The concept of low carbon, energy saving and sustainable design has been widely accepted all over the world. To increase the energy utilization efficiency, organic phase change materials (PCMs), which can store and release heat through phase change, have been recognized as an excellent candidate. To serve well for the purpose of energy saving and temperature regulation, high PCM content and high phase change rate are needed. Due to the poor thermal conductivity and thermal diffusivity of widely used organic PCMs, conductive fillers are often added to form composites to enhance the thermal performance. However, more filler will unavoidably decrease PCM content, increase fabrication difficulty and increase the cost, which are fatal for further applications of PCM products. Since c...[ Read more ]

The concept of low carbon, energy saving and sustainable design has been widely accepted all over the world. To increase the energy utilization efficiency, organic phase change materials (PCMs), which can store and release heat through phase change, have been recognized as an excellent candidate. To serve well for the purpose of energy saving and temperature regulation, high PCM content and high phase change rate are needed. Due to the poor thermal conductivity and thermal diffusivity of widely used organic PCMs, conductive fillers are often added to form composites to enhance the thermal performance. However, more filler will unavoidably decrease PCM content, increase fabrication difficulty and increase the cost, which are fatal for further applications of PCM products. Since classic Stefan model is only available for general phase change analysis but not for practical application, it is of emerging importance to develop a fundamental understanding of the heat transport in PCM and explore the strategies for optimizing the performance of PCM composites with constraint filler content to have higher phase change rate and maintain energy storage amount.

First, by using the equivalent melting temperature, a heat transfer model for semi-crystalline organic PCM is constructed based on classic Stefan solution. Meanwhile, this model concerns the phase change in a finite region. This model can serve as a fast tool to predict the one-dimensional heat transfer with phase change in an explicit form and get validated by the results of simulations and experiments reported in the literature.

Second, to achieve the optimized performance without sacrificing the thermal capacity, a novel numerical methodology has been developed to model the thermal behavior of phase change material composites, which has been validated by the experimental results for pure n-octadecane and n-octadecane/expanded graphite composites. Effects of different filler concentration distributions have been analyzed and compared. It is found that the phase change time is significantly affected by the filler distribution. An optimal polynomial filler distribution can reduce the phase change time by more than 50% with the same filling content, compared with the uniform distribution. Entropy analysis indicates that a shorter phase change time is correlated with a less entropy generation rate.

Moreover, quantitative analysis was conducted to predict the maximum enhancement potential and according optimal structure. Tri-layer epoxy/PCM/exfoliated graphite composite was fabricated and characterized to further explore the effect of functional gradient structure experimentally. Innovatively, the law of Cauchy inequality and variational principle were applied to investigate the optimized distribution function in discrete and analytical models. With a uniform latent heat distribution, higher filler content and higher filler thermal conductivity not only resulted in less phase change time but also led to higher enhancement potential. With a uniform thermal conductivity, enhancement potential by non-uniform latent heat distribution is dependent on the ratio of average latent heat density and maximum latent heat density allowed through fabrication.

Besides gradient structure, modeling and optimization of PCM composites with uniform filler distribution was numerically and experimentally explored. For novel application on electric vehicles (EV), the thermal performance is not always better with higher filler content and thermal conductivity. A numerical model was constructed and validated through small scale experiments. With the validated model, the performance of PCM board on EV was investigated with various material properties. The result shows low thermal conductivity will reduce indoor temperature, but high thermal conductivity will increase thermal leakage and reduce operation time. Furthermore, non-dimensional analysis was applied to investigate synthetic effect of materials properties and surrounding conditions. Criteria for material selection and structure design were thoroughly investigated.

In sum, the outcome of this study enriched fundamental knowledge of latent heat storage optimization methodology which will be of help for industry to increase the energy efficiency and reduce the cost. The optimization strategy explored in this research will be of great help to bridge the thermal performance and structure optimization of latent heat storage unit.