High density thermal energy stores utilizing phase change materials for shifting of peak cooling loads




Bourne, Stephen Frederick

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Air conditioning cooling loads consume 11% of the electricity produced in the United States, and can be as high as 27% of total electric production in warm, humid climates. During the summer months these HVAC cooling loads can comprise more than 50% of peak electric load, with much of the demand coming from small commercial or residential electric customers.

Thermal storage systems offer the ability to shift these peak cooling loads to non- peak periods, which allows for a more efficient operation of baseload electric power plants by better utilizing capital equipment and available capacity. In addition, thermal storage systems allow for the more effective use of intermittent, renewable energy sources by shifting cooling loads to periods of renewable energy availability. Finally, thermal storage systems reduce the capital equipment cost for HVAC compressor systems by allowing the equipment to be sized for average, rather than peak, loads.

Given that residential and small commercial buildings generate the majority of peak cooling load in warm, humid climates, a thermal storage system compatible with these structures and sites is necessary. These sites may not have the space necessary for common chilled water thermal storage systems, which utilize the temperature change of a working fluid (frequently water) to store sensible energy in large storage tanks. For residential and small commercial applications, a more compact, high-density thermal storage system will be required. Latent thermal storage, which utilizes the energy associated with a change in phase of a material to store thermal energy, shows promise for use in high-density thermal storage systems.

This research develops a latent thermal energy storage system based on phase change materials (PCM) suitable for use in applications where the size of the thermal store is critical, such as for existing residences and small commercial structures. Numerical, analytical, and experimental methods are used to design, test, and model a tube-encapsulated PCM-based thermal storage system with a capacity and performance suitable for HVAC applications. This research provides both general and specific design criteria sufficient to allow engineers to utilize this thermal store design for specific applications.


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