Just Imagine you could produce water out of thin air with no connection to any mains utility. Sounds incredible, especially if it operates where the air is very dry like deserts and steppe regions of the world. Such a device has actually been designed and the best thing about it is it uses almost no mechanical parts or complex electronics and works purely from the effects of direct solar radiation (not PV cells).
Researchers at MIT have been using a structure known as a Metal Organic Framework (MOF) to harvest water directly from the air (at humidity levels as low as 20%). This humidity level is commonly found in dry regions of the world. The prototype was able to extract 2.8 liters of water per day at an air humidity of 20 to 30%.
MOFs are network-like structures composed of organic compounds and metallic units and have been around since their invention about 20 years ago. Depending on the MOF composition and base materials, certain molecules can be deposited particularly stably into voids in the structure. Gases from hydrogen to methane are possible. Their storage density per volume is actually higher than if the gases were compressed into large hollow tanks. Since their invention at the University of California at Berkeley, over 20,000 different MOF variants have been developed. The MOF used for the water harvester consists of zirconium and adipic acid which binds water vapor.
In the first picture you can see the construction of the water harvester. In the interior, there is about 1 kg of MOF crystals pressed between an upper light absorbing layer and a lower condenser plate. As ambient air is drawn through the porous MOF, water molecules attach themselves to the interior surfaces. Sunlight entering through a translucent window in the top of the unit heats up the MOF and drives the bound water toward the condenser, which is at ambient temperature via a heat pipe and radiator arrangement below the unit. The vapor condenses and the water drips into a collector. The principle is both simple and effective. If the properties can also be designed for long-term stability, this is a real breakthrough in the sensible use of solar energy without solar PV cells.
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