Repeat Example 8.1 for a parabolic collector with an evacuated tube receiver (case E in Figure 8.18a).

Example 8.1

Figure 8.18a

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Derive a relationship for the efficiency of a parabolic collector with a receiver/absorber tube corresponding to the reference case in Figure 8.18 as a function of insolation and temperature difference between the collector surface and ambient. The reflectance of the concentrator surface, p, equals 0.92, the transmittance of the glass tube surrounding the inner tube, t, is 0.94, the absorptance of the coating on the receiver tub, , e equals 0.95, and the interceptor factor, y, equals 0.96. Assume that the average direct normal solar irradiance is 560 W/m, which is twice the NREL daily average spread over a 24 h day, total = 10.5 mrad, and the axis of the collector has an east-west orientation.