Doctor of Philosophy
Mechanical and Materials Engineering
Latent heat thermal energy storage systems have many advantages over sensible-heat-based thermal storage systems, such as an increased energy density with a narrower range of operating temperatures. The main barrier to implementing latent heat storage systems is the low thermal conductivity of typical storage materials, called phase change materials (PCMs). Furthermore, the heat transfer mode in the liquid phase is natural convection, which is influenced by the geometry and orientation of the heat source and sink, thus impacting the phase change pattern and heat transfer. It is critical to have a good understanding of the underlying fundamental physics over the entire phase change cycle to design effective latent heat thermal energy storage systems.
Current numerical techniques have limitations to accurately simulate this complex process due to the intricate coupling of many different physical processes. Experimental studies lack detail, mainly due to the instrumentation and measurement challenges in these systems. The present study addresses some of the experimental challenges to obtain a better insight into the underlying physical processes. The specific focus of this study is on PCM encapsulations that have a circular cross-section, such as horizontal cylinders and spheres. Two new, non-invasive experimental techniques were developed. The first uses single-exposure images of particles, such as the data taken for the purposes of particle image velocimetry (PIV), to digitally compile instantaneous streak visualizations. The second technique uses remote measurements of a thermal camera to predict the internal temperature of the PCM. These two techniques, along with high-resolution grids of thermocouples and particle image velocimetry (PIV), were then used to capture transient velocity and temperature fields during constrained melting and solidification within a circular encapsulation of PCM. Tests were done at three magnitudes of applied heating for melting and three magnitudes of applied cooling for freezing. The results showed strong similarity between the three respective applied conditions for both melting and solidification. Natural convection was shown to dominate the heat transfer process during melting, and two distinct convective structures were identified. Natural convection was also shown to be significant for solidification in the early stages.
Summary for Lay Audience
With a rising global energy demand and a projected reliance on fossil fuels for the foreseeable future, it is imperative to take measures to reduce global reliance on fossil fuels. There are two strategies for this: energy use needs to be reduced through conservation, or more power needs to be provided by renewables. A major barrier at this time faced by renewable energy sources is that most renewable energy sources can only produce power at certain times of the day. This issue can be alleviated by implementing large-scale energy storage, which stores surplus energy until needed. Storing the energy as heat is a good option since systems of this kind are low-cost, have a long lifetime, and are easy to scale to the necessary size. Additionally, thermal energy storage could be used separately to store waste heat until it can be used later, thus conserving energy.
Thermal storage systems that mainly store energy during a change of phase (typically freezing and melting) of storage material, called latent heat thermal storage systems, have advantages over storage methods that rely on a temperature change, such as a hot water tank. For example, these systems can store more heat in the same size of a system, and can provide opportunities for passive thermal regulation. The main issue with using latent systems is that the typical storage materials are poor conductors of heat. Therefore, it is crucial to understand the physics during freezing and melting to design better systems. The present study developed two new experimental techniques, which were then used with other techniques to study freezing and melting in a circular container. The flow within the liquid during phase change was measured, which had not been done previously for a circular container. Also, temperature measurements were taken with a higher resolution than had been done previously. The results showed that the process was predictable between the three applied temperatures tested for melting and freezing when the time was scaled using a new parameter. Two distinct flow regions were observed in melting, significantly affecting the process. In freezing, the flow affected only the beginning.
Teather, Kyle, "Experimental Investigation of the Phase Change Process within Circular Geometries" (2023). Electronic Thesis and Dissertation Repository. 9545.