Computation of glass-cover temperatures and top heat loss coefficient of flat-plate solar collectors with double glazing

A set of correlations for computing the glass-cover temperatures of flat-plate solar collectors with double glazing is developed. A semi-analytical correlation for the factor f2-the ratio of outer to inner thermal resistance of a double-glazed collector-as a function of collector parameters and atmospheric variables is obtained by regression analysis. This relation readily provides the temperature of the second (outer) glass cover (T2). For estimating the temperature of first (inner) glass cover (T1), another relation for the factor f1-the ratio of thermal resistance between the two glass covers to the thermal resistance between the absorber plate and first glass cover-is developed. A wide range of variables is covered in the present analysis. The results are compared with those obtained by numerical solutions of heat-balance equations. Using the proposed relations of glass-cover temperatures, the values of top heat loss coefficient (Ut) can be computed and are found to be very close to those obtained by numerical solutions of heat-balance equations. The maximum absolute error in the calculation of Ut by the proposed method is only 1.0%, so numerical solutions of heat-balance equations for the computation of Ut are not required.