . . . . . . . . . . . . . . . . . . . . . 148 5.4.1 Silicon Mining and Processing Risks . . . . . . . . . . . . . . . . 150 5.4.2 Cadmium Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 5.4.3 Lead Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 5.4.4 Sulphur Hexafluoride Environmental Damage . . . . . . . . . 152 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 6 The Energy Balance of Solar Electricity . . . . . . . . . . . . . . . . . . . . . . . . . 157 6.1 Embedded Energy in Photovoltaic Systems . . . . . . . . . . . . . . . . . . 157 6.1.1 Embedded Energy in the Processing of Materials . . . . . . 158
- 17. Contents xiii 6.1.2 EmbeddedEnergy in the Manufacturing of Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 6.2 Solar Electricity Production of a Photovoltaic System . . . . . . . . . 163 6.2.1 Electricity Production and Yield . . . . . . . . . . . . . . . . . . . . 163 6.2.2 Lifetime of Photovoltaic Systems . . . . . . . . . . . . . . . . . . . 168 6.3 Energy Payback Time and Energy Return on (Energy) Investment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 6.3.1 Energy Payback Time Definition . . . . . . . . . . . . . . . . . . . . 169 6.3.2 Technology Dependence of the Energy Payback Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 6.3.3 Geographical Dependence of the Energy Payback Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 6.3.4 Energy Return on (Energy) Investment . . . . . . . . . . . . . . . 173 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 7 Impacts of Solar Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 7.1 Human Health Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 7.2 Environmental Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 7.3 Land use, Water, Mineral, Fossil and Renewable Depletion Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 7.4 The Rapidly Evolving Impacts of Emerging PV Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 7.5 Size Dependant Impacts of PV Systems: Land Occupancy and Agrivoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 7.6 Impacts of Module Transportation During Manufacture, Installation and End of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 8 Recycling and End of Life of PV Technologies . . . . . . . . . . . . . . . . . . . . 199 8.1 Reusing PV Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 8.2 Recycling PV Modules: Recovery of Components and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 8.3 Recovery and Reuse of Substances Required for PV Module Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 9 Balance of System (BoS) and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 9.1 Life Cycle Assessment of BoS Electronic Components . . . . . . . . 216 9.2 Life Cycle Assessment of BoS Structural and Mechanical Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 9.3 Introduction to Electricity Storage for PV Systems . . . . . . . . . . . . 221 9.3.1 Electricity Storage Technologies . . . . . . . . . . . . . . . . . . . . 221 9.3.2 Battery Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 9.4 Overview of Life Cycle Assessment Applied to Batteries . . . . . . . 224 9.4.1 Phases in LCA for Batteries . . . . . . . . . . . . . . . . . . . . . . . . 224 9.4.2 Phases in LCA Including Second Life of Batteries . . . . . 225
- 18. xiv Contents 9.4.3 Resultsof LCA for Batteries . . . . . . . . . . . . . . . . . . . . . . . 226 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 Part III Beyond Life Cycle Assessment: Socioeconomics and Geopolitics of Solar Electricity 10 Socioeconomic Impacts of Solar Electricity . . . . . . . . . . . . . . . . . . . . . . 235 10.1 Cost of Ownership of Photovoltaic Systems . . . . . . . . . . . . . . . . . . 235 10.2 The Cost of Solar Electricity: A Steady Learning Curve . . . . . . . . 241 10.3 The Cost of Electricity Storage in Batteries . . . . . . . . . . . . . . . . . . 244 10.4 Employment Opportunities Linked to the Solar Electricity Sector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 11 Standardization and Regulations for PV Technologies . . . . . . . . . . . . 249 11.1 International Technical Standards for Photovoltaic Technology and Life Cycle Assessment . . . . . . . . . . . . . . . . . . . . . 249 11.1.1 International Organization for Standardization . . . . . . . . 250 11.1.2 International Electrotechnical Commission . . . . . . . . . . . 251 11.1.3 Other International and National Standardization Organizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 11.2 Regulatory Frameworks for Production, Recycling and End of Life of PV Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 11.2.1 China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 11.2.2 European Union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 11.2.3 United States of America . . . . . . . . . . . . . . . . . . . . . . . . . . 259 11.2.4 Other Countries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 11.3 Ecodesign, Ecolabelling and Green Public Procurement . . . . . . . . 261 11.3.1 Ecodesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 11.3.2 Ecolabelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 11.3.3 Green Public Procurement . . . . . . . . . . . . . . . . . . . . . . . . . 264 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 12 Solar Electricity and Globalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 12.1 World Electricity Consumption Per Cápita . . . . . . . . . . . . . . . . . . . 268 12.2 Access to Energy and Development . . . . . . . . . . . . . . . . . . . . . . . . . 269 12.3 Solar Electricity for Rural Electrification: When There is No Electricity Market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 12.4 Mitigation of Climate Change: From Kyoto Protocol to Paris Agreement and Beyond . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 12.5 Geopolitics of Photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
- 19. Acronyms AC Alternating Current AMAir Mass a-Si Amorphous Silicon BGS British Geological Survey BHJ Bulk Heterojunction BIPV Building Integrated Photovoltaics CdTe Cadmium Telluride CIGS Copper Indium Gallium (di)Selenide, chalcopyrite structure c-Si Crystalline Silicon CSR Corporate Social Responsibility CSS Closed Space Sublimation CZ Czochralski CZTS Copper Zinc Tin (di)Selenide, kesterite structure DC Direct Current EC European Commission ED Electro Deposition EPBT Energy Payback Time EPR Extended Producer Responsibility ETL Electron Transporting Layer EVA Ethylene-Vinyl-Acetate FAPI Formamidinium Lead Iodide, perovskite structure FF Fill factor FTO Fluor Tin Oxide FU Functional Unit FZ Floating Zone GHG Greenhouse Gases GRR Ground Requirement Ratio HIT Heterojunction with an Intrinsic Thin layer HTL Hole Transporting Layer Isc Short circuit current IBC Interdigitated Back contact Cell xv
- 20. xvi Acronyms IEA InternationalEnergy Agency IEC International Electrotechnical Commission III-V Elements of groups III and V of the periodic table IPCC Intergovernmental Panel on Climate Change IRENA International Renewable Energy Agency ISO International Organization for Standardization ITO Indium Tin Oxide I-V Current–voltage characteristic curve of a solar cell or module JRC Joint Research Centre (European Commission) LCA Life Cycle Assessment LCIA Life Cycle Impact Assessment LCOE Levelized Cost of Energy (or Electricity) LEC Liquid Encapsulated Czochralski LPE Liquid Phase Epitaxy MAPI Methyl Ammonium Lead Iodide, perovskite structure MBE Molecular Beam Epitaxy mc-Si Multi-crystalline Silicon MOCVD Metal-Organic Chemical Vapour Deposition mono-Si Mono-crystalline Silicon MOVPE Metal-Organic Vapour Phase Epitaxy mpp Maximum power point (in a I–V or P–V curve) NREL National Renewable Energy Laboratory (USA) OPV Organic Photovoltaics Pmpp Power at maximum power point P3HT Poly-(3-Hexyl-Thiophene-2,5-diyl) PANI Poly-Aniline PAR Photosynthetically Active Radiation PCBM Phenyl-C61-Butyric acid Methyl ester PCE Power Conversion Efficiency PECVD Plasma Enhanced Chemical Vapour Deposition PEDOT Poly-3,4-Ethylene-Dioxy-Thiophene PERC Passivated Emitter and Rear Cell PET Poly-Ethylene Terephthalate PPV Poly-(p-Phenylene-Vinylene) PR Performance Ratio PVD Physical Vapour Deposition PVF Poly-Vinyl Fluoride PVPS Photovoltaic Power Systems Programme (IEA) RFS Radio Frequency Sputtering sc-Si Single-crystalline Silicon SLS Soda Lime Silica UNFCCC United Nations Framework Convention on Climate Change USGS United States Geological Survey Voc Open circuit Voltage VALCOE Value-Adjusted Levelized Cost of Energy (or Electricity)
- 21. Part I Introduction Part Iis an introductory part which describes the main concepts regarding pho- tovoltaic technology and life cycle assessment. The book contents are built upon the combination of both areas of knowledge, and it is, therefore, important from the beginning to clarify the purpose and the scope of the study. This part also emphasizes the importance of the problem that the energy transition is facing: a huge amount of photovoltaic systems has been already deployed and many more are planned for the near future; many of these systems will have to be revamped, replaced or extended with new modules, and the old ones will need to be recycled or landfilled. In Chap. 1, the working scenarios proposed by the International Energy Agency are presented and the implications for photovoltaic capacity growth will be analysed in detail. In Chap. 2, the main components of photovoltaic systems are presented, ranging from cells to modules and then to whole systems; this chapter describes each component, its principles of work and the equations governing its main output (but not going into details of the physics behind semiconductor photogeneration and transport dynam- ics); the objective of this chapter is to define the main parameters used to evaluate photovoltaic (PV) cells, modules and system performance and to classify the “prod- uct” parts (a classification which is used for the Life Cycle Assessment (LCA) study of the different technologies). The “product” from the LCA perspective is the final PV system, which includes different steps: cells, modules and whole system (with Balance of System (BoS), components). In Chap. 3, the Life Cycle Assessment methodology is presented, with a special focus on its application to energy systems in general and photovoltaic systems in particular and also the inclusion of social and economic considerations for a broader LCA approach (methodologies still under discussion in the scientific community).
- 22. Chapter 1 Scenarios forSolar Electricity at the TeraWatt Scale A world shock has occurred in 2020, and it has strongly affected the energy sector. According to the preliminary estimations included in the most recent report from the International Energy Agency (World Energy Outlook 2020, [9]), the global energy demand dropped by 5% in 2020, and energy-related CO2eq emissions dropped by 7%. This shock in the demand side, concentrated in a single year, is higher in terms of energy demand reduction than the shock in the supply side that started in October 1973 due to an oil export embargo proclaimed by the Organization of the Petroleum Exporting Countries (OPEC) that lead to a sudden rise in oil prices. The impact of the oil crisis was long lasting, it reshaped the energy landscape worldwide and triggered the first steps to unlock the “carbon lock-in” and initiate an energy transition that is now fully fledged [1]. After a sudden shock, a well-established paradigm can be shifted if the policy response is clearly defined and enough investment is provided, initially in research activities and later in demonstration projects. The initial efforts triggered by the oil crisis put in place technological advancements that supported the early stages of the energy transition a few decades ago. In 2020, an external shock, the catastrophic COVID-19 pandemia led to public policies designed with strong investment efforts to reactivate the economy, and this “new deal” has created the opportunity to accelerate the energy transition with renewable mature technologies that are cost-competitive. Wind and photovoltaic technologies are already the cheapest source of electricity in many parts of the world. This combination of shock, new investment and tech- nological readiness could definitely move the world from the carbon lock-in to a renewables lock-in. Large investments have been announced worldwide to reacti- vate the economy, and a good share of this investment is oriented to reinforce the energy transition and to mitigate climate change. It is a great opportunity that will require new ambitious policies and a worldwide coordination of a good regulatory framework to support this move towards a more sustainable energy landscape. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. Urbina, Sustainable Solar Electricity, Green Energy and Technology, https://doi.org/10.1007/978-3-030-91771-5_1 3
- 23. 4 1 Scenariosfor Solar Electricity at the TeraWatt Scale But despite the brilliant perspective, this transition is still in its early stages. In 2019, total primary energy consumption in the world was 583.9 Exajoules,1 with an annual growth rate of 1.6% averaged for the past ten years. The renewables contribution to the total primary energy consumption was 66.64 Exajoules (11.3%, of which 6.4% from hydropower), while oil continues to hold the highest share (33.1%), followed by coal (27.0%), natural gas (24.2%) and then renewables (11.3%) that have already surpassed nuclear (4.3%). Electricity generation in 2019 was 27004.7 TWh2 (average annual growth in the past ten years was 2.7%, almost doubling the primary energy average annual growth, a clear indicator of the “electrification” of the global energy consumption), and renewable electricity generation was 7027.7 TWh (26.1%, including hydropower, its main contributor, with 4222.2 TWh equivalent to 15.6% followed by wind 1429.6 TWh, 5.2% and solar photovoltaic with 724.1TWh, 2.7%) [2]. According to the International Energy Agency Photovoltaic Power Systems Programme, world final electricity consumption was 24,700 TWh in 2019, with a share of renewable energy in the global electricity production of 28%, including 810 TWh produced from solar photovoltaic systems; thus, the solar electricity production share was 3.3% [11]. In 2020, due to the world reduced energy demand and the increment in photo- voltaic power installed capacity, around 3.7% of world electricity production has been supplied by photovoltaic systems and the avoided emissions have been 875 Mt of CO2eq (a calculation by the IEA-PVPS based on the emissions that would have been generated from the same amount of electricity produced by the different grid mixes in all countries and taking into consideration life cycle emissions of PV systems). This world average hides a large variation among countries, where a group of seven countries are in the range of 10% and another seven have already surpassed 5%. In this group, it is important to emphasize that the two most populated coun- tries in the world have already reached 6.5% (India) and 6.2% (China) share of its electricity supply from photovoltaic systems [10]. Despite the progress in rural electrification, still 733 million people are lacking access to electricity, three quarters in sub-Saharan Africa (580 million), and another 100 million people cannot afford electricity although they have access to the grid [5]. Either to substitute electricity from non-renewable sources or to supply new demand, the contribution of photovoltaic systems has been growing steadily since many years ago and has now become the fastest growing technology in terms of annual installed capacity. The share of world electricity supply from photovoltaics is going to increase significantly in the coming decades in all scenarios that are proposed by different institutions. The rate of growth and the cumulative capacity depend strongly on the assumptions for these scenarios, and in all of them, photovoltaic technology share is very high, in some cases the top of the list of annual installed capacity during several years. This fact emphasizes the urgent need of a detailed evaluation of the 1 1 Exajoule (EJ) = 1018 Joules; another broadly used unit for primary energy is tonnes of oil equivalent (toe), 1 toe = 4.1868×1010 Joules. 2 1 TWh = 1012 Wh = 3.6 × 1015 Joules.
- 24. 1 Scenarios forSolar Electricity at the TeraWatt Scale 5 sustainability of solar electricity massive deployment. This is the purpose of this book. In this introductory chapter, an overview of the world photovoltaic energy status and trends are presented. After showing the rough numbers of installed capacity and its most recent evolution, the world energy supply and demand in future scenarios proposed by the International Energy Agency are analysed, and the implications for the growth of photovoltaic installed capacity are commented. The analysis of the sustainability of the photovoltaic electricity generation is the subject of the whole book, and the methodological tools both for the calculation of the electricity that can be generated with different photovoltaic technologies and its environmental and economical impacts are the framework to organize the book into three parts and twelve chapters: Part I. Introduction. It is an introductory part which describes the main concepts regarding photovoltaic technology and life cycle assessment. The book contents are built upon the combination of both areas of knowledge, and it is, therefore, important from the beginning to clarify the purpose and the scope of the study. This part also emphasizes the importance of the problem that the energy transition is facing: a huge amount of photovoltaic systems has been already deployed and many more are planned for the near future; many of these systems will have to be revamped, replaced or extended with new modules, and the old ones will need to be recycled or landfilled. In Chapter 1, the scenarios proposed by the International Energy Agency are presented and the implications for photovoltaic capacity growth will be analysed in detail. In Chap. 2, the main components of photovoltaic systems are presented, ranging from cells to modules and then to whole systems; this chapter describes each component, its principles of work and the equations governing its main output (but not going into details of the physics behind photogeneration and charge transport in semiconducting materials); the objective of this chapter is to define the main parameters used to evaluate PV cells, modules and system performance and to classify the “product” parts (a classification which is used for the Life Cycle Assessment (LCA) study of the different technologies). The “product” from the LCA perspective is the final PV system, which includes different steps: cells, modules and whole system (with Balance of System (BoS), components). In Chap. 3, the Life Cycle Assessment methodology is presented, with a special focus on its application to energy systems in general and photovoltaic systems in particular and also the inclusion of social and economic considerations for a broader LCA approach (methodologies still under discussion in the scientific community). Part II. Life cycle assessment of solar electricity. The Life Cycle Assessment (LCA) of the photovoltaic systems (the product) and the electricity produced by them (the service) requires a very clear statement of the scope and the func- tional unit (FU) used for the LCA study. The main part of the book is devoted to the two stages of the whole life cycle of a PV system: first, the PV system manufacture phase (from cradle to gate), starting with raw materials production and ending with the PV module delivery (at the gate of the factory); then, the
- 25. 6 1 Scenariosfor Solar Electricity at the TeraWatt Scale second stage focuses on the use phase and the end-of-life phase (including recy- cling and landfilling) and requires additional tools to calculate the electricity produced during the operational phase. Part II starts with a detailed description of the manufacturing process of all PV technologies, either commercial or emerging (Chap. 4), and the requirements of raw materials (Chap. 5); the energy balance of the PV system life cycle (Chap. 6) will be presented and, together, they comprise a life cycle inventory of the PV technologies. Beyond the standard LCA approach, an analysis of the energy payback time (EPBT) has been included; it is a param- eter broadly used to assess the sustainability of electricity production but which is strongly dependant on the operational phase of the PV system life, including the geographical location where it is operated, and some authors consider that it is not a reliable parameter. The impact assessment in several LCA categories of the whole inventory (materials and energy) will be presented in Chap. 7 with a special focus on commercial technologies and a section devoted to emerging technologies. The focus will be shifted to end-of-life and recycling issues in Chap. 8 and the final chapter of Part II is devoted to Balance of System components with a more detailed analysis of the use of batteries for energy storage. Part III. Beyond Life Cycle Assessment: socioeconomics and geopolitics of solar electricity. Finally, Part III goes beyond the standard approach to LCA and includes economic and social assessment of impacts. Economic evaluation of the economic cost of installed capacity and produced electricity is accomplished in this part. Comments on the geopolitics of photovoltaics provide the closing remarks of the whole book. In Chapter 10, the definition of economic parame- ters used to evaluate the impact of PV systems is provided. Those comprise the levelized cost of electricity (also with the modern definition of IEA, called the “value-adjusted” LCOE). Employment opportunities by sector and by country are analysed, including investigation on socioeconomic networks that range from NGOs or other associations to small, medium or large companies linked to solar electricity. Chapter 11 provides a list of the regulatory framework worldwide, with a presentation of technical standards and regulatory policies, including a comparison between countries and a comment about its evolution. The book ends with Chap. 12 in which solar electricity will be put into the context of global- ization, when on the one hand still a large amount of population lacks access to electricity while on the other hand solar electricity is now subject of speculation by investment funds and big multinationals. Climate change mitigation and the related international agreements are the closing subjects of the book. 1.1 Evolution of Installed Photovoltaic Capacity At the end of 2020, the cumulative installed photovoltaic capacity in the world reached 760.4 GWDC , steadily approaching the landmark of 1 TW that could be reached in two years if annual installed capacity follows the growing trends of the past few years (see Fig. 1.1, reproduced from [8]). Despite the COVID-19 pandemic,
- 26. 1.1 Evolution ofInstalled Photovoltaic Capacity 7 Fig. 1.1 Evolution of cumulative installed capacity (GWp). Source IEA-PVPS (Reproduced with permission from [8]) the annual installed capacity in 2020 was 139.4 GWDC , with at least 20 countries installing more than 1 GW, indicating a sustained annual capacity installation of more than 100 GW/year since 2017 that seems to be accelerating (see Fig. 1.2). China alone represented 253.4 GW on cumulative installed capacity followed by the European Union (as EU27, 151.3 GW), the USA (93.2 GW), Japan (71.4 GW) and India (47.4 GW). Considering that China installed a third of global new capacity in 2020 and that Vietnam and Korea have seen their highest growth in one year, the trend is clear: Asia is going to be the leading photovoltaic region in the next decade, with Australia also becoming an important actor and reaching the first position in the ranking of PV installed per capita (749 W/capita), surpassing Germany which had been the leader in per capita PV capacity until 2019. The Asia- Pacific region installed 61% of new global PV capacity in 2020. The European Union have been leader for many years, but it seems that the trend is slowing down, with only a few European countries keeping a strong growth (Germany still clearly at the head of installed cumulative capacity with 53.9 GW, followed by Italy and the United Kingdom at some distance). In America, the new USA administration announced a strong investment in new renewable energy infrastructure that could reinforce its already strong position in the photovoltaic market; two countries in Latin America installed more than 1GW (Mexico and Brazil), but others presented a contraction in annual installations (Argentina) or very limited growth (Perú, Chile). Africa and the Middle East, with a large potential for PV (due to its very high annual irradiation), showed a limited growth with new installed capacity in 2020 of only 3% of world total. Still both annually installed and cumulative capacity are mostly concentrated in a few countries, with the rest of the world (ROW in Table 1.1) contributing only 6.8% and 0.3%, respectively. Details of world data for PV annual and cumulative capacity and energy generation can be found in the regular
- 27. 8 1 Scenariosfor Solar Electricity at the TeraWatt Scale Fig. 1.2 Evolution of annual installed capacity (GWp) (Reproduced with permission from [10]) reports from the International Energy Agency (IEA) “World Energy Outlook”, the IEA-International Renewable Energy (IRENA) “Capacity Statistics and Highlights”, the IEA-Photovoltaic Power Systems Programme (PVPS) “Trends in Photovoltaic Applications”, the IEA-PVPS “Snapshot of Global PV Markets” and the reports from the World Bank initiatives for off-grid electrification programmes “Energy Sector Management Assistance Programme (ESMAP)” and “Lighting Global”. There are two main categories of photovoltaic system size classification: roof-top or utility scales. Until 2014, the roof-top scale was predominant with more than 50% of annual installed capacity, which kept the cumulative capacity also above 50% for this kind of system; since 2015, the annual installations have been clearly dominated by utility scale (grid-connected PV plants at MW scale), although roof-top systems continued to grow and this application sector has seen an unexpected increase in 2020 due to the very large programme for roof-top systems in Vietnam (and a continuation in Germany and United States were it was already strong): in 2020, around 55GW of new PV systems were roof-top; regarding off-grid systems, further 180 million of roof-top solar home systems have been installed to date providing electricity to 420 million people, and 47 million people are connected to 19,000 photovoltaic powered minigrids in the world (mainly in Asia, with 85% of minigrids, while the future planning is centred in Africa) [3, 4]. Nevertheless, the trend seems to point to a future domination of medium to large size plants. On the other hand, the two broad categories need to be extended to incorporate variations: building integrated photovoltaics (BIPV) complementing the first group of “building attached” (BAPV) roof-top systems (small to medium power systems), or floating systems, agrivoltaics or other utility scale but with very flexible plant design adapted to multiple func- tionalities of medium to large size plants. Other small groups of applications are still not significant in terms of capacity, but represent targeted markets that could grow significantly in the future: vehicle integrated systems, indoor systems adapted
- 28. 1.1 Evolution ofInstalled Photovoltaic Capacity 9 Table 1.1 Annual installed and cumulative photovoltaic capacity in 2020, with data from IEA- PVPS “Snapshot of Global PV Markets 2021” [10]; the European Union grouped 27 countries in 2020; power is expressed in GWDC and when data are available in GWAC they have been converted for better cross country comparison of data Annual installed capacity Cumulative capacity GWDC % GWDC % 1 China 48.2 34.6 1 China 253.4 33.3 (2) European Union 19.6 14.1 (2) European Union 151.3 19.9 2 United States 19.2 13.8 2 United States 93.2 12.3 3 Vietnam 11.1 8.0 3 Japan 71.4 9.4 4 Japan 8.2 5.9 4 Germany 53.9 7.1 5 Germany 4.9 3.5 5 India 47.4 6.2 6 India 4.4 3.2 6 Italy 21.7 2.9 7 Australia 4.1 2.9 7 Australia 20.2 2.7 8 Korea 4.1 2.9 8 Vietnam 16.4 2.2 9 Brazil 3.1 2.2 9 Korea 15.9 2.1 10 Netherlands 3.0 2.2 10 UK 13.5 1.8 ROW 9.5 6.8 ROW 2.1 0.3 Total: 139.4 Total: 760.4 to indoor light, portable flexible and low weight systems, cladding systems inte- grated in paths or roads and a broad range of new system designs in an already old application class dedicated to supply power to signals, lighting or electronic devices. Off-grid systems, mainly for rural electrification in developing countries, represented an important market at the beginning of PV system deployment (80s and 90s), and now its share market is strongly reduced, although in terms of installed capacity, it is still a significant application and have a very large impact in human develop- ment in rural livelihoods without previous access to electricity; in 2030, the off-grid PV systems should be extended to provide electricity to 1.2 billion people [3]. The evolution of the broad classes of PV applications can be seen in Fig. 1.3. The massive deployment of PV capacity is already producing electricity from a renewable source at a lower price than grid electricity in some countries at some time intervals. The produced solar photovoltaic electricity has been growing steadily at a similar pace of installed capacity, in Fig. 1.4, and overview of the aggregated data for different world regions is presented, the data can be downloaded from the IRENA website, and it is updated regularly. Asia is now the leading country in solar electricity production followed by Europe which was surpassed in 2016, North America comes in third position and the rest of the regions are clearly lagging behind, but they are expected to grow significantly in the coming years due to strong cost reductions of PV systems.
- 29. 10 1 Scenariosfor Solar Electricity at the TeraWatt Scale Fig. 1.3 Annual share of centralized, distributed, off-grid and floating installations (GW). Source IEA-PVPS Trends in PV Applications 2020 (Reproduced with permission from [11]) Fig. 1.4 Solar photovoltaic electricity production (TWh) per region during the past eleven years (with most recent real production data from IRENA Renewable Energy Statistics website (last update April 5, 2021, www.irena.org/Statistics/Download-Data) The availability of cheap electricity from photovoltaics will also contribute to enhance the penetration of other technologies that are energy consumers required in an energy transition aimed at a 100% green electricity. These sectors are hydrogen production and electric vehicles. The developments of PV technologies are acting as a strong driver for the development of other technologies linked to the energy sector and
- 30. 1.1 Evolution ofInstalled Photovoltaic Capacity 11 it has created a synergy between the need for efficient storage of electricity produced from photovoltaics at time intervals where demand is lower than supply (similarly for other intermittent sources like wind) and the need for higher electrification of the transport sector. The use of hydrogen as a fuel “vector” and the charging of batteries in electrical vehicles require electricity produced from renewable sources. The link between this renewable intermittent electricity production and the transport sector is pushing the development of technologies for efficient charge storage and green fuel production. This link is still not clear and a strong effort in research and development is currently being carried out. 1.2 Photovoltaics in the Scenarios of the International Energy Agency The International Energy Agency scenarios are the basis for projections shown in the World Energy Outlook reports, and they are linked to socioeconomic scenarios set up by the United Nations and in particular, the Sustainable Development Goals now used by most countries to set up their own sustainable objectives and to contribute to international cooperation policies [5]. The Stated Policies Scenario (STEPS) is a baseline scenario that is built upon the policies announced by each country; the targets related to new renewable energy capacity installations or emission reductions are backed up by detailed technical and economical measures needed for their realization. In particular, the Nationally Determined Contributions (NDC) for emissions reductions that the countries are announcing as part of their commitment with the Paris Agreement are considered in the STEPS scenario only if they are backed by a clear plan of implementation. In contrast, many policies that have been announced with net zero pledges already reaching 70% of global GDP and CO2 emissions, but still with high level of uncer- tainty or no technical backing in its energetic policies, are not considered; in general, those lousy undefined pledges are not considered in the STEPS scenario. On the other hand, the STEPS scenario already includes the impact of COVID-19 pandemic in the economic activity of 2020 but considers that the pandemic is brought under control and the economy will recover its pre-crisis levels before the end of 2021. Prior to the crisis, energy demand was projected to grow by 12% between 2019 and 2030, and growth over this period is now estimated at 9% in the STEPS scenario. Additionally, economic policies have already been modified by recovery policies and stimulus packages including additional investments in the energy transition infras- tructure towards a low-carbon energy sector. Nevertheless, commitments declared so far, even if successfully fulfilled, will keep global annual emissions in the range of 34– 36 Gt CO2eq between 2020 and 2030, followed by a reduction that would still leave around 22 billion tonnes of CO2 emissions worldwide in 2050; the continuation of that trend is consistent with a temperature rise in 2100 of around 2.7 ◦ C (with a prob- ability of 50%), well beyond the limits set in the Paris agreement. Furthermore, the
- 31. 12 1 Scenariosfor Solar Electricity at the TeraWatt Scale United Nations Framework Convention on Climate Change (UNFCCC) was even more pessimistic and considered that the initial nationally declared commitments (NDC) for greenhouse gases (GHG) emission reductions of 119 countries could lead to a temperature increase in the range of 2.7–3.7 ◦ C, indicating that much greater emission reduction efforts than those associated with the NDCs will be required in the period after 2025 and 2030 to hold the temperature rise below 2 ◦ C above pre- industrial levels [12]. The updates of NDCs by 75 parties (representing about 30% of global GHG emissions) were recently assessed by the UNFCCC, but still are not on track to meet the Paris Agreement; the reality is that far from a reduction, the figures contained in the NDCs will lead in 2025 to GHG emissions around 14.04 Gt CO2eq, that is, 2.0% higher than the 1990 level (13.77 Gt CO2eq), 2.2% higher than the 2010 level (13.74 Gt CO2eq) and 0.5% higher than the 2017 level (13.97 Gt CO2eq). Nevertheless, the long-term mitigation measures announced by many countries for 2050 (still without detailed roadmaps for its fulfilment) are ambitious, and the UNFCCC considers that if implemented, the per-capita emissions by 2050 could be reduced by 87–93% compared to 2017 levels and this is consistent with the objective of a temperature rise in the range of 1.5–2 ◦ C with low overshoot scenarios (with the IPCC models for scenario SR1.5) [13]. In the STEPS, renewables meet 80% of the growth in global electricity demand to 2030, hydropower remains the largest renewable source of electricity, but solar is the main driver of growth as it sets new records for deployment each year after 2022, almost tripling from today’s levels and followed by onshore and offshore wind. The modelled change in global energy generation from 2019 to 2040 is expected to be 4813 TWh for photovoltaics in the STEPS scenario, a change in twenty years that is seven times larger than the change occurred in the previous twenty years (664 TWh from 2000 to 2019). This deploy- ment of PV capacity will require a fast development of smart, digital and flexible electricity networks and the requirement of new transmission and distribution lines is 80% larger for the next decade compared to the extension paths seen during the past ten years. Data about population growth in the STEPS is taken from the United Nations and considered that the total population rises from 7.7 billion in 2019 to 10.4 billion in 2070, an average growth of 0.6% per year, with almost three quarters of global increase up to 2070 occurring in Africa, and India accounting for a 10% share in the growth and becoming the most populous country in 2024. The DelayedRecoveryScenario(DRS) isdesignedwiththesamepolicyassump- tions as in the STEPS, but considering that a prolonged pandemic causes lasting dam- age to economic prospects. The global economy returns to its pre-crisis size only in 2023, and the pandemic ushers in a decade with the lowest rate of energy demand growth since the 1930s. Prior to the crisis, energy demand was projected to grow by 12% between 2019 and 2030. Growth over this period is now 9% in the STEPS, and only 4% in the DRS with the consequent slowdown of the economic activity in all end-user sectors and, therefore, in energy demand (with important impacts on transport, for example, where the number of cars in the DRS is 50 million lower than in the STEPS). The Sustainable Development Scenario (SDS), where a surge in clean energy policies and investment puts the energy system on track to achieve sustainable energy
- 32. 1.2 Photovoltaics inthe Scenarios of the International Energy Agency 13 objectives in full, including the Paris Agreement, energy access and air quality goals. The assumptions on population growth, GDP and other socioeconomic parameters are the same as in the STEPS. The SDS scenario is based on a stronger technological development of the energy sector that is modelled by using the Energy Technology Perspectives 2020 Model (ETP) of the International Energy Agency, which explores the evolution in energy supply (using an energy conversion model from primary energy, grouped in fossil, nuclear and renewables to final energy such as electricity, heat, gasoline and diesel) and in the three end-user sectors with the highest energy demand and largest greenhouse gas emissions (using models for industry, transport and buildings). The energy conversion step considers 400 technological options, described in terms of detailed technical and economical parameters including learn- ing curves, thus providing a broad range of possible combinations. Interestingly, the model also considers hydrogen-based fuels (synthetic hydrocarbon fuels from hydrogen and CO2 or ammonia) and direct air capture of CO2 from the atmosphere, though a cross-cutting technology option; but although these technological options have been demonstrated at small or medium scale, they are still not deployed com- mercially, and, therefore, some uncertainty is introduced in the model. The modelled change in global energy generation from 2019 to 2040 is expected to be 8135 TWh for photovoltaics in the SDS scenario. Details of the model can be found in the IEA report “Energy Technologies Perspective 2020 Model” (updated in 2021 from its previous 2016 version, [6]). The new Net Zero Emissions by 2050 case (NZE2050) extends the SDS analysis. The NZE2050 scenario is consistent with around a 50% chance of limiting the long- term average global temperature rise to 1.5 ◦ C, as stated in the Paris Agreement. A rising number of countries and companies are targeting net zero emissions, and all stated policies are considered to come into force although there is still not a clear commitment or detailed plans from governments to do so. The NZE2050 includes the first detailed IEA modelling of what would be needed in the next ten years to put global CO2 emissions on track for net zero by 2050. Reaching net zero globally by 2050 would demand a set of dramatic additional policies and actions over the next ten years, starting already in 2021 with no new oil and gas fields approved for development and no new coal mines or mine extensions; only new coal plants with carbon capture and storage could be approved beyond 2021. The NZE2050 scenario considers that total energy supply falls by 7% between 2020 and 2030, reaching a total of 550 exajoules (EJ) and remains at around this level until 2050, this reduction achievement occurs by reducing the energy intensity of GDP growth by 2% annually. Renewable sources will supply 80% of total energy supply by 2050, growing from 20% in 2020. Electrification is one of the key drivers towards a de-carbonization of the energy sector with global electricity demand more than doubling from 2020 to 2050. Bringing about a 40% reduction in emissions by 2030 requires that low-emission sources provide nearly 75% of global electricity generation in 2030 (up from less than 40% in 2019). Again, hydrogen and CO2 capture are essential for this horizon; 150 million tonnes of hydrogen should be produced with 650 GW installed capacity of electrolyzers by 2030 (rising to 435 million tonnes and 3000 GW, respectively, in
- 33. 14 1 Scenariosfor Solar Electricity at the TeraWatt Scale 2045);4GtofCO2 shouldbecapturedby2035(risingto7.6Gtby2050).Importantly, the NZE2050 model considers that by 2030 all world population will have access to electricity and clean cooking (at an estimated cost of 40 USD billion) and the cost of energy services for households will be affordable and stable even if an increase in energy consumption is produced. The achievement of the NZE2050 scenario will require of strong policy impulse for emission cuts already in 2030 and a constant technological development (most of the reductions beyond 2030 rely on technologies yet to come); only new international standards, regulations and intense cross-border cooperation could guarantee the needed framework for this ambitious objective. A large investment is required in the electricity generation, energy infrastructure for distribution and end-user sectors. In electricity generation, an initial surge from annual investment of about USD 0.5 trillion (average over the past five years) to USD 1.6 trillion in 2030 should be achieved, then annual investment in renewables in the electricity sector should be around USD 1.3 trillion (slightly more than the highest level ever spent on fossil fuel supply which was USD 1.2 trillion in 2014); after this peak in 2030, investment can be reduced to around 30% by 2050. Similarly, investment in energy infrastructure for distribution (electric vehicle charging stations, hydrogen) and carbon capture, transport and storage should increase from USD 290 billion over the past five years to about USD 880 billion in 2030 and for low-carbon technologies in end-user sectors should rise from USD 530 billion in recent years to USD 1.7 trillion in 2030. The NZE2050 scenario can be considered as an optimistic path for a more sus- tainable energy generation, and in particular electricity generation as indicated in Table 1.2; therefore, it is an scenario where photovoltaic electricity will play a sub- stantial role with a large increase both in installed capacity and electricity generation in the coming decades. In this scenario, the TeraWatt scale for PV capacity will be surpassed within two or three years, reaching almost 5 TW in 2030 and surpassing 10 TW in 2040. Beyond this point, new installed capacity will coincide with the decommissioning of several GW of previously installed capacity that would have reached its end of life and recycling could become an important industrial activity. The contribution of renewable electricity generation is key to achieve the ambi- tious objective of net zero emissions by 2050. The evolution of total CO2 emissions in Table 1.2 includes carbon dioxide emissions from the combustion of fossil fuels and non-renewable wastes, from industrial and fuel transformation processes (pro- cess emissions) as well as CO2 removals. The energy transition becomes evident in the evolution of the CO2 intensity (elec.) shown in Table 1.2, that refers to the CO2 emissions per each kWh of electricity generation; it will achieve a net zero balance before 2040 and become negative afterwards, with the electricity sector acting as a carbon sink for other sectors. Details of the scenario are provided in the Interna- tional Energy Agency report “Net Zero by 2050—A Roadmap for the Global Energy Sector” [7].
- 34. 1.3 The TeraWattScale of Photovoltaic Deployment: Is There Any Limit? 15 Table 1.2 Electricity capacity (GW) and generation (TWh) (total, renewables and solar PV) and energy-related CO2 emissions evolution for the NZE2050 scenario of the International Energy Agency. Data from the International Energy Agency report “Net Zero Emissions by 2050. A Roadmap for the Global Energy Sector” [7] Share (%) CAAGR∗ (%) Electricity 2020 2030 2040 2050 2020 2030 2050 2020– 2030 2030– 2050 Total capacity GW 7795 14933 26384 33415 100 100 100 6.7 5 Renewables capacity GW 2994 10293 20732 26568 38 69 80 13 7.5 Solar PV capacity GW 737 4956 10980 14458 9 33 43 21 10 Total generation TWh 26778 37316 56553 71164 100 100 100 3.4 3.3 Renewables generation TWh 7660 22817 47521 62333 29 61 88 12 7.2 Solar PV generation TWh 821 6970 17031 23469 3 19 33 24 12 Total CO2 Mt CO2 33903 21147 6316 0 –4.6 –55.4 CO2 (electricity + heat) Mt CO2 13504 5816 –81 –369 –8.1 n.a. CO2 intensity (elec.) kg CO2/kWh 0.438 0.138 −0.001 −0.005 –11 n.a. aCAAGR = compound average annual growth rate 1.3 The TeraWatt Scale of Photovoltaic Deployment: Is There Any Limit? The energy transition that slowly started after the oil crisis in 1973 has gained momen- tum and it will change the energy landscape in the coming years. The main driver for this change has been shifted from the fear of a supply risk of fossil fuels, sometimes linked to the frequent claim that fossil fuels were achieving their peak production and become more scarce and more expensive every year. This was not the case so far (although some oil fields have indeed reached their peak). But the main driver now is the need to reduce the demand and consumption of fossil fuels, due to the urgent need to reduce CO2 emissions and mitigate climate change, the biggest challenge for the twenty-first century. The contribution of renewable energies to the electricity mix and the increasing electrification of the global energy production and consumption for all end-user sectors create a synergy path where photovoltaic could become the main electricity supplier and perhaps the main global primary energy supplier. In all the International Energy Agency scenarios presented in Sect. 1.2, photo- voltaic deployment is going to reach the TeraWatt scale in the coming years, with the most optimistic NZE2050 scenario pointing to 2030 to nearly reach the 5 TW milestone. It seems that at the initial stages of the TeraWatt scale, no insurmountable limiting factor has been pointed out in the reports, although some barriers have been identified and policies have been recommended to overcome them, but: Is there any limit?
- 35. 16 1 Scenariosfor Solar Electricity at the TeraWatt Scale Throughout this book, the different potential insurmountable barriers from the point of view of the sustainability of solar electricity are explored. The reader will find asummaryofresultsthataimtoanswerthisquestion,butalsoprovidemethodological tools related to photovoltaic technology and to sustainability assessment that will allow any researcher to perform his or her own calculations in search for a response. The main factors that could pose a threat to a massive deployment of photovoltaic technology in the TeraWatt scale are grouped and briefly described below. All issues will be analysed in depth in the corresponding chapters. The risk of materials supply. A huge amount of photovoltaic modules will have to be manufactured in the coming decades. There are many different photovoltaic technologies based on different materials, but today the PV market is relying in more than 95% on one technology (crystalline silicon), the excessive dependence on one single option could be seen as a weakness. Other technologies require in some cases the use of scarce materials (for example, Indium or Tellurium). This possible risk will be assessed in Chap. 5. The risk of energy balance. Long ago, it was clearly established that the balance between the energy embedded in a PV module (materials processing and module manufacture) and the energy delivered by the PV module throughout the lifetime of any PV technology is overwhelmingly positive. In a few years of operation (depending on the technology), the energy is “recovered” and there is a net clean energy supply of decades before the module reaches its lifetime. This will be analysed in Chap. 6. The risk of environmental and health damage. This is an important issue that has been already addressed by many research groups by a detailed Life Cycle Assess- ment methodology, that is constantly updated and re-evaluated for the commercial technologies and newly developed for the emerging technologies, some of them including materials with potential toxicity risks (for example, cadmium in already commercial CdTe technology, or lead in emerging perovskite technology, just to mention two examples). These results are presented in detail in several chapters throughout the book (Chaps. 7, 8 and 9). The risk of high economic cost. The cost of PV modules was an important bar- rier for the deployment of PV systems and several policies were implemented to overcome this barrier. This is no longer the case, and currently in many countries, solar electricity from photovoltaic systems is cheaper than the electricity pur- chased from the grid. Furthermore, the International Energy Agency considers that solar photovoltaic electricity will become the cheapest source of electricity by mid twenty-first century. This was achieved thanks to an impressive learning curve that is analysed in Chap. 10. Geopolitical risks. Energy supply from oil was plagued by political risks, and the best examples were the two oil crisis of the 70s. Apparently, renewable energies in general, and specially photovoltaic energy benefit from the ubiquity of the energy source, but the supply chain for manufacture could face some geopolitical risks (materials supply chain, technological dependence, commercial wars, etc...); they are presented and discussed in Chap. 12.
- 36. References 17 References 1. AklinM, Urpelainen J (2018) Renewables. The politics of a global energy transition. The MIT Press. https://mitpress.mit.edu/books/renewables 2. BP (2020) Statistical Review of World Energy 2020 (69th edn). Tech. rep., British Petroleum. https://www.bp.com/content/dam/bp/business-sites/en/global/corporate/ pdfs/energy-economics/statistical-review/bp-stats-review-2020-full-report.pdf 3. ESMAP (2019) Mini Grids for Half a Billion People: Market Outlook and Handbook for Decision Makers. Tech. Rep. Technical Report 014/19, Energy Sector Management Assistance Program (ESMAP). World Bank. http://hdl.handle.net/10986/31926 4. GOGLA (2020) Global Off-Grid Solar Market Trends Report 2020. Tech. rep., GOGLA— Lighting Global - World Bank. https://www.lightingglobal.org/wp-content/uploads/2020/03/ VIVID%20OCA_2020_Off_Grid_Solar_Market_Trends_Report_Full_High.pdf 5. IEA (2020) Sustainable Recovery. Tech. rep., International Energy Agency—World Energy Outlook Special Report, world Energy Outlook Special Report in collaboration with the Inter- national Monetary Fund 6. IEA (2021a) Energy Technologies Perspective 2020 Model. Tech. rep., International Energy Agency, Paris. https://www.iea.org/reports/energy-technology-perspectives-2020 7. IEA (2021b) Net Zero by 2050. A Roadmap for the Global Energy Sector. Tech. rep., Interna- tional Energy Agency, net Zero by 2050 Interactive iea.li/nzeroadmap Net Zero by 2050 Data iea.li/nzedata 8. IEA (2021) The Role of Critical World Energy Outlook Special Report Minerals in Clean Energy Transitions. IEA—World Energy Outlook special report, International Energy Agency 9. IEA (2021d) World Energy Outlook 2020. Tech. rep., International Energy Agency. https:// www.iea.org/reports/world-energy-outlook-2020 10. IEA-PVPS (2021) Snapshot of Global PV Markets 2021. Tech. Rep. Report IEA PVPS T1 3 9 : 2021, International Energy Agency—Photovoltaic Power Systems Programme—Task1, iSBN 978-3-907281-17-8 11. Masson G, Kaizuka I (2020) Trends in Photovoltaic Applications 2020. Tech. Rep. Report IEA- PVPS T1-38:2020, International Energy Agency - Photovoltaic Power Systems Programme— Technology Collaboration Programme, iSBN 978-3-907281-01-7 12. UNFCCC (2016) Aggregate effect of the intended nationally determined contributions: an update. Tech. Rep. FCCC/CP/2016/2, United Nations Framework Convention on Climate Change. https://unfccc.int/resource/docs/2016/cop22/eng/02.pdf 13. UNFCCC (2021) Nationally determined contributions under the Paris Agreement. Synthesis report by the secretariat. Tech. Rep. FCCC/PA/CMA/2021/2. https://unfccc.int/documents/ 268571
- 37. Chapter 2 Photovoltaic Technology 2.1Introduction to the Physics of Solar Cells: Power Conversion from Sun to Electricity An energy technology can be considered renewable when the source of the supplied work is naturally available or replenished within a certain time frame. The availability of any renewable source is always variable in time, that is, intermittent with different periodicity depending on the technology. Also, the energy density of the renewable source may be low and disperse when compared with non-renewable sources like fossil fuels or radioactive fuels. Those are common characteristics of any renewable technology: wind, geothermal, hydro, tidal, etc…and specially evident for the case of solar photovoltaic technology. The source of photovoltaic energy is the Sun light; it is intermittent in its daily and seasonal cycling; it is low density but universally available on the Earth’s surface; it does not require replenishment since the Sun can be considered a permanent source within the human-scale time frame. Although it is not really permanent, since the evolution of a G-type, small to medium size main sequence star like the Sun indicates that it may be through approximately half of its life, and therefore, it will provide light to the Earth for another 4,500 million years before becoming a red giant whose radius will be probably larger than the Earth’s orbit. Solar photovoltaic energy is the technology which converts the Sun light power available on the Earth’s surface into useful electricity. It converts an intermittent, low power density resource into a reliable source of electrical work which can be delivered on demand at the required power density. According to this definition, solar photovoltaic is a renewable energy, although it is not a completely clean technology since, like any other energy technology (being it either renewable or not), it requires some input of energy to manufacture the devices that are able to convert the power from the Sun into useful work at the Earth. It is important to distinguish between a renewable energy technology and a clean, greenhouse gas (GHG) emissions-free, © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. Urbina, Sustainable Solar Electricity, Green Energy and Technology, https://doi.org/10.1007/978-3-030-91771-5_2 19
- 38. 20 2 PhotovoltaicTechnology technology. Within the lifetime of the devices, solar photovoltaic technology will provide much more energy than the one required to its manufacture: the balance is positive from the point of view of generated versus embedded energy, and it is also positive when the GHG emissions associated with electricity production is compared to any other means to produce the same amount of electricity by non-renewable sources. A quantification of this balance is one main conclusion of this book. An important characteristic of photovoltaic technology is its modularity, that is, its capability to work as an efficient power converter at all scales. A small solar cell is as efficient as a module, or as a generator, or even as a very large plant; in fact, a small laboratory cell is more efficient than the larger devices or systems. A device with 100 cm2 active area is more efficient than a 100 Ha PV plant. Of course, the small cell will provide a few Watts of power, while a solar plant may reach hundreds of Mega Watts (MW, even nowadays a few Giga Watts, GW), but the power conversion efficiency (PCE) when converting the Sun light power into electrical power is better in the small cell. In this chapter, an introduction of the working principles of the solar cell is presented, followed by the “scaling-up” from cell to module with a focus on its material components. Power management once it is converted from light into electricity requires additional elements of the photovoltaic system which are grouped in the so-called “balance of system” components, including electricity storage means. 2.1.1 A Brief History of the Development of the Solar Cell Three main stages can be proposed to summarize the development of photovoltaic technology.Oneearlystagecharacterizedbyslowexperimentalprogressduringnine- teenth century since the discovery of the photovoltaic effect by Edmund Becquerel in 1839 and culminating with the discovery of the electron by J. J. Thomson in 1897, followed by a second stage coincident with the quantum revolution, from Planck’s proposal of the Light Quanta in 1900 to the development of quantum solid-state physics, where theory and experiment progressed steadily with two interruptions caused by the First and Second World Wars. Two technological advances culminate this second stage: the discovery of the transistor in 1947 and the first solar cell with power conversion efficiency higher than 5% in 1954. These two stages are summa- rized in this subsection. The third one is a stage of technological development in pursuit of higher power conversion efficiencies, when experimental advancements in small laboratory solar cells have been quickly applied to commercial photovoltaic modules during fifty years and accelerated since the early 2000s with the onset of organic and hybrid technologies and the massive deployment of installed power capacity of inorganic technologies. This third stage of technological development during the past seventy years is summarized in the final subsection of this chapter. The first scientifically reported effect of the light on the electrical transport prop- erties of a material was presented by Edmund Becquerel in 1839 [3, 4]. These reports are considered the discovery of the photovoltaic effect. He observed an electrical cur- rent passing through a liquid electrolyte (aqueous alkaline, neutral or acidic) when
- 39. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 21 the Sun light illuminated a silver chloride or silver bromide coated platinum electrode and analysed the chemical reactions triggered by the action of light. It took almost forty years for a new report of a photovoltaic effect, in this case on a solid-state selenium sample; in 1876, Adams and Day were studying the photoconductivity of selenium and they observed an increase in photocurrent when the sample was illu- minated, but intriguingly, the current was also produced in the absence of a driving voltage: the current was produced by the action of light and not by an applied voltage [1]. They had invented the first solid-state photovoltaic cell: by using two platinum electrodes in the selenium sample, a metal-semiconductor rectifying Schottky bar- rier contact had been created, although those concepts were not known at that time. The same structures (a metal pressed on a piece of semiconducting material) was used by several scientists with the aim to develop a device which could work as a reliable, calibrated, light sensor: Charles Fritts, by coating the selenium with gold, created the first working solar cell in 1883 with 1% power conversion efficiency [7] which was pushed up to 2% shortly afterwards by Heinrich Hertz with more focus on the photodetector research that he was carrying out and which ultimately lead to the discovery of the photoelectric effect when ultraviolet light was illuminating a metallic plate and produced the effect of discharging the plate [10]. A decade later, and also by illuminating with ultraviolet light, J. J. Thomson discovered that the “cathodic rays” emitted by the metallic plate could be composed of tiny particles, that he called “corpuscles” and were later named electrons [25]. All experimental ingredients of the photovoltaic and photoelectric effects had been discovered by the end of nineteenth century, but the theoretical explanation and the full understanding of the difference between them was only possible after the full development of the quantum theory, which started in the first year of twentieth century (Fig. 2.1). The discovery of the electron by J. J. Thomson was followed by the revolution- ary proposal of Max Planck in 1900, the equation which describes the blackbody radiation in terms of Light Quanta [20]. The equation was successful in explaining experimental data about the radiation emitted by a body at temperature T and which had been elusive so far. Planck’s equation, written in terms of the light frequency, is B(ν, T ) = 2hν3 c2 1 e hν kB T − 1 . (2.1) Equation 2.1 is the spectral distribution of the radiation emitted by the blackbody, that is, the number of light quanta at each frequency interval from ν to ν + δν. Planck was awarethathisempiricalequationwascorrectsincehehadfirst-handinformationfrom experimental colleagues. He then tried to deduce the equation from first principles, which he did in a second article where the revolutionary proposal of light quanta was made in order to be able to deduce the equation proposed in his first 1900 paper. The light came in packages of energy, each light quanta with an energy proportional to its frequency ν [21]: E = hν = hc λ , (2.2)
- 40. 22 2 PhotovoltaicTechnology Fig. 2.1 Time frame of the theoretical and experimental developments during the first half of the twentieth century which led from the discovery of the electron and Planck’s quantum theory of light to the fabrication of the first solar cell with power conversion efficiency higher than 5% where h is Planck’s constant, h = 6.62607015 × 10−34 Js, the quantum of “action” (energy×time), c is the speed of light, c = 299792458 ms−1 and λ is its wavelength. Planck’s 1900 articles did not have a very strong impact in the first years of the twen- tieth century. Planck was always trying to keep a connection to classical thermody- namic theory via the concept of entropy and the inclusion of Boltzmann’s constant in his equation (kB = 1.380649 × 10−23 JK−1 ). It was only after Albert Einstein applied the light quanta revolutionary concept to his successful explanation of the photoelectric effect when the old quantum theory started to be broadly accepted [5]. The origin of the old quantum theory is, therefore, linked to photovoltaic technology by two fundamental concepts: first, the blackbody radiation describes the resource which is coming from the Sun, that is, the light and its spectral distribution in terms of the number of photons with given energies at each frequency (or wavelength inter- val), and second, the light quanta and Einstein explanation of the photoelectric effect that describes how ultraviolet light interacts with matter; it explains how the light quanta are absorbed by the material: in packages of well-defined energy, later called photons [5]. Nevertheless, the photoelectric effect should not be confounded with the photo- voltaic effect. In the photoelectric effect, high-energy photons (blue or ultraviolet) are absorbed by a metallic material and electrons are expelled from the material (in air or preferably in a vacuum chamber); its main application are in photomultiplier detectors or photoelectron spectroscopy (UPS, XPS). In the photovoltaic effect, the
- 41. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 23 electrons are not expelled from the material, the photons are absorbed and excite the electrons to higher levels of energy inside the material, and if these electrons can be effectively used to generate a current through an external load, they can supply work to this load; in this sense, the solar cell, driven by the photovoltaic effect, is acting as a current source where the amount of current delivered to the load is controlled by the light arriving at the cell. The explanation of this process had to wait for the development of the modern quantum theory. At the time of Einstein’s 1905 article and the confirmation of the corpuscular nature of both the cathodic rays (electrons) and light (photons), atomic models were being developed and proposed by J. J. Thomson (1904), E. Rutherford (1911) and N. Bohr (1913) in rapid succession, but it was not until the development of modern quantum theory a decade later that the deep understanding of the atom and, therefore, light–matter interaction was possible. First in 1925 with the matrix mechanics (W. Heisenberg, M. Born and P. Jordan) then in 1926 with the wave equation (E. Schrödinger) and finally in 1927 with the relativistic quantum equation of the electron, the discovery of spin and the first proposal for an anti-particle, the positron, was made by Paul Dirac. For the understanding of the behaviour of electrons and photons with the aim to explain the photovoltaic effect, the equations of modern quantum theory need to be complemented with the statistical description of both kinds of particles. This task was accomplished first by S. N. Bose and A. Einstein for particles with integer spin, called “bosons”; they proposed an equation to describe how these particles occupy states of a given energy. The bosons can condensate in the same energy state, and so do photons (with zero spin) which behave like bosons: fγ (ω, T ) = 1 e ω−μγ kB T − 1 , (2.3) where ω in the energy of the photon with = h/2π and ω = 2πν its angular fre- quency. The chemical potential of light is μγ , which is the average thermodynamical energy of the set of photons at a given absolute temperature T; the link with classical thermodynamics is provided by the energetic term kB T where kB is Boltzmann’s constant and T the absolute temperature (in Kelvin). This equation, when applied to a body at absolute temperature T which emits electromagnetic radiation (photons), recovers Planck’s blackbody radiation, Eq. 2.1. If the particles have half odd integer spin (s = 1/2, 3/2, etc…), they obey Pauli’s exclusion principle and are called “fermions”. This principle, proposed by W. Pauli in 1925, indicates that two or more identical fermions cannot occupy the same quantum state (of a given energy). Fermions obey the Fermi–Dirac statistics and electrons, with spin s = 1/2, behave like fermions: fe(Ee, T ) = 1 e Ee−EF kB T + 1 , (2.4)
- 42. 24 2 PhotovoltaicTechnology where Ee is the energy of the electron, and EF is the Fermi energy that indicates the energy level below which all states are fully occupied at T = 0. If T 0, a small amount of electrons is excited across this Fermi energy and occupy states with E EF . In intrinsic semiconductors, with well-defined conduction and valence bands, the Fermi level is given by EF = Ec − Ev 2 , (2.5) where Ec is the minimum energy level within the conduction band and Ev is the maximum energy level within the valence band. Both the Bose–Einstein and the Fermi–Dirac statistics recover at high temperatures (and low concentrations of par- ticles) the classical thermodynamic Maxwell–Boltzmann distribution function. With those statistical ingredients, the development of solid-state physics pro- gressed rapidly. Bloch’s theorem (1928) enabled the possibility of solving Schrö- dinger’s equation in crystalline solids and obtaining the wavefunction and eigenen- ergies of electrons within a solid. When solved for a large number of atoms, the atomic orbitals are very closely spaced in energy (around 1022 available states per eV1 ), thus creating some ranges of quasicontinuum energy called “bands”; these bands are separated by ranges of forbidden energy, commonly known as the “energy gap”, Eg, for which there is no solution of the wave equation, i.e. there is no wavefunction at this energy, and therefore, there is no available state to accom- modate any electron. The combination of the Bloch theorem and the progress in experimental solid-state physics enabled a very rapid progress in the understand- ing of the behaviour of electrons within solids, with the works of Eugene Wigner and León Brillouin on the atomic structure of materials and Arnold Sommerfeld which developed the first models of electrons in solids (Drude–Sommerfeld model, 1927) and later by Nevill Mott who proposed a full quantum theory for elec- trons within solids, including metal–insulator transitions and electrons in disordered semiconductors [15, 16]. A detailed description of band calculations and quantum electronic transport in solids is out of the scope of this book and can be found in very good solid-state physics books, like the classical Ashcroft and Mermin book [2] and with more focus on photovoltaic technology, in the excellent books by Jenny Nelson and Peter and Uli Würfel [18, 27]. Nevertheless, the concepts of Fermi energy and energy gap are at the core of semiconducting physics, and an understanding of the underlying physics of photogeneration requires at least a grasp of its physical meaning which is presented in the following subsections. The final steps of the second stage of the evolution of the solar cell are provided by two inventions. The first one is the fabrication of the first solid-state transistor by John Bardeen, William Shockley and Walter Brattain at Bell Labs in 1947 on a piece of germanium with metallic gold contacts; this experimental device opened the door to solid-state electronics based on semiconducting materials which was rapidly 1 The electron-volt, eV, is a very convenient energy unit in solid-state physics, it is defined as the energy that an electron acquires when accelerated in an electric field of 1V and, by definition, is equal to 1.602 × 10−19 J.
- 43. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 25 developed with the fabrication of diodes, transistors and ultimately the first silicon solar cell fabricated also at Bell Labs by Pearson, Chapin and Fuller in 1954 with power conversion efficiency of 6% which demonstrated the possibility of using them for power generation by converting Sun light into electricity. The key to this impres- sive performance was the ability of Fuller, a chemist, to efficiently dope the silicon semiconductor and create a controlled p/n junction. The two principal ingredients of a solid-state solar cell had been developed and combined: a semiconducting material with an energy gap and an asymmetry in doping which creates an internal electric field to drive the photogenerated electrons into the external metallic electrodes. 2.1.2 Solar Radiation The source of photovoltaic energy is the light arriving from the Sun. The total solar radiation includes photons and also several subatomic particles, such as electrons, protons, alpha particles and neutrinos, and some atomic nuclei such as carbon and nitrogen and others, comprising the solar wind plasma. Most of the solar wind par- ticles are deflected by the magnetosphere, which protects the Earth’s surface from the solar wind. When considering solar radiation with the purpose of evaluating the resource of solar energy for electricity production, only the photons are accounted for. The blackbody radiation model proposed by M. Planck (Eq. 2.1) provides a very good fit to the spectral distribution of the photons arriving at the Earth from the Sun, which is acting like a black body at temperature T = 5960 K. Some of the photons arriving at the outer part of the atmosphere are scattered by atoms, and others are absorbed (for example, by water in the clouds, producing dips in the wavelength range of 900, 1000, 1400 and 1900 nm or by carbon dioxide, producing dips in the wavelength range of 1800 and 2600 nm); finally, part of the radiation arriving at the surface is reflected. The spectral irradiance of the Sun’s light is the power density (in units Watts per square meter) and within wavelength λ and λ + δλ which arrives at the Earth’s surface; it is shown in Fig. 2.2. For an average distance between Sun and Earth of 1AU,2 the power density integrating all wavelengths is 1367 W/m2 , which is called the solar constant. Depending on the atmosphere thickness that the solar light has to cross before arriving at the surface, the spectral irradiance is slightly different. Air Mass (AM) zero is defined for the outer part of the atmosphere, while for any point on Earth’s surface, Air Mass is defined as the ratio between the optical path length to the Sun and the optical path length if the Sun were in the zenith, which is the inverse of the cosine of the angular height of the Sun on the horizon as seen from this point of Earth’s surface. For example, AM1.5 corresponds to the Sun elevated at an angle of 42◦ . The total (also called “global”) solar radiation includes direct (or beam), diffuse andalbedo(orreflected)components.Severalmodelsforitscalculationandempirical measurements have been presented in the past decades and important databases 2 Astronomical Unit (AU) is 149, 597, 870, 700 m, i. e. roughly 150 million kilometers.
- 44. 26 2 PhotovoltaicTechnology Fig. 2.2 Spectral irradiance outside the Earth’s atmosphere (AM 0), on the Earth’s surface for direct sunlight (AM 1.5D) and the direct sunlight together with the scattered contribution from atmosphere integrated over a hemisphere (AM 1.5 G) (according to ASTM G173-03 and in comparison to the spectrum used by Shockley and Queisser of a blackbody with a surface temperature of 6000 K (BB 6000 K). Reproduced with permission from reference [23] have been constructed and are available. Diffuse radiation is calculated by using isotropic and anisotropic models, where one circumsolar anisotropy component is considered, or an additional horizon-dependant second anisotropy is also included [9, 17, 19]. Albedo contributions are strongly dependant on geographical location and surrounding topography or structures; therefore, the best estimations are provided by empirical databases, like the Copernicus Global Land Service3 of the European Union, which is based on satellite observations. The most important solar radiation database is PVGIS,4 the Photovoltaic Geographical Information System of the Joint Research Centre of the European Commision, which provides free and open access to its irradiation and meteorological database including the following, among other data: • Solar radiation and temperature, as monthly averages or daily profiles (database and maps). • Typical Meteorological Year data for nine climatic variables. • Full-time series of hourly values of solar irradiance. Other databases, such as those of the National Renewable Energy Laboratory (NREL) and the National Aeronautics and Space Administration (NASA) (in the United States of America) or other national meteorological organizations are also available. With 3 Copernicus Global Land Service, https://land.copernicus.eu/global/products/sa. 4 PVGIS-JRC(EU), https://ec.europa.eu/jrc/en/pvgis.
- 45. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 27 these online tools, either solar radiation data or electricity production data by using different PV technologies and system configurations are easily available and a very accuratecalculationofthepotentialofsolarelectricityproductionatanygeographical location is within the reach of anyone with Internet access. 2.1.3 Metals and Semiconductors To classify materials according to their electrical properties, the best property to choose as the main criteria for the classification is resistance. First, resistance in metals is low, while in semiconductors, it is very high (and in insulators much higher), and secondly, resistance in metals increases when the material is heated, while in semiconductors, it is reduced when the material is heated. Therefore, this criteria is useful and easy to measure. Resistance is a parameter that is defined by Ohm’s law: a current passing through a conductor between two points is proportional to the applied voltage across those two points I = V R , (2.6) where R is the resistance and it is measured in Ohms (). Since the resistance of a piece of material depends on the shape and size of this material, it is better to define the resistivity: R = ρ L A , (2.7) where ρ is the resistivity and L, A are, respectively, the length of the conductor and the area of its cross section. The units of resistivity are m. The inverse of the resistivity is the conductivity, σ, with units of −1 m−1 also called “Siemens per meter” (S m−1 ). In Fig. 2.3, a summary of resistance values is included for some metals, semiconductors and insulators; note the huge span of values for the materials. For thin films, it is convenient to define a “sheet” resistance, Rs, when the thickness of the sample is small and uniform and the area of cross section can be considered as the product of a width (W) and a thickness (t). Then, the resistance can be rewritten as follows: R = ρ t L W = Rs L W , (2.8) where Rs = ρ/t is the sheet resistance and has units of , but in order to emphasize that it refers to a thin film, it is often indicated as /. In photovoltaic technol- ogy, since many materials are used in thin films (specially for the emerging organic and hybrid technologies), the sheet resistance is commonly used to characterize the materials used in those layers.
- 46. 28 2 PhotovoltaicTechnology Fig. 2.3 Draft of bulk and thin film materials for whom resistance and sheet resistance are defined in the main text, and the table includes resistivity and conductivity values for representative metals (grey), semiconductors (green) and insulators (yellow) The resistivities listed in the table of Fig. 2.3 have been measured at T = 20 ◦ C and present a very large span of values, with an extremely broad range of more than thirty orders of magnitude that could be enough to classify the materials. But most importantly, the temperature dependence is very different in metals and semiconduc- tors. For metals, the resistivity behaviour with temperature is well described by the model of Bloch and Grünesein, given by ρ(T ) = ρ(0) + A T θR n θR T 0 xn (ex − 1)(1 − e−x ) dx, (2.9) where θR is the Debye temperature and n depends on the kind of scattering interaction of the electrons within the material. This equation produces a constant growth of resistivity when the material is heated. For intrinsic semiconductors, an empirical model explains the behaviour of most materials: ρ(T ) = ρ(0)e−αT , (2.10) where α is an empirical coefficient. The exponential behaviour indicates that intrinsic semiconductors have a very broad range of resistivity, which can be strongly modified by using doping. When the values of resistivity are very high at room temperature (ρ 1010 m), the material can be considered as an insulator. The resistivity versus temperature behaviour provides a good empirical classifi- cation, but an understanding of the electronic transport mechanisms requires another classification based on the structure of the energy levels of the material. It was only
- 47. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 29 Fig. 2.4 Schematic representation of the energy levels of a metal (left) and a semiconductor (right), with the Fermi energy position within a partially filled band or within an energy gap, respectively with the onset of the quantum theory of solids that such an explanation was provided. The materials have energy bands with states that can be filled with electrons (fol- lowing Pauli’s exclusion principle and the Fermi–Dirac statistics), and the ultimate electron of a given material that is accommodated in a state within an energy band (ideally at absolute temperature T = 0 K) establishes the Fermi energy level of this material (energies are measured with respect to a “vacuum level” which corresponds to the energy of the electron immediately out of the material). In Fig. 2.4, a simplified draft of the energetic structure of bands is presented. In a metal, the Fermi energy lies within a band, and thus, at absolute temperature T = 0 K, all levels below the Fermi energy are full and levels above it are empty but there is no energy gap between the filled and the empty states; therefore, statistically speaking, the Fermi energy lies within a band of allowed states which are partially filled. In a semiconductor, at absolute temperature T = 0 K, one band is completely filled and the next one is completely empty, both bands being separated by an energy gap, with energy levels in which there is no available state to accommodate electrons (there is no solution of the wavefunction at the energies within the gap); statistically speaking, in a semicon- ductor, the Fermi energy lies within the energy gap, the band below the Fermi energy is the valence band and the band above the Fermi energy is the conduction band. Only the energy levels at the top and bottom of those bands are useful for calculations and for measurements and are labelled, respectively, EV and EC in Fig. 2.4. With the help of Fig. 2.4, some definitions can be made which will be useful to characterize the materials within the different parts of a solar cell: • Evac − EF is the Work Function, q m where q is the electron charge. • Evac − EC is the Electron Affinity, χ and does not depend on EF . • Evac − EV is the Ionization Energy (first, second, third, …binding energy.)
- 48. 30 2 PhotovoltaicTechnology • EC − EV is the Energy Gap, EG. The difference between a semiconductor and an insulatoristhesizeoftheenergygap.Roughlyspeaking,if EG 4eV ,thematerial is a semiconductor, and if EG 4eV , it is an insulator, the frontier between them being a diffuse one. The selection of metallic or semiconducting materials for the fabrication of the different parts of a solar cell will depend upon the relative values of all these mag- nitudes and how they are combined to optimize the process of generating excited carriers within the active layer of the cell and extracting them out of the cell. The energy band structure and the resistivity of the materials are enough to provide a link between the nanoscale quantum properties of the solid (the energy gap is a purely quantum phenomena) and the macroscopic classical characterization of an operating solar cell whose main parameters are described in the following paragraphs. 2.1.4 Equivalent Circuit and Parameters of the Solar Cell A solar cell requires two main ingredients, the energy gap of the material which enables the possibility of absorbing photons and excite electrons, and an internal electric field to drive the photogenerated electrons out of the device and deliver an electric current (at some voltage) to an external load. Semiconductors are required to provide the energy gap, and the combination of a metal/semiconductor interface or a semiconductor with two differently doped regions (homojunction) or two different semiconductors (heterojunction) is required to provide the internal electric field. All these ingredients are included in the diode, and if this diode is capable of absorbing photons, it will behave as a solar photovoltaic device when illuminated by light. The most simple electronic circuit to represent this combination is the parallel connection of a current source and a diode as shown in Fig. 2.5 (top). In this schematic view, the photogenerated current can be driven through an exter- nal load (RL) or “lost” through the diode, which in this case would represent a loss of power which is not available to make work at the load (this loss is mainly due to recombination). The sign of the currents in Eqs. 2.11 and 2.12 representing this equiv- alent circuit is arbitrary: in conventional electronic circuits, the current is considered positive when it flows through the diode from p-type material to n-type material (from anode to cathode within the diode), but in photovoltaics, the positive sign is applied to the photogenerated current and to the delivered current to the load; then, the current through the diode, also called dark current, is subtracted from the photo- generated current. Using the Shockley equation to describe the diode, the equation for the ideal solar cell is given by J(V ) = Jsc − Jdark(V ), (2.11) J(V ) = Jsc − J0 e qV kB T − 1 , (2.12)
- 49. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 31 Fig. 2.5 Top: Equivalent circuit of the ideal solar cell, summarized in the right-hand side by the symbol of the solar cell, which is connected to an external load RL . Bottom: Equivalent circuit of the real solar cell, with parasitic resistances, Rs and Rsh which is the equation of the ideal solar cell and current densities (J(V ), Jsc and Jdark) are used with most common units mA/cm2 . The diode is described by the saturation current J0 and an ideality parameter, β, which in this case of “ideal diode” is equal to one and not included in the equation for the ideal solar cell. Jsc is the short circuit current and J(V ) is the current delivered to the load and it is also called J-V or I-V characteristic curve of the solar cell. The shape of the J-V characteristic is shown in Fig. 2.6 where the dark current is shown in grey, with initially positive sign in the left-hand side of the figure (standard convention for electronics) but then it is flipped downwards (changing the sign of current) and a shift of the whole curve is applied upwards when the solar cell is illuminated and a photocurrent is created (positive sign). The photocurrent through the solar cell and through an ideal load of R = 0 (voltage across this load would be V = 0) is called short circuit current, Isc, or Jsc when referred to current density. When the circuit is open (RL = ∞), there is no current flowing through the load and the voltage between the terminals of the solar cell is called open circuit voltage, Voc. If the solar cell is illuminated, all other intermediate cases with 0 RL ∞ produce an electromotive force on the load with power density P(V ) = J(V ) × V . The open circuit voltage (Voc) does not appear explicitly in Eq. 2.12 because it refers to a single point, when the J-V characteristic curve crosses the voltage axis. It is the case when the current delivered to the load is zero, that is, J(V ) = 0 and V = Voc. Considering this particular point, an equation for the open circuit voltage is easily obtained:
- 50. 32 2 PhotovoltaicTechnology Voc = kB T q ln Jsc J0 + 1 . (2.13) The photocurrent generated by the solar cell at short circuit conditions, Jsc, is a quantum phenomenon which depends on the ability of the solar cell to absorb photons and use its energy to promote electrons from the valence to the conduction band to generate charge carriers that are delivered to the external load. Jsc is independent of the voltage between the solar cell electrodes. Using the incident spectral photon flux (bs(E)) which is the amount of photons with energy between E and E + dE per unit area and unit time arriving from the Sun, and the quantum efficiency (QE(E)) of the solar cell, which is the probability that a photon arriving into the solar cell with energy between E and E + dE generates an electron in the active layer that is collected by the negative electrode of the solar cell and delivered to the external load, then the short circuit current can be calculated as follows: Jsc = q ∞ 0 bs(E)QE(E)dE, (2.14) where q is the electron charge and the integral is calculated for all photon energies. Theshapeof QE(E)dependsonthematerialsinthesolarcellandthecellarchitecture for photons with energy larger than the energy gap of the semiconducting material, while for photons with energy below the energy gap of the cell is zero. Therefore, the lower limit of the integral in Eq. 2.14 can be replaced by EG, the energy gap of the material. A more detailed description of the real solar cell includes two parasitic resistances in the equivalent circuit of the ideal solar cell as shown in Fig. 2.5 bottom. They are the shunt resistance (Rsh) and the series resistance (Rs) which accounts for different losses thus reducing the delivered power to the load, mainly recombination losses and transport losses (voltage drop due to resistance of materials and of mismatch of energy levels from the active layer to the transporting layers and the electrodes). If the parasitic resistances are considered, the equation of the ideal solar cell must be modified; the voltage drop across Rs, which is J ARs, indicates that the voltage in the circuit branch to the left of Rs is higher than the one across the load, then V + J ARs is the voltage now biasing the diode and must be included in the Shockley equation of the diode instead of just V ; the diode is no longer considered ideal and therefore, the ideality factor β must also be included in the equation (typical values for β range from 1 to 2), and finally, there is a current loss through the shunt resistance given by (V + J ARs)/Rsh which reduces the current delivered to the load. With all these modifications, the equation of the real solar cell becomes J(V ) = Jsc − J0 e q(V +J ARs ) βkB T − 1 − V + J ARs Rsh , (2.15)
- 51. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 33 Fig. 2.6 J-V characteristic curve of a solar cell, and P-V curve which defines the maximum power point, the area of the green square in the J-V plot, divided by the area defined by the Jsc × Voc square illustrates the graphical ratio of areas which provides the value of the filling factor, FF where A is the active area of the solar cell. The effect of the parasitic resistance is to reduce the “squareness” of the J-V characteristic curve of the solar cell. From Eq. 2.15, it is clear that Rs must be small and Rsh must be large to recover the ideal case. Typical values for the parasitic resistances depend on the photovoltaic technology under consideration, but a general rule is that Rs must be lower than a few Ohms, and Rsh must be larger than a few hundred thousand Ohms. A good measurementofthequalityofthesolarcellisthe“squareness”ofitsJ-Vcharacteristic curve which can be quantified by the filling factor, FF, which is defined as F F = JmppVmpp JscVoc , (2.16) where Jmpp and Vmpp are the current density and voltage at which the maximum power is delivered to the load, and define a special point in the J-V characteristic curve called the maximum power point, mpp. The current and voltage at this mpp point are neither the maximum current (which is Jsc) nor the maximum voltage (which is Voc) that can be delivered to the load, but the combination in which P(V ) = J(V )V , the delivered power, is maximum (Pmpp). The relationship between the J-V characteristic and the P-V curve is graphically shown in Fig. 2.6, and the maximum point of the P-V curve defines the special mpp point at which the filling factor is defined.
- 52. 34 2 PhotovoltaicTechnology Table 2.1 Summary of electrical parameters used to characterize solar cells Parameter Symbol Units Power conversion efficiency η or PCE No units, % Peak or nominal power P Wp Power density at maximum power point Pmpp mW/cm2 Short circuit current Isc mA Short circuit current density Jsc mA/cm2 Current density at maximum power point Jmpp mA/cm2 Open circuit voltage Voc V Voltage at maximum power point Vmpp V Filling factor F F No units: between 0 and 1 or % Saturation current J0 μA or nA Diode ideality factor β No units, usually between 1 and 2 The power conversion efficiency, PCE (or η) of the solar cell is the ratio between the electrical power density delivered by the solar cell operating at the maximum power point and the power density arriving from the Sun on the active area of the cell: PC E = η = Pmpp Ps = JmppVmpp Ps = JscVoc F F Ps , (2.17) which is given in %. Since the solar cell efficiency depends on the irradiance and the temperature of the cell, all solar cells from different photovoltaic technologies must be characterized at the same ambient conditions for a fair comparison. The Standard Test Conditions (STC) have been set by the international standard IEC-60904-1 to provide the values of the solar cell parameters for any technology: solar irradiance 1 kW/m2 with spectrum AM1.5G (defined by the international standard IEC 60904- 3), cell temperature T = 25 ◦ C and wind speed lower than 1 m/s. The parameters measured at STC are often called peak parameters and indicated with a p subindex in the units: for example, a module delivering 300 W at STC is said to have a peak power or nominal capacity of 300 Wp. Table 2.1 summarizes the main parameters which are used by manufacturers to characterize the solar modules. Another group of parameters widely used are the thermal coefficients of the solar modules, which are needed to calculate thermal losses; they are empirical parameters measured in operating conditions different of the STC; for example, the nominal operating cell temperature, NOCT, which is the temperature of the cell measured when operating with irradiance 800 W/m2 and at ambient temperature T = 20 ◦ C. This NOCT parameter is used to calculate
- 53. 2.1 Introduction tothe Physics of Solar Cells: Power Conversion from Sun to Electricity 35 Table 2.2 Empirically determined coefficients used to predict module back surface temperature as a function of irradiance, ambient temperature and wind speed with the SANDIA model [12] Module type Mount a b Glass/cell/glass Open rack –3.47 –0.0594 Glass/cell/glass Close roof mount –2.98 –0.0471 Glass/cell/polymer sheet Open rack –3.56 –0.0750 Glass/cell/polymer sheet Insulated back –2.81 –0.0455 Polymer/thin film/steel Open rack –3.58 –0.113 22× Linear Concentrator Tracker –3.23 –0.130 the operating temperature of the cell at any other ambient conditions by using the linear Ross model given in Eq. 2.18, and the thermal coefficients of losses are fitted experimentally [22] Tm = Ta + N OCT − 20 800 G = Ta + K G, (2.18) where K is the Ross coefficient, it is expressed in units ◦ Cm2 /W and can be defined as K = (N OCT − 20)/800 when G is expressed in units W/m2 . The first value reported by Ross was 0.03 ◦ Cm2 /W for crystalline silicon and wind speed lower than 1 m/s2 (which delivers a NOCT around 47 ◦ C) [22]. For other thin film PV technologies such as a-Si, CIGS, CdTe in different orientations and even in BIPV applications, the obtained Ross coefficient is always around 0.03 ◦ Cm2 /W with small deviations; only organic technologies deliver lower values (around 0.02 ◦ Cm2 /W) but in this case, the value seems to be more dependant on the encapsulation and fram- ing material than the organic photovoltaic cell material [26]. Manufacturers always provide the empirical NOCT for the PV modules as the main thermal parameter. A more sophisticated thermal model for the solar cell includes the influence of wind and an exponential behaviour was proposed by researchers from Sandia National Laboratory (USA) in reference [12] and it is presented in Eq. 2.19; the two parameters a and b to be used are obtained empirically for different combinations of materials in cell, encapsulants, cover, backsheet and frames; they are found in scientific references, but very rarely reported by the manufacturers of modules; a summary is presented in Table 2.2. Tm = Ta + e(a+bWs ) G. (2.19) Outdoor tests carried out in different climatic regions have lead to more detailed models for NOCT in real operating conditions according to the international stan- dards IEC 61215 and IEC 61646, showing that natural convection can be neglected
- 54. 36 2 PhotovoltaicTechnology Table 2.3 Empirically determined coefficients used to predict cell temperature [13] U0 U1 c-Si 30.02 6.28 c-Sia 26.9 6.2 a-Si 25.73 6.24 CIS 22.64 3.61 CdTe 23.37 5.44 aUsed in reference [11] and calculated as an average of values reported in [13] Table 2.4 Summary of thermal parameters used to characterize solar cells Parameter Symbol Units Nominal operating cell temperature NOCT ◦C Power (Pmpp) thermal coefficient γ %/◦C (negative) Current (Isc) thermal coefficient αI %/◦C (positive) Voltage (Voc) thermal coefficient βV %/◦C (negative) for wind speeds above 2 m/s, that the main effect of radiation cooling can be found during night time which is not relevant for the solar energy gain and that the effect of wind gusts and fast temperature changes is low [13]. Therefore, yet another method was proposed to calculate module temperature in different ambient conditions; it is used by the PVGIS model (European Commission Joint Research Centre, [6]): Tm = Ta + G U0 + U1W , (2.20) where Ta is the ambient temperature and W is the wind speed. The coefficients U0 and U1 used in PVGIS have been obtained by fitting experimental data and are summarized in Table 2.3 by providing the average value for each PV technology [13]. Once the module temperature is calculated by using any of those simple models or others which include additional environmental variables such as wind direction and relative humidity, the temperature losses present a linear dependence such as thermal coefficient × T where the thermal coefficient is given as a relative loss in % (with respect to nominal STC values) per temperature degree (older PV module datasheets used to provide absolute thermal losses, but it is no longer the case). Typical values are around -0.3%/◦ C for power and voltage losses and +0.05%/◦ C for current gains when the temperature of the operating module is above 25◦ C (opposite effect when temperature is below 25 ◦ C) (Table 2.4).
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- 56. valittunsa, parhaimpansa tekevättiet ja portit avariksi, kun minä saavan milloin paratiisiin valtaani siellä uudistamaan taas… Pahan viimat suhahtelivat yhä kiivaammin. Autiona ja tyhjänä levittäytyi järven vesitanhua. Ylvästellen jatkoi suuri puhuja: — Niin, pappi oli minun aseenani!… Vaan eipä toisenlaista suosittajaa olisi uskonutkaan Litvan kansa. Hän, pappi, Jehovaansa puolustaessa epäilyn kylvi äsken Oolaviin!… Se kylvö itää nyt jo kyselynä… Hän mietti sekä jatkoi puhettaan: — Nyt on jo voitto varma, saavutettu… Hän lausui ensi iskun saatuansa: 'Minä panen vainon sinun ja vaimon välille'… Niillä sanoillaan Hän jätti käsiini vaimon vihollisenani. Ei vainon vallitessa ole säälin aika… Suuri viha välähti hänen silmistänsä. Kostonhaluisena vannoi hän: — Jos Martva nyt ei kohta minuun suostu, niin hänen kohtalonsa sekä Litvan osa on ratkaistu: se päättyy häviöön… Hän nousi mahtavana ja vannoi: — Armotta tuhosi Hän kaupunkini: Sodoman sekä kauniin Gomorrani… Nyt hänellä on täällä vuorostansa Tuukkala Sodomana, Rannisto Gomorrana… Ne tuhoan nyt minä vuorostani… Hän katsahti ympärillensä. Ihmishengen onni vierähti virtenä, nousi öisenä kauniina unena… Martva eli Litvan rantojen runona.
- 57. Kiukustuen jatkoi silloinPerkele valaansa: — Jos Hän täältä 'Lootin' pelastaakin, ja vaikka vaimo miestään seuraisikin, niin tytär, Martva, nyt ei seuraa häntä… Hän tällä karilla on ijät kaiket kuvaistaan kaipaava kuin suolapatsas, kuten jo 'Litvan laulu' ennustaa… Rantalehdoista nousi Litvan laulun sävel… Sen kaiku heräsi kallion koloista… Surullisena ennusti se ihmishengen kohtaloa… Kaiku ja sävel sulivat yhdeksi suruksi… Kerskaten lausui silloin Perkele: — Tuo laulu hämärine aatteinensa on minun tahdostani Litvaan tullut… Se avasi jo ensimäisen raon Martvaan ja myöskin hänen sulhaseensa: Se saattoi heidät ensin kyselemään, mikä on kuulun Litvan laulun henki… Sen johdosta voi minun Harhamani vapaammin heihin henkeäni kylvää… Ranta itki jo runonsa, ihmis-onnen lakastumista… Se tajusi suuren taistelun tulon… Elämän suuri hämäryys etsi jo itsellensä lymysijoja ihmis-onnen asuinmailta. Ylpeänä lopetti Perkele järkeilynsä: — Nyt täytyy joutua jo Tuukkalaan… Mies odottaa siellä vastausta hämäriin ihmishengen kysymyksiin, joita hän ei itse jaksa ymmärtää… Hän epäilee ja nuori epäilijä on harhaanjoutuneista kaikkein sokein. Hän umpimähkään, aivan sokeana epäilee päivänselvääkin ja uskoo valhetta sekä järjettömyyttäkin, varsinkin jos se epäilys on pelkkää semmoista pientä muotitavaraa, jommoista se on muoti-epäilijöille, jotka epäilevät mitään miettimättä, omaksi henkiseksi koruksensa, tai viisaudeksi, hengen peruukiksi, kun milloin hämmästyttävät seurapiiriänsä suurella henkisellä 'aarteellaan', jossa onkin kalju tekotukan alla…
- 58. Hänen äänessänsä väreilisuuri iva. Jumalien kylmillä eleillä lopetti hän: — Ne epäilijät ovat arvottomia… Tytölle kelpaavat ne, eivät jumalille… Ne selittävät ikivarmuudella syvintä olemisen ohjelmaa, jota ei kukaan ihmisistä tunne…Vaan Oolavi on toki aivan toista: Hän ei koskaan kaada leilistänsä syvien ongelmoiden selitystä, ennen kun leilin täyttänytkään on… Vaan kumminkin on Oolavikin vielä jaloista, kaiken tiedon kysyjistä toki nuori sekä vasta-alkavainen… Semmoiseen juuri pystyy kuiskaus… jo on aika hänen luokseen rientää… Hän lähti… Suuri elämä alkoi laskeutua hämäränä sumuna Litvanselälle… Järven rantojen runouden, Martvan, onnen ratkaisu alkoi jo lähetä… Järven syvyydessä virittelivät hyvän ja pahan voimat viulujansa… Kun elämänkoski vei jo venettä. Kello löi puoli kahta yöllä… Yön sumut muuttuivat elämän hämäriksi, unet ihmiskohtaloiksi. Taivaanrannan alta nousi kuun reuna valokannikkana. Se levitti punakirkkaan valonsa taivaalle, peittäen syys-öisen tähtikylvön, joka iti korkeudessa. Ihminen eli unessa. Salaiset voimat tekivät työtänsä hänessä. Oolavi nukkui sikeintä untansa. Hänen vuoteensa vierellä seisoi taas rietas käärme, Harhaman elämä, vaanien uhriansa pahan
- 59. voimien aseena. Turhaaoli toki ollut sen vaiva tähän asti. Jaloon sieluun ei pystynyt paha. Mutta nyt oli jo pahan pisto sattunut: Oolavi näki unta Harhaman kirjasta. Hän näki unta epäilystä ja elämän salaisesta kysymyksestä, jonka hän oli unohtanut olevan siihen kirjaan kätkettynä… Rietas käärme lirutteli inhottavaa pyrstöänsä lattialla. Se lipoi kieltänsä, joka riippui leuvalla verisenä… Kuun valopallo oli jo noussut kokonaan taivaanrannan yläpuolelle. Se loisti siinä, kuin alustalle asetettu tulikoru. Oolavin uni jatkui… Käärmeen hengähdys loihti hänen eteensä huumaavia kuvia: Epäily muuttui jalokiveksi… Joskus taas availi se hänelle enkelinä ihanimpia syvyyksiä… Käärme hymyili… uni jatkui: Syvyyksien pohjalla kimaltelivat elämänongelmat armaissa, salaperäisissä elämänhämärissä… Ne kimaltelivat kuin ihanat helmet… Ne odottivat häntä poimijaksensa… Hän oli valmis… Hän halusi heittäytyä syvyyteen, kauniiseen elämän salaiseen hämärään, ottamaan siitä ihanan helmen… Hyvän ja pahan voimat taistelivat… Rietas käärme iloitsi… Elämä kuvastui Oolaville yhä ja yhä kiehtovampana… Sen salaisuus veti kuin koski venettä… Hän hapuili jo käsin kaunista hämärää kiinni. Mutta silloin ilmestyi enkeli, varottamaan vaarasta, joka uhkasi. Se heitti silmäyksen elämän kiehtovaan hämärään ja oitis muuttui se oikeaan muotoonsa: Siitä sukeutui vihainen suuri, musta käärme. Se käärme kiemurteli ruumistansa lattialla, nukkuvan vuoteen vierellä… Se uhkaili nukkuvaa… Oolavi hätäytyi… Hyvän voima oli paljastanut hänelle paheen tien… Taistelu jatkui… Käärme matoi likemmäksi… Mairitellen liurutteli se häntäänsä… Kavalana valmistautui se jo
- 60. pistoon… Oolavi olijo likomärkä tuskanhiestä… Kauhu tukki häneltä suun… Yhä lähestyi vihainen, kavala käärme… Turhaan koetti Oolavi paeta sitä… Jäsen ei hievahtanut, ei irronnut hätähuuto kielenpäästä… Hirmu jähmetti hänet kokonansa… Käärme kohottautui jo pistoon… Sen pää nousi jo vuoteen reunan yli… Jo oli se kyljen tasalla… Jo avasi se kitansa… Jo näkyi sen verinen kieli… Se tähtäsi pistonsa suoraan sydäntä kohti… Oolavi kiemurteli kauhuissansa… Hän tunsi käärmeen kylmän hengityksen… Nyt oli sen verinen kita jo ammollansa… — Oi! — huudahti Oolavi kauhuissansa. Silloin kierähti tulikieli käärmeen avatusta kidasta, ja sen sävähteestä leimahti outo, viisaan näköinen mies Oolavin eteen, ja käärme hävisi. Perkele oli ilmestynyt ihmispoveen hyvän voimien aseesta, sen varottavasta käärmeestä. — Luojan kiitos! — huudahti kauhusta vapautunut Oolavi ja katsoi outoon tulijaan kuin suureen, taivaalliseen pelastajaan. Rauhotellen, oikean olemuksensa salaten lausui Perkele jatkoksi Oolavin sanoihin: — Niin… Saakoon Luoja aina ylistyksen!… Ei toki väärä, olematon Luoja!… Oolavi seisoi hänen edessänsä uudessa, henkisessä ruumiissa. Ihmetellen lausui hän hänelle:
- 61. — Nyt näenvarmaan jotain ihmeellistä!… En muista, missä olen sinut nähnyt… Myös ovi, josta tulit, oli outo… Suuri viettelys alkoi. Kavalana vastasi hänelle hurskaaksi tekeytynyt viettelijä: — Niin ehkä… Mutta tässä maailmassa ei ole viisaalle mikään ihmeellistä… Ja mitä oveen tulee: eihän viisas käy koskaan tavallista, tyhmän tietä, kuin lammas, joka seuraa kellokasta suin-päin ja aivan mitään miettimättä… Jos kellokas vain hyppää riuvun yli, niin koko lauma seuraa johtajaansa ja hyppää hypyn aivan sillä kohtaa, vaikka riuku onkin aikaa ollut poissa… Hänen katseensa oli lumoa täynnä. Terävä iva tunki hänen aseenansa ihmispoveen… Ihmeissään katsoi Oolavi häneen, lausuen: — Sinä et nähtävästi tahdo milloinkaan hypätä tielle pannun riuvun yli, et myöskään kellokasta jäljitellä… En totisesti ole vielä nähnyt kenenkään käärmeen suusta taloon käyvän… Oletko ehkä joku suuri viisas? Nerokkaana, mutta vaatimattoman näköisenä myönteli Perkele hänelle: — Ehkä!… Mutta viisaudella kerskuminen ei ole toki minun ammattini… Se työ on aina ollut tyhmän työtä. Sen verran sentään tässä uskaltanen sanoa, etten koskaan vielä ole hypännyt, enkä hyppää riuvun yli, vaan potkaisen pois tieltä moisen kepin, ja kuljen suoraan, kuten tekee mies… Kohtalon rihmat punoutuivat. Ne alkoivat kiertyä vähäpätöisistä säikeistä, ettei sitä huomaisi ihmishenki. Perkeleen miehuus lumosi
- 62. Oolavia. Hän huomauttiihmeissänsä: — Mutta entäs jos riuvun päässä onkin aikamies, joka ei salli sinun 'keppiänsä' noin ilman muuta potkaista vain pois? — Niin riuvun alitse silloin pujottaudun, — vastasi suuri kiusaaja, selittäen: Kuuluisalla viisaudella on aina ollut tämä tunnusmerkki: Se antaa hullun häntä narrinansa pitää ja sillä aikaa tekee narraajastaan sen puuhailuiden kautta aika narrin… Kai on myös tässä maassa viisaitakin, siis niitä, joilla on ikää hartioillaan?… Tietysti nuoret ovat viisautta täynnä siihen asti, kun huomaavat jo että raukat ovat harteillaan tyhjää leijaa kanneksineet… Viisaus on aina pitkän ijän lahja… Salaperäinen valo peitti puhujat hämyihinsä. Siellä täällä heilahti jotakin silmiin tuskin näkyvätä, Perkeleen sanat leikkelivät Oolavia, joka lausui mietteissänsä: — Ei minullakaan ole liikaa ikää… Siis ehkä olen mielestäsi hullu? Salaisen hämärän valot vaihtelivat… Kuului etäinen veden kohina. Sen humistessa selitti kiusaaja hänelle: — Ei nuoruus ole toki oma vika. Se on ainoastaan pieni välttämätön paha… Se kyllä itsestänsä ohi menee… Se tuottaa hyötyäkin elämässä, jos sitä itse tahdot oikein käyttää: et opeta, vaan itse opit, tutkit ja tunkeudut suureen salaisuuteen, kuin toukka joka puuhun tunkeutuu… Siis tutki, sekä tartu miehekkäästi viisauden jaloon, suureen epäilyyn!… Se sana pisti Oolaviin kuin puukon terä… Hän vavahti… Yhä koveni salainen veden soitto… Se muuttui jo koskenkohinaksi, joka kutsui…
- 63. Yhä himmeni outovalo… Perkele loihti siihen himmeät elämän ihanuudet… Sanojensa vaikutusta lisätäksensä asetti hän kädet ristiin rinnoillensa ja lausui hurskailla eleillä: — Kaikkeuden oikealle Jumalalle olkoon kiitos jalosta epäilystä! Hän painosti sanan oikealle, tarkottaen sillä itseänsä. Yhä lähempänä humisi huumaava koski. Oolavi alkoi hiljaa lumoutua. Sana epäily soi yhä hänen korvissansa. Perkele jatkoi hartaana: — Se epäily vie kyselyjen kautta syvimmän salaisuuden tutkimiseen… Niin avautuu suuri viisaus ja ihmissilmä oppii näkemään kaikkeuden ihmeet niin kuin Jumalakin… Kosken kohina soi kuin kaikkein kaunein soitto… Epäily sai kauniin ruumiin. Se häilähteli oudossa valossa elämän ihanuutena… Se kutsui ja veti… Perkele näytti Oolavista pyhimykseltä. Oudostellen lausui hän hänelle: — Sinä taidat olla pyhiinvaeltaja, kun Jumalaa noin hartain äänin kiität!… Valo muuttui. Sen lumous lisääntyi… Kavalana vakuutti Perkele: — En ole ikänäni muuta tehnyt, kuin perustanut suurta valtakuntaa hänelle: oikealle Jumalalle, ja kumonnut sen väärän kaikkivaltaa, joka kavalana rääkkää ihmislasta pimittäen siltä järjen kokonaan… Yhä lumoutui Oolavi. Hän huomautti Perkelettä tarkastaen: — Äänestä päättäen olet hurskas munkki.
- 64. — Kaikista puhtainmitä olla voi, — huudahti Perkele, lisäten: — Naisesta en tiedä enempää, kuin tiesi muinoin Neitsyt Mariakaan miehestä, nähdessänsä Gabrielin… Yhä kauniimpana vilahteli epäily ihanassa elämän hämärässä… Sillä oli jo koruina kaikki elämän ihanuudet… Se häilähteli kuin kaunis neidon kutri… Kosken kohina soi kuin kaunis taikasoitto… Sekin oli epäilyn suurta kutsua… Huumautuneena alkoi Oolavi jo epäilystä puhua… Hän lausui: — Niin: kyllä kunnioitan hurskauttasi… Vaan sinä puhuit äsken epäilystä… Se sana ei nyt ole aivan selvä: en tiedä mitä olisi epäiltävä… Pitäisikö minun sitä tehdessäni epäillä Häntä, suurta Kaikkivaltaa, joka on luonut taivaan sekä maan?… Valo vierähti punertavaksi… Koski soitti kauneinta kanneltansa… Kaikki se oli epäilyn kutsuvaa ääntä, ja se soi koskenlaskijalle… Kiehtovana selitti Perkele hänelle: — Varmaankin epäilet Hänen olemustaan, kun et uskalla Häneen luottaen laskeutua temppelinharjalta alas maahan asti: Et tohdi epäillä Hänen olemustaan… varmaankin pelkäät, että hän on heikko, semmoinen joka kaatuu taikka häviää, kun sokeasta uskostasi luovut ja alat Hänen töitään tutkistella… Tai et luota siihen, mitä kerran Hän on luvannut sanassaan, kun lausui: 'Hän lähettää enkelinsä vartioimaan sinua, ettet jalkaasi kiveen loukkaa'… Syvyydestä kuului kaunis laulu… Sieltä näkyi ihanin elämän hämärä, jossa nuoret naiset karkeloivat… Ne huiskuttelivat limoinansa suurta elämän salaisuutta… Koski kohisi yhä kuumemmasti…
- 65. Jo värisi Oolavinsielu… Jo veti sitä salainen voima… Nuhdellen lausui Perkele hänelle: — Sinä epäilet siis Hänen lupaustaan tai voimaansa, kun et niihin luottaen uskalla sokean uskon temppelinharjan päältä epäilyn jaloon kuiluun heittäytyä, töitään ja ihmeitänsä tutkimaan… No sekin 'epäily' on 'epäilystä'… Mutta muista: sokea epäilys ei ole siveellistä, vaan on se aivan yhtä arvoton kuin sokea usko… Entistä lumoavampana nousi soitto syvyydestä… Yhä armaampana huiskuttivat neidot siellä liinojansa… Elämän salaisuus nousi huumaavana höyrynä kultaisista kattiloista… Koski veti jo yhtenä lumona… Perkeleen viisautta ihaillen lausui Oolavi hänelle: — Sinähän olet pieni filosofi… Salaperäisenä selitti suuri henki siihen: — En sitä kiellä, enkä kerskaa sillä… Se vanha, kuulu filosofin arvo on miekka, jossa on kahdenlainen terä: Hulluuden filosofi tuntee tutkimatta syvyyden kaikki salat juurtajaksain… Järjen suuri filosofi pukee taas viisauden kaikkein konstikkaimpaan vaikeuden narrikaapuun… Hän tietää, että salaperäisyys on aina kiehtovinta… Sillä myöskin on helpoin peittää pieni järjen puute… Edellinen saa viisautensa äitinsä kohdussa ja halveksii tutkimista… Jälkimäinen kokoaa sen epäilyn ja tutkimuksen kautta… Kummanko tien ja arvon sinä tahdot?… Tohditko epäillä ja tutkistella? Hän loihti Oolaviin viisauden ja tiedon janon… Se jano poltti… Mutta syvyydessä lorisi jo kaunis tiedon lähde… Nuoret neidot istuivat sen ympärillä, kultaiset harput käsissä… Lähteestä näkyi neidon kuvain… Harput säestivät sen lorisemista… Kaunis neitonen
- 66. jakoi lähteen vettäkultalipillä… Mutta minkä se vesi sammutti tiedonjanoa, sen sytytti sitä harpun ihana soitto… Yhä kaunistui elämän hämärä… Yhä kiehtovampana kutsui epäily Oolavia. Hän lausui jo mietteissänsä: — Ei minulta koskaan rohkeutta puutu… Jo tuhannesti olen Hänen varassansa laskenut kotikosken suuret kuohut… Vaan enhän ole toki oikeutettu nyt Hänen armoaan ja kaitselmustaan niin julkeasti väärin käyttämään ja uskon temppelinharjalta epäilyn syvään kuiluun heittäytymään… Jeesuskaan ei kiusannut niin paljon suurta Isää, kun seisoi temppelinharjalla kiusattuna… Hyvän ja pahan voimat taistelivat hänen sielussansa… Mutta yhä kauniimmin soivat harput syvyydessä… Jano yltyi soitannosta… Lähde tarjosi vilpoista vettä… Iki-kavalana selitti Perkele hänelle: — Jeesus on toista, toista olet sinä… Se mikä sopi kerran Jeesukselle, ei sovi toki koskaan ihmiselle… Hän Itse käski kerran Pietarinsa koettaa Luojan sanan pätevyyttä: Hän käski hänen käydä haahdestansa myrskyiseen mereen… Kirkonharjajuttu siis oli siinä aivan pilkulleen… Tiedonlähteellä istuva neito ojensi jo Oolaville kultalipillä vettä… Harppu soitteli häneen yhä uutta janoa… Jo veti koski koko huumeellansa… Yhä kavalampana puhui kiusaaja hänelle. Se selitti: — Epäilyn kautta silloin Pietarille avautui jalo, siveellinen usko: Hänhän sen jalon epäilyksen kautta sai todistuksen Hänen voimastansa ja tuli kaiken uskon kallioksi… Epäily jalostaa siis heikon uskon, kuin tuli kullan…
- 67. Pietarin suuri elämäntyöoli loihdittu Oolavin eteen… Se alkoi Genezaretin järveltä, jossa Pietari kävi veden päällä, epäili ja rupesi vajoamaan… Sumuna puhalsi Jeesus hänestä epäilyn… Näky jatkui… Miljoonat kirkonkellot soivat. Miljoonat seurakunnat veisasivat kautta aikojen… Kaikkien niiden perusteena oli entisen epäilijän Pietarin usko… Silloin kirkastui Oolaville epäilyn jalous. Se kirkastuminen tuli kuin kirkkain salama, joka iskee kuivaan keloon… Jo seisoi hän koskenniskan luona… Jo veti vesi venettä… Miehen veri kuohahti suonissa… Mieli hapuili korkeutta tavotellen… Suuri henki janosi nähdä syvimmät salat… Veren kuohuessa lausui hän kiusaajallensa: — En pelkää hänen töitään tarkastella… Nyt minulla on myöskin rohkeutta epäilyn koskeen syöstä veneineni, kun näytät vain sen kosken kuohut nyt… Vaan enhän voi laskea koskea kalliolta, jossa ei ole edes vedentilkkaa: ei ole pienintäkään epäilystä. Vaan näytä nyt se koski vesinensä, vie minut temppelinharjalle, jonka alla on se koski, niin oitis olen valmis viskautumaan epäilyn kosken kuohuvimpaan ryöppyyn… Minä en arkaile Pietarin lailla, sillä minä olen Oolavi… * * * * * Syvyydestä näkyivät elämän suuret työpajat… Salaperäiset olennot takoivat oudossa satumaisessa valossa. Neidot lauloivat ja lähde lorisi… Oolavi seisoi jo matkavalmiina suuren saattajansa edessä. Kummat voimat vetivät häntä. Perkele heitti sauvansa maahan ja oitis leimahti siitä palava pensas. Hämmästyneenä huudahti Oolavi:
- 68. — Kas sitä!…Tämä peli oudostuttaa. Puutarhan pensas palaa, eikä kulu!… Sehän jo ihmetyötä muistuttaa!… Rauhallisena selitti Perkele hänelle: — Joka lähtee ihmeen syitä tutkimaan, hän saa nähdä ihmeitä itseäänkin… Huumaava savu nousi tulesta… Siinäkin soi kaunis soitto. Ihastuneena lähestyi Oolavi lieskaa ja koski siihen kädellänsä, mutta silloin hävisi tuli kuin rajutuulen sieppaamana ja sen sijalle avautui kuilu, johon laski vuolas koski, häviten kuilun pilkkopimeyteen: Suuri ihme oli Oolavin edessä. Hämmästyneenä huudahti hän: — Suuri Luoja!… Nythän muuttuvat jo alkuaineet!… Tulesta syntyy vesi!… Entistä viisaamman eleillä selitti Perkele: — Ei tämä ole vielä mikään ihme, vaan vanha, ennen tapahtunut seikka… Hän loihti Oolavin eteen maailmoiden syntymistoimen. Sitä osottaen selitti hän: — Maakin oli kerran tulinen pallo. Mutta eikö vesi nyt jo peitä sitä? Se vesihän on tulen synnyttämä. Ei ole se voinut tulla sateena avaruuden ikityhjyydestä… Kauniit neidot soittivat syvyydestä tiedon ja elämän suloista virttä… Koski kutsui laskijaansa… Suuri lumous veti Oolavia ottamaan salaisuutta käsin kiinni. Huumautuneena huudahti hän Perkeleelle:
- 69. — Nyt joluotan sinun puheisiisi!… Vie minut syvemmälle ihmeen sisään! Suo silmin nähdä ihmeen syyt ja juuret ja näytä miten oleva syntyy olemattomasta ja elollinen alkaa elottomasta! Minä tahdon nähdä kaikkeuden salat… — En jätä pyyntöäsi täyttämättä, — vakuutti Perkele. Voiton-ilo kirkasti taas hänen kasvojansa. Suuri kulku aikoi. Perkeleen käskystä syöksyi syvyydestä koskenniskaan tulinen vene… Sen hohde huikaisi silmää. Se keikkui kepeänä kuohujen päällä, koreili siinä kuin tulinen kukka… Jo astui Oolavi veneeseen… Hän astui siihen rohkeampana, kuin oli tähän asti viskautunut veneessänsä kotikosken kuohuihin, koettelemaan siinä käsivarren voimaa… Vene keikkui kepeänä… Jo syöksyi vene maan alle pimeään kuiluun… Kamala koski syöksyi synkkää tietänsä peninkulmittain, kierrellen maan pimeissä onkaloissa… Väliin viskautui se kalliolta äkkijyrkästi alas… Hyrskyn heittämänä syöksyi se taas toisten kallioiden yli, viskautuen oman vauhtinsa voimalla… Se teki äkkimutkia: raivoisana heittäytyi se joskus aivan päinvastaiseen suuntaan… Mutta aina syöksyi se toki alaspäin… Ryöppy ajoi ryöpyn päälle… Hirmuisimmat hyökyaallot löivät toisiansa kumoon… Ne paiskailivat toisiansa vesipaljouksina pimeän, jylhän holvin vuoriseinämiin… Kaikki ryski… huusi… ulisi… ärjyi… Kamala vesipauhu täytti holvin… Se tuntui repivän vuoria… Se vapisutti kaikkea hirmuisuudellansa… Se syöksyi syvyyteen kuin raivostunut vesipeto, kiemurrellen kamalaa tietänsä pitkin kuin hirvittävä vesikäärme, joka tulipihdissä raivostuu, nostaa harjansa ja
- 70. yrittää kietaista kiusaajansaruumiinsa hirmuvoimien rusennettavaksi… — Tämä on toista kuin Tuukkalan koski, — huudahti hämmästynyt Oolavi. Koski ärjyi yhä vimmatummin… Holvin kiviseinät viskoivat sen hyökyaaltoja kauvas luotaan… Pimeyden vimmastuneet voimat tappelivat keskenänsä… Joskus syöksähti koskeen sivulta pienempi koski… Hyrske pieksi silloin hyrskettä… Joskus kaareutui itse koski monikymmenhaaraiseksi, hajoten pimeisiin onkaloihin, kunnes taas kaikki yhtyivät ja syöksyivät kiljahdellen syvyyttä kohti… Holvi oli haljeta pauhinaansa… Pieni tulivene valaisi pimeyttä himmeällä valollansa, joka hohti punaisena ryöpyn seasta, joskus aivan häviten siihen… Sen valon hämärässä häämöittivät rosoiset kiviseinät kuin hornan jättiläiset… Vähin taas irvistelivät vuoren halkeamat kuin ammottavat pimeyden kidat… Ne uhkasivat joskus niellä veneen ja kaiken… Kaikki huusi, ärjyi, ulisi… Koko holvi oli voimaan haljeta. Kävi outo viima. Tulinen vene lensi nuolena kuohujen halki, Oolavi teljolla, Perkele perää pitämässä. Huumaavasti kiusattavaansa katsoen kysyi hän: — Pelkäätkö?… Epäiletkö? — En, — vastasi Oolavi. Koski veti ja huumasi häntä. Kiehtovana selitti Perkele hänelle: — Turhaa onkin katumus ja pelko sille, joka on kerran kosken kuohuun syössyt. Nääs: koskessa ei voi kukaan venettänsä kääntää ja vastakoskeen laskea. Siis täytyy koskelle herruus antaa…
- 71. Leimuavan tulen laillaajeli vene aaltoja pitkin. Viisaana jatkoi Perkele: — Sitäpaitsi on hulluutta lähteä tyventä kosken yläpuolelta etsimään, kun alapuolellakin on tyven… Se tyven on tuntematon ja siis ihana ja siihen vie myötävirta… Hän karkotti silmäyksellänsä viimeisenkin arkailun Oolavista. Kosken alapuolelta, tyvenestä, kuului jo heleä soitto… Sieltä vilahtelivat kauniit, kutsuvat tulet… Oolavin rohkea henki nautti kosken pimeyden voimaa… Salaisuuden ikävä täytti koko hänen sielunsa. * * * * * Outo tuuli puhalsi… Tulinen vene viskautui jo valtavaan luolaan, jonka holvit kohosivat suunnattomaan korkeuteen… Ne holvit nojailivat taitteissansa sateenkaaren-värisiin kaariin, jotka viskautuivat yli luolan valaisten sen salaisella värivalollansa… Kaikki peittyi sen valon himmeään hämärään… Vesi vilisi surullisena, seinät vanhoina satuina… Luola näytti autioksi jätetyltä jumalien asuinmaalta… Silmä etsi siellä kaikkialla jumalien vainajia, tai niiden hautuumaata… Pauhu lakkasi… Alkoi ikityven. Tulinen vene solui hämärissä luolan läpi, niin kuin kaunis vesilintu… Omin voimin suhahti se rannan hiekkaan runollisessa onkalossa… Käskevästi lausui Perkele Oolaville: — Astu maalle! Nyt on laskettu pieni koski… Ihmeissään huudahti Oolavi hänelle:
- 72. — Ei aivanpieni!… Riittää kerraksensa!… Sanopas miten monta peninkulmaa alemmaksi jo laskeuduimme!… — Emme tuumaakaan, — vastasi Perkele kuivasti. Kuului oudon linnun laulu. Oolavi tunsi seisovansa elämän ja kuoleman rajamailla, tai ihmeiden kotiperillä. Perkeleen vastaus ei häntä tyydyttänyt. Hän intti loukkaantuneena: — En usko sitä… Varmaankin teet nyt pilaa minusta!… Loukkautuneeksi tekeytyen selitti kiusaaja: — Minäkö ivaa! Ensi kertaa kuulen siitä puhuttavankaan. Iva on pikkusielun tavaraa. Se sekä suuret sanat ovat huntu, joilla peitetään älyttömyys ja tiedon puute, kuten nainen peittää hunnulla rumuutensa. Vannon sinulle että emme tulleet rahtuakaan alemmaksi emmekä etemmäksi, sillä eihän äärettömyydessä voi olla paikkaa eikä ylä- ja alapuolta… Outo valo välähti. Se sokaisi Oolavia. Hämmästyneenä huudahti hän: — Sitä en ole ennen ajatellut!… Nyt näen ettet kulje ensi kertaa järjen teillä… — En viimeistäkään… Järjen tie on aina ollut tienäni, — vakuutti suuri henki. He tulivat salaperäiseen luolaan. Siellä raatoivat salaisimmat luonnon voimat. Hämärässä ei erottanut silmä mitään selvästi nähtävää. Kaikkea peitti suuri salaisuus. Se salaisuus pani Oolavin sielun värisemään kuin puhtaan kukan, joka juuri avautuu auringon valolle ja lemmen ensi suutelolle, pelkää ja odottaa sen antimia. Hän
- 73. vapisi ja herkkeni…Hän odotti hengen lemmen antimia: syvimmän salaisuuden tietämystä, Jumaluuden olemusta. Värisevänä, janon polttamana kuiskasi hän: — Täällä on varmasti ihmeen alkukoti. Kylmänä selitti kiusaaja hänelle: — Täällä on vasta ihmeen esihuone. Ihmeen juuret kiinnittyvät kaikkeuden jokaiseen rahtuun… Salaiset voimat raatoivat taas Oolavissa… Taas levisi hänelle tiedonlähde… Taas kuului kaunis harpun soitto… Taas tarjosi kaunis tarutyttö kultalipillä vettä… Taas kuiskaili kiusaaja hänelle: — Sinä saat nähdä kaikki salaisuudet, jos itse tahdot… Sinun tarvitsee ainoastaan ottaa salaisuuden avaimet pois Jumalan käsistä ja itse nousta niiden herraksi Jumalan sijalle… Outo kello kumahti… Oolavin sielussa värähti salainen kieli… Yhä lumoavammin jatkoi silloin Perkele. Äänensävyä muuttaen selitti hän: — Hänellä onkin liian paljon työtä, kun kaikki on jättäynyt Hänen rasituksekseen: Hänen täytyy hoitaa pilven kulku, hoitaa linnunpoika ja pitää huolta ihmisestä… Hänen täytyy johtaa luonnon salaista työtä, kun ihminen on suruttomana jättänyt kaikesta huolenpidon ristiksi Hänelle… Kukaan ei tahdo auttaa Häntä kuten hyvä poika, joka ottaa isän talon hoitaaksensa, kun on päässyt isän turvissa miehen ikään… Usko ja kieltämys alkoivat suuren taistelunsa Oolavissa. Perkele oli puhunut hänelle vanhan asian uusilla sanoilla. Keskeyttäen huudahti kiusattava:
- 74. — Nyt sinäpuhut minulle himmeästi… Pitäisikö minun nousta Hänen istuimelleen ihmeen herraksi?… Ei Hän toki herruuttansa ihmisille luovuttane!… Yhä enemmän huumasi salainen soitto ja hämärä. Tiedonlähteestä nousi ihmeen kaunis sumu… Murheissansa selitti Perkele Oolaville: — Järkeä on aina sanottu himmeäksi… No, monelle se lieneekin himmeää. Sinun uskoin kumminkin käsittävän asian, mutta petyin. No katso: Eikö Hän kerran Itse luovuttanut Pietarille taivaan valtakunnan avaimia? Ja jos Hän Pietarille uskoi taivaan, eikö Hän silloin sinulle tarjoa maan ja ihmeen avaimia?… Harpun salainen ääni hiveli Oolavin korvia… Tiedonlähteen höyry huumasi häntä… Salaisuus tarjoutui hänelle ihanana… Hän näki kauniita näkyjä. Perkele lopetti kavalana: — Luomisesta asti on Hän sinua kehittänyt sitä varten, että julistaisit itsesi täysi-ikäiseksi, ottaisit ihmeen avaimet haltuusi ja päästäisit Hänet lepoon… Tiedonlähteellä karkeloivat kauniit neidot… Ne huiskuttelivat suurta salaisuutta liinoinensa… Kaikki kutsui Oolavia… Perkele lisäsi vielä: — Hän lausui luomisessa; 'Tehkää maa alamaiseksenne!'… Tahdotko totella Häntä?… Haluatko nousta Hänen istuimellensa, jonka Hän on tarjonnut sinulle?… Hänen sanansa ja katseensa lumosi… Oolavi värisi. Hän tahtoi. Sokeana lausui hän:
- 75. — Sinun puheesion kaikki oikeaa, vaikka se on sumuun käärittyä. Näytä minulle nyt koko ihmeen syvyys!… Minä tahdon tietää kaiken sen, mitä Hänkin tietää… Silloin voin päättää onko minulla oikeutta nousta Hänen istuimelleen… Minä tahdon nähdä kaikki… Kuuletko!… Näytä minulle nyt kaikki! Hän puhui kiihkoissansa, katse hurjana. Lumous painoi häntä. Kylmänä vastasi Perkele hänelle: — Minä en vielä tunne sinua… Suuriin tehtäviin ei kelpaa tuntematon toveriksi… Muistathan miten Judas petti kerran Mestarinsa suuressa tehtävässä… Se vilahti kuin salainen punainen vaate Oolavin edessä. Hän kiihtyi… Hän tavotti jo salaisuutta käsin kiinni. Sormet paloivat. Kaunis soitto tuntui olevan hänen käsistänsä pois pääsemäisillänsä… Kiihkoisena huudahti hän Perkeleelle: — Epäiletkö sinä minua nyt jo petturiksi?… Uskallatko?… Kauniimpina ja aina kauniimpina huiskivat salaiset liinat neitojen käsissä… Oolavin vavistessa vastaili Perkele hänelle: — En epäile, mutta en voi myöskään varomaton olla… Mutta jos vannot ja lupaat kuolemaasi asti olla altis, uskollinen ystäväni, niin tahdon koettaa… Oolavin ohi suhahti kaunis sävel… Hänen henkensä tapasi sitä kiinni… Sävel pääsi pakenemaan… Se lisäsi sielussa paloa. Kiihkoissansa huudahti hän: — Mies ei koskaan ystäväänsä petä…
- 76. Tiedonlähteestä nousivat salaisetsuudelmat. Harppu ei soinut enää säveltä, vaan lumoa. — Vannotko siis? — kysyi Perkele lumotulta. — Minä vannon. Näytä nyt kaikki minulle! — lausui Oolavi valansa. Tiedonlähteellä lauloivat neidot ylistystä ikuiselle hämärälle. Soitto soi. Se jakoi ainaista janoa. Lähde lorisi ja neito kurotti siitä kultalipillä vettä. Mutta yhä janoisempana kärsi ihmishenki juotuansakin… * * * * * Kulkijat tulivat ihmeen suureen kiviluolaan. Kaamea puoli-pimeä täytti sen suunnattoman suuruuden… Pelottava hiljaisuus näytti asustavan sen onkaloissa. Kylmänä, suurena puhui Perkele uudelle ystävällensä: — Tähän asti olet uskonut ukkos-ilmaa Jumalan ihmetyöksi… Ja uskosi olikin oikea, sillä se on Jumala, joka hallitsee ihmettä. Nyt saat sinä nousta sitä hallitsemaan… Saat nousta Hänen valta- istuimelleen… Ota tämä tomu ja heitä ilmaan, niin olet tehnyt ihmeen, kuten Jumala, ja olet tullut Jumalaksi… Oolavi totteli ja heitti tomun ilmaan. Se herätti oitis sähkön toimimaan… Kirkas salama sävähti. Hirmupauhu riehahti ankarana, kuin olisi se vuorta reväissyt. Salaman huikaisemana säikähtynyt Oolavi huudahti kauhuissansa: — Oi, ukkonen!… En tahdo minä nähdä enempää.
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