UC Merced

Energy storage is a pressing need throughout a range of applications. While electrical storage systems, such as for renewable power intermittency mitigation and electric vehicles, often receive considerable attention, the potential of thermal energy storage is equally significant. Storage of thermal energy is an important element in energy management. Thermal energy can be stored directly as sensible heat corresponding to a change in temperature of the storage media, latent heat corresponding to a change of phase in the storage media, or as heat of a chemical reaction in the media. Latent heat storage systems have significant advantages in terms of their energy density, exergetic efficiency, and practicality.

The focus of this thesis is a multi-scale examination of high-density polyethylene (HDPE) composites as potential phase change materials in latent heat thermal energy storage systems, specifically in a packed-bed setup suited for medium-temperature applications requiring heat below 125°C and involving direct contact heat exchange between the phase change materials and the heat transfer fluid.

The research commences with a bench-scale experimental evaluation, providing an in-depth thermal characterization of an HDPE composite fortified with surface-treated glass fiber fillers and coated with a thin layer of epoxy resin. This media is stable over repeated melting and solidification cycles and shows excellent thermal capacity, with more than 160 kJ/kg attributable to latent heat. The system can be charged and discharged at relatively high rates, e.g. > 100 W/kg. The performance of direct contact heat exchange between the storage media and heat transfer fluids, such as glycerol, flowing over the media is characterized. The performance study includes effects of the mass flow rate of the heat transfer fluid, the charging and the discharging temperature, and the initial bed temperature of the porous packed bed TES system. Significant advantages of this TES system are excellent energy density with high exergetic efficiency averaging approximately 79% (i.e. low temperature differences between charge and discharge). The direct contact heat exchange mode improves heat transfer performance, reducing the transport component of ΔTc/dc while eliminating the costs associated with the heat exchanger. Experimental characterization and models of conjugate heat transfer processes in a bed of storage media are presented. The simple construction also leads to a compact system for easy transportation and installation on site. The approach presented offers opportunities to enhance the use of thermal storage in medium temperature applications (e.g. a charge/discharge operational range between 120 °C – 140 °C).

The thesis transitions to a pilot-scale experimental assessment, introducing another novel composite consisting of HDPE and polyethylene terephthalate (polyester) fibers. Synthesis of this composite and its implementation in a pilot-scale thermal energy storage system is presented. Complexity associated with previous media composition and manufacturing is reduced. This refined media has improved manufacturability and lower cost of components. A strategy for in-house production of the composite and its installation in a media containment vessel is introduced and discussed. Thermodynamic characterization, including determination of latent heat of fusion using differential scanning calorimetry, reveals that the composite exhibits a thermal capacity with a latent heat of melting value of over 190 kJ/kg for HDPE. The thermal energy storage system, tailored for medium-temperature processes requiring heat below 120 °C, leverages the novel composite as a phase change material within a strategically designed containment system. This design offers efficient heat storage, promoting maximum capacity and heat transfer while minimizing parameters such as porosity, liquid volume, cost, and heat loss. Results indicate that the system can be discharged at rates exceeding 70 W/kg and maintains stability during extended thermal testing. This study also examines the composite's deformation, long-term stability, and leakage potential. The energy storage and discharge power, observed through bulk calorimetry tests, rely significantly on latent heat, confirming the composite's high performance. In conclusion, the composite shows potential for use in large-scale thermal energy storage for medium-temperature applications, with an operational charge/discharge range between 120 °C – 140 °C.

The final component of this work presents an analytical analysis of steam generation in pressure-drop steam accumulators with the incorporation of phase change material, in a packed bed configuration. The work employs a thermodynamic framework, featuring equations to quantify steam generation capacities under different working pressures. A quasi-steady approximation is also utilized to focus on latent heat effects during phase changes. The system's performance is assessed under two operational contexts—constant pressure output and constant thermal energy output. The study identifies conditions under which the phase change material-enhanced, packed bed, steam accumulator's performance surpasses that of a conventional steam accumulator, offering insights that are backed by derived equations and graphical representations.

Through this investigation that incorporates bench-scale testing, pilot-scale validation, and analytical modeling, the thesis provides a comprehensive understanding of the key performance metrics, operational constraints, and areas requiring further research, with respect to the use of polymeric composites as phase change materials in latent heat thermal energy storage systems for medium-temperature applications.