According to a UNICEF 2017 press release, about three (3) in ten (10) people lack access to potable drinking water; that’s at least 2 billion people worldwide at the time, with perhaps even more now that five (5) years have passed since. It’s thus pertinent that we address our concerns with providing all people with a clean water supply—a concern that researchers from the Massachusetts Institute of Technology (MIT) and Shanghai Jiao Tong University (SJTU) are ready to address if their study published in the journal Nature Communications is anything to go by.
The researchers’ novel desalination design makes use of solar power, meaning the device can be used entirely off-grid—a crucial feature that may allow future iterations of their novel technology to be used in communities that may otherwise struggle with sustaining a steady power supply.
The novel design also evades the problem of salt fouling, which is when salts and other impurities that are filtered out of water build-up on membranes and other surfaces within the device, limiting functionality and necessitating regular maintenance and frequent replacement.
“There have been a lot of demonstrations of really high-performing, salt-rejecting, solar-based evaporation designs of various devices,” said co-author and MIT mechanical engineering professor Evelyn 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.”
Collaborations between MIT and SJTU produced a layered desalination device that simply sits above the surface of a body of saltwater. The top layer contains heat-absorbing dark material, which sits atop a thin layer of water; this water was, in turn, let through by a third bottom layer of perforated material.
Here, the 2.5-mm (~0.1 in) perforations pull water up from the depths below it, exposing them to sunlight at its topside. This, of course, causes the water to evaporate and is helped along by the presence of the dark material that accelerates the heating process.
This vapor can then be collected and condensed to be used as desalinated water. What’s distinct about their novel setup, however, is the fact that the perforations are just big enough to allow natural convective circulation to take place within the water. The warmer water that sits atop the device, now made denser by the leftover salts after evaporation, is then pulled below the depths, which will then be replaced by less-dense saltwater ready for more desalination.
Testing revealed that the device is capable of reaching more than 80% efficiency in terms of conversion between solar energy and water vapor, which it can do with salt concentrations that reach up to 20% by weight. A week’s operation didn’t even leave any salt crystals behind in the device, delivering on its promise of avoiding salt fouling over time.
“[The device’s advantages are] both the high performance and the reliable operation, especially under extreme conditions, where we can actually work with near-saturation saline water. And that means it’s also very useful for wastewater treatment,” said co-author and MIT postdoc Xiangyu Li. “But in our case, we use really low-cost, almost household materials.”
Said University of Houston professor of chemical and biomolecular engineering Hadi Ghasemi, who’s unrelated to the work: “[This new approach] provides a promising and efficient path for desalination of high salinity solutions and could be a game-changer in solar water desalination. Further work is required for assessment of this concept in large settings and in long runs.”
Wang added that “a real opportunity is the developing world. […] I think that is where there’s [the] most probable impact near-term, because of the simplicity of the design. [But,] if we really want to get it out there, we also need to work with the end-users, to really be able to adopt the way we design it so that they’re willing to use it.”
(For more news concerning saltwater, read about how future house cooling may rely on it.)
References
- Chandler, M. & MIT News Office. (2022, February 14). Solar-powered system offers a route to inexpensive desalination. MIT News; Massachusetts Institute of Technology. https://news.mit.edu/2022/solar-desalination-system-inexpensive-0214
- Irving, M. (2022, February 15). $4 solar desalination system produces a family’s daily drinking water. New Atlas. https://newatlas.com/materials/desalination-family-drinking-water/
- Two billion people lack safe drinking water, more than twice lack safe sanitation. (2017, July 17). UNICEF. https://www.unicef.org/philippines/press-releases/two-billion-people-lack-safe-drinking-water-more-twice-lack-safe-sanitation
- Zhang, L., Li, X., Zhong, Y., Leroy, A., Xu, Z., Zhao, L., & Wang, E. N. (2022). Highly efficient and salt rejecting solar evaporation via a wick-free confined water layer. Nature Communications, 13(1), 849. https://doi.org/10.1038/s41467-022-28457-8
