Scientists at UC Berkeley and MIT have demonstrated a water harvester that uses only sunlight to pull liters of water out of the air each day in conditions as low as 20% humidity, a level common in arid areas.
The solar-powered harvester, reported in the journal Science, makes use of a special material – a metal-organic framework, or MOF. Pulling water out of low-humidity air right now requires an energy-intensive device like an electric dehumidifier, which is a pretty expensive way to get water. The new device uses only the energy from the sun beating down on it.
MOFs combine metals like magnesium or aluminum with organic molecules in a tinker-toy arrangement to create rigid, porous structures that are ideal for storing gases and liquids. Thousands of different MOFs have been created worldwide for various purposes, such as capturing CO2 from flue gases or separating petrochemicals in processing plants.
The new harvester consists of more than two pounds of dust-sized MOF crystals compressed between a solar absorber and a condenser plate, placed inside a chamber open to the air. As ambient air diffuses through the MOF crystals, water molecules preferentially attach to interior surfaces. Sunlight coming through a window heats up the MOF and drives the bound water into the cooler condenser where it drips into a collector.
The prototype could pull about 3 quarts of water out of rather dry air over a twelve-hour period. The researchers believe that improved MOF materials could double the efficiency of the device.
One can imagine a future in which homes in water-starved locations have an appliance that pulls all the water the household needs out of the air, using only the power of the sun.
Photo, posted August 9, 2006, courtesy of Will Powell via Flickr.