Optimizing the design of composite phase change materials for high thermal power density

Abstract

Phase change materials (PCMs) provide a high energy density for thermal storage systems but often suffer from limited power densities due to the low PCM thermal conductivity. Much like their electrochemical analogs, an ideal thermal energy storage medium combines the energy density of a thermal battery with the power density of a thermal capacitor. Here, we define the design rules and identify the performance limits for rationally-designed composites that combine an energy dense PCM with a thermally conductive material. Beginning with the Stefan-Neumann model, we establish the material design space using a Ragone framework and identify regimes where hybrid conductive-capacitive composites have thermal power densities exceeding that of copper and other high conductivity materials. We invoke the mathematical bounds on isotropic conductivity to optimize and define the theoretical limits for transient cooling using PCM composites. We then demonstrate the impact of power density on thermal transients using copper inverse opals infiltrated with paraffin wax to suppress the temperature rise in kW cm−2 hotspots by ∼10% compared to equivalent copper thin film heat spreaders. These design rules and performance limits illuminate a path toward the rational design of composite phase change materials capable of buffering extreme transient thermal loads.