The energy density and power density are critical properties of thermal energy storage systems. Use of a phase change material as the storage medium provides high energy density due to the ability to store energy as latent heat during the phase transition; however, the power density is limited by the low thermal conductivity. Insertion of a highly conductive graphite foam within the material can increase the rate of thermal response of the phase change material. As the graphite bulk density increases, the thermal conductivity increases, but the composite latent heat decreases due to displacement of the phase change material. This introduces a trade-off between energy density and power density of the composite.
In this work, a validated numerical model is used to study the trade-off between energy density and power density with respect to graphite bulk density under various imposed heat fluxes. Comparisons are made based on the melting time, junction temperature between the composite and the heat source, and the volumetric energy density. To simplify the complicated relationship between composite thermophysical properties and charging response, two non-dimensional numbers are used. The Fourier number provides a comparison between the heat storage and heat diffusion by considering the thermal conductivity, latent heat, sensible heat, density, melting time, and volume. A dimensionless temperature compares the junction temperature (temperature between the heat source and the composite) when the sample is fully melted to the melt onset temperature. These non-dimensional numbers can assist in the design of latent heat storage systems where some parameters are fixed (such as heat flux, junction temperature, mass, operating time, or required energy storage) and others must be determined by the design (such as required thermal conductivity or height).