## 2 Comparison of Experimental Devices
### Udl-GREiA III
#### 2.2.2 Type B--Tests on Supercooled PCM  
In the case of PCM with stable supercooling, the investigation of long-term stability consists mainly in determining the reliability of the state of supercooling over a certain period of time under various experimental conditions (size of the sample container or storage tank, maximum charging temperature, minimum discharging temperature, etc.). The desired stability of a supercooled PCM storage unit is achieved if no spontaneous crystallization occurs during the discharge phase of the storage unit or during the holding time of the long-term storage phase. Since PCM storage systems based on stably supercooling PCM are usually operated as long-term storage systems, the number of thermal cycles to be investigated is comparatively low. DTU Full-Scale  
\"DTU full-scale\" is a full-scale heat storage prototype testing installation [40; 41; 42]. The stability of the supercooled PCM in contact with the container material is investigated. The setup is shown in Figure 22.  
A typical temperature profile applied to study the supercooling of SAT consists in the following steps (Figure 23): charging from approximately 20 to 90 \({}^{\circ}\)C, cooling down to ambient temperature (approximately 20 \({}^{\circ}\)C), initialization of crystallization, and discharge to ambient temperature (approximately 20 \({}^{\circ}\)C). Figure 22: Testing device “DTU full scale”: PCM heat storage test facility at DTU (**a**), schematic of charge and discharge loop (**b**). During the experiments, the following data are measured: temperatures (inlet, outlet) and volume flow rate of the heat transfer fluid, ambient temperature, and heat storage surface temperature. DTU Heat Loss Method  
With \"DTU heat loss method\", determination of the heat content of (supercooled) PCM at a large sample size (approximately 200 g) is performed [43]. Pictures of the testing device are shown in Figure 24.  
In the case of supercooled SAT, a typical temperature profile consists in heating up the sample in the oven from approximately 20 to 90 \({}^{\circ}\)C followed by cooling down to ambient temperature at approximately 20 \({}^{\circ}\)C. Then, the crystallization of supercooled SAT is initiated inside an insulated chamber by means of a seed crystal, and the temperature-time curve during the temperature rises due to the crystallization enthalpy, and the succeeding cooling down to ambient temperature is measured. Figure 25 depicts the temperature development of a SAT sample, where the heat content was evaluated as the thermal energy dispersed to the ambience (green area). Figure 23: Temperature profile of a test cycle with supercooling of PCM (device “DTU full scale”). Figure 24: (**a**) Glass jar with lid, (**b**) the well-insulated box, (**c**) locations of temperature sensors (“DTU heat loss method”). DTU Multiple Storage  
The device \"DTU multiple storage\" consists of multiple (10) identical heat storage units [47] which are subjected to repeated heating and cooling cycles with various temperature levels and durations [44]. SAT has been investigated as PCM utilizing supercooling. The focus of the investigations is the effect of different temperature levels and duration of charge on the supercooling stability of SAT. Illustrations of the testing device are given in Figure 26.  
Regarding application conditions, discharge is conducted with cold water at 10 \({}^{\circ}\)C. As the focus of investigations is the supercooling behavior of SAT, a typical temperature profile consists of the following steps: charging in periods of 8 to 16 h from 10 to 90 \({}^{\circ}\)C, cooling down to 10 to 30 \({}^{\circ}\)C, 72 h observation period (looking for spontaneous crystallization of the supercooled SAT), intentional initialization of solidification and discharge (at 10 to 30 \({}^{\circ}\)C). During the experiments, the following data are measured: temperatures (inlet, outlet) and volume flow rate of the heat transfer fluid, ambient temperature, and heat storage surface temperature.