## 3 Results  
### Development of a General Systematic Approach  
#### 3.1.1 Selection of Suitable Fundamental Terms  
A systematic approach needs suitable terms, being summarized in Figure 1, and later discussed in detail. Some of them, specifically \"property\", \"degradation\", and \"condition\", are somewhat established in the field. However, they are not used by all authors, and sometimes even various terms are used in a single publication. Those terms marked in red in Figure 1 are new, introduced here to allow a comprehensive discussion of the topic.  
PCMs, PCM composite materials, and encapsulated PCMs have \"functions\" regarding their application, each function being described by a physical or chemical \"property\". E.g., the function for heat storage can be described by the physical property enthalpy. The value of the property can change during use in an application. If the property change is in an unfavorable way for the application, this is called \"degradation\". A change is caused by a \"degradation effect\", e.g., phase separation, due to an underlying \"mechanism\", like the formation of phases with different densities and their macroscopic separation by gravity. Degradation is affected by \"conditions\" during use, as they affect the chemical or physical mechanism. First is the temperature, automatically related to the number of storage cycles or time (possibly also without changing phase). Then come o exposure to forces (external pressure, gravitation, internal forces by thermal expansion, etc.) and exposure to other substances (gases, walls, etc.).  
Various terms are in use specifically to discuss the crucial \"property change\" during use in applications such as stability, aging, or degradation [14], with reference to a property, performance, etc. For a scientific-technical discussion, including specifically quality control, not all of them are useful. The term \"performance\" refers to a specific situation and action. It has no measurable value in general; the performance with respect to a required function is good or bad, or sufficient, etc. It is therefore not useful in a scientific-technical discussion. In contrast, a physical or chemical property has a value, for example, the enthalpy change in J/g, or the thermal conductivity in W/(m-K). Consequently, physical and chemical properties allow a quantitative and not just a qualitative discussion. \"Aging\", from its meaning refers to an effect with time; even more crucial is it has no measurable value. \"Stability\" is a term that expresses the optimum performance, but also has no quantitative, measurable value to describe changes. But it can be used with a \"yes\" or \"no\" answer regarding positive expectations. This makes it suitable for quality control on a customer level. Quality means a promised function is observed in reality with sufficient stability during use, so a \"yes\" answer on stability means a promised function is observed within guaranteed limits, while \"no\" means that the function is not as was promised. More suitable, and established for product testing [17], is the term \"failure\". \"Failure\" is defined by the property changing to values not within specified limits for customer use, so it is measurable. The quality criteria at PCM-RAL [16] define a specific value at which, due to degradation, a product fails specifications. This is crucial if producers guarantee a value of a property for a product. The term \"property degradation\" therefore seems to be the most suitable for scientific-technical discussions, including discussing quality control in a scientific-technical context. Property degradation refers to a measurable value, specifically of a physical or a chemical property, and expresses that it refers to a possible negative trend. So, for the discussion here, the term \"property degradation\" is used throughout. The fundamental terms are now introduced (and suitable and unsuitable ones discussed with explanation).