## 4 Results
### Experiments with Thermal Spreader
During the charging process, the temperature profiles did not differ much compared to the experiment with PCM package A, and the same conclusion can be drawn with one exception: Since dodecanoic acid has a higher melting temperature, it took until the end of the experiments (120 min) to fully melt all of the PCM in the system, compared to approximately 60 min in the case of \(n\)-eicosane. This is enough of a difference in the overall behavior to explain the difference of a few degrees in the temperature of the heater. Figure 7: (**a**) Temperature profile of the heater (\(T_{H}\)) and the cover surface (\(T_{C}\)). (**b**) Temperature profiles of the upper (\(T_{U1}\)-\(T_{U4}\)) and lower (\(T_{L1}\)-\(T_{LS}\)) surface of the PCM package A (\(n\)-eicosane) and thermal spreader for the experiment with a constant power input of 5 W for 120 min. The experiment with the thermal spreader was also performed without PCM. Figure 9 compares the measured temperature profiles of the heater and the center cover surface for the experiment without any PCM package, PCM package A, and PCM package B.  
Figure 8: **(a)** Temperature profile of the heater (\(T_{H}\)) and the cover surface (\(T_{C}\)). **(b)** Temperature profiles of the upper (\(T_{U1}\)–\(T_{U4}\)) and lower (\(T_{L1}\)–\(T_{LS}\)) surface of the PCM package B (dodecanoic acid) and the thermal spreader for the experiment with a constant power input of 5 W for 120 min. Figure 9: Temperature profile of the heater (TH) and the cover surface (TC1) for the experiment with no PCM package, package A, and package B with the thermal spreader and a constant power input of 5 W for 120 min. As illustrated in Figure 9, the maximum heater temperature and the maximum cover temperature were reduced by adding PCM packages compared to the experiment where no PCM package, but the thermal spreader only was used. In fact, the peak temperature of the heater was reduced by 31.5 \({}^{\circ}\)C when package A was used and 28.8 \({}^{\circ}\)C when PCM package B was used. The maximum cover temperature was reduced by 2.2 and 1.7 \({}^{\circ}\)C, respectively. As mentioned above, the reduction of these peak temperatures was mostly due to the increased thermal storage capacity when a PCM package was installed. In Figure 9, the heater temperature is similar in all three experiments during the first 15 min of charging. Once the PCM in the packages started to melt, the increase of the heater temperature lessened compared to the case where no PCM package was used. Additionally, the air gap between the thermal spreader and the cover presents a larger thermal resistance compared to the case when a PCM package was used and thus led to a large temperature increase of the thermal spreader and the heater. When comparing the temperature profiles obtained with PCM package A and PCM package B, one sees that the increase of the heater temperature was slightly higher for PCM package B, where the measured temperature profiles of the cover temperature were almost identical. It is assumed that this difference was due to the higher melting rate when PCM package A was used compared to PCM package B. The melting rate can be analyzed by comparing the time intervals, \(\Delta t_{melt}\), between the start of the experiment and the point in time when the temperatures of the surface of the PCM packages have reached the melting temperatures. The obtained \(\Delta t_{melt}\) are illustrated in Figure 10 for each temperature sensor and for both experiments with PCM package A and B.  
Figure 10: Time intervals (\(\Delta t_{melt}\)) for the thermocouple mounted (**a**) at the upper surface and (**b**) at the lower surface of both PCM packages to reach their respective PCM melting temperature for the experiment with the thermal spreader and a constant power input of 5 W for 120 min. The comparison of \(\Delta t_{melt}\) illustrated in Figure 10 indicates that PCM package A experienced a higher melting rate than PCM package B.