## 3 Results
### Development of a Detailed Systematic Approach for the Function of Heat Storage
#### 3.2.2 Degradation Effects and Underlying Mechanisms
\begin{table}
\begin{tabular}{c c} \hline
**Degradation Effect** & **Underlying Degradation Mechanism** \\ \hline change in the chemical composition of PCM to other chem. compounds (atomic/molecular level) & chemical reaction, e.g., decomposition of just the PCM, or a reaction of the PCM with a substance it is exposed to (a compatibility issue, after failed protection of the PCM) \\ \hline change in the chemical composition of PCM from a mixture to others on a small distance & unmixing of a mixture on cooling and microscopic separation, e.g., eutectic mixtures solidifying after supercooling, or change in the mixing ratio by exchange of matter with the ambient (a compatibility issue, after failed protection of the PCM) \\ \hline change in the chemical composition of PCM from a mixture by unmixing on a larger distance (called phase separation or phase segregation) & formation of more than a single phase, with different densities, and macroscopic separation by gravity to a relevant degree (thermal diffusion or mixing not sufficiently present, and the same for gelling or thickening materials in PCM composites) \\ \hline change in the physical composition of the PCM, specifically the low temperature phase at crystal scale, to one that is not wanted & formation of a crystalline phase not wanted (polymorphism), or even an amorphous phase, in whole or in parts (e.g., PE, water, salt hydrates, salt-water eutectics, if not given sufficient time for crystallization upon fast cooling) \\ \hline no or late (called supercooling), or early formation of the desired low-temperature phase of the PCM & no, or late, nucleation of the desired low temperature phase, e.g., due to a deactivated nucleator or inhibited crystal growth, or early nucleation if stable supercooling is desired \\ \hline \end{tabular}
\end{table} 
Table 2: Degradation effects and underlying degradation mechanisms for the function “heat storage”. The next degradation effect is a change in the chemical composition of the PCM a level beyond molecules (so at the same chemical compounds), possible if a PCM is a mixture of compounds, from an initial composition to another. In metallurgy, the effect and underlying mechanism is well known. As described by [26], an alloy of 35 wt.% Co-65 wt.% Ni, a binary isomorphous system, forms different microstructures at equilibrium and at non-equilibrium solidification. At non-equilibrium solidification, solid phases with locally different concentrations form because mass diffusion is too slow to result in a homogeneous mixture of the same concentration throughout the whole volume. It is important to note that this effect, resulting from non-equilibrium, is not indicated by phase diagrams, as they describe equilibrium states. For PCM, Ref. [27] investigated tetradecane and hexadecane mixtures and also classified them as a binary isomorphous system. Using DSC, Ref. [27] found that the mixtures melt and freeze over a temperature range, and specifically that the temperature range depends on the DSC ramp rate. They concluded that the behavior of binary mixtures of n-alkanes is far more complicated than considered in earlier studies. Similarly, eutectic mixtures solidify without supercooling into two different mixtures, at the same time, separated only by a very small distance. However, if there is supercooling, meaning a non-equilibrium, the mixing ratios could change further, at small distances, and consequently also the overall heat stored. In both cases, complete melting brings back the initial state, thus eliminating any previous degradation. The crucial question remains what happens if binary isomorphous mixtures are not melted completely. The mixing ratio can also change by the exchange of matter with the ambient. Then the mechanism is again a compatibility issue, after failed protection of the PCM. e.g., salt hydrates can change the mixing ratio by water exchange with the ambient, and as long as the changes remain small, like an impurity, this might not have consequences beyond the mixing ratio on short distances.