## 5 Conclusions  
The primary objective of this work was to directly investigate the thermal behavior of thin PCM packages and the combination of thin PCM packages with a thermal spreader. In order to characterize thin PCM packages and its combination with a thermal spreader, a set of experiments on a simplified setup with a constant heat power input were performed. Such a setup needed to be used to isolate the thermal behavior of the PCM packages, since a proper understanding of the behavior of PCM packages is impossible to fully isolate when used and studied in a real portable electronic system (including the screen, battery, and other complex internal components). This study is therefore the first one focused solely on the PCM package. An appropriate passive cooling system, which may be integrated in electronic devices, has to limit the temperature increase of heat-producing parts while keeping the cover surface within a comfortable  
Figure 12: (**a**) Temperature of the lower surface (\(T_{L1}\)–\(T_{L5}\)) after 60 min of charging vs. the radial distance to the center; (**b**) temperature of the cover (\(T_{C}\)) after 60 min of charging vs. the radial distance to the center for the experiment with PCM package A (\(n\)-eicosane) with a spreader and without spreader. temperature range. Therefore, it was particularly of interest to experimentally investigate the behavior and ability of PCM packages to reduce the peak temperatures of both the discrete heat source and the cover surface of the setup. Two different types of PCM were used: \(n\)-eicosane, a paraffin with a melting temperature of 35.6 \({}^{\circ}\)C, and dodecanoic acid, a fatty acid with a melting temperature of 43 \({}^{\circ}\)C. Thin heat-sealable aluminized film was used to encapsulate the PCM. The first set of experiments were performed with PCM package A (\(n\)-eicosane) and PCM package B (dodecanoic acid) without a thermal spreader. A constant power input of 5 W for 60 min was applied. It was observed that both PCM packages reduced the maximal heater temperature by 15 (package A) and 17 \({}^{\circ}\)C (package B) compared to the case where no PCM package was used. The maximum cover temperature was reduced by approximately 14 \({}^{\circ}\)C for both PCM packages. Furthermore, it was observed that both PCM packages did not melt completely during the charging process since heat was mostly transferred in the axial direction, although package A experienced a higher melting rate than package B due to its PCM's lower melting temperature. The PCM encapsulated in thin aluminized film therefore offered very little heat spreading capability, still leading to a hotspot on the cover of the system. The experiments confirmed that the inclusion of the heat spreader reduced both the peak heater and cover temperature by spreading this hotspot over a wider area. By facilitating lateral conduction, the spreader both increased the thermal mass of the PCM activated for thermal storage and the cover area available for convection. The use of both the heat spreader and PCM proved even more effective at reducing the temperature of the heater while maintaining a low cover temperature. The spreader and PCM was superior to either solution alone. Latent and sensible storage of the PCM increases the apparent thermal mass of the setup and the spreader facilitates the heat accessing that thermal mass. During the experiments with the PCM package and spreader, all of the thermocouples mounted on the PCM package showed a temperature increase above the melting temperature of the PCM used, which indicated that, most likely, the complete PCM melted. Furthermore, the experiment showed that PCM package A with a thermal spreader provided a slightly better performance than PCM package B. The overall comparison showed that when PCM package A and a thermal spreader were used, the temperature peak of the heater after 60 min of charging was reduced by 75 \({}^{\circ}\)C compared to the case where neither a spreader nor a PCM package were used.