Investigation of the influence of flux inhomogeneity on the heat transfer within a solar thermal receiver

Parabolic troughs are the most used technology for solar thermal concentrators. These systems consist of a parabolic mirror which concentrates solar radiation onto a centrally located metallic tube which absorbs this radiation. A heat exchange fluid flowing inside the tube utilizes a part of this absorbed radiation and can reach temperatures in excess of 400°C. A typical parabolic trough only concentrates solar radiation onto the lower side of the horizontally aligned absorber tube, which leads to an inhomogeneous flux distribution around the absorber circumference. This is an undesirable effect, as it will not only affect the heat exchange performance but will also introduce non-uniform thermal stresses within the absorber material. These stresses have been known to result in deflection of the absorber, which in turn negatively affects the amount of radiation the absorber is intercepting. The addition of a secondary reflector placed above the absorber provides a more spatially uniform flux distribution and allows for a reduction of the absorber diameter. In this work, absorber tube heat transfer is compared between a standard receiver with inhomogeneous flux distribution and one with a flat secondary reflector. Numerical modeling is used to analyze the heat transfer within the absorber. The novel secondary reflector employed allows for a 20% reduction of the absorber tube external surface area. The influence of this parameter on the temperature distribution across the absorber's surface is analyzed. Numerical simulations show that the heat transfer fluid receives 3% less energy in the secondary flat reflector absorber, but the bending expected in the standard absorber will produce a 10% of energy loss, so the overall efficiency of the secondary flat reflector is expected to improve the standard absorber.