3D Two Phases Reduced Model of a Rock Bed Thermocline Thermal Energy Storage Unit: Parametric Study on Thermal Performances

A numerical simulation was conducted to evaluate the performance of a structured compact rock-bed energy storage system. The study encompassed the analysis of various configurations of rock bed arrangements, including simple cubic, body-centred cubic, and face-centred cubic structures, along with their dimensions and the velocity of the heat transfer fluid (HTF) during the charging process. A transient three-dimensional reduced model incorporating symmetry planes was developed and subsequently validated. This approach has been shown to significantly minimize the extensive computational time typically required when the full model is adopted, while providing enhanced accuracy compared to commonly used models in existing literature, achieving an improvement in accuracy exceeding 5%. Moreover, the methodology that has been adopted enables a more comprehensive investigation of the storage system, thus facilitating the capture of local data. The findings indicated a pronounced effect of the arrangement of the rock bed, the rock dimensions, and the HTF velocity on the heat transfer within the thermocline rock bed thermal energy storage system. It was determined that the thermal energy storage was optimized when the rocks were arranged in a face-centered cubic configuration, which is associated with lower porosity. It was established that, at an HTF velocity of 3.84 × 10–4 m.s-1 and a rock diameter of 0.01 m, transitioning from a simple cubic arrangement to a face-centred cubic arrangement resulted in a 16.5% increase in capacity ratio and a 21.5% enhancement in exergy efficiency. Furthermore, it was determined that this transition also delayed the charging process by 39% (equivalent to 40 min). Moreover, a reduction in the rock diameter from 0.05 to 0.01 m resulted in a 44% increase in capacity ratio and a 54.5% rise in exergy efficiency for a simple cubic arrangement at the same HTF velocity, with a recorded 76% increase in charging duration. Furthermore, for a rock diameter of 0.03 m and a simple cubic arrangement, decreasing the HTF velocity from 9.233 × 10–4 m.s-1 to 2 × 10–4 m.s-1 resulted in a 26.5% increase in capacity ratio, a 28% increase in exergy efficiency, and a delay of 2.8 h in charging duration.