|Fig. 1: Top: Theoretical wavelength and intensity ranges of short-wave and long-wave radiation. Bottom: Theoretical transmittance of light through the atmosphere versus wavelength. The region of high transmittance between 8 and 13 μm (highlighted green) is the "atmospheric window," which selective radiators use to efficiently send energy into space. (Figure created using data calculated from HITRAN2004 spectroscopic database. (Source: Wikimedia Commons)|
Radiative cooling has been extensively examined as a means of energy-efficient cooling for buildings. The principle uses outer space as a practically infinite energy sink by emitting energy away from man-made structures through the atmosphere. The transmission of energy is caused by the emission of radiation over a spectrum of different wavelengths - a property exhibited by all surfaces - and by the significant difference between the temperature of space and that of objects on the Earth. This approach to cooling is "passive" in the sense that no additional energy input is necessary to induce cooling; rather, radiative heat transfer occurs continuously and spontaneously. But while the emission of radiation from surfaces like rooftops and driveways is constant, achieving cooling requires a net negative change in radiative energy. This condition means that during the daytime, incoming solar radiation typically overpowers outgoing radiation to space and creates a net gain of energy, heating the surface. For this reason, passive radiative cooling is most commonly used during the nighttime, when outgoing radiation to space is much greater than incoming radiation. 
An object radiates energy at a spectrum of wavelengths determined by the temperature of the object. The objects in question within radiative cooling systems can be simplified into two categories: hot objects (e.g. the sun) that emit short-wave radiation, and cold objects (e.g. buildings, Earth's atmosphere) that emit long-wave radiation.  (See Fig. 1) Radiative coolers on Earth's surface, having cool temperatures (relative to objects like the sun), emit long-wave radiation towards the atmosphere and space. Because the atmosphere is even colder, the amount of long-wave radiation it emits towards the surface is much smaller in magnitude than the long-wave radiation emitted from the surface. This means that at night, when there is no short-wave solar radiation penetrating the atmosphere and affecting the cooling apparatus, the difference in radiative emissions between the sky and the apparatus is sufficient to achieve significant cooling. 
The properties of the cooling apparatus can be designed to enhance this difference in net radiated energy between the sky and the apparatus. To do so, the cooler takes advantage of the so-called "atmospheric window," the spectral region of wavelengths between 7.9 and 13 μm where the atmosphere exhibits relatively low absorption.  (See Fig. 1) Because the atmosphere is mostly transparent to these wavelengths and therefore does not radiate them down to the Earth's surface, materials that selectively absorb and emit radiation between 7.9 and 13 μm yield a much greater difference between incoming and outgoing radiation. By fabricating specialized nano-materials with the desired emission spectra, cooling apparatuses can be designed to emit very intensely within the transparency window of the atmosphere, effectively transporting a greater amount of heat directly into the cold of space with minimal interference from the less-cold atmosphere. [3,4]
|Fig. 2: Simplified diagram of two strategies for passive cooling. Reflective coolers reduce cooling demand during the daytime by reflecting a large fraction of incoming solar energy, while selectively-emitting radiators send energy through the atmospheric transmission window to the night sky. (Source: M. Burnett)|
The great potential of passive radiative cooling lies in its ability to reduce the energy used for cooling systems like air conditioning. But in order to induce a useful cooling effect that could replace air conditioning, the cooling apparatus would have to emit energy until its temperature became lower than that of ambient air. 
Cooling below ambient air temperatures at night has been achieved often with materials designed to selectively emit narrow-band radiation through the atmospheric transparency window.  Under the night sky, cooling beyond ambient temperatures is more easily accomplished because the atmosphere serves as the sole weak source of incoming radiation; in fact, cooling below ambient temperatures has been accomplished even with broad-band emitting building materials and in non-clear weather conditions. 
Cooling below ambient air temperatures in the daytime presents a greater challenge, as radiators must deal with incoming solar radiation that can infiltrate the atmosphere mostly unhindered. Even when protected from direct sunlight, the diffuse sunlight scattered throughout the sky is absorbed by the radiator, heating it up.  Recent developments in nano-material radiators have succeeded in producing nearly 5°C of cooling below ambient air temperature under direct sunlight. To accomplish this feat, the radiator was coated with nano-scale layers of hafnium dioxide and silicon dioxide. This coating was designed to emit radiation selectively and intensely between 8 and 13 μm wavelengths, while at the same time maintaining a high reflectivity in the dominant wavelengths of sunlight. 
The use of passive radiative cooling technologies in energy-efficient buildings has been successfully demonstrated for many years. [1,4,6] However, these technologies can only create a cooling effect under the night sky because of the absence of incoming solar radiation (see Fig. 2). As peak cooling demand occurs during the day, most realized forms of energy-efficient "cool roofs" use high-albedo paints and materials to simply reflect incoming sunlight back towards the sky, which can result in significant savings in cooling energy.  (See Fig. 2) While such designs do not use selective emission to transfer heat to space, they can reduce daytime cooling demand more effectively than radiative cooling designs that must rely on nighttime energy emission. Recent nano-material designs that can both reflect most incident sunlight and selectively emit energy through the atmospheric transparency window could lead to even more energy-efficient buildings by combining both strategies for passive cooling.  The same research team has demonstrated the passive cooling of photovoltaic cells by an average of 13°C without negatively impacting the solar energy absorbed, which could have a significant impact on the temperature-dependent efficiencies of solar cells.  While the cost of such technologies is still prohibitive because of the complex manufacturing processes needed to produce the nano-material coatings, the concept shows promise that may allow humanity to practically harness the endless cooling power of space in the future. Air conditioning alone accounts for nearly 15% of energy consumption in US buildings, and global energy consumption for space cooling is projected to increase significantly as infrastructure is expanded and climate change progresses. [9,10] In this context, innovations in passive cooling for buildings are critical to ensure mankind's continued expansion is as sustainable as possible.
© Michael Burnett. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. Givoni, Passive and Low Energy Cooling of Buildings. (Wiley, 1994).
 A. R. Gentle and G. B. Smith, "Radiative Heat Pumping from the Earth Using Surface Phonon Resonant Nanoparticles." Nano Lett. 10, 373 (2010).
 M. De Zoysa et al., "Conversion of Broadband to Narrowband Thermal Emission Through Energy Recycling," Nat. Photonics 6, 535 (2012).
 S. Catalanotti et al., "The Radiative Cooling of Selective Surfaces," Sol. Energy 17, 83 (1975).
 A. P. Raman et al., "Passive Radiative Cooling Below Ambient Air Temperature Under Direct Sunlight," Nature 515, 540 (2014).
 B. Bartoli et al., "Nocturnal and Diurnal Performances of Selective Radiators." Appl. Energ. 3, 267 (1977).
 S. E. Bretz and H. Akbari, "Long-Term Performance of High-Albedo Roof Coatings," Energ. Buildings 25, 159 (1997).
 L. Zhu, A. P. Raman and S. Fan, "Radiative Cooling of Solar Absorbers Using a Visibly Transparent Photonic Crystal Thermal Blackbody," Proc. Nat. Acad. Sci. (USA) 112, 12282 (2015).
 J. Kelso, "2011 Buildings Energy Data Book," US Office of Energy Efficiency and Renewable Energy, March 2012.
 M. Isaac and D. P. van Vuuren, "Modeling Global Residential Sector Energy Demand for Heating and Air Conditioning in the Context of Climate Change," Energ. Policy 37, 507 (2009).