ATMOSPHERIC WATER GENERATION: CONCEPTS AND CHALLENGES

G. Raveesh

R. Goyal

S.K. Tyagi

Department of Energy Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India


The lack of access to clean water is a growing global challenge that threatens human lives and sustainable development. Many countries are facing their worst-ever water crisis and the situation will be more severe in the coming years owing to many factors including but not limited to growing population, urbanization, unsustainable use of existing water resources, and climate change. Therefore, new technological solutions are required to address the global water crisis on priority. Atmospheric water generation that generates water from the atmospheric humidity is an emerging solution to water scarcity and could be a lifeline to people living in water-stressed and landlocked regions. However, several challenges must be addressed, including energy consumption, cost, operation window, and scalability to ensure the viability and sustainability of atmospheric water generation as a solution for water scarcity. Changes in operating environment conditions have a profound impact on the performance of atmospheric water generation systems, thereby limiting their extensive application. In all climates, an energy-efficient and scalable approach to extract water from the atmosphere is not yet available. The techniques involved in generating water from the atmosphere are still in the nascent stage, providing enormous opportunities to further refine the technology. This article presents a brief introduction to the concept of atmospheric water generation, the underlying principles of various techniques involved and its practical challenges.

1. INTRODUCTION

Water scarcity represents an emerging, severe global concern. According to research conducted by the World Health Organization, one-third of the world's population lacks access to clean water and the situation is expected to be exacerbated due to climate change by the year 2050 (Boretti and Rosa, 2019). New technological interventions are urgently required to minimize groundwater stress and complement the suite of existing water generation technologies. In this context, atmospheric water generation (AWG), which extracts water from the humidity present in the atmosphere, could be viewed as a futuristic approach to address the issue of water scarcity. The water present in the atmosphere can be considered as a nearly inexhaustible resource for fresh water because at any given time approximately 13,000 km3 of fresh water is in the atmosphere (Gleick, 1993; Graham et al., 2010), which is naturally replenished through the hydrological cycle.

The amount of water present in the local atmosphere at any given time depends on the temperature, pressure, and relative humidity (RH) of a location. However, if the ideal gas law is assumed, then the amount of water present in air at different temperatures and RHs can be roughly estimated, as shown in Fig. 1. AWG offers several advantages over conventional, centralized technologies, such as minimal infrastructure, ease of installation, low space requirements, and rapid emergency response capability. In addition, AWG processes will not negatively impact the environment since the daily hydrological cycle will replenish any water extracted from the atmosphere. The recent pandemic has taught us the critical importance of clean water availability and hygiene, particularly for people living in landlocked or water-stressed rural locations. Given the need for new fresh water production technologies, this article attempts to provide a brief overview of the concept of atmospheric water generation, its working principles, and the challenges involved.

Amount of water present in air at different temperatures and relative humidities

Figure 1. Amount of water present in air at different temperatures and relative humidities

2. DIFFERENT APPROACHES FOR ATMOSPHERIC WATER GENERATION

Potentially extractable water is present in our atmosphere in the form of fog and water vapor. The approaches to extract atmospheric water can be broadly classified as follows: active refrigeration, passive cooling and collection, vapor concentration, and hybrid approaches (Raveesh et al., 2021), as shown in Fig. 2. If moisture-laden air is cooled below its dew point temperature by any of these means, water will start condensing. The majority of commercial AWG systems use active refrigeration with conventional vapor compression refrigeration to cool and condense the air. The difference between the air temperature and dew point temperature directly indicates the sensible cooling requirement and energy investment. Thus, the performance of systems using AWG technology critically depends on the climate, where these systems perform best in hot and humid environments. Under hot and humid conditions, the water content in the air will be high, and the dew point temperature will be close to the air temperature. Thus, the energy input required is less when the air has a high dew point, such that most of the energy supplied will be utilized in the condensation of moist air. When the dew point temperature of the air falls below 2°C, active refrigeration techniques are not feasible because of frost formation and the very high level of specific energy consumption. For comparison, under favorable conditions the specific energy consumption may fall below 1 kWh/L; however, under unfavorable conditions it may go up to 6 kWh/L (Bagheri, 2018). More specific details on the four AWG approaches are given in the following subsections.

Classification of AWG approaches

Figure 2. Classification of AWG approaches

2.1 Active Refrigeration

The active refrigeration approach is straightforward, which is based on well-established cooling technology. If moisture-laden air is drawn past an actively cooled coil, fin, or surface that is maintained below its dew-point temperature, condensation occurs (Al-Farayedhi et al., 2014; Habeebullah, 2009; Patel et al., 2020; Raveesh et al., 2023; Suranjan Salins et al., 2020). To facilitate a continuous process, as the air comes in contact with the cooling heat exchanger surface, the latent heat load must be continuously removed by the underlying cooling fluid (i.e., chilled water or an evaporating refrigerant). A schematic representation of an AWG system based on vapor compression refrigeration (VCR) is shown in Fig. 3. Commercial AWG systems are mostly based on VCR technology with additional components such as inlet air filters, draft fans, food-grade coating over the cooling coil, and water treatment units. The amount of water generation depends on many factors such as the air flow rate, heat exchanger effectiveness, cooling capacity, temperature, and humidity of the inlet air. Performance evaluation of VCR-based AWG systems shows that it attains optimal performance under hot and humid conditions with minimum ambient dew point temperature requirements above 2°C (Bagheri, 2018). The main performance indicators of AWG systems are the water generation rate (L/h) and specific energy consumption (Wh/L). The main limitations of active refrigeration systems are their inability to work under low RH conditions and their high specific energy consumption requirements under unfavorable climate conditions. The sensible load on the AWG system due to the larger temperature difference between the air and its dew point represents an overburden to the AWG system; in that type of climate a lot of energy is required just to pre-cool the air to its dew point (Peeters et al., 2020; Salehi et al., 2020). In addition, when the dew point temperature falls below 0°C under cold and dry conditions the cooling coils become covered with frost, which adds resistance to the heat and mass transfer rates and significantly reduces the efficiency of the AWG system. Small-scale portable AWG systems can be designed and fabricated using thermoelectric cooling, particularly for army or emergency applications (see Fig. 4), but their scalability is limited (Kadhim et al., 2020; Milani et al., 2011; Shourideh et al., 2018). Overall, the active refrigeration AWG configuration is best employed in hot and humid climates and should be avoided in dry and cold climates due to the drastic performance degradation under unfavorable climate conditions.

Schematic representation of a VCR-AWG system (Reprinted from Raveesh et al. with permission from Elsevier, Copyright 2023)

Figure 3. Schematic representation of a VCR-AWG system (Reprinted from Raveesh et al. with permission from Elsevier, Copyright 2023)

Thermoelectric cooling-based AWG system

Figure 4. Thermoelectric cooling-based AWG system

2.2 Passive Collection

In the passive collection approach, water generation is achieved through passive means with net-zero energy investment. Fog and dew harvesting methods are the two main techniques that have been proposed under this approach (Fessehaye et al., 2014; Korkmaz and Kariper, 2020; Sharan et al., 2017; Tomaszkiewicz et al., 2015). In fog harvesting, water in the form of fog or aerosol droplets is trapped in specially designed structures called fog nets (see Fig. 5). Fog nets, which are usually vertical mesh-like structures, allow fog droplet growth by coalescence and the removal of large drops is driven by gravity to catchments that collect them for further use. This approach is inspired by nature since several plants and animals have been found to have specialized materials capable of droplet harvesting (Park and Kim, 2019; Wen et al., 2019). Engineered fog harvesting is dependent on the geographical nature of the location where fog occurrence is predominant. Thus, the application of this technique is limited to hills and valleys, which are typically located near water bodies that are prone to frequent and favorable fog occurrence. Many countries with favorable topology have successfully implemented this technology (Abdul-Wahab et al., 2010; Clus et al., 2013). Additionally, since fog occurrence is seasonal in nature, the capacity factor for such systems is relatively low and the annual average daily productivity will be very low. Still, fog harvesting is promising due to its passive nature with practically zero energy investment.

Schematic illustration of fog harvesting

Figure 5. Schematic illustration of fog harvesting

Dew water harvesting is another passive approach, which is based on the radiative sky cooling concept (see Fig. 6). Radiative sky cooling allows a surface of interest to reach temperatures below the ambient environment by exchanging radiation with the cold temperatures of outer space via selective emissivity in the atmospheric transparency window in the infrared wavelength range of 8–13 μm. In this technology, the formation of dew typically happens during the night and depends on many factors such as the wind velocity, air temperature, RH, cloud cover, and dew forming material structure and properties. The cooling power by natural radiation is limited to less than 100 W/m2 (Beysens, 2018); hence, large surface areas at favorable locations with high RH at night are required for the successful deployment of this approach. The water output from the passive approaches will be low; hence, it is usually quantified based on the daily productivity per unit area. Radiative sky cooling can also work during the daytime if solar inputs are blocked and specially designed materials with selective optical properties are used (Chen et al., 2016; Raman et al., 2014). The passive cooling approach is again geographically limited and faces some cost issues due to the large surface area requirements, but it offers an energy efficient AWG option, even under low RH and daytime conditions. The major challenge to this approach involves the development of cost effective and scalable metamaterials suitable for efficient radiative cooling.

Schematic illustration of dew water harvesting

Figure 6. Schematic illustration of dew water harvesting

2.3 Vapor Concentration

In the vapor concrentration approach, water vapor from the atmosphere is selectively captured in a hygroscopic material and then regenerated either by using an external condenser or thermal energy to condense the water vapor captured under ambient conditions. The heart of this type of AWG approach is the sorbent material, which captures the water vapor through physisorption or chemisorption. The advantage of this approach lies in its ability to operate under low RH conditions (< 20%), which extends the application of AWG in arid climates. If the thermal energy or heat required for regeneration of the sorbent is provided by concentrating the solar radiation and condensation at ambient temperature, then this approach can offer a sustainable, passive AWG solution. Although many conventional hygroscopic materials exist, such as silica gel, zeolite, etc., the water uptake under low RH conditions, high regeneration temperature, sorbent stability, sorption kinetics, and cyclic operation are the major challenges to its potential use in AWG. Sorption-based AWG systems can work in continuous mode with multiple sorption beds or sorption bed rotation and batch mode with multiple sorption cycles in a day or in discontinuous mode with nighttime adsorption and daytime desorption (Poredoš et al., 2022). A schematic illustration of a typical sorption-based AWG system with discontinuous operation is shown in Fig. 7.

Nighttime adsorption (a) and daytime desorption (b) of a sorption-based AWG system

Figure 7. Nighttime adsorption (a) and daytime desorption (b) of a sorption-based AWG system

In a typical adsorption process, the process is utilized at night when the RH is comparatively high, even in desert regions. Air flows over a sorption bed placed inside an enclosure through the openings. Heat for adsorption is released into the atmosphere at night with the natural temperature difference. During the daytime, the glass enclosure is closed and solar radiation enters into the enclosure through a transparent cover. The solar radiation heats up the sorption bed, resulting in gradual desorption of the water vapor from the bed. Since this is a closed system, the local RH inside the unit becomes high, resulting in a dew point temperature that is higher than the ambient temperature and thereby facilitating condensation under the ambient condition. To obtain even more condensate, an external air-cooled condenser or a small capacity thermoelectric cooler unit may be incorporated. Research on the sorption-based AWG approach has gained more traction in recent years and many high-performance novel materials such as metal–organic frameworks (Hanikel et al., 2020; Kim et al., 2017), hydrogels (Li et al., 2018), and porous composites (Ejeian et al., 2020; Wang et al., 2018) have been developed and characterized specifically for this application. Each material differs in its sorption performance under different RH conditions and the wide array of material options can be considered as a pathway toward optimization in the development of sustainable AWG. The development of recently reported novel sorbents involves a complex synthesis process, in which mass transfer and scalability are challenging issues that still need to be addressed (Raveesh et al., 2021).

2.4 Hybrid Approaches

A universal AWG approach is to use multiple technologies in a single device such that it can work effectively under a wide operational band and ensure scalable water generation with minimal energy consumption (Raveesh et al., 2021). Hybrid AWG approaches pull together multiple AWG techniques to obtain a synergistic effect and improve overall efficiency (Shafeian et al., 2022). Desiccant wheels, solar inputs, and brine evaporation can be used to pre-treat the air to create hot and humid conditions before the air enters the cooling and condensation components, resulting in greater water production. Thus, active refrigeration systems can be operated under their best operating conditions with mixed pre-treatment systems to enable good performance regardless of weather conditions. Tu and Hwang (2019) showcased a configuration that had multiple desiccant wheels before a VCR-AWG system to increase the humidity ratio of the incoming air. Bahrami and Bagheri (2018) used a sorption column and a specially designed operational strategy with a VCR-AWG system. Kwan et al. (2020) fed exhaust flue gas from a fuel cell to an AWG system. In another study, Ghosh et al. (2015) demonstrated harvesting of water from cooling tower plumes using a fog net. Although the possibilities for such hybrid approaches have not been exhausted, they offer an excellent chance to make a breakthrough in the currently available AWG technologies.

3. CONCLUSIONS

In this paper, a brief overview of atmospheric water generation, the working principles for various approaches, and their importance and challenges, given the current concerns regarding global water scarcity, has been presented. This overview indicates that AWG technologies are still in their earlier stages, giving immense scope for further exploration. The authors suggest that the active refrigeration method is the simplest and most straightforward approach; however, its operation fails when the dew point temperature is at 2°C or below. Thus, this approach suffers from severe performance degradation under unfavorable climate conditions. Passive approaches offer net-zero energy investment, and some working options are available; however, they do not generate a lot of water. The locations of the potential deployment of these approaches are limited. One of the most promising, emerging approaches is vapor concentration with selective sorbents, and an international campaign is underway in the development of novel materials for atmospheric water capture. Since these materials can potentially work well under low RH conditions, passive and solar-assisted configurations may eventually bring these technologies to the forefront commercially. Finally, several hybrid approaches have been proposed to cover a broad range of environmental conditions. These approaches represent a promising pathway toward improving the performance of AWG systems in practice. The feasible production capacity of water from active refrigeration employing VCR is the highest among all of the approaches, which could have a production capacity in the range of 20–2000 L/day (Maithriaqua, n.d.; Watergen, n.d.) depending on the cooling capacity of the refrigeration unit. The capacity levels of fog and dew harvesting techniques are limited to 1.5–12 and 0.3–0.6 L/day/m2, respectively. Sorption-based AWG systems could generate water in the range of 1–3 L/day/m2 (Raveesh et al., 2021; Tu et al., 2018). The reported AWG approaches are still in nascent stages and have not yet emerged as energy efficient water solutions. Clearly, there is ample scope for innovation and further development of AWG technologies. If these techniques can be optimized and deployed in practice, they will provide critical freshwater production alternatives.

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Referencias

  1. Abdul-Wahab, S.A., Al-Damkhi, A. M., Al-Hinai, H., Al-Najar, K.A., and Al-Kalbani, M.S. (2010) Total Fog and Rainwater Collection in the Dhofar Region of the Sultanate of Oman during the Monsoon Season, Water Int., 35(1): 100–109.
  2. Al-Farayedhi, A.A., Ibrahim, N.I., and Gandhidasan, P. (2014) Condensate as a Water Source from Vapor Compression Systems in Hot and Humid Regions, Desalination, 349: 60–67. DOI: 10.1016/J.DESAL.2014.05.002
  3. Bagheri, F. (2018) Performance Investigation of Atmospheric Water Harvesting Systems, Water Resour. Ind., 20: 23–28.
  4. Bahrami, M. and Bagheri, F., inventors; Simon Fraser University, assignee (2018) Hybrid Atmospheric Water Generator, U.S. Patent Application 15/576,600, filed July 26, 2018.
  5. Beysens, D. (2018). Dew Water, Gistrup, Denmark: River Publishers.
  6. Boretti, A. and Rosa, L. (2019) Reassessing the Projections of the World Water Development Report, NPJ Clean Water, 2(1): 15. DOI: 10.1038/s41545-019-0039-9
  7. Chen, Z., Zhu, L., Raman, A., and Fan, S. (2016) Radiative Cooling to Deep Sub-Freezing Temperatures through a 24-h Day–Night Cycle, Nat. Commun., 7(1): 1–5.
  8. Clus, O., Lekouch, I., Muselli, M., Milimouk-Melnytchouk, I., and Beysens, D. (2013) Dew, Fog and Rain Water Collectors in a Village of S-Morocco (Idouasskssou), Desalin. Water Treat., 51(19–21): 4235–4238. DOI: 10.1080/19443994.2013.768323
  9. Ejeian, M., Entezari, A., and Wang, R.Z. (2020) Solar Powered Atmospheric Water Harvesting with Enhanced LiCl/MgSO4/ACF Composite, Appl. Therm. Eng., 176: 115396.
  10. Fessehaye, M., Abdul-Wahab, S.A., Savage, M.J., Kohler, T., Gherezghiher, T., and Hurni, H. (2014) Fog-Water Collection for Community Use, Renew. Sustain. Energy Rev., 29: 52–62.
  11. Ghosh, R., Ray, T.K., and Ganguly, R. (2015) Cooling Tower Fog Harvesting in Power Plants—A Pilot Study, Energy, 89: 1018–1028.
  12. Gleick, P.H. (1993) Water in Crisis, vol. 100, New York: Oxford University Press.
  13. Graham, S., Parkinson, C., and Chahine, M. (2010) The Water Cycle, NASA Earth Observatory. https://earthobservatory.nasa.gov/features/Water
  14. Habeebullah, B.A. (2009) Potential Use of Evaporator Coils for Water Extraction in Hot and Humid Areas, Desalination, 237(1–3): 330–345. DOI: 10.1016/j.desal.2008.01.025
  15. Hanikel, N., Prévot, M.S., and Yaghi, O.M. (2020) MOF Water Harvesters, Nat. Nanotechnol., 15: 348–355.
  16. Kadhim, T.J., Abbas, A.K., and Kadhim, H.J. (2020) Experimental Study of Atmospheric Water Collection Powered by Solar Energy Using the Peltier Effect, IOP Conf. Ser.: Mater. Sci. Eng., 671: 12155.
  17. Kim, H., Yang, S., Rao, S.R., Narayanan, S., Kapustin, E.A., Furukawa, H., Umans, A.S., Yaghi, O.M., and Wang, E.N. (2017) Water Harvesting from Air with Metal-Organic Frameworks Powered by Natural Sunlight, Science, 356(6336): 430–434.
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  19. Kwan, T.H., Shen, Y., Hu, T., and Pei, G. (2020) The Fuel Cell and Atmospheric Water Generator Hybrid System for Supplying Grid-Independent Power and Freshwater, Appl. Energy, 279: 115780.
  20. Li, R., Shi, Y., Alsaedi, M., Wu, M., Shi, L., and Wang, P. (2018) Hybrid Hydrogel with High Water Vapor Harvesting Capacity for Deployable Solar-Driven Atmospheric Water Generator, Environ. Sci. Technol., 52(19): 11367–11377.
  21. Maithriaqua. (n.d.). Retrieved August 25, 2020, from https://www.maithriaqua.com/.
  22. Milani, D., Abbas, A., Vassallo, A., Chiesa, M., and Bakri, D. (2011) Evaluation of Using Thermoelectric Coolers in a Dehumidification System to Generate Freshwater from Ambient Air, Chem. Eng. Sci., 66(12): 2491–2501. DOI: 10.1016/j.ces.2011.02.018
  23. Park, J.K. and Kim, S. (2019) Three-Dimensionally Structured Flexible Fog Harvesting Surfaces Inspired by Namib Desert Beetles, Micromachines, 10(3): 201.
  24. Patel, J., Patel, K., Mudgal, A., Panchal, H., and Sadasivuni, K.K. (2020) Experimental Investigations of Atmospheric Water Extraction Device under Different Climatic Conditions, Sustainable Energy Technol. Assess., 38: 100677. DOI: 10.1016/j.seta.2020.100677
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  26. Poredoš, P., Shan, H., Wang, C., Deng, F., and Wang, R. (2022) Sustainable Water Generation: Grand Challenges in Continuously Atmospheric Water Harvesting, Energy Environ. Sci., 15(8): 3223–3235.
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