## 5 Conclusions  
The state of the art today is that PCMs are characterized regarding their long-term performance with respect to some commonly used properties, mainly enthalpy change (sometimes also thermal conductivity, viscosity, and compatibility), usually under repeated phase change at application-typical T(t) conditions (sometimes to save time also done at much shorter cycle times). It was shown here that, in contrast to common thinking, future standardization of testing should not mean to simply agree to test always with the same methods at the same conditions because this is not suitable to the large variety of PCMs and applications. Instead, it was shown that testing should be tailored to each individual case following a standardized approach to select what should be tested and at which conditions. The new \"systematic approach to investigate property degradation\" developed here identifies first the functions and related properties that might be subject to testing, e.g., with a specific application in mind, and then gives an approach to find the related degradation effects and underlying mechanisms that allow tailoring and optimizing test procedures. This can lead to significant savings in time and testing effort and better results. As an example, the systematic approach was then applied to the most crucial function and its related property: heat storage and enthalpy. With regard to the degradation effects, by the systematic approach suggested here, a new one was found. The common way of testing today, to test only for degradation effects that are well known from experience, is thus at the risk of missing something crucial. With regard to the underlying degradation mechanisms, for most degradation effects the corresponding degradation mechanisms are known, but mostly without details. It was shown on the example of phase separation that this lack of deeper understanding of degradation has several serious consequences. Specifically with regard to choosing testing conditions, the example of phase separation showed degradation could even be reversed if the conditions were changed somewhat. Thus, current testing at more or less arbitrary conditions can lead to a completely wrong impression on the degree of degradation. The lack of deeper knowledge on the degradation effects and the mechanisms behind them, however, causes serious problems not only in testing. The lack of deeper understanding of how conditions affect the degradation mechanisms usually leads to the attitude to avoid degradation at all costs. Avoiding phase separation in PCM development at all, instead of just limiting it to an acceptable degree, is a significant limitation in the material options to choose from. Even more, it also leads to not studying possible counter measures. With respect to future R&D, the first goal must therefore be to gain deeper knowledge of the degradation effects, mechanisms behind them, and how they are affected by the conditions. Based on that knowledge, the second goal is then to apply the knowledge for individually tailored and optimized testing. The third goal is finally to apply the knowledge for a reduction in property degradation in applications, meaning, e.g., improving PCM by additives such that degradation is reduced. These goals require a different way of R&D. Up to now, the focus has mainly been on an existing PCM or a potential candidate, which is then tested for degradation of a property value with an established method. However, having the degradation effects and mechanisms behind them as the focus of study, specifically understanding them not only on a qualitative basis but qualitatively to make predictions, PCMs have to be selected that promise learning something about the degradation effects and their underlying mechanisms. While the common way of investigation promises short-term results for a given PCM and its application, this very different way of investigation is required for the long-term development of PCM technology. This research received no external funding. Not applicable.