Performance enhancement of Phase Change Materials (PCMs) for energy storage and thermal management

Phase Change Materials (PCMs) are widely used as potential storage mediums in thermal systems owing to their capabilities to store a substantial amount of thermal energy in a compact size within a limited temperature swing. However, integration of PCMs with energy storage and cooling systems is associated with slow thermal response due to the weak thermal conductivity of PCMs. The present thesis aims to suggest innovations to improve the charging-discharging performance of phase change materials for practical applications in both energy storage and thermal management systems.

In the first part of this thesis, latent thermal energy storage (LTES) systems have been developed and investigated under different real-life application conditions such as consecutive or simultaneous charging-discharging. Accurate and validated numerical (CFD) models have been employed to predict the melting-solidification behaviour of PCMs or nanoparticle enhance phase change materials (NEPCMs) where Brownian motion of nanoparticles, and the convection motion of liquid PCM have been taken into account. Heat storage and recovery enhancement ratios have been introduced to comparatively analyse the studied designs and enhancement methods. The results show that depending on the application condition, some regions of an energy storage unit suffer from weak natural convection effect, which leads to delayed charging and fast discharging of PCMs. Therefore, innovative methods have been introduced in this study combining the benefits of multiple PCMs, high-conductivity nanoparticles, and well-designed fins which significantly enhance the rate of energy storage and recovery in the TES. Specially, appropriate arrangements of multiple PCMs are suggested to accelerate the charging and discharging of TES units under different application conditions. Through extensive numerical studies, a number of innovative solutions are developed to enhance the rate of energy storage and recovery in the TES combining the benefits of well-arranged multiple PCMs, dispersed high-conductivity nanoparticles, and optimized low-volume fins. 

The computational results have been deployed in conjunction with Response Surface Methodology (RSM) in order to predict an optimum design with fast charging and fast discharging objectives. The present study offers a design breakthrough solution for real-life applications where the rate of energy storage and recovery are equally emphasized without causing any significant reduction in the storage density, nor any considerable raise in system’s weight, cost of production and maintenance.

In the second part of this thesis, passive thermal management of electronic devices has been investigated using heat sinks embedded with single and multiple PCMs, where RT58, RT44 and n-Eicosane have been selected as PCMs. A transient numerical model has been validated and developed with realistic consideration of heat loss by natural convection from the fin tip walls to the ambient, the interaction of PCM with ambient air, temperature-dependant properties of PCM, and the natural convection effect in the liquid PCM. Detailed graphical results from the CFD have shed light on the melting-solidification behaviour of PCM during the entire heating-cooling cycle. The results show that dual n-Eicosane/RT44 heat sink is a viable solution for electronic components subjected to high critical temperatures, since it represents the longest working period and the lowest average transient temperature among the studied cases. The offered dual-PCM heatsink is further improved in a parametric two-objective optimization study where fast-cooling and slow-heating have been objectified for an effective passive cooling. The optimized designs for three set point temperature (SPTs) deliver charging periods up to 7% longer and discharging periods up to 4.2% shorter than the reference geometry. Furthermore, total volume of the heat sink is decreased by up to 23% due to current optimizations, signifying a favourable compact design. 

To complement the passive thermal cooling, gallium has been subsequently utilized as a low melting point alloy (LMPA) to assist thermal management of an electronic heat sink with crossed copper fins subjected to cyclic ultra-high thermal shocks. Applying an LMPA with a high thermal conductivity has been discovered as a feasible method to resist against sudden release of heat in the electronic components which happens due to the technical problems in the circuits. RSM has been utilized to perform a two-objective optimization and to study the effect of different geometrical parameters on the peak temperature and cooling period of the system in presence of thermal shock. The cyclic evaluation shows that the two-objective optimized heat sink delivers peak temperature up to 8.6% and 36.3% lower than that of single-objective optimized systems after five cycles of thermal shocks within periods of 30–60 s. This study shows how LMPA-based heat sink may be optimized and demonstrates a potential approach in the design of novel thermal management systems to prevent overheating from thermal shocks. 

The existing work in the literature on the application of PCMs in latent heat storage and thermal management can help mitigate the problems associated with melting and solidification performances of PCMs. In this study, novel methods have been introduced to enhance the rate of charging and discharging of PCMs without reducing heat storage capacity, any significant extra cost of production and maintenance, and the extra weight of the system. 

The main contribution of this study is the development of several numerical optimization models that are applied to different challenges associated with integration of PCMs in thermal systems. The results from this study offer an insight into the design of novel energy storage and thermal management systems which are applicable for future solar energy harvesting, waste heat recovery, energy saving at buildings, and electronics industry. The numerical results could be also beneficial by being developed for other applications such as solar panels and batteries.