A passive solar-powered system has achieved a new level of efficiency in harnessing sunlight to make fresh drinking water from seawater. Such systems could potentially serve off-grid arid coastal areas to provide an efficient, low-cost water source.
The desalination system at work. Image courtesy of the researchers.
A completely passive solar-powered desalination system developed by researchers at MIT and in China could provide more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area.
The system uses multiple layers of flat solar evaporators and condensers, lined up in a vertical array and topped with transparent aerogel insulation. The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water, explains a statement.
At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team’s demonstration device can achieve an overall efficiency of 385 per cent in converting the energy of sunlight into the energy of water evaporation.
Releasing energy as heat
The device is essentially a multilayer solar still, with a set of evaporating and condensing components like those used to distill liquor. It uses flat panels to absorb heat and then transfer 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.
Whenever vapor condenses on a surface, it releases heat; in typical condenser systems, that heat is simply lost to the environment. But in this multilayer evaporator the released heat flows to the next evaporating layer, recycling the solar heat and boosting the overall efficiency.
“When you condense water, you release energy as heat,” said MIT professor Evelyn Wang. “If you have more than one stage, you can take advantage of that heat.”
Efficient production of drinking water
Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system. The team 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 of solar collecting area. This is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system, Wang says.
Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.