New Theoretical Approach of Flat Plate Solar Collector Considering the Glass Cover as a Participating Media Subjected to Solar and Thermal Radiations

Abstract

A new theoretical approach of black flat platesolar collector considering the glass cover as a participating media and taking into account the absorption and emission within a glass cover is presented. Two kinds of glasses commonly used as cover for such system, clear and low iron, have been studied. The glass material is analyzed as a non-gray plane parallel medium subjected to solar and thermal irradiations in one-dimensional case using the Radiation Element Method represented by Ray Emission Model. The optical constants of the complex refractive index, considering 160 values the pair of real part (n) and imaginary part (k), of a clear and low iron glasses reported by Rubin covering the range of interest for solar and thermal calculation have been used. The CPU times for predicting the thermal behavior of a solar collector using non-gray models were found to be prohibitively long. Therefore, suitable semi-gray (SG) models have been proposed for rapid calculation. We should mention that the results presented here are related to the low-iron glass. However, the instantaneous efficiency of the solar collector with low iron glass cover was compared with that obtained with clear one using SG models. It has been shown that the effect of the non-linearity of the radiative heat exchange, between the black plate absorber and the surroundings on the shape of the instantaneous efficiency curve is important. Indeed, the thermal loss coefficient is not constant but is function of temperature; due primarily to the radiative transfer effect. Therefore, when the heat exchange by radiation is dominant compared with the convective mode, the profile of the efficiency curve is not linear. It has been also shown that the instantaneous efficiency of the solar collector with a low iron glass cover is higher than the efficiency of the system with clear glass cover. It increases by approximately 8%.