Transient stresses at metallic solar tube receivers

Using highly concentrated solar radiation for power generation with gas turbine cycles is a promising way for cost effective solar power production. In order to reach the required high temperatures (800°C), a high concentration of the solar radiation is necessary. This is achieved in a solar tower plant where many movable mirrors ("heliostats") concentrate the direct solar irradiation on a focal spot on the top of a tower where the receiver is located. A possible type of such a receiver exists of directly irradiated metallic tubes. The receiver components, especially the absorber tubes, are subjected to complex thermo-mechanical loads including fast thermal transients due to clouds passing the heliostat field and slower transients due to startup and shutdown cycles. Furthermore, the solar irradiation varies over day and year. To determine the local transient temperature distribution in the receiver, a three-dimensional finite element model was developed. Ray-tracing was used to obtain the solar flux distribution on the FE grid. The heat transfer to the fluid was considered using a one dimensional thermal fluid element. Radiation exchange was modeled by the enclosure method. In order to predict the fatigue life of a receiver design a Chaboche-type plasticity model was used. The results showed a weakness of the first design. Following the receiver was re designed to improve the life time