Solar-powered clean water
A new way to use sunlight to produce clean water and even superheated steam has been developed...
Every day, thousands of people - most of them children - die from diseases picked up from dirty water. This is because cleaning and purifying water is technologically and energetically demanding, which makes it expensive. But now, thanks to a system developed by scientists at York University, Toronto, and MIT there might be a way to do this much more cheaply in future.
"When we look at the current energy landscape, solar is the one that excites me most because of its sheer availability," says York researcher Thomas Cooper. "But the challenge is how to develop technologies to best harness that energy in a useful way."
Most of us are familiar with solar photovoltaic panels, which convert sunlight into electricity. But there is a whole other realm of technologies that convert solar energy into heat. These are called solar thermal technologies and they're used for a range of purposes, including water desalination - the process of removing the salt from seawater.
One way to do this is to boil water and collect and condense the resulting steam. Since the salt is left behind by the process, it can yield clean water. But getting heat into water from a solar source can be tricky, because, being transparent, water doesn't absorb sunlight very well.
Normally, a material is needed that can absorb sunlight and transfer the energy it carries into the water. Effectively, as the material heats up, it heats the surrounding salty water to produce steam.
But most of the devices that can do this have to be in contact with the water and, as Cooper explains, "whenever you have a material, or a device, that is in contact with contaminated or salty water, there is always the risk that that structure itself is going to become contaminated with these salts or impurities." This fouling really limits the efficiency and lifetime of such devices.
To circumvent these issues, Cooper and his colleagues have developed a new "contactless" approach. Writing in Nature Communications, they have designed a system like a sandwich with a top layer made of a dark material that efficiently absorbs sunlight and heats up. The "sandwich filling" comprises a carbon-rich sponge material, which retains the heat and transfers it to the bottom layer. The bottom surface is coated with a material specifically designed to re-radiate the energy as long-wavelength infrared waves, which are well-absorbed by the water surface below.
"We are achieving an energy transfer that is essentially consisting of first absorbing the sunlight and then re-emitting that energy as longer-wavelength thermal radiation towards the water," says Cooper.
As the water warms it evaporates, rising upwards towards the underside of the device where it is directed through the apertures within the sponge-like middle layer, picking up more heat as it goes. The warming effect is sufficient to raise the temperature of the water vapour to well above its boiling point, producing what is called "superheated steam".
This can be used for cooking, cleaning, sterilisation and even drying other things before it's condensed to produce fresh water. "You can think of cooking in a remote area using this steam rather than burning things like fossil fuels or coal," points out Cooper. "There are also many industrial processes, for example drying of different materials or production of different chemicals that require heat at these elevated temperatures."