# Storage Capacity in Dependency of Supercooling and Cycle Stability of Different PCM Emulsions
## 3 Results and Discussion  
The used paraffins are characterised via DSC as well. Table 1 summarizes all DSC results and Figure 2 illustrates the variety of phase transitions and enthalpies within the dependency of temperature. All analysed paraffins show supercooling in the form of a hysteresis. Parafol 16-97 (P16-97), Parafol 18-97 (P18-97) and Parafol 20Z (P20Z) are linear \(n\)-alkanes, with an even number of 16, 18 and 20 carbon atoms. In the DSC, they show a single-phase transition or two-phase transitions (P20Z) that are very close together when crystallising. The phase transitions are sharp and take place in a narrow temperature range. Linpar 17 (L17), Parafol 17-97 (P17-97), Parafol 19-90 (P19-90) and Parafol 19-97 (P19-97) are \(n\)-alkanes with an odd number of carbon atoms. These \(n\)-alkanes show a clearly visible second phase transition in both the melting and crystallisation processes. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**PCM/Paraffin** & **T\({}_{\text{m-end}}\)** & **T\({}_{\text{m, onset}}\)** & **T\({}_{\text{c, onset}}\)** & **T\({}_{\text{m-start,1 st}}\)** & **T\({}_{\text{m-start-1 st}}\)** & **\(\Delta\)Hf\({}_{\text{m-tot}}\)** \\ \hline  & \({}^{\circ}\)**C** & \({}^{\circ}\)**C** & \({}^{\circ}\)**C** & \({}^{\circ}\)**C** & \({}^{\circ}\)**C** & **kJ/kg** \\ \hline P16-97 & 19.6 & 16.6 & 15.7 & 13.9 & - & 217 \\ P16-97/P18-97 & 22.3 & 18.7 & 21.1 & 15.4 & - & 151 \\ L17 & 24.1 & 20.5 & 21.1 & 17.4 & 6.1 & 212 \\ P17-97 & 24.8 & 21.2 & 21.6 & 18.3 & 9.3 & 210 \\ RT27 & 27.7 & 24.5 & 26.1 & 19.4 & - & 153 \\ P18-97 & 30.3 & 27.3 & 26.7 & 23.7 & - & 223 \\ P19-90 & 32.4 & 30.5 & 30.6 & 25.9 & 17.1 & 207 \\ P19-97 & 34 & 31.2 & 31.5 & 29 & 19.2 & 222 \\ P20Z & 36.5 & 34.3 & 34.6 & 29.1 & - & 211 \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of the most important properties of bulk PCM that are taken for the formulation of PCM dispersions. Figure 1: Example for the melting behaviour of a partly crystallised microencapsulated paraffin. The PCM shows two phase transitions in the crystallisation process. The 1 st transition is at the higher temperature. P17-97 has an onset temperature T\({}_{\rm onset}\) (melting point) of 21.2 \({}^{\circ}\)C, L17 of 20.5 \({}^{\circ}\)C, P19-90 of 30.5 \({}^{\circ}\)C and P19-97 of 31.2 \({}^{\circ}\)C. These paraffins show a smaller hysteresis than most of the pure even-numbered paraffins, but the melting and crystallisation also take place over a broader temperature range. Combined with the two-phase transitions, the temperature difference in an application must then be comparatively large to use the full latent of the storage capacity. For example, P17-97 requires a temperature range of 8 K when only the first phase transition is used and 15 K when the second phase transition should be used as well. The comparison between P19-90 and P19-97 shows how impurities influence the storage capacity and the phase transition temperature. The total storage capacity is reduced from 222 kJ/kg for P19-97 to 207 kJ/kg for P19-90, and the melting temperature (T\({}_{\rm onset}\)) decreases from 31.2 \({}^{\circ}\)C to 30.5 \({}^{\circ}\)C.  
RT27 is a purchasable blend of _n_-alkanes with paraffin, having a different numbers of carbon atoms in their hydrocarbon chains. The melting point T\({}_{\rm onset}\) is at 24.5 \({}^{\circ}\)C, which is close to that of P18-97 (27.3\({}^{\circ}\)C). The phase transition is in a temperature range that is larger than that of P18-97, and the enthalpy for the blend RT27 is 153 kJ/kg, which is 70 kJ/kg less than that of P18-97 (223 kJ/kg). For the comparison with L17 and P17-97, which are both high-purity odd-numbered paraffins, a blend was mixed using 40 wt.% P16-97 and 60 wt.% P18-97. It has a melting point T\({}_{\rm onset}\) of 18.7 \({}^{\circ}\)C and an enthalpy of 151 kJ/kg. All the investigated blends show a small hysteresis but a broad temperature range for the phase transition in comparison to the pure PCMs. P20Z is an _n_-alkane with twenty carbon atoms in the hydrocarbon chain.