EXPERIMENTAL INVESTIGATION AND THERMAL MODELING OF PCM BASED LATENT HEAT THERMAL ENERGY STORAGE SYSTEM WITH EMBEDDED HEAT PIPES

Abstract

Scientists worldwide are continuously seeking alternative solutions to address the growing global energy crisis, focusing particularly on technologies powered by renewable energy sources. One of the major barriers to the widespread adoption of renewable energy is its intermittent nature, which necessitates efficient energy storage systems. Among the various energy storage techniques under research and development, thermal energy storage (TES) systems have gained significant attention due to their ability to store energy in different forms such as sensible heat, latent heat, and thermochemical heat. Latent heat thermal energy storage (LHTES), in particular, has emerged as a promising solution due to its high energy storage density and ability to store energy at nearly constant temperatures using phase change materials (PCMs). However, the widespread utilization of PCMs in practical LHTES applications is severely restricted by their low thermal conductivity, which hinders the rates of thermal charging and discharging. This research is motivated by the need to enhance the thermal charging performance of PCM-based storage systems, thereby improving their feasibility for real-world energy management applications. In this comprehensive research work, both numerical and experimental approaches have been utilized to analyze the thermal charging performance of LHTES systems by integrating heat pipes (HPs), altering tube shapes, and embedding fins with various geometries and materials. Three types of PCM enclosures—circular shell, trapezoidal shell, and rectangular shell—have been systematically studied under three distinct problem scenarios. In the first study, a numerical investigation was conducted on horizontally-oriented shell-and-tube LHTES systems, analyzing the effect of different eccentric tube shapes—circular, semi-circular, square, triangular, inverted triangular, and C- shaped—on thermal performance. Heat pipes were incorporated to further augment heat transfer between the heat source and PCM. The results revealed that the C-shaped tube significantly reduced the maximum melting time by up to 30.3% compared to the circular tube. Furthermore, the addition of one, two, and three heat pipes led to reductions in thermal charging time by 45.5%, 57.5%, and 72.2%, respectively. The C-shaped tube consistently exhibited superior performance in terms of the enhancement ratio, mean power output, and thermal response compared to other vii geometries. Notably, although non-circular tubes achieved better thermal charging for partial melting, they also exhibited trade-offs in terms of total energy storage capacity. In the second study, a numerical investigation was performed on a trapezoidal PCM container integrated with variable-length fins and heat pipes. The effects of fin quantity and material—aluminium, copper, and steel—on thermal performance were systematically analysed. The results indicated that increasing the number of fins improved the melting rate and mean power; however, beyond a certain number of fins, the thermal enhancement effects began to diminish. Among the materials tested, aluminium fins achieved a better balance between thermal enhancement and cost- effectiveness, while steel fins offered the most uniform temperature distribution. Additionally, a higher number of fins was found to compensate for the lower thermal conductivity of certain fin materials. The results highlighted the importance of optimizing both the number and material of fins to maximize thermal performance while maintaining economic feasibility. The third study involved an experimental analysis of a rectangular shell-and-tube LHTES system integrated with inclined heat pipes, with configurations of 2, 4, and 6 HPs. The experiment evaluated the melting behaviour of the PCM by monitoring temperature profiles, melt fraction evolution, and visual tracking of the melting front. Findings revealed that increasing the number of heat pipes significantly enhanced the melting rate; the case with 6 heat pipes demonstrated up to 120% faster melting compared to the 2-HP configuration. However, diminishing returns were observed beyond a certain number of HPs, and overheating was detected in the upper PCM region, suggesting potential inefficiencies at high HP densities. Additionally, although the highest number of HPs yielded the highest mean power output, cases with fewer HPs demonstrated superior energy storage capacity and better cost-effectiveness. Throughout this thesis, advanced computational fluid dynamics (CFD) simulations were conducted using ANSYS Fluent, employing the enthalpy-porosity method to model the melting behavior of PCMs. The Boussinesq approximation was used to capture natural convection effects, and heat pipes were modeled as highly thermally conductive elements for computational simplicity. The numerical models were validated through comparisons with previous studies, demonstrating excellent agreement in terms of melting time and melt fraction profiles. The combined findings of the numerical and experimental investigations emphasize the critical role of heat pipes, optimized tube geometries, and fin configurations in viii enhancing the thermal performance of LHTES systems. The integration of these techniques can dramatically reduce thermal charging time, increase mean power output, and improve thermal uniformity within the PCM domain. However, optimal design requires careful balancing of heat transfer enhancement, energy storage capacity, system complexity, and economic feasibility. This thesis contributes valuable insights for the development of advanced LHTES systems, offering design strategies for effective thermal energy storage in renewable energy, waste heat recovery, and other thermal management applications. Additionally, the study highlights several promising avenues for future research, including the exploration of heat pipe parameters such as inclination angle, length, and diameter; combined enhancement techniques involving fins and heat pipes; and the experimental validation of novel LHTES designs under practical conditions. These future directions aim to advance the design of highly efficient, compact, and cost-effective thermal storage systems suitable for a wide range of engineering applications.