Numerical investigation of a packed bed thermal energy storage system with different heat transfer fluids

This paper presents the results of a numerical investigation on the transient behaviour of a packed bed thermal storage unit using different fluids: oil, molten salt and air. The storage material consists of loosely spherical particles of alumina packed in a reservoir wherein the heat transport fluid flows from the top to the bottom in the charging phase, and in the opposite way in the discharging phase. The process of charge/discharge of the storage system gives rise to a typical temperature distribution along the flow direction defined "thermocline". The main objective of this work is to analyze the temperature distribution along the storage system and the formation of the thermocline for repetitive consecutive cycles, evaluating the progressive reduction of the stored energy in the solid material for every new cycle. The numerical investigation is based on a two-phase one-dimensional modified Schumann model, where thermodynamic properties of the fluid are temperature dependent.