Water-from-air Extraction Processes

Water is an essential good that fulfills multiple roles in society. Water is used for drinking, agricultural irrigation, and it acts as a solvent, reactant, or cooling medium in industrial processes. Enormous amounts of fresh water are needed to maintain our standard of living. Water itself is plentiful, but most water on Earth is either saline, found deep underground, or is present as snow, ice, or water vapor and therefore unavailable for immediate use. A dynamic and uneven water distribution combined with high fresh water demand is a recipe for water scarcity. Reports show that one billion people already live in water-stressed areas; this number is likely to increase due to a growing world population, deterioration in water quality, declining groundwater levels, and the intensification of the water-energy nexus. Providing people with water at any time and any place presents the ultimate solution to water scarcity. Taking advantage of the currently untapped but ubiquitous water vapor from atmospheric air paves the way for solving local water scarcity. Water-from-air extraction processes for fresh water production are currently not considered in the water sector, although the technology has been around for some time. This observation necessitates a thorough evaluation of the bottlenecks hindering atmospheric water vapor from becoming a viable fresh water source. Water-from-air technologies are classified and sorted according to their underlying working principles to gain a thorough understanding of the water-from-air field in its current state. Conventional categories of water-from air technologies include direct air cooling and desiccant-based water harvesting. In the air cooling approach, the air cools down below the dew-point to allow for water condensation. Operating in subzero dew-point temperatures should be avoided and call for a different strategy: desiccant-based water harvesting. In this approach, water vapor is captured by a desiccant material. The water evaporates from this material in the next step by thermal heating, facilitating the final condensation step by raising the dew-point. Presented concepts are carried out in a passive operation mode in which the process is carried out based on natural phenomena or an active mode in which an auxiliary energy input is required to carry out the process. The underlying principles, advantages and disadvantages, a thermodynamic analysis, and the achieved efficiencies of devices are presented in this work. The climate-dependent specific water yield (L/kWh), an energy intensity measure, is introduced to evaluate the impact on the water-energy nexus. The low intrinsic specific water yields of water-from-air technologies create a major bottleneck for large-scale implementation. Water consumption in thermoelectric power plants drastically reduces the overall process efficiency in the scope of the water-energy nexus. In addition, current water-from-air extraction devices reach the thermodynamic boundaries, limiting further improvements. Alternative low energy-intensive atmospheric water harvesting concepts are needed to turn water vapor into a viable water resource. Conventional water-from-air harvesting processes are characterized by low maximal specific water yields. The phase transitions of desorption and condensation are energy-intensive process steps. Two alternative concepts are proposed which avoid these transitions and drastically reduce the energy demand. Thermo-responsive polymers are capable of immediately releasing trapped water as a liquid upon heating due to a hydrophilic-hydrophobic configuration change of the polymer chains. Another alternative is deliquescent salt reverse osmosis: the transfer of the seawater desalination concept to land. Deliquescent salts become liquid solutions above a certain relative humidity, making it possible to separate the solution into fresh water and a concentrated brine in a pressure-driven reverse osmosis unit. The concentrated brine can be reused to capture water vapor and close the process cycle. The process fundamentals, technological restrictions, and an energy analysis of these alternative concepts are provided. A world map indicating the optimal water-from-air extraction process is constructed based on regional climate conditions, maximum specific water yields, and practical considerations. The selection of water technologies used to evaluate the water-energy nexus has been updated with the air-based fresh water production technologies covered in this work. Most water-from-air extraction processes operate in two consecutive steps: water vapor capturing with a desiccant followed by a water separation step. The rate of water uptake is crucial for maximizing water production, but research focuses mainly on water uptake capacity and the separation step. A complete non-isothermal kinetic model of a desiccant bed of particles has been developed to elucidate the prevailing kinetic bottlenecks during water vapor adsorption. The mass transfer effects of particle diffusion and bed diffusion are investigated in conjunction with the heat transfer phenomena of bed thermal conductivity and air convection. The validity of the commonly held assumptions of neglecting thermal radiation and convective water vapor supply has been checked. In addition, the use of a complete multiphysics model allows for large step sizes of the boundary conditions, making it possible to determine the effect of the nonlinear adsorption isotherm shape on the mentioned mass and heat transfer phenomena. In general, limited improvements in water uptake rate are expected by enhancing heat and mass transfer. Practical solutions are given to improve the slow water vapor uptake of the passive desiccant-based technology.

Publication year:2021

Accessibility:Open