Researchers build new passive solar desalination system

Researchers from the Massachusetts Institute of Technology (MIT) and the Shanghai Jiao Tong University in China have developed a solar desalination system that they claim is both more efficient and less expensive than previous solar desalination methods.

The process could also be used to treat drinking water or contaminated wastewater — without requiring any power source other than sunlight itself.

The findings are described in the journal Nature Communications, in a paper by MIT graduate student Lenan Zhang, postdoc Xiangyu Li, professor of mechanical engineering Evelyn Wang, and four others.

“There have been a lot of demonstrations of really high-performing, salt-rejecting, solar-based evaporation designs of various devices,” said Wang. “The challenge has been the salt fouling issue, that people haven’t really addressed. So, we see these very attractive performance numbers, but they’re often limited because of longevity. Over time, things will foul.”

Many attempts at solar desalination systems rely on some kind of wick to draw the saline water through the device. However, these wicks are vulnerable to salt accumulation and are relatively difficult to clean. The team focused on developing a wick-free system instead.

The result is a layered system, with dark material at the top to absorb the sun’s heat, then a thin layer of water above a perforated layer of material, sitting atop a deep reservoir of the salty water such as a tank or a pond.

After careful calculations and experiments, the researchers determined the optimal size for the holes in the perforated material, which in their tests was made of polyurethane. At 2.5 millimeters across, these holes can be easily made using commonly available waterjets. The holes are large enough to allow for a natural convective circulation between the warmer upper layer of water and the colder reservoir below.

That circulation naturally draws the salt from the thin layer above down into the much larger body of water below, where it becomes significantly diluted and no longer a problem. Their test apparatus operated for a week with no signs of any salt accumulation.

“It allows us to achieve high performance and yet also prevent this salt accumulation,” said Wang.

The team says that the advantages of this system are its high performance and reliable operation under extreme conditions. They say that the system can actually even work with near-saturation saline water.

“The evaporation happens at the very top interface. Because of the salt, the density of water at the very top interface is higher, and the bottom water has lower density,” explained Zhang. “So, this is an original driving force for this natural convection because the higher density at the top drives the salty liquid to go down.”

The water evaporated from the top of the system can then be collected on a condensing surface, providing pure fresh water.

The rejection of salt to the water below could also cause heat to be lost in the process, so preventing that required careful engineering, including making the perforated layer out of highly insulating material to keep the heat concentrated above. The solar heating at the top is accomplished through a simple layer of black paint.

So far, the team has proven the concept using small benchtop devices. They plan to scale up to devices that could have practical applications. Based on their calculations, a system with just 1 square meter (about a square yard) of collecting area should be sufficient to provide a family’s daily needs for drinking water. The necessary materials for a 1-square-meter device could cost only about $4.

The team says that the necessary work to translate this lab-scale proof of concept into workable commercial devices, and to improve the overall water production rate, should be possible within a few years. The first applications are likely to be providing safe water in remote off-grid locations, or for disaster relief after hurricanes, earthquakes, or other disruptions of normal water supplies.