Nanocomposite phase change materials for high-performance thermal energy storage: A critical review

Phase change materials (PCM) are deemed to be a great option for thermal energy storage (TES) with high energy density, but the low thermal conductivity of numerous PCM candidates, especially organic PCMs, has remained an issue of low power density. Over the past two decades, the development of nanocomposite PCM with highly thermally-conductive carbon-based nano-additives, referred to as nano-enhanced PCM (NePCM), has achieved great progresses towards boosting the effective thermal conductivity. However, along with the ostensible success of thermal conductivity enhancement, two negative yet overlooked effects arise. First, the presence of the nano-additives lowers the latent heat of fusion of the matrix PCM, leading to a significant loss in energy density as a penalty. Secondly, dramatic additive-induced viscosity growth also occurs, which suppresses, or even eliminates, natural convection during melting of NePCM and may overwhelm the contribution by augmented heat conduction. If thermal conductivity were used as the single performance indicator, the low power density issue would not get actually resolved. Here we propose a novel way of enhancing hydrogen bonding between the matrix PCM, especially for alcohols rich in hydroxyl groups, and carbon nano-additives to make up the enthalpy loss. We also stress the adoption of close-contact melting mechanism, featuring high heat transfer rate through a thin film of molten PCM, with appropriate fin design in TES heat exchangers. The combination of enhancing strategies from molecular interactions to heat transfer structures sheds light on the approach to tackling the challenge of improving the overall performance of PCM-based TES systems.