A Novel Metal-Wool Phase Change Material for Thermal Energy Storage Applications with Exceptionally High Power Density

Phase change materials (PCMs) are widely recognized for their potential in thermal energy storage (TES) systems due to their high latent heat capacity. However, a significant limitation in their practical application is attributed to the low thermal conductivity of the majority of such materials, which is detrimental to the heat transfer rate, particularly during the solidification processes of the PCM. Conventional methods to enhance heat transfer, such as the use of fins, nanoadditives, and metal foams, have demonstrated limited effectiveness, as they are often plagued by issues such as significant volume occupation, segregation tendencies, theoretical versus practical performance gaps, and high production costs. To address these limitations, a novel heat transfer enhancement technique involving the incorporation of metal wool within the TES prototype is proposed. Metal wool consists of metal fibers entangled to form sheets with porosity exceeding 90%, creating a highly conductive three-dimensional path within the PCM matrix. This structure significantly enhances the heat transfer rate and reduces the solidification time, which is the main critical bottleneck in PCM-based TES applications. Moreover, being already present in the market for several applications, metal wools represent a relatively cheap, already available, and flexible solution for PCM-based TES applications, especially for those in which an already built TES tank is available. An experimental campaign was conducted by performing charging and discharging processes of a copper wool-PCM composite, by considering one complete and two partial copper wool patterns within the TES tank. Results demonstrated a substantial reduction in discharging times, ranging from 40% to 80% compared to the baseline case with bulk PCM. The maximum volume increase due to the inclusion of metal wool was less than 4%, ensuring that the same mass of PCM was maintained across all experiments. CFD simulations were performed using COMSOL to complement the experimental work. The comparison between the numerical simulations and experimental measurements was obtained by means of local temperature measurements within the PCM matrix at different locations of the TES tank. A significant challenge in these simulations is the variability in thermophysical properties of the PCM, which are often reported with scattered values in the literature due to differing experimental characterization techniques. These techniques typically involve highly controlled conditions that may not accurately represent mesoscopic application scales. To address this, the CFD model was coupled with an algorithm to explore the design space of the input parameters, accounting for their variability and enabling a sensitivity analysis to identify key input properties. This approach simplified the numerical model while ensuring accurate representation. The introduction of the copper wool within the numerical simulations required additional parameters to be considered, mainly due to the rough surfaces of the metal fibers and non-ideal contact between metal fibers and PCM, which created a thermal barrier to the heat flux. Validation of the numerical model was achieved for both the baseline case and the complete pattern with minimal discrepancies observed between experimental and numerical PCM (or metal wool-PCM composite) temperature curves over the discharging period. Following validation, the numerical model was used to explore different wool materials, providing a techno-economic overview of their potential impact in practical applications. Furthermore, a comparative analysis was performed between the experimentally tested copper wool and a hypothetical nanocomposite PCM with equivalent thermal performance. Numerical results indicated that a PCM nanocomposite with a thermal conductivity of 2.5 W/mK is required to match the thermal performance of the tested copper wool.