Consider a flat-plate solar collector placed horizontally on the flat roof of a house. the collector is 5 ft wide and 15 ft long, and the average temperature of the exposed surface of the collector is 124°f. the emissivity of the exposed surface of the collector is 0.9. determine the rate of heat loss from the collector by convection and radiation on a calm day when the ambient air temperature is 70 °f and the effective sky temperature for radiation exchange is 50°f. take the convection heat transfer coefficient on the exposed surface as 2.5 btu/h·ft2·°f.