One of the features that should be considered when designing a thermal energy storage (TES) system is its behaviour when subjected to non-continuous (partial loads) operating conditions. Indeed, the system performance can be sensibly affected by the partial charging and discharging processes. This topic is analysed in the present study by means of a two-dimensional axisymmetric numerical model implemented in COMSOL Multiphysics. A latent heat TES system consisting of a vertical concentric tube heat exchanger is simulated to investigate the effect of different partial load operating conditions on the system behaviour. The effects of different heat transfer distributions and evolutions of the solid-liquid interface, are evaluated to identify the optimal management criteria of the TES systems. The results showed that partial load strategies allow to achieve a substantial reduction in the duration of the TES (up to 50%) process against a small decrease in stored energy (up 30%). The close correlation between the energy and the duration of the TES cycle is also evaluated during the discharge using detailed maps related to the melting fraction. These maps allow for the evaluation of the most efficient load conditions considering both charging and discharging processes to satisfy a specific energy demand.

This paper presents the results of experimental and numerical research activities on a packed bed sensible thermal energy storage (TES) system. The TES consists of a cylindrical steel tank filled with small alumina beads and crossed by air used as the heat transfer fluid. Experimental tests were carried out while varying some operating parameters such as the mass flow rate, the inlet-outlet temperature thresholds and the aspect ratio (length over diameter). Numerical simulations were carried out using a one-dimensional model, specifically developed in the Matlab-Simulink environment and a 2D axisymmetric model based on the ANSYS-Fluent platform. Both models are based on a two-equation transient approach to calculate fluid and solid phase temperatures. Thermodynamic properties were considered to be temperature-dependent and, in the Computational Fluid Dynamics (CFD) model, variable porosity of the bed in the radial direction, thermal losses and the effective conductivity of the alumina beads were also considered. The simulation results of both models were compared to the experimental ones, showing good agreement. The one-dimensional model has the advantage of predicting the axial temperature distribution with a very low computational cost, but it does not allow calculation of the correct energy stored when the temperature distribution is strongly influenced by the wall. To overcome this problem a 2D CFD model was used in this work.

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