## 1 Introduction
PCM are classified in cycling categories according to the number of cycles performed without damage: from A category (10,000 cycles) to F category (50 cycles). However, the testing regulations of the \"RAL Quality Association PCM\" neither indicate details on the appropriate design of stability testing devices nor give recommendations on suitable experimental techniques to study the stability of relevant properties after thermal cycling (except calorimetric techniques to measure enthalpy-temperature curves). In addition, apart from defining different cycling categories, nothing is said about the long-term stability of PCM under application conditions. So far, other testing regulations dedicated to the thermal cycling stability of PCM have not been proposed. A concept for medium and long-term thermal energy storage is to utilize the ability of certain PCM to be stored as a liquid in the supercooled state. As investigated by [PERSON] et al. [10] and [PERSON] [11], the application of sodium acetate trihydrate (SAT) in closed containers permits the use of its sensible heat capacity after melting while preserving its heat of fusion at room temperature in a state of stable supercooling. The result is a thermal energy storage capacity that can be used on demand by the controlled initialization of crystallization. This storage concept has been demonstrated for a solar heating system [12; 13] and showed potential for compact thermal energy storage systems in buildings [14]. Recently, [PERSON] et al. reported Xylitol [15] and [PERSON] et al. reported micro-structured polyol-polystyrene composites [16] as further promising materials with stable supercooling properties. To investigate the stability of PCM with stable supercooling (type B tests), dedicated test procedures are required. In addition to temperature-time profiles of regular thermal cycling tests, both the degree and duration of supercooling are parameters of interest. Phase change slurries (PCS) are fluids composed of PCM and a carrier fluid [1]. A PCS is stable if the emulsified or encapsulated PCM droplets do not degrade during operation, e.g., due to an agglomeration of droplets or a PCM leakage in the case of capsules. Among others, the stability of PCS depends on the investigated sample and particle size and the applied flow rate. Stability testing of PCS is carried out with self-built experimental devices (type C tests) that provide application-oriented charging and discharging conditions. While existing studies reported on stability tests of a certain PCM tested with a certain device for a given number of thermal cycles (as e.g., in the review articles of [PERSON] et al. [6] and [PERSON] and [PERSON] [17]), this work focuses on the experimental devices and testing procedures that are being used to perform PCM stability tests under application conditions. The aims of this work are as follows: first, to give an overview of the variety of experimental devices that are being used to investigate the long-term stability of PCM; second, to compare the specifications and test conditions of these devices, and to identify differences or similarities of the applied methods; third, to increase awareness that PCM stability studies need to take application conditions into account in order to investigate possible degradation phenomena in a realistic manner;and fourth, to initiate a discussion of whether there is a need to define or develop standardized stability test devices and procedures. Following the comparison of investigated validation test devices of types A to C (Section 2), appropriate experimental methods to detect a possible degradation of the PCM are introduced (Section 3).