A method of test for the thermal performance rating of thermosyphon systems is developed in the present study. It is suggested that the overall performance rating of a solar thermosyphon system should include (1) system efficiency test during the energy collecting phase and (2) system cooling loss test during the cooling phase. Both the tests are performed outdoors. The cooling loss test is performed right after the efficiency test. A semi-empirical system efficiency model with a variable (TiTa)/Ht is derived to correlate the daily efficiency test results; while a simple first-order model with a cooling time constant τc is used to evaluate the loss parameter in cooling phase. A method of test is then proposed and an expert system is designed to perform the outdoor tests. It is shown that very good correlation for the system efficiency model is obtained and the system parameters obtained can be used to rate the thermal performance in energy collecting phase; while the thermal performance in cooling phase is rated by examining the time constant τc of the cooling loss model.

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