THESIS
2015
xix, 131 pages : illustrations ; 30 cm
Abstract
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...[
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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.
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