Performance assessment of Water-in-Glass ETC with integrated storage and internal heat exchanger: Experimental and numerical analysis

This study investigates the performance of a solar thermal installation composed of a 14-unit serial array of Water-in-Glass Evacuated Tube Collectors (WiG-ETC) with integrated storage and internal heat exchanger. The system is designed to provide heating for a livestock building in north-western France. The installation was instrumented and monitored over two consecutive years. Experimental results indicate an average collector global efficiency of 45.8%, while the solar fraction reached 25.9% of the building’s heating demand. To support system analysis, a dynamic model was developed in TRNSYS 18 to predict temperatures throughout the solar field and the daily useful solar thermal energy production. The model was calibrated using measurements collected over a representative two-week operating period during the summer of 2024. Following a global sensitivity analysis, the most influential model parameters were identified and estimated by minimizing the weighted Mean Absolute Percentage Error (W-MAPE) between measured and simulated daily energy production. The calibrated model was validated over 8-week operating period during the summer of 2025. Good agreement was obtained between simulations and measurements, with a Mean Absolute Percentage Error (MAPE) of 10.2% for the daily useful solar thermal energy production and a relative error of + 7.3% for the cumulative energy over the validation period. Simulated temperatures also showed high accuracy, with MAPE values below 1.8% for the mean temperatures of the integrated storage units and 3.3% for the solar field outlet temperature. The proposed model provides a reliable tool for analyzing the operation of WiG-ETC solar heating systems and supporting their design and optimization under real operating conditions.