Abstract:A radiative vapor condenser sheds heat in the form of infrared radiation and cools itself to below the ambient air temperature to produce liquid water from vapor. This effect has been known for centuries, and is exploited by some insects to survive in dry deserts. Humans have also been using radiative condensation for dew collection. However, all existing radiative vapor condensers must operate during the nighttime. Here, we develop daytime radiative condensers that continue to operate 24 h a day. These daytim… Show more
“…For example, at the most favorable condition for the convective condenser (black solid line in Figure 16C), the condensation rate of the radiative condenser almost doubles that of the convective condenser, reaching 2.5 L m À2 h À1 (red solid curve in Figure 16C), well above the theoretical limit of the one-sun evaporation rate. 133 Such a high condensation rate will also increase the vapor pressure gradient inside the water-harvesting system, further facilitating the water-production cycle.…”
Section: Harvesting Water From the Atmospherementioning
confidence: 99%
“…To analyze the theoretical upper bound of condensation rate, we assume a relative humidity of 100% throughout our calculation. Source : Reproduced with permission: Copyright 2021, PNAS 133 (D) Relative humidity dependence of the mass flux of dew‐harvesting for three different emitters under two representative scenarios: h = 2 W m −2 K −1 and 8 W m −2 K −1 . The near‐ideal emitter (blue) surpasses its blackbody counterpart (black) in both scenarios.…”
Section: Applications and Challenges Of Pdrcmentioning
Passive daytime radiative cooling (PDRC) dissipates terrestrial heat to the extremely cold outer space without using any energy input or producing pollution. It has the potential to simultaneously alleviate the two major problems of energy crisis and global warming. In this review, we summarize general strategies implemented for achieving PDRC and various applications of PDRC technologies. We first introduce heat transfer processes involved in PDRC, including radiative and nonradiative heat transfer processes, to evaluate the PDRC performance. Subsequently, we summarize the general material designs used for controlling PDRC performance, such as tuning the thermal mid-infrared emittance and solar reflectance. Finally, we discuss the diverse applications of PDRC technologies to overcome problems in space cooling, solar cell cooling, water harvesting, and electricity generation.
“…For example, at the most favorable condition for the convective condenser (black solid line in Figure 16C), the condensation rate of the radiative condenser almost doubles that of the convective condenser, reaching 2.5 L m À2 h À1 (red solid curve in Figure 16C), well above the theoretical limit of the one-sun evaporation rate. 133 Such a high condensation rate will also increase the vapor pressure gradient inside the water-harvesting system, further facilitating the water-production cycle.…”
Section: Harvesting Water From the Atmospherementioning
confidence: 99%
“…To analyze the theoretical upper bound of condensation rate, we assume a relative humidity of 100% throughout our calculation. Source : Reproduced with permission: Copyright 2021, PNAS 133 (D) Relative humidity dependence of the mass flux of dew‐harvesting for three different emitters under two representative scenarios: h = 2 W m −2 K −1 and 8 W m −2 K −1 . The near‐ideal emitter (blue) surpasses its blackbody counterpart (black) in both scenarios.…”
Section: Applications and Challenges Of Pdrcmentioning
Passive daytime radiative cooling (PDRC) dissipates terrestrial heat to the extremely cold outer space without using any energy input or producing pollution. It has the potential to simultaneously alleviate the two major problems of energy crisis and global warming. In this review, we summarize general strategies implemented for achieving PDRC and various applications of PDRC technologies. We first introduce heat transfer processes involved in PDRC, including radiative and nonradiative heat transfer processes, to evaluate the PDRC performance. Subsequently, we summarize the general material designs used for controlling PDRC performance, such as tuning the thermal mid-infrared emittance and solar reflectance. Finally, we discuss the diverse applications of PDRC technologies to overcome problems in space cooling, solar cell cooling, water harvesting, and electricity generation.
“…Therefore, this type of colored radiative cooling materials requires an even more stringent spectral selectivity than white cooling materials. [80][81][82][83][84][85][86][87][88][89][90] On the other hand, due to this inevitable optical absorption, the cooling power of these colored absorbing surfaces is generally lower than that obtained by white materials. Next, we will discuss another strategy to enable colorful radiative cooling surfaces with structural colors.…”
Daytime radiative cooling has attracted extensive research interest due to its potential impact for energy sustainability. To achieve subambient radiative cooling during the daytime, a white surface that strongly scatters incident solar light is normally desired. However, in many practical applications (e.g., roofing materials and car coatings), colored surfaces are more popular. Because of this, there is a strong desire to develop colorful surfaces for radiative cooling. We summarize the general design criteria of radiative cooling materials with different colors and discuss the limitations in cooling performance. Major efforts on this specific topic are reviewed with some suggested topics for future investigation.
“…The major drawbacks of condensation include the requirement for high relative humidity (RH) (13,14), e.g. fog harvesting, and/or sizeable energy input for cooling (15,16), which are not viable for arid and developing regions. In contrast to condensation, sorption-based AWH systems can work over a much wider RH range exploiting the hygroscopicity of adsorbents.…”
Emerging atmospheric water harvesting (AWH) technologies promise water supply to underdeveloped regions that have no access to liquid water resources. The prevailing AWH systems, including condensation- or sorption-based, mostly rely on a single mechanism and thus have a limited range of working conditions and inferior performance. In this study, we synergistically integrate multiple mechanisms, including thermosorption effect, radiative cooling, and multiscale cellulose-water interactions, and demonstrate a low-cost (~ 4 USD kg− 1) and high performance (up to 6.75 L kg− 1 day− 1 in field tests) AWH system requiring zero active energy input. The proposed system consists of a highly scalable and sustainable cellulose scaffold impregnated with hygroscopic deliquescent lithium chloride (LiCl) salt. Cellulose scaffold and coated LiCl synergistically interact with water at different scales from molecular, to nanometer, and micrometer scales, providing a fast harvesting rate and high yield over a wide operation range. Iterating radiative cooling and solar heating workflow achieves simultaneous enhancement of water capture and release through the so-called temperature-swing strategy. The captured water in return facilitates radiative cooling due to the intrinsically high infrared (IR) emissivity of the LiCl-cellulose composite. With our simple yet effective material design, the AWH working range extends to lower than ~ 10% relative humidity (RH), and the water uptake in the controlled lab test reaches 16 kg kg− 1 at 90% RH (sample at 19°C, i.e. 6°C below 25°C ambient temperature). In addition, we propose a theoretical model capable of elucidating the experimental water uptake curves and demonstrating the synergy among cellulose-LiCl-water-energy interaction. The promising performance emphasizes the potential of involving multiple AWH mechanisms and points to an alternative pathway to stimulate future improvement.
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