Rochester Institute of Technology



John VanDeLinde

MSD 1

P11401: 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.

[pic]

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 [pic] , 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 3 meters. This height was chosen to determine values at a low-end wind speed results.

[pic] [pic]

Table 2: Wind speed distribution at height of 2 meters

[pic]

Figure 1: Rochester wind probability distribution

This analysis shows that most probable wind speed for Rochester is about 2 m/s at a hub height of 3 meters. In order for the wind turbine to be able to operate for the maximum possible hours throughout the year, it will be important to design for the most probable wind speed of 2 m/s.

POWER REQUIREMENTS

The power required to charge the lithium ion battery is based on the amount of charge time desired. The target mission for the WOCCS family of projects is to operate for a total of three 2-hour uses per day with 30 minutes of off-time before and after each use period. This results in eight hours for the target mission day. The RF boards will draw 0.32 Amps of current for 30% of each two hour periods (transmit/receive mode), and 0.04 Amps of current for 70% of each two hour period (idle mode). This yields a total mission current draw of 0.22 Amps. The operating voltage will be a constant 3.7 Volts, which gives a total power consumption of 0.814 Watts during each two hour mission. A target day of operation will include three of these two hour missions, or 2.442 Watts consumed per day.

The lithium ion battery contains a total power of 8.14 Watts. Dividing this value by the total power consumed by the RF boards in one two-hour period yields 10 two-hour periods of life in the battery. This shows that the battery can last for 3.3 days of target mission operations.

The story of the mission profile is to supply each RF team with two batteries. One battery will be used in the transceiver while the other is charging. Since the battery life spans three days of target mission use, the charge rate for the spare battery could be set to 3 days. However, it was decided that it would be desirable to start each new day with a freshly charged battery. Thus, the charging requirement for the battery is set to a maximum of 8 hours.

[pic]

Table 3: Battery Life Analysis

In order to determine the power needed to charge the battery in 8 hours, Table 4 was created. This table shows the constant voltage and current values needed to charge the battery at hourly unit steps from 1 to 8 hours. In order to charge the battery in 8 hours, it will be necessary to supply the battery with a constant power of 1.155 Watts.

[pic]

Table 4: Battery Charge Rate Requirements

TURBINE BLADE ANALYSIS

Determining Blade Length

The power requirement to charge the battery in the desired 8-hr period is 1.155 Watts. This power set-point is the driving factor for the wind turbine design, and can be used to determine the blade length required at various wind speeds. Assuming efficiencies for the blades, generator, and electronics to be 35%, 75%, and 90% respectively, the power in the wind needed to yield the power requirement is 4.89 Watts. Power in the wind is a function of air density, blade swept area, and the cube of wind speed velocity: PW = ½ρAv3. Using this equation, the swept area “A” required at all wind speeds can be calculated. From this area, blade length needed to achieve 4.89 Watts of power in the wind can be calculated. The TurbineDesign.xls Excel file, “Blade Length” tab.

Looking back at the wind distribution probability curve, it is desirable to design for operation at just below the curve peak of 2 m/s. Using the Excel table, a blade length of 34.5 inches will achieve a power of 1.155 Watts at a wind speed of 1.5 m/s. It was decided to use this wind speed of 1.5 m/s as the minimum operating wind speed. The blade length dimension of 34.5 inches was then used as the target value in blade selection.

[pic]

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 are 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 35 inch WindGrabber blades are made of aircraft grade aluminum and designed for low wind areas. According to the manufacturer, this blade set starts turning in a wind speed of 2.5 mph, or 1.1 m/s.

[pic]



Blade Performance

According to the manufacturer, this 3-blade assembly can reach 200 rpm in 6 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 three blade wind turbines is 6-7. In order to determine the tip speed ratio of the blade set, the RPM value of 200 must be converted to mph. By using conversion relations, 200 rpm is equivalent to 41.65 mph at a blade length of 35 inches. This yields a tip speed ratio of 6.94. 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. A tip speed ratio of 7 was used for this analysis (see TurbineDesign.xls Excel file)

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).

[pic]

[pic]

[pic]

[pic]

[pic]

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.

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:

[pic]

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.

[pic]

Figure 2: Model of Hub

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Figure 3: Deflection of hub

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Figure 4: Alternate view of hub deflection

[pic]

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,1,4,0

KEYOPT,1,5,0

KEYOPT,1,6,0

KEYOPT,1,8,0

!*

R,1,.1875, , , , , ,

!*

!*

MPTEMP,,,,,,,,

MPTEMP,1,0

MPDATA,EX,1,,28000000

MPDATA,PRXY,1,,.25

!Create the keypoints for arms

K, ,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 circle

csys,1

K, ,1,30,0,

K, ,1,150,0,

!Create the lines

L, 7, 1

L, 1, 6

LSTR, 2, 3

LSTR, 3, 5

LSTR, 5, 4

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 holes

FLST,3,4,5,ORDE,2

FITEM,3,2

FITEM,3,-5

ASBA, 1,P51X

!Copy areas to form all three arms

csys,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 areas

FLST,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 part

SMRT,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 hole

FLST,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 forces

FLST,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,SOLU

SOLVE

FINISH

/POST1

!*

/EFACET,1

PLNSOL, S,EQV, 0,1.0

GENERATOR SELECTION

An appropriate generator size can be selected based on the rpm range at which the turbine is designed to run at. In a 1.5 m/s wind speed, the blades will be spinning at 110 rpm. It is desirable to have a generator that achieves a high voltage at low rpm.

The chosen generator motor is a metal gear motor with a free run speed of 6V:175rpm. Voltage is a linear relation with rpm, and therefore this generator will produce 3.8V at the start-up speed of 110 rpm.

[pic]



An alternate generator is a permanent magnet motor specifically designed for small wind turbine applications. It will be purchased from 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 36” pipe section using #10 bolts from the RIT machine shop.

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.

[pic]Figure 6: Turbine stand assembled

[pic]

Figure 7: Exploded view of assembly

Through research, it was found that the boom length of the turbine (the length of the tail) is recommended to be equal to the length of one blade. The area of the tail is recommended to be 5%-10% of the total swept area of the blades. These parameters allow for a sufficient moment arm to be created to turn the turbine into the direction of the wind in the event of a cross-wind.

[pic] [pic]

The turbine tail in the Figure 8 drawing below has a total area of 138 in2, which is equivalent to 7.17% of the total swept area of the blades. The boom length is 36 inches. The boom will be constructed from a 36” galvanized steel pipe, machined according to the drawing in Figure 9.

[pic]

Figure 8: Drawing of turbine tail

[pic]

Figure 9: Drawing of boom pipe

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.

[pic]

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.

POWER ELECTRONICS BOARD

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CHARGER CIRCUIT BOARD

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The charger board will be connected to the generator via a three phase power connector. This connector is located in the upper left corner of the PCB board in the drawing above. The battery holder will be mounted on the side of the board, leaving sufficient room for the inductor and diodes in the remaining space.

[pic]

The enclosure box (shown above) from Hammond, Inc. is made of clear plastic, which will allow for viewing of the LED indication of battery charge state. When the circuit LED is on, this indicates a fully charged battery. The PCB board mounts into the box by connector ridges A hole will be cut into the box to allow for the wire from the base of the turbine stand to be inserted and connected to the circuitry. This hole will be sealed with a gasket to keep moisture out of the box.

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