A simple solar heater consists of a flat plate of glass below which is located a shallow pan filled with water, so that the water is in contact with the glass plate above it. Solar radiation is passing through the glass at the rate of 156 Btu/(h ft2). The water is at 200°F and the surrounding air is 80°F. If the heat transfer coefficients between the water and the glass and the glass and the air are 5 Btu/(h ft2 °F), and 1.2 Btu/(h ft2 °F), respectively, determine the time required to transfer 100 Btu per square foot of surface to the water in the pan. The lower surface of the pan may be assumed to be insulated.
GIVEN
A simple solar heater: shallow pan of water below glass, the water touches the glass
Solar radiation passing through glass (qr/A) = 156 Btu/(h ft2)
Water temperature (Tw) = 200°F
Surrounding air temperature (Tˆž) = 80°F
Heat transfer coefficients
Between water and glass ( hcw ) = 5 Btu/(h ft2 °F)
Between glass and air ( hca ) = 1.2 Btu/(h ft2 °F)
ASSUMPTIONS
One dimensional, steady state heat transfer prevails
The heat loss from the bottom of the pan is negligible
The radiative loss from the top of the glass is negligible
The thermal resistance of the glass isnegligible