A solar water heater consists of a flat-plate collector that is coupled to a storage tank. The collector consists of a transparent cover plate and an absorber plate that are separated by an air gap.

Although much of the solar energy collected by the absorber plate is transferred to a working fluid passing through a coiled tube brazed to the back of the absorber, some of the energy is lost by free convection and net radiation transfer across the air gap. In Chapter 13, we will evaluate the contribution of radiation exchange to this loss. For now, we restrict our attention to the free convection effect.

(a) Consider a collector that is inclined at an angle of τ = 60° and has dimensions of H = w = 2 m on a side, with an air gap of L = 30 mm. If the absorber and cover plates are at T₁ = 70°C and T₂ = 30°C, respectively, what is the rate of heat transfer by free convection from the absorber plate?

(b) The heat loss by free convection depends on the spacing between the plates. Compute and plot the heat loss as a function of spacing for 5 < L < 50 mm. Is there an optimum spacing?

Transcribed Image Text:

Absorber plate, T Hot water supply Solar irradiation Back-up heater Storage tank Cover plate, T2 Cold water return