Engineers from MIT and China are collaborating to develop a pioneering solar-powered desalination device, inspired by oceanic processes. The device uses desalination to convert seawater into drinkable water. This system is small and produces 4 to 6 litres of drinking water per hour. Surprisingly, it performs better and is cheaper than tap water and researchers are taking this approach beneficial for coastal areas where access to traditional water supplies is limited.
This passive device uses natural sunlight to heat saltwater, according to a paper published by the research team. The device configuration allows water to circulate in swirling eddies, similar to the ocean’s thermohaline circulation. This circulation, along with the sun’s heat, causes water to evaporate, leaving behind salt. The evaporated water can be condensed and collected as pure, drinkable water. Meanwhile, the remaining salt keeps circulating through and out of the device, preventing accumulation and system clogging.
The new system produces more water and removes more salt than other passive solar desalination concepts being tested. When scaled up to the size of a small suitcase, it can produce about 4 to 6 litres of drinking water per hour and last for several years without needing replacement parts. At this size and performance, the system can produce drinking water more affordably than tap water.
A research scientist in MIT’s Device Research Laboratory, Lenan Zhang said, â€œFor the first time, it is possible for water, produced by sunlight, to be even cheaper than tap water.â€
The team wants to create a bigger version of the device that can generate drinking water for a small family. This device can also be used in coastal communities that don’t have access to a traditional water supply.
Strengthening the Process of Convection
This wasn’t their first attempt to develop a desalination device for this purpose.
The team improved the previous design by creating a new system. They tested this design on the roof of an MIT building, and it efficiently converted the sun’s energy to produce drinkable water. However, the salt leftover from this process would accumulate and clog the system after a few days.
The next system has multiple layers called stages, which contain an evaporator and a condenser. These stages used heat from the sun to separate salt from water. In a real-world situation, the user would have to frequently replace the stages, which would make the system more expensive.
In the follow-up design, they created a solution with a similar layer arrangement. A new feature has been introduced to enhance the circulation of incoming water and ensure complete removal of any remaining salt. This design prevented salt from settling and accumulating on the device, but it desalinated water at a slow pace.
The latest version of the design is the ultimate achievement. The team has successfully developed a system that not only produces a large amount of water at a rapid rate but also has an exceptional ability to reject salt. This means that the system is not only efficient but also trustworthy, ensuring a continuous supply of clean drinking water for a long period of time.
The new design incorporates the best of their two prior concepts: a multistage system with evaporators and condensers that effectively augments the water and salt circulation within each stage. The new system generates small circulations, resembling the thermohaline convection in the ocean. This remarkable phenomenon propels the movement of water worldwide, driven by variations in sea temperature (“thermo”) and salinity (“haline”).
Lenan Zhang explains, â€œWhen 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 phenomena in a small box, we can take advantage of this feature to reject salt.â€
The researchers placed the entire box at an inclined angle inside a larger container. They then connected a tube from the upper part of the box all the way down through the base of the container and finally floated the container in saltwater.
At the core of the design lies a sleek, slender box-like structure, with a dark material that effectively harnesses the sun’s radiant heat. It is divided into two distinct sections: an upper and lower portion.
- The top half of the structure features a specially designed evaporator layer on the ceiling, harnessing the power of the sun to warm and evaporate water upon contact.
- Afterwards, the water vapor is redirected to the lower section of the box.
- Here, an air-cooling layer condenses the vapor, transforming it into pure and safe liquid that is free of any salt.
This setup allows water to be naturally propelled upwards through the tube and into the box. The unique combination of the box’s tilt and the sun’s thermal energy not only prompts the water to swirl as it moves through, but also ensures that the small eddies continuously keep the salt circulating instead of settling and causing blockages.
In addition, these swirling motions assist in bringing the water into contact with the upper evaporating layer, optimizing the overall process.
The team created different prototypes:
- One with one stage
- Another with three stages
- Next with 10 stages
They tested these prototypes in water with different levels of saltiness, including natural seawater and water that was seven times saltier.
Based on these tests, the researchers have determined that scaling up each stage to a square meter would result in the production of an impressive 5 liters of drinking water per hour. What’s even more remarkable is that this system is designed to desalinate water without any salt accumulation for multiple years.
In addition to its longevity, the system operates entirely without electricity, making it incredibly cost-effective. In fact, the estimated cost of running this system is even cheaper than the production cost of tap water in the United States.
Yang Zhong says, â€œWe show that this device is capable of achieving a long lifetime. That means that, for the first time, it is possible for drinking water produced by sunlight to be cheaper than tap water. This opens up the possibility for solar desalination to address real-world problems.â€
This research on desalination converts seawater into drinkable water is conducted at Shanghai Jiao Tong University and received funding from the Natural Science Foundation of China.