Transient Model for Predicting Outlet Air Temperature of a Solar Air Collector with Recycled Concrete Sensible Heat Storage

Abstract

This study develops a transient mathematical model for predicting the outlet air temperature of a modular solar air collector integrating recycled concrete as sensible heat storage. The reference prototype used 18.8 kg of recycled concrete storage, with a volume of 0.01033 m3, density of 1820 kg/m3, and measured thermal conductivity of 0.885 W/(m·K). The model connects absorber, airflow, storage, external convection, and radiative heat-transfer mechanisms using a 600 s time step and measured meteorological boundary conditions. Validation was performed for clear, overcast, and scattered cloud conditions, producing absolute MBE values below 0.38 °C, MAE values of 0.64–1.17 °C, RMSE values of 0.87–1.39 °C, and R2 values of 0.83–0.96. The expanded uncertainty of the measured outlet temperature was ±0.58 °C, placing the lowest model error close to the experimental uncertainty level. The results support the use of recycled demolition concrete as a sensible storage medium and provide a modeling framework for subsequent investigation of different storage masses, geometries, airflow rates, and longer multi-day operating periods.