A two-glaze solar air heater with the second glazing of variable length

Solar air heaters use either one or two glazings. However, large absorber area collectors normally do not use a second glazing because of high cost of the two-glaze unit. An analysis of a solar air heater with a partial second glazing is presented. The efficiency of the system is calculated as a function of x, ratio of the length of the single glazing to the total length of the collector. The normalized efficiency (ηN), i.e., the ratio of the efficiency of the partial double-glazed air heater to the efficiency of the double-glazed system is represented by ηN = 1 + (b0 + b1y + b2y2)x + (c0 + c1y + c2y2)x2, where y is a function of flow rate. The coefficients b and c are determined by regression techniques. A set of these regression coefficients is presented for different plate lengths, flow-channel widths and absorptance of two glazings. These coefficients with suitable modifications can be used over a wide range of design and meteorological parameters to determine ηN. It is shown that a partial glazing is effective for long absorber plates.