## 3 Results
### Development of a General Systematic Approach
#### 3.1.2 Classification of Approaches for Testing and Identification of Optimization Potential
Even doing comprehensive testing if the variations to be tested are given by an application has a risk. Already what exactly is application-typical? Cycling requires a repeated temperature change with time. If the property to be tested has a degradation mechanism significantly affected by temperature or time, then what is application-typical? A temperature-time profile with a step? Or one with a linearly increasing ramp? These questions show that selective testing should be avoided without additional, deeper knowledge. Deeper knowledge, about degradation effects, underlying mechanisms, and conditions affecting them, is crucial for selective testing. A common and very important example is the function of heat storage, described by the property-specific enthalpy, and focus on the degradation mechanism of phase separation. The critical condition is temperature, specifically the temperature range where the PCM isn't fully solid. To reduce the effort, one can follow 3 steps: (1) Could degradation happen at all? If no, then why test? If yes, then (2) Does degradation happen (qualitative question) at the given conditions? If no, then stop. If yes, then (3) How much degradation (quantitative question) does happen? If the PCM used is known to be a single chemical compound, e.g., water, highly pure, degradation of the phase change enthalpy by phase separation cannot happen at all. Temperature is then not a critical condition, so why test its effect? If the PCM is known to be a salt hydrate, degradation by phase separation could happen. Then, does degradation happen? Phase separation in a salt hydrate is only possible in the liquid phase. Temperatures where the PCM is solid are not relevant and can therefore be eliminated from testing, at least for a first, qualitative answer. The PCM could be kept liquid at constant temperature above the phase change temperature, for longer than a typical cycle, to check if different phases separated from each other occur. If phase separation is observed, then afterwards, cycling under a variety of temperatures and temperature-time profiles can be performed at last. The detailed investigation, with the biggest effort, is now only performed if it is required, as the last step. In the example, in the 1 st step, a prediction is done, requiring theoretical knowledge or a phase diagram. In the 2 nd step, the experimental effort is reduced and the sensitivity of testing increased by not cycling at all, requiring theoretical knowledge on mechanisms behind the individual degradation effect. If a PCM might decompose at high temperatures, it is similarly possible to select first a temperature higher than in its application for testing, and if no decomposition is observed, testing under application-typical conditions, meaning cycling, is not needed. This is commonly done by TGA, scanning a large temperature range in a single, continuous heating test. Generally, to limit the effort and the time of testing, the functions looked at and conditions investigated and their variations are selected based on interest and on experience of what is important and/or critical. This is why generally the heat storage capacity is investigated, sometimes thermal conductivity, compatibility, leakage, etc. ; why typically the sample is subject to repeated phase change (called cycling), sometimes substances the sample is in contact with, etc. ; and why a fixed T(t) profile is used for cycling. Without thinking much about it, all real testing is already a selection, automatically done, often unaware. This explains why the reviews on the used testing methods [12; 13; 14] show an overwhelming majority of testing PCMs under cycling and checking for changes in the heat storage capability, typically phase change enthalpy and temperature, e.g., by DSC. And it also explains why cycling is performed in a wide variety of individual ways; without defining a specific application, there cannot be an agreement on application-typical conditions to be used for testing.