New Solar Technology Promises Safe Drinking Water in a Compact Off-Grid Footprint

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Promo image. Jeff Fitlow/Rice University

More than 18,000 desalination plants operate in over 150 countries, but these aren't helping the estimated 1 billion people who lack access to safe drinking water, or the 4 billion who suffer water scarcity at least one month per year.

Many desalination plants use distillation processes, which require heating water to boiling temperature and harvesting the purified water vapors, or reverse osmosis, in which strong pumps suck energy to pressurize the liquids. A newer option, membrane distillation, reduces the energy inputs by using saltwater heated to lower temperatures flowing on one side of a membrane while cold freshwater flows on the other. Vapor pressure differences due to the temperature gradient transport water vapor out of the saltwater across the membrane, where it condenses in the cool water stream.

In traditional membrane distillation, there is still a lot of heat lost, as the cool water constantly draws heat away from the warmer saltwater. And the saltwater is cooling constantly as it flows along the membrane, making the technology inefficient to scale up in size.

Enter the researchers of the Rice University-based multi-institutional Center for Nanotechnology Enabled Water Treatment (NEWT). They have integrated nano-particles of carbon black into a layer on the saltwater side of the membrane. The high surface area of these low-cost, commercially available black particles collect solar energy very efficiently, which provides the heating needed on the saltwater side of the membrane.

They named the resulting process "nanophotonics-enabled solar membrane distillation (NESMD)". When a lens is used to concentrate the sunlight striking the membrane panels, up to 6 liters (over 1.5 gallons) of clean drinking water can be produced per hour per square meter of panel. Because the heating increases as the saltwater flows along the membrane, the unit can be scaled up quite effectively.

The technology can be applied as well to cleaning up waters with other contaminants, which might give the NESMD wide applicability in industrial situations, especially where power infrastructures are not readily available. The only question remaining is: will the US still be committed to developing these leading edge technologies? The press release on this breakthrough notes:

"Established by the National Science Foundation in 2015, NEWT aims to develop compact, mobile, off-grid water-treatment systems that can provide clean water to millions of people who lack it and make U.S. energy production more sustainable and cost-effective. NEWT, which is expected to leverage more than $40 million in federal and industrial support over the next decade, is the first NSF Engineering Research Center (ERC) in Houston and only the third in Texas since NSF began the ERC program in 1985. NEWT focuses on applications for humanitarian emergency response, rural water systems and wastewater treatment and reuse at remote sites, including both onshore and offshore drilling platforms for oil and gas exploration"

The National Science Foundation wasn't mentioned in Trump's original 'skinny budget' in March but is tagged with an 11% cut in the more fleshed out version released in May, certainly less severe than the 31% cut to EPA or 18% redlined at the National Institutes of Health. This could be the technology that prevents the wars of the future -- seems like an investment worth making even if you don't count the value of the many lives it could save along the way to preventing water from becoming our most precious resource.

Read more at PNAS: doi: 10.1073/pnas.1701835114