## 1 Introduction
### Long-Term Performance
That a liquid phase changes to different crystal structures in the solid, depending on the conditions, is also possible, e.g., in erythroid [9]. Crucial is also if these effects are reversible. The formation of an amorphous or other crystalline phase in the solid is already reversible by melting. Reversibility of phase separation can depend on the specific conditions, or be impossible. More elementary is overall chemical change, e.g., if a salt hydrate absorbs water from the atmosphere. Similarly, oxygen from the atmosphere leads to a chemical reaction at high temperature in sugar alcohols [10; 11]. The number of possibilities rises with new PCM material classes, with use of additives, or the development of shape-/form-stable PCMs and PCM composite materials. Investigations of specific PCMs from many material classes, with a variety of methods, are numerous. Ref. [12] reviewed the methods previously used for \"stability characterization\" of PCMs. They state that this means the stability of thermal, chemical, and physical \"properties\", after a number of repeated melting/freezing cycles, commonly also called cycling stability. They focused on four issues: equipment used to perform the cycling tests, techniques used to characterize the PCM before and after cycling and to check for \"degradation\", the number of cycles performed, and the temperature-time profile used for the melting/freezing cycles. For these, they listed what has been used, and tried to find out a common, useful methodology. They concluded that no common standard for thermal cycling stability tests was available at the time. Recent updates were done by [13; 14]. Summarizing, a PCM sample is placed in a temperature-controlled ambience, the temperature changing with time in a defined way. The temperature-time profiles used vary strongly regarding maximum and minimum temperature, the heating/cooling rate to reach them, and the time the temperature is kept constant, which define a single storage cycle, as well as number of cycles or time. Besides the temperature, other conditions are the exposure to gases, specifically air, N\({}_{2}\), Ar, or vacuum, and the container wall materials being glass, ceramic, metallic (aluminum, stainless steel), or plastic. While cycling is rarely done in a calorimeter due to cost and lack of flexibility of test conditions, differential scanning calorimeters (DSC) or other calorimeters are generally used to check the heat storage capability after a number of cycles. An indication of changes is mass change, even giving some hint on the reason of the change observed. Whether the mass of a sample has changed can be checked after a number of cycles. More detailed is, however, a separate measurement by thermo-gravimetric analysis (TGA), giving information on the temperature where the change occurs. To find out potential causes of any change, information regarding structural changes often comes from X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and also chromatography. An attempt to develop a methodology to test PCM candidates in R&D was published by [13], based on methods used by researchers to test new PCMs. But testing commercial PCMs differs, specifically if for quality assurance. Here, minimum quality requirements are needed, defining when a product fails specifications. An analysis of reasons for failure is not needed, quite contrary to R&D. In 2004, several companies founded the RAL Quality Association PCM to develop such minimum performance requirements and suitable procedures for quality assurance of commercial PCM products. These Quality and Testing Specifications for PCM (RAL-GZ 896) were published in 2006 in the 1 st edition, later updated [15; 16], and contain information on issues discussed here. What is the current state? PCMs are generally characterized regarding their heat storage capability, sometimes their thermal conductivity (specifically if enhanced), sometimes their viscosity (in slurries), and their compatibility, specifically with regard to corrosion.