## 3 Results
### Development of a Detailed Systematic Approach for the Function of Heat Storage
#### 3.2.1 Property Basics and Related Property Measurement  
The behavior of a PCM can differ significantly from the ideal case with a phase change temperature \(\mathrm{T_{PC}}\). A PCM can show a phase change temperature range (Figure 4), even behave differently upon heating and cooling, called hysteresis. A case of hysteresis is if upon cooling the new phase is not formed until a sufficiently low temperature, the nucleation temperature \(\mathrm{T_{nuc}}\), is reached. This is called subcooling, supercooling, or undercooling, and depends on the sample size, cooling rate, and (if any is used) the nucleator.  
Phase change temperature \(\mathrm{T_{PC}}\) and enthalpy change \(\mathrm{\Delta_{pc}}\) are thus not sufficient data to characterize heat storage. Proper characterization of a PCM is done using heating and cooling curves of the specific enthalpy h(T), typically mass or volume-specific values, e.g., in J/g or J/cm\({}^{3}\), and the sample-size-dependent nucleation temperature \(\mathrm{T_{nuc}}\). The same applies to a PCM composite material. Encapsulated PCMs are individual objects, thus the description is done by an object-specific enthalpy, e.g., in J, and object-specific nucleation temperature.  
Methods to determine the heat stored are called calorimetric methods. Commonly used for PCMs are differential scanning calorimetry (DSC), T-history, 3-layer calorimetry, or modified heat flow meters [16]. For composite materials, being by nature inhomogeneous, methods with sufficient sample size to be representative have to be used. An encapsulated PCM is not a material but instead it is an object; therefore, calorimetric measurements must be done on it as a whole, e.g., on a whole pouch. Ideally, measurements give a resolution better than 1 \({}^{\circ}\)C. When doing a calorimetric measurement, the heat flux into or out of a sample is associated inevitably with a heat flux within it, and therefore an internal temperature gradient. Because of this temperature gradient, heat exchanged by the sample can only be attributed to a sample temperature within some temperature range. This effect, always present, increases with the heating/cooling rate as well as sample mass and its dimensions [19]. For PCMs, when changing phase the effect is already relevant even in small samples, so even for DSC measurements on small samples, a small heating rate has to be chosen for a good temperature resolution. PCM composite materials must be measured on samples large enough to be representative for inhomogeneities of the composite, so typically larger than for PCM, so this effect is hard to keep small. And for large, encapsulated PCMs, good temperature resolution is often impossible to get at acceptable effort. It is then common to use calorimetric methods which give no temperature resolution at all but are simple. The simplest setup, and extremely flexible regarding the sample size and shape, is a mixing calorimeter. In a mixing calorimeter, the sample  
Figure 4: Phase change with a phase change temperature range and hysteresis incl. supercooling.  
exchanges heat with a reference liquid, and the heat stored by the sample between initial and final temperature is calculated from the same for the reference liquid [20; 21; 22].