## 3 Results
### Development of a Detailed Systematic Approach for the Function of Heat Storage
#### 3.2.3 Testing of Degradation  
For an individual case, based on the selected property to be tested and knowing the possible degradation effects, final decisions on the test design can be made. Testing can be done with regard to a specific degradation effect or be general such that the possible degradation effects help to understand observations. This is largely a matter of the interest and goal of testing. In any case, it must be kept in mind that the selected conditions (T, p, etc.) affect the result of degradation testing. For the function of heat storage, described by the property enthalpy h(T) on heating and cooling (covering the full phase change), testing is generally done by calorimetric methods. Further on, besides the T(t) profile used for the calorimetric measurement, the sample is of course exposed to a T(t) profile for testing degradation with time (e.g., called cycling). It is technically possible to do both in the calorimeter, and for many reasons, the exposure to a T(t) profile for testing degradation is however commonly in an individual setup. The most common way of testing is simply testing under application-typical conditions for degradation and only determining the property enthalpy in well-defined intervals, for example, after every 100 cycles. However, as discussed before, what is application-typical?And where are the limits of the conditions where degradation starts? And without deeper understanding, accelerated testing is also not possible. Thus, a deeper knowledge (Figure 1) of the degradation effects, the degradation mechanisms behind them, and the conditions affecting them is needed. These issues are now briefly addressed with regard to the two most commonly investigated degradation effects: changes in the chemical composition of a PCM to other chemical compounds and changes in the chemical composition of a PCM from a mixture by unmixing on a larger distance (called phase separation or phase segregation). A change in the chemical composition of a PCM to other chemical compounds by a chemical reaction can be achieved by decomposition of just the PCM or by a reaction of the PCM with a substance it is exposed to. Therefore, the first issue to decide for testing is whether the testing is with respect to exposure to a substance, e.g., the atmosphere, or not. However, it is advisable to test the PCM in any case first for decomposition just by itself, in vacuum or inert atmosphere, even if later testing under exposure is desired. Otherwise, if degradation under exposure happens, it is unclear if the reason is a chemical reaction with the exposed substance or if it is simply the decomposition of just the PCM. In any case, verifying if a chemical reaction has taken place by chemical analysis is needed to make sure that no other degradation effect took place. Better is of course even finding out which chemical reaction took place. Care has to be taken when using thermogravimetric analysis, TGA, which determines mass change of a sample exposed to T(t) profiles. First of all, mass change could also be simply due to evaporating sample material into the atmosphere or absorbing, for example, water from the atmosphere, which are both not due to a chemical reaction. Also, a chemical reaction of the sample material itself could happen by thermal decomposition and therefore without overall mass change. The systematic approach used in the previous section, which allows identifying all possible degradation effects, now allows here to identify options for misinterpretation. Besides, it is necessary to also select other testing conditions properly. It is today common knowledge that small heating and cooling rates in calorimeters must be chosen to avoid significant temperature gradients in the sample, which would otherwise lead to inaccurate h(T) results. But that a similar effect is present also in thermogravimetric analysis, and could become significant, has not been tested yet. With respect to accelerated testing the situation is for this degradation effect already well investigated.