Simple, solar-powered water desalination

A completely passive solar-powered water desalination system developed by researchers at MIT and in China could provide more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area. Such systems could potentially serve off-grid arid coastal areas to provide an efficient, low-cost water source.

Solar-Powered Water Desalination System Developed by MIT and Chinese Researchers

The system uses multiple layers of flat solar evaporators and condensers, lined up in a vertical array and topped with transparent aerogel insulation. It is described in a paper appearing today in the journal Energy and Environmental Science, authored by MIT doctoral students Lenan Zhang and Lin Zhao, postdoc Zhenyuan Xu, professor of mechanical engineering and department head Evelyn Wang, and eight others at MIT and at Shanghai Jiao Tong University in China.

The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate water using solar power. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.

The device is essentially a multilayer solar still, with a set of evaporating and condensing components like those used to distill liquor. It uses flat panels to absorb heat and then transfer that heat to a layer of water so that it begins to evaporate. The vapor then condenses on the next panel. That water gets collected, while the heat from the vapor condensation gets passed to the next layer.

Whenever vapor condenses on a surface, it releases heat; in typical condenser systems, that heat is simply lost to the environment. But in this multilayer evaporator, the released heat flows to the next evaporating layer, recycling the solar heat and boosting the overall efficiency.

“When you condense water, you release energy as heat,” Wang says. “If you have more than one stage, you can take advantage of that heat.”

Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system. The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area. This is more than two times as much as the record amount previously produced by any such passive solar-powered water desalination system, Wang says.

Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.

Solar-Powered Water Desalination: A Cost-Effective Solution to Water Scarcity

According to researchers, unlike some desalination systems, solar-powered water desalination eliminates the need for disposal of salt or concentrated brines. Any salt that accumulates during the day in a free-floating configuration is carried back out at night through the wicking material and back into the seawater.

The demonstration unit is built mainly from readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry water into contact with the solar absorber. The solar absorber material and the wicking material have been decoupled, which is not the case for other passive solar desalination systems. Wang, the lead researcher, says this approach requires specialized and expensive materials.

The prototype’s most expensive component is a layer of transparent aerogel used as an insulator at the top of the stack, which could be replaced by less expensive insulators. The team’s contribution is a framework for optimizing multistage passive systems, which they call thermally localized multistage desalination. The formulas they developed could apply to a variety of materials and device architectures for different scales of operation or local conditions and materials.

One possible configuration would be floating panels on a body of saltwater, constantly and passively delivering fresh water through pipes to the shore. Other systems could serve a single household, using a flat panel on a large shallow tank of seawater that is pumped or carried in. A system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person. They estimate a family-sized system could be built for around $100.

The researchers plan further experiments to optimize the choice of materials and configurations, testing the system’s durability under realistic conditions. They will also work on translating their lab-scale device design into something suitable for use by consumers. Solar-powered water desalination could play a crucial role in alleviating water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.

According to Ravi Prasher, an associate lab director at Lawrence Berkeley National Laboratory and an adjunct professor of mechanical engineering at the University of California at Berkeley, this new approach is significant as it eliminates the loss of significant energy in condensation. By efficiently harvesting the condensation energy, the overall solar-to-vapor efficiency is dramatically improved, reducing the cost of produced water.