Chapter 1, Problem 105P

Solar radiation is incident on a 5-m² solar absorber plate surface at a rate of 800 W/m2. Ninety-three 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 ${40}^{\text{o}}\text{C}$ with an emissivity of 0.9 that experiences radiation exchange with the surrounding temperature of $-5^{\text{o}}\text{C}\text{.}$

In addition, convective heat transfer occurs between the absorber plate surface and the ambient air of ${20}^{\text{o}}\text{C}$ with a convection heat transfer coefficient if ${\text{7 W/m}}^{\text{2}\text{.}}\text{K}\text{.}$

Determine the efficiency of the solar absorber, which is defined as the ratio of the usable heat collected by the absorber to the incident solar radiation on the absorber.