Capturing water from air with less energy

As the world’s population explodes, fresh water is getting harder to come by in drier regions of the world. One innovative way to produce potable water in these areas is to harvest moisture from the atmosphere. But most methods developed to do this so far this guzzle large amounts of energy. Israeli researchers have proposed a new scheme that takes up to 65 percent less energy than previous vapor-collecting methods.

Several alternative techniques to produce drinking water exist today. Desalinating seawater is the most promising technology and is deployed extensively in California, Saudi Arabia, and Israel. But it needs large capital investment and, of course, a saltwater body close by.

The atmosphere is a vast water source to tap, since it contains just as much water as all of the surface and underground freshwater on Earth. And 98 percent of this moisture exists as water vapor. But harvesting water vapor is tricky. One commercially available system relies on directly cooling air and condensing it on a surface that is kept cold through electrical refrigeration, which consumes a large amount of energy.

So a group of civil and environmental engineers from Technion – Israel Institute of Technology instead propose using a liquid desiccant, lithium chloride, to separate the vapor from air first and then cool only the vapor. Their system consists of six major components and requires low-grade or solar heat.

The researchers used a computer model to analyze the three interconnected cycles of the system—air, desiccant, and water—under different ambient conditions. They found that, depending on ambient conditions, the liquid desiccant system uses 5 – 65 percent less energy than direct air-cooling systems. The results are reported in the journal Environmental Science & Technology.

The new system is more complicated and could have slightly higher capital costs, but once the core of the system is installed, it should be easy to scale up, the team says. Using a cheaper desiccant such as calcium chloride could also bring down costs. —Prachi Patel | 28 July 2016

Source: Ben Gido, Eran Friedler, and David M. Broday, Liquid-Desiccant Vapor Separation Reduces The Energy Requirements of Atmospheric Moisture Harvesting, Environ. Sci. Technol. 2016. DOI: 10.1021/acs.est.6b01280

Photo: Luc Viatour / www.Lucnix.be

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