MOFs Enable Harvesting Water from Air for the Warfighter

Scientists and engineers at framergy proposed a novel system to which could allow forward deployed soldiers to pull drinking water directly out of the air – even in dry deserts.

Figure 1: framergy’s MOFs-based space origami design to harvest atmospheric water at both unit and squad level

The warfighter can survive three weeks without food but only three days without water. As the military moves towards more mobile, flexible, and self-sufficient operations, reducing the water resupply requirements will have even more important tactical implications.   In order to liberate the warfighter from the water supply chain, framergy is using its Metal Organic Frameworks (MOFs), which function as incredible non-expanding desiccants, and can release water with temperatures far below the dew point.

Commercially available atmospheric water harvesting (AWH) systems operate use vapor compression refrigeration (VCR) to condensate water from ambient air by cooling it below its dew point temperature. Studies conducted on commercial units showed that water harvesting yield drops significantly by the simultaneous decrease of water content and dewpoint temperature, and increase of temperature. Dew-harvesting systems become impractical at relative humidity (RH) levels less than 50%. Consequently, due to changes in operating climates, VCR-based harvesters can experience water collectionrate drops as high as 92% while energy consumption increases by five times. 

framergy Solution

framergy’s MOF-based strategy eliminates AWH limitations by utilizing materials with isotherms showing a steep increase in water uptake within a narrow range of RH, which enables maximum sorbent regeneration with minimal temperature increase. MOFs also eliminate the need to reduce the condenser temperatures all the way to dew point. This eliminates the energy intensive, VCR cooling, because with MOFs, hot desorbed vapor can condense at slightly cooler temperatures compared to ambient temperatures by using a passive heat sink.

Several MOFs whose water sorption properties were assessed under operating conditions including their cyclic ability. All of these candidates, which included MIL-125-NH2, show a stepwise increase below 40% RH, capable of operating both in arid and humid climates. Among these alternatives, MIL-100(Fe) shows one of  the highest water uptake capacity with a stepwise response below 30% RH within arid climate conditions. The hydro-stable material, marketed by framergy as AYRSORBTM F100, is easily manufactured at a reasonable cost.

framergy has been working on developing atmospheric water harvester technologies, leveraging the unprecedented cyclic water uptake properties of MOFs. This self-contained, high efficiency harvester design can assist disaster relief activities to generate clean, microorganism free water - much needed for sustenance, cooking and sanitation in the field. To allow for efficient space management, framergy designed a space-origami-based harvester which can adapt to natural diurnal temperature swings by opening up to adsorb water during the cooler night and folding into itself to release water during the warmer day.

Figure 2: framergy’s MOFs-based harvester in adsorption and desorption cycles, unfolded and folded surfaces

In order to test this concept, the space-origami design contactor was built and placed inside a hand-held prototypical thermoelectric (TE) cooler with a cylindrical chamber. The preliminary tests showed that the contactor temperature, while in its folded form, could be quickly reduced and made ready for the next adsorption cycle. The TE cooler does not require refrigeration and the generated heat at the Peltier can be rejected via heatsinks. The thermal images taken during cyclic tests showed that while the condensation chamber temperature drops below ambient temperatures, the rejected heat from the TE peltier element increased the temperature of the heatsink fins over the ambient temperature.
 

The thermoelectric cooling effect evaluations successfully proved that the MOF chamber walls could be cooled down efficiently while the chamber base was still warm, which matched to the harvester design principle. This finding was particularly helpful to utilize solar energy to aid sorbent desorption steps in the future. Concentrating solar rays on the contactor’s base helps achieving both adsorption and desorption steps during the day, increasing the total amount of water harvested per day.

Conclusion

The framergy team proposed this technology to U.S. ARMY and other military teams. Delivering water to remote theaters poses great risk to the military and its warfighters. The personal water harvesting unit shown above for individual warfighters or squad units can help prevent casualties associated with delivering water for sustenance.

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