Solar radiation is incident on the glass cover of a solar collector at a rate of 700W/m². The glass of collector absorbs 88 percent of the incident radiation on it and has an emissivity of 0.9. The entire hot water needs of a family can be provided by a single collector of 1.2m height and 2m width. The temperature of the glass cover of the collector is measured to be 35C° on a day when the surrounding air temperature is 25C° and the wind is blowing at 30km/h. The effective sky temperature for radiation exchange between the glass cover and the open sky is -40C°. Water enters the tubes which attached to the absorber plate at a rate of 1kg/min. Assuming the back surface of the absorber plate to be heavily insulated and the only heat loss to occur through the glass cover, determine (a) the total rate of heat loss from the collector, (b) the collector efficiency, which is the ratio of the amount of heat transferred to the water to the solar energy incident on the collector, and (c) the temperature rise of water as it flows through the collector. Air 25°C Tsky = -40°C Solar collector 135°C Solar radiation

2. Consider steady heat transfer between two large parallel plates at constant temperatures of T1 = 295 K and T2 = 155 K that are L = 2 cm apart. Assuming the surfaces to be black, determine the rate of heat transfer between the plates per unit surface area assuming the gap between the plates is (a) filled with atmospheric air,(k = 0.01979 W/m- o C) (b) evacuated,(c) filled with superinsulation having an apparent thermal conductivity of 0.00015 W/m · °C.

A mercury-in-glass thermometer having a diameter of 6 mm is placed in a verti- cal position to measure the air temperature in a metal warehouse in the summer. The walls of the warehouse are at 50°C. The convection coefficient is 7 W/m² - °C, and the emissivity of the thermometer may be taken as € = 0.9. Calculate the temperature indicated by the thermometer if the air temperature is 32°C.