Solar-powered desalination

A team of Chinese researchers have announced an efficient solar-powered desalination technique that directly uses the Sun’s energy to clean water instead of reverse osmosis.

Current desalination plants usually operate by two ways: distilling water by boiling or evaporating it, or using a membrane to separate salt particles from water (reverse osmosis). There has been a lot of research into improving the newer reverse osmosis method, but a paper published in AIP Advances suggests a technique that could improve the energy efficiency of distillation.

This new technique uses a transparent container with a sloped quartz roof, filled with seawater. Floating in the seawater is a device made of a substance called titanium nitride oxide (TiNO), an insulating foam, and a specific type of paper.

The TiNO is the thing that harnesses the Sun’s energy. “In the solar energy field, TiNO is a common commercial solar absorbing coating, widely used in solar hot water systems and in photovoltaic units,” says Chao Chang, first author on the paper and a researcher at the Institute of Marine Engineering and Thermal Science, at the Marine Engineering College in Dalian, China.

“It has a high solar absorption rate and a low thermal emittance and can effectively convert solar energy into thermal energy.”

When sunlight hits the TiNO, it heats quickly and vaporises the layer of water on top of it. The fresh water then condenses on the roof of the container, trickles down the slope and can be collected.

The researchers used a method called magnetron sputtering to deposit the TiNO across an insulating foam, which traps heat and lets the device float. The base of the device is airlaid paper, which is porous and supplies water to the TiNO layer. It also appears to wick salt away from TiNO surface, preventing a salt build-up which would make it less efficient.

“The porous airlaid paper used as the substrate for the TiNO solar absorber can be reused and recycled more than 30 times,” says Chang.

Water collected from this solar-powered desalination unit had a salinity of 2 parts per million – that is, two milligrams of salt per litre of water. In comparison, the Australian Drinking Water guidelines state that water begins to taste salty at around 180 parts per million, and seawater can have salt levels of over 70,000 parts per million.

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