Phase Change Materials and Storage Design for Pumped Thermal Energy Storage

Abstract

Pumped Thermal Energy Storage (PTES) is an emerging energy storage technology; a key component of the system is the Thermal Energy Storage (TES) unit. This thesis investigates the optimization of a multi-tube shell-and-tube TES unit through both numerical simulation and experimental material characterization to enhance thermal performance. A numerical model was developed to evaluate the performance of nine bare-tube configurations, varying from dual to oct-tube arrangements. The addition of longitudinal fins was explored to improve the charging time, while the impact of the length of the TES was also studied. In parallel to the initial numerical studies, potential Phase Change Materials (PCMs) were identified, and four selected materials, adipic acid, D-mannitol, hydroquinone, and LiNO3-LiOH (81:19 wt%), were characterized experimentally using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). The numerical results indicated that the Oct-4F configuration delivered the most efficient charging performance and highest energy storage density. It was found that a system length between 0.50 m and 0.75 m balanced a sustained power output with maximal latent energy utilization. Experimentally, the synthesized LiNO3-LiOH (81:19 wt%) eutectic proved to be the most viable PCM, exhibiting excellent thermal stability with a melting temperature of 176.24 °C and a latent heat of fusion of 385.28 J/g. However, when the Oct-4F geometry was simulated using the LiNO3-LiOH (81:19 wt%) eutectic and Therminol 59 as the Heat Transfer Fluid (HTF), the system was unable to complete charging within the target eight-hour window. This was driven by the combination of the PCMs extremely high latent heat of fusion and the substantially lower specific heat capacity of the HTF, indicating that further geometric or operational optimizations are required to fully harness the material’s storage potential.