Optimizing phase change composite thermal energy storage using the thermal Ragone framework

Optimal design of thermal energy storage devices can allow systems to approach specific design objectives while maintaining desired levels of performance. The thermal Ragone framework has been applied to the design of thermal energy storage heat exchangers, specifically identifying relationships between their power and energy capabilities. In this paper, a finite-difference model is used to optimize thermal storage heat exchanger designs for three objectives given a discharge power constraint. The three objectives are maximizing energy density, minimizing energy-specific capital costs, and minimizing the levelized cost of storage. This study focuses on the design of planar thermal energy storage heat exchangers with phase change materials and thermal conductivity additives. Key design parameters identified included the conductivity additive volume fraction, spacing between heat transfer fluid tubes, and the phase transition temperature. The optimal design is found to depend strongly on the required thermal power, with higher powers requiring transition temperatures further from the use tem-perature, more conductivity additives, and closer tube spacing. There is also a trade-off between these design parameters, where changing one (e.g., closer tube spacing) can allow another parameter to be relaxed (e.g., less conductivity additive). To aid in future device design, models of reduced complexity were developed and evaluated for their ability to predict optimal designs. These simplified models can predict the performance of thermal energy storage heat exchangers up to 5000 times faster than the finite-difference model. For the design objectives of maximizing energy density, minimizing costs, and minimizing levelized cost of storage, the simplified models predicted optimal designs with average absolute deviations below 2 % in relation to the optimal designs chosen by the finite difference model optimization. These models can serve as tools to further study thermal storage heat exchangers incorporating the realistic trade-off between the device's power and energy.