## 3 Results and Discussion
### Thermal Management Experiment
#### 3.2.2 Cooling Run  
The cooling curves for PCMs under NV and FV are presented in Figures 9-11 to study PCM regeneration to solid state for a comprehensive understanding of PCM performance. Figure 9 presents the cooling curve for paraffin wax when cooled from 50 \({}^{\circ}\)C under NV and FV.  
Figure 9 shows that under NV the paraffin wax starts solidification at 40 \({}^{\circ}\)C and completes it at 34 \({}^{\circ}\)C with a wide solidification range of 6 \({}^{\circ}\)C. However, in the case of FV, the start and end of solidification are not evident due to rapid heat removal so that the phase change boundaries cannot be traced.  
Figure 10 shows that salt hydrate starts solidification at 25 \({}^{\circ}\)C and finishes at 24 \({}^{\circ}\)C with a narrow melting range of 1 \({}^{\circ}\)C. However, salt hydrate has to sub-cool down to 17 \({}^{\circ}\)C before it starts solidification. This phenomenon reduces the utility of the salt hydrate, as it would need salt to be cooled far below room temperature (25 \({}^{\circ}\)C), at which the electronic devices typically operate. Adding extra water can prevent the formation of the heavy anhydrous salt that can suppress sub-cooling but reduces the heat storage intensity of the PCM [17]. Addition of nucleating agents such as borax [26] can suppress the sub-cooling effect but it settles down due to higher density, and thus further research is required to solve this problem [27].  
Figure 11 indicates that milk fat did not show any clear phase change since the melting range was very wide (10-40 \({}^{\circ}\)C) and could find it difficult to regenerate. This problem can be solved by refining milk fat samples to retain only contents with the higher melting point and narrow melting  
Figure 10: Transient cooling curve for salt represented by HS bottom surface temperature under NV and FV.  
Figure 9: Transient cooling curve for paraffin represented by HS bottom surface temperature under NV and FV.  
range, which will be studied further. Results show that milk fat and paraffin wax do not exhibit such sub-cooling effect and would offer an advantage over salt hydrate during regeneration back to solid (if melting range for butter is fixed which is potentially easy to realize).  
It is generally observed that the PCM takes less time to complete melting and regulate electronic temperature than it takes to get back to solid to be ready for the next heating cycle. In case of 4 W, the time to complete melting:solidification was 27 min:85 min for salt hydrate, 21 min:33 min for paraffin wax while it is difficult to indicate the melting and solidification time for milk fat due to its wide melting range. This suggests that the PCM would be used for shorter time for temperature regulation than it would have to be idle to release heat. It also clearly suggests that the PCM based cooling technology cannot be applied for continuous device operation. Such technology only suits applications with periodic working/idle cycles. The time taken to melt and solidify depends on the heating/cooling rates, the nature of the PCM, the amount of the PCM and ambience around the PCM which would be optimized for each specific case in future research.