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Towards solar-powered desalination: a path of research and challenges

  • Towards solar-powered desalination: path of research and challenges
    Photo by AIO SIDS IWRM Project on Foter

Desalination holds the promise of producing enough freshwater water to overcome water scarcity, but two hurdles are in the way: it is energy intensive and the resulting brine needs to be safely disposed of. Research into using renewable solar energy to power desalination processes is underway across the world, focusing mostly on using photovoltaic (PV) systems to generate electricity and on solar-thermal technologies.

Solar-thermal technologies generate direct heat used to evaporate water and then condense the vapour into freshwater; their advantage over reverse osmosis, currently the most advanced desalination technique used on a large scale, is that they can handle high salinities, potentially even concentrated brine resulting from other desalination methods. Coming up with a solar-thermal system that is simple and inexpensive would make desalination more accessible. To encourage such efforts, the U.S. Department of Energy launched the Solar Desalination Prize, an initiative to accelerate the commercial development of thermal desalination systems powered by low-cost solar-thermal energy. Last April, six semi-finalists were selected to compete in the third phase of the competition, receiving $250,000 and a $100,000 technical assistance voucher. The winner will receive a $1 million cash prize.

In the Middle East, the UAE also launched a biennial $1 million award, the Mohammed bin Rashid Al Maktoum Global Water Award, to recognise innovators from around the world for their models to produce freshwater using solar power and other renewable energy sources.

In a recent paper in Nature Communications, researchers from the MIT and China introduce a desalination system that is more efficient and less expensive than previous solar desalination methods, and addresses the issue of salt fouling, which limits the longevity of designs based on solar evaporation. The system is made of low-cost materials: a 1 square metre device would only cost $4 and provide enough drinking water for a family’s daily needs.

Another promising development is the use of concentrating solar power technology. A pioneering solar dome technology, by UK based Solar Water Plc., uses heliostat mirrors to gather and focus sunlight on a solar dome. The heat generates evaporates seawater, which is then condensed and precipitated as freshwater.

Although the benefits of using solar power as a green source of energy for desalination are clear, an article in Earth.org points out some challenges that are still in the way.

Challenges common to all solar-powered desalination types relate to solar energy storage when sunlight is not available, during night hours and non-sunny weather, location – of the desalination output, the solar energy output, and the water users –, connecting to the grid, existing regulations and fossil-fuel subsidies, scaling up the size of operations, and cost-effectiveness.

In fact, the intermittence of solar energy supply may be the biggest obstacle to solar powered desalination, as battery technology is improving but is still expensive. According to The World Bank, PV-based SWRO solar desalination is the leading solar energy choice and is the main focus of further research. Many planned desalination plants include using a renewable source of energy part of the time, using electricity from the available grid the rest of the time. When they generate more energy than they use, they can sell it to the grid. This ensures a reliable water supply for countries that rely heavily on desalination. The plants can also store excess solar energy as freshwater when electricity demand is low.

In the future, we expect to see large scale renewable desalination facilities fully independent from the grid. “In less than five years, battery technology will have developed further and then we can have an independent solar and photovoltaic desalination plant. I have no doubt about that,” thinks Carlos Cosín, CEO of Almar Water Solutions.

Beside further development of battery technology, other strategies to overcome solar energy storage issues could include storage as thermal energy. On the other hand, combining desalination methods in hybrid plants can help mitigate the downsides of each one, such as pairing an inland concentrated solar power plant – very efficient at treating high salinity water, and able to store thermal energy onsite – with a coastal PV-powered reverse osmosis plant, which needs better quality water but generates easy to transmit PV-generated power. Other options being explored are using a hybrid – wind and solar – renewable energy source and, of course, increasing the efficiency of desalination processes to reduce their energy consumption.

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