Combined System Gives Abundant Solar Electricity And Clean Water

In dry environments, the future of solar could be producing water at the same time from waste heat. Wenbin Wang

In a world desperate for non-polluting energy and water free from pollutants, a dual system can provide both at once. Frequently, technology designed to tackle two problems simultaneously handles both poorly, and that's been true for past efforts in this area. However, Professor Peng Wang of the King Abdullah University of Science and Technology, Saudi Arabia claims that attaching membrane distillation systems to the back of solar panels allows both to work at least as well as they would on their own.

Wang placed multi-stage membrane distillers behind conventional solar cells so that light falls on the cells as normal, producing electricity. The waste heat drives the three-stage distillation. In Nature Communications, Wang reports that trials of this combination demonstrated high efficiency both for water and for power.

Photovoltaic cells work best when kept cool, which is one of the reasons floating solar farms are gaining popularity despite the flotation costs. Just as the lake keeps floating panels cool, Wang's distillers can draw heat away that would otherwise interfere with solar performance.

Meanwhile, the heat radiated from the packs of the solar cells heats salty or polluted water to encourage evaporation. When the water vapor condenses back to liquid, it is largely cleansed of its impurities. The heat of condensation released by the first stage is captured to evaporate water in the second and so on. This allows for the production of much larger amounts of water per unit area than conventional solar sills, while avoiding the enormous energy demands of desalination plants that boil the water.

Schematics of two designs for combined solar cells and water distillers. The waste heat from the solar cells evaporates polluted water in the top layer, and when this condenses, it not only produces clear water, but the heat of condensation is used for the level below. Wang et al/Nature Communications 

Wang experimented on a system built with off-the-shelf solar cells and recorded 11 percent efficiency – matching what he measured without the distillation backing. Meanwhile, his self-made distillers turned salty solution to fresh water three times faster than conventional solar stills. Combining the systems also saves both land and mounting costs compared to operating each independently.

Most existing sources of electricity consume a lot of water. Coal and gas boil enormous amounts of water to drive turbines. Nuclear power plants do the same and use even more water for cooling. “In the United States and Western Europe, about 50 percent of water withdrawals are for energy production,” the paper notes.

Even solar panels need to be cleaned, particularly in dusty environments, although their requirements are tiny compared to fossil fuel plants. Wang's distillers produce far more clean water than is needed for cleaning, making the combined system a potential lifeline for people who have solar farms in dry environments.

Plenty of other people have had the idea of combining electricity and water production in deserts, but their efforts have usually been so inefficient on at least one of these factors as to be far from commercially viable.

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