The influence of daily radiation collected on the daily heat gain of an evacuated tube solar water heater

Abstract

Q17，daily solar heat gain corresponding to 17MJ／m2 of daily collectible radiation is one of key parameters to evaluate the thermal performance of a solar water heater．In this paper, the influence of daily collectible radiation on the daily heat gain of an evacuated tube solar water heater is studied experimentally in the radiation condition described in GB／T 18708. Results showed that the daily heat gain linearly increases with the increase of daily colllectible radiation, but Q17 and system's efficiency coefficient almost kept constant. It was also found that when the testing condition with the daily coUectible radiations being not less than 17 MJ／m2 changed into that of not less than 16 MJ/m2，change of Q17 and the system's energy efficiency coefficient was not insignificant, implying that the testing conditions about daily collectible radiation specified in the revised version of GB／T19141 was reasonable.