Thermal Modeling of Water-in-Tube Type Evacuated Tube Solar Collectors to Predict Outlet Water Temperature: An Experimental Validation

Thermal model is developed to predict the outlet water temperature with respect to time for water-in-tube type evacuated solar collector connected in series. Developed mathematical expression is validated for the single collector and two collectors connected in series. In each collector, there are 20 evacuated tubes connected to the storage tank. Coefficient of determination (R²), reduced chi-square (χ²), and root mean squared error (RMSE) have been calculated to show the prediction accuracy of the developed model. For single and two collectors connected in series, R² is 99.73% and 99.90%, χ² is 0.46 °C and 0.39 °C, and RMSE is 0.70 °C and 0.65 °C, respectively. Predicted value shows good agreement with the experimental value. At a constant mass flowrate, the maximum outlet temperature reaches 53.10 °C and 71.50 °C for single and two collectors in series, respectively. The maximum energy for the single collector is 4.12 kW and 4 kW, while for two collectors connected in series, the maximum energy is 7.08 kW and 6.69 kW. Average thermal efficiency is 4.45% and 4.51% and average exergy efficiency is 9.66% and 15.17% for single and series-connected collectors, respectively. Developed model can design energy-efficient “water-in-tube type evacuated tube collector” for domestic and industrial applications.