The Technology
The increasing demand for freshwater is overstretching the natural water resources around the globe. Desalination of seawater by reverse osmosis is among the most promising technologies for intensive freshwater production. However, it requires a large saline water source and large capital investments for building the desalination plant and the piping infrastructure.
Atmospheric moisture is another potential source of freshwater, which sums up to a significant amount and is accessible essentially everywhere. However, the limiting factor of fog harvesting is the global unavailability of the necessary meteorological conditions that support frequent fog occurrence. Similarly, dew formation is limited by the surface radiation properties, and is highly affected by the ambient conditions. Active atmospheric moisture harvesting (AMH), where the heat interactions involved in the process are handled by, e.g., a standard electrical compression−expansion refrigeration unit, can ensure continual water production for varying ambient conditions. However, the energy requirement and the lack of commercially-available high-performance membrane modules (and hermetically sealed low-power fans), make this technology not currently feasible.
An alternative approach for separating water vapor from the air is by using a desiccant. The novel technology is an atmospheric moisture harvesting system, in which water vapor is separated from the air using a liquid desiccant subsystem. The moisture charged liquid desiccant enters a desorber and condensation system, where the moisture is extracted. Cooling and condensation are performed on the extracted water vapor only, thereby saving the energy for cooling air containing the water vapor, as is done in direct cooling systems. The system thus operates as a single stage vapor separation system since the vapor concentration remains in the liquid desiccant which is circulated to the separation and condensation subsystems without air laden with the moisture. Low grade or solar heat can be used as the energy source for the vapor desorption. A heat exchanger utilizes unwanted heat from regenerated desiccant to heat up charged desiccant entering the regenerating stage.
Advantages
- Substantial energy savings over prior art systems using direct cooling of air
- No need for treating water for bacteria and contaminants
- Stable and foreseeable energy consumption and water production
Applications
- Decentralized water production plants in remote communities and stations
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