An unpowered device that can harvest water vapor in a desert
Luke Skywalker may have been unimpressed with the life of a Tatooine moisture farmer, but a simple device that could economically harvest water from desert air would really be pretty exciting. According to Wookieepedia, the “moisture vaporators” the young Skywalker tended utilized refrigeration coils to chill air to the dew point and collect the water that condensed. We can certainly do that today (as they could “a long time ago… ”), but the amount of energy required makes collecting condensation impractical.
Enter a new study device developed by MIT’s Hyunho Kim. His idea is to work with a unique class of materials called “metal-organic frameworks.” Organic, carbon-based molecules form links between metallic ions to create interesting 3D structures that can have lots of open space internally. This allows the structures to do strange things, like make a high-pressure tank hold far more hydrogen gas after it’s first filled with granules of the right metal-organic framework material.
Kim worked with a zirconium oxide paired with an organic molecule. The combination has the useful quality of grabbing and holding onto water vapor at lower temperatures, but also letting go of that water as the heat rises. So the basic idea is that a device based on this material could passively harvest water vapor from the air at night and then release it (to be collected) in the heat of the day.
For a first test, the researchers tried out the process in the laboratory. A thin square of the metal-organic framework material was placed in a chamber of air at just 20-percent relative humidity. After a while, it was hit with Sun-strength light. Behind the square was a small surface kept at a constant, cooler temperature. This surface allowed the released water vapor to condense in droplets. The setup worked as predicted.
Next came the construction of the demonstration prototype seen in the photo at the top of the page. It was taken out of the lab and onto the roof. A layer of the water-absorbing material sat atop a chamber with a viewing window on the side. The bottom of the chamber held a condensing plate, again cooled a few degrees below the surrounding air temperature, but only to ensure the water vapor condensed on the plate rather than fogging up the viewing window. In practice, a passive heat sink (think radiator fins) at ambient temperature would be used to help the water condense without the need for any energy input.
The researchers left the metal-organic framework layer out overnight to absorb water vapor. Then they placed it inside the prototype chamber and watched as droplets of water appeared on the condenser in the daytime sunshine.
Because the demonstration prototype was a small device, it only captured a small amount of water, but the potential is there. Scaled up to one square meter, this version would harvest roughly 0.2 liters per day. But if the water-absorbing layer is kept thin, it can “fill up” in as little as an hour, so a once-per-day collection cycle is perhaps unnecessarily limiting. Shading the device periodically over the course of a day would increase the number of cycles and get you close to a liter per day per square meter.
The output could be increased in a number of ways, but the bigger lesson is that the Sun alone can drive a pretty impressive process, however it’s optimized. Even in dry desert air, there’s some “water, water everywhere”—and this could get you a drop to drink.