UC Davis

As the world moves towards a more sustainable future with increased deployment of renewable energy sources, thermal energy storage offers a solution to the intermittent issue of the primary source of renewable energy: the sun. Latent heat storage, a type of thermal energy storage system, utilizes a phase change material to absorb and release thermal energy. In this study, we aim to achieve two objectives. Firstly, we seek to test a small-scale prototype of a latent heat system using Al-Si as a phase change material (PCM). Secondly, we aim to optimize parameters of the thermal energy storage tank for a household in the United States to sustain eutectic Al-Si at its phase change temperature (577 °C) for 24-30 hrs. We explore various tank structure layer configurations and manipulate parameters to achieve optimal values for target parameters such as heat loss and time. To build the small-scale latent heat system, we assessed the subsystems within the prototype design to ensure their compatibility with the overall system. The design was initiated by research done by a previous lab member, and the necessary equipment was procured and tested for viability. To optimize the parameters for storage tank, we employed a sensitivity analysis approach. This involved defining both the input and target parameters, observing how the adjustments to the input parameters impacted the target variables, and devising strategies to achieve the desired target values. We also explored the practical feasibility of implementing these manipulations. Through the experiments, thermoelectric generator test highlighted issues with imperfect contact and lower output efficiency, necessitating higher quantities of TEG for more energy conversion. Additionally, 304 stainless steel was deemed unsuitable for long-term containment of molten Al-Si due to prolonged exposure. Moreover, the heating setup was inefficient due to thermal inconsistencies and non-uniform heating. While the demonstration of the benchtop LHB wasn’t entirely successful, it provided valuable insights into the small-scale latent heat system using Al-Si as a phase change material and emphasized the need for extensive preliminary research in this interdisciplinary field. Secondly, the theoretical analysis provides insights into optimizing thermal storage tank parameters to sustain phase change material at 577 °C for 24-32 hours. Temperature across the layers, material thickness, and emissivity are the key parameters affecting the system’s performance. Case 1, without vacuum layer, achieves desired outcome with adjustment of few parameters, notably the thickness of kaowool insulation. On the other hand, Case 3 required manipulation of numerous parameters but fails to offer significant advantage over Case 1 . Temperature of the alumina in contact with the molten Al-Si (which is known as T_alsi in this study), temperature before vacuum (which is the temperature of the surface before vacuum), emissivity, and thickness of kaowool was observed to have significant effect on the target parameters. Notably, the system greatly depends on the low value of emissivity (0.0045) which is 85% smaller than emissivity of polished silver (0.03) which is one of the lowest values of emissivity at operation temperature practically possible in the ideal case. Therefore, achieving an extremely low emissivity value is currently unattainable. In addition, the mechanical structure needed to maintain vacuum layer complicates the system, increasing system cost and complexity. Thermal energy storage is a multidimensional and multidisciplinary framework and while our study does not propose novel solutions to the challenges of thermal energy storage, both experimental and theoretical methodologies offer insights into the fundamental hurdles facing such systems. In comparison to Case 3, Case 1 emerges as a more feasible option for optimizing thermal storage tanks employing Al-Si as phase change materials, presenting a practical solution with fewer complexities.