Efficient Solar Still Makes Seawater Drinkable

By Sunny Lewis for Maximpact

CAMBRIDGE, Massachusetts, February 13, 2020 (Maximpact.com News) – A new level of efficiency in using sunlight to turn seawater into fresh drinking water has been achieved. The completely passive solar-powered desalination system could provide more than 5.6 liters (1.5 gallons) of potable water an hour for every square meter of solar collecting area, more than twice as much as the record amount previously produced by any passive solar desalination system.

Developed by researchers at the Massachusetts Institute of Technology (MIT) and in China, such a system could serve off-grid arid coastal areas to provide an efficient, low-cost water source. They hope it could play a role in easing water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.

The device is essentially a multilayer solar still, with a set of evaporating and condensing components like those used to distill liquor.

The passive system, with no moving parts, uses multiple layers of flat solar evaporators and condensers, lined up in a vertical array and topped with transparent aerogel insulation.

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.

The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water. 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 system is described in a paper entitled “Ultrahigh-efficiency desalination via a thermally-localized multistage solar still,” published February 6 in the journal “Energy and Environmental Science.” It was authored by MIT professor of mechanical engineering and department head Evelyn Wang, MIT doctoral students Lenan Zhang and Lin Zhao, postdoc Zhenyuan Xu, and eight others at MIT and at Shanghai Jiao Tong University in China.

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,” says Wang, an expert in high-efficiency energy and water systems. “If you have more than one stage, you can take advantage of that heat.”

She says that adding more layers increases the conversion efficiency for producing drinkable water, but each layer also adds cost and bulk to the system.

Proving the Concept

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 (1.52 gallons per 11 square feet) of solar collecting area.

Wang says this is more than twice as much as the record amount previously produced by any passive solar-powered desalination system.

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

Unlike other desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.

The demonstration unit was built mostly from inexpensive, readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry the water into contact with the solar absorber.

In most other attempts to make passive solar desalination systems, the solar absorber material and the wicking material have been a single component, which requires specialized and expensive materials, Wang says. “We’ve been able to decouple these two.”

Wang emphasizes that the team’s key contribution is a framework for understanding how to optimize such multistage passive systems, which they call “thermally localized multistage desalination.”

Adaptable System Design Fits Multiple Situations

The formulas the team developed could be applied to a variety of materials and device architectures, allowing for further optimization of systems based on different scales of operation or local conditions and materials.

One possible configuration would be floating panels on a body of saltwater such as an impoundment pond. These could constantly and passively deliver fresh water through pipes to the shore, as long as the sun shines each day.

Other systems could be designed to serve a single household, perhaps using a flat panel on a large shallow tank of seawater that is pumped or carried in. The team estimates that a system with a roughly one-square-meter solar collecting area could meet the daily drinking water needs of one person.

The human body is about 60 percent water, and health authorities say we each need about two liters, or half a gallon of water a day.

With this level of production in mind, the MIT researchers think a system built to serve the drinking water needs of a family might be built for around US$100.

The most expensive component of the prototype is a layer of transparent aerogel used as an insulator at the top of the stack, but the team suggests that other less expensive insulators could be used.

The researchers plan further experiments to continue to optimize the choice of materials and configurations, and to test the durability of the system under realistic conditions.

They also will work on translating the design of their lab-scale device into something that would be suitable for use by consumers.

“This new approach is very significant,” says Ravi Prasher, an associate lab director at Lawrence Berkeley National Laboratory and adjunct professor of mechanical engineering at the University of California at Berkeley, who was not involved in this research.

“One of the challenges in solar still-based desalination has been low efficiency due to the loss of significant energy in condensation. By efficiently harvesting the condensation energy, the overall solar to vapor efficiency is dramatically improved,” said Prasher.

“This increased efficiency will have an overall impact on reducing the cost of produced water,” he said.

For years, Professor Wang has been working towards efficient drinking water systems in arid environments. Her work on the development of a device that can extract fresh water from the air in arid environments was selected by “Scientific American” and the World Economic Forum as one of 2017’s 10 promising emerging technologies.

This research was supported by the National Natural Science Foundation of China, the Singapore-MIT Alliance for Research and Technology, and the MIT Tata Center for Technology and Design.