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Water

Superefficient solar desalination

System harnesses waste heat to produce enough drinkable water for an adult in one hour

by Prachi Patel
February 13, 2020

20200213lnp2-roof.jpg
Credit: Massachusetts Institute of Technology
A 10 cm × 10 cm solar-powered desalination device produced 72 mL of drinking water from salty water in 4.5 hours when tested on a rooftop at MIT under partly cloudy conditions. In full sun, the device can purify 5.8 liters per hour.

Portable devices that use sunlight to remove salts from seawater could provide affordable drinking water to people in remote or arid coastal areas. But solar-powered desalination technologies have suffered from low efficiencies. Researchers now report an ultraefficient, multistage desalination system that can produce 5.8 liters of fresh water per hour in full sun, almost three times the amount that similar devices made in the past have produced (Energy Environ. Sci. 2020, DOI: 10.1039/C9EE04122B).

Conventional desalination plants, which rely on membranes, are expensive or use a lot of energy. Solar thermal desalination is a less expensive option but is inefficient and requires immense parabolic mirrors to focus light on seawater to evaporate it. Neither technology is suited for remote or resource-poor areas. Portable solar-powered desalination devices address this problem. These typically use less expensive sunlight-absorbing materials that heat and evaporate seawater. The vapors are collected on a condenser, which gathers fresh water. The best systems can convert over 90% of the sun’s energy to generate water vapor.

Now Evelyn Wang of MIT, Ruzhu Wang of Shanghai Jiao Tong University, and their colleagues made a multistage system that goes way beyond this, boasting an efficiency of 358% when tested under ideal conditions. It does that by reusing the heat that is released when water vapor condenses to liquid. It’s the first system to take advantage of this energy, which is usually wasted.

The device contains ten identical 3D-printed nylon frames stacked and placed vertically on top of a water reservoir. Each frame has a paper towel that acts as the evaporator and an aluminum film that serves as the condenser. On the sun-facing side of the stack, the researchers place a black solar absorber that heats up and evaporates the water that wicks up into the paper towel. As vapors condense on the aluminum film, the heat released gets channeled to the second paper towel layer, with the process repeating down the stack. Such a multi-stage approach has been used in large-scale solar thermal desalination plants, says Wang’s graduate student Lenan Zhang. “We transferred this idea into a portable design.”

In the laboratory, the 10 cm × 10 cm device produced 5.8 L of water per hour. In less than ideal conditions, on an MIT rooftop in summer, the device’s performance was lower, about 50% because of occasional clouds and wind. On the roof, the device produced 72 mL of water in 4.5 hours. To meet the average daily water needs for an adult or a small family, you could create an array of the multistage devices, Zhang says.

The team now wants to optimize the device and to further reduce material cost. The prototype now costs around $1.50, but 70% of that cost is the 3D-printed nylon frames, Zhang says, so there’s room for improvement.

This a highly efficient and cost-effective approach for solar desalination, says Hadi Ghasemi, a mechanical engineer at the University of Houston. “This work brings the concept of solar heat localization a step closer to large scale implementation.” Using different wicking materials that have antifouling characteristics will help for long-term consistent performance of this device, he says.

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