# Storage Capacity in Dependency of Supercooling and Cycle Stability of Different PCM Emulsions
## 4 Conclusions  
In this study, even- and odd-numbered paraffins were used to prepare PCM emulsions and mixtures. The DSC analyses of the paraffins showed the two-phase transitions of the odd-numbered paraffins. The even-numbered paraffins shown in the DSC were only one-phase transitions, except for P20Z, which was a two-phase transition that was close together in the measurements. Furthermore, all used paraffin mixtures showed less enthalpy than the pure materials and a larger temperature range for the phase transition from a solid to liquid and, also, less supercooling. While all used pure paraffins offered total enthalpies in the range between 207 and 223 kJ/kg for the phase transition from a solid to liquid, the enthalpies of the mixtures P16-97/P18-97 and RT27 were 151 kJ/kg and 157 kJ/kg, respectively. This was on the same level with the enthalpies of the first transition of the measured odd-numbered paraffins, which were about 75-77% of their total enthalpy. The supercooling and storage factors of the PCM emulsions were influenced by the emulsified PCM and nucleation agent. The odd- and even-numbered paraffins showed different crystallisations in the bulk but similar behaviours when emulsified. Odd-numbered paraffins kept the two-phase transitions of the bulk materials, while the investigated even-numbered paraffins showed a second phase transition (different from the bulk materials) that indicated a phase transition from the liquid to a rotator phase (first transition) and from the rotator phase to the stable phase (second transition). This behaviour led to a reduction of up to 27% of the bulk paraffin's phase transition enthalpy for the first transition, as the emulsion P16-97, for example, showed. It was shown that the pure materials exhibited more supercooling when emulsified than the blends or mixtures. The results revealed that, due to this effect, the mixtures and blends were better in terms of the SF when small temperature intervals were given, which was shown by using a temperature difference of 6 K (comparing RT27 with P18-97 and P19-90 with P19-97). Additionally, the mixture of P16-97 and P18-97 had a SF within a 6-K range that was on the same level as P17-97 (2.11 versus 2.19). If a large melting enthalpy of a bulk material suggests a large SF for a PCM emulsion, these examples showed that this could not be transferred to the PCM emulsion directly, because the supercooling and limited temperature intervals of the applications needed to be taken into account and strongly influenced the SF and, therefore, the advantage of the emulsions over conventional heat transfer fluids. Figure 14: Comparison of the melting and crystallisation behaviours before and after the thermal-mechanical stability test. One aspect of the analysed PCMs that could be transferred to the PCM emulsions was its tendency of supercooling with a decreasing melting temperature. The shorter the PCM's hydrocarbon chain, the larger the degree of supercooling. Furthermore, the supercooling of the PCM emulsions was higher using purer materials. This effect was shown for Parafol 19-90 and Parafol 19-97, as well as for RT27 and P18-97 emulsions. Therefore, in terms of costs, it is favourable to have knowledge about the specific temperature interval of an application to consider whether it is more cost-efficient to use not-so-pure but cheaper PCMs providing a good storage capacity within small temperature intervals or to choose the more expensive, purer PCMs to provide more storage capacity when the usable temperature interval is a bit broader. All the investigated PCM emulsions had similar particle sizes. This indicated that, for the investigated emulsions, the particle size distribution was dependent on the formulation rather than on the used PCM. The thermal-physical properties were measured before and after cycling and showed comparable results for P16-97, L17, P18-97, P19-90 and P20Z emulsions.