Development and Characterization of Stable Low-cost Salt Hydrate-based Phase Change Materials for Thermal Energy Storage Applications

Energy storage technologies are gaining attention due to rising utilization of renewable energy sources. One of the viable energy storage technologies is thermal energy storage (TES) in which system stores and releases thermal energy for various uses. Applications for TES systems include building systems, space heating and cooling, and refrigeration. Several TES systems use phase change materials (PCMs) to operate near-isothermally owing to phase change latent heat. Inorganic salt hydrate PCMs are popular because to their inexpensive cost, high energy density, and near ambient phase transition temperature. However, salt hydrate PCMs have phase separation, low thermal conductivity, and supercooling issues that reduce their long-term energy storage capacity and reliability. This work explores the synthesis, characterization, and property enhancement mechanisms of low-cost salt hydrate PCMs using various additives. Using empirical approach, dextran sulfate sodium (DSS) polyelectrolyte was found to stabilize sodium sulfate decahydrate (SSD) over repeated thermal cycles, compared to other additives. Atomistic simulation was used to unravel the stabilization mechanism at the molecular level. Findings show that there is an electrostatic interaction between SSD and the DSS polyelectrolyte which keeps the salt hydrate in domains that prevent phase separation, ultimately leading to improved thermal stability. The low thermal conductivity of salt hydrates has limited their practical application in TES. This work explored the enhancement of the properties of calcium chloride hexahydrate PCM using strontium chloride hexahydrate, graphene nanoplatelets (GNP) and cellulose nanofiber (CNF). Findings showed the amphiphilicity of CNF aids the dispersion of GNP thereby forming a compact PCM composite with improved thermal conductivity and thermal cycling stability. Salt hydrate PCMs are also attractive for near-ambient TES applications due to their suitable phase change temperature. This work further explored the synthesis and development of salt hydrate based eutectic PCMs with high heat storage capability and favorable phase change temperature for potential applications in near-ambient TES. Overall, this work provides promising methods to improve the properties and performance of salt hydrate PCMs by utilizing polyelectrolytes and bio-based nanocellulose, and eutectic approach for thermal energy storage applications.