# Storage Capacity in Dependency of Supercooling and Cycle Stability of Different PCM Emulsions
## 1 Introduction
After 100 cooling/heating cycles in a test rig, the DSC measurements of the samples revealed an increased supercooling again and indicated an unstable nucleation agent. [PERSON] and [PERSON] [12] reported the development of a PCM slurry for cooling applications. A paraffin with a high melting temperature was used as a nucleation agent to reduce the supercooling of the emulsified PCM. The storage stability was tested by storing a sample over eight months and applying 50 heating and cooling cycles. The stability was analysed by comparing the DSC curves before and after the experiment. The emulsion showed no reduction of the effectiveness of the nucleation agent, but separation or creaming of the paraffin droplets was observed, which was, in further experiments, supressed by using thickeners. [PERSON] et al. [2] dispersed n-octacosane with a melting temperature of 60 \({}^{\circ}\)C for use in heat storage applications. As emulsifiers, they used non-ionic surfactant different combinations of Tween and span emulsifiers with a HLB of 12, as well as hydrophobic SiO\({}_{2}\) particles, as nucleating agent to reduce supercooling. The stability was tested first by storing samples for up to six months and, second, by applying five freezing and melting cycles in which the sample was heated to 70 \({}^{\circ}\)C and then cooled down again to room temperature. The breaking ratio, which is the ratio between the separated PCM to the whole tested volume, was used to quantify the stability. After the storing experiment, a creaming was observed but no clear trend in the changes of the droplet sizes. In the thermal cycling experiment, separated oil was observed on the samples, and a breaking ratio was determined in a range between 40% and almost 90% for the different emulsions. [PERSON] et al. [1] stated that the smaller and more homogenous the particle size distribution is, the more stable a PCM emulsion is for thermal-mechanical cycling and storage time-induced effects. [PERSON] et al. [13] described the development of PCM emulsions with particle sizes in the nanometre scale using hexadecane as the PCM. The emulsion showed a high stability against phase separation, which was tested under repeated thermal cycles in which the emulsion was cooled below the solidification temperature in an ice bath and heated up again by taking it out of the ice bath and exposing it to room temperature at 25 \({}^{\circ}\)C. A high stability was proven by applying this procedure. The DSC measurements revealed supercooling, which was reduced in further experiments by using SiO\({}_{2}\) particles, but nevertheless, the supercooling was still in the range of about 5-10 K.  
[PERSON] et al. [14] investigated a PCS based on microencapsulated paraffin with a melting range between 22 and 26 \({}^{\circ}\)C and a melting enthalpy of 50 kJ/kg. The PCS had a fraction of 30 wt.% of microcapsules and was tested on stability within a hydraulic test rig in which the PCS was cycled. The test rig contained plate-type heat exchangers and a centrifugal pump. In every cycle, the slurry was cooled below the solidification temperature and heated up above the melting temperature of the PCM again. Nearly 100,000 cycles were applied without observing a significant number of broken microcapsules. The storage capacity was investigated in a storage tank containing 5 m\({}^{3}\) of the slurry, and the results were compared with experiments using water as the storage fluid in the same storage tank. It was shown that it was possible to store 55% more energy than using water, which leads to a 95-min-longer discharge duration. [PERSON] et al. [15] reported the experimental investigation of a PCM emulsion with 35 wt.% n-octadecane in a 500-L storage tank. The storage capacity was compared to water, which was also investigated in the same storage tank.