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

Deuk-Won Kim, Wangje Lee and Kyoung-ho Lee ()
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Deuk-Won Kim: Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
Wangje Lee: Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
Kyoung-ho Lee: Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea

Energies, 2025, vol. 18, issue 20, 1-19

Abstract: 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.

Keywords: evacuated tube solar collector; mass flow rate variation; thermal performance; international standards; instantaneous efficiency model (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2025
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