Transient discrete-granule packed-bed reactor model for thermochemical energy storage

Abstract

Thermochemical storage of high-temperature (450–1000°C) thermal energy can be applied to concentrated solar power systems to ensure round-the-clock electricity dispatchability. Reversible, non-catalytic gas–solid reactions are used to convert thermal into chemical energy during endothermic charging and vice versa during exothermic discharging. To assist the experimental investigation of such reactors, a numerical model of the heat and mass transfer in a packed-bed tubular reactor has been developed. The fluid serves as both heat-transfer medium and gas-reactant supplier and is modeled as a homogeneous phase using unsteady one-dimensional mass and energy conservation equations. Solid reactants are modeled as porous spherical granules. The unsteady energy and mass conservation equations are solved in the granules. This allows for the accurate treatment of larger granules with a diameter in the range of mm to cm in which radial gradients may occur. The temperature profile within a granule is calculated from an energy balance, whereas mass balances track the local concentration of the gases within the pores, affected by diffusion and a mass source/sink due to the gas–solid reaction. The fluid and granule phases are coupled through local mass and energy exchange terms including interphase resistances. Our approach allows for temporal changes of internal porosity and granule volume as a function of solid conversion, the incorporation of a particle size distribution, variable fluid properties and velocity, as well as accounting for the pressure drop in the bed. The numerical implementation of the model is tested through a code-verification study and a general assessment of mass and energy conservation within the coupled fluid and granule phases. The model is validated through comparison with experimental data from the literature for a non-reactive and a reactive packed bed.