Thermo-mechanical modelling of solar central receivers: effect of incident solar flux resolution

Abstract

The most important constraint for central receiver design is to keep the intercepting solar flux within the tube mechanical safety limits. An error in the heliostat aiming strategy or an inaccurate estimation of the tube temperature can produce overheating of the tubes, putting at risk the power plant operation. Therefore, the development of thermo-mechanical models to predict the incident solar flux, the temperature and the thermal stresses is mandatory. The large number of tubes in the receivers makes a detailed simulation computationally expensive, and simplifications are usually done. These simplifications consist on selecting a representative tube in each panel of the receiver and reducing the spatial resolution of the incident radiation to a scale equal to one panel size. This "coarse grid model" simulation can be used with confidence to obtain the solar tower yield production with an error lower than 2.5%. However, the small resolution of this model results in errors in the wall temperature and the thermal stresses estimation, and hence in the aiming strategy selection. In this study a 'fine grid model", whose cells have a size equal to the tube pitch (distance between centres of the tubes) width in the receiver, has been developed for an accurate estimation of the tube wall temperature. Using this fine grid model the temperature and the thermal stress profiles in each tube, for a given panel of the receiver, can be indirectly obtained without assuming that all the tubes of the panel have the same temperature profiles. Therefore, for modelling receiver, the fine grid model is required to determine with more accuracy the maximum tube stress and the aiming strategy, especially in the real situation of a highly non-uniform solar flux onto the receiver.