Effect of Air Gap between two Glass Covers on the Performance of Plane Solar Air Heaters

Due to the increasing importance of using clean and renewable energies, one of the simplest mechanisms for converting solar radiant energy into thermal energy, on non-industrial applications, is the use of solar air heaters, which has always been an attractive idea for researchers. Therefore, the present study investigates the effect of air gap on the performance of plane solar air heaters with two glass covers by means of numerical simulations. A similar study has not been yet reported in various research resources. For this purpose, the equations governing the forced convection flow of air passing through the heater duct and also the natural convection flow caused by the buoyancy force in the air gap between two glass covers are numerically solved by applying the finite element method and using COMSOL Multiphysics software. In addition, in order to determine the temperature field in the solid elements of the air heater (including glass cover, absorber plate and insulation layer), conduction heat transfer equations are simultaneously solved with the flow equations. The accuracy of the numerical simulations performed in this research has been evaluated by comparing those with the theoretical and experimental findings of previous researches, and an acceptable consistency and agreement has been obtained in this regard. The results demonstrate the desired effect of the presence of air gap between the two covers on the performance of plane solar air heaters, such that changing the air flow rate from 2 to 14 g/s, the performance of this type of air heaters at a radiant heat flux of 1000 W/m2 compared to the simple model (without air gap) has shown an increase in efficiency about 4 to 6 percent. Also, in the conditions of inlet flow rate of 9.6 g/s and radiant heat flux varying between 400 to 1200 W/m2, the use of two glass covers has increased the outlet air temperature about 2 to 6 °C, compared to the simple model.