# Storage Capacity in Dependency of Supercooling and Cycle Stability of Different PCM Emulsions
## 1 Introduction  
PCM emulsions or, more generally, PCM slurries (PCS) are heat transfer fluids consisting of a PCM that is dispersed in a fluid (in this study, water). Compared to pure water, they showed a high storage capacity due to the usage of the PCM melting and solidification process, in addition to the sensible heat capacity of the water and the PCM. To utilise this benefit, they have to be used within the PCM melting and crystallisation temperature range [1]. This offers the advantage of reduced volumes for energy storage due to a greater storage density [2]. PCS, in general, offer the advantage of being liquid-independent of the PCM's state of matter. This property makes it possible to convey PCS in hydraulic systems equally in both physical states of the PCM. Hence, PCS can act as heat transfer fluids and as a storage fluid. With this functionality, PCS contributes as a storage material to thermal systems that are operated within small temperature differences, such as cooling or temperature-buffering applications. PCS can transfer this heat homogeneously at an almost constant temperature [3]. The fine dispersion of the PCM in water leads to ahigh surface-to-volume ratio that contributes to a high heat transfer rate between water and PCM [4,5]. Over time, researchers have investigated different paraffins for use as PCM in emulsions. Already, in the 1990s, [PERSON] and [PERSON] [6] investigated the application of tetradecane with a melting temperature of 14.75 \({}^{\circ}\)C in water as a PCM emulsion for cold storage. A mixture of anionic and non-ionic emulsifiers stabilised the emulsion. Paraffins are available over a wide range of melting temperatures. Additionally, melting temperatures between that of pure paraffins can be achieved by mixing two different paraffins [7]. He and [PERSON] [7] mixed, for example, n-tetradecane and n-hexadecane and found melting temperatures between those of the pure materials. They also found that a mixture of about 90 mol% n-tetradecane and 10 mol% n-hexadecane showed a melting temperature at 2 \({}^{\circ}\)C. This makes it possible to adapt the melting point to the demand of different applications by choosing a paraffin with an appropriate melting temperature or mixed paraffins with higher and lower melting temperatures. According to [PERSON] and [PERSON] [8], n-alkanes with an even number of carbon atoms in the range from 12 to 26 crystallise in the triclinic form, and those with an odd-numbered carbon chain show a phase transition from liquid to a metastable orthorhombic rotator phase and, in the second step, at a lower temperature, a transition to the stable phase. It was also observed that even-numbered n-alkanes show a phase transition to the rotator phase when emulsified in the nanometre scale. [PERSON] [9] observed this as well for octadecane emulsions. It was found that the transition to a rotator phase accounts for 70-75% of the total crystallisation enthalpy. Beside this change in the phase transition, it was observed that, due to the confined volume of the paraffin droplets, it was crystallising at a temperature below its bulk phase-change temperature from solid to liquid. This is due to a separation of the natural given crystallisation seeds [10]. This hysteresis effect is called supercooling. [PERSON] et al. [11] reported that a PCM emulsion with a melting temperature of 18 \({}^{\circ}\)C had to be cooled down to 7 \({}^{\circ}\)C to obtain the full latent heat. Nucleation agents were used to supress the supercooling. [PERSON] et al. [5] developed, for example, a PCM emulsions based on the paraffin blends RT6 with a melting temperature of 6 \({}^{\circ}\)C, RT10 (10 \({}^{\circ}\)C) and RT20 (20 \({}^{\circ}\)C) from Rubitherm Technologies GmbH. Non-ionic emulsifiers--in particular, ethoxylated fatty alcohols--were used for the stabilisation of the PCM droplets/particles. A paraffin with a melting point at 50 \({}^{\circ}\)C was used as a nucleating agent that was able to decrease the supercooling.