2015 Angus R. Gentle andGeoff B. Smith. 'A Subambient Open Roof Surface under the Mid-Summer Sun' in Advanced Science. Article first published online: 26 MAY 2015. DOI: 10.1002/advs.201500119. Online ISSN: 2198-3844
If a surface is under intense sunlight and open to warm air, cooling to below ambient has up to now been an elusive target. The technical goal is super-cool roofi ng with thermal performance well above that of existing cool roofs. Stringent spectral requirements at solar and infrared (IR) wavelengths are needed, leading to quite limited choices for suitable coating materials and layer structure. Metal alone, except silver, cannot provide the required level of solar reflectance of above 96% and the thermal emittance of common metals is far too low to cool. Placing silver under a glass or polymer material with very low solar absorptance may cool well as high emittance E r results. However, options such as PMMA and most glasses absorb too much solar radiation. Low iron glass and various polyesters including PET absorb very little solar energy if thin, but their IR spectral response is not ideal for this task. Cooling is enhanced if IR spectral response in the thermal emission band involves a mix of moderate refl ectance at those wavelengths where the atmosphere irradiates the earth under clear sky conditions and very high absorptance hence emission in the range from 7.9 μm < λ < 14 μm, called the “sky window,” which is largely free of incoming radiation as it views the cold of space. Super-cool surfaces require solar refl ectance and sky window absorptance to be close to 100%. The admix of IR reflectanceand absorptance in the down-welling atmospheric radiation band is less stringent but together with the sky window setting determines the value of overall thermal emittance E r which dictates the heat output rate at the roof temperature T r . This radiative rate is an important practical consideration. Its final choice depends on total rate of heat input including the contribution of absorbed down-welling atmospheric radiation. We concluded that the desired combination of solar and IR criteria could be met using two or more specially chosen polyesters on a silver layer. At the set thickness, their IR absorptance had to be very strong in slightly different sections of the sky window with moderate transmittance at incoming wavelengths. A suitable surface was produced and set up outdoors. It remained subambient throughout a hot summer day including under the peak intense solar intensity of 1060 W m −2 , with ambient at 27 ÅãC, and high IR intensity from the atmosphere of 400 W m −2 .