# Passive Thermal Management of Tablet PCs Using Phase Change Materials: Intermittent Operation
## 4 Conclusions  
This study experimentally justified the effectiveness of using PCM TES units in a tablet PC. The PCM TES units were tested under different orientations of the device, different heat input levels, different latent heats, and different TES unit sizes. To represent real applications, the intermittent regime was studied for three cycles of operation. Two PCMs, \(n\)-eicosane and PT-37, were selected, based on melting temperatures that best suit the operating comfort limit required by the tablet PC users. Smaller PCM-based TES units, placed only on the heat generating components (i.e., heater) were introduced and investigated to access the viability of component-level selective thermal management concept. The following conclusions resulted from the current work:  
1. Applications of PCM TES units lead to significant temperature reduction after each 15-min charging cycle. Back-cover temperature reduction is more than 20 \({}^{\circ}\)C at 6 W power input. At 8 W heat input level, the tablet PC temperature was maintained below the comfort limit of 40 \({}^{\circ}\)C at all times with PCM TES units, although the SOC temperature exceeded the allowed maximum.
2. The effect of tablet PC orientation on thermal performance of TES units is negligible.
3. PCM TES units provide a comfortable back-cover temperature at all tested power input levels (2 W to 8 W). At higher power input level, more PCM melts, storing more latent heat. As a result, a longer time is required to release latent heat in the discharging phase at higher power input levels. Though PCM TES units provide acceptable back-cover temperature of the tablet PC at 8 W power input, the chip temperature rises beyond 85 \({}^{\circ}\)C. Hence, for the system studied here, power input of 8 W and higher should be avoided for reliable operation of the tablet PC combined with current PCM TES units.
4. Larger PCM TES units result in better thermal performance due to the removal of most of the air gap and higher thermal storage from more PCM.
5. Smaller PCM-based TES units placed on heat-generating components (i.e., heater) provide the best temperature reduction of the heater, even compared to regular PCM TES units, since the temperature reduction is significantly increased due to larger amounts of PCM melting during the charging phase. However, the back-cover temperature suffers from slightly higher temperatures (hot spot) with these types of PCM TES units.
6. The thermal behavior of the tablet PC does not show significant sensitivity to the variety of PCMs tested, although both PCMs have slightly different thermophysical properties.  
PCMs encapsulated in the aluminized laminate film have been proven to be a practical solution for portable electronic device thermal management. This study did not look at the impact of ambient temperature on the initial temperature of the device, the impact of ambient temperature on the cooling of the device, or the possible influence of a change in ambient temperature on the optimal selection of PCM. Those questions will be investigated in future work.  
Conceptualization, D.G., M.A.W.; methodology, T.A., D.G.; formal analysis, T.A.; investigation, T.A.; writing--original draft preparation, T.A.; writing--review and editing, M.B., D.G., M.A.W.; supervision, M.B., D.G.; project administration, D.G.; funding acquisition, D.G., M.A.W.  
This research was funded by Intel Corporation, grant number 9388323.  
**Acknowledgments:** The authors thank Intel Corporation for the financial and technical support which made this work possible, along with the Natural Sciences and Engineering Research Council of Canada (NERC), the Nova Scotia Government (Graduate Scholarship program) and the Canada Foundation for Innovation (CFI) for further laboratory financial assistance.  
**Conflicts of Interest:** The authors declare no conflict of interest.