Numerical analysis of the influence of side wall shape on the efficiency of thermal energy storages based on granular phase change materials

This work is devoted to a numerical study of processes in thermal energy storages based on granular phase change materials. The influence of the side walls shape on the efficiency of such heat accumulators is studied when the plane-parallel flows of gas heat transfer fluid take place. The shape of the energy storage affects the heat transfer fluid flow in the object, and this affects the heat transfer, heat accumulation and heat recovery. Using the novel numerical model, the influence of narrowing and expansion of the side walls on the charging and discharging processes of thermal energy storages with rectangular cross sections are studied under two types of boundary conditions: the known mass flow rate of gas at the object inlet and the known gas pressure drop at its open borders for different phase change temperatures of the phase change material .Different efficiency criteria are used to estimate the preferred shape of a heat storage. For the charging process, the preference criteria considered are the maximum instantaneous storage efficiency, the maximum cumulative storage efficiency, and the minimum time to fully charge the device. For the discharge process, the considered preference criteria are the maximum energy recovery efficiency, the maximum total utilization ratio, and the maximum time to maintain the temperature of the heat transfer fluid at the outlet not lower than the desired value. It is shown that the preferred shape of the energy storage depends on the choice of the efficiency criterion and specific process conditions such as boundary conditions, phase transition temperature, etc. Narrowing and expanding thermal energy storages have an advantage in rare cases, and storages with straight walls are often most preferable.