Solar desalination device produces drinking water at high rate

The system mechanisms

The system's design incorporates a fusion of two principles: a multistage setup of evaporators and condensers. 

Furthermore, the functioning of this low-cost, novel solution is inspired by the thermohaline circulation of the ocean, a process caused by temperature and salinity variations.

Notably, the device's design enables water to flow in small swirls or eddies.

The combination of circulation and solar heat causes water to evaporate, leaving the salt behind. This creates water vapor, which can be condensed into clean, potable water. Simultaneously, the surplus salt continues to flow through the device rather than accumulating and obstructing the system’s performance.

“When seawater is exposed to air, sunlight drives water to evaporate. Once water leaves the surface, salt remains. And the higher the salt concentration, the denser the liquid, and this heavier water wants to flow downward. By mimicking this kilometer-wide phenomenon [thermohaline] in small box, we can take advantage of this feature to reject salt,” explained Lenan Zhang, a research scientist in MIT’s Device Research Laboratory. 

The basis of this unique design is a small box sealed with a highly effective heat-absorbing material. This box is separated into upper and lower portions on the inside.

The official release briefly explains the device’s functioning: “Water can flow through the top half, where the ceiling is lined with an evaporator layer that uses the sun’s heat to warm up and evaporate any water in direct contact. The water vapor is then funneled to the bottom half of the box, where a condensing layer air-cools the vapor into a salt-free, drinkable liquid.”

This advancement builds upon the team's earlier efforts. However, in their previous design, they grappled with the issue of excessive salt buildup. However, this improvised design successfully conquers that challenge.

The suitcase-sized device could produce 4 to 6 liters of drinking water per hour

The engineers developed prototypes with varied stages (one, three, and ten) and evaluated their effectiveness using water with varying levels of salinity.

According to their testing, increasing each stage to a square meter in size might yield up to five liters of drinking water every hour.

When scaled up to the size of a small suitcase, the system could generate 4 to 6 liters of drinking water per hour and remain operational for several years. At this size, it may be more cost-effective than tap water to provide drinking water.

The team hopes that in the near future, the scaled-up system will be able to generate a sufficient amount of drinking water to fulfill the daily needs of a small household. Additionally, this system could serve off-grid coastal communities as well. 

The results were reported in the journal Joule.

Study abstract:

Recent advances in multistage solar distillation are promising for the sustainable supply of freshwater. However, significant performance degradation due to salt accumulation has posed a challenge for both long-term reliability of solar desalination and efficient treatment of hypersaline discharge. Here, inspired by a natural phenomenon, thermohaline convection, we demonstrate a solar-powered multistage membrane distillation with extreme salt-resisting performance. Using a confined saline layer as an evaporator, we initiate strong thermohaline convection to mitigate salt accumulation and enhance heat transfer. With a ten-stage device, we achieve record-high solar-to-water efficiencies of 322%–121% in the salinity range of 0–20 wt % under one-sun illumination. More importantly, we demonstrate an extreme resistance to salt accumulation with 180-h continuous desalination of 20 wt % concentrated seawater.