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Temperature measurement in electronic cooling
Published in Kaveh Azar, in Electronic Cooling, 2020
From an atomic point of view, it can be seen that good absorbers of radiation are also good emitters. If radiation of a certain wavelength produces an atomic transition in a material, then the radiation will be efficiently absorbed. Similarly, if thermal energy agitation results in atomic excitation, then the atoms will emit radiation at the same wavelength. It can, in fact, be shown that the wavelength dependent absorptance a(λ) and emittance ε(λ) of a material are equal, a(λ) = ε(λ), (Eckert and Drake, 1972). The absorptance, a(λ), is defined as the fraction of the incident radiation in a wavelength interval dλ about λ which is absorbed by the sample. In addition, at some wavelengths, a material may transmit radiant energy instead of absorbing or reflecting it. Si is an example of a material which reflects or absorbs in the visible but transmits in the infrared.
Physical Laws of Solar–Thermal Energy Harvesting
Published in Ashutosh Kumar Dubey, Amartya Mukhopadhyay, Bikramjit Basu, Interdisciplinary Engineering Sciences, 2020
Ashutosh Kumar Dubey, Amartya Mukhopadhyay, Bikramjit Basu
The absorptance can be represented as the ratio of the absorbed radiation by any material to the incident flux. The following equation expresses the absorptance as, αλ(λ)=Gλ,abs(λ)Gλ(λ) where Gλ,abs(λ) and Gλ(λ) are the absorbed and incident radiations, respectively.
Introduction to Organic Electrochromic Materials and Devices
Published in Sam-Shajing Sun, Larry R. Dalton, Introduction to Organic Electronic and Optoelectronic Materials and Devices, 2016
Two types of reflectance measurements are of primary interest: specular and diffuse reflectance. All materials that reflect radiation can do so specularly, like a mirror, in a fixed direction, or uniformly in all directions, i.e., diffusely. The former case is that of an ideal polished, reflecting surface, whereas the latter case is that of an ideal matte, scattering surface. In practice, most materials scatter both specularly and diffusely. In the case of an electrochromic material, which is deposited on a metallic surface and has high transparency in one redox state accompanied by high contrast between electrochromic states, the specular measurement is of greater interest; this case would typically find use in flat panel communication displays. Specular measurements can be carried out for various incidence angles, but a fixed angle measurement (typically at 16°) is preferred for practical and comparative reasons. In the case of a coarse electrochromic coating, e.g., one which is to be used for camouflage applications, the diffuse measurement is of greater interest. A material can either absorb, emit, or reflect radiation, and a very crude relation between these three parameters, not taking into account directional and other effects, is absorptance = emissivity = 1 – reflectance. (Emissivity or spectral emissivity differs from emittance, which is an integrated measure.) Thus in very broad terms, %-reflectance data can parallel %-transmission data.
Hybrid solar photovoltaic thermal systems in Nearly-Zero Energy Buildings: the case of a residential building in Greece
Published in International Journal of Sustainable Energy, 2022
The efficiency of the solar thermal collector is defined as the ratio of the useful thermal energy to the available incident solar radiation on the surface area. More specifically, it is calculated from Equation (3) (Gagliano et al. 2019): The useful thermal energy is defined by Equation (4) (Duffie and Beckman 2006): where Ac: total installed collectors’ area [m2]; : collector heat removal factor; : monthly average transmittance – absorptance factor; : intensity of solar radiation ; : collector’s overall energy coefficient factor ; : the temperature difference between the heat transfer fluid and outside air [K].
Parameter identification of a Round-Robin test box model using a deterministic and probabilistic methodology
Published in Journal of Building Performance Simulation, 2018
Marta Fernández, Borja Conde, Pablo Eguía, Enrique Granada
The studied parameters were classified into five groups. Those corresponding to the wall properties were the thickness of two layers of the walls, and the external convective heat transfer coefficient, to which a multiplier was applied to modify the value calculated as a function of the instantaneous wind speed. Five parameters of the window were considered for the GSA: U-value, solar absorptance and emissivity of the frame; a shading factor to adjust the solar radiation entering through the glass and a multiplier applied to the external convective heat transfer coefficient. Two parameters affect the heating device: the load percentage corresponding to the power radiative part and a multiplier which scales the power of the lamp. The building indoor air capacitance was also assessed, since TRNSYS initial model only considers the volume of the indoor air, ignoring internal masses – in this case the lamp – which affect the effective capacitance. Finally, two external factors were studied both representing the reflectivity of the ground below the test box that were calculated from the outdoor data registered during the experiment. As the ground reflectivity is used by two types of TRNSYS, it was necessary to define two different parameters for the GSA in order to assess their influence independently: parameter x12, ground reflectance, is one of the inputs of Type 99, which calculates the weather parameters for the period of the simulation, while parameter x13, ground reflection, is used by Type 56, which represents the multi-zone Building. The weather parameters are controllable variables which were used as data, not as modifiable parameters in the calibration process, and hence, were not considered in the GSA.
Analytical modeling and performance analysis of a solar cooker cum dryer unit
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Muhammad Zeeshan Siddique, Abdul Waheed Badar, Shan Ali Jakhrani, Fahad Sarfraz Butt, M. Salman Siddiqui, Muhammad Yasin Khan, Khalid Mahmood
Where; UL represents the overall heat loss coefficient from all sides of cooker compartment, incorporating all the active modes of heat transfer, () is the effective transmittance absorptance product which accounts for optical losses through double glazing, I represent the global solar irradiance, and S is the total amount of solar radiations absorbed by absorber surface.