Numerical Study of Charging and Discharging in a Sand Battery

Sammanfattning

In recent years, sand-based thermal energy storage (TES) has attracted research interest in high temperature energy storage applications due to its low cost, material availability, and more importantly, thermal stability at high temperatures. The present thesis investigates both the discharging and charging sides of the sand battery developed by Solstice Storage. The numerical study was conducted in ANSYS FLUENT 2022 R2, which involved steady state thermal hydraulic analysis with Therminol VP-1 as HTF, evaluation of air as a heat transfer fluid (HTF) for steam generation, transient conjugate heat transfer of discharging, and transient charging of the sand battery. Therminol VP-1 and air were adopted as HTFs for the transient discharging study, while transient charging was studied using an aluminium plate heater. The results of thermal hydraulic analysis demonstrated that the serpentine configuration enhances convective heat transfer performance with the help of curvature induced Dean vortices, thereby establishing it as a promising starting point for this TES. In the case of steam generation study, the pressurised air displayed the potential to generate 43 kg/hr of steam compared to air at atmospheric conditions, which generated 2.3 kg/hr. The transient studies identified slow conductive heat diffusion as the main bottleneck of the system that limits both charging and discharging of the sand battery. Localised cooling near the pipe during discharge and localised hotspots near the plate heater during charging indicated the effect of slow heat diffusion. Overall, the study demonstrates the suitability of the serpentine configuration while highlighting the main bottleneck and future direction that Solstice storage must pursue.