Development and validation of a 3D CFD model for simulation of porous volumetric receivers in solar concentration systems

Abstract

In recent years, increasing attention has been given to the use of porous structures as thermal receivers (porous volumetric receivers) in high-temperature concentrated solar power (CSP) plants technology. This increasing interest is mainly due to the capabilities of these receivers to achieve high temperatures and thermal efficiencies, which make them a promising technology to improve the thermal performance of CSP plants. This work addresses the Computational Fluid Dynamics (CFD) modelling and thermal performance analysis of porous volumetric receivers coupled to solar concentration systems. A cylindrical receiver element made of open-cell SiC ceramic foam was considered, with the concentrated solar radiation and fluid flow entering in one of its tops and with the side wall insulated. Fluid flow and heat transfer processes in the porous media are modelled through volume averaged mass, momentum and energy conservation equations, considering the local thermal non-equilibrium (LTNE) approach, while the thermal radiation transfer is described by the P1 spherical harmonics method, using an open source software (OpenFOAM). An in-house algorithm based on the Monte Carlo Ray Tracing (MCRT) method was developed and coupled to the CFD mesh to model the 3D propagation and absorption of solar radiation, which is included as a source term in the energy conservation equation for the solid matrix structure. To validate the global model (MCRT and CFD), results were compared against data from the literature for a test case. An experimental study is under progress to validate the MCRT method. A detailed analysis of a reference configuration of a receiver element with 5 cm of diameter and 5 cm of height coupled to a parabolic dish with a concentration ratio of 500 is conducted. The obtained values of thermal power output, thermal efficiency, mean fluid temperature at outlet and pressure drop for this reference configuration are 628.92 W, 85.46%, 474.22 K and 103.10 Pa, respectively. The use of receivers with high porosity and pores size increases the thermal efficiency and decreases the pressure drop. The convergent incidence of solar rays on the inlet of the receiver leads to high temperature peaks in the porous structure and fluid. A way to decrease these peaks is to design the concentration system or place the receiver in such way to obtain lower incidence angles at the inlet. Another ways are imposing a non-uniform distribution of fluid velocity at the inlet, or having different porosities and pores sizes in the core and peripheral annular regions of the receiver.