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°C with an emissivity of 0.9 that experiences radiation exchange with the surrounding temperature of ??5°C. In addition, convective heat transfer occurs between the absorber plate surface and the ambient air of 20°C with a convection heat transfer coefficient of 7?W/m2•K. 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.
What will be an ideal response?
A flat-plate solar absorber is exposed to an incident solar radiation. The efficiency of the solar absorber (the ratio of the usable heat collected by the absorber to the incident solar radiation on the absorber) is to be determined.
Assumptions 1 Steady operating conditions exist. 2 Temperature at the surface remained constant.
Properties The absorber surface has an absorptivity of 0.93 and an emissivity of 0.9.
Analysis The rate of usable heat at the absorber plate can be expressed as
Expressed in terms of heat flux, we have
Thus, the efficiency of the solar absorber is
Discussion The efficiency of the solar absorber is influenced by the surrounding and ambient temperatures, as well as the convective heat transfer coefficient. If the weather is particularly windy, thus causing higher level of heat loss via convection, then the efficiency of the solar absorber could be adversely affected.
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