Question

An unglazed flat plate solar water heater is orientated perpendicular to the sun such that the incident radiation reaching the plate is 1100 W/m2. The plate has a semi selective coating such that the plate absorptivity and emissivity is equal to unity for all wavelengths up to 2µm, and 0.13 for all λ > 2µm. Coolant flowing in the collector tubes removes heat at such a rate as to maintain the plate temperature at 80°C. Considering the sun to be a blackbody at 5760 K, and the sky to be at 0 K (i.e. non participating), calculate the rate of heat removal by the coolant. If the system pump fails, what would be the plate stagnation temperature. Furthermore, calculate the rate of heat removal required to maintain the plate at its initial operational temperature if the surface were black, that is, α = ε for all λ. What would be the plate stagnation temperature if the pump were again to fail.

Answer 1

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