## 2 Comparison of Experimental Devices
### Descriptions of Testing Devices
#### 2.2.1 Type A--Thermal Cycling Stability Tests  
CIEMAT I  
The experimental device \"CIEMAT I\" is an oven that allows performing thermal tests under controlled heating and cooling rates with stand-by periods at constant temperatures up to 350 \({}^{\circ}\)C. Pictures of this device are shown in Figure 2. Thermal cycles can be performed between two temperature levels with transitions via linear ramps with rates from 1 to 20 K-min\({}^{-1}\). These ramps can be combined with stand-by periods up to a maximum test time of 24 h.  
The temperature inside the oven is accurately controlled thanks to a fan located on one side, which provides forced ventilation (Figure 2b). Various samples with different sizes can be tested simultaneously under static ambient air. During the experiments, the temperature of both sample(s) and oven are monitored with calibrated T-type thermocouples. CIEMAT II  
The \"CIEMAT II\" device consists of a vertical oven with a cylindrical ceramic cavity as shown in Figure 3. This oven can perform thermal cycles up to 500 \({}^{\circ}\)C at controlled heating rates (between 1 and 20 K-min\({}^{-1}\)) with stand-by periods at constant temperature. To cool down the sample, the heating system automatically switches off so that only natural cooling can be achieved. This device also allows performing thermal tests at constant temperature for indefinite periods of time. Moreover, it can be installed inside an extraction hood so that PCM thermal degradation tests under air can be carried out either at constant heating rate or constant temperature. As shown in Figure 3b, only one sample can be tested at a time remaining under atmospheric air. During the experiments, the sample temperature is monitored with calibrated T-type or J-type thermocouples. Typically, one thermal cycle per day is carried out. Figure 2: Setup outside (**a**) and oven inside (**b**) of the device “CIEMAT I”. CIEMAT III\" is a thermal cycling device shown in Figure 4. It consists of a vessel enclosed inside a furnace that allows performing cycles under controlled heating (between 1 and 20 K\(\cdot\)min\({}^{-1}\)) up to 500 \({}^{\circ}\)C and natural cooling (Figure 4a) with stand-by periods in between. This device also allows performing thermal tests at constant temperature for indefinite periods of time. The testing vessel is a metallic cavity closed by two flanges where only one sample can be introduced. In the upper covering flange, a J-type thermocouple is installed to record the sample temperature during the experiments, and the lower flange has two connecting pipes for gas inlet and outlet, respectively (Figure 4b). Each gas pipe is connected to a valve that allows gas entrance and exit and keeps the pressure in the device at a user-defined pressure range. The gas facility is prepared for using Ar or N\({}_{2}\), but other gases such as air might also be used. EHU  
The testing device \"EHU\" is comprised of a thermostatic bath and airtight tubes. The samples are inserted inside the tubes, and these are immersed into the thermostatic bath, which uses silicone oil as a heat transfer fluid. The tubes are made of glass and include a nylon screwed cap with an O-ring sealing that ensures tightness. They can handle pressures up to 10 bar. The usual sample size is approximately 5 g, although tubes with variable dimensions can be employed (Figure 5). Figure 4: Setup outside and furnace (**a**) and setup inside (**b**) of the device “CIEMAT III”. Figure 3: Setup outside (**a**) and oven inside (**b**) of the device “CIEMAT II”. The arrangement and configuration are flexible and adapted to the investigated PCM and the kind of degradation expected. The temperature program usually comprises consecutive heating and cooling cycles if physical phase separation is expected. When the material under study might undergo thermal decomposition, then the program normally consists of submitting different samples to a constant temperature. The samples are extracted after different periods of time and analyzed. Several complementary techniques are used.