## 3 Results
### Development of a General Systematic Approach
#### 3.1.2 Classification of Approaches for Testing and Identification of Optimization Potential  
An analysis of what is done for testing shows that basically three approaches can be distinguished (Figure 1). As they are crucial for a systematic approach, and to understand why testing is done the way it is done, an attempt is made to explain and define them here. Various aspects are relevant for testing. First of all, what is tested, why it is tested, and what is the focus. For example, a whole system can be tested (maybe for air-conditioning) that includes an ice storage, or just the ice storage, or one macroencapsulation with ice as a component of it, or just the ice as PCM. The focus can be one or several functions, for example, a PCM-graphite composite is used for heat storage, but as a composite with graphite, specifically if used for higher thermal conductivity, also for heat transfer. The  
Figure 1: Overview on fundamental terms and relation to testing. property that describes the function, for example, the enthalpy h(T) that is describing heat storage, is investigated with one or several conditions in mind, specifically temperature. If there is a specific application in mind on testing or not strongly affects the focus chosen for testing. For example, if testing an air-conditioning system, application-typical conditions can be used for testing, like typical room temperatures and outside temperatures, thus a range of values, but also extreme values. Also, with an application in mind, typical lifetimes and/or number of usages are expected and tested. If testing is for R&D or for quality control, so for degradation of a property value or for failure (meaning that a property value fails at given specification), is also crucial. Is there interest to know degradation effects, maybe also underlying mechanisms, such that an analysis is of interest beyond just the plain testing? And related to that, is there previous knowledge? For example, in R&D, it is often known what is tested, while in quality control of a product, usually its construction and composition are largely unknown. The most comprehensive testing would be testing with respect to all functions of what is to be tested, with regard to all conditions, their possible variations including extreme limits, and for full required lifetime, respectively, the number of usages or until failure. Having an application in mind, the application already reduces the variations significantly from the beginning. For example, a transport box with PCM to stabilize the temperature typically has a specific application case, for example, the transport of vaccines, world-wide, up to a week. The function is clear, the conditions of use too, and testing for typical lifetime including at extreme conditions is feasible. So, comprehensive testing is possible, and usually it is done. The reason that comprehensive testing is possible is because the application already limits variations. Knowing, e.g., the type of PCM is not necessary at all for testing. Testing a new PCM for transport boxes can, however, be different. If no information on the PCM is available, it should be tested comprehensively. But if the PCM to be tested is known to be a pure substance, then testing for phase segregation isn't needed. Or if the PCM is water, then thermal decomposition at typical application temperatures is impossible. Therefore, with increasing knowledge on what is to be tested and why, e.g., for which application, it is possible to limit the functions to be tested to those known to be crucial, and knowing the possible as well as impossible degradation effects, the number of conditions and range of their values including extremes can also be limited to those being crucial. Effort and time required for testing all variations is excessive. Therefore, selective testing, by selecting the functions and conditions on testing, saves effort and also time. However, limiting variations by an additional selection is at the risk of missing something crucial.