Water covers 70% of the Earth’s surface, but most of it not suitable for drinking. All rest of it is salty or brackish water, which can be made accessible after desalination. Thus the interest in desalination technology in places around the world has been increased.
Now, a team of researchers at MIT and Shanghai Jiao Tong University in China has managed to create a completely passive solar-powered desalination system with a record efficiency of 385%, producing more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area. Such a system could potentially enable an efficient and low-cost water source for off-grid coastal areas.
In detail, the system consists of multiple layers of flat solar evaporators and condensers. Each of the layers, aligned in a vertical array and topped with transparent aerogel insulation, plays an important role in this process. Sunlight passes through a transparent insulating layer at left, to heat up a black heat-absorbing material, which transfers the heat to a layer of wicking material (shown in blue), where it evaporates and then condenses on a surface (gray) and then drips off to be collected as fresh, potable water.
Instead of being lost in the environment, the heat of the vapor is transferred to the next evaporating layer. Recycling solar heat boosts overall efficiency. The team found in the first experiment that the demonstration device can achieve an overall efficiency of 385% in converting the energy of sunlight into the energy of water evaporation.
The most interesting aspect is that, unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. Any salt that collected during the day would simply be carried back out at night through the wicking material and back into the seawater.
The addition of multiple layers increases efficiency but also adds cost and bulk to the system. That is why the team opted for a 10-panel system for its prototype, which was tested on the roof of an MIT building. The system provided pure water that exceeded the city’s consumption standards at a rate of 5.78 liters per square meter of solar collection surface. This is more than double the record reached by passive solar desalination in recent years.
According to the researchers, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700-800%. And by replacing some expensive components (for example, the aerogel) with low-cost alternatives, a family-sized system could be brought to the market for only $100.