## 2 Materials and Methods  
To perform the measurements, a PCM had to be selected, the necessary equipment for measuring the values had to be prepared and the right conditions had to be ensured.  
The PCM chosen was a material of organic nature, specifically, paraffin, with a phase change in the temperature range of 48.2-52 \({}^{\circ}\)C for the melting process and 53.5-51 \({}^{\circ}\)C for the solidification process. Based on the above, it can be concluded that the phase change process for the material shows a hysteresis curve.  
The Comet MS3 measuring and recording panel was used to obtain the most accurate temperature measurement. The measuring and recording panel was designed to measure, record, evaluate and subsequently process input electrical quantities that are subject to relatively slow changes (>1 s). In conjunction with appropriate sensors and transducers, it is suitable for monitoring physical quantities. The device can measure up to 16 channels simultaneously. It can also work independently without the use of a PC as it has its own memory for 150,000 data. The data can be measured cyclically and non-cyclically, and the desired time interval of data measurements can be set. K-type thermocouples were used for temperature measurement; the tolerance is \(\pm\)0.15 \({}^{\circ}\)C at the required temperature range of measurement (25 to 65 \({}^{\circ}\)C). Temperature sensors of this type are a combination of nickel and chromium alloys. The thermocouples have good mechanical flexibility and strength and are resistant to acids and liquids with adverse effects.  
The equipment shown in Figure 1a was used to heat (charge) the samples. For measurements to detect energy transfer profiles, the samples were placed in the constructed calorimetric experimental device. The device monitors the transfer of thermal energy from the water and PCM to the heat transfer medium (water), and is shown in Figure 1b. The parameters of the selected material are shown in Table 1.  
The storage tank was a 12 L transparent tank with a controllable heat source (1000 W) with a temperature range from 20 \({}^{\circ}\)C to 99 \({}^{\circ}\)C in steps of 0.1 \({}^{\circ}\)C and a deviation from the desired temperature of approx. 0.2 \({}^{\circ}\)C. The heat source also allowed the water in the tank to circulate. The experimental device was designed to monitor the state of phase change of the PCM from solid to liquid and vice versa. The experimental setup thus allowed the samples to be monitored with the required temperature parameters while also ensuring a uniform flow of water around the samples with the PCM. At the same time, the transparent tank allowed the possibility of creating time-lapse image recordings, enabling the evaluation of the individual stages of phase change.  
\begin{table}
\begin{tabular}{c c} \hline
**Property** & **Value** \\ \hline Melting phase change temperature & 48.2–52 \({}^{\circ}\)C \\ Solidification phase change temperature & 53.5–51 \({}^{\circ}\)C \\ Specific heat capacity & 2.22 kJ/kg\(\cdot\)K \\ Latent heat capacity & 180 kJ/kg \\ Thermal conductivity in solid state & 0.22 W/m\(\cdot\)K \\ Thermal conductivity in liquid state & 0.7 W/m\(\cdot\)K \\ \hline \end{tabular}
\end{table}
Table 1: Parameters of selected PCM.  
Figure 1: Sample-charging device (**a**) and experimental calorimeter (**b**).