The Influence Of Wind On The Heat Losses From Solar Cavity Receivers

Abstract

The principal objective of the present study is to assess the influence of wind on the heat loss from a generic cavity solar receiver under different geometrical configurations and operating conditions. This understanding is needed to provide insight into approaches with which to increase the thermal efficiency of a solar cavity receiver. The results from this work can be used to reduce the cost of concentrating solar energy and increase the rate of penetration of sustainable and renewable energy sources. A purpose built modular and cylindrical cavity receiver was mounted in a large-scale wind tunnel in order to quantify heat losses under the different conditions. The cylindrical cavity was lined up with 16 well controlled and separated heating strips to investigate the effect of the internal temperature and its distribution on heat losses. A systematic experimental study was performed to assess the influence of wind speed (𝑉= 0, 3, 6, 9 and 12 m/s), yaw angle (α = 0°, 22.5°, 45°, 77.5° and 90°), tilt angle (𝜑= 90°, 45°, 30°, 15° and -90°), cavity aperture ratio (0.33, 0.5, 0.75 and 1), internal walls temperature (𝑇=100, 200, 300 and 400 °𝐶) and 4 combination of temperature distribution inside the cavity. The data was analysed, then the total heat loss, normalised heat loss, and heat loss distribution through the internal walls of the heated cavities are presented. To further our understanding of the heat transfer and fluid flow inside and outside a cavity receiver, a numerical model of a solar cavity receiver was developed to assess the effect of aspect ratio (0.5 to 3) and head-on wind speed on the forced and natural (combined) convective heat loss and area-averaged convective heat flux from a cylindrical solar cavity receiver. The temperature distribution and velocity of air in the cavity are also presented. The present study found that increasing the cavity aspect ratio leads to a reduction in the influence of wind speed on the combined convective losses per unit of cavity internal area. Consequently, the overall efficiency of a solar cavity receiver increases with the cavity aspect ratio for the conditions assessed in this study (aspect ratio below 3). The influence of head-on wind speed on the heat losses was found to be ~ 4 times higher than the side-on wind for (1/𝑅𝑖 > 19). Decreasing the aperture ratio 𝐷𝑎𝑝/𝐷𝑐𝑎𝑣 from 1 to 0.33, acts to reduce the natural convective losses (at zero wind speed) by up of to a factor of 5, while the effect of this ratio diminishes as wind speed is increased. For high wind speed, the heat loss from the upward facing cases (𝜑=−90°) is approximately 3 times lower than the downward tilted cases (𝜑=15°) for a head-on wind condition. The heat losses from the upward facing cases are similar with the side-on wind conditions. For a downward tilted solar cavity receiver, an “upper or rear surface hotter” cavity has less overall heat losses compared with the other cases. There is also a slight advantage with respect to heat loss in keeping the tilt angle of a solar cavity between 15° and 30°.