Resource scarcity is a pressing issue in the 21st century. A paper published in the journal ACS Materials Letters has asked the question: how can we harvest water vapor in the air to provide fresh drinking water? The authors have reviewed current technological progress and proposed a novel water vapor harvesting system, and provided a brief outlook of the advantages of the system and its associated challenges.
Study: Water Harvesting from Air: Current Passive Approaches and Outlook. Image Credit: OnD/Shutterstock.com
Water Scarcity in the 21st Century
Currently, around four billion people worldwide do not have access to safe and sanitary drinking water, with this figure expected to grow sharply in the coming decades as the world population increases and humanity continues to consume finite resources.
Traditionally, rivers, lakes, and reservoirs have provided the world population with access to fresh water, but these systems are becoming overburdened by rapid urban growth and industrial activities. Climate change and contamination from heavy industry and raw sewage have further exacerbated the problem by making previously reliable drinking water sources unsafe for drinking and sanitation purposes.
The lack of access to safe water is a key concern in today’s world. It is such a critical issue that the UN has named access to clean water and sanitation as one of its seventeen Sustainability Goals. Other goals include reducing poverty, food insecurity, and gender inequality, and promoting education, health, well-being, and sustainable innovation as well as protecting nature and biodiversity.
Tackling the Issue with Technology
Technological solutions have been developed over the years to improve access to safe and sanitary drinking water for urban and rural populations. Key technologies include wastewater treatment and seawater desalination, but these solutions can be expensive and complex and suffer from issues with water transportation capacity. The use of these technologies is particularly challenging in underdeveloped and landlocked locations.
There are significant freshwater reserves contained in ice caps, glaciers, and deep aquifers, but these are difficult to reach and, in the case of glaciers and ice caps, environmentally problematic to exploit. Plus, utilizing these resources requires expensive and technologically sophisticated solutions.
There is one other water resource that is abundant, ubiquitous, and critically underexploited, which can solve many of the water scarcity issues that modern society faces: atmospheric water vapor. 1.29x1013 m3 of this life-giving resource is contained in the atmosphere, and its ubiquitous nature overcomes transportation and geographical issues which hinder traditional technologies. Water vapor can be utilized to provide drinkable water in coastal, inland, developed, or underdeveloped areas alike.
The Study
The research has focused on current advances and perspectives in the field of passive atmospheric water harvesting (AWH) technologies. Dew and fog collection using panels and nets are the conventional approach to passive water vapor collection. However, these strategies only produce modest yields and are significantly influenced by environmental conditions. Additionally, during droplet liquefaction, heat is produced, which has an adverse effect on condensation.
Strategies for improving passive AWH performance investigated recently include developing novel engineered materials and surfaces for condensers, improving dewing occurrence by cooling condensing substrates, and using sorbent-assisted systems to overcome environmental conditions and concentrate moisture, increasing the yield.
The study has investigated recent advances in strategies for surface morphology and wettability optimization of passive AWH systems, substrate cooling, and assistive sorbent-based systems. The performance of each of these systems has been summarized in the review, and the authors have analyzed their advantages and limitations in-depth.
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It has been noted in the research that natural dew condensers cannot perform as well as humidifier-assisted setups, but by utilizing aerodynamics, the yield can be improved by orders of magnitude. The use of advanced materials with high infrared emissivity can help to improve yield when used as AWH components. An interesting recent development is a dual daytime/night-time desalination/dew collection system that utilizes multi-walled carbon nanotubes on flexible substrates.
A Proposed Multifunctional AWH Panel
A key contribution of the current paper to the field of atmospheric water harvesting is the introduction of a proposed multifunctional passive AWH panel. The system uses black silicon, which has shown promise as a material in previous bodies of research, and the system was inspired by solar energy harvesting panels. The proposed panel possesses a unique surface morphology and superior radiative cooling ability.
Black silicon has a radiative cooling effect at night which promotes condensation formation. Being superhydrophobic, condensed water vapor easily runs off the surface of the black silicon panel into a collector, thus reducing evaporative loss. A pluviometer evaluates the water harvesting capacity of the system.
During the daytime, the system can absorb solar irradiation, and due to the silicon-based substrate, it can act as a solar panel to harvest energy, making the system self-sufficient and multifunctional; additionally, the optimized microstructure surface can help with self-cleaning. The proposed harvester can be installed in any geographical locale.
Further research into the mechanical robustness and self-cleaning abilities and using windshields or hollow structures to mitigate wind-induced heat convection have been identified as future opportunities to improve the performance of the proposed device. Overall, the research has provided a significant review of the current progress in passive AWH strategies and technologies which will help to solve the pressing issue of water scarcity in the 21st century.
Further Reading
Liu, X, Beysens, D & Bourouina, T (2022) Water Harvesting from Air: Current Passive Approaches and Outlook ACS Materials Letters 4 pp.1003-1024 | pubs.acs.org. Available at: https://pubs.acs.org/doi/10.1021/acsmaterialslett.1c00850
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