A parabolic trough of length 10 m, aperture D= 2m and surface reflectivity rhoc=0.95, concentrates solar radiation on an evacuated tube absorber of absorptivity α=0.97 and glass transmissivity τ=0.9. Given that the effective thermal resistance from plate(absorber surface) to ambient, Rpa= 0.04 K/W, the ambient temperature Ta = 20 degrees C, and that the incident solar radiation normal to the aperture Gb= 600 W/m2 , do the following:
i) sketch the thermal resistance diagram when there is no fluid flowing, denoting the convective resistance from glazing to atmosphere, Rc, ga , and the radiation resistance from plate to glazing Rr,pg and from glazing to atmosphere Rr,ga .
ii) calculate the maximum realizable plate temperature under those conditions.
iii) calculate the water outlet temperature when pressurized water flows through the evacuated tube at a rate of 0.02 kg/s with an inlet temperature of 150 degrees C. You may assume that the local plate temperature is 10 degrees C above the local water temperature, along the entire length of the tube.
iv) calculate the collection efficiency under the conditions of iii)