John VanDeLindeMSD 1P11401: Portable High Power-Density Energy System

Mechanical Design of a Portable Wind Energy System

OVERVIEW

The objective of this project is to design a portable renewable energy system that will charge lithium ion batteries to be used in wireless transceiver applications. In order for this project to be successful, the mechanical components of this system must meet the following customer needs:

Portable Tactical Supply Power Efficient

Robust Reliable Safe Obtain Energy from the Environment

The specifications outlined by our team during the development phase of this project are listed below. These engineering specs are related to the mechanical components of the system, and must be met by the design criteria outlined in this paper.

Net weight less than 20 pounds System volume less than 5 cubic feet Energy generation efficiency of 40% Impact resistance of up to 5 foot drop Maximum surface operating temperature of 60°C Achieve power of 60 Watts

CONCEPT SUMMARY

The concept selected to meet these customer needs and engineering specifications is a wind turbine generator. This turbine will generate power (i.e. a current and voltage) by magnetic inductance achieved by use of a permanent magnet (PM) generator attached to a hub/blade assembly. Generated power will be directed into a charging circuit, which will contain a lithium ion battery and the necessary circuitry required to charge the battery. This includes but is not limited to a rectifier, flyback controller, and IC charger/micro-controller. The purpose of this circuitry is to regulate the dynamic power output from the generator harvested from the wind energy to achieve a constant voltage across the battery terminals.

WIND ASSESSMENT

In order to begin the turbine design, it is necessary to first examine the typical wind speed ranges that can be expected in the Rochester, NY area. A wind energy potential report from the New York State Energy Research and Development Authority (NYSERDA) was obtained from their online resource database. This wind report provides the average annual wind speed at various heights. The table below displays these values.

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Table 1: Data obtained from NYSERDA energy report

Using this information, the probability of wind speeds at various heights can be generated. Assuming a Rayleigh case of the Weibull probability density function, this probability is defined as:

V = exp(-(V1/C)2) – exp(-(V2/C)2)

where C=2Vavg/√ π , V1 & V2 are wind speeds, and Vavg is the average wind speed at any given height obtained from relating the ratio of heights with the ratio of velocities and a friction coefficient.

The probability of each wind speed can be multiplied by the total number of hours per year (8760) to yield the total hours that each wind speed value is expected to occur each year. Table 2 below shows a sample calculation table, and Figure 1 shows the expected wind speed distribution in Rochester at a height of 2 meters. This height was chosen to determine values at a low-end wind speed results.

V Probability at V Hours at Vm/s % hrs/yr

0.0 -0.5 0.156 1370.81.0 0.276 2421.91.5 0.337 2954.42.0 0.337 2949.32.5 0.290 2541.03.0 0.221 1934.73.5 0.150 1318.44.0 0.092 810.14.5 0.051 451.05.0 0.026 228.25.5 0.012 105.36.0 0.005 44.36.5 0.002 17.07.0 0.001 6.07.5 0.000 1.98.0 0.000 0.68.5 0.000 0.29.0 0.000 0.09.5 0.000 0.010.0 0.000 0.010.5 0.000 0.011.0 0.000 0.011.5 0.000 0.012.0 0.000 0.0

Turbine Height (m) 2Vavg @ Height 2.15C 2.42

Density (kg/m3) 1.225Power Density (W/m2) 6.07Blade Diameter (m) 1.2Blade Area (m2) 1.13

Varibles

Table 2: Wind speed distribution at height of 2 meters

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-500

0

500

1000

1500

2000

2500

3000

3500

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

hrs/

yr

Windspeed (m/s)

Windspeed Distribution at 2 m

Rochester

Figure 1: Rochester wind probability distribution

This analysis shows that the typical wind speeds which this turbine design must operate at will be within the range of 2m/s – 9m/s, or about 5 mph – 20 mph.

TURBINE BLADE ANALYSIS

Blade Selection

In order to achieve the maximum efficiency in harvesting wind power, it is crucial to have an efficient blade design. Blades are the key components in transferring the linear motion of wind speed to angular velocity of a spinning generator. Some of the design challenges faced with wind turbine blades is balancing and shape. The blades must provide a good amount of torque and power to the shaft of the generator. They must also be perfectly balanced to spin quietly and smoothly with minimal vibration. After researching various blade manufacturers, it was decided to purchase a set of blades from WindyNation. The 24 inch TurboTorque high RPM blades are made of aircraft grade aluminum and designed for low wind areas.

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Blade Performance

According to the manufacturer, this 3-blade assembly can reach 500 rpm in 8-9 mph winds. From this given information, a tip speed ratio can be calculated. Tip speed ratio (TSR) is defined as the ratio of the velocity at the tip of the blade with the velocity of the wind (TSR = Tip speed of blade ÷ Wind Speed). A typical TSR value for wind turbines is 6-7. In order to determine the tip speed ratio of the TurboTorque blade set, the RPM value of 500 must be converted to mph. By using conversion relations, 500rpm is equivalent to 71.4 mph at a blade length of 24 inches. This yields a tip speed ratio of 7.93. This value was verified by contacting the manufacturer.

This derived tip speed ratio can be used to calculate the rpm range that can be expected from the TurboTorque blades at any wind speed. To be conservative, a tip speed ratio of 7 was used for this analysis. As can be seen in the Turbine RPM Calculations of Table 3, the 24 inch blades will operate in a range between 245 rpm and 980 rpm in the wind speed range of 5 mph and 20 mph.

TABLE 3: TURBINE RPM CALCULATIONSConversions:

Tip Speed Ratio = 7 1 m = 39.37 in1 m/s = 2.24 mph

TABLE 1: Turbine RPM Values in Metric Units

Blade Length (m) 2 4 6 8 10 12 14 16 18 20 220.1 1337 2674 4011 5348 6685 8021 9358 10695 12032 13369 147060.2 668 1337 2005 2674 3342 4011 4679 5348 6016 6685 73530.3 446 891 1337 1783 2228 2674 3119 3565 4011 4456 49020.4 334 668 1003 1337 1671 2005 2340 2674 3008 3342 36760.5 267 535 802 1070 1337 1604 1872 2139 2406 2674 29410.6 223 446 668 891 1114 1337 1560 1783 2005 2228 24510.7 191 382 573 764 955 1146 1337 1528 1719 1910 21010.8 167 334 501 668 836 1003 1170 1337 1504 1671 18380.9 149 297 446 594 743 891 1040 1188 1337 1485 16341.0 134 267 401 535 668 802 936 1070 1203 1337 1471

TABLE 2: Turbine RPM Values in English Units

Blade Length (in) 1 5 10 15 20 25 30 35 40 45 5012.0 98 490 980 1471 1961 2451 2941 3431 3922 4412 490214.0 84 420 840 1261 1681 2101 2521 2941 3361 3782 420216.0 74 368 735 1103 1471 1838 2206 2574 2941 3309 367618.0 65 327 654 980 1307 1634 1961 2288 2614 2941 326820.0 59 294 588 882 1176 1471 1765 2059 2353 2647 294122.0 53 267 535 802 1070 1337 1604 1872 2139 2406 267424.0 49 245 490 735 980 1225 1471 1716 1961 2206 245126.0 45 226 452 679 905 1131 1357 1584 1810 2036 226228.0 42 210 420 630 840 1050 1261 1471 1681 1891 210130.0 39 196 392 588 784 980 1176 1373 1569 1765 1961

Windspeed (m/s)

Windspeed (mph)

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Blade Forces

Another parameter that must be considered with the turbine blades is the forces that they will encounter. In order to analyze these forces, principles of fluid dynamics were applied. Forces imposed on the blades due to the wind can be found analytically by considering the case of fluid flow over a curved plate. If we examine a cross-section of the blade mounted to a rotor as if the tip were pointed directly in the line of sight, the linear velocity of air moving from left to right enters a control volume surrounding the blade with a certain momentum. This momentum is equivalent to the product of the velocity of the entering fluid, the fluid density, and the area of the jet stream. From the law of conservation of momentum, the momentum of the fluid entering the control volume must be equal to that of the fluid leaving the control volume. Assuming uniform properties at the entrance and exit, steady flow, negligible body forces, and incompressible flow, the force in the x and y directions can be found by the derivation shown below (hand calculations). The x component of displacement is set to zero, since this motion is restricted by the rotor. The -y component of displacement represents the blade spinning motion. The resulting equations yield forces Rx and Ry in terms of wind speed (V), jet area (A), blade angle of curvature (θ), and air density (ρ).

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Input different variable values to optimize blade design

steady flowuniform properties at sections 1 and 2incompressible flow

Windspeed, V 7 m/sBlade Angle ϴ 70 degreesBlade Length 0.61 mFluid Area, A 0.0610 m2

Density, ρ 1.225 kg/m3

Mass, m 0.7 kg Gravity, g 9.81 m/s2

Tip Speed Ratio 7

Force on Blade (metric units) Blade TorqueRx Ry Fx Fy Single NetN N N N Nm Nm

-2.41 10.31 2.41 -10.31 3.14 9.43

Force on Blade (English units)Rx Ry Fx Fy

lb f lb f lb f lb f

-0.54 2.32 0.54 -2.32

CONSTANTS

Assumptions

BLADE FORCE ANALYSIS

TURBINE BLADE END CROSS SECTION

This analysis was performed in an excel file in order to have flexibility in changing variable values. This particular blade force analysis shown above evaluates the forces on a single blade at a wind speed of 7 m/s and a blade curvature of 70°. The resulting forces are 2.41 N in the x direction, and -10.31 N in the y direction. Constant values are set to reflect the TurboTorque 24 inch blade set.

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Hub Analysis

The hub for the blades comes with the TurboTorque assembly. This hub is made from stainless steel with thickness of .1875 inches. The image below shows a picture of the hub:

An finite element analysis was performed on this hub in order to examine the stresses and deflections that will occur in the part at various wind speed conditions. Using the Blade Force Analysis calculations described above, a wind speed of 30 m/s (about 67 mph) was selected to analyze the stresses in a very high wind condition. The force calculated in the x-direction was about 10 lb. This is the force that each arm of the hub will experience at the end.

ANSYS Finite Element Modeling software was used to analyze the solution. The results are shown in the figures below. The hub was modeled using English units, and values for deflection are in inches. From Figure 3, the maximum deflection reaches about 0.02 inches at the outer tips. The stress distribution is shown in Figure 5. A maximum stress of 18.558 ksi occurs at the stress concentration points around the holes. The conservative published value for ultimate tensile strength of stainless steels is about 102 ksi. This result shows that there is no risk of hub failure due to stress, even at very high wind speeds. Running iterations of this finite element model shows that the hub can withstand a force up to about 54 lb.

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X

Y

Z

NOV 3 201017:43:43

AREAS

TYPE NUM

Figure 2: Model of Hub

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1

MN

MX X

Y

Z

-.019334-.017186

-.015037-.012889

-.010741-.008593

-.006445-.004296

-.0021480

NOV 3 201017:42:08

NODAL SOLUTION

STEP=1SUB =1TIME=1UZ (AVG)RSYS=0DMX =.019335SMN =-.019334

Figure 3: Deflection of hub

1

MN

MX XY

Z

-.019334-.017186

-.015037-.012889

-.010741-.008593

-.006445-.004296

-.0021480

NOV 3 201017:45:14

NODAL SOLUTION

STEP=1SUB =1TIME=1UZ (AVG)RSYS=0DMX =.019335SMN =-.019334

Figure 4: Alternate view of hub deflection

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1

MN

MX

X

Y

Z

230.911

22674304

63408376

1041212449

1448516521

18558

NOV 3 201017:41:23

NODAL SOLUTION

STEP=1SUB =1TIME=1SEQV (AVG)DMX =.019335SMN =230.911SMX =18558

Figure 5: von Mises stress distribution

ANSYS Macro written to model hub:

!JOHN VANDELINDE!MSD-1!P11401

!THIS MACRO CREATES THE TURBO TORQUE HUB TO ANALYZE!THE STRESSES AND DEFLECTIONS

F = arg1 !Force on the arms due to the blades (wind)

/PREP7 !*

!Define the element type

! Shell93 element type is for 3 dimensional plate theory

ET,1,SHELL93!* KEYOPT,1,4,0KEYOPT,1,5,0KEYOPT,1,6,0KEYOPT,1,8,0!* R,1,.1875, , , , , , !* !* MPTEMP,,,,,,,, MPTEMP,1,0 MPDATA,EX,1,,28000000MPDATA,PRXY,1,,.25

!Create the keypoints for armsK, ,0,0,0,K, ,-.5,1.75,0,K, ,-0.5,4,0, K, ,.5,1.75,0, K, ,0.5,4,0,

!Keypoints for circlecsys,1 K, ,1,30,0, K, ,1,150,0,

!Create the linesL, 7, 1 L, 1, 6 LSTR, 2, 3 LSTR, 3, 5 LSTR, 5, 4

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LSTR, 2, 7 LSTR, 4, 6 FLST,2,7,4 FITEM,2,3 FITEM,2,4 FITEM,2,5 FITEM,2,7 FITEM,2,2 FITEM,2,1 FITEM,2,6 AL,P51X

!Form the holes to subtract

CYL4,-.2,1.65,.125 CYL4,-.2,2.65,.125 CYL4,-.2,3.75,.125 CYL4,0,0,.25

!Subtract the holesFLST,3,4,5,ORDE,2 FITEM,3,2 FITEM,3,-5 ASBA, 1,P51X

!Copy areas to form all three armscsys,1 FLST,3,1,5,ORDE,1 FITEM,3,6 AGEN,2,P51X, , , ,120, , ,0 FLST,3,1,5,ORDE,1 FITEM,3,6 AGEN,2,P51X, , , ,240, , ,0

!Add areasFLST,2,3,5,ORDE,3 FITEM,2,1 FITEM,2,-2 FITEM,2,6 AADD,P51X

!Create fillets

LFILLT,21,64,3, , !* LFILLT,65,51,3, , !* LFILLT,30,66,3, ,

ADELE, 3 FLST,2,6,4,ORDE,6 FITEM,2,7 FITEM,2,22 FITEM,2,25 FITEM,2,-26 FITEM,2,29 FITEM,2,46 LDELE,P51X, , ,1

FLST,2,60,4 FITEM,2,17 FITEM,2,16 FITEM,2,19 FITEM,2,18 FITEM,2,13 FITEM,2,12 FITEM,2,15 FITEM,2,14 FITEM,2,9 FITEM,2,8 FITEM,2,11 FITEM,2,10 FITEM,2,40 FITEM,2,39 FITEM,2,42 FITEM,2,41 FITEM,2,36 FITEM,2,35 FITEM,2,38 FITEM,2,37 FITEM,2,32 FITEM,2,31 FITEM,2,34 FITEM,2,33 FITEM,2,55 FITEM,2,54 FITEM,2,53 FITEM,2,52 FITEM,2,59 FITEM,2,58 FITEM,2,57 FITEM,2,56 FITEM,2,60 FITEM,2,62 FITEM,2,61 FITEM,2,63 FITEM,2,67 FITEM,2,68 FITEM,2,49 FITEM,2,69 FITEM,2,28 FITEM,2,23 FITEM,2,51 FITEM,2,24 FITEM,2,65 FITEM,2,5 FITEM,2,4 FITEM,2,3 FITEM,2,64 FITEM,2,6 FITEM,2,21 FITEM,2,20 FITEM,2,2 FITEM,2,1 FITEM,2,30 FITEM,2,27 FITEM,2,66 FITEM,2,45 FITEM,2,44

FITEM,2,43 AL,P51X

!Mesh the partSMRT,6 SMRT,5 SMRT,4 MSHAPE,0,2D MSHKEY,0!* CM,_Y,AREA ASEL, , , , 1 CM,_Y1,AREA CHKMSH,'AREA' CMSEL,S,_Y !* AMESH,_Y1 !* CMDELE,_Y CMDELE,_Y1 CMDELE,_Y2

!Apply displacement in Z-direction to 0 for center holeFLST,2,6,4,ORDE,5 FITEM,2,23 FITEM,2,28 FITEM,2,49 FITEM,2,67 FITEM,2,-69 !* /GO DL,P51X, ,UZ,0

!Apply the forcesFLST,2,6,3,ORDE,6 FITEM,2,3 FITEM,2,5 FITEM,2,20 FITEM,2,22 FITEM,2,44 FITEM,2,-45 !* /GO FK,P51X,FZ,-F

!Solve and plot stresses

FINISH /SOL/STATUS,SOLUSOLVE FINISH /POST1 !* /EFACET,1 PLNSOL, S,EQV, 0,1.0

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GENERATOR SELECTION

An appropriate generator size can be selected based on the rpm range (~250 to ~ 980) at which the blades will operate. The battery charging circuitry requires a minimum voltage of about 5 V to operate, and it is desired to achieve a typical operating voltage in the range of 5V to 40V.

The selected generator is a permanent magnet motor specifically designed for small wind turbine applications. It will be provided by the USA Windgen company. It is a 50 Watt motor that generates 24 V and 2 A at 260 rpm. The generator has a ¼” flat mill shaft onto which an arbor can be mounted with an Alan set-screw.

TURBINE STAND ASSEMBLY

The design of the turbine stand assembly is critical in meeting the “portable” customer need. The need for the device to be portable means that the system must be simple to disassemble and pack into a shoulder bag or loaded onto a truck to transportation. The application can be related to military missions where soldiers set up base in one location, and then must move to another location via vehicle. The particular engineering specs that this design will help to meet are the net weight and system volume. The stand design also meets the customer need of robustness, in particular the specification for withstanding a vertical drop test.

The turbine stand is designed using readily available ½” galvanized steel tubing and fittings. The main shaft will consist of a four foot section at the base coupled with a three foot section. A flange will be bored out to slide freely on the 3’ pipe section. From this flange will hang four sections of 1/16” galvanized steel cable, which will each be staked into the ground using 10” galvanized spikes. This will serve as the support for the turbine.

At the top of the 3’ pipe section will be a ½” x 3” galvanized pipe section attached with a ½” coupling. A ½” cap will top this section off. A ½” black cross will be bored out to slide freely on this 3” section. This black cross will serve as the pivoting mechanism for the turbine as wind direction changes. A turbine fin, or tail, will be threaded into one side of this black cross. A fin will be machined from a sheet of 12” x 18” aluminum sheet, and will be attached to a ½” x 18” pipe section using bolts.

From the remaining side of the black cross will be an elbow attached to a flange. The generator will be mounted onto this flange using U-brackets bolted to a plate that mounts onto the flange. An arbor allows for the hub and blades assembly to attach to the generator shaft.

Wire from the generator will be run down into the main shaft of the stand, out the bottom through a ½” hole drilled into the pipe. The location of the generator and pivot design will allow for tangle free functionality as the wind direction changes.

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Figure 6: Turbine stand assembled

Figure 7: Exploded view of assembly

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Figure 8: Drawing of turbine fin

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CABLE FORCE ANALYSIS

In order to correctly size the cables that will support the turbine stand, the following analysis was performed. The cables that will be used are 1/16” galvanized steel with a tensile rating of 96 lb. To determine the max force that will be placed on any cable at a given time, the x-component of the blade forces was found at an excessively high wind speed of 22 m/s. At this speed, each blade will exert 8.13 lb perpendicular to the stand. The total force is then three times that, or 24.4 lb. The force on the cable can then be determined by dividing by the cosine of ϴ. The table below shows the varying cable forces, and corresponding necessary lengths, that will result for different ϴ values.

CABLE FORCE ANALYSIS FxMAX 108.5 NFxMAX 24.4 lbf

Angle θ Fcable Fcable Lcable

degrees N lbf ft20 115.5 26.0 4.322 117.0 26.3 4.324 118.8 26.7 4.426 120.7 27.1 4.528 122.9 27.6 4.530 125.3 28.2 4.632 127.9 28.8 4.734 130.9 29.4 4.836 134.1 30.1 4.938 137.7 31.0 5.140 141.6 31.8 5.2

1/16" Galvanized Steel Cable 42 146.0 32.8 5.4Rating: 96 lb 44 150.8 33.9 5.6

46 156.2 35.1 5.848 162.2 36.5 6.0

FxMAX = 3Fx blade 50 168.8 37.9 6.252 176.2 39.6 6.5

Fcable = Fx max/cos( )θ 54 184.6 41.5 6.856 194.0 43.6 7.2

Lcable = 4ft/cos(θ) 58 204.7 46.0 7.560 217.0 48.8 8.062 231.1 52.0 8.564 247.5 55.6 9.166 266.8 60.0 9.868 289.6 65.1 10.770 317.2 71.3 11.772 351.1 78.9 12.974 393.6 88.5 14.576 448.5 100.8 16.5

4ft

θ

A cable length of 6 ft will be used for this design. From the table, the force on the cable in this scenario would be 36.5 lb. This yields a factor of safety of about 2.5, and an angle ϴ = 48°.

The cables will be looped at each end using 1/8” wire rope clips. One end of the cable will be permanently looped to the flange hole, and the other will slide into a 10” galvanized stake. The stake will be pounded into the ground, providing support to the turbine stand assembly.

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POWER ELECTRONICS BOARD

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