MIT researchers have dramatically improved the performance of a system capable of extracting drinking water directly from the air, even in dry regions, using heat from the sun or another source.
The system, which is based on a design originally developed three years ago at MIT by members of the same team, brings the process closer to something that could become a convenient water source for remote areas with limited access to water and electricity.
Wang and coworkers’ first device took advantage of a temperature difference inside the device to allow an absorbent material – which collects liquid on its surface – to draw moisture from the air for at night and release her the next day. When the material is heated by sunlight, the temperature difference between the heated top and the shaded bottom causes water to be released from the adsorbent material again. The water then condenses on a collecting plate.
But this device required the use of materials called MOFs, which are expensive and of limited supply, and the water production of the system was not sufficient for practical use.
Now, by incorporating a second step of desorption and condensation, and using readily available adsorbent material, the efficiency of the device has been dramatically increased and its scalability has been greatly improved, the researchers say.
Instead of MOF, the new design uses an absorbent material called zeolite, which in this case is made of microporous iron aluminophosphate. The material is widely available, stable, and has adequate adsorbent properties to provide an efficient water production system based solely on typical day-night temperature fluctuations and heating in sunlight.
The two-stage design developed by LaPotin intelligently uses the heat generated each time the water changes phase. Heat from the sun is collected by a solar absorption plate on top of the box-like system and heats the zeolite, releasing the moisture that the material has captured overnight. This vapor condenses on a collector plate, a process which also gives off heat. The collector plate is a sheet of copper directly above and in contact with the second layer of zeolite, where the heat of condensation is used to release vapor from this back layer. Water droplets collected from each of the two layers can be channeled together into a collecting tank.
While similar two-stage systems have been used for other applications such as desalination, Mr. Wang says: “I don’t think anyone really went this route“Use such a system for atmospheric water capture (AWH), as these technologies are known.
Current AWH approaches include fog collection and dew collection, but both have limitations. Fog collection only works at 100% relative humidity and is currently only used in a few coastal deserts, while fog collection requires energy-intensive cooling to provide cool surfaces on which moisture condenses, and always requires a humidity of at least 50%, depending on the ambient temperature.
Instead, the new system can operate with humidity levels as low as 20% and requires no energy input except sunlight or other low quality heat source available.
LaPotin says the key is this two-step architecture; Now that it’s proven to be effective, people can look for even better absorbent materials that could further increase production rates. The current production rate of around 0.8 liters of water per square meter per day may be sufficient for some applications, but if this rate can be improved with some adjustments and materials, it could be practical on a large scale, he said. Materials are already under development which have an adsorption about five times higher than this zeolite and which could lead to a corresponding increase in water production.
More information: mit.edu