One-Dimensional Heat and Mass Transfer and Discrete Granule Model of a Tubular Packed-Bed Reactor for Thermochemical Storage of Solar Energy

Thermochemical storage of high-temperature (450°C – 1000°C) thermal energy can be applied to concentrated solar power systems for 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 chemical reactors, a numerical model of the heat and mass transfer in a tubular packed bed 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 balances. Solid reactants are modeled as spherical porous granules. The unsteady radial energy and mass balances 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 an eventual mass source/sink due to the gas-solid reaction. The fluid and granule phases are coupled through local mass and energy exchange terms. The numerical implementation of the model is tested through a thorough code-verification study and a general assessment on mass and energy conservation within the coupled fluid and granule phases.