Thermal energy storage based on cold phase change materials: Discharge phase assessment

In an energy scenario characterized by strong requirements in terms of flexibility and readiness, the integration of thermal energy storage in energy systems could play an important role in demand-supply management and allows novel operational schemes. Thermal Energy Storages based on latent heat are characterized by their compactness and small temperature swing. However, there is still a lack of performance analysis on large-scale setups. This work aims to fill this gap. In this paper, a shell & tube latent heat-based cold thermal energy storage was characterized in the discharge configuration, considering different temperatures and mass flow of the heat transfer fluid, representing an opportunity to understand the behavior of a full-scale system in different operative conditions. Sensitivity analyses on heat transfer fluid flow rate, flow direction, and inlet temperature were performed. The results show that peak power increases by approximately 25 % with doubled mass flow rate, and it doubles with increased inlet temperature by 6 °C. Since the impact of the buoyancy effect occurs when the liquid phase is predominant over the solid one, there is no strong impact on the direction of the Heat Transfer Fluid, from top to bottom or viceversa. Despite no metastability phase being detected, many discontinuities in the thermal power and temperature profiles were identified and analyzed, providing new insights into full scale latent heat storage. Finally, in this work, the thermal round-trip efficiency was estimated to reach above 90 %.