## 3 Results
### Development of a Detailed Systematic Approach for the Function of Heat Storage
#### 3.2.2 Degradation Effects and Underlying Mechanisms
The next degradation effect is a change in the chemical composition of the PCM from a single mixture to more than one, as before, but now followed by unmixing on a larger distance, called phase separation or segregation. The underlying degradation mechanism is a formation of more than one phase with different mixing ratios, different densities, and the successive separation by gravity or centrifugal forces. This requires, different than before, that at least one phase is liquid to allow separation on a larger distance. Phase separation is a complex effect because the underlying mechanism depends on a series of steps: first, the formation of phases of different densities, then their macroscopic separation and lack of effective opposing mechanisms like thermal diffusion or mixing and limiting measures like gelling or thickening. Phase separation is commonly discussed using phase diagrams, which show if a PCM changes phase in a congruent, semi-congruent, or incongruent way. Congruent melting occurs if all liquid and solid phases have the same composition [8]; this means that for congruent melting already the initial step needed for phase separation cannot happen. However, knowledge of what happens in all the other cases, and the consequences for heat storage, is still far from being sufficient. Phase separation can occur if phases of different densities form and if at least one of them is liquid to allow motion on a large distance. However, already phase diagrams show that in many cases just heating to a temperature somewhat above phase change leads to remixing, in equilibrium, if only sufficient time is given for the process to happen. What \"sufficient\" time means depends on the effectiveness of remixing, by thermal diffusion or by free or forced convection (mixing), and the distance that phases have separated, limited by the height of the container or encapsulation or further limiting measures like the use of foams, gelling, or thickening. Phase diagrams show if phase separation can happen. But phase separation is not a problem by itself. Crucial is if separation accumulates to a degree that significantly reduces the heat storage capability, thus how far degradation proceeds. Actually, in ice slurry generation from brines, phase separation is even used intentionally. All options regarding a change of the chemical composition of a PCM, being a single chemical compound or a mixture of more than one chemical compound, have now been treated: first the change to other chemical compounds, possible in pure substances as well as in mixtures, then, for mixtures, the change to other mixtures without large distance separation, and then also with large distance separation. The physical composition is the arrangement of the atoms/molecules at a given chemical composition. Physical composition thus refers to the crystal fraction, called crystallinity, and the specific crystal structure. A change in phase generally means a change in the chemical composition and/or physical composition. For heat storage, a PCM undergoes preferably a change in the physical structure, not the chemical one. The change in the motion and in the position of atoms/molecules between the phases of a phase change determines the enthalpy change \(\Delta_{\text{pc}}\)H as well as the entropy change \(\Delta_{\text{pc}}\)S. At equilibrium between phases, both are related by \(\Delta_{\text{pc}}\)H = T\({}_{\text{pc}}\)-\(\Delta_{\text{pc}}\)S, and consequently, they determine the phase change temperature T\({}_{\text{pc}}\). How much heat is stored, given by the phase change enthalpy, increases with entropy change. For this reason, the low-temperature, solid phase should be crystalline, because crystals have low entropy due to their regular arrangement of atoms/molecules. This leads to two further degradation effects. The next degradation effect is thus a change in the physical composition of the PCM on the crystal scale, specifically with regard to the low temperature phase, by formation of a phase that is not the one wanted.