## 3 Experimental Work
### Measurement of the Experimental Hybrid Tank of Water-Pure PCM  
A cylindrical accumulation tank (heat transfer medium reservoir) made of low-alloy ferritic-pearlic steel (steel grade 13--Cr-Ni-V) was used for the measurements (Figure 24a). The volume of the tank was 30 L and the dimensions were a diameter of 320 mm and a height of 420 mm. The tank was insulated with two layers of glass wool. The heating of the heat transfer medium was provided by an electric coil with an input of 1000 W. A heat exchanger (radiator) with an output of 800 W was chosen as the load. The tank was filled to 25 L as there was a smaller tank with PCM inside.  
A smaller cylindrical tank with a capacity of 5 L served as a reservoir with the PCM for energy storage. The tank was also made of low-alloy ferritic-pearlic steel of grade 13 (Cr-Ni-V). The dimensions of the vessel were a diameter of 160 mm and a height of 250 mm. The vessel material had a thermal conductivity of \(\lambda\) = 80 W/m-K. The container contained  
Figure 23: Hysteresis curves of PCM–steel twisted matrix measurements.  
Figure 24: Accumulation tank for heat transfer medium (**a**) and thermocouple location in the tank (**b**).  
3.7 kg of paraffin, which, based on its density, corresponds to 5 liters, while the percentage of the copper matrix to PCM volume was 1.3%.  
A total of 12 thermocouples were used in the measurement tanks, 3 of which were used to measure the temperatures in the storage tank with the heat transfer medium and 9 of which were used to measure the temperatures in the PCM tank. The locations of the thermocouples in the vessels are shown in Figure 24b. A cylindrical system was chosen as the method of thermocouple placement.  
Figure 25 shows the charging process of the water and PCM tank. The phase change starts at 3155 s at the part where the PCM4 thermocouple is located. Gradually the phase change passes from the shell of the tank to the core of the tank. From the graph, the heat transfer dissipation between HTF and the paraffin shell and core is clearly visible; this is due to the low thermal conductivity, especially in the semi-solid phase.  
The total time to melt the entire 3.7 kg of PCM is 28,185 s. The end of the phase change occurs in the core of the tank with the PCM9 thermocouple. It is evident from Figure 25 that the charging is non-uniform throughout the volume.  
Figure 26 shows the solidification process of the PCM in the storage tank and the charging of the heat transfer medium of the accumulation tank. Solidification starts at a time of 1310 on the PCM9 thermocouple. During this measurement, the heat transfer medium was discharged to a temperature of 43.4 \({}^{\circ}\)C. Thus, the temperature of the heat transfer medium is still at approximately 48.9 \({}^{\circ}\)C at this point. The phase change ends at a temperature of 51.00 \({}^{\circ}\)C at 43,050 s on the PCM5 thermocouple. The total solidification time is 41,740 s. The heat transfer medium was heated to a maximum temperature of 47.0 \({}^{\circ}\)C. This temperature was recorded approximately at the midpoint of the phase change. Overall, the heat transfer medium was heated by \(\Delta\)t = 3.6 \({}^{\circ}\)C. This can be calculated as follows:  
\[Q\ =\ m\cdot c\cdot\Delta\mathrm{t}\ =\ 3.7\cdot 4180\cdot 3.6\ =\ 55677.6\ \mathrm{J} \tag{29}\]  
Figure 25: Charging rate of the hybrid tank of water–pure PCM.