3.10 A parabolic trough collector has a tubular steel receiver with a glass cover and the space between the receiver and the glass is evacuated. The receiver is 10m long and has an outside diameter of 5 cm and an inside diameter of 4cm. The glass cover diameter is 8cm. If the receiver sur- face is selective with &= 0.11 and is at 250°C, determine the overall heat loss coefficient of the receiver when environmental temperature is 24°C, the wind speed is 2 m/s, and the glass emissivity is 0.92. 3.11 For the previous problem, if the collector aperture is 4m, the receiver tube is steel, the inside convection coefficient is 280 W/m²-K, and the absorbed solar radiation is 500 W/m² of aperture area, estimate the use- ful energy gain of the collector and the collector outlet temperature. The circulating fluid is oil with a specific heat of 1.3kJ/kg-K, circulating at a flow rate of 1kg/s and entering the receiver at 210°C.