| Summary: | Subambient cooling is vital for promoting human health and well-being, driving sustainable economic growth, and minimizing food waste. Providing these benefits however, comes with major energetic and environmental costs. The over 1.6 billion air conditioning units currently in use around the world today already consume over 2000 TWh/year of electricity, or the equivalent of half the United States yearly electricity consumption, straining existing electrical systems and contributing to 3% of global CO2 emissions. With growing space cooling demand stemming from economic and population growth in hot and developing parts of the world, more efficient air-conditioning and refrigeration systems are urgently needed.
One promising solution to help address existing and future global cooling challenges is to use passive cooling solutions such as passive radiative or evaporative cooling to provide electricity-free subambient refrigeration for food produce or to improve the efficiency of existing air conditioners and refrigerators. Passive radiative cooling relies on the rejection of naturally occurring infrared radiation emission of terrestrial objects to the cold (3 K) outer space through earth’s transparent atmospheric spectral window (8-13 µm) to achieve passive cooling to subambient temperatures. On the other hand, evaporative cooling leverages the large enthalpy of vaporization of water, and the difference in water vapor concentration between a liquid surface and the ambient to generate high cooling power and subambient cooling. While promising, the cooling performance of these systems has traditionally suffered from important parasitic solar absorption during the day and parasitic heat gain from the warmer ambient air when operating at subambient temperatures. In this work, we propose to tackle these two longstanding challenges by optimizing and using polyethylene aerogel, a solar reflecting, infrared transparent, and vapor permeable thermal insulator, as a cover for radiative and evaporative coolers.
We first present the development, characterization, and optimization of polyethylene aerogels to achieve a low thermal conductivity material with high solar reflectance and infrared transmittance. We then theoretically and experimentally investigate the benefits of using polyethylene aerogel covers in outdoor radiative coolers exposed to direct sunlight. We demonstrate significant passive cooling below the ambient temperature and high subambient cooling power over a continuous 24h period. Next, we show how ZnS nanoparticles inside polyethylene aerogel covers can help increase the solar reflectance of the cover, improving the daytime cooling performance of radiative coolers. Next, we propose a hybrid cooling architecture combining passive evaporative and radiative cooling, leveraging heat rejection mechanisms of both approaches to achieve lower subambient and higher cooling power passive cooling. Finally, we explore the potential impact of our proposed hybrid evaporative-radiative cooler in applications such as off-grid food produce storage and building air-conditioning and refrigeration systems. We show that our passive hybrid cooler can meaningfully extend the lifetime of perishable fruit and vegetables and provide important energy savings for cooling and refrigeration in commercial buildings across the United States with low water consumption. Successful development and commercialization of our hybrid cooling structure have the potential to reduce food-related wastes in developing countries while reducing building cooling energy and water consumption, and CO2 emissions.
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