EXPERIMENTAL AND NUMERICAL ANALYSIS OF CONVECTIVE HEAT LOSSES FROM SPHERICAL CAVITY RECEIVER OF SOLAR CONCENTRATOR

Spherical cavity receiver of solar concentrator is made up of Cu tubing material having cavity diameter 385 mm to analyze the different heat losses such as conduction, convection and radiation. As the convection loss plays major role in heat loss analysis of cavity receiver, the experimental analysis is carried out to study convective heat loss for the temperature range of 55-75 degrees C at 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees inclination angle of downward facing cavity receiver. The numerical analysis is carried out to study convective heat loss for the low temperature range (55-75 degrees C) as well as high temperature range (150-300 degrees C) for no wind condition only. The experimental set-up mainly consists of spherical cavity receiver which is insulated with glass wool insulation to reduce the heat losses from outside surface. The numerical analysis is carried out by using CFD software and the results are compared with the experimental results and found good agreement. The result shows that the convective loss increases with decrease in cavity inclination angle and decreases with decrease in mean cavity receiver temperature. The maximum losses are obtained at 0 degrees inclination angle and the minimum losses are obtained at 90 degrees inclination angle of cavity due to increase in stagnation zone in to the cavity from 0 degrees to 90 degrees inclination. The Nusselt number correlation is developed for the low temperature range 55-75 degrees C based on the experimental data. The analysis is also carried out to study the effect of wind speed and wind direction on convective heat losses. The convective heat losses are studied for two wind speeds (3 m/s and 5 m/s) and four wind directions [alpha is 0 degrees (Side-on wind), 30 degrees, 60 degrees, and 90 degrees (head-on wind)]. It is found that the convective heat losses for both wind speed are higher than the losses obtained by no wind test. The highest heat losses are found for wind direction alpha is 60 degrees with respect to receiver stand and lowest heat losses are found for wind direction alpha is 0 degrees (side-on wind). The heat losses obtained for wind direction, alpha, is 30 degrees condition are higher than the heat losses obtained for wind direction alpha is 0 degrees (side-on wind) condition, while the heat losses obtained by wind direction alpha is 90 degrees (head-on wind) condition are less than the heat losses obtained for wind direction, alpha, is 60 degrees condition.