MIT scientists have created a new solar desalination system that provides more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area.
Developed in collaboration with China’s Shanghai Jiao Tong University, the passive device was built using inexpensive, readily available materials such as a commercial black solar absorber, and paper towels for a capillary wick to carry the water into contact with the solar absorber.
The prototype’s most costly component is a layer of transparent aerogel that has been used as an insulator at the top of the stack.
The scientists however contend that other less expensive insulators could also be used as an alternative.
According to the scientists, the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water.
At each stage, heat released by the previous stage is harnessed instead of wasted.
This allows the device to achieve an overall efficiency of 385% in converting the energy of sunlight into the energy of water evaporation.
The device is essentially a multi-layer solar still, with a set of evaporating and condensing components like those used to distill liquor.
It uses flat panels to absorb heat and transfers that heat to a layer of water so that it begins to evaporate.
The vapor then condenses on the next panel. That water gets collected, while the heat from the vapor condensation gets passed to the next layer, thereby boosting the device’s overall efficiency.
The U.S.-Chinese team led by MIT professor Evelyn Wang, settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop.
The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area.
According to Wang, this is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system.
Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800%, says study co-author Lenan Zhang.
Another big plus for the new system: there is no accumulation of salt or concentrated brines to be disposed of.
Any salt accumulated during the day can be streamed out at night through the wicking material and back into the seawater.
Image and content: MIT