Abstract
The main goal of this paper is to explore the radiative cooling and solar heating potential of several materials for the built environment, based on their spectrally-selective properties. A material for solar heating, should have high spectral emissivity/absorptivity in the solar radiation band (within the wavelength range of 0.2–2 μm), and low emissivity/absorptivity at longer wavelengths. Radiative cooling applications require high spectral emissivity/absorptivity, within the atmospheric window band (8–13 μm), and a low emissivity/absorptivity in other bands. UV-Vis spectrophotometer and FTIR spectroscopy, are used to measure, the spectral absorption/emission spectra of six different types of materials. To evaluate the radiative cooling potential of the samples, the power of cooling is calculated. Heat transfer through most materials is not just a surface phenomenon, but it also needs a volumetric analysis. Therefore, a coupled radiation and conduction heat transfer analysis is used. Results are discussed for the selection of the best materials, for different applications on building surfaces.
Highlights
Buildings are responsible for about 35–40% of all energy used globally
The way the building surfaces emit and absorb radiative energy during daytime or nighttime, determines the energy loss or gain by them, which is correlated to the energy they use for heating or cooling purposes
In situations where a surface is to be kept cool while subjected to the Sun, it is necessary to have the maximum reflection of solar energy within the wavelength range of 0.2–2.0 μm, where incoming radiative energy is maximum, and to have maximum radiative emission from the surface in the 8–13 μm wavelength spectrum [1]
Summary
Buildings are responsible for about 35–40% of all energy used globally. The way the building surfaces emit and absorb radiative energy during daytime or nighttime, determines the energy loss or gain by them, which is correlated to the energy they use for heating or cooling purposes. If a surface has high absorptivity within the visible and ultraviolet spectrum (i.e., the wavelength range of 0.2–2.0 μm), it can absorb most solar radiation similar to a blackbody. In order to attain an equilibrium temperature below the ambient during daytime radiative cooling, it is important to reflect more than 88% in-coming solar radiation [2] This so-called spectrally-selective reflection, can be achieved using special coatings. Their results showed that the CuO coating displays a dark, black-color, the performance of CuO pigment was much higher than that of Titanium dioxide white pigment Another class of selective emitters is based on rare earth metals, due to their high absorption in the infrared spectrum [10,11,12]. As explained in References [1,19,20], some of these methods are extremely time-consuming for ordinary engineering calculations; so we selected the gray box model, which is simple and effective for non-gray calculations
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