The ongoing quest for an energy-efficient means of making salt water drinkable has recently spawned interest in the prospect of using solar radiation to drive evaporation. This strategy must confront a variety of heat loss challenges, but Thomas Cooper, an assistant professor in York University’s Department of Mechanical Engineering, was part of a team that successfully demonstrated how desalination could be achieved with solar steam.
The concept is hardly new, but previous attempts maintained contact between the water and heating arrays, with results that were routinely compromised when salt crystals fouled the inner workings of the system. In contrast, Cooper’s group constructed a fully-contained unit that separated water from heat-gathering structures, which allowed the liquid to rise past the boiling point, reaching temperatures as high as 133C and generating superheated steam with nothing more than a single sun’s worth of illumination.
In fact, as the researchers reported in Nature Communications, they achieved this outcome on the roof of a building at the Massachusetts Institute of Technology in October, which would hardly qualify as the optimal setting for any kind of solar undertaking.
“It’s not about a new material,” explains Cooper. “It’s about using existing materials in a new way. It’s a configuration and clever thermal design that allows for this new contactless mode.”
This contactless solar evaporation structure (CSES) was surrounded with heavy insulation to minimize both radiation and conduction and heat loss. The clear top consisted of several layers of highly transparent polymer film, which was no more than 50µ thick. With an index of refraction much lower than typical household glass, these layers had essentially no convention between them but still transmitted more than 80% of available sunlight. This energy warmed a high-performance absorber with low infrared emissions, another way of preventing radiant heat loss.
As steam formed, it was channeled into a faucet that produced a steady trickle of water as it condensed. Even in a CSES with interior dimensions on the order of several square centimetres, this flow could amount to more than a litre of water per day. By scaling up, Cooper suggests, water could be produced at much greater rates.
Nor would the use of this technique be limited to desalination. The researchers argue that a CSES could be used for cooking, laundering, and even the sterilization of medical equipment in locations without access to electricity for such tasks.
“The demonstrated method of steam temperature control through radiative shielding would allow the device to output a constant steam temperature even during cloudy and low solar flux periods,” they conclude. “Given the wide range of potential applications, we believe this demonstration of contactless solar steam generation will open up new avenues for harnessing solar energy and transforming it into useful forms.”
A fully insulated container maintains a separation between salt water and a solar heating absorber, allowing for the production of superheated steam.