Water Extraction Technologies:
Water from Air

Sciperio is applying the application of proven liquid desiccant technology to the challenge of energy-efficient water harvesting from air.

On-site water harvesting addresses the issue of distribution, which comprises the single most difficult challenge associated with water logistics and management. Conventional water purification systems, such as reverse osmosis, may be adequate if water source and utilization locations are geographically near to one another. In most geographic areas, however, impaired water sources are likely to be far from the actual utilization point. In such cases, being able to extract water from air offers substantial advantages because the water does not have to be transported from a distant source to a local storage facility. If water is continuously harvested, local storage requirements are also greatly reduced.

Another opportunity area for water-from-air extraction is in those regions of the world where sources of condensed water are scarce or nonexistent. Given the ubiquitous nature of water in the vapor phase, it is possible to establish a sustainable potable water supply at virtually any location if one can develop a technology that efficiently harvests water from air. Possession of such technology will provide a clear logistical advantage for soldiers, community workers, healthcare officials, and others in the field .

Under DARPA funding, the proposed work at Sciperio, Mechatronic Solutions Inc and Spectra Watermakers seeks to develop a new approach for harvesting potable water from the air. The system being developed is based on the application of liquid desiccant and advanced energy recovery RO technologies. It provides clear energy efficiency advantages over current state-of-the-art water-from-air systems, and can be used to produce water under ambient air conditions (e.g., hot and dry) in which current systems are incapable of producing condensed water from air.

For purposes of water harvesting, we are using liquid desiccants to absorb water vapor from the air. Liquid desiccants have at least two major advantages over conventional solid desiccants:

Liquid desiccants have significantly smaller absorption-evaporation cycle hysteresis characteristics than the adsorption-desorption cycle of conventional solid desiccants. This means that the energy losses per hydration-dehydration process cycle are smaller for liquid desiccants than they are for typical solid desiccants. A water-from-air system based on the application of liquid desiccants is inherently more energy efficient than a system based on conventional solid desiccant surfaces.

Liquid desiccants have a much higher relative water mass uptake capacity than conventional solid desiccants. Upon dissolution, for example, a LiCl ion pair generates two hydration shells comprised of a total of 26 water molecules (e.g., 26 moles of water per molar equivalent of dissolved solute). Only a few water molecules may be condensed and held within a traditional solid desiccant crystallite. As a result, liquid desiccants exhibit a 15-100 fold mass uptake advantage over traditional solid desiccants as shown in the figure below. This means that liquid desiccant-based water-from-air systems, in principle, will be lighter and more compact than comparable systems based on traditional solid state adsorbents.

Hygroscopic salts, like LiCl, make excellent liquid desiccants. The free energy of hydration, ∆G hyd, is so large for such salts that, upon exposure to typical ambient room air, they dissolve in their own waters of hydration. Figure 5 represents a timed-sequence series of photographs, shot at Sciperio, showing the dissolution of solid LiCl crystals and the formation of a desiccant solution due to water vapor extraction from a flowing air stream (70 oF / 50% RH, 0.5 standard L/min flow rate):

Click on the picture for a larger version.

Below is an example of the water collection in our liquid desiccant solutions as a function of time at 50% relative humidity.

Potable water is produced by extracting solvent water from the desiccant solution, which is achieved in the proposed system through the use of an evaporation/condensation cycle The primary functional elements of the Evaporator/Condenser Module being built are

liquid desiccant reservoir;

closed loop scavenger air plenum;

an air-to-liquid water vapor mass exchanger;

a heat exchanger / water vapor condenser stage;

a heat pump which is used to heat liquid desiccant in the evaporator stage and cool water vapor in the condenser stage; and

a produced water collection reservoir.

Simply put, the key advantage of a liquid desiccant cooling system (versus a pure refrigeration-type) lies in the fact that water vapor may be extracted from the process air stream without first removing the sensible heat. Liquid desiccant cooling systems are beginning to become popular for humidity control and HVAC applications in the US and internationally because it is only the latent heat of vaporization for condensed/extracted water that must be managed. We are applying the application of proven liquid desiccant technology to the challenge of energy-efficient water harvesting from air. As with building air-handling applications, this approach has the immediate advantage of eliminating the need to remove sensible heat from a process air stream.