Articles techniques et présentations

Metal hydride based thermal energy storage systems belong to the category of heat storage systems which rely on reversible thermo-chemical reactions to store and release heat. Specifically, these systems utilize metal/alloy hydride decomposition (endothermic) and formation (exothermic) reactions to store and liberate heat respectively. They have received considerable attention in areas like solar thermal systems as they promise high energy storage density (both volumetric and gravimetric) and discharge temperatures. The core components of these systems are reactors enclosing the solid-gas reaction system constituted by unreacted metals/alloys and their hydride (solid phase) and hydrogen (gaseous phase). Invariably, the solid phase is in pellet or powder form making a porous bed. The desired features in the design of the reactors include effective hydrogen transport into the porous pellet/powder bed, efficient heat supply to and removal from the reaction system and safe structural stability in the operating pressure and temperature range which case of materials like magnesium may go up to 40 bar and 400°C. Simulation of conceptual designs of reactors aid in design improvements by analyzing these requirements, qualitatively and quantitatively, but require handling the chemical reaction, fluid flow and heat transfer simultaneously with appropriate material models. In the present study, a reactor concept for heat storage, with hydrogen supply at specified pressure conditions and with heat exchanger coils carrying heat transfer fluid is analysed using COMSOL Multiphysics®. The geometry is created in COMSOL Multiphysics® and required physics models are obtained from the CFD module, heat transfer module and mathematics module. The Brinkman Equations physics from CFD module solves for the velocity field in hydrogen flow through the porous bed. It is coupled with a Heat Transfer in Porous Media physics from Heat transfer module which solves for temperature field in the whole reactor domain (solid or fluid phases are added and assigned appropriately to non-porous domains). The Coefficient Form PDE physics from Mathematics module, tuned and reduced to a known model of metal/alloy-hydrogen reaction kinetics which depends on gas pressure from Brinkman Equations and temperature field from Heat Transfer in Porous Media, solves for the field of absorbed hydrogen. The hydrogen and heat absorption/liberation due to reaction of hydrogen with metal/alloy are modeled respectively by adding a mass source term to the Brinkman equations and a heat source term to the heat transfer in porous media physics (both the terms depend on the reaction kinetics model). The flow of heat transfer fluid is modeled by Laminar/Turbulent Flow physics which is also coupled to the earlier mentioned heat transfer physics. Results obtained from the time dependent studies are, the distributions of absorbed hydrogen, temperature, hydrogen pressure with heat transfer fluid outlet temperature and heat transfer rates.