Compact PCM-based thermal stores for shifting peak cooling loads

Thermal energy stores, when used as part of a demand management system, can shift peak cooling loads to off-peak hours or to periods when intermittent renewable energy sources are available. This enables more efficient use of baseline and renewable electric capacity, and reduces dependence on less efficient and more costly peak energy power plants. High density thermal stores can be an effective retrofit option for existing buildings and environmental control systems. This paper investigates the development of a simple, compact, high density thermal storage unit suitable for use in small to medium residential or commercial applications that utilize, or can be retrofitted to use, chilled water cooling coils. This thermal store design is based on the use of a widely available phase change material (PCM), macro-encapsulated into cylindrical tubes that are packed into a containment tank. An experimental scale model of this thermal store is constructed for test purposes, and tests are conducted to determine the feasibility of this design and to characterize its operation. A numeric model is developed to demonstrate the feasibility of modeling such a store using simplified assumptions, both for design purposes and real-time predictive control strategies. The results show that such a design increases thermal capacity over that of a simple chilled water tank, and that a simplified numeric model for use by control systems can predict the short-term performance of the store in near real-time.