5 cells-modules-and-arrays

 2012 Jim Dunlop Solar
Chapter 5
Cells, Modules and Arrays
Principles of Operation ● I-V Characteristics ● Response
to Irradiance and Temperature ● Series/Parallel
Connections ● Specifications and Ratings

 2012 Jim Dunlop Solar Cells, Modules and Arrays: 5 - 2
Overview
 Describing the photovoltaic (PV) effect and comparing the fabrication of
solar cells from various manufacturing processes.
 Defining the current-voltage (I-V) characteristic for a PV device and the
key operating parameters.
 Understanding how sunlight, temperature and electrical load affect the
output of a PV device.
 Determining the electrical output of similar and dissimilar PV devices
connected in series and parallel.
 Explaining the purpose and operation of bypass diodes.
 Describing the performance rating conditions and labeling requirements
for PV modules.

Cells, Modules and Arrays
Cell
Module
Array

Solar Cells
 Solar cells are semiconductor devices that convert sunlight to DC
electricity.
Photovoltaic cell
Phosphorous-doped silicon
(N-type) layer ~ 0.3 μm
Electrical
Load
(-)
(+)
Boron-doped silicon
(P-type) wafer < 250 μm
DC current flow

The Photovoltaic Effect
 The photovoltaic effect is the process of creating a voltage
across charged materials that are exposed to electromagnetic
radiation.
 Photons in sunlight impart their energy to excess charge carriers
(electrons and holes) allowing them to freely move about the
material.
 Charge opposition between the two materials creates an
electrical field that provides momentum and direction to the free
charge carriers, resulting in the flow of electrical current flow
when the cell is connected to a load.

Silicon Solar Cells
 Silicon solar cells produce about 0.5 to 0.6 volt independent of
cell area, depending on temperature.
 The current output of a solar cell depends primarily on the cell
area, its efficiency, and the incident solar radiation.
 Modern silicon solar cells are up to 8 inches in diameter and
produce up to 4 watts and 8 amps under full sunlight.
Monocrystalline cell Polycrystalline cell

Crystalline Silicon Wafer Production
 The following processes are commonly used to create P-type
silicon wafers:
 The Czochralski method produces a single or monocrystalline wafer.
 The cast ingot method produces a multigrain or polycrystalline wafer.
 The ribbon method produces polycrystalline wafers by drawing molten
silicon between dies in a continuous process.
 Wafer are additionally processed to produce a complete solar
cell.

Moncrystalline Wafer Production:
Czochralski Method
 Single crystal or monocrystalline
silicon wafers are grown in the
form of a cylindrical ingot,
creating a perfect crystal.
 A seed crystal is inserted into
molten polysilicon doped with
boron, rotated and drawn
upward allowing the P-type
silicon material to cool into a
cylindrical ingot.
Czochralski Method

Polycrystalline Silicon Wafer
Production
 Polycrystalline or multi-
crystalline silicon wafers are
cast, forming a block-shaped
ingot that has many crystals.
 Molten polysilicon doped with
boron is poured into a
rectangular crucible, and slowly
cooled at controlled rate.
 Polycrystalline wafers are also
made using the ribbon method.
Cast Ingot Method

Solar Cell Manufacturing
 Once a P-type silicon ingot is produced, a number of additional
steps are required to create an actual solar cell.
SolarWorld USA
Cropping
Phosphorous diffusion
Screen printing Electrical testing
Sawing

Flat-Plate PV Modules
 Flat-plate PV modules respond to both direct and diffuse solar
radiation, and are the smallest field installable generating unit.
Single (mono) crystalline Polycrystalline
SolarWorld

Flat-Plate PV Modules
Single (mono) crystallinePolycrystalline
36 cell modules
60 cell polycrystalline module

Typical PV Module Construction
Continuous silicone seal
Tempered glass
EVA embedding
Solar cells
Tough polymer
back sheet
High strength frame
SolarWorld USA

Emerging PV Module
Technologies
 Thick wafer silicon P-N junction solar cells are considered first
generation PV devices.
 Second generation devices are thin-film devices including:
 Amorphous silicon (a-Si)
 Cadmium Telluride (CdTe)
 Copper indium gallium selenide (CIS or CIGS)
 Other advanced PV module designs include:
 Concentrating PV modules
 AC modules
 Polymer and organic solar cells

Thin-Film PV Modules
 Thin-film PV modules are
produced by depositing ultra-
thin layers of semiconductor
materials on a flexible or rigid
substrate.
 Thin-film modules have
significant potential for cost and
weight reductions.
 Disadvantages include lower
efficiencies and higher
degradation rates than
crystalline silicon modules.

Concentrating PV Modules
 Use optics to focus sunlight on
solar cells up to 200-500 X.
 Employ advanced multijunction
solar cells approaching
efficiencies of up to 40%.
 Utilize only direct component of
total global solar radiation, and
employ two-axis sun tracking.
 Design challenges include
managing high temperatures and
high DC currents.
NREL, Bill Timmerman

AC Modules and Micro-Inverters
 Alternating-current (AC) modules
are an integrated PV module and
inverter product intended for
installation as a single unit.
 AC modules do not have any
field-installed DC wiring.
 Micro-inverters are separate
module level inverters intended
for field installation.

Photovoltaic Arrays
 PV arrays are constructed from building blocks of individual PV
modules, panels and subarrays that form a mechanically and
electrically integrated DC power generation unit.

Photovoltaic Arrays
Ground-mounted rack array
Pole-mounted tracking array
Standoff roof-mounted array
Building-integrated array

Photovoltaic Arrays
Roof-mounted rack array
Roof-mounted standoff array

Leading Manufacturers of
PV Cells and Modules
 BP Solar
 First Solar
 JA Solar
 Kyocera
 Mitsubishi
 Motech
 Q-Cells
 Sanyo
 Schott Solar
 Sharp Solar
 SolarWorld
 SunPower
 Suntech
 Trina
 Yingli

Current-Voltage (I-V)
Characteristic
 The electrical performance of a
PV device is given by it current-
voltage (I-V) curve.
 Represents an infinite number of
I-V operating points.
 Varies with solar radiation and
device temperature.
Voltage (V)

Key I-V Parameters
 PV device performance is
specified by the following I-V
parameters at a given
temperature and solar irradiance
condition:
 Open-circuit voltage (Voc)
 Short-circuit current (Isc)
 Maximum power point (Pmp)
 Maximum power voltage (Vmp)
 Maximum power current (Imp)
Voltage (V)
Isc
Voc
Imp
Vmp
Pmp
Area = Pmp

Power vs. Voltage Curve
Voltage (V)
Isc
Imp
Vmp Voc
Pmp = Imp x Vmp
Pmp
Current vs. Voltage
Power vs. Voltage

PV Module Rating Conditions
 The electrical performance of PV
modules is rated at Standard Test
Conditions (STC):
 Irradiance: 1,000 W/m2 , AM 1.5
 Cell temperature: 25°C
Source: SolarWorld USA

Fill Factor
 Fill factor (FF) is an indicator of the quality of a solar cell.
 FF = (Vmp x Imp) / (Voc x Isc) = Pmp / (Voc x Isc)
Voltage (V)
Isc
Imp
Vmp Voc
Pmp = Imp x Vmp
Isc x Voc

Efficiency
 Efficiency of a PV device is the
ratio of the electrical power
output and the solar irradiance
input over the device area,
expressed as a percentage:
 Example:
 What is the efficiency for a PV
module that has a surface area of
1.4 m2, and produces 200 W
maximum power when exposed to
1000 W/m2 solar irradiance?
2
2
efficiency
maximum power rating (W)
solar iradiance (W/m )
surface area (m )
mp
mp
P
E A
where
P
E
A
η
η
=
×
=
=
=
=
2 2
200 W
(1000 W/m 1.4 m )
0.143 14.3%
mpP
E A
η
η
η
=
×
=
×
= =

Response to Electrical Load
 The electrical load connected to a PV device determines its
operating point.
 If a battery is connected to a PV device, the battery voltage sets the
operating voltage for that PV device.
 In a grid-connected PV system, the inverter loads the PV array at its
maximum power point.
 From Ohm’s Law, the electrical load resistance that operates a PV
device at its maximum power point is equal to Vmp/Imp (ohms).
PV Device
+
-
Electrical
Load

Operating Point
Voltage (V)
Isc
Imp
Vmp Voc
Pmp = Imp x Vmp

PV Modules for
Battery Charging
PV module maximum power voltage must be higher
than battery voltage at highest operating temperature
Voltage (V)
Current(A)
Module with 36 series-connected cells
operating at temperature of 50°C (optimal)
10 20
Operating voltage range for 12-volt lead-acid battery:
11.5 to 14.5 volts.
Maximum power points
Module with 30 series-connected
cells at 50°C (voltage too low to
deliver maximum current to battery)
Module with 42 series-connected cells at 50°C
(voltage is more than adequate for charging,
but power is wasted)

Effect of Electrical Load on
Operating Point
Voltage
Decreasing
resistance
Constant Temperature
Increasing
resistance
R = 0
R = ∞
Load lines of constant
resistance

I-V Measurement Methods
PV Device
Variable
resistor
Electrolytic
capacitor
Variable
battery
V
A

Solmetric PVA-600 PV Analyzer

Raydec DS-100 IV Curve Tracer

Spire 4600 SLP Flash Simulator

Response to Electrical Load:
Example
 The maximum power voltage (Vmp) and maximum power current
(Imp) for a PV module are 35.8 volts and 4.89 amps, respectively.
What is the maximum power and load resistance required to
operate at maximum power?
 The maximum power is calculated by the product of the
maximum power voltage and maximum power current:
35.8 volts x 4.89 amps = 175 watts
 From Ohm’s Law, resistance is equal to the voltage divided by
the current:
35.8 volts / 4.89 amps = 7.32 ohms

Solar Cell Equivalent Circuit
 A solar cell equivalent circuit consists of a current source in
parallel with a diode and shunt resistance, connected to a series
resistance.
Shunt resistance Load resistance
Series resistance
DiodeCurrent
source

Series Resistance
Voltage
Moderate Rs
Low Rs
Increasing Rs decreases
Pmp, Vmp and Imp, and
also reduces FF and
efficiency.
Rs = - ∆V / ∆ I
High Rs
∆V
∆I

Shunt Resistance
Voltage
Moderate Rsh
High Rsh
Decreasing Rsh decreases
Pmp, Vmp and Imp.
Rsh = - ∆V / I
Low Rsh
∆V
∆I

Response to Solar Irradiance
Voltage
1000 W/m2
750 W/m2
500 W/m2
250 W/m2
Current increases with
increasing irradiance
Voc changes little
with irradiance
Maximum power increases
with increasing irradiance
Maximum power voltage
changes little with irradiance

Irradiance (W/m2)
1000
Isc increases with
increasing irradiance
Voc changes little with
irradiance above 200 W/m2
8006004002000

 The power and current output of a PV device are proportional to
the solar irradiance:
 Example: A PV module produces 200 watts maximum power at
1000 W/m2. Assuming constant temperature, the maximum power
output at an irradiance level of 600 W/m2 would be:
2 2 2
1 1 1
E I P
E I P
= =
2
2 1
1
600
200 120 W
1000
E
P P
E
= × = × =

Response to Temperature
 For crystalline silicon PV devices, increasing cell temperature results in a
decrease in voltage and power, and a small increase in current.
Voltage
T = 25°C
T = 50°C
T = 0°C
Increasing temperature
reduces voltage
Increasing temperature
reduces power output
Increasing
temperature
increases current

Temperature-Rise Coefficient
 The temperature-rise coefficient relates the temperature of a
given PV array to the ambient air temperature and solar
irradiance:
 At peak sun, the difference between PV array and ambient air
temperature can vary from 20 to 40°C, depending on the array
mounting system design.
2
( )
cell temperature ( C)
ambient air temperature ( C)
temperature-rise coefficient ( C/kW/m )
cell amb T rise
cell
amb
T rise
T T C E
where
T
T
C
−
−
= + ×
=
=
=




Temperature Coefficients
 Temperature coefficients relate the effects of changing PV cell
temperature on voltage, current and power.
 Percentage change coefficients are commonly used to translate voltage,
current and power from one temperature condition to another
temperature.
 For crystalline silicon PV, percentage change temperature coefficients
are approximately:
 CV = -0.4%/°C (voltage decreases 1% for 2.5°C increase in temperature)
 CI = +0.04%/°C (current increases 1% for 25°C increase in temperature)
 CP = -0.45%/°C (power decreases 1% for 2.2°C increase in temperature)
 Since the temperature coefficient for current is an order of magnitude
less than for voltage or power, the effects of temperature on current are
not usually considered in systems design.

Temperature Translation
Equations
cell
ref
cell
[ ( )]
[ ( )]
translated voltage at T (V)
reference voltage at T (V)
translated power at T (W)
reference
trans ref ref V cell ref
trans ref ref P cell ref
trans
ref
trans
ref
V V V C T T
P P P C T T
where
V
V
P
P
= + × × −
= + × × −
=
=
=
= refpower at T (W)
voltage-temperature coefficient (% per C)
power-temperature coefficient (% per C)
cell temperature ( C)
reference temperature ( C)
V
P
cell
ref
C
C
T
T
=
=
=
=





Response to Temperature:
Example 1
 A 72-cell crystalline silicon PV module has a rated open-circuit
voltage of 44.4 V at 25°C, and a voltage-temperature coefficient of
-0.33 %/°C. What would the open-circuit voltage be at a cell
temperature of 60°C?
 If the same PV module operates at -10°C (35°C lower than the
reference temperature), the translated voltage is:
[ ( )]
44.4 V + [44.4 V × -0.0033/ C × (60-25) C]
44.4 V - 5.19 V = 39.2 V
trans ref ref V cell ref
trans
trans
V V V C T T
V
V
= + × × −
=
=
 
44.4 V + [44.4 V × -0.0033/ C × (-10 - 25) C]
44.4 V + 5.19 V = 49.6 V
trans
trans
V
V
=
=
 

Response to Temperature:
Example 2
 A crystalline silicon PV array has a power-temperature coefficient
of -0.45 %/°C and rated maximum power output of 50 kW at 25°C
and solar irradiance of 1000 W/m2. What would the array
maximum power be at a cell temperature of 50°C?
 If the same PV module operates at 0°C (25°C lower than the
reference temperature of 25°C), the translated power is:
[ ( )]
50 kW + [50 kW × -0.0045/ C × (50-25) C]
50 kW - 5.6 kW = 44.4 kW
trans ref ref P cell ref
trans
trans
P P P C T T
P
P
= + × × −
=
=
 
50 kW + [50 kW × -0.0045/ C × (0 - 25) C]
50 kW + 5.6 kW = 55.6 kW
trans
trans
P
P
=
=
 

Building PV Arrays
 PV modules are connected
electrically in series to build
voltage output.
 Series strings of PV
modules are connected in
parallel to build current and
power output.
Voltage (V)
Current(A)

Monopole and Bipolar
PV Arrays
 Monopole PV arrays consist of two output circuit conductors.
 Bipolar PV arrays combine two monopole arrays with a center
tap.
Bipolar ArrayMonopole Array
PV Array
Positive (+) Negative (-) Center Tap
PV Array
Positive (+) Negative (-)
PV Array

Connecting Similar
PV Devices in Series
(-)
Pos (+)
Neg (-)
1 2(+) (-)(+) n (-)(+)
Vseries string = V1 + V2 ….. + Vn
Vseries string = V1 x n
Iseries string = I1 = I2 ….. = In (for similar
devices)

I-V Curves for
Similar PV Devices in Series
For similar PV devices in series:
Vseries = V1 + V2 ….. + Vn
Vseries = V1 x n
Iseries = I1 = I2 ….. = In
Voltage (V)
Current(A)
1 device
2 devices
in series
“n” devices
in series

Dissimilar PV Devices in
Series
 When dissimilar PV devices are connected in series, the voltages
still add, but the current is limited by the lowest current output
device in series.
 Not acceptable.
Vseries = VA + VB
Iseries = IA < IB
Pos (+) (-) (+) Neg (-)
Pos (+)
Neg (-)
A B

Connecting PV Devices
in Parallel
For PV devices in parallel:
Vparallel = V1 = V2 ….. = Vn (for similar devices)
Vparallel = (V1 + V2 … + Vn) / n
Iparallel = I1 + I2 ….. + In
Pos (+)
Neg (-)
n
(-)
(+)
1
(-)
(+)
2
(-)
(+)

I-V Curves for
Similar PV Devices in Parallel
Voltage (V)
Current(A)
Device 1+2
independently
Devices 1+2
in parallel
For PV devices in parallel:
Vparallel = V1 = V2 ….. = Vn (for similar
devices)

Connecting Dissimilar PV
Devices in Parallel
 When PV devices with the same voltage but with different current
output are connected in parallel, the individual currents add, and
the voltage is the average of devices.
Vparallel = (VA + VB) / 2
Iparallel = IA + IB
B
A
Pos (+)Neg (-)

 Series strings of PV modules with similar voltage but having
different current output may be connected in parallel.
Connecting Dissimilar PV
Devices in Parallel
Voltage (V)
Current(A)
Device 1
Devices 1+2
in parallel
For dissimilar current PV
devices in parallel:
Device 2

Bypass Diodes
 Bypass diodes are connected in parallel with series strings of
cells to prevent cell overheating when cells or parts of an array
are shaded.
When cells are not shaded, the
bypass diode is reverse biased and
does not conduct current
Shaded cell
When a cells is shaded, the bypass
diode is forward biased and conducts
current
Pos (+)Neg (-)

Without Bypass Diodes
Voltage (V)
Current(A)
Module 1
Module 2
0
operating current
Power produced by module with
higher current (healthy module)
Large power dissipation in module with
lower current (failing module)
<< Negative voltage (reverse bias) Positive voltage (forward bias) >>

With Bypass Diodes
Voltage (V)
Current(A)
Module 1
Module 2
0
operating current
Power produced by module with
higher current (healthy module)
Low power dissipation in module with
lower current (failing module)
<< Negative voltage (reverse bias) Positive voltage (forward bias) >>

Module Junction Box with
Bypass Diodes

 The electrical performance of PV
modules is rated at Standard Test
Conditions (STC):
 Irradiance: 1,000 W/m2 , AM 1.5
 Cell temperature: 25°C
Source: SolarWorld USA

Other PV Module Ratings
 Standard Operating Conditions (SOC)
 Irradiance: 1,000 W/m2
 Cell temperature: NOCT
 Nominal Operating Conditions (NOC)
 Irradiance: 800 W/m2
 Cell temperature: NOCT
 Nominal Operating Cell Temperature (NOCT)
 Irradiance: 800 W/m2
 Ambient Temp: 20°C
 PV Array: open-circuit
 Wind Speed: 1.0 m/s
 PVUSA Test Conditions (PTC):
 1000 W/m², 45°C, 1 m/s

Voltage
STC
SOC
PTC
NOC

Approved Modules
 Certain listed PV modules have been approved as “eligible
equipment” for California incentive programs.
 See: www.gosolarcalifornia.org
 These modules have had additional independent performance
tests for PTC ratings.
 Many other states refer to this list for eligible equipment for their
incentive programs.

Photovoltaic Module Standards
 Installation Requirements:
 National Electrical Code, NFPA 70
 Must be installed in accordance with manufacturer’s instructions
 Product Listing
 UL 1703: Standard for Safety for Flat-Plate Photovoltaic Modules and Panels
 Design Qualification (reliability testing)
 IEC 61215: Crystalline Silicon Terrestrial Photovoltaic (PV) Modules - Design
Qualification and Type Approval
 IEC 61646: Thin-Film Terrestrial Photovoltaic (PV) Modules - Design Qualification
and Type Approval
 Performance Measurement
 ASTM E1036: Standard Test Methods for Electrical Performance of Non-
concentrator Terrestrial Photovoltaic Modules and Arrays Using Reference Cells

PV Module Markings
 All PV modules must be marked
with the following information [NEC
690.51]:
 Open-circuit voltage
 Short-circuit current
 Operating voltage
 Operating current
 Maximum power
 Polarity of terminals
 Maximum overcurrent device
rating
 Maximum permissible system
voltage

Fire Classification
 PV modules may be evaluated for external fire exposure for
building roof covering materials.
 The fire class is identified in the individual Recognitions as class
A, B or C in accordance with UL's Roofing Materials and Systems
Directory.
 Modules not evaluated for fire exposure are identified as NR (Not
Rated), and not suitable for installation on buildings.

PV Module Design Qualification
 PV modules attaining optional design qualification undergo
additional reliability testing that validates long-term warranties.
 The tests include:
 Thermal cycling tests
 Humidity - freezing tests
 Impact and shock tests
 Immersion tests
 Cyclic pressure, twisting, vibration and other mechanical loading tests
 Wet/dry hi-pot, excessive and reverse current electrical tests
 Other electrical and mechanical tests.

Module Installation Instructions
 Listed PV modules must be installed in accordance with
instructions provided (shipped) with product.
 Includes safety information, working with PV modules during sun
hours (energized electrical equipment), mounting configurations,
and electrical wiring and grounding instructions.

PV Module Safety
 Most manufacturer’s literature states that module installation
should be done by qualified, licensed electrical professionals.
 Safety precautions for installing PV modules include:
 Do not insert electrically conducting parts into the plugs or sockets.
 Do not wear metallic jewelry while performing installation.
 Do not fit solar modules and wiring with wet plugs and sockets. Tools and
working conditions must be dry.
 Exercise extreme caution when carrying out work on wiring and use the
appropriate safety equipment (insulated tools/gloves, fall protection, etc.)
 Do not use damaged modules. Do not dismantle modules. Do not remove
any part or label fitted by the manufacturer. Do not treat the rear of the
laminate with paint, adhesives or mark it using sharp objects.
 Do not artificially concentrate sunlight on modules.

Handling PV Modules
 Care in handling, transporting, storing and installing PV modules
includes the following:
 Leave modules in packaging until they are to be installed.
 Carry modules with both hands, do not use connectors as a handle
 Do not stand modules on hard ground or on their corners
 Do not place modules on top of each other or stand on them
 Do not mark or work on them with sharp objects
 Keep all electrical contacts clean and dry
 Do not install modules in high winds

Module Selection Criteria
 The selection of PV modules for a given project may be based on
any number of factors, including:
 Module physical and electrical specifications
 Manufacturer certification to quality standards (ISO 9000)
 Module warranty and design qualification (IEC 61215/61216)
 Customer satisfaction and field results
 Company ownership and years in business
 Costs and availability

Summary
 Photovoltaic (PV) cells are semiconductor devices that produce
electrical output when exposed to sunlight.
 The current-voltage characteristic (I-V curve) is the basic
descriptor of PV device performance.
 The output of a PV device is dependent upon sunlight intensity,
temperature and electrical load.
 PV devices are connected in series to build voltage, and in
parallel to build current and power output.
 PV modules are installed in accordance with installation
instructions and local building codes.

Questions and Discussion

5 cells-modules-and-arrays

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