A Micro Wind Turbine for Energy Harvesting Zhixin Alice Ye, EE247A Final Project, 2015

Department of Electrical Engineering and Computer Science University of California, Berkeley

Berkeley, CA alice.ye@berkeley.edu

Abstract—A micro wind turbine design is proposed for fabrication on a polyMUMPS process. The wind turbine consists of 6 blades angled using a scissor hinge. A pin joint keeps the rotating turbine in place while the inner ring of the turbine acts as an electrostatic generator to couple out energy. A preliminary layout and design are included assuming simplified physics. Using a simple model for a wind turbine system, a power conversion rate in the microwatt range is predicted at a wind speed of 10 m/s and a turbine rotation rate of 160 rotations per second.

Keywords—polyMUMPS; wind turbine; energy harvesting; microelectromechanical systems (MEMS).

I. INTRODUCTION Micro wind turbines have become an increasingly attractive

option for powering small devices, for example when light is not available for solar power, or for recharging small handheld smartphone devices [1]. Though commercial large-scale wind turbines are a well-established field offering a wide variety of designs, the study of turbines at a micro scale creates new challenges in both fabrication and simulation of turbine performance. Design of a micro wind turbine for use in energy harvesting involves achieving a design for the wind turbine blades that couples well with a rotor design to transform wind energy to electrical energy.

During typical large-scale wind turbine design, there are two major classes of wind turbines: horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT). Typically HAWTs are more popular because of their higher efficiency which is closer to the Betz Limit of 59.3%. However, VAWTs pose a number of advantages, including greater stability and robustness, as well as potential for increased power output density compared to HAWTs [2].

Recently, S. Rao et al. have developed a small scale (1.8 mm in diameter) working prototype of a windmill using a stacked nickel-alloy MEMS process provided by WinMEMS [3]. This work demonstrates the feasiblity of developing small scale windmills; however, for proper energy conversion it is still necessary to demonstrate a method of coupling the wind turbine motion to a rotor to generate power.

A number of different on-chip energy harvesters have been proposed including magnetic and vibrational harvesters [4]. Reference [5] examines different methods of electrostatic energy scavengers that operate along a single axis (X, Y, or Z) and weighs the pros and cons of options such as an in-plane overlap compared to the in-plane gap closing type.

In this work, a micro wind turbine design is proposed that couples the rotation of the turbine with an rotating electrostatic capacitive generator. Power and peak rotational rates are predicted and plotted. In addition, test structures are constructed to verify assumptions and compare experimentally obtained quantities.

II. BACKGROUND

A. Flow of Air Over a Perpendicularly Oriented Flat Plate For simplicity, the analysis of the force of a single flat

plate VAWT is first conducted, with the flow of wind traveling perpendicular to the plate, as shown in Fig 1.

Fig. 1. Schematic diagram (side view) of wind traveling perpendicular to a flat plate and exerting force due to a difference in air pressure on two sides of the plate.

A VAWT operates on the principle of drag force, where

the exerted force varies with the drag coefficient ("#) of the wind-facing object. For air flowing perpendicular to a flat plate, a typical CD value is between 1.28-1.97 [6]. This value is not dependent on the Reynolds number, and can be assumed to be roughly constant for each structure geometry. Using a "# of 1.5, the drag force can be calculated as:

%# ='

()*+,-.

( /01234"# ≈ 6.982<= (1)

Here, %#is the total drag force on the plate, ) is the density of fluid flowing over the plate (in this case, air), *+,-.is the wind velocity, and /01234is the area of the plate. Due to the low operating Reynolds numbers, it is still expected that there will be a boundary layer formed along the top surface of the plate. However, since the plate thickness will only be 2 µm, this can be considered negligible, as illustrated in Fig 2.

Windvelocity=10m/s

Flatplate

Area=7.2×10-8m2

Width=400µm Height=180µm

Fig. 2. Experimental imaging of fluid flow dynamics over a parallel plate for Re = 0.014. The primary drag force is produced by turbulent flow of air through the flat plate [7].

The applied force exerts a torque on the rotor, causing

acceleration until frictional force matches the rotational force, or the rotor moves so quickly to damage the device, thus limiting ω. Torque on the rotor can be calculated to be, with the mean radius of the fin being 400 µm:

> = %#×@ ≈ 2.79×10DE=F (2)

B. Electrostatic Rotor From [5], it is shown that it is possible to develop an

electrostatic generator based on variation of capacitance, as shown in Fig 3.

Fig. 3. Plot of the electrical circuit used in the electrostatic rotor. Cv is the variable capacitor in the wind rotor [5].

The wind turbine can be used to operate a variable

capacitor, thus creating a changing Cv. Then, the expected energy conversion per cycle will be:

G04HIJI14 ='

(K,-( ("L2M − "L,-)(

OPQR

OPST) (3)

To estimate the power generated:

UV4- = G04HIJI14×#XYIJI14Z

HX323,X-×#HX323,X-Z

Z4IX-. (4)

Letting N = number of capacitive fingers on one circle (in this case, 180), we can then solve for the number of rotations per second, assuming a tip-to-speed ratio (λ) of 0.1:

U = G04HY,-×=Y,-Z[ = G3X321\]^ST_

`≈ 8.3<b (5)

III. DEVICE DESIGN

A. Turbine Design The wind turbine was designed in a polyMUMPS process

that allowed for multiple polysilicon layers in design. Using scissor hinges similar to [8], the design for the turbine blades aims to pop out of the surface of the micromachined plane.

The final design measures 2.1 mm in diameter. It consists of 6 blades as shown in Fig. 4 that are raised out of the plane of the chip surface. The outer ring has a tooth pattern in order to maximize the overall area of the device that overlaps with the electrostatic generator. Since the drag coefficient of the perpendicular flat plate is equivalent in both the forward and reverse rotational directions, a physical barrier would need to be constructed over half the wind turbine to block air flow in one direction.

Fig. 4. Preliminary design of the electrostatic turbine and anchor (top view). A central pin is used to hold down a rotor with 6 flat blades that will held perpendicular to the rotor using a scissor hinge. Around the outer edge are 180 pairs of capacitive fingers.

The rotor design consists of a dual-plane overlap of fins to

maximize the capacitance difference of the design. The fins in POLY2 layer are overlapped with those of POLY1 to create a difference in capacitance, as shown in Fig 5. The capacitive fins could also have been created between the POLY1 and POLY0 layers, which may occur in a future design iteration.

Laminar flow

Turbulent flow

Boundary layer formation

Electrostatic Fingers

Vertical Wind Blade

Pin

Scissor Hinge

Fig. 5. Side view of the difference between the minimum overlapping capacitance between fingers (left) and maximum overlapping capacitance (right).

Excluding parasitic capacitance, "L2M can be calculated as

the total overlapping area of the capacitive fingers.

"L2M =cd

.= 9.45×10D'(% (6)

"L,-can also be calculated by approximating the fringe capacitance on each edge of overlap of a capacitive finger (where the width of the fringe is approximately equal to half the thickness of the gap).

"L,- = =cd

.= =

c(gh.i

gh.)(`hD`g)

.= 4.779×10D'j% (7)

B. Test Structures Several test structures were created to experimentally

determine the drag coefficient, estimated parasitic finger capacitance, residual stress gradient of the fingers, and optimal hinge structure. These are outlined in the following figures.

Fig. 6. Drag coefficient of wind blade. Several different shapes are proposed for the blade with equivalent areas of 0.072 mm2. These can be built and tested in a small wind tunnel or a large scale model using viscous liquid.

Fig. 7. Capacitance of fingers. Various finger configurations are done to estimate parasitic capacitance. An RC circuit and oscilloscope can be used to measure time delay and C. Capacitive test structures comparing max and min overlapping capacitance and finger length effect.

Fig. 8. Residual stress gradient of fingers. Cantilever beams are constructed and the resulting deflection of the fingers can be measured using an optical microscope.

Fig. 9. Optimal hinge structure. Hinge beam structure and spacing are varied to ensure beam functionality.

IV. RESULTS AND DISCUSSION An estimation of expected performance of the device is

included below and has yet to be verified.

A. Turbine Performance and Optimal Efficiency With an assumed input voltage of 1V, we can calculate

GHX3, the energy produced per rotation, where [ is the rotational speed of the turbine:

G3X321 = G04HY,-× =kFlm@noopqr ×[ = 8.32qs (8)

Here, note that energy is in units per full rotation of the turbine. Thus, the total energy is directly dependent on the rotational speed of the device.

To estimate the power produced by the rotor:

U = G3X321×[ = G3X321\]^ST_

` (9)

t is the tip-to-speed ratio (TSR), which equals the rotational speed of the turbine divided by the speed of wind. R is the radius of the turbine.

The tip to speed ratio is dependent on the wind speed, the inertial mass of the rotor, the maximum shear stress on the plates, and the forces due to friction. For a drag-style turbine, a conservative λ is around 0.1. Therefore, an estimate for the total producible power is around 8.3 µW. In comparison, the total theoretical power available in the wind flowing over that area (Pwind):

U+,-. = 12 )/*+,-.

j = 44.1<b (10)

"L,- "L2M

Thus, the total efficiency can then be calculated as:

u = vwxT

v^ST_= 18% (11)

Note that no losses due to friction are included in this calculation. In Fig 10, the relationship between the tip-to-speed ratio, wind speed, and total theoretical output power is plotted. The total convertible power in the rotor exceeds the theoretical wind power below the around 5 m/s. Overall efficiency ultimately depends on the wind conditions of the surroundings and the maximum rotational speed of the rotor.

Fig. 10. Plot and comparison of the theoretical available wind power vs. the output converted power of wind for various tip-to-speed ratios. If the TSR exceeds the theoretical wind power, then the rotational speed cannot be achieved.

B. Discussion of Frictional Losses Several forms of friction may occur in the plate during

rotation that would cause further loss in power efficiency. Dry friction (kinetic friction of the plate) may occur in the rotor surface and also between the dimple contacts of POLY0-POLY1. Fluid frictional losses might also occur due to the boundary layer formed by the plate-air interface. The force of friction will vary greatly depending on the surface shape, roughness, and process variation.

C. Discussion of Stiction Stiction is a potential drawback for this design. While a

number of potential sources of stiction may occur, the main concern is between the POLY1 and POLY2 layer (the stiction between POLY0 and POLY1 will be reduced using dimples). One method for reducing stiction between the POLY1-POLY2 fingers during fabrication would be to use CO2 critical drying after structure release.

It is also possible that stiction would occur during operation of the chip. To estimate whether or not the beams may contact

each other, the deflection of the beams during the operation of 1V is approximated as a fixed cantilever with a uniformly applied load. The total force due to attraction between two charged parallel plates on a single finger is calculated to be:

%01234 =dcz{|h

(.h= 4.92q= (12)

Maximum beam deflection is calculated as:

Δ~L2M,0X1J( =ÄÅ(Å

Ç)

ÉÑÖ≈ 0.039<F (13)

This is less than 2% of the space between the gap size, and indicates that pull-in of the POLY2 layer should not occur due to the electrostatic force.

The other possibility is that the spinning POLY1 rotor could be attracted to the POLY2 layer by electrostatic forces. Assuming careful construction, the mass of the rotor is sufficient to overcome this.

V. CONCLUSION The proposal of a design for an energy harvester that relies

on wind power can open up a variety of opportunities in energy scavenging and beyond, especially in small device cases. Similar technology might be used to sense wind speeds for weather purposes. This design proposed represents a first attempt at creating a micro wind turbine in a polyMUMPS process. Preliminary analysis shows that power could be generated from the device on the microwatt scale. Further analysis should be conducted to optimize the design for specific wind-loading conditions and to ensure greater reliability.

REFERENCES [1] D. Kim and M. Gharib, “Horizontal-Type Wind Turbine with an

Upstream Deflector,” 2014.

[2] J. O. Dabiri, “Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays,” J. Renew. Sustain. Energy, vol. 3, no. 4, pp. 1–12, 2011.

[3] S. All, N. Topics, and S. Rao, “Technology uses micro-windmills to recharge cell phones,” pp. 15–17, 2014.

[4] S. Chalasani and J. M. Conrad, “A survey of energy harvesting sources for embedded systems,” Conf. Proc. - IEEE SOUTHEASTCON, pp. 442–447, 2008.

[5] S. Roundy, P. K. Wright, and C. Hall, “Micro-Electrostatic Vibration-to-Electricity Converters,” pp. 1–10, 2002.

[6] N. Hall, “Shape Effects on Drag,” vol. 90840. National Aeronautics and Space Administration, pp. 2–5, 2007.

[7] J. Nedić, B. Ganapathisubramani, and J. C. Vassilicos, “Drag and near wake characteristics of flat plates normal to the flow with fractal edge geometries,” Fluid Dyn. Res., vol. 45, no. 6, p. 061406, 2013.

[8] M. Ross and K. Pister, “Micro-windmill for optical scanning and flow measurement,” Sensors Actuators A Phys., pp. 576–579, 1995.

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