numerical study of temperature control in tablet

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touch glass removed. The majority of sources are located on the mother board at the rear of the tablet model. These sources include the system on a chip (SOC), ...
Proceedings of the 4th International Forum on Heat Transfer, IFHT2016 November 2-4, 2016, Sendai, Japan

IFHT2016-1871

NUMERICAL STUDY OF TEMPERATURE CONTROL IN TABLET COMPUTERS USING PHASE CHANGE MATERIAL THERMAL ENERGY STORAGE Benjamin N. Sponagle, Dominic Groulx* Dalhousie University, Halifax, NS, Canada

ABSTRACT The integration of phase change material (PCM) thermal energy storage into the temperature control system of tablet computers was investigated through a numerical study. A finite element simulation of a modern tablet computer was constructed using COMSOL Multiphysics 5.0. Previous work showed that a PCM thermal storage unit with a melting temperature of 31.8°C will significantly improve the temperature control during heating. This work is a more detailed analysis of the transient thermal behaviour of a tablet computer with integrated PCM thermal storage. The study includes heating and cooling phases with PCMs having melting temperatures between 35°C and 47°C. Results show that integrating PCM thermal storage into the temperature control system of tablet computers is both beneficial and feasible.

KEY WORDS: Phase change material, Portable electronics, Thermal storage, Temperature control 1. INTRODUCTION Thermal control systems for portable electronics are designed to maintain the temperature of internal components within their operating range while also keeping the surface of the tablet at a temperature which is comfortable for the user. In modern portable electronics, heat is transferred via conduction from the main sources to the surface of the device where it is dissipated through natural convection. Present temperature control systems mainly utilize heat spreading; while heat spreading is critical, future performance improvements will require the development of innovative temperature control techniques. In this paper, the integration of phase change material (PCM) thermal energy storage into the overall thermal design is proposed. Portable electronics often have a periodic use profile, producing peak power levels for short periods of time and experiencing long periods of inactivity. Therefore, during a period of high heat dissipation, a portion of the heat is stored in the PCM storage module. The stored heat is then dissipated at a later time when the device is less active. Several researchers have investigated the use of latent heat thermal storage in the thermal control of electronic devices. Many studies have been conducted on PCM hybrid heat sinks, both numerically and experimentally. Kandasamy et.al. [1] investigated the melting of a relatively large rectangular reservoir (80 × 132 × 20 mm) of paraffin wax by a discrete heat source. The author experimentally produced transient temperature profiles within the PCM as well as doing a 2D numerical simulation. The same authors also published a paper where they experimentally and numerically tested three different finned heat sinks (the smallest being 16 ×14 × 12.5 mm) which were filled with paraffin wax [2]. Fok, et al. [3] also investigated the melting in a PCM filled heat sink. In their paper, they experimentally investigated the melting of eicosane in an aluminium heat sink with varying fin numbers. Again the heat sink was relatively large (85 × 72 × 21 mm). Fan, et al. [4] experimentally investigated the use of an aluminium heat sink (80 × 80 × 30 mm) filled with PCM (n-eicosane and 1-hexadecanol) at heating loads ranging from 60 to 120 W. Researchers have analysed many aspects of this scenario: heat sink design, PCM melting temperature, different heating profiles, etc. However, these works *Corresponding Author: [email protected]

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IFHT2016-1871 focus on heat sinks which are far too large to be used in modern portable electronics and may be more suited to the cooling of stationary electronics like the traditional personal computer. Additionally, due to the size of the heat sinks used in much of this research, the melting behaviour of the PCM is strongly effected by convection in the liquid phase. In contrast, PCM thermal storage, on the scale of modern portable electronic devices, will likely be very thin (on the order of 1 mm) and therefore the heat transfer will be dominated by conduction. Research has also been done on the use of PCM thermal storage in the thermal control of portable electronics. Alawadhi and Amon [5] built a representative experimental and numerical model of a wearable computer, developed at Carnegie Mellon University, and used a PCM thermal storage system (TES) to help cool the device. They found that the PCM thermal storage helped to control the temperature of the system while the PCM was melting, and that the melting time increased with an increase in Stefan number. This is excellent work showing the viability of the concept but it cannot be directly applied to modern portable electronics devices simply because of the change in scale and form factor which has occurred. The thermal storage unit used by Alawadhi and Amon [5] was 12 mm thick in its thinnest dimension while modern portable electronics are often less than 9 mm thick. Hodes et al. [6] conducted a similar investigation into the use of a PCM thermal storage unit in the cooling of a telephone handset. They found that even small amounts of PCM can increase the duration of use before overheating by several times. For example, they found that adding only 9.5 g of tricosane increased the time it took for the case temperature to reach 62ºC by a factor of five. The volumes of PCM used in this study are much more realistic, however, the shape, form factor and level of complexity of their handset model are not similar to modern devices. Tomizawa et.al. [7] investigated the use of a sheet of microencapsulated paraffin to control the temperature of a simple model of a mobile electronics device both experimentally and numerically. In previously published work, the author has shown that placing a thin PCM thermal storage module near the back cover of a tablet computer has a positive effect on the temperature control of the processor and the devices exterior surface when a PCM with a melting temperature of 31.3ºC was used [8]. However, PCM storage modules with melting temperatures of 43.3 and 55.8ºC were found to be much less effective. The effectiveness of such thin PCM-TES in a similar tablet computer has been investigated and shown in the authors’ laboratory [9]. The current study expands on previous work investigating the effect of melting temperature on the performance of a PCM thermal storage module in a tablet computer. A finite element model of a tablet computer is used to investigate the performance of a PCM storage module with a melting temperature ranging from 35 to 47ºC. This study focuses mainly on the melting of the PCM but includes a preliminary look at the solidification of the PCM as well.

2. METHODOLOGY 2.1 Geometry The model geometry used in this work is representative of a modern tablet computer and contains five main regions or assemblies which include the: display assembly, stiffener, battery, motherboard and external cover. Figures 1 and 2 show a front and back view of the model with the back cover and front touch glass removed. The majority of sources are located on the mother board at the rear of the tablet model. These sources include the system on a chip (SOC), memory and power management integrated circuit (PMIC). Table 1 gives the power dissipation of each of the components used in this simulations. This dissipation profile was provided by our industrial partners and is designed to mimic the heat dissipation of a tablet while shooting very high resolution video. Table 1 Power dissipation profile. Associated Domain SOC Memory COG PMIC Backlight Misc. Motherboard Camera Total

Heat Generation 3.687 W 0.805 W 0.765 W 0.742 W 0.672 W 0.479 W 0.353 W 7.503 W

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Fig. 1 Front view of the tablet computer Fig. 2 Back view of the tablet computer geometry geometry with the touch glass removed. with the back cover removed. The PCM thermal storage module is located at the back of the tablet in direct contact with the inside of the back cover. It runs the full width of the tablet and is between 1 mm and 0.5 mm thick depending on the space available. The simulation does not currently include encapsulation of the PCM domain. The geometry used in this work is the same as used in previous work, more details on the model, including material properties, can be found in that paper [8].

2.2 Numerical Model A finite element model was constructed using COMSOL Multiphysics 5.0. Due to the thinness of the PCM domain, convection in the melt was ignored. The governing equation is therefore the transient conduction equation: ∙ (1) A modified Cp method was used to simulate phase change [10, 11]. In this method, the Cp is increased at the transition temperature of the PCM in order to account for the latent heat of fusion. An instantaneous jump in specific heat causes computational instability; to avoid this, the change in Cp is spread over a temperature difference according to a smoothed distribution function. Effectively, this results in the PCM changing phase over a range of temperatures centred on the actual transition temperature, the so called mushy zone. In the current simulation, the transition temperature range is 3K. Equation 2 defines the value of specific heat (2) , (3) where L is the latent heat of fusion and αm is defined by Eq. 3 ( being the melt fraction, 0 for solid and 1 for liquid). The boundary conditions on the outer surface of the tablet were modelled as natural convection (havg = 3.5 W/m2K) with the tablet oriented vertically in air at 25ºC. The initial temperature of all domains in the model were set to 25ºC.

2.2 Phase Change Material Properties This study is focused on the effect of the PCM transition temperature. In order to isolate its contribution from the other properties of the PCM, an average organic PCM was used and then the melting temperature was freely changed within the range of 35-47°C. Three organic PCMs representing a wide range of melting temperatures were selected and their properties were used to produce the average PCM. Table 3 shows the properties of each of the example PCMs as well as the average PCM. Table 3 Properties of the mean organic PCM. PCM Nonadecane dodecanoic acid 1-octadecanol Average PCM

Tm(⁰C) 31.8 43.3 55.8 -

ρ (kg/m3) 769.9 867.5 814.1 817.2

k (W/m·K) 0.1488 0.1468 0.1798 0.1585

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Cp (J/kg·K) 2381 2052 2596 2343

∆Hfusion (J/kg) 181 184 179 177.2

IFHT2016-1871 3. RESULTS Seven melting temperatures ranging from 35-47°C were investigated. The effectiveness of the PCM thermal storage modules were evaluated by comparing the transient temperature history of the tablet to two comparison cases: no PCM thermal storage (labelled “No PCM”), and a module with the same thermophysical properties as the PCM but with no latent energy storage (labelled “No Melt”). The second comparison case is important to differentiate the effect of latent heat storage from the effect of introducing a solid conductive domain into the tablet computer. The goal of a PCM thermal storage module is to delay the time which it takes the SOC and the cover of the tablet to reach their respective temperature limits. The researchers have adopted conservative temperature limits of 80°C at the SOC and 40°C at the cover. Simulations include both a heating and a cooling phase. All of the sources in the tablet model are active for a period of 45 minutes after which they are shut off and the tablet is allowed to cool for 75 minutes. Figures 3 and 4 show the maximum temperature of the SOC and the back cover over the entire simulation. The temperature of the SOC is reduced by introduction of the latent heat storage module. However, this trend requires a closer examination as the No Melt simulation also shows a significant decrease in the maximum temperature of the SOC. Introducing a solid medium into the air gap between the back cover and the SOC has increased conduction to the back of the case. From Fig. 4, it is evident that for the No Melt case and several of the PCM thermal storage modules, the back cover temperature is increased compared to the No PCM case. Figures 3 and 4 also show the cooling phase of the tablet. From 45 minutes onward the sources are inactive, the tablet cools, and the PCM solidifies releasing the stored heat. Modules with lower melting temperature PCM take longer to solidify and therefore will require a longer off time to dissipate heat. It is challenging to do a one to one comparison of solidification behaviour between modules, given the current simulations. Future simulations with cyclic heating and cooling cycles may offer more insight. The primary goal of a latent heat thermal storage device is to delay the time that it takes for the system to reach its maximum operating temperature. Figures 5 and 6 show the first 20 minutes of the simulation in which the SOC reaches 80°C for all melting temperatures. All of the latent heat storage modules significantly increase the time required for the SOC to reach 80°C. Figure 6 shows the total time until over temperature for each case as well as the corresponding delay when compared to a tablet with no PCM thermal storage.

. Fig. 3 Temperature history of the SOC for the various cases studied throughout the simulation.

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Fig. 4 Temperature history of the back cover for the various cases studied throughout the simulation.

IFHT2016-1871

Fig. 5 Temperature history of the SOC for the various Fig. 6 Total time for the temperature of the SOC to reach cases studied during the first 20 minutes of the simulation. 80°C and the over temperature delay for each of the PCM thermal storage modules. These simulations concur with previous results [8] in showing that the delay time for the SOC increases with decreasing melting temperature and at higher melting temperatures approaches the “No Melt” case. The performance of a module is directly correlated with the amount of melting which happens during this critical period of time. For comparison, the latent heat thermal storage module with a melting temperature of 35°C is 73% melted at 20 minutes while the module with a melting temperature of 47°C is only 6% melted at 20 minutes. Figure 7 shows the maximum temperature of the back cover for the first 20 minutes. Introduction of the latent heat storage module decreases the resistance from the major sources to the back cover of the tablet resulting in the “No Melt” simulation and latent heat storage modules with melting temperatures above 37°C decreasing the operating time before overheating. Figure 8 shows the time to overheat and the delay for each simulation.

Fig. 7 Temperature history of the back cover for the Fig. 8 Total time for the temperature of the back cover to various cases studied during the first 20 minutes of the reach 40°C and the over temperature delay for each of the simulation. PCM thermal storage modules.

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IFHT2016-1871 These simulations clearly show that, for this specific design, lower melting temperature PCMs perform better. It should be noted that for cases where the PCM thermal storage module detrimentally effected the heating of the back cover it did increase the total time for the system to overheat. For example, the storage module with a melting temperature of 41°C increased the time for the SOC to overheat from 3.4 to 13.5 minutes but decreased the time for the back cover to overheat from 10.9 to 8.6 minutes. Therefore, the total safe operating time went from 3.4 minutes to 8.6 minutes and the location of the critical temperature shifted from the SOC to the back cover. However, this is only marginally better than the no melt scenario.

4. CONCLUSIONS These simulations are the first step in designing PCM thermal storage modules for use in the temperature control of portable electronic devices. A representative thermal model of a tablet computer was constructed. This model was used to evaluate the effectiveness of seven PCM thermal storage modules with melting temperatures ranging from 35-47°C, each placed near the back cover of the tablet computer. It was found that a PCM with a melting temperature below 39°C was superior to higher temperature PCM modules because it melted sooner and more completely providing better temperature control for the SOC and the cover of the tablet computer. There are obvious concerns about using low melting temperature PCMs in portable electronics devices including unintended melting of the storage module and a slower solidification phase. However, the results contained in this work prove that integrating latent heat thermal storage modules into portable electronics is a very promising technique that deserves further study. Future work will move in two directions. Firstly, the experimental validation of the simulation. This includes the validation of simulation methodology but also the inclusion of elements which cannot easily be modelled without experiment: interface resistance, convection coefficients, etc. Secondly, the use of a more representative power dissipation profile, starting first with cyclic dissipation profiles and eventually moving towards increasingly realistic profiles.

ACKNOWLEDGMENT The authors would like to 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 (NSERC) and the Canadian Foundation for Innovation (CFI) for further laboratory financial assistance.

REFERENCES [1]

Kandasamy, R., Wang, X. Q., Mujumdar, A. S., "Application of phase change materials in thermal management of electronics," Appl. Therm. Eng., 27, pp. 2822-32, (2007). [2] Kandasamy, R., Wang, X. Q., Mujumdar, A. S., "Transient cooling of electronics using phase change material (PCM)-based heat sinks," Appl. Therm. Eng., 28, pp. 1047-57, (2008). [3] Fok, S C, Shen, W, Tan F L, "Cooling of portable hand-held electronic devices using phase change materials in finned heat sinks," Int. J. Therm. Sci., 49, pp. 109-17, (2010). [4] Fan, L. W., Xiao, Y. Q., Zeng, Y., Fang, X., Wang, X., Xu, X., Yu, Z. T., Hong, R. H., Hu, Y. C., Cen, K. F., "Effects of melting temperature and the presence of internal fins on the performance of a phase change material (PCM)-based heat sink," Int. J. Therm. Sci., 70, pp. 114-26, (2013). [5] Alawadhi, E., Amon, C., "PCM Thermal Control Unit for Portable Electronic Devices: Experimental and Numerical Studies," IEEE Trans. Compon. Packag. Technol., 26, pp. 116-25, (2003). [6] Hodes, M., Weinstein, R. D., Pence, S. J., Piccini, J. M., Manzione, L., Chen, C., "Transient Thermal Management of a Handset Using Phase Change Material (PCM)," J. Electron. Packag., 124, pp. 419-26, (2002). [7] Tomizawa, Y., Sasaki, K., Kuroda, A., Takeda, R., Kaito, Y., "Experimental and numerical study on phase change material (PCM) for thermal management of mobile devices" Appl. Therm. Eng., 98, pp. 320–29, (2016). [8] Sponagle, B., Groulx, D., "Thermal Modeling of Tablets: Temperature Management using Phase Change Materials," 1st Thermal and Fluid Engineering Summer Conference, TFESC, 14 p. (2015). [9] Ahmed, T., Bhouri, M., Kahwaji, S., Groulx, D. and White, M. A.,"Experimental Investigation of Thermal Management of Tablet Computers using Phase Change Materials (PCMs)," Proc. of the ASME 2016 Summer Heat Transfer Conference, 10 p. (2016) [10] Groulx, D. and Ogoh, W., "Solid-Liquid Phase Change Simulation Applied to a Cylindrical Latent Heat Energy Storage System," Proc. of COMSOL Conference 2009, 7 p. (2009). [11] Kheirabadi, A. C. and Groulx, D., "Simulating Phase Change Heat Transfer using COMSOL and FLUENT: Effect of the MushyZone Constant," Comp. Therm. Sc., v. 7, pp. 427-440 (2015).

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