## 1 Introduction  
Phase change materials (PCM) can store large amounts of thermal energy in a small temperature range around their phase change temperature. Therefore, latent heat storage systems based on PCM can provide high thermal energy storage densities [1; 2; 3; 4]. An important prerequisite to select a reliable phase change material (PCM) for thermal energy storage applications is to investigate its stability under application conditions. Since appropriate testing devices are not commercially available, lots of different self-built devices exist in various research institutions. Many of these institutions are currently participating in IEA ES Annex 33/SHC Task 58 \"Material and Component Development for Compact Thermal Energy Storage\", which is a joint working group of the \"Energy Storage\" (ES) and the \"Solar Heating and Cooling\" (SHC) Technology Collaboration Programmes (TCP) of the International Energy Agency (IEA) [5]. The main goal of this working group is to support an application-oriented development of innovative and compact thermal energy storage materials including PCM and thermochemical materials (TCM). Therefore, the participants have proposed sharing their experience regarding PCM stability testing. In total, 18 mostly self-built devices from 10 institutions are presented in this survey. The experiments performed can be divided in three types: type A: thermal cycling stability tests; type B: tests on PCM with stable supercooling, and type C: tests on the stability of phase change slurries (PCS). In the case of solid-liquid PCM, heat is stored and released under repeated melting and solidification processes, which are also referred as thermal cycles. To assess the suitability of a PCM for a thermal storage application, testing the variation of its thermal properties under repeated thermal cycling (type A tests) taking application conditions into account is crucial [6; 7; 8]. Relevant experimental conditions are the temperature interval, heating and cooling rates, contact to atmosphere, contact to container, heat exchanger material, etc. To study thermal cycling stability, the PCM under investigation is typically subjected to a temperature-time profile, as shown in Figure 1.  
Figure 1: Idealized temperature–time curves of a thermal cycling stability experiment of a solid–liquid phase change material (PCM); PCM temperature (red dashed line); and heat transfer medium temperature (black solid line). Two temperature levels, one above (\(T_{\rm high}\)) and one below (\(T_{\rm low}\)) the melting temperature (\(T_{\rm m}\)), are applied alternately for a defined number of cycles. Certain periods of time are necessary to obtain a complete phase change from solid to liquid and vice versa. In between these two temperature levels, transition steps are carried out, either using temperature ramps with controlled temperature gradients, allowing natural cooling, or by heating and cooling as fast as possible. The curves plotted in Figure 1 are idealized. In reality (e.g., as shown in Figure 20), both the temperature-time profile of the heat transfer medium (solid black line) and the temperature response of the PCM under investigation (dashed red line) will not change suddenly between isothermal segments and transition segments. In addition, in many experiments, supercooling of the PCM will occur at least to a small extent. Among the thermal properties to be checked after (or during) thermal cycling, the enthalpy-temperature curves upon melting and solidification are particularly relevant. In this context, a material can be regarded as stable after a certain number of thermal cycles if melting temperature and melting enthalpy variations remain within specific tolerance values. For example, according to the \"RAL Quality Association PCM\" [9] testing regulations, a decrease of the melting enthalpy by less than 10% and a melting temperature variation of \(\pm 1\) K can be tolerated after a certain number of cycles.