Solar desalination

Solar desalination

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  Biswajit Biswas Department ofChemical Engineering Indian Institute of Technology (BHU) Varanasi DESALINATION OF SEA WATER BY SOLAR ENERGY
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  MILLIONS LACK FRESHWATER AROUND THE WORLD!! 9 Million people without water access in developed countries. 36 Million people in Latin America and Caribbean. 358 Million people in Africa. 180 Million people in South , West and Central Asia. 186 Million people in Southeast, East Asia and Oceania. Overall about 769 Million people lacks fresh water all over the globe. Source: water.org
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  RELEVANCE TO WATERCRISIS : Human population is growing at a rate that puts stress on our current freshwater supply. Global water use is 9 trillion cubic meters a year and this rate is expected to increase by approximately 60 billion cubic meters more per year. Desalination allows to people to have access to water that was previously not potable. Coastal cities can use seawater for their municipal water supply or landlocked cities can use brackish groundwater for the same purpose. Desalination combined with Water recycling are both integral components of UN’s “water crisis” solution.
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  WHAT IS THEWAY OUT? In the light of increased water usage and depleting natural water supplies, the world is facing an impending water shortage. To combat this, many countries have turned to desalination to derive usable water from the sea. In this report, we have researched the current desalination technologies available in order to provide a comparative analysis of different methods of solar desalination. From the research, we have concluded that although solar desalination is not a viable option at the present, further research will make solar desalination most economic.
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  SOLAR STILL A solarstill is a simple way of distilling water, using the heat of the Sun to drive evaporation from humid soil, and ambient air to cool a condenser film. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic. In desert environments water needs can exceed 3.8 L per day for a person at rest, while still production may average 240 mL per day. Even with tools, digging a hole requires energy and makes a person lose water through perspiration; this means that even several days of water collection may not be equal to the water lost in its construction.
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  Multi-Stage Flash Distillation(MSF) Salt water is heated by solar energy under extreme pressures and lead through a series of chambers. Upon leaving the first chamber the salt water enters several more chambers each with a lower pressure than the previous one allowing even more of the pressurized salt water to vaporize. The water that did not vaporize leaves the system with a higher saline concentration than when it entered; this is discarded properly as waste while the distilled water is put into the municipal water supply as drinkable water.
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  Process of multistage flash distillation: Its temperature increases from sea temperature to inlet temperature of the brine heater. At the outlet of the brine heater, when entering the first cell, sea water is overheated compared to the temperature and pressure of stage 1. Thus it will immediately "flash" ie release heat, and thus vapour, to reach equilibrium with stage conditions. The produced vapour is condensed into fresh water on the tubular exchanger at the top of the stage. The cumulated fresh water builds up the distillate production which is extracted from the coldest stage. Upon leaving the evaporator, part of the warmed water is rejected to the sea, part is used as the make-up for the plant. The warmed water may be used in winter to warm up the cooling sea-water, thus enabling the evaporator be designed for a high temperature.
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  Multiple Effect Distillation(MED) Salt water is heated under pressure and and forced through a chamber. However, in this system the water vapour from the first chamber is used to heat the water in the next chamber. This produces distilled water (the condensed water vapor) and more water vapor (the cycle repeats).
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  Process of MultipleEffect Distillation plant: The steam enters the plant and is used to evaporate heated seawater. The secondary vapour produced is used to generate tertiary steam at a lower pressure. The primary steam condensate is returned to the boiler of the power station since it is of extremely high quality that is needed for turbine steam production. The MED technique is based on double- film heat transfer. Latent steam heat is transferred at each stage by steam condensation through the heat transfer surfaces to the evaporated falling film of seawater. The process is repeated up to 16 times in existing plants between the upper possible temperature and the lower possible cooling water. A compressor is used to maintain the gradual pressure gradient inside the vessel by removing the accumulated non condensable gases together with the remaining water vapour. The pressure gradient along the MED effects is dictated by the saturation pressure of the feed stream and the saturation pressure of the condensing steam exiting the last stage. It is condensed by cooling with seawater. Typical pressure gradients of 5- 50 kPa across the system (less than 5 kPa/stage) are typical.
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  Future option ofDesalination: Desalination needs to be implemented in any ocean bordering region . As Saudi Arabia has demonstrated, water can be piped inland to landlocked cities which means desalination is not limited to coastal cities . If technology continues to produce new methods and better solutions to the issues that exist today, there would be a whole new water resource for more and more countries that are facing water problem. With complete reliance on sea water, it would undoubtedly be at least an option for many people struggling to survive or maintain their standard of living.
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  COST/ BENEFIT ANALYSIS: Thebenefits of desalination are straightforward: more water. The drawback, unfortunately, is the cost. Regardless most people would avoid paying 3-4 times more for anything if they do not have to . If a region’s water supply is currently meeting the needs of the people in a sustainable manner, then switching to desalination, is not an urgent issue. If the supply is unsustainable and the rate consumption is high, then some action must be taken to insure that future generations will have access to water. Desalination will be appropriate if the area is then either rapidly running out of water or if the effort it would take to make the water supply sustainable is not feasible.
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  SOLUTIONS TO THEISSUE !! People must know what awaits in future!! • Countries will have to first do some preliminary research to make sure that there are locations within their borders where it is feasible to construct a desalination plant; Market Demand ? • To address these issues the respective countries have to develop an initiative based system to promote the further use of desalination. Organise!! Co- operation !! • If a country meets these qualifications they can apply for the desalination plant. They will have to determine the most appropriate way to organize the funds for the construction of desalination plants on a case to case basis by cooperating with business, banks, and other resources willing to participate. Make Blue Prints !! • Additionally, drawing up the plans to implement desalination as well as physically making the blueprints for a plant will take another 2-3 years. • Finally, building the plants themselves would take anywhere from another 3-4 years . As a result, they may expect to see the working effects of desalination as soon as 2023 if they begin as early as possible.
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  Can a desalinationplant be a solution for poor countries affected by drinking water scarcity? A desalination plant requires substantial investment towards building and operation which could be costly and may not be affordable to poor countries. Sea-level rise is a major threat to freshwater resources in a number of countries including small island nations and both developing and developed countries. One of the options is desalination.Here also the poor country must look for an alternate solution. Geography of Desalination is currently used by countries that have an extreme need for fresh water, have enough money to fund it, and posses the amount of energy required to produce it. The technology is expected to spread increasingly, particularly in the United States, Libya, China, and India. Saudi Arabia is currently the world’s number one producer of desalinated water.
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  THE INDIAN SCENARIO!!
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  Location of Desalinationplant in India Source: Bhabha Atomic Research Centre
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  Negatives of SolarDesalination Dumping the wasted high temperature solution back into the ocean makes the process more complicated. The energy required to start up and power desalination plants is a huge expense and is a matter of great concern for poorer countries. Within the energy issue, solar energy is potentially the most cost-effective energy source, but remains largely untapped due to public opinion. If regions situated away from the coast or in a high altitude try to use desalinated water, it is an even more expensive process. There are major environmental concerns from desalination, is related to discharge of brine which can damage the local ecosystem and biodiversity.
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  SO WHAT TODO ?? Solar stills were in fact the first method used on a large scale to process contaminated water and convert it to a potable form. Of the estimated freshwater being produced a day through desalination processes worldwide, less than 1% is made using solar energy. The prevailing methods of desalination are energy intensive and rely heavily on fossil fuels. Because of inexpensive methods of freshwater delivery and abundant low cost energy resources, solar distillation has, up to this point, been viewed as cost prohibitive and impractical. With the approach of peak oil production, fossil fuel prices will continue to increase as those resources decline; as a result solar energy will become a more attractive alternative for achieving the world’s desalination needs. Environmental implications of desalination regarding safe brine disposal will be significant by 2050. It is important for countries to jointly plan and implement national environmental legislation. So Solar Desalination is the one of the possible option left in near future to meet the demand of fresh water.
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