Geometrical and PCM optimization of a thermocline energy storage system

This work presents an optimized thermal energy storage (TES) system based on thermocline technology. A prototype of a single-medium (molten salt) thermocline storage system was built and tested at the ENEA Casaccia Research Center, which consists of a single tank equipped with an internal vertical channel to drive the salt motion by natural convection. At the channel ends, two serpentines are installed for charging (bottom serpentine) and for discharging (top serpentine). In this study, the thermocline prototype was numerically optimized by modifying its geometry and introducing a phase change material (PCM) in the design. First, the tank geometry was modified to involve the entire tank in the thermal stratification and to avoid undesirable heat transfer during the charge/discharge processes. Then, sets of PCM toroidal tubes were inserted at different heights inside the modified tank, increasing the thermal storage capacity of the system by a maximum of about 23 %. In addition, PCM inserts are expected to stabilize the salt temperature close to the serpentines. A transient 2D CFD model was developed to determine heat losses and the salt temperature distribution in the tank. This model was coupled with a lumped-parameter model aimed at computing the PCM temperature and the corresponding heat transfer coefficient. The storage tank geometrical optimization resulted in an increase of 9.6 % and 22.7 % in the stored and recovered energy, respectively. This optimization also indicated enhancement in the TES system in terms of charging/discharging efficiency and heat storage/release ratio. The PCM integration demonstrated a further improvement in the thermal performance of the TES system due to the PCM latent heat. For the charging process, configurations with PCM inserts achieved the highest charging efficiency and heat storage ratio, corresponding to an increase of ~18 % in the stored energy by inserting PCM tubes at both top/bottom locations. For the discharging process, the PCM integration did not lead to a considerable change in discharging efficiency and heat release ratio. However, including the PCM inserts at both top/bottom locations increased the recovered energy by ~14 % compared to the case without PCM.