According to estimates, two-thirds of the population is affected by water scarcity, while many parts of the developing world also face a lack of reliable electricity. As a result, more research efforts are focused on methods that use solar energy to desalinate seawater or brackish water. However, many research efforts have encountered equipment fouling problems caused by salt deposition, which often increases the complexity and cost of desalination of saltwater.
Now, a team of researchers at the Massachusetts Institute of Technology (MIT) and China has come up with a solution to the problem of salt deposition. They developed a desalination system that is more efficient and less costly than previous solar desalination methods. This process can also be used to treat contaminated wastewater or to generate steam for disinfecting medical devices. All of this doesn't require any energy, except for sunlight itself.
The results of this study were recently published in the journal Nature Communications. It is reported that many attempts at solar desalination systems rely on some kind of core to attract salt water through the equipment, but these cores are easily affected by salt accumulation and are relatively difficult to clean, which in turn affects their service life. Therefore, the team focused on developing a coreless system.
The result is a layered system with a dark material at the top absorbing the sun's heat, and then a thin layer of water on top of the porous layer of material, located above a deep salt reservoir such as a tank or pond. After careful calculations and experiments, the researchers determined the optimal size of the holes drilled through the perforated material, which in their tests were made of polyurethane. These holes are 2.5 mm in diameter and can be easily fabricated with common waterjets.
The holes are said to be large enough to create a natural convection cycle between warmer upper water bodies and cooler lower water bodies. This cycle naturally sucks the salt from the thin layer above into the larger body of water below, where the salt is well diluted and is no longer a problem.
According to the researchers, it's like hot air rising and cold air falling, with natural convection driving the device's desalination process. Evaporation occurs at the very top of the interface. Due to the presence of salt, the water density at the uppermost interface is higher, and the water density at the lower interface is lower. So, this is the original driving force of this natural convection, because the higher density at the top drives the salt liquid to fall. "The water that evaporates from the top of the system can be collected onto the condensed surface, providing pure fresh water.
The researchers say the advantages of this system are high performance and reliable operation, especially under extreme conditions, and can actually work with near-saturated brine. And that means it's also very useful for wastewater treatment.
In addition, they say, much of the current work on solar desalination technology is focused on new materials. But in this case, the researchers were using really low-cost, almost household materials. The key is to analyze and understand the convection that drives this completely passive system, the first to achieve desalination without a water-absorbing structure.
Hadi Ghasemi, a professor of chemical and biomolecular engineering at the University of Houston, said the new approach "offers a promising, effective path to desalination of highly salinity solutions and could be a game-changer for solar seawater desalination." He was not involved in the work, adding, "However, further work is needed to assess the concept in the broader context and in the long term." ”
So far, the team has validated the concept using small desktop devices, so the next step will begin to expand to devices with real-world applications. According to their calculations, a system of only 1 square meter of collection area should be enough to provide a family's daily drinking water needs, they said. According to the researchers' calculations, the materials required for one square meter of equipment only cost about $4.
The researchers said their test equipment ran for a week without any signs of salt accumulation. And this device is very stable. "Even if we apply some extreme disturbances, such as seawater or waves on the surface of a lake, it can quickly return to its original equilibrium position," they say.
The researchers believe it should be possible to translate this lab-scale proof of concept into operational commercial equipment and improve overall water production in the next few years. The first application might be to provide safe water in remote areas far from the grid, or to provide disaster relief after a hurricane, earthquake or other disruption of normal water supply.
They also added, "If we can concentrate the sunlight a little bit, we can use this passive device to generate high-temperature steam and medically sterilize rural areas off the grid." We see real opportunities in developing countries, which are where the short term is most likely to have an impact because of the simplicity of design. ”-