Design of spatial variability in thermal energy storage modules for enhanced power density

Abstract

Peak load shifting requires strategies to efficiently and cost effectively absorb and discharge various forms of energy, including thermal energy. The energy storage rate of a thermal energy storage (TES) module containing phase change materials (PCMs) depends on the module geometry and dimensions, the internal distribution and orientation of PCMs and thermally conductive elements, the thermophysical properties of the materials composing the module, the characteristics of the heat transfer fluid responsible for transporting heat to the module, and the rate of conjugate heat transfer between the heat transfer fluid and the internal TES volume. Due to the complex interactions between design variables in such a system, routes for efficient optimal design of a TES module remain unresolved. To overcome these challenges, we implement and validate a reduced-order dynamic model of a metal-PCM composite TES module that accounts for spatial variability of a composite PCM layer. This manuscript reports a sensitivity analysis to understand the effects of key design parameters on performance metrics of interest, using the Ragone plot formalism to characterize tradeoffs in energy and power density. Finally, a case study is presented in which the model is used to optimize a tube-fin TES module to maximize its power density at different timescales. Improvements in both power density and average power are demonstrated when the metal fraction within the PCM is allowed to vary spatially.