Various Approaches of AWHs
Various technology for water production from humid air [3]
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[3] Renewable and Sustainable Energy Reviews 2020, 2, 120, 109627
Collecting fog water is placing a rectangular mesh perpendicular to the wind.
In 1956, the Catholic University of the North in Antofagasta in Northern Chile conducted the first conclusive experiments with nets. Since 1987, several experiments following various ideas but on larger scales were conducted by Schemenauer and colleagues.
A viable and efficient fog extraction project should meet the following criteria:
(1) Fog must occur frequently throughout the whole year. And fog should persist for a relatively long time.
(2) Fog should contain relatively high liquid water content in arid lands.
(3) Fog collection must be carried by wind to achieve a higher efficiency.
Today, the biggest challenge for fog collection is the low efficiency.
Fog Water Collection
Dew Water Collection
Compared with fog water collection, it is necessary to be minimally affected by climatic and geographical constraints.
So, dew water collecting should be also developed to be more cost-effective than the cloud seeding in less cloudy areas.
There are two approaches: Passive radiative condenser and active dew condenser.
I. Passive Radiative Condensers
Passive radiative condensers do not need extra energy input. These condensers normally work during the night since the solar radiation in the daytime. Dew forms on surfaces when the surface temperature is lower than the dew point temperature.
To condense dews, several environmental conditions must be met [4]. (1) A high relative humidity, (2) high sky visibility to infrared radiation, and (3) low wind speed are required. However, this means that the volumes of dew formed are highly dependable on the environmental conditions. Radiative dew condensers rely solely on the physical processes that induce dew formation naturally.
To maximize water condensation without any external source of energy [4], radiative condensers can be optimized in terms of their shape, size, material (hydrophilic properties, mass, infrared emittance), and position (inclination, shading, sky exposure, and orientation).
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The maximum recorded yields of dew water in arid and semi-arid climates typically fall within a range of 0.3–0.6 kg/day/m2 of surface area. Additionally, yields will remain low, since the scaling up the condenser size from 1 m2 has been found to decrease efficiency.
Radiative cooling ability available for condensation, the upper limit of dew yield is 0.8 kg/day/m2
This process is limited by (1) the rate of radiative heat exchange, (2) the weather conditions, and (3) the surface properties.
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[4] Sustainable Water Resources Management · March 2016, DOI: 10.1007/s40899-015-0038-z
II. Active Dew Condensers
Active dew condensers typically use cooling condensation or regenerative desiccation to bring trapped air to the dew point temperature, thus causing the water vapor to condense for collection [4].
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Rapid and wide innovations did not occur until the commercialization of mechanical refrigeration after the 1980s.
Active condensers work in a manner similar to that of a dehumidifier, which extracts water from the air.
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Inefficient heat-exchange process and the low amount of water obtained by small units
The yields can reach 20 L/day by portable devices and up to 200,000 L/day by larger agricultural water devices.
In general, the efficiency is 650–850 Wh (electricity)/kg : the best efficiency is ca. 250 Wh (electricity)/kg.
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To reduce the fossil fuel energy cost, researchers began to use solar energy to drive the AWHs. There are two approaches:
(1) sorption-regeneration-condensation method, and (2) dew water harvesting by a solar-powered sorption chiller.
(a) Sorption-regeneration-condensation systems (Desiccant)
Desiccants capture the moisture from the humid air during nighttime and then desorb the water out of the desiccant under the sun. So, the regeneration process occurs during daytime utilizing solar heat.
- It was preferred because of its system continuity à All day harvesting water.
- However, an additional large cooling surface is necessary to cool the reactivated solution.
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There are three kinds of basic configurations of the sorption-regeneration-condensation systems:
(1) Glass-covered greenhouse sorber, (2) Sandwich plate sorber, (3) Packed columns sorber.
At night, the window or the glass cover is opened and the desiccants capture the moisture in the feed air by natural or forced convection. During daytime, the sorber absorbs the incident solar radiation and thus the desiccant temperature increases [5].
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[5] Environ. Sci. Technol. 2021, 55, 10, 6542–6560
Schematic illustration of sorption-based atmospheric water harvesting (SAWH)
(b) Solar photovoltaic-powered thermoelectric condensers
The Peltier cooler is a solid-state active heat pump that transfers electrical heat from one side to the other side [6]. Peltier machine or the thermoelectric generator is used to reach the dew point temperature.
The collection of water from atmospheric humid air with a Peltier module is based on the concept of condensation phenomena on the cold surface.
Water condensation systems in which the solar panel is used as an energy source have a significant economic benefit [7], because the power source is free and the need for maintenance is virtually nonexistent.
Solar thermoelectric refrigeration system Scheme representing the water collector
[6] Journal of Cleaner Production 209 (2019) 1376-1395
[7] IOP Conf. Series: Materials Science and Engineering 671 (2020) 012155
(c) HVAC (heating, ventilation, and air-conditioning) systems
An integration of an air-conditioning system and a water-harvesting system could be a potential solution to achieve optimized energy consumption and sustainable use of resources.
The water production can be considered almost as a free benefit, or in other words is obtained as the ‘‘by-product’’ of a smart HVAC system design [8].
Example of an AWG integrated system scheme
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[8] Energies 2021, 14, 8528
Water production with nature structures
Currently, a great deal of highly effective fog harvesting methods based on optimizing the physical structure of the fog harvesting devices (FHDs) has been reported, such as kirigami cutting, electrospinning, 3D printing, etc. In addition, inspired by the unique wettability of the Namib desert beetle, much attention has been paid on the surface wettability modification, including spray coating, selective grafting, photomask, etc [9].
However, the fog capture efficiency of FHDs inspired by a single creature is extremely limited. à The development of FHDs is undergoing the process from single biomimetic to multiple biomimetic.[9].
The state-of-the-art development of fog harvesting devices (FHDs): introduction of typical creatures, fog harvesting mechanisms, and devices fabrication inspired from single creature to multiple creatures.
[9] Adv. Funct. Mater. 2022, 2200359