Dual-use devices offer a different path for more sustainable living
Article Source: Science
Article Link: https://www.science.org/doi/10.1126/science.add1795
Photo Credit: A. Fisher / Science
A fivefold increase in cooling-related greenhouse gas emissions is expected by 2050 (1) from a combination of improved standards of living, especially in countries with humid climates, and the ongoing overall warming of the planet. Alternative approaches to efficient cooling are required to meet these needs. Although a large effort is underway to improve the energy efficiency of cooling more generally, a potentially important strategy is found by looking at the small water droplets that emerge on cold surfaces. Most people have experience with this process, from the condensation formed from air conditioners to cold drinks taken from a refrigerator. Although conventional chillers are becoming more efficient, integrating a passive radiative cooling strategy in which the sky is used as a cold sink could not only improve performance but also produce fresh water. This may help with cooling and potentially also with water scarcity issues in many regions of the world.
The water vapor dewing process has a large barrier owing to its inefficiency. An established figure of merit known as a coefficient of performance (COP) represents the ratio of useful cooling energy generated to the work provided [COP = Qcool(TE)/W(TE,TC)]. In general, the COP of chillers depends on the temperature difference between the hot (TC) and cold (TE) sides. Air conditioning requires air dehumidification, but water vapor dewing introduces a larger temperature difference and thus decreases the COP. This likely requires a reconceptualization of humidity management.
Synergistic and multifunctional effects can be realized by focusing on the air-water-energy nexus, primarily driven by water separation from the air. Pulling the water vapor from the air achieves air dehumidification while also creating a source of fresh water. Several studies already have demonstrated more than a twofold energy efficiency increase by decoupling latent from sensible cooling, predominantly in humid climate conditions (2). Splitting the water vapor–capturing process from sensible cooling sparked a pertinent question concerning the viability of dual-use devices that promote efficient cooling and freshwater generation (3). Although the dual nature of devices is relatively straightforward in terms of economic and performance efficiency for hot and humid regions, severe challenges exist for deploying these systems in hot and dry places.
Arid regions typically have abundant natural sunlight under clear sky conditions, offering a sustainable way to either increase or decrease the internal energy of a material relative to the ambient. The elevated temperature of a wet material accelerates a drying process, with water vapor released to the ambient. By contrast, the reversibility of that process is facilitated by hygroscopic porous materials that capture water from the atmosphere owing to the water concentration difference, according to Fick’s law. In 2017, a proof of concept showed water harvesting from the air in outdoor arid conditions by using a porous metal-organic framework (4). In just a few years, atmospheric water harvesting has come a long way, from diurnal devices to ones using multiple cycles per day (5), even in the desert environment (6), swiftly converging to continuously operated devices not only by sorbent–desiccant but also using daytime radiative sky cooling materials (7). Attaining the dual-use feature of existing water harvesters with cooling capabilities could potentially be accomplished by reengineering existing devices rather than reinventing materials.
Sustainable cooling in arid regions requires a two-step approach (see the figure) owing to intrinsic material and heat-transfer characteristics. Sorbent at subambient temperature lowers outdoor air temperature while simultaneously promoting water capture, culminating in cooled and dehumidified air. As the adsorption process generates heat, both sensible and adsorption heat (QADS) can be taken away by using radiative cooling (QC,RC) (8) or a desiccant-based heat pump (QC,HP) (2). The addition of the latter device with its capability of adsorbing moisture from the handled air in the evaporator introduces cooling-adsorption dehumidification that results in comfortable air conditioning. By leveraging the intrinsic heat pump capability with simultaneous cooling and heating energy production, the condensing heat (QH,HP) as a by-product is not wasted, but rather is used for the heating-desorption process (QDES) at a reduced temperature. Hence, elevated evaporation (TE → TE,HP), as well as reduced condensation (TC → TC,HP) temperatures, creates a positive impact on the COP. Thus, the latent heat load handled by low-temperature evaporation cooling (condensing dehumidification) in conventional air-conditioning devices can be managed by a system based on sorption technology in which sorbent materials could be regenerated by solar heating or condensing heat from a heat pump, having a positive impact on the device performance.
Dual-use device for cooling and water generation in arid places
Hygroscopic sorbent material captures water vapor from the outdoor air, while also lowering air temperature with a heat pump or radiative cooler. Upon saturation, sorbent materials are regenerated by using solar thermal energy or a heat pump. Released high-temperature water vapor is dewed. Humid indoor exhaust air and vapor in ambient air are also water sources, where the latter (vapor in ambient air) is dewed during the nighttime by using radiative roof coating. The continuous operation of the dual-use device relies on solar thermal storage. Radiative sky cooling materials provide a cooling effect continuously owing to the atmospheric window (8 to 13 µm) during the day.
Alternatively, solar energy (QH,SOL), coupled with technological advances in thermal energy storage, emerges as another potential source, capable of ejecting the water stored in sorbent material in a continuous manner. The final step in collecting a liquid form of water is established through a thermal pathway (QV,C) between the high-temperature water vapor and a heat sink, served by the ambient, residual cooling capacity of a heat pump (QC,HP), or radiative cooler (QC,RC). Solarpowered harvesters could play a large role in water generation (9). Radiative coatings deployed on rooftops in general may be helpful in dry regions and would take some of the burden off either dual-use devices or conventional air conditioning for buildings (10). The reduced cooling requirements during the night offer an effective solution for providing liquid water by using photon flow to outer space (11). In addition, mass recovery of exhaust indoor air with latent heat load from humans or other wet sources emerges as a different source of water in extremely dry ambient conditions. Such air-humidity recovery has already been used in spacecraft or space stations for months-long or year-round water supply to astronauts.
Variable ambient and indoor conditions throughout the year require a flexible strategy concerning device adaptability. For instance, temperature-enabled precise regulation of the metal-organic framework’s stepwise position showed promise with the prospect of operation under harsh ambient conditions with relative humidity down to 10% (12). This approach seems suitable for desiccant-based heat pumps owing to the straightforward regulation of evaporation temperature. At micro- and nanometer scales, real-time manipulation of the sorbent pore volume is poised to bring tunable water-uptake behavior as well as desorption temperature (13). Radiative coatings, by contrast, can potentially utilize switchable temperature-modulated thermal emittance (14), extending the usage scenario of the dual-use devices in providing all-year thermal comfort. On e issue that makes it more difficult to chart a path forward is the need for a consensus in evaluating devices with relevant metrics to associate useful products such as generated cooling energy and water with the provided work. Other factors, such as total and sorbent weight, besides radiative cooling, solar, and total areal footprint, might add another layer in ensuring a fair basis for comparison of competing devices. In addition, dual-use devices ought to be globally evaluated for arid and semiarid regions to clearly show potential for widespread deployment. Such an engineering opportunity requires further quantitative analysis and deeper physical insights, with clear descriptions about the theoretical and practical improvements over conventional single-use devices.
The widespread implementation of the dual-use approach may encounter various challenges. One of these is a decreased potential of sorbents to capture the water caused by lower relative humidity. An effective solution to enhance the adsorption rates and kinetics of sorbents is to use composite materials, such as MOF+LiCl (15), and regulate the evaporation temperature (12). By combining these material- and system-level strategies, the potential of the sorbents to adsorb water can be substantially improved. By contrast, lower moisture content in the atmosphere can improve the performance of radiative sky cooling materials (10). Because sustainable cooling with atmospheric water harvesting in a single dual-use device is interfaced with diverse technologies, a special effort needs to be addressed to finding the best solution for commercial attractiveness, along with a low-carbon path for sustainable living.
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