Passive solar heat device could be hygiene boon in remote regions

Using solar heat to turn water into superheated steam without optics, MIT discovery hopes to enable sterilisation and provide clean drinking water

Hygiene is a persistent problem in remote regions of poorer countries. Medical facilities require sterilisation, and clean drinking water is often in short supply. A device developed by MIT engineers has the potential to provide both, using only the sun as a power source.

solar heat
The suspended heating device being tested at MIT, where it generated steam at 146°C and conveted seawater into pure drinking water. Image: Thomas Cooper et al

Led by mechanical engineer Thomas Cooper, the researchers report their results in Nature Communications. Their device is a development of previous research which resulted in a foamed graphite material that could turn water it absorbed into steam. Concerns over degradation of material caused by impurities in the water led Cooper to develop the system further, and he and his colleagues have now incorporated it into a device that can be suspended over a container of water, left in sunshine, and will generate superheated steam above 100°C. “It’s a completely passive system – you just leave it outside to absorb sunlight,” Cooper said.

The device is a flat tile about the size of a small tablet computer. It consists of three layers. The top layer is a metal-ceramic composite that is highly efficient at absorbing solar radiation. The bottom layer is perforated aluminium, painted with a coating that efficiently emits infrared energy. Sandwiched between these is a layer of reticulated carbon foam, a form of graphite riddled with a network of winding tunnels and pores. This is suspended above a container of water so that the lower layer is not in contact with the liquid.

Sunlight hitting the top layer is converted into heat, which warms up the reticulated carbon and is transmitted to the layer of aluminium. The coating on the metal radiates infrared onto the surface of the water, which absorbs the radiation far more readily than absorbs heat from sunlight and evaporates.

The vapour rises through the perforations in the base layer and is absorbed into the carbon foam, where stored energy heats it further, taking it to superheated temperatures. The steam escapes through a small tube fixed into the side of the foam, allowing it to be either used directly or condensed into purified water.

“It’s this clever engineering of different materials and how they’re arranged that allows us to achieve reasonably high efficiencies with this non-contact arrangement,” Cooper said.

The team tested the device on the roof of one of MIT’s laboratories, adding a simple curved mirror to concentrate sunlight onto the top layer. Over a three hour test, the device produced steam at 146°C, and when used on a container of seawater, was not contaminated with salt crystals. According to Cooper and laboratory director Prof Gang Chen, a device of this size would be sufficient to produce clean drinking water for a single family or sterilise one operating theatre and its equipment with no need for any additional infrastructure or connection to external services.