Heat Transfer Aspects of Using Phase Change Material in Thermal Energy Storage Applications

Abstract: Innovative methods for providing sustainable heating and cooling through thermal energy storage (TES) have gained increasing attention as heating and cooling demands in the built environment continue to climb. As energy prices continue to soar and systems reach their maximal capacity, there is an urgent need for alternatives to alleviate peak energy use. TES systems allow decoupling of energy production from energy utilization, both in location and in time. It is shown in this thesis that successful implementation of TES in the built environment alleviates peak energy load and reduces network expansion as well as the marginal energy production cost. This thesis analyzes phase change material (PCM) based TES systems in terms of material property characterization, numerical modeling and validation of thermal storage, as well as case specific techno-economic feasibility studies of system integration. The difficulties identified in latent heat TES design, such as heat transfer aspects, subcooling and identification of phase separation, have been analyzed through Temperature-History mapping and TES numerical modeling with experimental validation. This work focuses on the interdependency between resource availability, thermal charge/discharge power and storage capacity. In a situation where resource availability is limited, e.g. when using free cooling, waste heat or off-peak storage, the thermal power and storage capacity are strongly interrelated and should always be considered in unison to reach an acceptable techno-economic solution. Furthermore, when considering TES integration into an existing thermal energy distribution network, three adverse aspects are revealed in the Swedish case study: the single tariff system, the low-return temperature penalty, and the low storage utilization rate. These issues can be overcome through better adapted policies and optimized storage control strategies. Finally, despite the currently unfavorable conditions in the Swedish energy system, it is shown that TES has the potential to mitigate climate change through greenhouse gas emission reduction by displacing fossil-fuel based marginal thermal energy production.