Review of stability and thermal conductivity enhancements for salt hydrates

•Technical challenges for salt hydrate PCMs are reviewed and summarized.•Numerous enhancement techniques are presented, discussed, and compared for supercooling, phase separation, and thermal conductivity.•In-depth review on calcium chloride hexahydrate, sodium sulfate decahydrate and sodium acetate trihydrate.•Longevity data in the literature for salt hydrates is summarized.•Future challenges in salt hydrates are identified. Salt hydrates can be used as phase change materials for thermal energy storage. Critical technical challenges for their widespread deployment include poor cycling stability, large supercooling, and low thermal conductivity. In this work, numerous enhancement techniques are reviewed. Related to stability, numerous existing methods to characterize the number of thermal cycles are summarized. Following this, 11 techniques to mitigate phase separation (plus 9 studies on macro-, micro-, and nano-encapsulation types) are reviewed. For supercooling, 38 nucleator-salt hydrate combinations to minimize subcooling are reviewed. The empirically observed effect of isomorphism in minimizing subcooling is explored and discussed, with cross-study trends depicted for 38 nucleators across 9 salt hydrates. In addition, several studies reporting combined effects on stability and supercooling are presented. Related to thermal conductivity, 32 combinations of conductivity enhancement material and salt hydrate are reviewed. For those using graphite, the dependence of conductivity enhancement on graphite mass fraction is shown across numerous studies for different graphite types. The performance and stability of calcium chloride hexahydrate, sodium sulfate decahydrate, and sodium acetate were explored and are discussed in-depth. Finally, the status of enhancement to salt hydrates is summarized, and remaining challenges are identified.