Thermal stress and fatigue damage of central receiver tubes during their preheating

Abstract

• Transient temperature and thermal stress in tubes of CSP receivers are modelled. • The Vant-Hull algorithm is used to define the incident heat flux during tube preheat. • Large temperature gradients and a stress peak occur in the first 120 s of preheat. • The standard Vant-Hull algorithm may be unable to preheat tubes in windy conditions. • Fatigue damage due to preheat is less than 5%, avoiding creep-fatigue interaction. Preheating of the absorber tubes of a central receiver is a crucial operation in the start-up of current solar power tower plants (SPT) working with molten salt as a heat transfer fluid. In the preheating, a subset of the heliostat field is aimed at the receiver to produce an incident heat flux capable of heating the tube walls before they are filled with the molten salt. Incident radiation should be kept above a minimum as an insufficient level of preheating may lead to the freezing of the incoming flow of molten salt, interrupting the receiver operation. However, too much incident heat flux in the preheating causes severe thermal gradients in the walls of the tubes, leading to extremely high thermal-stresses and the eventual fatigue damage of the receiver. The present work numerically characterizes the temporal evolution of the temperature and thermal-stresses in the absorber tubes made of Haynes 230 alloy. This is done for two examples of SPT plants, i.e. Gemasolar and Dunhuang, with the aim of detecting when and where the largest temperature gradients and mechanical stresses appear during the tube preheating and evaluate their impact on the fatigue damage of the receiver tubes. The results show that during the first seconds of preheating, the front side to the tube is rapidly heated but the temperature at rear side of the tube is barely modified, causing a great non-uniformity of temperatures. This effect is enhanced when the tube diameter is increased because the heat requires more time to reach the rear side of the tubes. The peak values of the temperature gradient and the von Mises stress are reached during the first minutes of the preheating. Besides, under windy conditions, the preheating procedure proposed by Vant-Hull may be insufficient to preheat the receiver. Furthermore, in all the cases analyzed, the estimated fatigue damage of the tube is much less than the 5 % upper limit to avoid significant creep-fatigue interaction. This indicates the Vant-Hull algorithm does not compromise the structural integrity of the studied receiver tubes.