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 0.05 mm in diameter with 2-mm
pitch and a porosity of 0.999. The experimental apparatus consisted of a copper 300 mm 300 mm 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.