Produce drinking water from salt water in relatively large quantities and at a price even lower than the usual tap water, simply thanks to the use of solar energy. For people of advanced age or diversified culture, it may seem like a proposal from the historic and friendly section The Great Inventions of the TBO. For the youngest or digital natives, it can be understood as a new installment of the usual fake news (fake news, for those who are not used to normal Spanish).
But in this case it seems that he is serious. At least because it is endorsed by the Massachusetts Institute of Technology (MIT, in Cambridge, United States) and the Shanghai Jiao Tong University (China). Furthermore, the results obtained in the design of this innovative desalination plant and the tests with the most advanced prototypes have been formally presented – so that they can be verified by other researchers – in the journal Joule, from the prestigious Cell publishing group.
It is not the first time that an invention of this type has been presented – using the sun’s energy to produce drinking water economically from sea water – but, as the authors of this new research explain, the novelty is that A design has been achieved that avoids one of the main problems of this type of alternative: the accumulation of salts that blocks the production of drinking water until filtration is unviable.
“Recent advances in multistage solar distillation are promising for sustainable freshwater supply,” recalls this team led by Jintong Gao of the Solar Energy and Refrigeration Engineering Research Center at Shanghai Jiao Tong University. “However, significant performance degradation due to salt accumulation has posed a challenge to both the long-term reliability of solar desalination and the efficient treatment of hypersaline discharge,” they detail. As a solution, the authors indicate that they have been inspired “by a natural phenomenon, thermohaline convection” and have managed to “demonstrate a multistage membrane distillation powered by solar energy with extreme salt resistance performance.” “Using a confined salt layer as an evaporator, we initiate strong thermohaline convection to mitigate salt buildup and improve heat transfer.”
Although it is a technical explanation that is difficult to understand for laypeople in hydraulic engineering, we point out that the summary of this study indicates that, “with a ten-stage device, record solar-water energy efficiencies of 322% to 121% have been achieved. in the salinity range of 0 to 20% by weight under sunlight.” “With high freshwater production and extreme salt resistance, our device significantly reduces the cost of water production, paving the way towards the practical adoption of passive solar desalination for a sustainable water economy,” state the authors of this investigation.
In an informative article published in the MIT News Bulletin, Jennifer Chu explains that “the team’s new system improves on their previous design: a similar concept of multiple layers, called stages.” Each stage contained an evaporator and a condenser that used heat from the sun to passively separate salt from incoming water. That design, which the team tested on the roof of an MIT building, efficiently converted the sun’s energy to evaporate water, which was then condensed into drinking water. But the salt that remained quickly accumulated in the form of crystals that clogged the system after a few days. “In a real environment, a user would have to clean the stages frequently, which would significantly increase the overall cost of the system.”
In a follow-up effort, they came up with a solution with a similar layered configuration, this time with an added feature that helped circulate incoming water as well as leftover salt. While this design prevented salt from settling and accumulating in the device, it desalinated the water at a relatively low rate.
In the latest version, which is now presented as a result of the work, the team has arrived at a design that achieves both a high water production rate and high salt rejection, meaning the system can produce drinking water of quickly and reliably over a long period of time. The key to their new design is a combination of their two previous concepts: a multi-stage system of evaporators and condensers, which is also configured to drive water (and salt) circulation within each stage.
The main equipment of the prototype tested resembles a box topped with a dark material that efficiently absorbs heat from the sun. Inside, the box is divided into an upper and lower section. Water can flow through the top half, where the roof is lined with an evaporative layer that uses the sun’s heat to heat and evaporate water on direct contact. The water vapor is then channeled to the bottom half of the box, where a condensation layer cools the vapor with air and turns it into a salt-free drinkable liquid. The researchers placed the entire tilted box inside a larger empty container, then connected a tube from the top half of the box to the bottom of the container and floated it in salt water, explains the MIT Bulletin.
In this configuration, water can naturally rise through the tube and enter the box, where the tilt of the box, combined with thermal energy from the sun, induces the water to circulate as it flows. The small swirls help bring the water into contact with the top evaporation layer while keeping the salt circulating, rather than settling and clogging.
The researchers calculated that if each stage were expanded to one square meter, up to 5 liters of drinking water would be produced per hour, and that the system could desalinate water without accumulating salt for several years. Given this long lifespan and the fact that the system is completely passive and does not require electricity to operate, the team estimates that the total cost of running the system would be cheaper than what it costs to produce tap water in the United States.
“We showed that this device is capable of achieving long lifespan,” says Yang Zhong, co-author of the study and an expert in the Department of Mechanical Engineering at MIT. “That means that, for the first time, drinking water produced by sunlight may be cheaper than tap water. This opens the possibility for solar desalination to address real-world problems,” concludes Yang Zhong.
