Phase-change materials for thermal energy storage
Sodium thiosulfate hydrates cif files.zip (54.40Kb)
Oliver, David Elliot
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There is a current requirement for technologies that store heat for both domestic and industrial applications. Phase-change materials (PCMs) represent an important class of materials that offer potential for heat storage. Heat-storage systems are required to undergo multiple melt/freeze cycles without any change in melting-crystallisation point and heat output. Salt hydrates are attractive candidates on account of their high energy densities, but there are issues associated with potential crystallisation of lower-hydrates, long-term stability, and reliable nucleation. An extensive review of the PCMs in the literature, combined with an evaluation of commercially available PCMs led to the conclusion that many of the reported PCMs, lack at least one of the key requirements required for use as a heat-storage medium. The focus of this research was therefore to identify and characterise new PCM compositions with tailored properties. New PCM compositions based of sodium acetate trihydrate were developed, which showed improved properties through the use of selective polymers that retard the nucleation of undesirable anhydrous sodium acetate. Furthermore, the mechanism of nucleation of sodium acetate trihydrate by heterogeneous additives has been investigated using variable-temperature powder X-ray diffraction. This study showed that when anhydrous Na2HPO4 was introduced to molten sodium acetate trihydrate at 58°C the hydrogenphosphate salt is present as the dihydrate. On heating to temperatures in the range 75-90°C the dihydrate was observed to dehydrate to form anhydrous Na₂HPO4. This result explains the prior observation that the nucleator is deactivated on heating. The depression of melting point of sodium acetate trihydrate caused by the addition of lithium acetate dihydrate has also been investigated using differential scanning calorimetry and powder X-ray diffraction. It has been possible to tune the melting point of sodium acetate trihydrate thereby modifying its thermal properties. Studies of the nucleation of sodium thiosulfate pentahydrate, a potential PCM, led to the structural characterisation of six new hydrates using single crystal Xray diffraction. All of these hydrates can exist in samples with the pentahydrate composition at temperatures ranging from 20°C to 45°C. These hydrates are: α-Na₂S₂O₃·2H₂O, which formed during the melting of α-Na₂S₂O₃·5H₂O; two new pentahydrates, β-Na₂S₂O₃·5H₂O and γ-Na₂S₂O₃·5H₂O; Na₂S₂O₃·1.33 H₂O, β-Na₂S₂O₃·2H₂O and Na₂S₂O₃·3.67 H₂O, which formed during the melting of β- Na₂S₂O₃·5H₂O. Furthermore, new PCMs in the 75-90°C range were identified. The commercial impact and route to market of several of the PCMs are discussed in the final chapter.