## 3 Results
### Development of a General Systematic Approach
#### 3.1.3 Selection of the Basic Functions and Corresponding Properties  
The literature reviews [12; 13; 14] have identified many things that are commonly tested. Looking specifically at PCMs, PCM composite materials, and encapsulated PCMs, they are tested for their phase change enthalpy, temperature, density, mass loss, thermal conductivity, viscosity, compatibility, corrosivity, leakage, etc. Each \"function\" of a PCM, PCM  
Figure 2: Property degradation data from tests, extrapolated beyond test time, and from accelerated testing.  
composite material, or encapsulated PCM in a heat storage application should be subject to testing, so described by a \"property\" (Figure 1). What is the systematics behind here?  
The crucial, basic functions in an application are heat storage and, related, the needed heat transfer (Table 1). The function of \"heat storage\" can be described by the specific enthalpy h(T) with regard to mass m or volume V for materials, such as for PCM and PCM composite materials. Encapsulated PCMs are not materials but objects; heat storage is here described by the enthalpy H of a single object, so of a single capsule. The \"heat transfer\" is described by the thermal conductivity \(\lambda\) for PCMs and PCM composite materials. For encapsulated PCM's heat transfer is not described by the thermal conductivity; actually, it cannot be described by a single property. Instead, heat transfer depends on a set of properties, including the thermal conductivity of the PCM and capsule wall, and the shape/form and surface area of the encapsulation.  
The more technical functions and corresponding properties are not so obvious, despite how they seem at first. Encapsulations have several functions: to avoid loss of the PCM, specifically liquid phase flowing away, to avoid contact with the ambient, and to keep the shape/form (specifically rigid encapsulations). But avoiding loss and contact are not basic functions with respect to applications. Basic functions are regarding heat and mass transfer, specifically to keep the shape for heat transfer, e.g., using rigid encapsulations, and the flow behavior for mass transfer, e.g., in liquid slurries, and additionally, specifically to protect from degradation and related issues, protecting the PCM from the ambient as well as protecting the ambient from the PCM. \"Protection of the PCM\" comprises avoiding loss of PCM, and avoiding changes in its composition by contact with the ambient, e.g., by taking up or loosing water. \"Protection of the ambient\" means avoiding contact to the PCM, e.g., due to leakage. And both also contain compatibility, e.g., corrosivity. The function to \"keep the shape\" is described by the modulus of elasticity E for shape-stabilized or encapsulated PCM, and the function \"flow\" by the viscosity \(\eta\) for PCM slurries. The function \"protecting the PCM\" must be described depending on the situation: regarding loss of PCM, it can be described by the PCM \"mass loss\" to the ambient, regarding uptake of harmful substances from the ambient by the \"mass uptake\" and its specific effect (e.g., water uptake).  
If necessary, more functions can be added to thermal and mechanical ones, e.g., electrical functions for the case of battery thermal management, specifically the electric conductivity, or optical functions for daylighting modules [18], like the optical transmittance. Further on, other properties can be specified, e.g., with regard to protecting the environment, properties describing safety risks.  
To tailor testing to the specific case, e.g., with an application in mind, it is now possible to select the basic functions and related properties which are relevant to test from this systematic overview.