Thermal Energy Storage Using Phase Change Materials and their Composites

Abstract

Commercial thermal energy storage (TES) systems require consistent thermal performance over their operational lifespan. However, thermal cycling of phase change materials (PCMs) in TES composites results in volume changes that disrupt thermal contacts among the fillers, such as expanded graphite (EG), leading to unstable thermal conductivity (k) that deteriorates with cycling. To overcome this limitation, we have addressed two key elements: (a) establishing highly stable filler connections using binders and (b) minimizing the influence of PCM volume changes on filler network degradation. Through a scalable synthesis methodology, we demonstrated the production of exceptionally robust EG/PCM composites with strong interconnectivity of filler particles within a stable matrix. The corrosion-resistant matrix, formed by carefully selecting binders based on PCM-binder compatibility, stabilized thermal networks, and ensured reliable k maintenance over thousands of thermal cycles. In this work, we introduced an innovative in-operando technique for real-time visualization of salt hydrate PCM crystallization, facilitating analysis of interactions among PCM, nucleator, and thickener within TES systems. Leveraging fundamental insights gained from crystallization studies on eutectic PCM of zinc nitrate hexahydrate (ZNH) and KNO3, we discovered that the formation of large, sharp-cornered PCM crystals during freezing dislodged EG particles, yet carboxymethyl cellulose (CMC) thickener enabled the formation of smaller crystal networks, thereby preserving thermal contacts. With our two-pronged approach, we achieved minimal k fading up to 10,000 thermal cycles for 10 vol% EG/paraffin wax composites and 1,000 cycles for 7 vol% EG/ZNH eutectic composites. This study provides valuable insights into cooperative interactions among TES system components (PCM, filler, thickener, binder) for exceptional thermal properties and robustness.