Passive daytime radiative cooling: Principle, application, and economic analysis
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PERSPECTIVE Passive daytime radiative cooling: Principle, application, and economic analysis
Yuan Yang, Program of Materials Science and Engineering, Department of Applied Physics and Applied Mathematics, Columbia University, New York 10027, USA Yifan Zhang, Program of Materials Science and Engineering, Department of Applied Physics and Applied Mathematics, Columbia University, New York 10027, USA Address all correspondence to Yuan Yang at [email protected] (Received 24 March 2020; accepted 5 May 2020)
ABSTRACT Passive daytime radiative cooling (PDRC) is an electricity-free method for cooling terrestrial entities. In PDRC, a surface has a solar reflectance of nearly 1 to avoid solar heating and a high emittance close to 1 in the long-wavelength infrared (LWIR) transparent window of the atmosphere (wavelength λ = 8–13 μm) for radiating heat to the cold sky. This allows the surface to passively achieve sub-ambient cooling. PDRC requires careful tuning of optical reflectance in the wide optical spectrum, and various strategies have been proposed in the last decade, some of which are under commercialization. PDRC can be used in a variety of applications, such as building envelopes, containers, and vehicles. This perspective describes the principle and applications of various PDRC strategies and analyzes the cost, and economic and environmental consequences. Potential challenges and possible future directions are also discussed. Keywords: photonic structures; radiative cooling; thermal management
Background and principle of radiative cooling Cooling is critical to a wide range of human activities, such as food preservation, air conditioning, and large-scale computation. Currently, compression-based cooling systems are prevalently used for cooling. However, they consume substantial amounts of electricity and generate a large quantity of CO2. The typical gaseous media used in compression is either ozonedepleting or has a strong greenhouse effect.1 Moreover, such a cooling strategy only moves heat from one location to another on the earth surface, together with converting work to heat. Therefore, the net effect is actually heating instead of cooling, leading to various issues such as the urban heat island (UHI) effect and thermal pollution.2 These issues are getting worse under global warming, which requires more energy for cooling. Hence, inexpensive and eco-friendly approaches with net cooling capability are desirable for reducing energy costs and
DISCUSSION POINTS • What are the cost, economic benefits, and payback time of PDRC? • How to enhance the durability of PDRC? How important is it? • How to make ideal PDRC device?
associated adverse effects above. The earth itself has been deploying such an approach since its formation, which is to radiate heat to the cold outer space. This is why the earth surface’s temperature gradually decreases during the night since its temperature (∼300 K) is significantly higher than the outer space (∼3 K). In general, if a surface absorbs sunlight less than the energy it radiates to th
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