When clean water isn’t easily accessible, solar stills can be used to purify available water that might be dirty or salty. These devices absorb heat from sunlight and use it to evaporate water, leaving behind contaminants. The water vapour is then condensed in a separate container. Such stills can be relatively inefficient and expensive.

Researchers at the University of Buffalo have developed a new type of solar still using carbon-coated paper that they say is cheaper and more than twice as efficient as existing devices.

The main problem with conventional devices is that they lose heat energy due to heating the bulk liquid during the evaporation process. One solution has been to use optical concentrators, such as mirrors and lenses, to concentrate the sunlight. But these are costly.

The research team addressed these issues in a solar still about the size of mini-refrigerator. The device is made of expanded polystyrene foam and porous paper coated in carbon black. When the still is floated on a body of water, a sheet of the paper absorbs water, while the carbon black absorbs sunlight and transforms the solar energy into heat used during evaporation.

The new solar still coverts water to vapor very efficiently – only 12% of the available energy is lost during the evaporation process because the device heats only a thin layer surface water, which evaporated at 44°C.

The researchers say that the still is capable of producing 3 to 10 litres of water per day compared with 1 o 5 litres per day produced by current stills of a similar size. Materials cost about $us1.60 per square metre compared with about $us200 for systems using optical concentrators.