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Source : Gemar official website Release time：2019/08/27
【Energy frontier】APR Review: The Principle, Material and Application of Radiation Sky Cooling
Original： Yaohui Feng Energy Elsevier 6月1日
Editor Notes: The Physics World Top 10 Breakthrough of 2017 was announced by the British institute of physics in 2017. The metamaterial with no power input for radiative cooling developed by the group of Chinese American scholars Ronggui Yang and Xiaobo Yin from Boulder School of Colorado University was selected as one of the top 10. The group once published an article about a new type of radiative cooling film on Science in 2017 (Science 355, 1062–1066, 2017). It’s said that the film can radiate energy to the outer space by the electromagnetic infrared wave to realize refrigeration, which caused an extensive discussion from the academic circles and the business world.
After that, the group of Ronggui Yang and Xiaobo Yin issued the first KW radiative cooling system capable of whole day running to realize “cooling in the sun”, and the super log used for radiative refrigeration to realize “send thermal to the sky “(published together with Liangbin Hu of Maryland University) on Joule and Science(Joule 3,111–123, 2019； Science 364, 760–763, 2019) successively.
Recently, Appl. Phys. Rev. announced the review of Prof. Ronggui Yang group “Radiative sky cooling: Fundamental principles, materials, and applications”，which comprehensively discussed the fundamental principles, materials and applications of radiative sky cooling, provided a brand new idea and approach for the realization of energy-saving and emission reduction, the retard of greenhouse effect and urban heat island effect. The paper was chosen as the “Editor’s Pick”.
Along with population growth, industry development and people’s pursue of more comfortable environment, there is a booming demand of power in the refrigerator field in the 21th century. The current traditional vapor press cooling technology is facing the problems of large energy consumption and greenhouse effect triggered by the refrigerant etc. In fact, to realize cooling by the circle of thermal is to discharge cooling capacity to the environment, which will result in the hotter of the earth. That’s why to raise the efficiency of the current cooling system and look for new type of cooling technology becomes a widespread concern of the academic circles. As everyone knows, any object will produce radiation, and according to the Stefan-Boltzmann law, the higher the object’s temperature the stronger the object’s emission power. Consider the temperature of the earth’s surface is about 300k while that of the atmosphere is about 2.7k, the large temperature difference between the earth and the atmosphere makes it to be possible to utilize infrared radiation to cool the earth’s surface.
Differ from the traditional cooling technology which discharge heat to the environment, radiation cooling technology is extremely attractive because it can emit thermal to the sky directly. It focuses on the research of a kind of new material with ideal radiation characteristics and the exploration of its possibility to apply radiative sky cooling to various situations, which will have an extensive application on construction energy-saving, solar battery, water condensing from air, cooling of outdoor personal instrument and the condensation of refrigerators and large power stations.
Chart 1 Fundamental principle of radiative sky cooling (a) the earth gets energy from the sun and radiates thermal to the universe to keep energy balance; (b) thermal transmission on radiative cooling surface.
The interaction of the atmosphere and the environmental radiation constituted by solar radiation and infrared radiation from the earth’s surface maintains energy balance and decides temperature on the surface of the earth(chart 1). Short wave from solar irradiation (0.3~2.5μm) can be reflected, absorbed or scattered by the gas and cloud in the atmosphere. Similarly, according to the Planck’s law, temperature on the earth’s surface is about 300k, and the range of radiative wave length generated by the earth is 2.5~50μm, the earth radiation will also be reflected, absorbed and scattered by the atmosphere during its upward transmission.
In the past, radi-cooling was generally applied to obtain low temperature at night, but in the current years, day time cooling becomes possible along with the development of new micro-naro material and radiation research. It can be discovered that there is only a very small overlapping part between the wave length of solar radiation shown in chart 2(a) and the 300k black radiation shown in chart 3, so it’s possible to realize cooling under direct solar radiation by the research of near zero low emissivity (high reflectivity) in solar spectrum and spectral selective surface that can keep high reflectivity in the infrared area.
Temperature on the earth’s surface is about 300k (blackbody radiation wavelength is about 3~50μm), so the atmosphere emissivity in the range caused people’s attention. It can be seen from chart 3 that the least range of atmosphere wavelength which is called “atmosphere window” is 8~13μm due to the comprehensive effect of atmosphere components, that means most of the thermal radiation on the earth surface transmitting through the atmosphere without resistance is in this range. Therefore, the special material with high infrared emissivity in the range (emitting wave length 8-13μm) can pass through the atmosphere without any resistance and will not be reflected, absorbed and scattered to enter the outer space directly thus to realize refrigeration. In other words, this kind of material becomes a bond of energy transmitting between human’s residential environment and the outer space.
Chart 2 Solar radiation intensity absorbed by the cold surface from the horizontal radiation
Chart 3 300k blackbody surface spectrum (black) and atmosphere transmittance of the mid-infrared area (blue). 8~13μm wavelength is called atmosphere window due to its high transmittance and its spectrum is consistent with the 300k blackbody radiation curves.
The article deeply analyzed the principle of radiative sky cooling and all kinds of influencing factors from the aspects of solar irradiation, atmosphere radiation, thermal radiation on solid surface and the characteristics of radiative sky cooling (such as ideal transmitting curves, thermal test and instruments, selective radiation surface, the impact of humiture on radiation cooling, non radiative heat transfer like convective heat transfer and the strengthen of construction design of sky cooling). Furthermore, it also has a comprehensive review of the material used for sky radiation in the daytime and at night, such as polymeric materials, inorganic film, nanometer materials and bionic materials etc. and discussed the manufacture of the materials from economy and practicability at the same time, thus to realize massive application of super high emissivity in the atmosphere window under low cost.
(a) Metallic polyethylene fluoride film; (b) compound of polyethylene film and amorphous sic nano-particles; (c) monox and silicon nitride film on aluminum surface (d) selective infrared emitted ethylene gas panel.
Chart 5 nano-bionic materials for daytime radi-cooling
Glass-polymer metamaterial. Silver thin film on the back. Silver film reflects most of the emitting sunlight and has high emissivity in the atmosphere window; (b) integrated spectral performance of metamaterial: reflect 96% solar irradiation, emissivity in the atmosphere window>0.93; (c) continuous test of cooling power in 3 days shows, average power>110W/m2; (d) produced at 5m/min, get 300mm 50μm metamaterial film without silver coating. (e) silver coating metamaterial
Take the United Station as example, construction energy consumption takes up 40% of the total energy consumption. While the consumption mainly comes from HVAC system, so in this regard the cooling consumption takes most part of it. As for radiative sky cooling, it can combine with the buildings with little or even no power consumption so as to have great influence on the realization of energy conservation and emission reduction. The article summarized the radiative sky cooling combined with buildings and discussed the opportunity and challenge facing this technology.
In terms of solar battery, the commercial solar battery has a transfer efficiency of 12%-20% which makes it be heated by unnecessary solar energy to shorten its lifetime. At present, water cooling, wind cooling, thermal pipe, thermoelectrical module and phase change material are usually used on solar battery. The radiative sky cooling, however, provides a new idea for the cooling of solar battery. It can be realized by the thin film materials with high transmissivity in the solar spectrum and high emissivity on infrared spectrum under working temperature.
Radiative sky cooling also provides some certain thoughts for water condense from air technology. Nowadays absorption type watering and membrane separation technology are usually used. There are still an enormous potential application of the new technology in the arid and semiarid areas, remote villages and outlying island areas.
In addition, the article also discussed the potential application of radiation sky cooling on waste heat cooling in power plants, small cooling box for remote areas, fixed bed condenser distilled by solar energy, thermal generator and the cooling of human body equipment etc.
Chart 7 passive radiative sky cooling for buildings- cold roof: (a) reduction of heat in buildings by stronger solar reflection and infrared thermal radiation; (b) compared to low reflection galvanized roof (right), the high reflectivity cold roof has far more low surface temperature; (c) cold roof can reduce building’s heat island effect to cities.
Chart 8 comparison of polymeric metamaterial cold roof developed by Bould school of Colorado university with common roof: (a) the comparison rooms are 2.4m*1.8m*2.4m (length*width*height); (b) a highest temperature difference of 28.6℃between the temperature change of metamaterial cold roof and common roof in 24 hours; (c) a highest difference of 11.2℃ of the room air temperature during test; (d) environment temperature and solar radiation during test.
Chart 9 nighttime radiative sky cooling system of building integration: (a) use the roof as radiator; (b) combined with construction air conditioner; (c) use roof ventilation designed NightSolar® system to realize summer cooling and winter heating; (d) use nighttime radi-cooling and convectional cooling system; (e) pump cooling water into the floor at night and store the cooling capacity in the buildings; (f) cold water providing nighttime radiation system.
Chart 10 daytime radi-cooling buildings: (a) radi-cooling system use air as refrigerant to reduce building temperature; (b) radi-cooling system use air as refrigerant to precool air suction of buildings; (c) daytime radiative air condenser developed by Bould school of Colorado university; (d) test result of developed condenser.
Chart 11 daytime radiative cooling system use water as refrigerant: (a) photon radi-cooling system; (b) compound radi-cooling system; (c) daytime radi-cooling system for air conditioner condenser; (d) use daytime and nighttime radi-cooling system to raise efficiency furthest.
Chart 12 radiative sky cooling system combined with heat transfer fluid to generate continuous cooling effect: (a) equipment chart of radi-cooling system with heat transfer fluid; (b) series connected radiative cooling panels system use water as heat transfer fluid; (c) test result of 3 days continuously; (d) net-cooling power change in the 3 days’ test; (e) comparison of monthly electricity consumption of business offices with or without radiative cooling system; (f) monthly power-saving cost after the application of radiative cooling system in the three cities.
Chart 13 radi-cooling system for power plant
Chart 14 spectrum selected nanocomposite textile for outdoor personal cooling
As a conclusion, this article summarized the development of radiative sky cooling technology from the aspects of principle, material and application. The technology has been greatly promoted by the exploitation of advanced radiative cooling materials and the research of daytime radiative cooling. It has broken through traditional cooling mode and opened up a new type of cooling technology, which has an enormous potential in building energy conservation and large scale power plant. Along with the increasingly severe source situation and environment problem, this technology will play an important part in the aspects of relieving city heat island effect, solving water and environment problems and even in the aspect of coping with global warming effect.
Applied Physics Reviews 6, 021306 (2019);https://doi.org/10.1063/1.5087281