Analytical models of thermal performance of solar air heaters of double-parallel flow and double-pass counter flow

The development of two analytical models that describe the thermal behaviour of solar air heaters of double-parallel flow and double-pass counter flow is presented. Based on the local energy balances, algebraic expressions for the efficiency factor F′ and the overall heat loss coefficient, UL, as well as air temperature distributions along the collectors were obtained. In addition, the expressions of the mean temperatures of the two air streams and of the absorber plate were determined. The model corresponding to the solar collector with double-parallel flow predicts that the mass flow of air flowing between the absorber plate and the transparent cover is greater than the mass flow circulating through the other channel. The model corresponding to the solar collector of double-pass counter flow indicates that increasing the airflow proportionally increases the percentage temperature rise in the inlet channel. The analytical expressions developed are valuable tools that can be included in computational codes for the design and thermal analysis of solar air heating facilities or to improve current facilities. The paper include a graphic to estimate the heat transfer coefficients involved in energy balances.