In order to solve the problem of sustainable fresh water supply, researchers have developed many solar desalination systems in recent years. However, a frequently encountered difficulty is salt accumulation, which can clog the system and affect water production rates. To solve this problem, researchers at MIT and Shanghai Jiao Tong University took inspiration from a natural phenomenon: how deep ocean currents are driven by differences in the density of seawater, a process known as thermohaline circulation.
Researchers have developed a new solar-powered desalination system that can produce large quantities of drinking water and uses an ocean-inspired technology to avoid salt clogging issues. When scaled up, the system can provide enough drinking water to meet the daily needs of a small family.
The researchers' new system improves on their previous design, a similar concept consisting of multiple layers called "stages." Each stage contains an evaporator and a condenser, using sunlight to passively separate salts from the incoming water. While it effectively uses solar energy to evaporate water, it can clog after a few days due to salt buildup. Therefore, researchers tried a thermohaline circulation method to alleviate the accumulation of salt.
The newly designed single-stage unit looks like a thin box topped with a dark material that absorbs the sun's heat. The inside of the box is divided into upper and lower parts. Water flows through the upper section, and a layer of evaporators in the ceiling uses the sun's heat to heat and evaporate the water in direct contact with it. The water vapor is transported to the lower half, and the condenser layer air-cools the water vapor into salt-free drinking water.
The entire box is tilted, and the heat from the sun creates vortices when the water flows through. This movement helps bring water into contact with the upper evaporation layer while maintaining salt circulation and preventing salt precipitation and clogging.
Xu Zhenyuan, one of the corresponding authors of the study, said: "We have now introduced a more powerful convection that is similar to the kilometer-long scale convection we usually see in the ocean. When seawater is exposed to the air, sunlight causes the seawater to evaporate. Once the seawater leaves the sea surface, the salt remains. The higher the concentration of salt, the denser the liquid, and this heavier water will flow downward. By simulating this kilometer-scale phenomenon in [a] small box, we can use this feature to repel the salt."
The researchers found that their system could produce fresh water in environments with varying salt concentrations, from natural seawater to water that is seven times more saline. If scaled up to the size of a small suitcase, the system could produce 4 to 6 liters (1.1 to 1.6 gallons) of water per hour and last for several years before parts need to be replaced, they said.
The overall cost of running the system will be lower than the cost of producing tap water in the United States because of its high water production rate, high salt rejection, long lifespan, and the fact that it is solar-powered and requires no electricity, the researchers said.
Yang Zhong, co-author of the study report, said: "Our research shows that this kind of equipment can achieve a long service life. This means that for the first time, drinking water produced using sunlight may be cheaper than tap water. This provides the possibility for solar seawater desalination to solve practical problems."
The research was published in the journal Joule.