## 3 Experimental Work
### Sample with Copper Matrix  
Measurement of the PCM sample with a copper matrix was carried out, to find out the percentage efficiency of the melting and solidification of PCM and the acceleration of energy transfer to the heat transfer medium. The role of the copper matrix or copper spiral is to ensure uniform heat transfer within the PCM while at the same time providing an increase in heat transfer between the PCM and the PCM tank shell primarily through conduction (for solid-phase PCM), and in a combined manner through conduction and convection (for liquid-phase PCM). The transition state of the mixture during phase change melting is characterised by a transition from primary transfer through conduction to combined transfer. Within the conductive structure, melting occurs at its surface, which, from a heat transfer point of view, represents the formation of convective flow at the boundary layer. In cooling, the process is more complex due to the formation of a solid phase not only on the inner surface of the tank shell, but also on the surface of the structure towards the core. In view of the above, these processes are described by a set of cooling curves, which generally produce hysteresis. When the samples were measured with PCM, a copper matrix was added to PCM due to its high thermal conductivity. The copper matrix had a mass of 36 g. The thermal capacity of the copper matrix was not considered in these measurements. In Figure 12, the melting of pure PCM with the copper matrix is presented graphically. The thermocouple was placed in the paraffin core. Figure 11: Simulation of charging pure PCM sample. In the first part of Figure 12, the curve of sensible heat (State A) is presented, which lasts for 715 s; then, at a temperature of 48.2 \({}^{\circ}\)C, the phase change transition (melting of the PCM in the core) starts in the second part of the graph with latent heat (image record State B). The end of the phase change occurs after the core melts at 52 \({}^{\circ}\)C at a time of 1215 s. In this case, the whole phase change takes 500 s. In the third part of the graph, the curve again shows the acceptance of sensible heat up to the time of the end of the measurement. In the case of the measurement, the paraffin received the same amount of energy as the reference sample, namely 10,800 J of latent heat. The measurement at Figure 13 shows us the curve of latent heat transfer to the heat transfer medium over time. The solidification process of the PCM starts at a time of 260 s and a temperature of 53.5 \({}^{\circ}\)C. The temperature of the heat transfer medium at this point is 29.9 \({}^{\circ}\)C. The phase change ends at a temperature of 51.00 \({}^{\circ}\)C at a time of 1190 s. The heat transfer medium was heated to a temperature of 34.2 \({}^{\circ}\)C.  
In total, the phase change took 930 s. As can be also observed here, the phase change consists of two parts. The latent heat part lasts for 505 s, and the mixed-phase part lasts for 425 s, which means that the latent heat part accounts for 54.30% of the total phase change  
Figure 12: Charging PCM–copper matrix sample. Figure 13: Discharging PCM–copper matrix sample. time. During this period, the PCM heated the heat transfer medium from 29.9 \({}^{\circ}\)C to 33.0 \({}^{\circ}\)C, i.e., \(\Delta\) = 3.1 \({}^{\circ}\)C. This can be calculated as follows:  
\[Q\ =\ m\cdot c\cdot\Delta\mathrm{t}\ =\ 0.4\cdot 4180\cdot 3.1\ =\ 5183.2\, \mathrm{J} \tag{23}\]  
The PCM supplied 5183.2 \(\mathrm{J}\) to the heat transfer medium during the latent heat part of the phase change. During the mixed phase, the sample heated the heat transfer medium to a final temperature of 34.2 \({}^{\circ}\)C. In total, the two phases together heated the medium by 4.3 \({}^{\circ}\)C:  
\[Q\ =\ m\cdot c\cdot\Delta\mathrm{t}\ =\ 0.4\cdot 4180\cdot 4.3\ =\ 7189.6\, \mathrm{J} \tag{24}\]  
The total energy transferred was 7189.6 \(\mathrm{J}\).