By Dr. Dan Sellan, The University of Texas at Austin, Nanomaterials and Thermo-Fluids Laboratory
Big Picture – Phase Change Materials for Thermal Energy Storage
High-performance thermal energy storage technologies are envisioned to play a broad and critical role in the sustainable use of energy in heating and cooling applications, including building and vehicle heating and cooling; solar energy harvesting; and thermal management of electrochemical energy storage and electronic devices. Thermophysical energy storage based on phase change materials (PCMs) is one of the technologies being actively pursued.
One major barrier currently preventing the broad adoption of many PCM-based technologies, however, is the very low thermal conductivity of available PCMs, which significantly limits the power capacity (charging and discharging rates). Increasing the PCM thermal conductivity, kPCM, without affecting other performance criteria such as energy density or thermal cycling stability has been the focus of much research in recent decades, but only incremental performance advancements have been realized. Though dispersing high-thermal conductivity nanotubes and graphene flakes in PCM has shown to increase kPCM, the enhancement is limited by interface thermal resistance between the nanofillers, among other factors such as detrimental surface scattering of phonons.
Overview – Enhancing PCM Thermal Conductivity with Ultrathin Graphite Foams
Recently, our group at The University of Texas at Austin demonstrated significant thermal conductivity enhancement of PCMs with negligible change to other thermophysical properties by using continuous ultrathin-graphite foams (UGFs) to fabricate UGF–PCM composites. We showed that embedding continuous ultrathin-graphite foams (UGFs) with volume fractions as low as 0.8–1.2 vol% in a PCM can increase kPCM by up to 18 times, with negligible change in the PCM melting temperature or mass specific heat of fusion. The increases in kPCM, thermal cycling stability, and applicability to a diverse range of PCMs suggest that UGF composites are a promising route to achieving the high power capacity targets of a number of thermal storage applications. These findings are an important step toward high-performance thermal energy storage technologies.
Figure: Comparison of thermal conductivity for various PCM fillers; and SEM micrographs of ultrathin graphite foam, before and after filling with PCM. Courtesy Dan Sellan.