The absorber surface of a solar collector is made of aluminum coated with black chrome a s = 0.87 and e ir = 0.09). Solar radiation is incident on the surface at the rate of 600 W/m2. The air and the effective sky temperature are 25 and 15°C respectively. The convection heat transfer coefficient is 5 W/m2 K. If the absorber surface temperature is 70°C, what is the net rate of solar energy delivered to the working fluid (water)? (261.4 W/m2) (<-- this was placed at the end of the question but don't understand why.)
a s = Absorptance = fraction of energy absorbed