Reduction of heat loss from solar thermal collector by diminishing natural convection with high-porosity porous medium

Abstract

We studied the reduction of natural convection heat loss from a solar thermal collector by placing a high-porosity porous medium above the collector plate in a solar thermal collector system. It is known that natural convection can be diminished in a porous medium. In order to use a porous medium in a solar thermal collector, it is necessary to minimize the shading effect of solar radiation caused by the porous medium. In this work, we used a series of offset wire screens made of fine nylon fishing lines of 0.05mm in diameter with 2-mm pitch and a porosity of 0.999. The experimental apparatus consisted of a copper 300mm×300mm collector plate with a selective absorption film on the surface. We measured the reduction rate of the convection heat loss from the collector plate and the shading effect for actual sun radiation by changing the condition of the porous medium, the temperature of the collector plate, and the inclined angle of the collector plate. Experimental results of the Nusselt number of natural convection in the high-porosity porous medium agreed with the equation proposed by Gupta et al. The effect of the inclined angle on the Nusselt number was small. The net reduction rate of natural convection heat loss was 7% by placing the high-porosity porous medium above the collector plate when the temperature of the collector plate was 100°C.