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Graphene-Based Thermal Emitter
Published in Yaser M. Banadaki, Safura Sharifi, Graphene Nanostructures, 2019
Yaser M. Banadaki, Safura Sharifi
An idealized blackbody absorbs all radiation that falls into the range enforced by the temperature of the object. Kirchhoff’s law of thermal radiation states that the emittance and absorptance of an object must be equal for a given frequency, direction, and polarization under the condition of thermal equilibrium. As such, the normalized power radiated per unit area and unit wavelength by a non-blackbody in the normal direction as a function of wavelength and temperature can be calculated as follows: μ¯(λ)=∈Total(λ)B(λ,T)maxλ[B(λ,T)]
Non-concentrated solar thermal
Published in Volker Quaschning, Understanding Renewable Energy Systems, 2016
The temperature of the sun is 5,777 K. Most of the solar spectrum is wavelengths shorter than 2 μm. Here, the absorber should have a high absorption coefficient. Sunlight heats up the absorber to around 350 K, the maximum for the spectrum being far above wavelengths smaller than 2 μm. Kirchhoff’s law of thermal radiation states that absorption coefficient α is identical with emission coefficient ε, which should be as low as possible above 2 μm to ensure that a hot absorber does not give off much energy to its surroundings as thermal radiation.
Foundations of Heat Transfer
Published in Sadik Kakaç, Yaman Yener, Anchasa Pramuanjaroenkij, Convective Heat Transfer, 2013
Sadik Kakaç, Yaman Yener, Anchasa Pramuanjaroenkij
Thus, for a blackbody ε = 1. For a real body exchanging radiation only with other bodies at the same temperature (i.e., for thermal equilibrium), it can be shown that α = ε, which is a statement of Kirchhoff’s law in thermal radiation [10]. The magnitude of emissivity depends upon the material, its state, temperature, and the surface conditions.
Effects of pigment volume concentration on radiative cooling properties of acrylic-based paints with calcium carbonate and hollow silicon dioxide microparticles
Published in International Journal of Sustainable Energy, 2023
Sarun Atiganyanun, Pisist Kumnorkaew
Infrared irradiation is a mechanism that allows heat exchange between paints and the environment. Most research in radiative cooling focuses on achieving selectively high emissivity in the wavelength range of 8–13 µm for sub-ambient cooling. On the contrary, this work aims to achieve high broadband emissivity, i.e. thermal emissivity, which is suitable for above-ambient cooling. In this case, the atmosphere serves as a heat sink, and high thermal emissivity indicates efficient heat loss from the surface to the sky. The measured thermal emissivity of the paints is shown in Figure 4. According to the thermal emissivity of the 0:1 CaCO3:SiO2 paint, the addition of hollow SiO2 at the expense of acrylic resin is beneficial to thermal emissivity. This is due to two reasons. First, at a wavelength of 9.5–9.8 µm where the thermal radiation of a blackbody at a room temperature is strongest, the extinction coefficient of SiO2 is ∼1.0 (Kitamura, Pilon, and Jonasz 2007), whereas that of acrylic is less than 0.1 (Jitian and Bratu 2012). High extinction coefficients confer high absorptivity. By Kirchhoff’s law of thermal radiation, absorptivity is equal to emissivity at a thermal equilibrium. Second, the void volume of the SiO2 microparticles reduces the internal reflection of the infrared irradiation by reducing the effective refractive index of the coating in the IR region. Conversely, according to the thermal emissivity of the 1:0 CaCO3:SiO2 paint, the addition of the CaCO3 microparticles at the expense of acrylic resin reduces the paint’s thermal emissivity. While CaCO3 has a small, similar extinction coefficient to that of acrylic in the wavelength range of 9.5–9.8 µm, CaCO3 has a greater refractive index (∼1.9) (Jarzembski et al. 2003) than acrylic (∼1.56) (Jitian and Bratu 2012) at this wavelength. This leads to greater internal reflection of IR emission and less thermal emissivity when CaCO3 is added.