A proposed modified efficiency for thermosyphon solar heating systems

Abstract

Conventionally, the overall performance rating of a thermosyphon solar water heater considers the thermal performance of the system during the energy-collecting phase and the system cooling loss during the cooling phase. However, this study suggests that the performance rating should also take the heat removal efficiency of the system during the system application phase into consideration. This study modifies the CNS 12557 B7276 test standard and employs a precise, on-line operation to derive the heat removal efficiency of a system. The thermal performance and heat removal efficiency of 12 systems with capacities in the range of 102–446 L are evaluated. An efficiency coefficient, η 0, is defined, which represents the synthesis of the characteristic thermal performance, η s ∗ , and the characteristic heat removal efficiency, η R ∗ . The proposed modified efficiency coefficient is given by η 0=η s ∗×η R ∗ , and represents the quasi-overall performance of a solar heating system. The coefficient provides an effective measure of the amount of energy provided to the user from a system which collects and stores heat from solar radiation. According to prevailing regulations in Taiwan, commercial solar heating products should have a value of η s ∗ in excess of 0.5 in order to attract a government subsidy. The proposed modified efficiency, η 0, is a more practical and representative indication of the actual thermal performance of a system, and accordingly, the present study suggests that the regulations should adopt a value of η 0⩾0.41 as the standard for qualification rather than the current criterion of η s ∗⩾0.5 .