solar powered water harvester

Solar-powered system harvests drinkable water from dry air

Image credit: MIT

Output from a solar-powered system that can extract drinkable water from 'dry' air has been significantly boosted by MIT researchers.

The system, which builds on a design initially developed three years ago, brings the process closer to something that could become a practical water source for remote regions with limited access to water and electricity.

The earlier model harnesses a temperature difference within the device to allow an adsorbent material, which collects liquid on its surface, to draw in moisture from the air at night and release it the next day.

When the material is heated by sunlight, the difference in temperature between the heated top and the shaded underside causes the water to be released back out of the adsorbent material. The water is then condensed on a collection plate.

That device required the use of specialised materials called metal organic frameworks (MOFs), which are expensive and limited in supply, and the system’s water output was not sufficient for a practical system.

By incorporating a second stage of desorption and condensation, and by using a readily available adsorbent material called zeolite, the device’s output has been significantly increased. This should make it much more scalable for broad rollouts in areas lacking clean drinking water.

The two-stage design makes clever use of the heat that is generated whenever water changes phase.

The overall productivity of the system, in terms of its potential litres per day per square metre of solar collecting area, is approximately doubled compared to the earlier version, though exact rates depend on local temperature variations, solar flux and humidity levels.

In the initial prototype of the new system, tested on a rooftop at MIT before the pandemic restrictions, the device produced water at a rate “orders of magnitude” greater that the earlier version, the researchers said.

The new system can work at humidity levels as low as 20 per cent and requires no energy input other than sunlight or any other available source of low-grade heat.

The present production rate of about 0.8 litres of water per square metre per day may be adequate for some applications, but if this rate can be improved with some further fine-tuning and materials choices, this could become practical on a large scale, the researchers added.

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