Electrohydrodynamic (EHD) Cooled Laptop

Electrohydrodynamic (EHD) Cooled Laptop N. E. Jewell-Larsen, H. Ran, Y. Zhang, M. Schwiebert and K. A. Honer Tessera 3025 Orchard Parkway, San Jose, CA 95134

A. V. Mamishev University of Washington Seattle, WA 98195 Abstract Forced air cooling with fans remains the most popular cooling solution for electronic products because of its simplicity. However, increasing heat generation in microelectronics, and the demand for ever smaller portable devices, has resulted in heat fluxes that push the limits of conventional fan-based air cooling technology. Electrohydrodynamic (EHD) ionic wind pumps offer an attractive alternative to fans. In this technique, applying a voltage to a sharp electrode ionizes nearby air molecules, which are then propelled by the electric field and transfer momentum to neutral air molecules, thus creating airflow and cooling. The fundamental principles of this technology have been investigated theoretically and experimentally. However, few real world applications have been produced to date due to technical challenges such as device miniaturization, high voltage generation, EMI, reliability and others. This paper discusses the successful integration of an EHD cooling system into a laptop computer. The stock rotary fans in the laptop cooling system were removed and replaced with EHD blowers. Airflow and thermal performance measurements, both at the device and at the system level are presented. Keywords Electrohydrodynamic cooling

(EHD),

ionic

cooling,

laptop

Nomenclature CCFL Cold Cathode Florescent Lamp CPU Central Processing Unit GPU Graphics Processing Unit IC Integrated Circuit EHD Electrohydrodynamic EMI Electromagnetic Interference EFA Electrostatic Fluid Accelerator COP Coefficient of Performance TDP Total Dissipated Power 1.

Introduction

All electronic devices consume power and generate heat. Ultimately, all waste heat is dumped to the air, making heatsink-to-air heat transfer critical to system performance. Frequently, the heat dissipation requirements of modern electronic systems demand active thermal solutions such as forced convection. Traditionally, airflow has been provided by various forms of mechanical fans with rotating blades. However, mechanical fans have many fundamental limitations

such as size, form factor, noise, etc., which result from the need for high speed rotating parts. The electronics industry, as a result of its continued effort to miniaturize applications and enhance performance, needs a new air cooling technology. Electrohydrodynamic (EHD) ionic wind pumps may become a critical element of future electronic thermal management solutions. EHD based solid-state devices offer silent operation, dynamic airflow profiles, and controllable air velocities [1-4]. The EHD based air movement can be used to accelerate bulk air flow [5-8] or disturb the thermal boundary layer at the solid-fluid interface for heat transfer enhancement [9-11]. Significant developments over the last half century have been made in the EHD field as a whole [12-15]. However, investigation into meso-scale EFA cooling has been limited, with most previous work focused on modeling efforts [16,17] or basic proof-of-concept heat transfer enhancement experimental studies [18-21]. The present work focuses on the practical implementation of EHD technology in a compact consumer electronic system. This paper will first discuss the fundamentals of EHD air pumps. Following will be a discussion of numerical modeling to better understand the effect of various design variables and their affect on device coefficient of performance (COP). Furthermore, a laptop retrofitted with an EHD system is discussed, and the thermal performance and unit functionality are presented. The reliability of this technology in the consumer electronics domain will be addressed. Finally, future work will be discussed A laptop with a TDP of ~60 W was chosen as a test vehicle to demonstrate the EHD proof-of-concept thermal management system. To integrate the EHD system, the two 65 mm rotary fans in the laptop were removed and two EHD blower systems and their associated control electronics were designed and fabricated to fit in their place. For simplicity, the EHD system in this study was constrained to the footprint of the mechanical blowers. As such, the performance for the EHD thermal solution does not represent the full capabilities of the technology. Rather this study focused primarily on demonstrated that EHD technology could be successfully integrated into a laptop and not a demonstration of ultimate performance. 2.

Background The mechanism of DC positive corona-induced ionic wind with pin-rod geometry is illustrated in Figure 1. Application of voltage between a high tip-curvature corona electrode and a low tip-curvature collecting electrode creates a high electric 25th IEEE SEMI-THERM Symposium

field gradient at the corona electrode, ionizing its surrounding air molecules. The ionized air molecules are then propelled by the electric field, transferring momentum to neutral air molecules via collisions, thus resulting in bulk air movement toward the collecting electrode. The operating voltage range of an EHD device lies between the corona discharge onset and the complete air gap breakdown voltages [22].

for steady state incompressible air flow ρair Ui∇U = −∇p + μ∇2 U − ρ∇V

(5)

∇iU = 0 (6) where ρair is the air density, p is the air pressure, and μ is the air dynamic viscosity.

Heat transfer can then be described by thermal conduction and convention q = ∇i(−k ∇T ) + ρair C p Ui∇T

(7)

where q is the heat generation from the corona, k is the thermal conductivity of the medium, T is the temperature, and Cp is the specific heat capacity of air.

Figure 1: Schematic of ion stream and airflow generated by a DC electrohydrodynamic ionic wind pump, where a high voltage is applied between the corona and the collector electrodes. 3.

Device Modeling

Model driven design was conducted using a coupled physics EHD model and solved with finite element modeling software. The EHD model was adapted from previous published modeling work of the authors [17]. The objective of the modeling effort was to maximum heat transfer within the required form factor and system constraints by varying device geometry. EHD flow induced by corona discharge and the resulting heat transfer is described by the following equations. The electric potential V is governed by Poisson’s equation ∇ 2V = −

ρ ε0

The system of equations (1), (4), (5), (6), and (7) was subjected to appropriate boundary conditions for the forced convection heat transfer design optimization investigated in this study. Space charge generation from corona discharge is modeled using Peek’s equation and Kaptsov’s assumption as described in [17]. Example simulation results for EHD driven forced convection are shown in Figure 2. Figure 2 (b) shows the space charge flux stream lines, which is the trajectory of ion movement. Throught the trajectory, ions exchange momentum with neutral air molecules via collisions, and cause air flow in the same direction. If heat is applied to the metal collector surface, it will be removed by the air flow. The cooling effect of the EHD generated air flow is demonstrated in Figure 2 (d). In this simplified simulation, the temperature of the collector surface was assumed to be uniform at 70 oC. The ambient temperature was 20 oC. The above modeling approach was used to design the EHD blowers for the laptop shown in Figure 4.

(1)

where ρ is the space charge density and ε0 is the dielectric permittivity of free space. The electric potential is defined from electric field intensity E as E = −∇V

(2)

Electric current in the drifting zone is a combination of three effects: conduction (motion of ions under electric field relative to entire airflow), convection (transport of charges with airflow), and diffusion. Therefore, current density J is given by J = μE Eρ + Uρ − D∇ρ (3) where μE is the air ions mobility in an electric field, U is velocity vector of airflow, and D is the diffusivity coefficient of ions. The requirement of current continuity gives the equation for current density ∇i J = 0 (4) The hydrodynamic part of the problem is described by the Navier-Stokes equations and momentum continuity equation Jewell-Larsen, Integrated EHD laptop thermal solution

25th IEEE SEMI-THERM Symposium

D L

h

d

a)

q

normalized pressure

q

3 1 stage 2 stage

2

3 stage

1 0 0

1 2 normalized flow rate

3

b) Figure 3: Measured flow rate vs. pressure head, normalized to a single stage device. c)

EHD Blower d) Figure 2: Modeling of convective heat transfer of an EHD device. (a) Schematic of the device, with the emitter electrode at the far left with a wire with diameter d; the collectors are two parallel plates with a rounded leading edge facing the emitter, the gap between the two plates is h, the distance between the emitter wire and the leading edge of the collector is D, and the length of the collector along the flow direction is L. The collectors are grounded, and a positive voltage V0, is applied to the emitter wire. (b) The colored surface plot shows the distribution of electric potential, with color level from 0 (Blue) to 3.0 kV (Red), and space charge flux stream lines are shown. (c) Air velocity vector generated by the EHD flow is shown as arrows, with the average velocity at the exit plane being 3.56 m/s. (d) Temperature distribution within the air as a result of heat convection from the heated collector to the air, with colored surface map level from 20oC (Blue) to 70oC (Red). 4.

Integrated EHD laptop computer design

The EHD blower and power supply were designed to fit within the quasi circular ~26 cm2 by 1 cm tall cavity intended for the mechanical fan. A multistage EHD blower was used to maximize airflow within the existing system. As with axial fans, EHD blowers can operate in multiple stages to increase pressure and flow rate. In an ideal case without resistance or electric field interference, an N stage device has N times the static pressure of a single device, and N1/2 times the flow rate. Figure 3 shows the measured P-Q curve (pressure head vs. flow rate) of a single stage, two stage, and three stage EHD device, at the same input voltage. The pressure head and flow rate are normalized by the static pressure and free flow rate of a single device. A three stage device was chosen for this study to enable greater airflow at a minimum operating voltage, and is shown within the laptop in Figure 4. Jewell-Larsen, Integrated EHD laptop thermal solution

HVPS

Figure 4: Picture of EHD thermal system integrated into an operational laptop, showing both EHD blower and miniature high voltage power supply (HVPS) built into the mechanical fan cavity. One of the integration challenges of this technology lies in the design of a compact voltage converter capable of converting the 12V DC voltage of the laptop battery to approximately three thousand volts required to operate the EHD blower. A CCFL power supply topology with a voltage multiplier at the output was used and is shown in schematic form in Figure 5. The power supply had an approximate footprint and power output of 3 cm2 and 1.5 watts respectively, and is shown within the laptop with the keyboard removed in Figure 4. Newer versions are significantly more compact


Temperature ( C)

Voltage Multiplier

Royer Oscillator

CCFL Driver Chip

100 90 80 70 60 50 40 30 20 10 0

Stock Fans

EHD

CPU

Figure 5: Schematics of first generation miniature HV power supply

Figure 6: Steady state temperature while looping a 1080p movie trailer 50 45

EHD-cooled laptop operation and its performance comparison with the stock fans

The retrofitted laptop was tested with a closed case and with no modifications other than simply replacing the rotary fans with the EHD thermal system. In all tests the retrofitted laptop was compared against a stock version of the same model, which had no modifications and was cooled using the two stock mechanical fans. The major heat sources of the laptop are an Intel Core2Duo CPU, GPU, and the chipset. In operation, the retrofitted laptop performed similarly to the stock unit with no apparent impact on laptop functionality. There was no discernable impact on electrically sensitive systems such as wireless communication and track pad human interfaces etc. Thermal performance of the prototype system was measured by running several benchmark programs including GeekbenchTM and a looped 1080P movie trailer. Hardware MonitorTM, an off the shelf system utility application, was used to monitor the real time temperature of major components, such as the CPU, GPU and their respective heatsinks. Figure 6 shows the steady state CPU and GPU temperature while playing a HD movie in full screen mode. The CPU and GPU temperature of the retrofitted EHD cooled laptop was found to be approximately 10oC higher than the stock fan cooled laptop, with an overall temperature rise of Benchmark results demonstrated approximately 60oC. comparable overall performance scores, with a variation of less than one percent between stock and EHD retrofitted laptops while running at an 1800 MHz clock speed. Figure 7 shows the comparison of skin temperature for both the keyboard and bottom surface of the laptop, showing a temperature difference less than 5oC.

Jewell-Larsen, Integrated EHD laptop thermal solution

Skin Temperature (C)

5.

GPU

Stock Fans @ 4000 RPM

EHD

40 35 30 25 20 15 10 5 0 Top Case

Bottom Case

Figure 7: Skin Temperature Comparison 6.

Reliability considerations Corona based EHD technology has been used in several mature industries such as xerography, electrostatic air cleaners, and other industrial applications. EHD technology does not have moving parts or the associated failure modes of rotary fans, such as bearing and rotor wear, thermal and mechanical fatigue, or stiction, among others. Some failure modes, such as dust accumulation, are shared between fans and EHD blowers, which can act to both reduce the performance of the blower and increase heatsink thermal and flow resistance. The authors employ an enclosed dust chamber, shown in Figure 8, to simulate a variety of dust conditions. The system is being used to evaluate the dust sensitivity of EHD devices and comparable rotary fan-based cooling systems. EHD has some unique failure modes associated with the emitter electrode. Electrode degradation can be caused by the corona discharge surrounding the corona electrode, leading to surface degradation from effects such as metal sputtering at high electric fields and surface oxidation. Proper selection of emitter material, which can overcomes these challenges, is necessary to meet the required longevity.


removed up to 38% more heat than a fan, as illustrated in Figure 10. Another advantage of using the collector as the heat transfer surface is that there is no additional pressure drop, weight or volume associated with a separate heatsink, as is the case with a fan.

7.

Second generation EHD system

The first generation cooling solution merely replaced the stock rotary fans with EHD blowers and a HVPS. This approach achieved promising results and proved that the technology could function within a realistic laptop environment. However, because the EHD blowers were force fit into a cooling system optimized for a mechanical fan and not EHD, they did demonstrate the technology’s full potential. The design and testing of a second generation prototype has been undertaken. For this prototype, the constraints of fitting within an existing fan footprint and using an off-theshelf spreader, heatpipe, and heatsink, have been removed. The second generation solution dramatically increases heat transfer but still manages to reduce overall size. Two of the ways in which it does this are by using the collector as the heat removal surface and by increasing its total area. Air blowing across a surface forms a boundary layer that limits heat transfer. Figure 9 shows the idealized velocity profiles of air in a channel. The black curve shows a classic parabolic profile due to a pressure differential flow from a fan, the blue curve plots a non-parabolic profile due to EHD driven flow, with higher velocity near the boundary. The altered profile reduces boundary layer thickness and enhances heat transfer. In this case the channel walls are acting as collector electrodes.

EHD

0.8 0.6

Fan Flow 0.4 0.2 0 0

0.2

0.4

0.6

0.8

1

Normalized Flow

Figure 10 Cooling power of a heatsink when the air flow was generated by an EHD device as compared with a fan. In an EHD blower, static pressure P and air velocity V are functions of power density per unit length of emitter wire W. To first order, pressure (P) is proportional to W2/3 and velocity (V) is proportional to W1/3. Figure 11 shows the change of static pressure and total flow rate, the product of air velocity and total device area, as a function of total wire length. The total power to the EHD device was 1W in each case. The figure shows that it is possible to adjust the P-Q curve of the EHD blower to emphasize either static pressure or maximum flow rate by adjusting the total wire length. However, favoring flow rate over pressure is advantageous. When the collector is used at the heat removal surface, increased wire length corresponds to both an increased flow rate and a larger surface area and more heat removal capability.

EHD Fan

Figure 9: velocity profile between two parallel plates.

Normalized Pressure

1

1

0.8

0.8

Flow Rate 0.6

0.6

0.4

0.4

Pressure

0.2 0

0

0

Using a reference fin geometry, we measured the heat removed using EHD and fan-driven flow. For a given flow rate and the same temperature drop, EHD generated air flow Jewell-Larsen, Integrated EHD laptop thermal solution

0.2

Normalized Flow

Figure 8 Enclosed dust testing chamber

Normalized Cooling Power

1

0.2

0.4

0.6

0.8

1

Normalized Emitter Length


In addition to performance gains, EHD blowers offer other valuable advantages such as: • Effectively silent operation • Flexible form factor able to fit around electronics • Reduced thermal solution height and volume requirements These advantages among others make EHD cooling an intriguing technology for forced air cooling in space constrained applications.

Normalized Pressure

1 0.8 0.6 0.4 0.2 0 0

0.2

0.4

0.6

0.8

1

Normalized Flow Rate

Figure 11 (top) static pressure and flow rate as a function of total wire length, with fixed power consumption; (bottom), change of fan curves as wire length increase.

COP

The COP of the second generation EHD cooling system is improved by a factor of five over the first generation. The performance of the improved EHD system exceeded that of the stock cooling system in free air. In addition, the total foot print of the cooling system is more than 50% smaller and the thickness is less than 6 mm. These characteristics give EHD cooling significant advantages in thin laptop applications. 90 80 70 60 50 40 30 20 10 0

[4] N. E. Jewell-Larsen, E. Tran, I. A. Krichtafovitch, and A. V. Mamishev, "Design and Optimization of Electrostatic Air Pumps," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 13, no. 1, pp. 191-2003, Feb. 2006.

Stock Fan

20

25

30

35

Cooling power (W)

Figure 12 Comparison of COP between improved EHD cooling system and the existing stock fan solution. 8.

Conclusions An EHD cooling system with compact blower and power supply was successfully integrated into the existing cooling system of a performance laptop, replacing the mechanical fans. The retrofitted laptop was found to operate with no apparent effect on system functionality, including subsystems such as wireless communication and touchpad human interfaces. The performance of the laptop was compared with an unmodified stock laptop of the same model. Even with an un-optimized design, the EHD system shows promising cooling performance with reduced thermal solution volume and acoustics. By further design optimization and modification of the existing cooling solution, it is expected that the performance can be further improved to exceed that of laptop rotary fans. Jewell-Larsen, Integrated EHD laptop thermal solution

[2] L. Leger, E. Moreau, and G. G. Touchard, "Effect of a DC Corona Electrical Discharge on the Airflow Along a Flat Plate," IEEE Transactions on Industry Applications, vol. 38, no. 6, pp. 1478-1485, Nov. 2002. [3] Ran, H., Jewell-Larsen, N. E., Zhang, Y., and Honer, K. A., "Emerging Technologies in Forced Convection Air Cooling", Thermal News, 11-1-2008,

Improved EHD

15

References [1] G. M. Colver and S. El-Khabiry, "Modeling of DC Corona Discharge Along an Electrically Conductive Flat Plate With Gas Flow," IEEE Transactions on Industry Applications, vol. 35, no. 2, pp. 387-394, Mar. 1999.

[5] F. Yang, "Corona-driven air propulsion for cooling of microelectronics," Master Thesis, Department of Electrical Engineering, University of Washington, Seattle, WA, 2002. [6] B. L. Owsenek, J. Seyedyagoobi, and R. H. Page, "Experimental Investigation of Corona Wind HeatTransfer Enhancement With a Heated Horizontal Flat-Plate," Journal of Heat Transfer-Transactions of the Asme, vol. 117, no. 2, pp. 309-315, May 1995. [7] B. L. Owsenek and J. Seyedyagoobi, "Theoretical and Experimental Study of Electrohydrodynamic Heat Transfer Enhancement Through Wire-Plate Corona Discharge," Journal of Heat TransferTransactions of the Asme, vol. 119, no. 3, pp. 604610, Aug. 1997. [8] M. E. Franke and L. E. Hogue, "Electrostatic Cooling of a Horizontal Cylinder," Journal of Heat Transfer-Transactions of the Asme, vol. 113, no. 3, pp. 544-548, Aug. 1991. 25th IEEE SEMI-THERM Symposium

[9] D. J. Schlitz, S. V. Garimella, and T. S. Fisher, "Microscale Ion-Driven Air Flow Over a Flat Plate," Proceedings of ASME Heat Transfer/Fluids Engineering Summer Conference, 2004, pp. 463468. [10] N. E. Jewell-Larsen, "Optimization and miniaturization of electrostatic air pumps for thermal management," Master Thesis, University of Washington, 2004. [11] N. E. Jewell-Larsen, C. P. Hsu, I. A. Krichtafovitch, S. W. Montgomery, J. T. Dibene, and A. V. Mamishev, "CFD Analysis of Electrostatic Fluid Accelerators for Forced Convection Cooling," Dielectrics and Electrical Insulation, IEEE Transactions on, vol. 15, no. 6, pp. 1745-1753, 2008. [12] M. Huang and F. C. Lai, "Effects of Joule Heating on Electrohydrodynamic-Enhanced Natural Convection in an Enclosure," Journal of Thermophysics and Heat Transfer, vol. 20, no. 4, pp. 939-945, 2006.

Accelerators," Journal of Microelectromechanical Systems, vol. 16, no. 4, pp. 809-815, Aug. 2007. [19] D. Go, S. Garimella, T. Fisher, and R. K. Mongia, "Ionic Winds for Locally Enhanced Cooling," Journal of Applied Physics, vol. 102, no. 053302, 2007. [20] C. P. Hsu, N. E. Jewell-Larsen, A. C. Rollins, I. A. Krichtafovitch, S. W. Montgomery, J. T. Dibene II, and A. V. Mamishev, "Miniaturization of Electrostatic Fluid Accelerators," ASME International Mechanical Engineering Congress and Exposition, 2006. [21] C. P. Hsu, C. Sticht, N. E. Jewell-Larsen, M. Fox, I. A. Krichtafovitch, and A. V. Mamishev, "Characterization of Microfabricated Cantilever-toPlane Electrostatic Fluid Accelerators for Cooling in Electronics," ASME International Mechanical Engineering Congress and Exposition, 2007. [22] F. W. Peek, Dielectric Phenomena in High Voltage Engineering, New York:McGraw-Hill, 1929.

[13] S. Moghaddam, K. T. Kiger, and M. Ohadi, "Measurement of Corona Wind Velocity and Calculation of Energy Conversion Efficiency for Air-Side Heat Transfer Enhancement in Compact Heat Exchangers," HVAC and R Research, vol. 12, no. 1, pp. 57-68, 2006. [14] M. Robinson, "Movement of Air in the Electronic Wind of Corona Discharge," AIEE Transactions, vol. 80, pp. 143-150, May 1961. [15] Y. Yue, J. Hou, Z. Ai, L. Yang, and Q. Zhang, "Experimental Studies of the Enhanced Heat Transfer From a Heating Vertical Flat Plate by Ionic Wind," Plasma Science and Technology, vol. 8, 2006, pp. 697-700. [16] D. Schilitz, S. Garimella, and T. Fisher, "Numerical Simulation of Microscale Ion Driven Air Flow," ASME IMECE, Article 41316, Washington DC, 2003. [17] N. E. Jewell-Larsen, P. Q. Zhang, C. P. Hsu, I. A. Krichtafovitch, and A. V. Mamishev, "CoupledPhysics Modeling of Electrostatic Fluid Accelerators for Forced Convection Cooling," 9th AIAA/ASME Joint Thermophysics and Heat Transfer Conference, 2006. [18] C. P. Hsu, N. E. Jewell-Larsen, I. A. Krichtafovitch, S. W. Montgomery, J. T. Dibene, and A. V. Mamishev, "Miniaturization of Electrostatic Fluid Jewell-Larsen, Integrated EHD laptop thermal solution
