A team of researchers in the US made headlines some time back by developing a device that can extract potable water from ambient air¹ using only sunlight as the source of energy. K. S. Jayaraman got curious on how that could work for India. He spoke to Sameer Rao, an Indian postdoc involved in the project at the the Device Research Laboratory of Massachusetts Institute of Technology (MIT), and Omar Yaghi, who led the project at University of California, Berkeley.

Here’s Jayaraman’s guest post on the solar-powered water harvester and implications for India.

This is almost reminiscent of the famous magic trick — the “Water of India” — which India’s legendary magician P C Sorcar Jr. performed without fail in each of his stage shows. He would wave his hand in thin air, say the magic words in his characteristic style and, “by magic”, an empty jug kept on the table would constantly fill itself up. Again and again.

Now, a new research promises a potential new technology for harvesting drinking water from this hitherto neglected source – air.

Atmosphere actually contains an estimated 13,000 trillion litres of water – equivalent to nearly 10% of all fresh water present in lakes worldwide – that remains untapped. A solar-powered water harvester, which the MIT-UC Berkeley team has built, works even when relative humidity (RH) is as low as 20%, the level common in arid areas and deserts of the world.

Sameer Rao tells me their device could work well in most parts of India. Large scale use of this device can change the landscape of water utilisation in India where sunlight is abundant, Rao, a post-doctoral associate in Evelyn Wang’s lab at MIT says. The heart of the new device is a metal-organic framework (MOF) that belongs to a class of unique materials exhibiting extremely high porosity that Yaghi’s team had pioneered in the 1990s at Berkeley.

Yaghi, a chemistry professor at UC Berkeley and one of the research leads, calls this a major breakthrough in the long-standing challenge of harvesting water from the air at low humidity. “We used only ambient sunlight, with no electricity – this is a major improvement over most other air-water harvesting devices which require energy input, and therefore are economically not viable.”

“They have high affinity to water molecules, to pull them out of ambient air, but do not hold on to them too tightly so that water can be concentrated and released with a slight temperature change (induced by sunlight),” Yaghi says. The adsorbed water thus released is then stored by the device in a condenser.

The adsorption-desorption experiments performed in a RH-controlled chamber in the laboratory found that the device was able to pull 2.8 liters of water from the air over a 12-hour period at RH levels as low as 20% using one kilogram of MOF. Roof top tests at MIT confirmed that the device works equally well outdoors. The scientists have found the experimental data to be in “good agreement” with a theoretical framework they had developed.

The quantity of harvested water can be scaled up by finding better MOF materials “with enhanced sorption capacity and high intra-crystalline diffusivity” that could absorb more water. The current MOF can absorb only 20 percent of its weight in water. Evelyn Wang, who led the MIT team says they “continue to improve the harvesting system to produce more water..”

“…If you are cut off somewhere in the desert, you could survive because of this device,”Wang says.

The developers hope their device, when commercialised, could help every household obtain drinking water it needs out of the air using only the power of the sun.

Rao particularly feels the social impact of this in India would be far-reaching. “It would enable greater success for government’s grass-root level programmes seeking to increase literacy and education of women and children in villages as they would get more time not having to travel long hours to fetch water from far off reservoirs for cooking and drinking.

How much would the solar water harvester cost? “We have made the significant first step by building the demonstration model combining chemistry and engineering to show how it works,” Yaghi says. “The economic aspects as well as the production of these materials and devices are the obvious next step.”

1. Kim, H. et al. Water harvesting from air with metal-organic frameworks powered by natural sunlight. Science. doi: 10.1126/science.aam8743 (2017)