Thermal Performance Representation and Testing of Air Solar Collectors

The thermal performance of a plate-type air collector was evaluated experimentally. Tests were performed at the two ASHRAE Standard 93-1986 recommended flowrates and at a variety of inlet gage pressures to cover the three possible cases of collector leakage (inward, outward, inward-outward). These tests show that the thermal performance of air collectors depends on flowrate and inlet gage pressure (or the associated leakage rate). If the collector is used at an inlet gage pressure near atmospheric pressure as is often the case in the no-storage type of systems, then it is recommended to test the collector at an inlet gage pressure of zero. For collectors operating at various inlet gage pressures it is suggested to test them at three inlet gage pressures. As for the test flowrate, the present work confirmed the results of other studies and emphasizes the need to test the collectors at the design flowrate. Three methods of thermal performance representation were used and compared: One method bases the efficiency on the inlet flowrate, another on the outlet flowrate, and the third involves an overall enthalpy balance. When plotted in the classic way, that is, η vs. (T_i − T_a)/G the three methods exhibit significant differences, especially for the inward leakage case. An uncertainty analysis on the data obtained for this study indicates that for the inward leakage case, unacceptable uncertainties occur at high values of (T_i − T_a)/G.