Modelling strategies for porous structures as solar receivers in central receiver systems: A review

An international effort is being made to contribute to greener electricity production. Solar Thermal Electricity (STE) has emerged as the favourite candidate due to the advantages associated with it such as dispatchability, maturity and scalability. Particular interest is raised by Central Receiver Systems (CRSs) due to their ability to work at higher temperatures and concentration factors than Parabolic Troughs. Among the different CRS technologies, Volumetric Absorbers (VAs) working with air have received renewed research interest. VAs consist of porous structures where air is heated directly by the porous matrix. An optimised morphological configuration is essential to increasing the thermal efficiency and minimizing thermal losses. The literature presents a large number of works dealing with VA issues and potentialities, and most of them focus on numerical simulation in order to assess an optimal geometrical design or to point out the best directions in terms of thermal behaviour. This work presents a comprehensive literature review of the main simulation strategies adopted to evaluate VA performance for use in solar towers. The main methodologies, detail simulation and the homogeneous equivalent method, are presented and discussed. Furthermore, different model strategies such as Computational Fluid Dynamics (CFD) and one-dimensional (1D) models are described in detail, together with the importance of the equilibrium state between the fluid phase and the porous phase (local thermal equilibrium and non-equilibrium). Then, the main methods to determine the radiative heat transfer inside the porous phase are described. The study concludes with a discussion of the main trends in the field, where the homogeneous equivalent method, together with the CFD model and local thermal non-equilibrium, make up the most widely used strategies, in addition to silicon carbide material and foam geometry.