The stuff science fiction is made of has come closer to reality, with the possibility of an appliance that can pull enough water for a household out of the air, even in a desert or dry climates. The device only needs the power of the sun to operate.
A water harvester that does just that was demonstrated this week. The harvester only uses ambient sunlight and pulls liters of water out of the air each day. This can even be achieved with only 20 percent humidity, a level commonly found in arid areas.
Omar Yaghi is a faculty scientist at Lawrence Berkeley National Laboratory and one of two senior authors of the paper. Yaghi also holds the James and Neeltje Tretter chair in chemistry at UC Berkeley. He described the study results as a major breakthrough in the long lasting challenge of harvesting water from the air at low humidity and noted that there is currently no other way to do that, except by using extra energy. The water ‘produced’ by an electric dehumidifier is very expensive.
The prototype pulled 3 quarts (2.8 liters) of water from the air in 12 hours under conditions of between 20 and 30 percent humidity, using 2.2 pounds (1 kg) of metal organic framework (MOF). Rooftop tests at MIT established that the device is able to work in real world conditions.
Yaghi is also the founding director of the Berkeley Global Science Institute and a co-director of the Kavli Energy NanoSciences Institute and the California Research Alliance of BASF. He believes the results of the experiment will ultimately make it possible to produce water off the grid by having a device running on ambient solar at home and delivering enough water to fulfill the needs of a household.
More than 20 years ago, Yaghi invented metal organic frameworks by joining metals like aluminum or magnesium with organic molecules in a tinker toy arrangement. This resulted in porous, rigid structures, perfect for storing liquids and gases.
More than 20,000 different MOFs have since been created by scientists worldwide. Some can store chemicals such as methane or hydrogen. BASF, the chemical company, is currently testing one of Yaghi’s MOFs in trucks that use natural gas as fuel. Tanks filled with MOFs can hold three times the volume of methane that can be pumped into an empty tank under pressure.
MOFs are used in a wide variety of applications, from catalyzing the reaction of adsorbed chemicals, capturing carbon dioxide from flue gases, or separating petrochemicals in processing plants.
Wang and her students designed a system consisting of 2.2 pounds of dust sized MOF crystals compressed between a condenser plate and a solar absorber. This assembly was then placed inside a chamber open to the air. When ambient air spreads through the porous MOF, the water molecules in the air attach to the interior surfaces. X-ray diffraction studies demonstrate that the water vapor molecules often gather in groups of eight to form cubes.
Sunlight heats up the MOF through a window. The bound water moves toward the condenser, which is at the temperature of the outside air. The vapor then condenses and drips into a collector.
Wang explained that this new way of harvesting water from air does not need high relative humidity conditions and is much more energy efficient than existing technologies.
Yaghi noted the harvester only proves the concept and it could be improved dramatically. The MOF they’re using can only absorb 20 percent of its weight in water, but this could possibly be improved to 40 percent or more by using different MOF materials. The material can also be optimized to be more effective at either lower or even higher humidity levels.
He added that they’ve not simply made a passive device that sits somewhere collecting water, but have laid both the theoretical and experimental foundations for future development. Thousands of other MOFs can now be screened, or new ones manufactured, to find better materials. There is massive potential for scaling up the amount of water that can be harvested. It now simply boils down to further engineering.
While Yaghi and his team are focusing on improving the MOFs, Wang is working on finding ways to improve the harvesting system so that it produces more water.
To have running water all the time, it might be possible to design a system that absorbs the humidity during the night and evolves it during the day. The design of the solar collector could also be changed to enable a much faster water extraction rate, where more air is pushed in. Yaghi noted that a person only needed about 350 ml of water per day and this could be collected in less than an hour with this system. The experiment demonstrated that a person cut off in the desert somewhere could survive because of this device.