Stanford & Intel: Sub-Ambient Cooling Project

Transcription

1 Stanford University ME 113: Mechanical Engineering Design Professor Drew Nelson & Professor John Howard Stanford & Intel: Sub-Ambient Cooling Project Final Report Aaron Oro, Alex Le Roux, Jamie Young, Jeff Sarsona June 4th, 2016

2 Contents 1 Abstract 3 2 Introduction 4 3 Objectives Use Case Scenarios Overall Goals Proposed Approaches Device Overview Proposed Solutions Selection Criteria Concept Evaluation Chemical Reactions Vapor Compression Peltier Cooling Summary of Concept Evaluation Peltier Cooling Approach Benchmark Testing Peltier Element Surface Pro 4 m Design Objectives Thermal Models Peltier Thermal Model Fan Thermal Model Heat Transfer Plate Thermal Model Final Design Thermal Design Electrical Design Mechanical Design Final Prototype Final Testing Subambient Cooling External Dock Testing Comparison Benchmark Testing Final Results Analysis

3 8 Future Works Peltier Subambient Cooling Prototype Improvements Convective Subambient Cooling Approaches Our Proposed Way Forward Appendices 32 A Product Specifications 32 A.1 Peltier Element A.2 Cooling Fan B Vapor-Compression Refrigeration Cycle Calculations 33 B.1 Final Prototype Images C Project Timeline 35 2

4 1 Abstract Working for Intel s Disruptive Innovations Solutions Team, we were tasked with exploring sub-ambient cooling solutions for tablets. We were given two major objectives: (1) research the various ways in which a tablet can be cooled sub-ambiently and (2) construct a prototype sub-ambient cooling dock utilizing one of the researched processes. For the first objective, we focused our research on three sub-ambient cooling processes: Peltier elements, vapor-compression cycles, and chemical reactions. Taking into account the timeline of the project and the expertise of the group, we chose to use Peltier elements in our prototype because of their prototyping feasibility and implementability. We constructed a sub-ambient cooling dock functional for a wide range of tablet form factors using Peltier elements. The design of this dock was informed by thermal models and simulations created in MATLAB and SolidWorks. The dock functions by locally cooling the processor location on the back of a tablet. This sub-ambient cooling is achieved by coupling Peltier elements with a pin fin heat sink and forced convection through fans. The Peltier elements directly contact a thin aluminum heat transfer plate that contacts the back of the tablet to apply cooling. When tested with a Microsoft Pro 4 running at 100% CPU load, our device dropped the tablet surface temperature above the processor by 8.6 C while running at 9.24 Watts. This temperature drop was better than a commercial fan cooler running at 1.9 Watts. Moving forward, we are confident that our conductive Peltier solution could be optimized to further reduce tablet surface temperatures with improved power efficiency. Conversely, we believe that a convective sub-ambient cooling system would better reduce tablet surface temperatures as it could provide a constant sub-ambient temperature gradient. We would also advise Intel to research and prototype a sub-ambient cooling solution utilizing vapor-compression as this process provides the most potential cooling capacity. 3

5 2 Introduction The Disruptive Innovative Solutions Team, part of the Advanced Client Solutions Team at Intel, is focused on creating and disrupting current product solutions and technologies. The Intel team is tasked with pioneering, exploring, refining, and prototyping consumer electronic experiences for client R&D. The team has recently focused on sub-ambient cooling approaches as a method of cooling consumer electronic devices. In sub-ambient cooling, some part of the thermal system or the ambient around it is cooled below the ambient temperature to enhance heat transfer. This is accomplished by inserting a refrigeration cycle or comparable method between the processor and heat sink. While sub-ambient cooling techniques have been understood for over two decades, there has been little to no implementation in small form factor devices. Instead, the majority of system solutions have been applied to server systems and large computer systems. Over the past five years, computing devices have decreased in size while running more powerful processors. An example of this type of device is the Microsoft Surface, which runs Intel Core processors, has a native operating system, and functions as a full computer at a smaller and lighter form factor. However, in these types of personal computing devices, thermal management is crucial to both computing performance and user experience. As processor power increases and device form factor decreases, the ability to dissipate this generated heat from the processor is integral to device performance. As the thermal system fails to effectively reject heat, a decrease in performance can be noticed by users in slower functioning computers and hotter touch temperatures on the outside of the device. Sub-ambient cooling is therefore an attractive approach as a way to supplement heat dissipation in a consumer electronic device. As part of this project partnership, Stanford and Intel hope to develop a sub-ambient cooling approach for tablet form factor devices to improve computing performance. Our team will research sub-ambient cooling approaches and design and prototype a small form factor, external, sub-ambient cooling system to increase heat dissipation from the device. 4

6 3 Objectives 3.1 Use Case Scenarios In the design and prototype stage, we identified two different products aimed at addressing different use case scenarios. The intent was to later select one use case based on feasibility. Workstation Case This use case was targeted towards working professionals. We envisioned the need for a cooling device that could act as a semi-permanent workstation that the tablet could be placed into when the user was performing GPU or CPU intensive work at his/her desk but that could also easily be accessible when the tablet had to be taken to a meeting. Secondly, we also considered the need of this dock when the user traveled abroad and still needed extra cooling for increased performance. Portability Case This use case was targeted towards everyday consumers. Here the tablet is used as an entertainment hub that could provide high level functionalityfor running applications, working online or watching movies, which is not often done with the tablet in hand. Therefore, a tethered cooling dock would not be ideal. Our goal for this scenario was to provide a portable, battery powered sub-ambient cooling system that ould attach to the back of a tablet when needed. 3.2 Overall Goals In order to determine the feasibility of delivering a product, we defined a list of clearly defined goals and requirements that we wanted to meet in the final product. The specific goals for this prototype are listed below and are based on the two overarching use case scenarios discussed in the Section 3. Workstation Case 1. Size: Similar footprint to tablet (11.5 x 8 ) but no strict constraint on height, which depends on final dock orientation. 2. Noise: Less than 25dBA at 1 meter. 3. Usability: One action mechanism for quick docking and removal of tablet. 4. Nice-to-Have: Portable and deployable for extended travel. 5

7 Portability Case 1. Size: Similar footprint to tablet (11.5 x 8 ). Less than 1.4 in thickness. 2. Power: Able to perform 2 hours of sub-ambient cooling per full battery charge. 3. Weight: Less than 1.5 lbs in weight. 4. Noise: Less than 25 dba at 1 meter. 5. Nice-to-Have: One action mechanism for quick docking and removal from tablet. 4 Proposed Approaches 4.1 Device Overview In order to better understand the thermal designs of market tablets and computers, our group performed a case study on the processor location in competitive devices. This allowed us to explore the internal architecture of market devices to understand the location of the processor and the associated hot spots. Figure 1 shows a Microsoft Surface Pro 4 m3 running an dual-core Intel Core m3-6y30 processor. Figure 1: X-Ray image of a Microsoft Surface Pro 4 m3 with dual-core Intel Core m3-6y30 processor Figure 1 illustrates the processor location in the upper right quadrant of the device. This point can then be mapped across a range of competitive devices. As illustrated in Figure 2, a noticeable trend appears in the processor location for competitive devices. Most of the CPU and GPU components exist in the upper half quadrant of the device. This is primarily because the device batteries are placed in the lower half quadrant to improve weight balance and reduce tipping. This trend can be further seen visually in Figure 3 of an IR camera image of the back view of a Surface Pro 4. As seen, the hot spot location on the back matches well with the processor location displayed in the architecture case study. 6

8 Figure 2: Processor Location for Competitive Devices. Figure 3: Back view IR camera image of Surface Pro 4 m3 illustrating temperature gradient around processor location 4.2 Proposed Solutions Peltier Cooling Systems Peltier cooling, also categorized as thermoelectric cooling, uses an electric device which serves as a solid-state heat pump that moves heat from one side of the device to the other. These devices, called Peltier devices or thermoelectric coolers (TECs), are placed directly on top of an object to be cooled, such as a CPU or heat spreader. By running current through the device, heat is directed from the CPU to the other side of the TEC. On the other side of the TEC module, another cooling device, such as a heat sink, is placed to drive heat transfer and maintain the TEC efficiency. Peltier devices are good solutions for the scope of our design problem because our primary goals were to design a quiet and compact cooling system for small form factor tablets. They have no moving parts and their small, flexible form factor allowed us to design a portable cooling system that meets our goals. The disadvantages 7

9 of these devices are high power consumption and low thermal efficiency, especially at higher temperature gradients, both of which may limit the portability of our design. Chemical Reactions Reversible chemical reactions have two main advantages for this project. First, the reversibility means that the reaction is endothermic in one direction and exothermic in the other. By strategically forcing the reaction in specific locations of the cooling device, we can extract heat from the tablet with the endothermic reaction and reject heat with the exothermic reaction. This control can be attained through electric impulses or air compression. The second advantage of a reversible chemical reaction is that the cooling device would be selfsustained and would not require replenishing of reactants. The disadvantage of using a reversible chemical reaction is that these reactions usually operate at excessive temperatures and pressures that can require costly equipment and procedures. An alternative would be to use an irreversible, endothermic chemical reaction that readily reacts given the correct concentration of reactants. While this can be a cheap and safe solution if the right reactants are found, the main disadvantage is that the cooling station would need periodic replenishing of reactants and disposing of the products. Vapor Compression Refrigeration A vapor compression system is the most common refrigeration cycle for standard air conditioning systems in cars, houses, and commercial buildings due to its high efficiency. This cooling method makes use of a four-component thermodynamic system consisting of a refrigerant flowing through a compressor, condenser, expansion valve, and evaporator. Low pressure refrigerant vapor enters the compressor and gets compressed to a high pressure and temperature vapor, which then goes to the condenser where it undergoes an isothermal phase change by rejecting heat to the ambient environment. The high pressure and temperature liquid refrigerant then reaches the expansion valve, where it decreases pressure and temperature. The refrigerant then flows to the evaporator where it absorbs heat by undergoing isothermal phase change from liquid to vapor, thereby providing cooling to the target heat source. This cooling technique would be useful for our design given its widespread use and powerful cooling capability. Many of the components necessary for this system can be purchased off the shelf and implemented in a custom design for our use case. On the other hand, the disadvantage of this cooling method is that it is typically used in heavy-duty applications, meaning the system is physically larger, louder, and higher output than necessary for small portable devices. In order to adapt this method for our specific design problem, we needed to drastically reduce the scale of the vapor-compression system to fit the form factor of a portable tablet. This was difficult to do given that incorporating even the smallest vapor-compression system currently on the market would push the limits of a form factor defined as portable. These constraints forced us to evaluate the balance between thermal efficiency and system scale for an appropriate means of providing sub-ambient cooling for tablet devices. 8

10 4.3 Selection Criteria We used the following criteria to evaluate our proposed solutions: 1. Viability: Can this sub-ambient cooling solution adequately cool a tablet and improve computing performance? 2. Prototype Feasibility: Given our time and knowledge base, can we generate a functional prototype using this solution? 3. Implementability: Can this solution be safely used by consumers in a commercial setting? 4. Research: Is there an opportunity to provide system specifications at an industrial level without actually building a prototype? 4.4 Concept Evaluation Chemical Reactions While endothermic chemical reactions present a plausible and novel approach to sub-ambient cooling, our group does not possess the expertise to safely and efficiently implement this approach in the project time-frame Vapor Compression As discussed in Section 4.2, vapor compression refrigeration produces a much larger cooling capacity than necessary for our application, so scaling such a system down to an appropriate size would entail reducing the specifications for each component in the system. Specifically, the largest and most limiting component in a vapor compression cycle is the compressor. If we could minimize the operating pressures and compression ratio, we would be able to reduce the amount of work the compressor has to do and therefore reduce its size and noise output to levels suitable for our application. Such a system using a smaller compressor is limited by the ambient temperature of the system. Looking at the cycle diagram in Figure 4, ideally the refrigerant must be a saturated liquid coming out of the condenser at stage 3. At the condenser, however, the refrigerant is rejecting heat to the ambient environment, so T 3 must be greater than the ambient temperature for a temperature difference to exist and allow heat to be rejected. This specifically limits the minimum compression ratio of the cycle to at least 7:1 for a given system 1 using R-134a refrigerant, a commonly used refrigerant for low-pressure applications. This in turn would require us to design and prototype a compressor operating at pressures of at least 700 kpa, which wouldn t be feasible given our machining resources. Furthermore, had we proceeded with the intent to minimize operating pressures, the cooling capacity of the system would be 7.2 kw, which would be overkill for the application of cooling tablets. The alternative would be to pressurize the system even more to reduce the compression ratio and thus reduce the system cooling capacity to a value more appropriate for cooling a tablet. However, the resulting increase 1 See Appendix B for calculations for an example system with desired design specifications. 9

11 in operating pressures would require a more robust compressor design, further adding to the infeasibility of prototyping such a solution. With the aforementioned ambient temperature constraints, a prototyped solution implementing vaporcompression refrigeration was not feasible for the scope of this class. Manufacturing a custom pressure vessel would put our safety at risk, and to outsource such a component would exceed the scope of this class in terms of budget and lead time. Figure 4: Vapor-Compression Cycle Diagram Peltier Cooling We identified Peltier cooling as the most practical and implementable sub-ambient cooling method for our project timeframe and scope. Using Peltiers allowed us to prototype a tangible deliverable that provides sub-ambient cooling to tablets. From our models and calculations, we believed that we could provide an optimized sub-ambient approach with this technology. 4.5 Summary of Concept Evaluation Selection Criterion Chemical Reactions Vapor Compression Peltier Cooling Viability Prototype Feasibility X X Implementability X Research Table 1: Sub-ambient cooling concept evaluation. As Peltier cooling was the only sub-ambient cooling approach that satisfied all of our selection criterion, we used it as the sub-ambient cooling approach for our first prototype. In addition, for our proof of concept prototype, we designed a sub-ambient cooling solution for the Workstation Case detailed in Section 3.2. How- 10

12 ever, in parallel we worked to provide industrial specifications of sub-ambient cooling systems that used vapor compression. 5 Peltier Cooling Approach 5.1 Benchmark Testing Peltier Element The Peltier element we chose to use for the project was the 5V, 1A Peltier Thermo-Electric Cooler Module supplied by Adafruit. For device specifications see Appendix A.1. Characterization of our Peltier elements involved connecting them to a power supply and running them at 3V and 5V. Temperature measurements were taken by attaching a thermocouple to each side of the element using thermal paste and copper tape to reduce contact resistance. The Peltier passively convected the generated heat to ambient air inside the lab (T ambient 24 C). We ran the Peltier test until it reached steady state temperature values on either side. We then switched the power off and allowed the Peltier to cool passively. T hot side, T cold side and T as a function of experiment time for both the 3V and 5V cases can be seen in Figure 5. Figure 5: Steady State Peltier Characterization for 3V and 5V. For the 3V case, we saw that the cold side of the Peltier remains at sub-ambient for 120 seconds after power was supplied, and that T between the hot and cold sides ranged from 28 C to 33 C. For the 5V case, we saw that the cold side of the Peltier remained at sub-ambient for 50 seconds after power was supplied, and that T between the hot and cold sides ranged from 42 C to 52 C if outlier data points were omitted. When power was delivered to the Peltier element, each side maintained a constant temperature difference. However, when power was turned off, the Peltier equlibrated to a uniform temperature and the temperatures on each side equilibrated to the same value. These cool-down temperatures over time for both the 3V and 5V case can be seen in Figure 6. Note how the y-axis initial values correlate with the steady state values seen in Figure 5. 11

13 Figure 6: Cool Down Characterization of Peltier Stacks at 3 and 5 Volts Surface Pro 4 m3 Intel sent our group a Surface Pro 4 m3 to benchmark and use in testing our sub-ambient cooling approach. As seen in the x-ray image of the Pro 4 M3 in Figure 1, the device relied solely on passive cooling to dissipate heat (i.e., no fan). The Surface Pro 4 m3 fit was a good fit for our project objectives. Note that the Pro 4 is built with the Intel Core m3-6y30 2. This four thread, 2 core processor is capable of an average thermal design power (TDP) of 4.5 W. A major factor in quantifying the performance of our sub-ambient cooling method was the ability to create a constant, high intensity workload for the CPU to run over the course of the experiment. In order to accomplish this, the Surface Pro 4 was charged to 100% battery life and given particular settings (Wi-Fi turned off and screen brightness set at 50%). The Pro 4 15V power charger was plugged into a power wall meter that measured the current draw from the device. In addition, we placed a thermocouple on the back side hot spots of the device as shown in Figure 7. This setup allowed us to see how much power the system was drawn at a given time. In order to simulate graphics or CPU-intense workloads, our team used a software tool ( CPU Stress ) that throttled all four cores of the CPU in order to maximize CPU workload at 100%. In addition, in the background, we ran two 4K videos on loop 3 4. Our initial bench marking illustrated the success of this approach both in generating high, yet constant, power draw from the CPU. The metrics from this test can be seen in Table 2. Test Type Total Power Draw (W) Underside Temperature (C) Surface Pro 4 m3 Stand Alone Table 2: Initial benchmark testing of the Surface Pro 4 under a tested workload

14 Figure 7: Test setup using Surface Pro 4 m3 device and a thermocouple to measure the backside hot spot temperature. As seen in this initial benchmark test, our group we were able to develop a method to throttle the CPU to close to its TDP. 5.2 Design Objectives As discussed in Section 5.1.1, we initially saw sub-ambient temperatures on the cold side of the Peltier when power was supplied. At 3V, we saw sub-ambient temperatures for 120 seconds, and at 5V, we saw sub-ambient temperatures for 50 seconds. This was promising as it proved that Peltier elements could generate sub-ambient temperatures for a continuous amount of time. However, without any additional heat dissipation system, the temperature on the hot side of the Peltier continued to rise over time until it reached a steady state temperature value dependent upon power input (T SS,3V 64 C and T SS,5V 100 C). As evidenced by Figure 5, for a given voltage our Peltier element retained a fairly consistent T between the hot and cold sides (28 C ă T 3V ă 33 C and 42 C ă T 5V ă 52 C). Therefore, in order to maintain sub-ambient temperatures on the cold side of the Peltier, we needed to keep the hot side of the Peltier at or below a threshold temperature. Based on Figure 5, T threshold,3v 53 C and T threshold,3v 67 C. These threshold temperatures were well below the steady state temperatures of the hot side of the Peltier. This presented our main design objective: we needed to actively cool the hot-side of the Peltier so as to maintain sub-ambient temperatures on the cold side. We planned to achieve this cooling using heat sinks with forced convection. 13

15 5.3 Thermal Models Peltier Thermal Model In order to make data driven decisions throughout the design process and understand how to actively cool the Peltier elements, our group developed a thermal model for the proposed system. Utilizing our data in Section 5.1.1, as well as an understanding of thermal resistive networks, the thermal model was based on a number of underlying assumptions: Peltier elements maintain a constant T for a given voltage level (i.e. T 5 T 4 T ) One-dimensional heat transfer Steady state heat transfer Constant heat flux from the processor (i.e., 9q CP U is constant) Only contact resistance between Pro 4 chassis and external stand Ambient temperature is 298 K These boundary conditions allowed us to build a thermal resistance model for steady state heat transfer as illustrated in Figure 8 with the following variables: T1 - the processor temperature (K) T2 - the hot spot temperature on the outside of the Surface Pro 4 m3 (K) T3 - the temperature of the top side of the heat transfer plate (K) T4 - the temperature at the junction of the Peltier cold side and heat transfer plate (K) T5 - the temperature at the junction of the Peltier hot side and heat sink (K) T6 - the temperature at the base of each pin fin in the heat sink (K) TA - the ambient temperature inside of our sub-ambient cooling solution (298K) Q CP U -the heat dissipated from the underside of the Surface Pro 4 (W) W peltier - the electrical work inputted to Peltier elements to maintain sub-ambient cooling (W) Q H - the heat dissipated from heat sink to ambient (W) R Cond1 - the conductive resistance of the magnesium chassis of Surface Pro 4 ( L ka ) R Surface - the contact resistance between the magnesium chassis and the aluminum heat transfer plate R Cond2 - the conductive resistance of the aluminum heat transfer plate ( L ka ) R Cond3 - the conductive resistance of the aluminum heat sink base ( L ka ) R F in - the resistance of the pin fins in the aluminum heat sink 14

16 Figure 8: Thermal Resistive Network Model. Based on our understanding of Peltier models, we knew that Q H Q CP U ` W peltier. Knowing that Q CP U 4.5W for the Intel Core m3-6y30 processor as discussed in Section and that our Peltier element drew 1.5 Watts at 3 Volts, we characterized the heat flux from the hot side of the Peltier element, Q H. Using this information, we built a main script in MATLAB that iterated over design variables as well as a number of functions to define contact, conductive and fin resistance based on known equations. In addition, a major factor in improving the thermal efficiency of our system was minimizing the contact resistance between the Pro 4 and our heat transfer plate. This could be accomplished by minimizing air gaps between the two surfaces. Some design variables that our group also attempted to optimize included heat sink surface area, cylindrical pin fin diameter, pin fin spacing and length, convective coefficient h, and fan CFM. In addition, we used the SolidWorks thermal simulation tool to optimize the design of our heat sink. While it was difficult to model a Peltier element in SolidWorks, we used the tool to optimize and visualize heat sink geometry and performance. As seen in Figure 9, we iterated over convective coefficients to see how the temperature of the hot side of the Peltier element, T 5, changes. By knowing the T of a Peltier, we then backed out the cold side temperature. The SolidWorks thermal modeling tool assumed steady state condition and constant heat flux. It also took into account two-dimensions heat transfer. By running SolidWorks simulations for five convective coefficients from 10 to 40 (W {m 2 K) we were able to fit a curve to the SolidWorks simulation data for a given heat sink design. Figure 10 compares the predicted cold side Peltier temperature of both models for a given convective coefficient. As seen in Figure 10, a convective coefficient above 22 pw {m 2 Kq allowed us to provide sub-ambient cooling to our Surface Pro 4 m3 model. Since a convective coefficient of 10 pw {m 2 Kq roughly equates to a light draft in a room, a fan was required to reach this convective coefficient value. These models were scalable to any design iterations. In addition, we plotted actual experimental data against the models to see how closely they correlated and how we could optimize the model to accurately reflect our system. 15

17 Figure 9: SolidWorks Thermal Simulation for Various Convective Coefficients Fin Plate X-Y 2"x2" - Tablet Heat Flux = 4.5W - Peltier Power = 1.5W 70 Peltier Cold Side - Fin Heat Sink Solidworks Simulation Model Subambient Cooling Limit 60 Temperature,T, (C) Convective Coefficient, h, ( W/(m 2 K)) Figure 10: Comparison of SolidWorks and Matlab Thermal Models Relating Peltier Cold Side Temperature to Convective Coefficient. 16

18 5.3.2 Fan Thermal Model As mentioned in Section 5.3.1, our ability to maintain a steady state sub-ambient cooling solution largely depended on our convective coefficient. Therefore, we used fans that provided the necessary convective coefficient at a low db level. We used an anemometer to measure air flow velocity and a known equation for air relating velocity to convective coefficient as detailed below. h v air ` 10? v air Using this equation and our collected data, we related the convective coefficient of our fans to voltage. We then related these values back to our Peltier thermal model. This gave us an understanding of how much we could cool the cold side of the Peltier. Figure 11 illustrates that for the low db fans used in our various prototypes, we could generate a convective coefficient of approximately 22 pw {m 2 Kq. 30 Convective Coefficient vs Air Velocity - h = v + 10v 1/ Convective Coefficient, h, (W/m 2 K) Model Proto Fan 1 at 12V Current Fan at 6V Current Fan at 12V Cooler Master Fan Air Flow Velocity, v, (m/s) Figure 11: Air flow velocity vs convective coefficient for a range of fan types and voltage levels Heat Transfer Plate Thermal Model A final thermal consideration for our system was the size, thickness, and material of our heat transfer plate. From initial testing, we realized that we could maintain a extremely low temperature on the cold side of the Peltier ( 6 ). However, when this Peltier cold side interfaced with a heat transfer plate, it began to suck heat from the heat transfer plate and the ambient environment. This heat transfer is governed by the equation Q mc p T where m is the mass of the plate, c p is the specific heat capacity of the heat transfer plate material and T is the change in temperature of the heat transfer plate relative to the ambient temperature. The mass of the heat transfer plate is dictated by the thickness and surface area of the plate. While copper has a lower c p value than 17

19 aluminum (0.39 vs 0.91 kj kgk ), it is also more dense than aluminum (8.96 vs 2.70 g cm 3 ). Since the heat capacity equation is linear, our group opted to use an aluminum heat transfer plate; for the same heat plate dimensions and change in temperature, the Peltiers needed to remove less heat from the heat transfer plate. This meant that our Peltiers were doing less work to maintain the same temperature. These calculations using hypothetical values can be seen below: Heat Transfer Plate Dimensions - X = 2 Y = 7.25 Z = 0.05 T 15 Q Cu mc p T V ρc p T J Q Al mc p T V ρc p T J In addition, using data from our initial testing, we developed a model to relate how changing plate thickness and surface area would change the steady state sub-ambient temperature of the heat transfer plate. This allowed us to iterate over various surface areas and thicknesses to optimize our heat transfer plate. While we wanted to minimize the steady state temperature of the heat transfer plate, we also wanted to maximize surface area, rigidity and flatness of the heat transfer plate. Our optimization model can be seen in Figure 12. ]Eventually, we decided on final plate dimensions of X = 2 Y = 7.25 Z = made out of aluminum for a predicted steady state temperature of approximately Thermal Model for Cooling Plate Geometry vs Heat Transfer Plate SS Temperature 20 Heat Transfer Plate SS Temperature,T, (C) t = 1.25 mm Al t =1 mm Al t = 0.85 mm Al t = 0.6 mm Al t = 0.85 mm Cu Current SA " x 2" Final Design Surface Area Footprint, SA, in 2 Figure 12: Heat transfer plate optimization model for various surface areas and materials. 18

20 6 Final Design 6.1 Thermal Design Using the thermal models described in Section 5.3, we optimized the heat transfer plate and heat sink design to maintain the lowest possible sub-ambient steady state temperature on our heat transfer plate at the quietest fan speed and largest heat transfer plate surface area. We decided to use three Peltier arrays to maximize cooling and designed and machined a heat sink array with three 2 x 2 pin finned heat sinks. Each heat sink has sixty diameter, 0.52 long cylindrical pin fins. The final CAD of our heat sink design can be seen in Figures 13 and 14 below. The heat sinks were machined as one continuous part with three distinct sections to improve machinability while maintaining disc. Figure 13: Orthographic view of our final heat sink array design for three peltier elements. CNC machined from 6061 Aluminum. Figure 14: Top view of our final heat sink array design for three peltier elements. CNC machined from 6061 Aluminum. 19

21 In addition, as detailed in Section 5.3.3, we decided on an aluminum heat transfer plate with dimensions of 7.25 x 2 x This heat sink geometry allows us to maintain a steady state sub-ambient heat transfer plate temperature of approximately 12 C. This represents a steady state cooling solution that is approximately 13 C below ambient temperature. Finally, we decided to run our fans at 6V to maintain a silent cooling solution. We used two PWM 12V fans from Arctic 5. As our fans were rated at 21.5 dba at 12V, by running them below 12V we know that our system has a dba level below 21.5 which meets our initial product specifications detailed in Section 3.2. Our final thermal stack can be seen in Figure 15 below. Figure 15: Orthographic view of our thermal stack, including our heat transfer plate, three Peltier modules and cylindrical pin fin heat sink array. 6.2 Electrical Design The main goal of our electrical design was to run our system off of a single power supply so as to easily measure total system power draw and reduce the number of total components. Our sub-ambient cooling system runs off of a single BK Precision DC Power Supply set to 6 Volts. Three Addicore 3.3V LD33V linear voltage regulators 6 step down the voltage for the three Peltier elements. The DC Power Supply allowed us to measure the power required by the system during testing (i.e. P required V i). An EE schematic of our final electrical design can be seen in Figure 16 below

22 Figure 16: Electrical schematic for the final prototype. 6.3 Mechanical Design Our final design incorporated a few key mechanical features for enhancing heat transfer. These included air vents throughout the housing for improved airflow, features in the housing that encourage correct orientation of the tablet in the cooling device, and a spring-loaded mechanism to improve contact between the the heat transfer plate and the back of the tablet. Air vents throughout the housing served to increase airflow. The internal fans of our device pull in air from underneath the dock chassis and force it over the heat sink to convectively cool the hot side of the Peltiers. This convection requires vents along the sidewalls to reject the heated air back to the ambient environment and vents in the bottom plate to allow airflow to the fans. The sidewall vents, rectangular cutouts along three sides of the device, allow adequate airflow without sacrificing structural support. These vents can be seen in Figure 29 in Appendix B.1. The bottom plate vent designs allow for airflow through the fans, and required two iterations for design optimization. Our initial bottom plate vents seen in Figure 17 mimicked the rectangular cutouts in the sidewalls, however, this design had a poor ratio of open vent area to closed vent area. This resulted in limited and choppy airflow and added to the noise output of our device. For our second iteration we created a circular vent design, also shown in Figure 17, defined by a 2:1 open vent area to closed vent area ratio. This reduced noise levels and improved airflow. Figure 17: Air vent designs for the bottom plate. Initial design on the left, final design on the right. 21

23 We also designed for users that would dock the tablet in the wrong orientation, thus reducing the effectiveness of the cooling dock. The heat transfer plate targets the CPU hot spots on the back of the tablet, so a tablet rotated 180 in the dock would leave the hot spots insulated against the top plate of the housing instead of being cooled by the heat transfer plate. To prevent this, we blocked off access to the tablet s power and volume buttons when it s in the wrong orientation. In order for the user to access these buttons, they will notice that they need to rotate the device if it s in the wrong orientation, and from then on they will be wary of its orientation in the dock. This intuitive feedback was achieved by lengthening the wall constraints along the edge of the top that already serve to hold the device in place. This can be seen in Figure 18. Note that the charging port of a Surface Pro 4 is usable in any orientation. Figure 18: Wall constraints along the top edges of the dock block access to Surface Pro 4 buttons if placed in the wrong orientation, encouraging the user to correctly place the tablet in the dock. Lastly, we designed a mechanism that spring loaded the heat transfer place against the back of a tablet. This spring mechanism forces the heat transfer plate to sit slightly proud of the top surface of the dock. Upon placing the tablet in the dock, the springs compress to push the plate up against the tablet. This reduces the air gap between the tablet and plate, minimizing contact resistance at this main thermal interface. The first iteration of this mechanism incorporated foam strips that served as springs, shown in Figure 19. This version worked well to show that a spring-loaded plate did ensure better contact between the heat transfer plate and the tablet. However, it was not a long term solution. Therefore, the mechanism was redesigned to use compression springs instead of foam. As seen in Figure 20, the compression springs are held between the heat sink and a standoff on the bottom of the dock housing. The heat sink contains four mounting holes (1 in each corner) that hold screws that extend down into the housing standoffs. The end of these screws are constrained by hex nuts that bottom out on the underside of this standoff when the heat transfer plate is slightly proud of the top plate of the dock. 22

24 Figure 19: First iteration of the spring-loaded mechanism for the heat transfer plate using foam. Figure 20: Side profile of the Peltier assembly with final spring loaded mechanism. This new mechanism had several advantages over the foam solution. Mounting against the bottom plate rather than the top plate further reduced the deflection in the top plate of the dock, which would have contributed to an increased contact resistance. The compression springs also improved the compliance of the unit compared to the foam springs, allowing the entire heat transfer plate to compress more easily under the weight of the tablet while still providing an upward force to maintain contact. Lastly, the nuts used to lock the bolts down to the housing could be used to adjust the height of the heat transfer plate as well as its parallelism with the top plate of the dock. 6.4 Final Prototype Our final prototype was constructed of laser cut, 1/8 acrylic and had dimensions of x x 2.12 (length x width x height). Its front plate contained an 11.6 wide port for our Microsoft Pro 4 m3 as well as a 7.35 x 2.1 cut-out port for our heat transfer plate. This prototype can seen in Figure

25 Figure 21: Final prototype of a sub-ambient cooling dock. Small tabs and tab grooves were built into the sides of the acrylic pieces, allowing them to connect and mate in the correct orientation with other parts. Super glue was applied to these tabs to ensure rigidity when the final design was assembled. 1-1/4 tall rubber bumpers were placed on the back of the prototype to raise it off the ground and allow for adequate air flow to the fans. These can be seen in Figure 28 in Appendix B.1. Furthermore, vents were built into the side, top and back walls of the assembly to allow for air intake and rejection. Specifically, a concentric circle design was used on the back plate immediately below the fans to allow for proper airflow into the fans. This design incorporated a 2:1 ratio between open and closed vent space, and can be seen in Figure 17. The side vent design can be seen in Figure 29 in Appendix B.1. Two 80mm Arctic F8 PWM Fans were connected to the inside back of our device with #6-32 machine screws and hex nuts. Also attached to the inside bottom plate was our Peltier assembly. This assembly consisted of our CNC machined heat sink, three 5V thermo-electric Peltier cooling modules, a 7.25 x 2 x aluminum heat transfer plate and the spring loaded mechanism described in Section 6.3. This Peltier assembly can be seen in Figure 22 below and Figure 20 in Section

26 Figure 22: Front Profile of the Peltier Assembly. 7 Final Testing 7.1 Subambient Cooling External Dock Testing As mentioned in Section 5.1.2, we developed a reliable way to stress the CPU and GPU of the Surface Pro 4 m3 to simulate high intensity workloads that would require sub-ambient cooling. Using this test setup, our group tested the effectiveness of our sub-ambient cooling solution. As our two main objectives were to decrease the temperature of the outside of the Pro 4 and improve computing performance, our testing revolved around using thermocouples to measure temperature and a power wall meter to measure power draw. The Surface Pro 4 m3 was charged to 100% and set with a screen brightness of 50% with Wi-Fi turned off. Once fully charged, we started our high intensity workload and measured the steady state hotspot temperature on the backside of the Pro 4 with a thermocouple at the location shown in Figure 23 below. Figure 23: Thermocouple location on the backside of the Surface Pro 4 m3 used for final testing (marked with a white X). 25

27 Once the temperature reached steady state, we turned on our sub-ambient cooling dock and allowed the dock to reach a steady state temperature. We measured the steady state temperature of the heat transfer plate at approximately 13.2 C with an ambient temperature of 25.6 C. This indicates a 12.4 T between our sub-ambient cooling solution and the ambient air. For this test, we wanted to understand how the temperature of the heat transfer plate changed over time. Therefore, we routed a thermocouple through a vent in the external device and attached it to the underside of the heat transfer plate as seen in Figure 24. Figure 24: Thermocouple location on the underside of the heat transfer plate used to understand how the interface temperature changed during cooling. We then placed the Surface Pro4 m3 onto the external sub-ambient cooling dock and measured temperature values every 20 seconds until the device reached a steady state temperature value. This took approximately 2.5 minutes. The transient temperature values during the test can be seen in Figure 25 below Subambient Cooling Dock - Tablet Change in Temperature vs Time Microsoft Surface Pro 4 Temperature Subambient Cooling Dock Plate Temperature Initial SS Subambient Cooling Dock Plate Temperature 35 Temperature, T, (C) Time, t, (minutes) Figure 25: Surface Pro 4 and thermal plate temperatures during subambient cooling. 26

28 As seen in Figure 25, the external temperature of the Surface Pro 4 decreases by 8.6 C (42.2 C to 33.6 C) when placed on our sub-ambient cooling solution. In addition, the heat transfer plate quickly rose from its steady state sub-ambient value to approximately 28.0 C. Finally, using the wall power meter, we measured an increase in power draw of 0.24 W (16W to 16.24W). As the Surface Pro 4 m3 is a 4W TDP system, this represents an approximate 6% increase in processor power. This illustrates that for the same CPU and GPU workload, the tablet is able to have more computing power as it is not as thermally limited. We can also see the effectiveness of our solution in dissipating heat through IR camera images of the front display. In passive thermal designs like the Pro 4 m3, heat is also dissipated to the front of the device. We took IR camera images of the front display of the Surface Pro 4 before and after the test and this comparison can be seen in Figure 26. Figure 26: Comparison IR images of the front display of the Surface Pro 4 before and after testing. The IR camera images illustrate that we effectively sunk heat to the backside of the device and consequently the external sub-ambient solution. Additionally, even though we are targeting the processor location of the device, we can see that there is a uniform decrease in screen temperature, illustrating that targeting the hottest part of the device will also reduce the temperature throughout the whole device. 7.2 Comparison Benchmark Testing In addition to testing our sub-ambient cooling solution, we wanted to understand how our solution compared to market available solutions. We tested two other solutions: (1) Cooler Master 7, a laptop cooling pad and (2) our cooling solution without the Peltier elements, effectively creating an ambient conductive heat sink design. We ran the same test mentioned in Section 7.1 on both these devices. We first brought the Pro 4 to a steady state temperature value without the cooling solution, and noted the power and temperature values. We then measured the change in temperature of the backside of the Surface Pro 4 as well as the change in power draw at the same given workload. The final testing results of our various products can be seen in Table true&ref_=oh_aui_detailpage_o09_s00 27

29 Test Type Pro 4 Power Draw (W) T Pro 4 (C) Required Power (W) Subambient Cooling Dock Cool Master, Ambient Convective Cooling Heat Sink Cooling Dock Table 3: Final testing results for our cooling solutions. Figure 27: Surface Pro 4 sitting on the Cool Master, an ambient convective cooling solution. 7.3 Final Results Analysis As seen in Table 3, our sub-ambient cooling dock performed well, decreasing the exterior temperature of the Surface Pro 4 m3 while increasing the computing power. In comparison to an ambient convective cooling solution, our design decreased the external temperature by an extra 1.1 C and increased the power draw of the Surface Pro 4 by 1.85 times (0.24 vs 0.13 W). However, our cooling solution requires 4.86 times more power than a market available ambient convective cooling pad. However, these values are a little skewed as we are losing approximately 3.5 W while stepping down the voltage in our linear regulators. If we were to design a sub-ambient system with fans at 3.3V, the required power would be approximately 5.12 W, 2.69 times more than the market available convective cooling solution. Compared to the heat sink cooling dock (i.e. our system with the Peltier devices turned off), we have marginally better gains (0.6 C lower temperature and 0.01 W more computing power). This illustrates that while our Peltiers are effective in cooling the tablet and improving computing performance, they are extremely inefficient as they require lots of inputted power. More discussion about ways to improve our design and future works can be found in Section 8. 28

30 8 Future Works 8.1 Peltier Subambient Cooling Prototype Improvements If given more time, we would have improved a couple of key aspects of our design: Minimize the Z stack of the product: The fans that we used for our final prototype were off the shelf components purchased from Amazon. These were primarily chosen for their ability to produce the necessary CFM and convective coefficient. However, moving forward we would want to work with a fan supplier to get thinner fans to reduce the z height of our product. Minimize air re-circulation: The current prototype blows air perpendicular to the face of the heat sink and simply relies on the turbulent flow to exit through the outlet vents placed all around the product. Unfortunately, this method creates re-circulation and inefficiencies in the cooling of the heat sink. The future prototype will address this issue by creating a fully constrained inlet to outlet duct pathway for the air to move through the device and heat sinks. Implement dynamic Peltier control: Looking forward, this cooling method has the potential to be made portable and battery-powered. However, miniaturizing the device does impose a constraint that the current prototype is free of: power consumption. The future prototype will address this issue by implementing internal temperature sensors that would provide temperature readings of the Peltiers. At a specified threshold, Peltiers that have become too inefficient because of their high temperature could be turned off and allowed to cool before operating again. This would maintain a more efficient system. Sourcing electrically-compatible fans: As mentioned above, the current prototype uses off the shelf fans that are rated to operate at 12 volts. However, for circuit and flow re-circulation issues they are run at 6 volts in the current prototype. Future fans for the prototype will be sourced to run at 3 volts to be at the same voltage level as the Peltiers but will maintain the same flow velocity and convective coefficient. This will also allow the system to be powered by 3.82 V Lithium Polymer batteries for a more portable solution. Minimizing contact resistance between heat transfer plate and backside of the Surface Pro 4: As discussed throughout the report, our largest thermal resistance in the final prototype is the contact resistance between the heat transfer plate and the backside of the Surface Pro 4. We attempted to minimize this resistance by using a spring loaded mechanism to apply contact pressure and minimize any air gaps. We could further minimize this contact resistance in future works by using thin microfilms, conductive rubbers, and gap pads. 8.2 Convective Subambient Cooling Approaches As discussed in Section 7.1, the steady state heat transfer plate temperature when there was no tablet docked was 13.2 C, a value well below ambient temperatures. However, once a device was docked, the heat transfer plate temperature increased to a steady state temperature of approximately 28 C. While this conductive method of 29

31 cooling allows for a rapid cooling of the tablet s surface, the temperature gradient between the tablet and the heat transfer plate is eliminated at steady state. This minimizes the effectiveness and efficiency of a conductive subambient cooling approach since the surfaces will converge to an equilibrium point. In other words, a conductive approach acts more like a heat sink with a faster initial drop in temperature. Moving forward, a conductive approach might not be the most effective. A explorable solution is using convective cooling by blowing sub-ambient air over the tablet directly. A possible implementation of this method would consist of lining the interior of the air inlet and ducts with Peltier elements to cool the air before it is ejected directly to the tablet s surface. By using forced convection with subambiently cooled air, the constant flow of air across the tablet surface would maintain a temperature gradient at steady state. The heat transfer is also dictated by the surface area exposed to the air. The drawback of blowing air directly at the tablet is that the surface area is limited to the tablet geometry. Convective cooling in parallel with heat sinks, as we used in the final prototype, increased the available surface area by approximately 3.25 times. An alternative and improved method of using sub-ambient convective cooling would be to conduct heat to heat sinks and convectively cool them with sub-ambient air. The external device would have a heat transfer plate similar to the one utilized in the final prototype. Sub-ambient air would then be blown through heat sinks internal to our device. In terms of creating a sub-ambient fluid for convective cooling, we see two possible solutions: (1) vapor compression and (2) Peltier cooling. Vapor compression will be discussed further in Section 8.3 since this is a promising way forward. We also believe that air could be cooled using Peltiers. We noticed through our testing and work that Peltiers remained efficient when sucking heat from the environment rather than sinking heat (i.e. maintaining a steady state heat transfer plate temperature). This is the case for cooling air. We foresee a potential prototype using Peltiers to cool copper tubing. Just as we did with our plate optimization model in Figure 12, we could calculate the thickness of the copper tubing necessary to maintain a low sub-ambient temperature on the interior of the pipe. The pipe could be insulated on the outside to prevent temperature loses. Air would enter one end of the copper tubing and sucked out the other end by a fan blowing onto the backside of the tablet or internal heat sink. As the air moves through the sub-ambient copper tubing, it would be cooled. This is a concept for another convective cooling method. 8.3 Our Proposed Way Forward As mentioned in Sections 8.1 and 8.2, there are shortcomings with our final prototype s sub-ambient cooling method. Moving forward, our group would plan to pivot to a convective cooling approach. Although not within the scope of this class, we want to further explore vapor compression cycles as a solution. We would work directly with a compressor manufacturer to build a custom, small, low power compressor. The base line calculations and required compressor specifications are compiled in Appendix B. Even though most on the market compressors are designed for high cooling capacities (i.e. refrigerators, etc.), we believe it is feasible to create a compressor around our requirements. After working with a supplier, we would implement a sub-ambient convective cooling 30

32 system. Although our future work would involve this design pivot, we feel like we have identified issues and positives with sub-ambient cooling approaches. Any sub-ambient approach will require inputted work and will inevitably be less power efficient than an ambient cooling approach such as a laptop cooling pad. However, a sub-ambient system with a power requirement of less than 6 W seems reasonable to obtain. In addition, as processors become more powerful and computers and devices reduce in form factor, alternative methods of cooling will be required. Sub-ambient cooling represents a real solution to effectively decrease surface temperatures and maximize computing performance. We see that this type of external sub-ambient cooling solution should scale effectively with increasing processor power in computing devices. For instance, Intel Core i5 and i7 processors have much higher TDP than the Intel Core m3. However, with convective sub-ambient cooling, we should be able to maintain the same cooling efficiency across all devices. While no current sub-ambient cooling solution exists for portable consumer electronics, a solution for this scale of devices would represent an innovative step in thermal cooling. 31

33 Appendices A Product Specifications A.1 Peltier Element 5V, 1A Peltier Thermo-Electric Cooler Module: Max current: 5V; Suggested Voltage range: 5V to 7V; Maximum temperature differential Qc = 0): 65 C; 127 Peltier elements; DC Resistance: 2.5 ohms Silicone seal Dimensions: 30mm (1.18 ) x 30mm (1.18 ) x 3.29mm (0.12 ) Wire Length: mm / 11.4 Weight: 11.43g A.2 Cooling Fan ARCTIC F8 PWM Fluid Dynamic Bearing Case Fan: Model: AFACO-080P0-GBA01; Start-up Voltage: 5.5V; Nominal Voltage: 12V; Current: V and 100% PWM Dimensions: 80mm (3.15 ) x 80mm (3.15 ) x 25mm (0.98 ) ; Motherboard Connection: +12V DC PWM control signal, 4-pin socket RPM: Air Flow: RPM Sound: RPM Cable Length: 400mm / 15.7 Weight: 71 grams 32

34 B Vapor-Compression Refrigeration Cycle Calculations An example vapor compression refrigeration cycle using R-134a refrigerant with mass flow rate of 50 grams/second and compressor inlet pressure of 100 kpa is used in calculations below. These values correspond to a standard mass flow rate for refrigerators, and a low inlet pressure so as to minimize the operating pressure range. It follows that at stage 4, P 4 is approximately atmospheric pressure and refrigerant is saturated vapor. At stage 3, T 3 ě T ambient and refrigerant is a saturated liquid. Additional assumptions for ideal cycle include use of an isenthalpic expansion valve, i.e. h 3 h 4, isobaric condenser and evaporator, and isentropic compressor. This example system is used to calculate estimated cooling capacity ( 9 Q L ) and required power ( 9 W ), two system parameters that drive the design of a scaled-down refrigeration cycle. Enthalpy and entropy values used are from Table A-11 and Table A-13 of Cengel and Boles Thermodynamics: An Engineering Approach. 8 Notation follows that of the cycle diagram in Figure 4, i.e. temperature going into compressor equals T 1. 9m.05 kg{s, P kpa, T C h kj{kg (saturated liquid at P kpa) s 2 s kj{kgk h 4 h kj{kg (saturated vapor at T C) ñ 9 Q L 9mph 1 h 4 q p.05 kg{sqp kj{kg kj{kgq kw Using P 2 and s 2 for superheated R-134a at 700 kpa in Table A-13: h kj{kg ñ 9 W 9mph 2 h 1 q p.05 kg{sqp kj{kg kj{kgq 1.53 kw Q COP 9 L 7.28 kw 9W 1.53 kw 4.76 These calculated values for cooling capacity ( 9 Q L ) and required power ( 9 W ) illustrate just how overdesigned a vapor compression refrigeration cycle would be for the application of cooling small form factor tablet devices. With the desired minimal operating pressures, the system still produces 7.3 kw of cooling and draws 1.5 kw of power. These numbers can be tweaked to produce slightly more accurate values by decreasing the amount of refrigerant used in the system, however a mass flow rate of 50 grams/second was used here to follow standard values in normal refrigeration applications. Regardless, decreasing mass flow rate to a more accurate value for a miniaturized system would likely still produce cooling capacity and required power values on the order of kilowatts, which far exceeds the cooling needs of an Intel processor, which is on the order of 10 W ă 9 Q L ă 30 W. 8 Cengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. Boston: McGraw-Hill, Print. 33

35 B.1 Final Prototype Images Figure 28: Rubber standoffs on the bottom plate of our final prototype. Figure 29: Side vent design of our final prototype design. 34