Correlations for Surface Temperatures of the Glass Cover for Estimation of Heat-Transfer Coefficients in Upward Heat-Flow in Solar Collectors with Single Glazing

Abstract

A set of correlations for predicting the temperatures of inner and outer surfaces of the glass-cover of flat-plate solar collectors with single glazing is developed, enabling estimation of the heat-transfer coefficients. The temperatures of the inner surface of the glass-cover (Tgi) and outer surface of glass-cover (Tgo) are computed using the proposed relations and the results are compared with those obtained by numerical solution of heat balance equations. Using the proposed relations maximum computational errors in Tgi and Tgo are within 1.5 degrees and 3.0 degrees respectively: for the range of glass-cover thicknesses 3 mm to 5 mm.(Using the previously available relation in the literature computational error in glass-cover temperature is 9 to 11 degrees.) As a result the maximum errors in calculation of convective and radiative heat-transfer coefficients between the absorber-plate and the glass-cover are reduced from about 8.8 percent to 1.1 percent and from 3.7 percent to 0.5 percent respectively. With such low errors in computation of the heat-transfer coefficients, numerical solution of heat balance equations is not required. The proposed method can also take into account absorption in the glass-cover.