## 2 Materials and Methods
### Testing the Heat Pipe  
In order to preliminary test and verify the heat pipes, a test rig consisting of a thermostat, a heat exchanger, and thermal energy storage was made. The schematic of the test rig is shown in Figure 8.  
The heat exchanger was printed using 3D printing technology using PLA filament. It is a 3-piece construction, glued and fully disassembled. In addition, the heat exchanger is equipped with 4 threaded rods, the purpose of which is to prevent unsealing of the structure. Part 1, the main body, is equipped with 10 threaded glands for heat pipes, while part 2 and part 3 are the inlet and outlet, respectively. Figure 9 shows a diagram of the 3D model, and Figure 10 shows the actual view of the heat exchanger. Figure 8: Diagram of the heat pipe verification station: 1—thermostatic bath; 2—circulating pump; 3—validation module equipped with heat pipes; 4—T-type thermocouple. Figure 9: Diagram of the heat exchanger design: 1—proper body; 2—inlet part; 3—outlet part; 4—fixing brackets; 5—threaded rods; 6—nut; 7—glands fixing heat pipes. The test thermal storage was made using 3D printing technology, and PLA was used as the material. The thermal storage was equipped with 10 cells with a volume of 47 mL each, which are flooded with coconut oil during the tests. There is only 1 heat pipe in a single cell. Figure 11 shows a diagram of the test heat storage and the cell numeration. The procedure of testing the heat pipes consists of providing a constant level of temperature to the heat pipe evaporator. For this purpose, using a thermostat to heat the water to 40 \({}^{\circ}\)C, and then connecting the thermostat pump, water was pumped through the heat exchanger for 35 min. In order to verify the evaporator wall temperature behavior of the heat pipe, the temperature of the junction of the type \"T\" thermocouple next to the display was monitored on the downstream side of the heat exchanger, which indicated 40 \({}^{\circ}\)C \(\pm\) 0.2 K. Thermal energy storage using coconut oil was used as a heat pipe condenser. At the start of the research, the coconut oil had a temperature of \(-\)17 \({}^{\circ}\)C and started in a solid state, with the condenser of the tube immersed in coconut oil. The room where the heat pipes were checked was maintained at 20 \({}^{\circ}\)C. Ten heat pipes were placed in the experimental module. The heat pipes were verified using the mass method. After observing the tests, melted coconut oil was poured from the thermal heat storage, and then the storage was placed in the refrigerator for 60 min in order to coagulate the coconut oil remaining in the storage. In the next stage, a test rig equipped with a Steinberg SBS-L W laboratory scale and two beakers were used, as shown in Figure 12. The storage was placed on a tared scale, and each cell of the test thermal storage was flooded with cold water at a temperature of 15 \({}^{\circ}\)C, as shown in Figure 13, and weight indications are shown in Figure 14. Then, the weights of  
Figure 11: Test thermal storage dimensions (**left**) and their cell numeration (**right**). Figure 10: View of the 3D-printed heat exchanger. individual heat pipes installed in individual cells were compared with the reference heat pipe. If inequality 1 was met, the tube was accepted and ready for use. \[m_{HP,n}\geq m_{e,n} \tag{5}\]  
where:  
\(m_{HP,n}\)--mass of water left in cell \"\(n\)\" of the heat pipe;  
\(m_{e,n}\)--mass of water left in the cell of the empty heat pipe. Figure 12: View of the heat pipe verification station: 1—test thermal storage; 2—precision balance; 3—200 mL laboratory beaker; 4—500 mL beaker. Figure 13: Charging a cell with water 1. In order to determine the references for each of the cells of the proposed thermal energy storage, 10 blind copper tubes were prepared, devoid of a capillary insert and a working medium. The test was performed three times, reproducing the procedure for heat pipes. Then, the results for each cell were averaged.