Numerical modelling of back-pass triangular solar air heaters and its experimental validation. Comparison against different geometries

A numerical model was developed to predict the instantaneous thermal efficiency, the incident angle modifier curves, and airflow humidity and temperature in the Solar Air Heaters (SAHs) with triangular ducts. The model applies the control volume method with discretization along both axial and longitudinal directions, incorporating empirical correlations to solve the resulting system of algebraic equations. Validation was performed using 24 instantaneous thermal efficiency data points specifically collected for this purpose, yielding an average error of ± 1.63 % between experimental and simulated results. Additionally, a weighted least-squares regression was conducted to compare experimental and numerical thermal efficiency values. These analyses demonstrate that the proposed model is a reliable tool for the design and optimization of back-pass SAHs with triangular duct geometries.

Additionally, an experimental comparison between SAHs with triangular and rectangular ducts constructed from the same materials revealed that triangular configurations are, on average, 3.73 % more efficient than rectangular ones. The comparison of instantaneous thermal efficiency curves of triangular SAHs with two distinct opening angles (60° and 90°) indicated that a 60° angle yields, on average, 9.35 % higher efficiency, and with an incident angle of 50°, the 60° configuration is approximately 3 % more efficient than the 90° angle.