Experimental study and numerical modelling of high temperature gas/solid packed-bed heat storage systems

Abstract

Two pilot-scale regenerative heat storage systems have been tested by the French Alternative Energies and Atomic Energy Commission (CEA). The first one is a 1.1-MWhth structured packed bed consisting of ceramic plates forming corrugated channels. The second one is a 1.4-MWhth granular packed bed consisting of basaltic rocks enclosed by refractory walls. The two regenerators were tested over a hundred of thermal cycles between 80 °C and 800 °C with different fluid mass flows. Both systems showed their ability to store heat efficiently and to provide thermal energy at a stable temperature for the most part of the discharge process. The granular packed bed exhibited large transverse thermal heterogeneities due to flow channelling in the corners of the cross section. However, this phenomenon appears not to have degraded significantly the thermal performances, and the average one-dimensional thermal behaviour of the system may be assessed by the surface weighted average of the temperature over the bed cross section. Compared to the granular packed bed, the structured bed showed comparable thermal performances while inhibiting flow heterogeneities and reducing by up to 54% the average pressure drop. Furthermore, at the end of the test campaign, the packed beds were observed and compared from a mechanical point of view. The thermal results were successfully simulated over numerous charge/discharge cycles by using a one-dimensional numerical model. This is significant since the discrepancies between experimental and numerical results are likely to accumulate from a cycle to the other. The model considers the packed beds as continuous and homogeneous porous media but takes the conduction resistances within the solid filler and the walls into account. The pressure drop of the beds was computed using a correlation developed from a previous CFD study for the structured packed bed, and the Ergun equation for the granular packed bed. Compared to experimental data, these correlations enabled to estimate the order of magnitude and the evolution trend of the pressure drop with an average deviation ranging from −7.2% to +61.9%. For the granular packed bed, these deviations are ascribed to the flow heterogeneities and the shape of the rocks which are not taken into account in the Ergun equation.