## 4 Discussion  
The state of the art today is that PCMs are generally characterized regarding their heat storage capability, sometimes also their thermal conductivity, their viscosity, and not to forget also their compatibility. Long-term performance is tested usually by cycling, meaning by repeated phase change. If possible, cycling is done at application-typical T(t) conditions, but to save time, it is also done at much shorter cycle times. Even accelerated testing by a modification of test conditions, e.g., the temperature a sample is exposed to, has been investigated. However, most of the discussion is on what is tested and how. Ref. [14] concluded that, considering the diversity of the used devices and the wide range of experimental parameters, further work toward a standardization of PCM stability testing is needed. The analysis thus shows that standardization of testing should not mean to try to define testing in a way that testing is always done with the same methods at the same conditions. This is not applicable as already different applications impose different conditions. But it is also not possible to test everything for reasons of time and experimental effort. Therefore, it is clear that testing should be tailored to the individual case. As the analysis has shown, this is already done in some aspects, however, not systematically. Therefore, here, a \"systematic approach to investigate property degradation\" with focus to tailor and optimize testing was developed. In Section 3.1.3, the basic functions and related properties that might be subject to testing, e.g., selected with a specific application in mind were first identified. Afterwards, in Section 3.1.4, an approach was given to find the possible degradation effects and underlying mechanisms, to allow tailoring and optimizing test procedures. As an example, the systematic approach was then applied to the most crucial function and its related property: heat storage enthalpy. With regard to the degradation effects, most of the degradation effects identified by the systematic approach suggested here have already been known; however, surprisingly, also a new one was found: it is the change in the chemical composition of a PCM from a mixture to others just on a small distance. The effect is known in metallurgy; a binary isomorphic system forms different microstructures at equilibrium and at nonequilibrium solidification. With regard to PCM, it has not been investigated yet, and this should be done in the future to complete knowledge on degradation effects. At the moment, this possible degradation effect is not being tested at all due to a lack of awareness. Thus, just testing for degradation effects that are well known from experience is at the risk of missing something crucial. It can, e.g., lead to a misinterpretation of the cause of an observed degradation. With regard to the underlying degradation mechanisms, for most degradation effects the corresponding degradation mechanisms are known, but mostly without details. This lack of deeper understanding of degradation has several serious consequences. For example, for phase separation, the reason, being the first step in the mechanism, is very well known. Opposing measures like thermal diffusion and mixing, and limitation of the distance of phase separation by gelling, thickening, or shallow containers, are also known. What happens in a dynamic situation of repeated phase change has been investigated qualitatively, but is not quantitatively understood at all. And only if degradation by phase separation accumulates with repeated phase change to a point where the function heat storage has decreased significantly does the degradation become relevant for applications. At the moment, the lack of deeper understanding leads to testing in a very limited way: tests identify if there is phase separation at all and how it affects the enthalpy, all at more or less arbitrary conditions. However, the outcome of testing can strongly depend on the choice of conditions.