Thermal Performance of Double-Glass Evacuated Tube Solar Collectors: Flow Rate Variation Under International Standard Testing Conditions

Renewable energy sources are among the most promising alternatives to fossil fuels, and solar thermal energy stands out due to its high conversion efficiency and direct thermal utilization. The performance of solar collectors is evaluated under standardized procedures, including ISO 9806:2025. In the Republic of Korea, KS B 8295:2023 is applied for certification; however, it lacks clear guidance on the selection of the working fluid mass flow rate during experimental testing. This study experimentally investigates the thermal performance of a double-glass evacuated tube solar collector under varying flow rates, tested in accordance with both KS B 8295:2023 and ISO 9806:2025 standards. Three flow rates (0.042, 0.067, 0.092 kg/s) were tested at four inlet temperature levels. Unlike most previous studies, which were primarily based on simulations and lacked standardized experimental validation, this work provides empirical results obtained under fully standard testing conditions, thereby filling an important research gap. Instantaneous efficiency curves were derived, showing that increasing the flow rate enhanced the average thermal output by approximately 6%. These results highlight the necessity of defining optimal flow rate conditions in KS B 8295:2023, and the empirical correction factor proposed herein can support future standard revisions and promote international harmonization.