For more efficient solar water heating, cover foam with bubble wrap

Boiling water typically requires burning fuel. Solar water heating, while more sustainable, uses large, expensive metal sheets, mirrors, or lenses to concentrate sunlight in order to heat up water.

Researchers have now made a surprisingly simple technology that does not need heat or light concentrators to bring water to boiling temperatures of 100°C. The low-tech device is a piece of copper-coated foam covered with bubble wrap that floats on water, absorbing sunlight and converting the water underneath to steam.

This advance could lead to a cheap, effective way to sterilize medical equipment or disinfect water especially in low-resource settings. It could also be an environmentally friendly approach to residential water heating or desalination.

The concept, which a group of MIT engineers first came up with in 2014, relies on carbon foam with a graphite layer on top. Both layers are riddled with tiny pores. The graphite heats up when exposed to sunlight. Water soaks up through the carbon foam, and when the droplets hit graphite, they quickly turn into steam.

The original design required sunlight concentrated to 10 times normal levels, and the material was able to convert 85 percent of the light to heat. That’s because while the graphite absorbed light, it radiated heat back into the air.

So the researchers have come up with a better design, reported in the journal Nature Energy, which works in ambient sunlight by trapping solar energy better. They swapped the graphite layer with a heat-conducting copper sheet that’s coated with a thin layer of a blue metal and ceramic composite. This composite, commonly used in solar water heaters, absorbs sunlight and also traps heat.

But the team found that wind was still sweeping some heat off the top surface. So they covered the device with bubble wrap as an insulator. The device can boil water without a solar concentrator—and it even works on cloudy days.

MIT-Sun-Steam-2

The material could cost as little as $2 per square meter, the engineers estimate, and it could generate steam at about 5 percent of the cost of traditional light-concentrating approaches. They envision large sheets of the foam floating on small bodies of water to desalinate or treat the water. —Prachi Patel | 1 September 2016

Source: Ni G, et al. Steam generation under one sun enabled by a floating structure with thermal concentration. Nature Energy. 2016.

Header photo: Takeshi Kawai/Flickr. Ambient steam generator photo: George Ni.

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