Development of Dynamic Thermal Modeling for Evacuated U-Tube Solar Collectors

Evacuated U-tube solar collectors (ESC) are highly efficient devices for converting solar energy into heat. In this study, a mathematical model was developed for the dynamic thermal analysis of ESCs designed for low and medium-temperature applications. Carbon dioxide (CO₂), chosen as the working fluid in solar collectors, possesses several superior properties: excellent heat transfer capabilities, non-flammability, non-explosiveness, environmental friendliness, and low critical temperature and pressure. The developed mathematical model was validated against experimental results, demonstrating a deviation of 6.3% between theoretical predictions and experimental measurements. For the dynamic analyses, the seasonal and annual performances of the collector were assessed using meteorological data of Isparta, Türkiye. The calculated CO₂ exit temperatures from the collector on specific dates (15 January, 15 April, 15 July, and 15 October) were 129.19 °C, 149 °C, 205 °C, and 170 °C, respectively. The maximum CO₂ temperature was observed in July and June, whereas the minimum temperature occurred in January and December based on monthly average meteorological data. The analyses indicated that fluid temperatures could reach approximately 250 °C. Furthermore, temporal variations in temperature across the collector's layers were studied.