A novel computational model and OpenFOAM solver for simulating thermal energy storages based on granular phase change materials: Advantages and applicability

In this paper, we consider hydrodynamic and heat transfer processes in thermal energy storages based on granular or encapsulated phase change materials. Such heat storages can be applied in renewable and conventional energy, safety systems, civil building, and so on. A computational model, which is based on the assumption of interpenetrating interacting continua, has been developed to simulate the processes of phase transitions, heat transfer and fluid flow in the packed bed latent heat storages. To implement this model, we chose OpenFOAM - the leading free, open source software for computational fluid dynamics and heat transfer. Solver porousHeatTransferFoam, which is based on the SIMPLE algorithm, and additional libraries have been developed in the OpenFOAM framework. The solver and the libraries allow one to model granular materials with complex enthalpy-temperature diagrams during phase transitions, thus expanding the applicability of OpenFOAM. The advantage of the solver is high computational speed - its calculation time varies from a minute to a few minutes for 1D simulations and from a few minutes to a few hours for 2D simulations depending on the mesh sizes. Comparison between numerical results obtained and known experimental data has shown that the model based on the assumption of interpenetrating interacting continua adequately describes the heat transfer processes in the packed bed latent heat storages not only for the large ratio of the storage characteristic size to the particle diameter, but also for the small one (about 7-8). In contrast to many existing computational models, which applicability is usually limited to rather narrow class of the processes, our model covers a variety of hydrodynamic and heat transfer problems in the thermal energy storages based on granular phase change materials. This computational model and OpenFOAM solver developed allow one to simulate incompressible and compressible (liquid or gaseous) flows through the packed bed latent heat storages under various boundary conditions, with isothermal and non-isothermal phase transitions, with various properties of granular materials and coolant, in wide ranges of flow velocities, operating temperatures and particle sizes.