Optimization of Solar Tower Hybrid Pressurized Air Receiver Using CFD and Mathematical Optimization

Abstract

Solar receivers used for central tower Concentrated Solar Plants (CSPs) use either a surface-based or volumetric heat transfer region. Volumetric receivers are more efficient but require more sophisticated and expensive materials that can withstand the elevated temperatures (in excess of 1000°C). If pressurized air is used as heat transfer fluid (HTF) in order to increase both the density and heat capacity of the HTF, the volumetric receiver needs to be located in a pressure vessel with a pressurized quartz window located at the aperture of the concentrator. An alternative approach to utilize pressurized air in a pseudo-volumetric fashion is to populate a volumetric region with piped pressurized air (Kretzschmar & Gauché, 2012). This tubular-type volumetric receiver provides the challenge of enabling maximum heat transfer without causing hot spots on the side of the solar irradiation source. Heat transfer would not be as effective as for a true volumetric receiver because the heat transfer area is limited by the tube wall. A Computational Fluid Dynamics (CFD) model is generated of the solar receiver cavity. The commercial CFD code ANSYS Fluent v14.5 is used to evaluate the heat transfer between the incoming solar flux and the HTF. The incoming solar irradiation is modeled using the Discrete Ordinates (DO) radiation model ANSYS Fluent. The DO model also predicts the resulting thermal radiation in the conjugate heat transfer problem. The geometry is parameterized in order to allow for the selection of design variables in a formal design optimization formulation. Results include typical CFD results of a candidate geometry to illustrate the solar irradiation input, the effect of tubular layout as well as temperature and heat flux distributions that provide candidate objective functions for optimization. Recommendations are made for the implementation of the optimization process.