## 4 Results
### Experiments with Thermal Spreader  
The experimental results presented in the previous section showed that installing PCM packages led to a reduction of both the heater and cover peak temperatures. In this section, the experiment with a thermal spreader (thin aluminum sheet with a thickness of 0.4 mm) will be presented. For each experiment, the thermal spreader was placed on top of the heater. The same PCM packages as in the previous experiments were used and placed above the spreader. For each experiment, a constant heat input of 5 W for 120 min was applied. Figure 7a presents the measured temperature profiles of the heater (\(T_{H}\)) and the cover surface (\(T_{C}\)) for the experiment with package A (\(n\)-eicosane) and the spreader. Figure 7b shows the temperature profiles of the PCM package at the various positions on the upper and lower surface. The maximum temperature of the heater was 63 \({}^{\circ}\)C after 120 min of charging. The maximum temperature of the cover was observed to be 41.8 \({}^{\circ}\)C. The lower temperatures of both the heater and surface points to the usefulness of the thermal spreader, which moves part of the heat produced from the heater to the further corners of the system, enabling the reduction in the temperature of the warmest hot spot on the surface and an increased rate of energy storage in the entire PCM. To that last point, it is observed that the temperature profiles of all the installed sensors on the PCM package increased above the melting temperature within the charging time of 120 min, which indicates that all of the PCM  
Figure 6: Temperature profile of the heater (\(T_{H}\)) and the cover surface (\(T_{C1}\)) for the experiment with no PCM package, package A, and package B with a constant power input of 5 W for 60 min. melted during the experiment. It is also noticed that all those recorded temperatures had reached the melting point of the PCM after 60 min, which was not achieved in the previous experiments without the spreader. Therefore, the thermal spreader distributed the heat coming from the heater mostly in the radial direction. The shape of the temperature profiles, in particular during the discharging process, were much different compared to the experiment where no spreader was used. All of the temperature profiles, except \(T_{C6}\), showed a clear plateau where the temperature stayed nearly constant. This observation is caused by the fact that all the PCM melted, and therefore, during cooling, all of the PCM must solidify; the plateau is a sign of the isothermal phase transition (solidification) of the PCM. During the phase transition, the stored latent heat in the PCM is released and conducted to the cover of the setup at a nearly constant temperature. During the additional time span when the PCM is solidifying (plateau), the surface temperature of the system remains higher, which also leads to increased overall heat transfer by convection from the surface, since the temperature differential remains higher for a longer period. This helps explain why it takes approximately the same amount of time to bring the system back to its original state (room temperature), even though much more energy was generated by the heater and more of it was stored in the PCM. Figure 8a presents the measured temperature profiles of the heater (\(T_{H}\)) and the cover surface (\(T_{C1}\)-\(T_{C6}\)) for the experiment with package B (dodecanoic acid) and the spreader. Figure 8b shows the temperature profiles of the PCM package at the various positions on the upper and lower surface. The heater reached a maximum temperature of 65.7 \({}^{\circ}\)C after 120 min of charging. The maximum cover temperature was 42 \({}^{\circ}\)C after 121 min.