A Study of the Thermal Performance of Short-Term Thermal Storage Using Phase Change Materials in Integrated Community Energy and Harvesting Systems

Abstract

Thermal energy storage systems are essential for bridging mismatches between energy supply and demand, with sensible storage in water tanks being the most common storage medium. In large community systems, however, the size of these tanks becomes a major limitation. Integrating phase change materials (PCMs) into water tanks has emerged as a promising solution to overcome this challenge, as PCMs can significantly increase storage density by utilizing latent heat. The benefit of the latent heat energy storage is, however, hindered by the low specific heat of PCMs relative to water. Moreover, PCMs typically have low thermal conductivity, which influences heat transfer performance. As such, careful design of hybrid water-PCM tanks is required to ensure that the full latent heat capacity is utilized and that high energy density (relative to water alone) is achieved. The design of hybrid water-PCM tanks requires a systems context since the energy storage within the tank is determined by the operating temperature variations of the system. For example, because of the relatively low specific heat of PCMs, the benefit of a hybrid tank is enhanced when the operating temperature variation is low. In addition, due to the low conductivity of the PCMs, careful design of the PCM module is necessary to ensure that the PCM fully melts (or solidifies) within the available charging (or discharging) time. There is thus a need for mathematical models that can accurately predict the heat transfer and phase change characteristics of PCM modules while maintaining computational efficiency to allow for their incorporation into system analysis computer codes. The application of PCMs in long-term simulations, such as in community heating networks coupled with borehole fields, has been hindered by the computational cost of existing models which remain too expensive for simulations spanning months or years. This research addresses that gap by developing a reduced hybrid PCM–water tank model that is both accurate and computationally efficient, enabling the integration of PCM modules in community-scale energy systems to be studied in realistic, long-duration scenarios. This thesis presents a new reduced-order PCM model for spherical encapsulations. The model builds on the quasi-stationary approximation but introduces an effective-latent-heat formulation that captures both sensible and latent contributions. This modification significantly improves accuracy while reducing computational demand by up to two orders of magnitude compared to enthalpy-based models. The reduced PCM model was verified against a detailed benchmark conduction-dominated enthalpy model that is validated with experimental data from the literature. It was then integrated into a stratified tank framework, producing a reduced hybrid water-PCM tank model that is accurate, efficient, and suitable for long-duration simulations. To facilitate practical use, the model was implemented into the commercial code TRNSYS as a new component, Type 250, and applied in the context of integrated community energy systems to investigate the impact of adding PCM spheres to the water tank integrated with a borehole field thermal storage system. Parametric studies were performed on a hybrid water tank, revealing that its performance is strongly dependent on design and operating conditions. Higher mass flow rates shortened charging and discharging times but did not alter the total stored energy. Smaller spheres improved the melting rate, while larger capsules slowed it down. Lower temperature differences enhanced the PCM's latent heat contribution, increasing storage density but also extending charging times. Doubling the tank volume doubled the storage capacity, but it required higher flow rates to achieve a similar charging time. To guide system designers, a dimensionless contour map of PCM energy gain was developed, providing a simple tool for estimating the impact of capsule size, flow rate, and operating temperatures on the utilization of PCM. The reduced hybrid tank model was also applied to a community-scale Integrated Community Energy system with combined heat and power (CHP) units, short-term storage tanks, and a borehole field seasonal thermal storage. The results showed that PCM integration increased both the energy transferred to the borehole and the useful energy recovered from the CHP, especially at low CHP supply temperatures (i.e., 50°C). At higher CHP supply temperatures, the benefit of PCM diminished because most of the stored energy was in the form of sensible heat rather than latent heat. Notably, a 2 m³ tank with 50% PCM achieved higher energy density than a 4 m³ water-only tank, demonstrating the clear advantage of PCM integration in reducing the required tank volume. PCM also improved borehole performance by raising the average storage temperature, which enhanced the coefficient of performance of heat pumps and reduced compressor work during discharge. In summary, the contributions of this work include a new reduced PCM model that captures both sensible and latent heat while remaining highly computationally efficient; the development of a hybrid PCM–water tank model suitable for annual system simulations at low computational cost; the implementation of a new TRNSYS component (Type 250) to integrate the model into a widely used simulation platform; and the creation of a dimensionless contour map as a design tool for quickly estimating PCM performance under varying conditions. Finally, through a system-level case study, it demonstrated the tangible benefits of PCM integration in terms of tank storage density, CHP utilization, and borehole field performance, effectively overcoming the major barrier of high computational cost in detailed PCM models and enabling realistic deployment of PCM-enhanced hybrid tanks in community-scale energy systems.