## 3 Results
### Development of a General Systematic Approach
#### 3.1.4 Approach to Identify Degradation Effects and Underlying Mechanisms  
Having decided on the functions and related properties to be tested in an individual case is, however, only the easy part. To tailor testing of the selected properties afterwards to what is important/critical is much more complicated, specifically with regard to the selection of the test conditions. As a first step, however, always the relevant degradation \begin{table}
\begin{tabular}{l l l l l l} \hline \hline  & **Thermal** & \multicolumn{2}{c}{**Mechanical**} & \multicolumn{2}{c}{**Protecting Functions...**} \\ \cline{2-6}
**Basic Function** & **Heat Storage** & **Heat Transfer** & **Keep the Shape/Flow** & **Protecting the PCM** & **Protecting the Ambient** \\ \hline PCM & h(T) & \(\lambda\) & E, \(\eta\)... & m/V,... & m/V,... \\ PCM composite m. & h(T) & \(\lambda\) & E, \(\eta\)... & m/V,... & m/V,... \\ encapsulated PCM & H(T) & \(\lambda\), A, shape & E,... & m,... & m,... \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic functions of PCM, PCM composite materials, and encapsulated PCM in applications for heat storage and their corresponding physical or chemical properties for description. effects and underlying mechanisms with regard to the property must be identified. A systematic approach to find them is now given. Each function is described by at least one corresponding physical or chemical property (Figure 1, Table 1) to be measurable when testing, for example, the property enthalpy describes the function of heat storage. Degradation of a property can be due to different degradation effects, e.g., the enthalpy can degrade due to phase separation, nucleator deactivation, or thermal decomposition, or possibly more than one act. The mechanisms underlying the degradation effects are affected by the conditions, e.g., the temperature. For example, the temperature can cause the formation of different phases and nucleator deactivation, both leading to a degradation of the enthalpy. At the same time, temperature also affects other properties. Selective testing, which focuses on selected properties and conditions, often a range of values of a single condition, requires deeper knowledge about the PCM, PCM composite material, or encapsulated PCM to be tested, specifically possible degradation effects, underlying mechanisms, and how conditions affect them. Besides testing an application at application-typical conditions, basically all real testing is done for a selection. Without knowledge, a selection means to risk missing what is critical, or wasting time on testing the obvious. Accelerated testing, e.g., investigating the enthalpy when cycling at a temperature-time profile which is not application-typical to accelerate degradation and finally to make predictions for application-typical conditions, requires identifying an acceleration factor, meaning a critical condition, sometimes more. Deeper knowledge, meaning understanding properties, degradation effects, and mechanisms behind them, is needed. To identify possible degradation effects and underlying mechanisms, a systematic approach can, e.g., be checking from the small, the atomic, and molecular levels to the large levels, like external surfaces (Figure 3). Different functions, as being described by physical and chemical properties, are based on underlying structures, like bonds between atoms, crystals, etc. That properties are often connected is thus no surprise. The approach in Figure 3 is useful for the function of heat storage, as the discussion in Section 3.2 will show. It is also useful for the function of heat transfer as well as mechanical ones to keep the shape and flow. However, Figure 3 seems not useful for compatibility issues, like corrosion. Nevertheless, as it is the common systematic way to think of materials in physics, chemistry, and engineering, the presented approach should be useful, maybe with some modification, for most properties and their degradation.