Investigation and optimization of thermal characteristics of a vertical shell-and-tube phase change energy storage system

Latent heat thermal energy storage is a promising technology to solve the mismatch problem between demand and supply in renewable energy utilization due to its high energy density and a nearly constant temperature heat storage or release. In this thesis, the research aims to investigate and optimize the thermal characteristics of a vertical shell-and-tube latent heat thermal energy storage (LHTES) system which receives the most intensive research in the last decades. A comprehensive literature review was first conducted to evaluate the current research status and gaps. It was found that optimization in heat exchanger design and configurations is very important to improve the thermal energy storage performance. It can eliminate the reliance on expensive highly conductive material embedded in the phase change material (PCM) without reducing the energy storage capacity and density. This motivate this study to focus on investigating and optimizing the designs and configurations in a vertical shell-and-tube LHTES system. A comprehensive numerical model was then established by integrating the heat transfer fluid (HTF) flow model, HTF tube conduction model and heat transfer model in the PCM. The model eliminates the limitations in previous numerical studies that the PCM heat transfer model is subject to constant temperature boundary conditions during melting and solidification cycles. The model also enables evaluating both natural convection in PCM, and turbulent and lamina flows in HTF. It allows to study the effect of the HTF flow on the thermal performance of the LHTES system under a wide range of working conditions. The validation work showed that the predicted results agreed well with the experimental data. It demonstrated that the numerical model can reliably and accurately predict the thermal and heat transfer characteristics in the LHTES system during the phase transition cycles. The proposed model was then used to investigate energy storage and release performances in a vertical cylindrical LHTES system with different lateral tilting angles. The optimal tilting angle was examined for both energy storage and release processes. The results revealed that the total melting time was substantially shortened by up to 43% along with the tilting lateral surface angle varying from 0° to 7°; however, the increase in the tilting angle adversely affected the total solidification time. The optimal tilting angle selection must trade off between the heat storage and release performances. The tilting shell design stores heat energy much faster and can be applied for areas with short sunshine duration to enhance the storage of solar energy. The results regarding the tilting lateral surface demonstrated that the performance of the LHTES system is highly determined by the heat exchanger design. It further motivates research to investigate effects of the shell-to-tube radius ratio and unit height on the thermal characteristics of the cylindrical LHTES system. For this purpose, two series of LHTES system configurations were evaluated and studied. The first series varies the PCM shell radius (R) under a fixed HTF-tube radius (r\\(_f\\)) and the second series changes the HTF-tube radius with the PCM shell radius fixed. Based on the developed PCM-HTF conjugate analytic model, the numerical investigation assessed the vertical LHTES system using the energy storage/release ratio (E), total and average stored/release energy rate (\\(^-_{qch}\\)/\\(^-_{qdis}\\)), and energy storage/release density (Q\\(^{tot}_{ch}\\)/Q\\(^{tot}_{dis}\\)) as performance indicator during the charging and discharging processes. The investigation on the first series of configuration indicated that the optimal R/r\\(_f\\) is close to 5 under long charging/discharging time and close to 4 under short charging/discharging time. The results from the second series revealed that the R\\(_f\\)/r range of 4 to 5 can substantially accelerate both energy storage and retrieval processes without significant effect on the energy storage capacity. Furthermore, the optimal radius ratio is found to be nearly independent on the unit height. The optimal radius ratio obtained from the numerical investigation was applied to build a vertical multitube LHTES rig to demonstrate the efficacy of the theoretical study. The study then proceeds to an experimental investigation of the multiple tube heat exchanger. This experimental study first validated a well-known numerical solution to simplify the multiple-tube physical model into a single-tube one. This was done by comparing the experimental results between the multiple-tube heat exchanger (MTHX) and a single-tube heat exchanger (STHX). The STHX's geometrical parameters are the same as those of the virtual cylindrical domain in the MTHX, being similar to the single-tube model formulated by the simplifying numerical solution. The comparison concludes only an experimental study or a three-dimensional numerical modelling can reliably address the thermal characteristics of the multitube heat exchanger. The different inner tube numbers in the MTHXs were also experimentally compared. The results showed the added tubes in vertical MTHX boosted both charging and discharging cycles without hindering the PCM natural convection flow. Finally, the experimental results of five-tube MTHX were compared and analysed subject to different HTF operating conditions. The result confirms the numerical finding which indicates the PCM melting is dominated by natural convection while the solidification is mainly influenced by thermal conduction. The shell tilting angle, the shell-to-tube radius ratio and inner-tube arrangement are prominent design parameters to be considered when designing a shell-and-tube LHTES heat exchanger. The analysis of these parameters and operating conditions will provide the useful guidelines and implications for future research and practical designs.