Solar radiation is incident on a 5 m solar absorber plate surface at a rate of 800 w/m. Ninety -percent of the solar radiation is absorbed by the absorber plate, while the remaining 7 percent is reflected away. The solar absorber plate has a surface temperature of 40 c with an emissivity of 0.9 that experiences radiation exchange with the surrounding temperature of -50 c. In add tion,convection heat transfer occurs between the absorber plate surface and the ambient air of 20 c with a
Convection heat transfer coefficient of 7 w/m.k. determine the efficiency of the solar absorber, which is define as the ratio of the usable heat collected by the absorber to the incident solar radiation on absorber.