Numerical analysis of partial charging effect in a latent heat thermal energy storage unit

Abstract

In a context of energy transition and energy efficiency optimization, thermal storage plays a key role in managing energy resources, reducing dependence on fossil fuels and minimizing CO2 emissions. As a storage technology, latent heat storage stands out for its high energy density and its ability to store and release heat at quasi-constant temperature, making it particularly attractive. However, to ensure optimum operation of latent heat storage units, it is essential to understand and master the associated operating modes, i.e., full and partial charge-discharge. These modes influence their global performance, as well as their flexibility and adaptability to fluctuating demands and varying quantities of available thermal heat to be stored. In this context, the present work aims to investigate the partial charge effect on the performance of a finned multitube latent heat thermal energy storage unit (FM-LHTES). To this end, an 80% partial charge is compared with a full charge (100%) operating condition, with the level of charge defined by the liquid volume fraction of the phase change material (PCM). In addition to examining the effect of partial charging on the charging process, this investigation also extends to the analysis of its effect on the subsequent discharge, with both local and global assessments. Phase change duration and stored/released energy are chosen as performance indicators of the latent energy storage unit. Erythritol and hytherm 600 are utilized as phase change material and heat transfer fluid, respectively. This study is carried out numerically on the commercial CFD code Star CCM+, where the phase is modeled in 3D using the well-known enthalpy-porosity method. Besides, a preliminary grid and step time sensitivity study is conducted to optimize computational time and resources, without compromising precision. The obtained results show that the melting of the remaining 20% takes a considerable amount of time, and that interrupting the charging process at a level of charge of 80%, saves a substantial charging time of 39.83%, with a corresponding unexploited energy of only 25.48%. However, for the subsequent discharge process, after partially charging the storage unit, the time reduction is not significant with only 12.4%, for an equivalent energy reduction of 25.8%, implying that partial charging is more advantageous during charging process. Furthermore, the solid volume fraction contours show that the remaining solid PCM from the charging process continues to melt during the discharging period, thereby changing the local phase change front compared to the case after full charge.