Dual-mode water thermal energy storage system with differentiated seasonal operation. Experimental characterization of the thermocline TES mode

A dual-mode water-based thermal energy storage (TES) prototype, referred to as ELSA (Sensible and Latent Energy Storage System) and designed for differentiated seasonal building applications is presented. It operates as a liquid thermocline storage in winter mode (hot water thermocline storage) and as a water/ice phase change material (PCM) storage in summer mode (cold storage). The present paper study focuses exclusively on the water thermocline mode of the ELSA prototype, with main concern on the validation of the embedded instrumentation and the characterization of the thermocline behavior. Experimental results demonstrate typical thermocline temperature profiles and good radial and angular temperature homogeneity, supporting a predominantly one-dimensional axial thermocline behavior. Global energy balance, comparing inlet/outlet energy transfer with internally reconstructed energy, shows very good agreement, with discrepancies generally below 5%, thereby validating the embedded instrumentation (220 thermocouples). Experimental results highlight a three-steps process of thermocline: formation near the distributor, establishment and transport in the tank, with linear thickening rate. A study of the influence of mass flow rate indicates only limited impact on thermocline behavior and thickening (less than 8% of variations of thermocline thickness), except at low flow rate/high thermal losses (+40%). The influence of this operating temperature difference could not be reliably quantified because of regulation issues during the experiments.