Stand Alone Micro Wind Electric Generator

STAND ALONE MICRO WIND ELECTRICGENERATOR

A PROJECT REPORT

submitted by

AM 105 ME 072 LINKESH DIWAN

In partial fulfillment for the award of the degreeof

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

AMRITA SCHOOL OF ENGINEERING

AMRITA VISHWA VIDYAPEETHAM

AMRITAPURI, 690 525

May 2010

Dedicated to Truth. . .

and to my Sister,

who abides in Truth.

AMRITA VISHWA VIDYAPEETHAMAMRITA SCHOOL OF ENGINEERING,

AMRITAPURI, 690 525

BONAFIDE CERTIFICATE

This is to certify that the project report entitled “Stand Alone Mi-

cro Wind Electric Generator” submitted by Linkesh Diwan (AM 105 ME

072) in partial fulfillment of the requirements for the award of the Degree of

“Bachelor of Technology” in MECHANICAL ENGINEERING is a

bonafide record of the work carried out under my (our) guidance and supervi-

sion at Amrita School of Engineering, Amritapuri.

SIGNATUREJoshua D. FreemanSUPERVISORAssistant ProfessorDepartment of Electrical & Electronics Engineering

Dr. Balakrishnan Shankar (Date)

This project was evaluated by us on (date):

EXAMINER

AMRITA VISHWA VIDYAPEETHAMAMRITA SCHOOL OF ENGINEERING,

AMRITAPURI, 690 525Department of Mechanical Engineering

DECLARATION

I, Linkesh Diwan (Registered Number AM 105 ME 072) hereby declare that

this project report, entitled “Stand Alone Micro Wind Electric Generator” is a

record of the original work done by me under the guidance of Joshua D. Free-

man, Department of Electrical & Electronics Engineering, Amrita School of

Engineering, Amritapuri, and that this work has not formed the basis for the

award of any degree / diploma / associatship / fellowship or similar award, to

any candidate in any university, to the best of my knowledge.

Place: Signature of Student

Date:

COUNTERSIGNED

Joshua D. FreemanSUPERVISORAssistant ProfessorDepartment of Electrical &

Electronics Engineering

ACKNOWLEDGEMENTS

First and foremost, I wish to thank my Mother, Dr. P. Kamala Willey. With-

out her support and encouragement, I would not have undertaken such a chal-

lenging project, and without her practical wisdom, I would not have finished

it.

My heart’s love and gratitude is due to Holy Mother Amma, in who’s

auspices I have been priviledged and honoured to grow up. Her teachings

and presence have shaped my life; I pray to be worthy of Her grace, already

bestowed.

I also thank my Uncle, who provided me with Internationally Accepted

Social and Business Lubricant, also known as money, today so necessary for

any undertaking.

Thanks are due to many people whom I have not yet had the priviledge

of meeting, in particular Dan Fink of U.S.A., and Andre Espaze of France.

Both have been rich resources of assistance, experience and advice, available

to me via the wonders of modern day technology. In the same vein, I must give

thanks to the millions of people around the world who selflessly contribute to

the many Open-Source projects that have made this possible.

This project would not have been the same without Abhijith, who spent a

week carving a beautiful model blade out of wood, with just my hand drawings

and excited explainations to go by.

I must thank my good friend Rajan, driver of the white Ambassador marked

KLA-3411, for lending his motorbike whenever needed. If you ever need a taxi,

think of him! Trips to our workshop in Karunagappaly were so much easier

(and more frequent) because of him.

Gopi (the mechanic in the garage on the way to the BioTech canteen) got

things started for real, with a heads-up about where spare vehicle parts were

available cheaply. He also undertook to get the hub hydraulically pressed out

of its seating in the Esteem rear hub assembly.

i

I wish to thank Babu, the proprieteer and staff of Super Engineering Work-

shop in Karunagappaly, for his willingness to spend time fulfilling all the odd

demands that this project produced. His skill and expertiese in all types of

fabrication is amazing, and saved us many a headache.

Without Suresh and Shivakumar, two very skilled and honest carpenters,

the blades would not have been done. They did a beautiful job.

As a team, we thank Ani, the carpenter in Achu Furniture in Vallikavu for

letting us use his tools and his workshop.

We owe many thanks to all the laboratory staff at our college, in particular

to Srinivasan Sir, Kanakarajan Sir, and Trivikram Sir, who helped us in many,

many ways.

We owe thanks to Thiyagarajan Sir for the use of what was the fluid lab.

We all but lived there for a few hectic days of work.

We thank Joshua D. Freeman for his hands-off guidance and advice, and

through him, the entire WINSOC project for the use of copious amounts of

Araldite.

Bala Sir is owed thanks for helping us with necessary permissions, and

being supportive throughout the project.

I also want to thank my whole team: Manjusha, Pradeep, and Sanjay, for

being just that — a great team to work with. This is your project report as

much as it is mine; your names should feature somewhere!

I know that many more people than mentioned here have assisted this

project in some way; though I remember you not while writing this, please

know that you have my gratitude.

I want to acknowledge all the future students who will use this project as

a stepping-stone to further research and understanding in the field of wind

energy. I pray that this report will be a valuable tool for you.

As a team, we are honour-bound to give thanks to the wonderful creation

we live in. This project is a small step towards lightening the burden we add

to our Mother Earth through our human greed. Thanks must be given to the

intangible laws that operate to give the wind that turns our wind turbine.

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ABSTRACT

Although renewable energy is said to be the ‘energy source of tomorrow,’ it is

being used to meet almost 160TW of global energy production today[1]. With

Peak Oil already behind us (in 2008, see [2]), small-scale power generation, via

wind turbines and other renewable sources, will be much more widespread in

the future. Such systems are typically used for charging batteries to run small

electrical applications, often in remote locations.

The objective of this project is to design and build a horizontal axis upwind

wind electric generator, with the aim of delivering power to small loads. A wind

turbine is a rotating machine which converts the kinetic energy of the wind into

mechanical energy which is then converted to electrical energy by a generator.

Wind resource assessment has been carried out at Amritapuri Ashram as part

of this project. Various configurations of wind electric systems and different

methods of electricity generation are studied to find that horizontal turbines

with permanent magnet generators are the best option.

The mechanical design has been modelled in Pro-E, analyzed for saftey, and

found to be adequate. Tedious calculations involved in the design have been

performed in Python. The generated plans hve been used for manufacture of

the final components. Blades for the turbine were dimensioned using Schmitz

methodology, and carved out of wood. Airfoil data was obtained using XFOIL

software. A permanent magnet generator has been fabricated, and based on

test data, the entire setup is expected to have a CP of 0.28.

A secondary objective of this project is to encourage the exploration of

wind and alternative energy systems at Amrita, by providing a base on which

other projects can be developed. To this end, suggestions for enhancements,

improvements, and future projects based on the work done in this project are

listed.

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TABLE OF CONTENTS

TITLE PAGE

Acknowledgements i

Abstract iii

List of Figures ix

List of Tables xi

List of Symbols & Acronyms xii

1 Introduction 1

1.1 Why Renewable Energy? . . . . . . . . . . . . . . . . . . . . . . 1

1.2 The Scope of this Project . . . . . . . . . . . . . . . . . . . . . 2

1.3 Project Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Literature Survey 4

2.1 Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 A General Look at Wind Turbines 6

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2 Grid-Connected Systems . . . . . . . . . . . . . . . . . . . . . . 7

3.3 Stand-Alone Systems . . . . . . . . . . . . . . . . . . . . . . . . 7

3.4 Reasons for Choosing a Stand-alone system . . . . . . . . . . . 7

3.5 Components of a Wind Electric System . . . . . . . . . . . . . . 8

3.5.1 Rotor or Wind Turbine . . . . . . . . . . . . . . . . . . . 8

3.5.2 Tail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.5.3 Generator . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.5.4 Tower and Foundation . . . . . . . . . . . . . . . . . . . 9

3.5.5 Maximum Power Point Tracker . . . . . . . . . . . . . . 9

3.5.6 Power Converters . . . . . . . . . . . . . . . . . . . . . . 9

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3.5.7 Energy Storage . . . . . . . . . . . . . . . . . . . . . . . 10

3.6 The Wind and the System . . . . . . . . . . . . . . . . . . . . . 10

3.6.1 Cut-in Wind Speed . . . . . . . . . . . . . . . . . . . . . 10

3.6.2 Rated Wind Speed . . . . . . . . . . . . . . . . . . . . . 10

3.6.3 Cut-out or Furling Wind Speed . . . . . . . . . . . . . . 10

3.7 Wind Turbine Configurtions . . . . . . . . . . . . . . . . . . . . 11

3.7.1 Vertical Axis Wind Turbines (VAWTs) . . . . . . . . . . 11

3.7.2 Horizonal-Axis Wind Turbines (HAWTs) . . . . . . . . . 12

3.7.3 Advantages of HAWT over VAWT . . . . . . . . . . . . 12

3.8 Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.8.1 Lift and Drag Forces . . . . . . . . . . . . . . . . . . . . 13

3.8.2 Angle of Attack . . . . . . . . . . . . . . . . . . . . . . . 14

3.8.3 Tip Speed Ratio . . . . . . . . . . . . . . . . . . . . . . 15

3.8.4 Power Available in the Wind . . . . . . . . . . . . . . . . 16

3.8.5 Performance Over a Range of TSR’s . . . . . . . . . . . 16

3.9 Losses in Wind Turbines . . . . . . . . . . . . . . . . . . . . . . 17

3.9.1 Efficiency Losses . . . . . . . . . . . . . . . . . . . . . . 17

3.9.2 Cp and the Betz Limit . . . . . . . . . . . . . . . . . . . 17

3.9.3 Friction Losses . . . . . . . . . . . . . . . . . . . . . . . 18

3.9.4 Magnetic and Electrical Losses . . . . . . . . . . . . . . 18

3.10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Wind Resource Measurement 19

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.2 Wind Distributions . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3 Wind Resource Assessment . . . . . . . . . . . . . . . . . . . . . 19

4.3.1 What is Wind Resource Assessment? . . . . . . . . . . . 19

4.3.2 Performing the Assessment . . . . . . . . . . . . . . . . . 20

4.3.3 Analysis of the Results . . . . . . . . . . . . . . . . . . . 20

4.4 Rated Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5 Blade Design and Manufacture 23

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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5.2 Selection of Preliminary Values . . . . . . . . . . . . . . . . . . 23

5.3 Required Swept Area . . . . . . . . . . . . . . . . . . . . . . . . 23

5.4 Selection of Airfoil Shapes . . . . . . . . . . . . . . . . . . . . . 25

5.5 Dimensioning of the Blade . . . . . . . . . . . . . . . . . . . . . 26

5.5.1 Steps to Dimension a Blade . . . . . . . . . . . . . . . . 26

5.5.2 Formulæ Used . . . . . . . . . . . . . . . . . . . . . . . . 27

5.5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.6 Manufacturing the Blades . . . . . . . . . . . . . . . . . . . . . 28

5.6.1 Alterations to the Design . . . . . . . . . . . . . . . . . . 29

5.6.2 Selection of Wood . . . . . . . . . . . . . . . . . . . . . . 30

5.6.3 Carving the Blades . . . . . . . . . . . . . . . . . . . . . 30

5.6.4 Post-Manufacture Treatment . . . . . . . . . . . . . . . . 31

5.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6 Mechanical & Stuctural Design & Manufacture 33

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.2 Objectives for the Design . . . . . . . . . . . . . . . . . . . . . . 33

6.3 Selection of Materials . . . . . . . . . . . . . . . . . . . . . . . . 34

6.4 Important Points about the Design . . . . . . . . . . . . . . . . 35

6.4.1 Furling Tail . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.5 3D Modelling in Pro-Engineer . . . . . . . . . . . . . . . . . . . 36

6.6 Manufacturing the Components . . . . . . . . . . . . . . . . . . 36

6.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

7 Failure Analysis 38

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

7.2 Buckling Failure of Tower . . . . . . . . . . . . . . . . . . . . . 38

7.3 Shear Failure of Rotor Bolts . . . . . . . . . . . . . . . . . . . . 40

7.4 Shear Failure of the Fulcrum Pin . . . . . . . . . . . . . . . . . 42

7.5 Crushing Failure of Tower . . . . . . . . . . . . . . . . . . . . . 42

7.6 Shear Failure at Blade Root . . . . . . . . . . . . . . . . . . . . 43

7.7 Tensile Failure of Blades . . . . . . . . . . . . . . . . . . . . . . 44

7.8 Crushing Failure at Blade Root . . . . . . . . . . . . . . . . . . 45

7.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

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8 Tests on Various Generator Types 46

8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.1.1 Generators . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.1.2 Alternators . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.2 Wind Turbine Alternators . . . . . . . . . . . . . . . . . . . . . 47

8.2.1 RPM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

8.2.2 Permanent Magnets v/s Electromagets . . . . . . . . . . 47

8.3 Experimental Analysis of Different Generators . . . . . . . . . . 48

8.3.1 Experiment on a Universal Motor . . . . . . . . . . . . . 48

8.3.2 Experiment on Squirrel-Cage Induction Motor . . . . . . 48

8.3.3 Conversion of Induction Motor to PMG . . . . . . . . . 50

8.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

9 Design and Construction of the PMG 53

9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

9.2 Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

9.2.1 Design of the Rotor . . . . . . . . . . . . . . . . . . . . . 53

9.2.2 Neodymium Magnets . . . . . . . . . . . . . . . . . . . . 54

9.2.3 Placing the magnets . . . . . . . . . . . . . . . . . . . . 55

9.2.4 Preparing the Mould . . . . . . . . . . . . . . . . . . . . 56

9.2.5 Casting the Rotor . . . . . . . . . . . . . . . . . . . . . . 57

9.3 Stator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

9.3.1 Winding the Coils . . . . . . . . . . . . . . . . . . . . . . 58

9.3.2 Preparing the Mould . . . . . . . . . . . . . . . . . . . . 58

9.3.3 Casting the Stator . . . . . . . . . . . . . . . . . . . . . 59

9.3.4 Connecting the Stator Coils . . . . . . . . . . . . . . . . 60

9.4 Assembly and Testing of the PMG . . . . . . . . . . . . . . . . 60

9.4.1 Selection of Load . . . . . . . . . . . . . . . . . . . . . . 61

9.4.2 Converting AC to DC . . . . . . . . . . . . . . . . . . . 61

9.4.3 Testing of the Generator . . . . . . . . . . . . . . . . . . 62

9.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

10 Conclusions 65

10.1 Project Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . 65

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10.2 Future Projects based on this Platform . . . . . . . . . . . . . . 66

10.2.1 Doubling the Magnetic Flux . . . . . . . . . . . . . . . . 66

10.2.2 Creating a New Stator . . . . . . . . . . . . . . . . . . . 67

10.2.3 Researching Different Blades . . . . . . . . . . . . . . . . 67

10.2.4 Creating a Maximum Power Point Tracker . . . . . . . . 67

10.2.5 Implementing Active Pitch Control . . . . . . . . . . . . 68

10.2.6 Using a Permanent Magnet DC Motor as a Generator . . 68

10.2.7 Investigation into the Furling Tail . . . . . . . . . . . . . 68

10.2.8 System Modelling in Simulink . . . . . . . . . . . . . . . 69

10.3 A Final Word . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Bibliography 70

A Bill of Materials & Cost Breakdown 72

B Derivation of Equations Related to Blade Dimensioning 74

B.1 Power in the Wind . . . . . . . . . . . . . . . . . . . . . . . . . 75

B.2 The Betz Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

B.3 Schmitz Dimensioning Formulæ . . . . . . . . . . . . . . . . . . 77

B.4 Blade Tip Velocity w.r.t. Windspeed . . . . . . . . . . . . . . . 80

B.5 Rotor RPM w.r.t. Windspeed . . . . . . . . . . . . . . . . . . . 80

B.6 Reynould’s Number w.r.t. Windspeed and Radius . . . . . . . . 81

B.7 Mach Number w.r.t. Windspeed and Radius . . . . . . . . . . . 81

B.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

C The MH100 Family of Airfoils 82

C.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

C.2 MH102 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

C.3 MH104 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

C.4 MH106 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

C.5 MH108 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

C.6 MH110 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

D Drafts of Mechanical Parts 88

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LIST OF FIGURES

FIGURE PAGE

Figure 3.1 Power Flow Block Diagram . . . . . . . . . . . . . . . . . . 8Figure 3.2 Vertical axis wind turbine . . . . . . . . . . . . . . . . . . 11Figure 3.3 Horizontal axis wind turbine . . . . . . . . . . . . . . . . . 12Figure 3.4 Angle of Attack. . . . . . . . . . . . . . . . . . . . . . . . . 15Figure 3.5 Cp v/s TSR graph . . . . . . . . . . . . . . . . . . . . . . . 17Figure 4.1 Wind speeds being recorded by Weather Display. . . . . . . 21Figure 4.2 Wind speed frequency chart. . . . . . . . . . . . . . . . . . 21Figure 4.3 Wind speed power chart. . . . . . . . . . . . . . . . . . . . 22Figure 5.1 Solidity versus TSR . . . . . . . . . . . . . . . . . . . . . . 24Figure 5.2 CP,Real versus λD . . . . . . . . . . . . . . . . . . . . . . . 25Figure 5.3 Blade Cross-Sections for Manufacture . . . . . . . . . . . . 31Figure 5.4 Before and after images of the blades. . . . . . . . . . . . . 32Figure 6.1 3D Rendering of Design . . . . . . . . . . . . . . . . . . . . 36Figure 7.1 The equivalent model for the tower in buckling. . . . . . . 38Figure 7.2 The free body diagram for the bolt. . . . . . . . . . . . . . 40Figure 7.3 The shear planes for the blade failure analysis. . . . . . . . 43Figure 7.4 The place where tensile failure of the blades is most likely. 44Figure 8.1 Experimental setup to test a squirrel cage induction motor. 49Figure 8.2 Magnets Placed on Rotor . . . . . . . . . . . . . . . . . . . 50Figure 8.3 Voltage versus RPM test on PMG. . . . . . . . . . . . . . 51Figure 8.4 Load test on PMG. . . . . . . . . . . . . . . . . . . . . . . 52Figure 9.1 Neodymium magnets. . . . . . . . . . . . . . . . . . . . . . 54Figure 9.2 Template placed on the rotor disc. . . . . . . . . . . . . . . 55Figure 9.3 Placing the magnets on the rotor disc. . . . . . . . . . . . . 56Figure 9.4 Workbench. . . . . . . . . . . . . . . . . . . . . . . . . . . 56Figure 9.5 The finished rotor disc. . . . . . . . . . . . . . . . . . . . . 57Figure 9.6 Casting the rotor. . . . . . . . . . . . . . . . . . . . . . . . 58Figure 9.7 The casted rotor. . . . . . . . . . . . . . . . . . . . . . . . 58Figure 9.8 Stator windings. . . . . . . . . . . . . . . . . . . . . . . . . 59Figure 9.9 The rotor and stator moulds. . . . . . . . . . . . . . . . . . 59Figure 9.10 The casted stator. . . . . . . . . . . . . . . . . . . . . . . . 60Figure 9.11 Coils placed in the stator mould. . . . . . . . . . . . . . . . 60Figure 9.12 The generator on the work bench after assembly. . . . . . . 61Figure 9.13 Open circuit voltage versus windspeed. . . . . . . . . . . . 62Figure 9.14 Load test at 4.17 m/s equivalent windspeed. . . . . . . . . 63Figure B.1 Velocity Diagram for a Wind Turbine . . . . . . . . . . . . 74Figure B.2 Wind flowing through the wind turbine. . . . . . . . . . . . 75Figure B.3 CP versus v3

v1, showing the best possible value of CP . . . . . 77

Figure C.1 MH100 Family of Profiles . . . . . . . . . . . . . . . . . . . 82Figure D.1 Aerodynamic Profiles . . . . . . . . . . . . . . . . . . . . . 89

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Figure D.2 End View of Blades . . . . . . . . . . . . . . . . . . . . . . 90Figure D.3 Blade Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . 91Figure D.4 Magnetic Rotor Disk . . . . . . . . . . . . . . . . . . . . . 92Figure D.5 PMG Stator . . . . . . . . . . . . . . . . . . . . . . . . . . 93Figure D.6 Hub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Figure D.7 Spindle Weldment . . . . . . . . . . . . . . . . . . . . . . . 95Figure D.8 Yaw Mount Weldment . . . . . . . . . . . . . . . . . . . . 96Figure D.9 Furling Tail . . . . . . . . . . . . . . . . . . . . . . . . . . 97Figure D.10 The Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . 98Figure D.11 Ground Anchor . . . . . . . . . . . . . . . . . . . . . . . . 99Figure D.12 Fulcrum Pin . . . . . . . . . . . . . . . . . . . . . . . . . . 100

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LIST OF TABLES

TABLE PAGE

Table 5.1 Calculated properties of the five blade sections by Schmitzdimensioning. . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table 5.2 Calculated properties of the five blade sections for manu-facture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Table A.1 Bill of Materials . . . . . . . . . . . . . . . . . . . . . . . . 72Table A.2 Cost Breakdown . . . . . . . . . . . . . . . . . . . . . . . . 73

xi

LIST OF SYMBOLS & ACRONYMS

β The blade twist, or the angle between a blade element’s chord and

the plane of rotation.

Cp The coefficient of performance of the wind turbine system.

LD

The Lift to Drag ratio. This ratio varies with the angle of attack.

HAWT Horizontal Axis Wind Turbine. The blades rotate about a horizon-

tal axis.

KISS “Keep It Simple, Sir.”.

λD The Tip Speed Ratio (TSR); the ratio of the blade tip velocity to

freestream wind velocity.

Ma The Mach number, or the ratio of the velocity of a body to the

velocity of sound in the same medium.

MPPT Maximum Power Point Tracker.

NACA The US National Advisory Committee for Aeronautics. This orga-

nization has been absorbed into NASA, the National Aeronautical

and Space Agency.

ω Rotational speed in radians per second.

PMG Permanent Magnet Generator.

PyXFOIL A Python wrapper for XFOIL. XFOIL is a Fortran program writ-

ten by M. Drela of the Aerospace Department, University of Illinois,

used to mathematically mix, analyze, and create new airfoils.

R The radius of the swept area, or length from blade tip to blade hub

center.

Re The Reynould’s number, a measure of the turbulence in the flow.

xii

ρ The density of air at sea level. ρ = 1.225kg/m3.

V A unit of potential difference in electrical circuits.

VAWT Vertical Axis Wind Turbine. The blades rotate about a vertical

axis.

W A unit of power, equal to one Joule of energy per second.

xiii

1. INTRODUCTION

1.1 Why Renewable Energy?

Only recently it was announced that analysts have determined that Peak Oil

has already happened (see [2]). This means that in the near future, new

resources of oil will not be available to meet the growing greed for energy.

We will be forced to find new ways to power our gadgets, necessities, and our

consumer lifestyle.

Perhaps the most important and simplest way to create energy is to save

it. However, this approach often rubs contrary with most people, who are

determined that their share of energy is more than what the world can support.

A more politically correct solution is to research alternative ways to create

energy. That is partly the inspiration behind this project.

More importantly, however, is the fact that today’s main energy sources, oil

and coal, are contributing to the rape and murder of Mother Earth. As this is

written, an oil spill is continuing to grow at the rate of more than one million1

gallons of oil per day in the Gulf of Mexico, the direct and disasterous result

of increased offshore exploration for oil. Mountains are being blown up for

coal in Appalachia, ruining nature’s beauty, clogging and polluting streams,

killing scores of animals and plants, and endangering human lives. Arboreal

forest is being removed in Canada, and oil is being squeezed from sand in

what is perhaps the most environmetally and ecologically damaging method

of oil-extraction.

The discovery, harvest, production, use, and disposal of today’s energy

sources are killing the planet. This year, 2010, has been the hottest year on

record, in the past few centuries. Today’s energy sources pollute the planet

and are the reason for tomorrow’s mass extinction.

1According to Ian MacDonald, Oceanographer, Florida State University.

1

Stand Alone Micro Wind Electric Generator

It is in recognition of this crises that this project was undertaken, to gain

an understanding of how to produce clean energy. Small scale electricity gen-

eration is the method of the future.

1.2 The Scope of this Project

This project has been undertaken with the following overall objectives:

1. To study wind turbines and to gain practical experience in building a

Stand Alone Micro Wind Electric Generator.

2. To encourage renewable energy research and implementation at Amrita.

3. To create a base which future students can use to do their own projects

in wind energy. Suggestions for these projects have been included in

chapter 10.

This project report has been written primarily to assist future students in

their endevours, should they choose to discover the free energy blowing in the

wind.

1.3 Project Philosophy

In the execution of this project, a certian philosophy regarding the engineering

design and implementation was realized. In the interest of completeness and

for the benefit of future students, it is included here.

KISS is a well known acronym for “Keep It Simple, Sir.” This is one of

the deciding factors in this project. When considering the addition of any

components, evaluation was done to see if there was a simpler, more elegant

way to accomplish the same end, and needless complexity was done away with

as much as possible. This has contributed to a design that can be replicated

by semi-skilled people.

Redundant safety and control systems is necessary for any engineering

project, but so often overlooked as “wasted effort.” For the proposed de-

sign in this project to be usable by a wide range of people, it is important that

2


it be as safe as possible. Redundant safety and control systems ensure that

safety is maintained even with partial failure of the machine.

The mechanical design was heavily influenced by the objective of having a

flexible and modular design, that would allow future students to make mod-

ifications, replace parts, and innovate on this platform, without having to

re-create the entire setup.

As far as possible, open source solutions were employed in the execution of

this project. This included the creation of a well commented LATEX document

class for Amrita School of Engineering project reports, which was used by

at least four project teams this year. Open source solutions are preferred

for reasons of quality, customizability, security, and standards support. With

the exception of the three dimensional modelling, everything else has been

accomplished in open source. Furthermore, the code, models, drafts, and

documentation from this project will be released under open source licenses,

such as Creative Commons, General Public License, and the LATEX Project

Public License.

Low cost is the driving objective behind any design, and this project is no

exception.

This project documentation, and generated data, is released under the

Creative Commons Attribution-Noncommercial-Share Alike 2.5 License. For

more information about this license, please see http://creativecommons.

org/licenses/by-nc-sa/2.5/in/.

Computer code, and the 3D models are released under the GNU General

Public License version 3.0. For more information about this license, please see

http://www.gnu.org/licenses/gpl.html.

3

2. LITERATURE SURVEY

This chapter gives a brief insight on the reasons that inspired this project, and

gives an overview of various materials, books and articles that were helpful in

gaining knowledge of the related topics.

Magazines, blog articles, discussion forums, text books, and dreams were

all part of the vast array of resources that were devoured in the quest for

knowledge that this project entailed. Of particular note are the following

resources:

1. Wind Power Plants by R. Gasch and J. Twele.

2. Homebrew Wind Power by Dan Bartmann and Dan Fink.

3. Aerodynamics of Wind Turbine Blades by Martin C. Jischle, University

of Oklahoma.

4. Optimization of a low speed Wind Turbine by John Nathaniel Wise, Uni-

versity of Stellenbosch.

5. OtherPower.com — forums for Small Scale Wind Turbine enthusiasts.

For a more complete (but not exhaustive) list of our references, please see

the bibliography.

2.1 Findings

The literature survey blossomed into a full-scale fast-track course on the me-

chanical and electrical aspects of wind energy. Much more than can be related

here has been learnt. In particular, the following points are felt to be useful

to remember for a beginner:

4


• The Betz Limit of 59.26% is the maximum ratio of energy that can be

extracted from any flow, with any device.

• Neodymium-Iron-Boron (Nd-Fe-B) magnets are the strongest type of

magnet easily available, and are ideal for use in small permanent magnet

generators.

• A Maximum Power Point Tracker (MPPT) is necessary for proper control

and operation of the wind turbine at all windspeeds. A simple MPPT

can be designed and constructed which requires only the AC frequency

input from the PMG.

• Power available in the wind is proportional to the cube of the windspeed,

and the square of the rotor radius.

• Passive mechanical control can and should be used for small scale tur-

bines. This includes a furling tail, stall-controlled blades, and centrifu-

gally activated active pitch control.

• Large wind turbines in overspeed conditions can go supersonic at the

tips of the blades, generating large stresses in the blades, and very ob-

jectionable noise.

Further information gathered from the literary review is presented in the

rest of this report, in particular, in chapter 3.

5

3. A GENERAL LOOK AT WIND TURBINES

3.1 Introduction

Wind power is the conversion of wind energy into a useful form of energy, such

as electricity, using wind turbines. At the end of 2008, worldwide nameplate

capacity of windpowered generators was 121.2 gigawatts (GW). Wind energy

as a power source is attractive as an alternative to fossil fuels, because it is

plentiful, renewable, widely distributed, clean, and produces no greenhouse gas

emissions.

A wind electric generator converts the kinetic energy of the wind into useful

electricity which can be used for various applications like battery charging,

household lighting, etc.

Small wind energy systems can be used in connection with an electricity

transmission and distribution system (called grid-connected or grid-tie sys-

tems), or in standalone applications that are not connected to the utility grid.

Wind turbines come in two main flavours: the more common horizontal

axis, or the vertical axis design. Small scale wind turbines vary in size with

a range of models available, from less than 100 watts (W) up to 50 kilowatts

(kW). Micro wind turbines (less than 100W), are often used to charge 12 volt

(V) or 24V battery banks, for use on standalone systems. Turbines ranging

from 0.6kW to 50kW can be used to provide electricity generation for individ-

ual houses and businesses, with rooftop models varying from 0.5kW to 2.5kW

in size. Large scale systems can produce megawatts of power. Such systems

are typically deployed off-shore, either floating or ground-anchored, and are

used widely in Europe.

6


3.2 Grid-Connected Systems

In grid connected wind systems the output of the wind turbine is directly con-

nected to the existing mains electricity supply. This type of system can be

used both for individual wind turbines and for wind farms exporting electric-

ity to the electricity network. A grid-connected wind turbine can be a good

proposition if there is excess generation, and the electricity company offers

feed-in tariffs.

A grid-connected wind turbine can reduce the consumption of utility sup-

plied electricity for lighting, appliances, and electric heat. If the turbine can-

not deliver the amount of energy you need, the utility makes up the difference.

When the wind system produces more electricity than the household requires,

the excess can be be returned to the grid. With the interconnections available

today, switching takes place automatically.

One big issue with grid connected turbines is the expensive extra equip-

ment, utility company and government regulations, inspections and permits

that are required to connect to the grid.

3.3 Stand-Alone Systems

Small wind turbines have traditionally been used to generate electricity for

charging batteries for small electrical applications, often in remote locations

where it is expensive or not physically possible to connect to a mains power

supply. Such examples include rural farms, island communities, boats and

caravans. Typical applications are electric livestock fencing, small electric

pumps, lighting or any kind of small electronic system needed to control or

monitor remote equipment, including security systems.

3.4 Reasons for Choosing a Stand-alone system

• The site under consideration has an average annual wind speed of at

least 4m/s.

• A grid connection is not available or can only be made via extra expense.

7


• One is interested in gaining energy independence from the grid.

• To reduce the environmental impact of electricity production.

Figure 3.1: Block diagram showing power flow in a Stand-Alone Wind ElectricSystem.

3.5 Components of a Wind Electric System

The following are the components of a wind electric system:

3.5.1 Rotor or Wind Turbine

The rotor is what extracts energy from the wind, through blades, and turns

a shaft. Most wind turbines have an ‘upwind’ configuration (where the ro-

tor is pointed into the wind) and use three blades for electricity generation.

Higher torque applications such as wind pumps use more blades, at the cost

of efficiency and speed.

3.5.2 Tail

The tail is an indispensable part of upwind turbines; it keeps the rotor pointed

into the wind. Tails are not simply vertical boards placed downwind from

the rotor. Depending on the configuration, a tail may be required to furl

(fold out of the way) at a certain windspeed to protect the wind turbine from

overspeeding.

3.5.3 Generator

A generator, very simply, is an arrangement of components designed to cause

relative motion between a magnetic field and the conductors in which the

emf is to be induced. Those conductors, out of which flows electric power,

8


form what is called the ‘armature’. Most large generators have the armature

windings fixed in the stationary portion of the machine (called the ‘stator’),

and the necessary relative motion is caused by rotating the magnetic field. A

type of electrical machine that is being used more frequently in wind turbine

applications is the permanent magnet generator. This is now the generator of

choice in most small wind turbine generators, up to around 10kW. In these

generators, permanent magnets provide the magnetic field, so there is no need

for field windings or supply of current to the field.

3.5.4 Tower and Foundation

The tower and foundation is often the most expensive part of a wind turbine

system. It is also the most crucial, for without a proper tower and foundation,

the turbine can crash, destroying itself and potentially causing damage and

injury to others.

3.5.5 Maximum Power Point Tracker

The maximum power point tracker for a wind turbine monitors the speed of

rotation, and regulates the applied load to avoid stalling the machine. Stall

happens when excessive load is applied, and the blades slow down and stop.

This condition is best to avoid, as after stalling, the machine must spin up

and gather momentum before it can be used to generate electricity, which

translates to lost power. Advanced MPPT units can double as wind turbine

controllers, which will apply dummy loads as needed, and even shut down the

turbine in overspeed conditions.

3.5.6 Power Converters

Power converters are devices used to change electrical power from one form to

another, as in AC to DC, DC to AC, one voltage to another, or one frequency

to another, Power converters have many applications in wind energy systems.

They are being used more often as the technology develops.

9


3.5.7 Energy Storage

Energy is most often stored in batteries. In grid-connected systems, the grid

can be envisioned as a battery of infinite storage capacity.

3.6 The Wind and the System

Any wind electric system has a few characteristic wind speeds, which define

its operating parameters. These have been included here.

3.6.1 Cut-in Wind Speed

The cut-in wind speed is the speed at which the wind turbine starts to generate

electricity.

Lowspeed winds may not have enough power to overcome friction in the

drive train of the turbine and, even if it does and the generator is rotating, the

electrical power generated may not be enough to offset the power required by

the generator field windings (for certain designs).

Therefore, the wind turbine is allowed to rotate at no load in low wind-

speeds, and allowed to spin up to a certain cut-in speed before useful power is

generated.

3.6.2 Rated Wind Speed

As velocity increases above the cut-in wind speed, the power delivered by

the generator rises with the cube of wind speed. The rated windspeed is the

speed at which the wind turbine will produce its maximum power, that it was

designed to produce.

3.6.3 Cut-out or Furling Wind Speed

As wind speed continues to increase, it may pose a danger to the turbine,

causing it to rotate at speeds which the mechanical components cannot with-

stand. This is called ‘overspeeding’, and is a dangerous condition for the wind

turbine. Before the situation gets dangerous, the wind turbine must be shut

10


down, either with mechanical brakes (large scale systems) or by shorting the

terminals and creating an electrical brake (PMG systems). This windspeed is

called the cut-out windspeed.

The term ‘furling wind speed’ stems from the fact that many smaller wind

turbines use a furling tail. A furling tail is a tail that, in normal operating

wind speeds, keeps the wind turbine pointed into the wind, but at high wind

speeds, ‘furls’ (folds out of the way), causing the turbine to turn out of the

wind. This is a type of passive mechanical overspeed protection, and is very

effective in protecting the turbine.

3.7 Wind Turbine Configurtions

3.7.1 Vertical Axis Wind Turbines (VAWTs)

This configuration of wind turbines have the main rotor shaft arranged verti-

cally. The important feature of this arrangement is that the turbine does not

need to be pointed into the wind to be effective, and can harness wind from

any direction. This is an advantage in sites where the wind direction is highly

variable.

Figure 3.2: Vertical axis wind turbine

VAWTs are one of the oldest configurations, having been used in Persia

thousands of years ago to grind grain. They have recently begun to gain

11


popularity due to the lower cost and perceived versatility over the horizontal

configuration.

3.7.2 Horizonal-Axis Wind Turbines (HAWTs)

The HAWT configuration has the main rotor shaft and electrical generator at

the top of a tower. In ‘upwind’ configurations, the front of the rotor must be

pointed into the wind, whereas for the ‘downwind’ variety, the rotor orients

itself correctly automatically. The rotation about the vertical axis is called

‘yaw’. Small turbines use a yaw tail to keep them oriented into the wind,

while large turbines use wind sensors with servo motor drives.

Figure 3.3: Horizontal axis wind turbine

3.7.3 Advantages of Horizontal Axis Wind Turbine Over Vertical

Axis Wind Turbine

It is theorized that VAWTs can never be as efficient as HAWTs. This is due to

the very nature of the design. Because half of the machine is moving counter

to the direction of the wind, the machine must sweep twice as large as area as

a HAWT to generate the same amount of power. And since VAWT designs

are generally half as efficient as horizontal axis turbines due to the slow blade

speeds, for the same power, a VAWT would require four times the swept area

as an HAWT.

Furthermore, there are other advantages, as listed here:

• Variable pitch angle, which gives the optimum angle of attack. The

angle of attack can be adjusted such that it collects maximum amount

of energy.

12


• The tall tower allows access to stronger wind outside the wind shear

boundary layer.

• Horizontal axis turbines have higher efficiency, since the blades always

move perpendicular to the wind, receiving power through the whole ro-

tation. Most vertical axis turbines produce energy at only 50% of the

efficiency of the horizontal axis turbine because of the additional drag

that they have as their blades rotate into the wind.

For these reasons, it was decided to choose a HAWT wind turbine config-

uration for this project.

3.8 Parameters in the Design and Operation of Wind

Turbines

The difficulty in extracting energy from wind compared to other resources like

hydro power, solar, or fossil fuels, is that the power available in wind changes

rapidly and wildly, with the wind. The power available from the wind that can

be extracted by the turbine largely depends on the swept area of the turbine

and the wind speed. Therefore larger the swept area of the turbine, greater

is the power obtained. Another factor that influences the power output is the

height of the tower. The wind speeds will be more at greater heights and hence

the power available is also more.

The basic terms associated with the design of a wind turbine are:

• Lift and Drag Forces

• Angle of Attack

• Tip Speed Ratio

Each of these is explained in detail further on.

3.8.1 Lift and Drag Forces

The wind flowing past the surface of the turbine exerts a force on it. Lift is

defined as the component of this force that is perpendicular to the direction

13


of the wind. The component of this force which is parallel to the direction of

the wind is called ‘drag force’.

Both of these forces can be used to power wind turbines, using different

design methodologies.

Drag based wind turbines provide high torque at low rpm from the drag

forces. This makes them suitable for pumping water or grinding grain, both

being high torque operations.

Drag based turbines perform very poorly for making electricity. This is

because, in a drag-based design, the blades can never move faster than the

wind. At higher speeds, the efficiency and ease of power generation both

increase, making the turbine blade speed a critical factor.

In Lift-based wind turbines, the lift is created by diverting air from the

axial direction, or by inducing a vortex in the windstream. The force required

to divert the air is matched by the reaction of the air on the blade, causing the

blade to rotate and generate power. The key concept of lift is that it allows the

blade tips of a wind turbine to move faster than the wind. This is important

because doubling the forward speed of the blades quadruples their effect on

the wind.

Most modern wind turbines are lift based. These wind turbines also expe-

rience a drag force, but the objective is to minimize drag and maximize lift for

the turbine blades.

3.8.2 Angle of Attack

The angle of attack of a wind turbine is the angle between the chord line of

the blade section and the apparent velocity of the wind. This is shown in

figure B.1. Each airfoil shape (the cross-sectional profile of the blade) has lift

and drag components which depend on the angle of attack. If the angle is too

much or too little, drag forces increase, lift forces decrease and the air behind

the blade becomes turbulent; the blade is said to stall (or stop producing lift).

Figure 3.4 gives the general idea.

14


Figure 3.4: Angle of Attack.

3.8.3 Tip Speed Ratio

Tip speed ratio (λD) is the ratio of the speed of the rotating blade tip to the

speed of the free stream wind. The overall efficiency of the blades (how much

power they take out of the wind and transfer to a spinning shaft) has been

found to depend on the TSR. Drag based machines have a TSR of one or less,

which is why they are not used for electricity generation purposes. The best

TSR range to maximize blade efficiency while keeping tip speeds from getting

out of hand is a TSR of five to six.

TSR =ωR

V(3.1)

where:ω is the rotational speed in radians per second.

R refers to the swept area radius.

V refers to the wind freestream velocity.

15


3.8.4 Power Available in the Wind

The power in the wind that is available for harvest depends on the wind speed,

air density and the area that is swept by the turbine blades. The power avail-

able in the wind that can be converted into useful energy can be determined

using the following parameters:

• Air Density (ρ) = 1.225 kilograms per cubic meter (at sea level).

• Swept area in square meters, A.

• Wind velocity in meters per second, V.

The kinetic energy flowing through the swept area per unit time (also

known as power) is calculated as:

P =1

2Av3 (3.2)

The wind power per unit area of the turbine (wind power density) is:

P

A=

1

2v3 (3.3)

From the above relations it can be noted that:

• The wind power density is directly proportional to the density of air, for

standard condition it is equal 1.23 kg/m3

• Power from the wind is proportional to the area swept by the rotor.

• The wind power is directly proportional to the cube of wind velocity.

When the wind speeds are low, the available wind power is also low. The

only way to increase the available wind power in low wind speeds is by sweeping

a larger area with the blades. Power available increases by a factor of four,

when the diameter of the blade is doubled.

3.8.5 Performance Over a Range of TSR’s

Turbine rotors are designed to run best at a particular TSR, but in reality

the running speed depends how they are loaded. If the generator draws more

power than the rotor has to offer, it slows and often stalls. It is to avoid this

stall that an MPPT is employed.

16


Figure 3.5: Cp v/s TSR graph

3.9 Losses in Wind Turbines

Wind energy, though available in plenty, is not easily harnessible. The power

harnessed from the wind is much less than the total power available. This

difference can be attributed to the losses in the wind turbine.

3.9.1 Efficiency Losses

The wind turbine generator does not operate at rated power all of the time.

It can only create as much power as the rotor is able to extract from the wind,

ideally. The blades cannot transfer all the available power in the wind into shaft

power, as they are not 100% efficient. The generator has it’s own inefficiency,

causing additional power loss. The power loss due to the inefficiencies of the

individual components are known as efficiency losses.

3.9.2 Coefficient of Power (Cp) and the Betz Limit

The final ratio of how much power a wind turbine can extract from the wind

to how much power is available in the wind is called the coefficient of power

(Cp). Less efficient turbines (lower Cp) would need a larger swept area to make

the same power from the same wind speed.

17


The Betz limit is the absolute maximum that can be extracted from the

available power in wind. It applies to any wind powered device. The value of

the Betz limit is 59.26%.

3.9.3 Friction Losses

The bearings upon which the wind turbine spins are designed to reduce friction,

but it cannot be avoided completely. Friction losses end up as heat inside the

bearings and noise during operation.

3.9.4 Magnetic and Electrical Losses

Due to imprecision during manufacture, wind turbines can have magnetic and

electrical (I2R) losses as well. This is particularly true for small scale wind

turbines.

3.10 Conclusion

In this project, an attempt has been made to be as thorough as possible.

Designing and building a wind turbine is a complex task, and no doubt some

aspects have been overlooked. Here, basic information that all should be aware

of, and that was gained in the literary review stage, has been presented.

18

4. WIND RESOURCE MEASUREMENT

4.1 Introduction

The economic viability of constructing a wind turbine system at a particular

site depends most strongly on the quality of the wind resource available. Gen-

erally, average annual wind speeds of at least 4.0–4.5 m/s (14.4–16.2 km/h or

9.0–10.2 mph) are needed for a small wind turbine to produce enough electric-

ity to be cost effective. A very useful resource for evaluating a site for its wind

energy potential is wind resource assessment technique.

4.2 Wind Distributions

The statistics used to calculate wind speeds are complicated, but the results

are easy to understand. Most winds come to at lower and moderate speeds,

and higher winds are relatively rare. In most locations world wide, wind speeds

keeps fairly close to a Rayleigh distribution. There are non-Rayleigh locations

where the curve takes on other shapes but these are relatively rare.

4.3 Wind Resource Assessment

4.3.1 What is Wind Resource Assessment?

In wind resource assessment the wind speed measurements recorded at the site

under consideration represent the wind potential at that site.

The most important component of a wind resource evaluation system is

an anemometer. A common type of anemometer is the “Cup Anemometer,”

which has small hemi-spherical cups mounted on short arms, connected to a

rotating vertical shaft. The anemometer rotates in the wind and generates a

signal that is proportional to the wind speed.

19


It is very important that the measurement equipment is set high enough to

avoid turbulence created by trees, buildings or other obstructions. This height

is roughly taken to be 30 meters above the ground, or 10 meters above any

nearby obstacles, whichever is higher. Readings are most useful when taken

at hub height, or the planned elevation for the proposed wind turbine hub.

The anemometer is connected to a data logger which continuously records

wind speeds measured by the anemometer. The data logger can be anything

from a simple graph to a computer system.

4.3.2 Performing the Assessment

For this project, the wind resources on top of the ‘B’ building in Amritapuri

Ashram were measured using an anemometer for a period of six months, from

March 2009 to August 2009. After August, the room used to house the data

logger was required by the accomodation office, and the anemometer broke

down, effectively ending the assessment. Due to unavoidable reasons, out of

six month’s data, only five months were usable.

The magnitude and direction of the wind speeds at the proposed site were

measured and recorded as per-minute averages with the help of a software

called Weather Display. Weather Display is a full-fledged weather station pro-

gram, which can accept inputs for temperature, rainfall, wind, wind direction,

humidity, etc. The data thus collected was tabulated and analyzed to guage

the power available from that location the site.

The weather station used to record the wind speeds is shown in figure 4.1.

4.3.3 Analysis of the Results

The frequency distribution of the wind speeds for a period of five months were

plotted and analyzed to obtain the average wind speed of the site. It must be

noted that the wind speeds vary from time to time and the wind speed that

prevails for the maximum amount of time is useful for harnessing power. The

average wind speed for the five months of data was calculated to be 5.26m/s.

Figure 4.3 shows the power available with the wind measurements obtained.

20


Figure 4.1: Wind speeds being recorded by Weather Display.

The average windspeed is important as it is the defining windspeed for

the proposed wind turbine system. All calculations will be done based on the

average windspeed, to ensure that for most of the time, the system will be able

to generate power.

Since the data aquisition period did not complete a full year, the data is not

sufficient to create a representative distribution curve. The wind distribution

is plotted in figure 4.2.

Figure 4.2: Wind speed frequency chart.

21


4.4 Rated Output

Rated output power (or plain “Rated Power”) is the maximum sustained out-

put that the turbine can produce. It is not a sufficient metric to be used for

wind turbines, as they seldomly attain full-power output. A wind turbine is

entirely at the mercy of the wind; no wind means no power, no matter what

the rated power is. Therefore it can be said that “Rated power is overrated.”

More important is the cut-in speed for the wind turbine, and the operating

range of windspeeds. These must be matched to the wind resources, to ensure

optimum power generation. The equation for power (equation (B.1)) shows

that power is proportionate to the third power of the windspeed. Thus the

power availability depends directly on the wind speed of the site considered.

Figure 4.3: Wind speed power chart.

4.5 Conclusion

The wind analysis performed for this project is inconclusive, when analyzed

for the viability of installing commercial grade wind power generators on the

top of B building. This is not, however, indicative of inviability; rather it is

due to the fact that the wind resource assessment was not completed due to

unavoidable reasons.

22

5. BLADE DESIGN AND MANUFACTURE

5.1 Introduction

The blades on a wind turbine are what extract power from the air. They

work in the same way as an airplane’s wing, generating lift in the direction

of rotation by diverting air in the opposite direction. In the linear system of

an airplane wing, the diverted air forms a downwash behind the wing; in the

rotary system of the wind turbine blade, the diverted air forms a spiralling

vortex in the air behind the turbine.

5.2 Selection of Preliminary Values

In designing a wind turbine blade, preliminary estimates or guesses must be

made regarding some parameters, which are then revised at a later stage.

The first values to be determined are the number of blades and the tip speed

ratio (λD). The only way to choose these values is by studying the various

configurations already documented, and selecting accordingly.

For this project, a tip speed ratio of six (λD = 6) and a three-bladed

configuration (n = 3) were selected. The selection was influenced, in part, by

figure 5.1.

5.3 Required Swept Area

The equation for power from the wind is:

PReal =ρ

2πR2V 3

1 CP,Real (5.1)

23


Figure 5.1: Solidity versus tip speed ratio. Courtesy [11].

wherePReal represents the real power available from the blades,

ρ is the density of air (1.225kg/m3),

R is the radius of the swept area,

V1 is the upstream velocity of the wind, and

CP,Real is the overall coefficient of performance of the blades.

For the purpose of calculating the required swept area, suitable values

for the remaining variables must be assumed. The velocity is taken to be the

average freestream velocity, V1 = 5.26m/s. At the average velocity, the desired

power is 80W.

A suitable value for CP,Real taken from figure 5.2. The number of blades

having already been decided as n = 3, a value for ε was assumed. Effectiveness

of the blade is represented by ε, which has the value of CL

CD. As a conservative

value, ε = 20 was selected. These determinations combine to give, via figure

5.2, a CP,Real of 0.35.

Therefore, the value of the required radius is found to be R = 0.906m,

corresponding to a swept area of 2.096m2. For convenience, the desired radius

24


Figure 5.2: CP,Real versus λD for wind turbines, varying the number of bladesand effectiveness of the blades. Chart courtesy of [11].

is adjusted up to 0.95 meters.

5.4 Selection of Airfoil Shapes

The selection of the airfoil for the blade determines the blade’s performance.

There are hundreds, if not thousands, of airfoils to choose from, ranging from

general purpose NACA airfoils to very special purpose airfoils like the Whitman

Supercritical airfoil.

Each airfoil, though seemingly akin to all the others, has very individual

performance characteristics of stall angle, best lift-to-drag ratio ( LD

ratio), op-

erational range of attack angles, etc. Therefore, the selection of proper airfoils

proved a daunting task.

Practically, it is sufficient to choose any airfoil with a decent LD

ratio, and

build the blade from that. High performance wind turbines, however, employ

different airfoils at different radial sections of the blade, thereby obtaining a

wider range of operating conditions for the wind turbine.

In this project, it was decided to design the blades incorporating passive

25


stall control, whereby the blades themselves stop producing power in overspeed

conditions. This feature is part of our redundant protection, and when coupled

with a furling tail, gives very good protection to the wind turbine.

The decision to incorporate passive stall control necessitated that the air-

foil(s) chosen should have a sharply decreasing LD

ratio after stall. The MH100

family of airfoils were tested in XFOIL and compared to standard NACA air-

foils, and found to possess the desirable characteristics. In fact, the MH100

family was specifically designed for stall controlled wind turbine blades.

The MH100 family is found in its entirety in appendix C.

5.5 Dimensioning of the Blade

Blade dimensioning is a complex process. In short, there are two methods

of blade dimensioning: Betz dimensioning, and Schmitz dimensioning. Both

of these methodologies have originated in Germany, and are a product of the

famous German precision and quality engineering.

For this project, Schmitz dimensioning was adopted. Schmitz dimensioning

is an improvement on Betz dimensioning, and as such, is more accurate for

higher tip speed ratio wind turbines.

5.5.1 Steps to Dimension a Blade

Schmitz dimensioning specifies the principle equations to be evaluated at each

defining blade section. These equations define the shape of the blade and the

operating conditions of each section. Dimensioning the blade is accomplished

by the following steps:

1. Based on performance characteristics and prior experience, the airfoil

sections to use are selected and assigned axial positions (r) along the

blade. These are the principle blade elements.

2. Approximate values are assumed for the chord (c) at each section.

3. Using c and r, approximate values of the Mach (Ma) number and Rey-

nould’s number (Re) at that section are calculated. Equations relating

26


Ma and Re to freestream velocity have been included in appendix B.

These values are calculated using the average freestream velocity, in this

case, 5.26 m/s, and are required for analyzing the airfoil in PyXFOIL.

4. Using PyXFOIL, the coefficients of lift (CL) and drag (CD), and the

polar diagram for each airfoil section is created. This polar diagram is

used to determine the best angle of attack for the blade element under

consideration.

5. Having selected the best angle of attack, equations (5.2), (5.3), and (5.4),

are used to calculate the theoretically optimum values of chord and blade

twist (β).

6. Each blade is represented as a series of airfoil cross sections, as specified

axial locations, each with it’s own chord and twist angle, as shown in

figure 5.3.

5.5.2 Formulæ Used

The angle of apparent wind is calculated by the following formula:

φ1(r) = tan−1

(R

λDr

)(5.2)

The value of φ1(r) for each blade element (or cross section) is required to

find the minimum chord, using the formula:

c(r) =16πr

nCLsin2

(φ1

3

)(5.3)

Blade twist at each airfoil section is calculated as:

β(r) =2

3tan−1

(R

λDr

)− αA (5.4)

These formulæ have been derived in appendix B.

27


5.5.3 Results

The MH100 family of airfoils is a set of five airfoils, with specific axial positions.

The analysis was carried out keeping those positions, and results were obtained

as shown in table 5.1.

# Profile Distance from Root r (m) Chord c (m) Blade Twist β ()1 MH102 0 0.27 34.52 MH104 0.188 0.07 8.53 MH106 0.232 0.06 4.54 MH108 0.476 0.05 15 MH110 0.620 0.04 0

Table 5.1: Calculated properties of the five blade sections by Schmitz dimen-sioning. Note that blade twist is calculated with relation to the plane ofrotation of the blade.

5.6 Manufacturing the Blades

A number of methods for manufacture were proposed for the blades. Among

these were:

1. Aluminium skin wrapped around plywood pieces cut to the shape of the

profiles, mounted on fully threaded rods.

2. Aluminium skin shaped on a wooden blade and filled with “puff,” an

expanding foam used in boat-building.

3. Aluminium casting manufactured in Coimbatore.

4. Balsa wood cut and shaped, then waxed or painted for strength.

5. Wood carving.

After a proper examination of the various proposals and evaluating the

complexity and environmental impact of each, it was decided to contract a

carpenter to carve the blades out of solid or laminated wooden planks.

28


5.6.1 Alterations to the Design

For large mega-Watt range wind turbines, with blades 50 or 60 meters long, and

complex specifications that were derived are applicable, and are necessary for

optimum performance. However, for micro scale (less than 100W) installations,

such complexity can hamper manufacture.

The blade, as calculated, was found to be exceedingly precise. Struc-

tural considerations and input from carpenters caused the calculated results

shown in table 5.1 to be used merely as a reference for designing the final,

manufacture-ready blade.

Employing five different profiles was found to be difficult to carve from

wood, not the least because the profiles must be interpolated to obtain the in-

termediate cross-sectional shape along the blade. Additionally, the calculated

dimensions would have caused the blade to be exceedingly thin and delicate

at the tip, which would introduce significant structural problems.

These problems were overcome by simplifying the design with the following

constraints:

• Only two airfoil profiles were employed, MH102 at the root, and MH104

for the remaining sections.

• The chord of the MH104 was kept constant throughout the blade.

• The length of the blade was increased to 0.8m.

• Chord lengths and twist angles were rounded and approximated to near-

est convenient values.

These specifications necessitated re-calculating the blade twist for the third,

fourth, and fifth sections, as the profile shape was changed. After re-calculation,

the values given in table 5.2 were adopted for manufacture. Due to the com-

plex shape of the blade, three-view drawings were hand drawn and given to the

carpenters. A quasi-isometric view with all the relevant information is shown

in figure 5.3.

29


# Profile Distance from Root r (m) Chord c (m) Blade Twist β ()1 MH102 0 0.27 36.52 MH104 0.188 0.07 8.53 MH104 0.232 0.07 44 MH104 0.476 0.07 1.55 MH104 0.620 0.07 0

Table 5.2: Calculated properties of the five blade sections for manufacture.Note that blade twist is calculated with relation to the plane of rotation of theblade.

5.6.2 Selection of Wood

The carpenter was given the freedom to choose a wood that he deemed suitable,

given the following requirements:

1. High tensile strength.

2. High compressive strength.

3. Good properties for carving.

Based on his experience, Ani (the carpenter) recommended a wood known

locally as “Sheelanthi,” which has been translated as “Tulip-Tree.” This is

the wood that was used for carving all the blades. Wood was procured from

a sawmill in Alumkadavu. The blade blanks were made by laminating planks

of wood together with glue, as shown in figure 5.4.

5.6.3 Carving the Blades

The first blade was carved by Abhijith, from Italy, from drawings and extensive

explainations. A cardboard mock-up was fabricated to show the placement of

profile sections. Two fixtures were cut from aluminium to be used as guides

for carving, with angles and horizontal guides marked on them.

Abhijith spent a week carving the first one very carefully, first rough cutting

the shape, and then using a hammer and chisel to get an approximate outline.

Finally, he finished the blade with a rasp and sandpaper. Figure 5.4 shows the

blade blank, and the finished blade.

30


Figure 5.3: Diagram showing the cross sections and dimensions of a blade.Note that the twist angle shown here is w.r.t. the chord of the root profile, forthe purpose of manufacture.

Abhijith unfortunately did not have the time to carve all three blades.

His one blade was used as an example, and taken to carpenters Suresh and

Shivakumar, who were able to carve four more (one as a spare and one for

display purposes) in a two week period. Each blade seems to take one week to

carve, due to complexity.

5.6.4 Post-Manufacture Treatment

Dan Fink, in [10] recommends coating the blades liberally with linseed oil at

regular intervals, but not to coat them with paint or other sealant (like varnish).

This is because the blades must adjust internally to changing atmospheric

conditions throughout the years of operation, and a sealant would hamper the

process. Linseed oil waterproofs the wood, while at the same time, keeping

them open to the atmospheric conditions. For this project, it has been elected

to follow this advice.

31


Figure 5.4: Before and after images of the blades.

5.7 Conclusion

The Schmitz dimensioning scheme, used in this project to dimension the

blades, produces complex blade shapes that can harness optimum power from

the wind, based on the characteristics of the airfoil profile chosen. For large

scale wind turbines, the effort required to manufacture the blade to the spec-

ifications obtained from the Schmitz method is compensated for by increased

performance.

Whether small scale turbines experience a similar benefit is debatable. In

all probability, a blade with very approximate angles and any suitable profile

would perform similarly. The benefits of increased precision are only realized

in large scale applications.

That said, for the purpose of this project, performing the analysis and

manufacturing the blades to the specifications was a worthwhile exercise.

32

6. MECHANICAL & STUCTURAL DESIGN &

MANUFACTURE

6.1 Introduction

The mechanical design and mathematical testing against failure theories is a

crucial step. The design used for this project was adapted from an existing

design, given in [10], that has seen many years of craft evolution and imple-

mentation in the United States, Europe, Nicaragua, and other countries. The

design is simple to assemble, has minimal parts, minimal weight, and has many

thousands of hours of testing in real wind conditions.

Given the project objective of creating a base on which multiple variations

and different designs can be tested, and given local availability of materials,

the above mentioned design had to be modified, and this necessitated mathe-

matical verification against theoretical failure models for critical components.

6.2 Objectives for the Design

The following major objectives were determined, to be accomplished by the

mechanical and structural design:

Safety The safety of the design is paramount. No part may at any time

come loose which might cause catastrophic failure, or may injure people.

The mechanical design must be able to compensate for overloading, and

protect itself against adverse conditions. Additionally, at no point in the

construction and operation of this wind turbine may people be subject

to bodily harm.

Modularity This stems from the project objectives: to produce a flexible

and re-configurable wind turbine system. To encourage further research

33


and future projects based on this platform, ease of reconfiguration is

necessary. To this end, the design must be modular, with standardized

components and interfaces.

Compact As a final objective, components must be packaged and functions

executed in as simple and compact a manner as possible.

All of the decisions taken while adapting the design to this project were

evaluated against each above objective in turn. This proceedure dictated many

aspects of the design.

To ensure a safe design, the following design decisions were taken:

Tilting Tower The tower on which the turbine is to be mounted must tilt

at the base. Thus at no time will any person be required to ascend to

the top of the tower. All maintenance can be accomplished by gently

lowering the tower and performing necessary actions at ground level.

Redundant Overspeed Protection Overspeeding is a condition that oc-

curs when the windspeed becomes excessive. At very high speeds, the

centrifugal force developed in the blades due to rotation may cause the

blades to break, which can precipitate catastrophic failure. To avoid this,

we use redundant overspeed protection measures.

6.3 Selection of Materials

The parts were selected largely based on local availability, ensuring that they

met the minimum requirements of the proposed design. Some important

choices taken were:

1. Maruti Esteem Rear Hub and Spindle assembly as main hub for the

PMG.

2. 40mm x 4mm Mild Steel Angle Iron for most of the fabrication.

3. 5mm thick Mild Steel Disks for the rotors.

4. 5 & 6 inch heavy Galvanized Iron pipe for the tower.

34


5. Stainless Steel components for use on and around the rotor assembly.

6. Tulip-tree wood (“Sheelanthi”) for the blades, based on the recommen-

dation of the carpenters.

7. Polyester Resin for casting the stator.

6.4 Important Points about the Design

There are some important points about the design that must be kept in mind.

These were further constraints that had to be worked around.

6.4.1 Furling Tail

The decision to include a furling tail in the design as an additional saftey

device dictated some of the design, to accomodate for it. The tail, in normal

operation, sits downwind of the turbine, and is subjected to buffetting by the

vortex induced in the air by the blades. By an obscure interaction between

the blades, the wind, and the tail, the tail furls out of the way in high speeds

only when the turbine is subject to proper loading conditions. Furthermore, the

dimensioning of the tail and salient parts of the wind turbine are defined by

the following rules of thumb:

1. The horizontal offset of the horizontal turbine axis from the vertical yaw

axis should be one twelth of the blade length.

2. The tail should have surface area equal to about 7% of the turbine’s

swept area.

3. The length of the boom should be about the same as that of a blade.

4. The weight of the tail can be adjusted to fine tune the furling windspeed.

5. The tail, being in the vortex trail, is subject to extreme vibration and

cyclic loading. Therefore wood or a similar damping material should be

used.

35


6.5 3D Modelling in Pro-Engineer

As a first step, the planned project was roughly drafted by hand, and the

drafts were transferred to Pro-Engineer Wildfire 4.0 (Educational Edition)

over a three week period. The resulting three dimensional model enabled the

visualization of the assembly process, and precipitated several design changes

to increase modularity, and to decrease weight, cost, and complexity.

Figure 6.1: 3D rendering of the top of the proposed design, final iteration,from Pro-E.

6.6 Manufacturing the Components

The final draft with plan, elevation, and profile outline views was created and

exported to Adobe’s PDF file format, and printed on A3 paper. Copies of these

plans were given to Babu at Super Engineering Works (metals fabrication shop

in Karunagappally), and the parts, with minimal modification to the plans,

were manufactured accordingly. Copies of these plans have been included here,

in appendix D.

Stainless steel electrodes were used to weld the stainless steel blade hub.

All welding was done using arc welding. Some difficulties were encountered in

the manufacturing process, since the plates purchased were not perfectly flat.

They required extensive flattening proceedures with a sledge hammer.

36


The plans for fabricating the furling tail, yaw mount, and the tower for

erecting the turbine have been supplied as part of this project.

6.7 Conclusion

The mechanical design was completed and used as a basis for the actual manu-

facturing process. Last minute modifications to the design while manufacturing

were minimal, proving the usefulness of planning ahead and three-dimensional

modelling.

37

7. FAILURE ANALYSIS

7.1 Introduction

Failure analysis is a mathematical method to ensure that the proposed struc-

ture or design is safe. Sufficient research has been done on materials that

accurate results can be obtained by mathematical models, and the safety or

robustness of a design can be predicted with accuracy and precision.

Failure analysis was carried out on components identified to be critical and

susceptible to failure under normal and stressed use conditions. Data and

equations were taken from [25].

7.2 Buckling Failure of Tower

Figure 7.1: The equivalent model for the tower in buckling.

The tower is represented by a slender beam, fixed vertically at both ends,

with an eccentric axial load applied at the top. Assuming two ends to be fixed

is accurate, as the bottom, though pivoting on a fulcrum pin, is locked into

the vertical position, and the top is constrained in horizontal motion by guy

wires. It can be modelled as shown in figure 7.1. The failure analysis varies

based on the slenderness ratio, calculated as the ratio of length over radius.

38


Slenderness Ratio =L

r(7.1)

where,

L is the Lenght of the pipe (L = 9m) , and

r is the outer radius of the pipe (r = 6inches = 15.24cm.

Therefore, the slenderness ratio assumes a value of 90015

= 40. Using this

value as a reference, page 6.8 of [25] gives the following formula to calculate

the buckling load:

Pc =aσy

1 + eyr2

sec(Le

2r

√Pc

aE

) (7.2)

where,

Pc is the buckling load,

a is the cross-sectional area of the pipe,

σy is the yield strength of Galvanised Iron,

e is the eccentricity of the load to the tower axis,

y is the maximum fibre distance from the neutral axis,

Le is the effective length of the tower for buckling analysis, and

E is the Young’s Modulus of Galvanised Iron.

In this failure analysis, the end constraints dictate that the effective length

Le = 0.5× L. Thus, we find the following values:

a = 47.1cm2

σy = 3800kgf/cm2

e = 25cm

y = 15.24cm

Le = 4.5m = 450cm

E = 2.06× 105N/mm2

Using these values, the maximum load is found to be:

Pc = 11, 724.928kgf

Taking a factor of safety of 10 for the tower, the design max load is calcu-

lated to be:

[Pc] = 1, 172kgf

39


Analysing the proposed design, it is found that the maximum mass of the

wind turbine, yaw mechanism, PMG, and furling tail put together is not more

than 20 kg, corresponding to a load of 20kgf. Thus the tower, under buckling

analysis, is deemed to be safe.

7.3 Shear Failure of Rotor Bolts

The rotor bolts are made of 12mm fully threaded stainless steel rod. They are

fixed at one end to the hub with bolts, and are free at the other end. Each

bolt is 20 centimeters long, and four such bolts are employed to fix the blades,

blade hub, rotor blank, and rotor disk to the main hub. The blade assembly

rests at the farthest point from the fixed end. To fix the bolts, they are put

under tensile stress by two nuts.

For this analysis, it was assumed that only one bolt was present, and was

taking the entire load by itself. This analysis ensures that, should three of the

four bolts become loose, the turbine will still remain safe, though perhaps not

operational. Additionally, an inherent factor of safety of four is automatically

included, over and above other factors.

Figure 7.2: The free body diagram for the bolt.

The first task is to calculate the reaction forces and moments on the bolt.

The reaction force at the support is found to be FR = 43.5N , and the reaction

moment is found to be MR = 5.885Nm. These forces are shown in the free

body diagram, figure 7.2.

The area of the bolt, not including the threads, comes to 1.57× 10−4.

40


Calculating stresses from these values, we have τx = 0.55385N/mm2. Ap-

proximating the tensile force due to tightening the bolts to be around 10 kgf,

we find σx = 0.6366N/mm2.

The stress tensor is representated as:

[σ] =

σx τxy τxz

τyx σy τyz

τzx τzy σz

Since, in this analysis, there are no stress components in the z direction,

this matrix simplifies to the following:

[σ] =

[1.23 0.553859

0.553859 0

]To find the principle stresses, the following formula is used:

σ1,2 =1

2

(σx ±

√σ2x + 4τ 2

xy

)(7.3)

Using this equation, we find the principle stresses as:

σ1 = 1.4776N/mm2, σ2 = −0.2076N/mm2

The Von Mises-Henry failure stress theory has been selected, due to it’s

enhanced precision compared to other failure theories. This theory states that:

σ21 + σ2

2 − σ2σ2 = σ2y (7.4)

Using the principle stesses obtained above, the maximum stress applied to

the bolt is found to be:

σmax = 1.591587N/mm2

The strength of the bolt is found after acknowledging that the bolt is under

a completely reversed cyclic load, when the machine is in operation. Therefore,

the fatigue strength of the bolt must be the deciding factor.

The fatigue strength for the bolt is taken from page 7.7 of [25], and found

to be, for infinite life, [σ] = 500kgf/cm2 = 50N/mm2.

Since σmax < [σ], the configuration is deemed to be safe. The effective

safety factor here is more than 100, when considering the four bolts.

41


7.4 Shear Failure of the Fulcrum Pin

The fulcrum pin bears the entire weight of the wind turbine plus the vertical

component of the tension in the guy wires, in a double shear configuration,

between the ground support and the tower pipe. Should this component fail,

the entire tower will crash, most likely destroying the turbine and possibly

causing injury to people. Thus it is extremely important that this fulcrum pin

not fail.

For this analysis, it has been assumed that the fulcrum pin is undergoing

only single shear. This assumption inherently adds a saftey factor of 2.

The area of the pin, as per the proposed design, is A = 314.159mm2. This

area must distribute a stress induced by the entire weight of the tower and

turbine. This weight is estimated to be under 105kg. Taking 105kg, the shear

stress is found by:

τ =F

A=

105× 10

314.159= 3.34225N/mm2 (7.5)

The allowable stress for mild steel (C35) is obtained from [25], and found

to be [σy] = 500N/mm2. This gives:

[τ ] = 0.5[σy] = 250N/mm2 (7.6)

Thus it is shown that τmax < [τ ], which means that the fulcrum pin is safe

from shear failure.

7.5 Crushing Failure of Tower

The entire weight of the tower and turbine is passed through the walls of the

tower to the pin. There is a chance that the fulcrum pin hole may enlarge itself

due to the supporting part of the tower being crushed under its own weight.

For the purpose of analysis, the crushing area was taken as a rectangle of

dimensions equal to the thickness of the pipe and the diameter of the hole.

Since the pin passes through the pipe on both sides, the equivalent crushing

area is twice the area of one rectangle.

42


Area under crushing stress = 2× t× d = 2× 5× 20 = 200mm2

Finding the crushing stress,

σc =Force

Area=

1050

200= 5.25N/mm2

This stress is found to be well within the allowable limits, giving an effective

factor of safety of 70.

7.6 Shear Failure at Blade Root

The blades are perhaps the most delicate part of the entire turbine. They are

also under significant stress, due to their rotational motion. One way they can

fail is that the wood may shear, leaving the blade anchor bolt attached to the

turbine, while the main part of the blade flies off. The shear planes are shown

in figure 7.3.

Figure 7.3: The shear planes for the blade failure analysis.

The shear in this case would take place parallel to the grain. Shear strenght

of Sheelanthi wood parallel to the blade is estimated at 30N/mm2. Assuming

that the blade shears in two rectangular planes on either side of the bolt, this

gives a total shear area of:

Shear Area = 2× 60× 40 = 4800mm2

At windspeeds of 16m/s (the maximum encountered), the blade tips will

have a velocity of:

Vtip = λD × 16 = 96m/s

43


This corresponds to a rotational velocity of ω = 101.035rad/s. The cen-

trifugal force on the blade, assuming the mass of the blade is concentrated at

the tip (it is actually concentrated at the root) is found to be:

F = mrω2 = 0.75× 0.95× 101.0352 = 72.76kN (7.7)

This force produces a shear stress of τ = 15.16N/mm2, giving a total factor

of safety of two.

7.7 Tensile Failure of Blades

Another way that the blades can fail is by cracking and breaking, most likely

at the second section, due to the tensile stress developed from the centrifugal

force.

The area on which this tensile force would act is the area of the second

cross-section, or the area of the MH104 profile with a 70mm chord. This are

is approximatly 350mm2.

Figure 7.4: The place where tensile failure of the blades is most likely.

The centrifugal force would be developed due to the mass of the blade in

sections 3, 4, and 5. This is generously estimated to be 0.5kg. Using equation

(7.7), the centrifugal force is calculated to be F = 4877N , which corresponds

to a tensile stress of σ = 13.933N/mm2.

Given that Sheelanthi has an approximate tensile strength parallel to the

grain of 90N/mm2, the factor of saftey is found to be more than six.

44


7.8 Crushing Failure at Blade Root

Besides impact with some external body, there is only one other way that the

blades can fail. That is that the wood between the embedded nut and the

external nut may get compressed or crushed, causing the nuts to loosen, and

possibly causing the blade to become loose.

This is actually to be expected, and can be remedied by periodic servicing

and tightening of the external nut. The crushing strength of the wood will

increase as it is compressed, and the situation will stabilize after a few months

of use.

7.9 Conclusion

The failure analysis carried out on the proposed design has proven it’s robust-

ness and fitness for implementation. Each component has been designed with

a factor of safety over and above nominal values, and the entire structure has

been found to be safe.

45

8. TESTS ON VARIOUS GENERATOR TYPES

8.1 Introduction

There are two categories of machines that are used to create electrical power

from shaft power. These are generators and alternators.

8.1.1 Generators

We know that moving coils next to magnets can be used to change kinetic

energy from a spinning shaft into electrical energy in a wire. The devices

used for these are called generators and alternators. The magnetic field can

be either provided from permanent magnets or electromagnets. Either the

coil of wire or the magnet can be the moving part while the other sits still.

Generators (also known as dynamos) make direct current (DC) and alternators

make alternating current (AC).

8.1.2 Alternators

DC generators are used to charge batteries in a remote home. Batteries can

be charged only using DC, and generators make it. DC generators are much

more complicated to design and build than AC alternators. The electricity

flow is changing in direction along with the changing magnetic fields. So, gen-

erators use a device called a commutator to keep the energy flowing in only

one direction. This is a mechanical rotating switch and is not easy to build.

Most generators use electromagnets to make the magnetic field for generat-

ing electricity. Modern DC generators also use permanent magnet instead of

electromagnets but a commutator is still needed to keep the energy flow in

unidirectional.

46


8.2 Wind Turbine Alternators

In a wind turbine smooth power is not produced. Usually wind turbine manu-

facturers build their own alternators to match the power curve of their blades.

DC generators are hardly used along with wind turbines. For good results, the

output power curve of the alternator has to be matched with the input power

curve from the blades and the spinning shaft. The match is fairly simple to

accomplished can be very efficient when the shaft rpm and the torque are con-

stant. However, wind speeds are not constant, so matching the wind speeds

will be a compromise. The range of rpm and torque input from the spinning

shaft that the the alternator design will be most efficient must be decided by

the designer.

8.2.1 RPM

It is easier to make electricity from a fast spinning shaft than from a slow

spinnig shaft since the output power is directly proportional to the speed.

Even though the low rpm shaft carries the same amount of kinetic energy,

more of it will be in torque than in speed. An alternator to make electricity

from a low rpm shaft will be larger and more expensive than its high rpm

counterpart. Thus rpm plays an important role when the design of a wind

electric generator is to be done.

8.2.2 Permanent Magnets v/s Electromagets

When alternators with electromagets are used, a part of the power output goes

to them to provide the electric field. The flow of current to the load takes place

only after the electromagnets are magnetized. Also brushes or slip rings are

required to take the electricity produced by the armature windings. Another

problem with the alternators using electromagnets is that when they fail during

the operation the wind turbine will be free spinning, with nothing to extract

the energy coming in. Such a condition can be dangerous and can even lead

to the total failure of the system. Therefore permanent magnet generators are

best suited for low speed applications like wind systems.

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8.3 Experimental Analysis of Different Generators

The fact that permanent magnets are more efficient than the electromagnets

was ascertained by conducting various experiments on different types of gen-

erators and motors.

8.3.1 Experiment on a Universal Motor

Universal motors are unique motors that can be run using both AC and DC

supply. They are most commonly found in mixers. A universal motor when

given a field excitation will run as a generator. To test this aspect of the motor

an experiment was done where we used an external DC supply to produce a

magnetic field within the universal motor. An induction motor connected to an

autotransformer was used as the prime mover. The autotransformer is used to

vary the speed of the induction motor. The rotor was rotated using the prime

mover at various speeds and the output was observed through an oscilloscope.

8.3.1.1 Inference

• The universal motor was made to work as a generator and the various

voltage and current magnitudes at various speeds where studied.

• The Universal Motor produces voltage when an external excitation is

applied to the field.

• The output voltage increases when the field current increases.

• This system is not suited for electricity generation from the wind.

8.3.2 Experiment on Squirrel-Cage Induction Motor

The idea of this experiment was to convert a squirrel cage induction motor into

an induction generator and to study the output produced at various speeds. A

420rpm squirrel cage induction motor was acquired and converted to a gener-

ator to observe the power generated by it at various speeds. A 2HP induction

motor connected to an autotransformer was used as the prime mover for this

48


Figure 8.1: Experimental setup to test a squirrel cage induction motor.

experiment. The squirrel cage motor was coupled with the prime mover on a

work bench and the output wires connected to an oscilloscope. Ideally, induc-

tion motors behave as generators when the rotor is rotated at a speed greater

than their synchronous speeds. When the set up was run at a speed greater

than 420rpm without using any external excitation, the squirrel cage motor

produced very little voltage. But when an external voltage of 30V was applied

to the windings and the rotor rotated at a speed greater than 420rpm, an

output of about 90V was obtained. The experimental setup is shown in figure

8.1.

The voltage gain was later observed to be due to transformer action as the

squirrel cage motor was a dual speed motor with two sets of running windings

and thus the voltage, ie the excitation voltage, on the high speed winding

was induced in the slow speed running winding from which the output was

observed.

8.3.2.1 Inference

Squirrel cage induction motors are not suitable as wind electric generators

as external excitation is required. For standalone systems producing limited

power they are not the ideal generators.

49


Figure 8.2: Magnets placed on the rotor of a squirrel cage induction motor.

8.3.3 Conversion of Induction Motor to PMG

The idea of this experiment was to convert a squirrel cage induction motor

into a permanent magnet generator and study the output at various speeds.

An exhaust fan motor (squirrel cage induction motor) was acquired and

slots were cut into the rotor of the motor to hold magnets. In an ongoing

effort to reduce environmental impact by reusing materials, these magnets were

sourced from old hard disks. Hard Disk magnets are neodymium magnets and

are very powerful. They were mounted on the rotor of the fan motor.

The converted PMG, when rotated using the prime mover, produced volt-

age at even very low speeds. The magnets were placed in such a way that the

rotor and the stator magnetic field do not get locked and get cogged. Cogging

occurs when the stator poles and rotor poles are equal in number or are integer

multiples of each other. The stator had four poles and thus the magnets where

arranged in a way as to give ten poles on the rotor. The setup is seen in figure

8.2.

The advantages of using such a generator is:

• Voltage is produced even at very low speeds.

• External excitation is not required to produce power.

8.3.3.1 Open Circuit Test on the PMG

The PMG was run using a 2HP motor and the output was observed at various

speeds. The PMG produced upto 50 V at a speed of 800 rpm. Even at low

speeds of 100 or 200rpm the PMG was capable of producing power. The values

of the open circuit voltages for different speeds are measured and recorded.

50


These results were used to plot a graph which gives the relationship between

speed and the open circuit voltage.

The open circuit is done by running the PMG at different speeds without

any load connected to it. Thus it gives the maximum voltage that it can

produce without the load. The speed, which is a major part of the system is

what determines the voltage.

The measurement of voltage can be done using multimeters or voltmeters.

Since no load is connected, the actual voltage produced at the output is ob-

tained through this one experiment. The results are shown in figure 8.3.

Figure 8.3: Voltage versus RPM test on PMG.

The graph shows the variation of output voltage with speed. From the

graph it is seen that the output voltage increases with increase in speed of the

prime mover. Thus the power output from the generator is directly dependent

on the speed of the prime mover.

8.3.3.2 Load Test on the PMG

The PMG was loaded using a 50 ohm rheostat and a 1K resistor. The speed

was kept constant at 220rpm throughout the test and the values of load current

were tabulated to draw the graph. As seen from the graph the load current

decreases as the load decreases (voltage increases). The results are shown in

51


figure 8.4.

Figure 8.4: Load test on PMG.

8.4 Conclusion

Experiments were done on various motors to convert them into generators

and check their performance for application in this project. The universal

motor and induction generator were found to be unsuitable, as they require a

field excitation for producing voltage. The conversion of induction motor to

permanent magnet motor yields greater power at lower speeds when compared

to the other options. Thus it has been concluded that permanent magnet

alternators are best suited for wind energy applications.

52

9. DESIGN AND CONSTRUCTION OF THE

PERMANENT MAGNET GENERATOR

9.1 Introduction

Permanent magnet generators are the most effecitive to be used in wind electric

systems. This chapter decribes the design and construction of the axial flux

permanent magnet generator used in this project.

9.2 Rotor

The rotor is the rotating part of the generator. It contains the permanent

magnets, which are the source of flux for power generation.

9.2.1 Design of the Rotor

There are two magnetic rotors in the axial flux PMG, with magnets placed

on one of them, the other left blank. The rotor discs are made of plates of

mild steel. Mild steel is chosen because ferrous metals concentrate and channel

magnetic fields, and the rotors are placed such that the flux is concentrated

and channeled into the space where the coils are placed, producing maximum

flux linkage. If the magnets for the rotor are mounted on wood instead of steel

plates, the power output would be less than half of the power from steel magnet

rotors at any rpm. The discs are 6mm thick with a diameter of 150mm.

The magnets used are neodymium-iron-boron magnets (rare earth mag-

nets) 2 inches long, length, 1 inch wide and 1/2 an inch thick. The magnets

are face polarized, and are positioned on the rotor so as to alternate the north

and south poles. The 12 poles are placed at 30 degree intervals around the

53


Figure 9.1: Neodymium magnets.

disk. The magnets positioned using a template cut from plywood. The tem-

plate is a 12 inch diameter disc, with four 12mm holes to match those of the

rotor disk, and 12 equally spaced cutouts in which to place the magnets.

9.2.2 Neodymium Magnets

Neodymium magnets are the strongest permanent magnets available today.

They are made from rare earth elements, and are therefore sometimes referred

to as “rare earth magnets.” The strength of these magnets can be very dan-

gerous if handled improperly.

9.2.2.1 Identifying the Poles of Neodymium Magnets

There are 3 different ways in which the poles of a magnet can be identified.

• Using a simple directional compass, the north and south poles of the

magnet can be easily found out.

• The simplest way is to use another Neodymium magnet that is already

marked. By virtue of magnetic properties, the North Pole of the marked

magnet will repel the North Pole of the unmarked Neodymium magnet

and attract the South Pole.

• Dangling a magnet by a string so that it is free to rotate will cause the

north pole of the magnet to point south (being attracted to the Earth’s

south pole).

54


9.2.2.2 Attributes of Neodymium Magnets

There are several attributes of Neodymium magnets that differentiate them

from other magnets.

• Neodymium magnets have a very high resistance to demagnetization.

This makes them very useful in many different kinds of industrial appli-

cations.

• They work in ambient temperature conditions, and do not require special

handling.

• Neodymium magnets, unlike other high strength magnets such as tiny

cobalt-samarium magnets, have a high flux-to-cost ratio, making them

cost effective for experimenters with limited budgets.

• Neodymium magnets are easily corroded and hence have to be handled

carefully. They are supplied with a coating of chromium for protection

from the elements.

• Neodymium magnets demagnetize at around 80C, and the dust made

from cutting them is highly inflammable.

9.2.3 Placing the magnets

The plywood template is put on one of the steel discs and the 1/2 inch holes

are lined up. The screws on the rotor are adjustable so that the air gap can

be changed. The output voltage varies with change in the air gap.

Figure 9.2: Template placed on the rotor disc.

55


The magnets were then placed in the slots of the template, and stuck to

the rotor with the help of Araldite, an epoxy adhesive. This is shown in figure

9.3. The rotor disc after placing the magnets is shown in figure 9.5.

Figure 9.3: Placing the magnets on the rotor disc.

Figure 9.4: Workbench.

9.2.4 Preparing the Mould

The design used in this project employs fibre glass cloth to help strengthen the

castings. Two rings were cut from the fibre glass mat with diameter slightly

56


Figure 9.5: The finished rotor disc.

greater than that of the rotor. The interior of the mould was coated with

mould release compound (automotive grease). This facilitates the removal of

the finished casting. A bead of caulk was run around the interior seams of the

mould, to protect against leakage.

9.2.5 Casting the Rotor

Polyester resin was used for casting the rotor. The resin was first mixed with

an accelerator (a cobalt solution) and silica powder. The rotor was then placed

inside the mould, and a plywood core was placed in the center. Just before

pouring the resin, adequate hardener (methyl ethyl ketone peroxide or MEKP)

was added and the resin was mixed thoroughly. Then the resin was poured

into the mould and a fibre glass cloth placed on top of the casting as shown

in figure 9.6. The fibre glass cloth enhances the strength of the casting. Resin

was again poured over the magnets till the whole mould was filled. The whole

set up was then covered with a wooden plate and tightened with screws. It

was left to cure for about 12 hours.

After the resin was completely set up, the lid from the mould was removed

and the rotor cast is taken outside as shown in figure 9.7. The edges of the

casting were then smoothened and cleaned with a file and hacksaw blade.

9.3 Stator

The stator is the stationary part of the alternator, which contains the coils. It

has been configured to work as a 3 phase alternator, in which the wiring can

be switched from star to delta at ease.

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Figure 9.6: Casting the rotor.

Figure 9.7: The casted rotor.

Each phase consists of three coils in series, with both ends exposed via

terminal bolts that protrude outside the casting.

9.3.1 Winding the Coils

The stator is a casting that contains and protects all nine coils of wire across

which the magnets spin. It is cast using the same resin as the rotor.

The template for the stator coil winding was designed according to the size

of the magnets. The stator coils were wound with 19 gauge copper wire, and

are each 5cm by 7cm in size. Each coil has 40 turns. The number of turns was

chosen such that the coil width does not exceed the width of the magnet. The

shape of the coil was adjusted such that maximum flux lines from the magnets

cut the stator coils when the rotor rotates, creating maximum emf the stator

windings. The stator windings are shown in figure 9.8.

9.3.2 Preparing the Mould

The mould for the stator was constructed out of layers of plywood. The mould

has an outer radius of 175 mm with a core at the centre of 75mm radius. The

58


Figure 9.8: Stator windings.

Figure 9.9: The rotor and stator moulds.

core creates a hole where the rotor hub will fit and rotate relative to the stator.

The mould was greased and all joints were sealed with silicone caulk, as

was the rotor mould.

Six holes were drilled in the mould lid to accomodate the six terminals

bolts (two from each phase) from which the outputs are taken. The moulds

are shown in figure 9.9.

9.3.3 Casting the Stator

After placing the stator coils into the mould, the resin mixed with the hardener

is poured into the mould and is allowed to cure for 12 hours. The casted stator

is shown in figure 9.10. After the resin has hardened the stator cast containing

the stator coils with the six terminals is taken out and filed to bring it to a

proper shape.

59


Figure 9.10: The casted stator.

Figure 9.11: Coils placed in the stator mould.

9.3.4 Connecting the Stator Coils

The stator coils were placed inside the mould with the three coils of each

phase connected in series placed 120 degrees apart as shown. The ends of

the coils from each phase were connected to the terminals as shown on figure

9.11. These terminals from the three phases can be star or delta connected

to obtain different configurations. The output can also be obtained from each

phase directly. The output voltages obtained from these three phases will have

a phase difference of 120 degrees between them.

Star connections are used for low speed applications, whereas for high

speed, the delta connection is preferred.

9.4 Assembly and Testing of the PMG

The rotor containing the magnets was fixed onto the hub, which carries all

the moving parts via four lengths of all-thread. The stator was bolted directly

onto the spindle weldment which is the base for the whole system. The stator

was carefully brought as close as possible to the magnetic rotor, to reduce the

60


Figure 9.12: The generator on the work bench after assembly.

air gap. The second rotor was lowered very carefully along the bolts to the

other side of the staor. The magnetic forces attract the second rotor to the

magnetic one and thus the alignment must be done very carefully. The whole

setup was adjusted to bring the airgap to a minimum. The experimental set

up is shown in figure 9.12.

9.4.1 Selection of Load

The power from a wind electric generator is not constant and varies with

the wind speeds. So this power cannot be fed into an electrical appliance

directly. Most wind electric generators are attached to asynchronous loads.

The various asynchronous loads include battery banks, remote communication

equipment, cathodic protection for buried pipelines, and direct space heating

or domestic hot water heating applications. The best option which requires

less maintenance is using a lead acid battery as the load. The output from the

wind electric generator is thus fed into the battery which charges the battery.

The battery can be further used for applications like room lighting.

9.4.2 Converting AC to DC

The three-phase AC output from the wind turbine alternator is often called

‘wild AC’ because the frquency and voltage vary with the alternator’s rpm,

and therefore with the wind speed. This wild AC cannot be used to power

61


the electrical appliances, so it is converted to DC and used to charge batteries.

Loads can use the charged battery as a power source. A commomly used power

converter is a three phase bridge rectifier.

Each output wire from the three phases of the alternator are connected to

the full wave bridge rectifier and the DC output leads are directly connected

to the battery through a diode. The diode is connected to prevent the back

flow of current whhen the battery is not being charged.

9.4.3 Testing of the Generator

Various tests were conducted to determine the available output, the efficiency

and the loading charateristics of the generator.

9.4.3.1 Speed v/s Output Voltage

The generator was run and the output voltage at various speeds were noted.

A voltmeter was connected across the output terminals. The results are shown

in figure 9.13.

Figure 9.13: Open circuit voltage versus windspeed.

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9.4.3.2 Load Test

The output terminals of the generator was loaded using a 55Ω rheostat and run

at a speed of 250rpm (corresponding to a windspeed of 4.17m/s). An ammeter

was connected in series with the load so as the get the load current flowing

through the circuit. A voltmeter was connected in parallel with the load and

the voltage at various loads were noted. The results are shown in figure 9.14.

Figure 9.14: Load test at 4.17 m/s equivalent windspeed.

9.4.3.3 Efficiency

As mentioned in section 3.9.2 the maximum power that can be derived by the

wind turbine blades from the wind is only 59.26% of the available power in

the wind. The power is even further lost in the rotating parts and in the shaft

connecting the blades to the generator. The maximum efficiency attained so

far by modern wind electric systems are about 45%. From the results of these

tests, it is estimated that this project has achieved almost 30%.

Available power developed by the blades at 4.1m/s = 50W

Power generated with a load of 55Ω = 14W

Efficiency of the system = 1450

= 28%

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9.5 Conclusion

The wind electric generator was designed as per the requirements of the site

and constructed to obtain a considerable amount of power. The system was

also tested under no load and full load conditions to study the performance

and the efficiency of the system was found to be 28%. Scope exists for further

modification, to improve the generator performance. Some suggestions are

given in section 10.2.

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10. CONCLUSIONS

10.1 Project Synopsis

This project, the design and construction of a stand-alone micro wind electric

generator, has been a learning experience. Due to the able help and guidance of

all involved, no critical mistakes were made, and the project has been executed

without problems.

As part of this project, a wind resource assessment was carried out in the

Amritapuri Ashram for six months. The wind resources were found to indicate

favourable conditions for the installation of wind electricity generation systems,

but the short duration necessitates that the results be labelled as inconclusive.

The authors recommend that a conclusive wind study be undertaken, and that

Amritapuri begin to generate its electricity from the abundantly available free

and clean resources.

Blade design was found to be a vast subject in itself, and therefore a deep

study of blade design was not undertaken. Sufficient knowledge was gained,

however, to enable the authors to design the blades, and giving them an ad-

vantage in the subject for the future. The blades for the wind turbine were

dimensioned using Schmitz algorithms, and were created by carving them out

of wood. The blades were designed to incorporate stall control at high wind

speeds, as part of the redundant saftey control scheme.

The mechanical design was created, based on existing designs and unique

objectives for this project. Three dimensional models were created of each

part and assembled in Pro-Engineer. This process enabled the visualization

of the actual assembly process, and the evaluation of the design. In all, four

iterations of the design were created, the final iteration being selected for

manufacture. The drafts produced from the 3D models were used as a basis

for manufacturing the components.

65


Designing and manufacturing the alternator was a very intensive process.

Plywood for the moulds were salvaged as scrap from a construction site in

Karunagappaly. Having no prior experience in resin casting, test casts were

made to understand the casting process. The results of the test castings

were very encouraging; they were found to be practically indestructable. The

moulds were created at Achu Furniture, borrowing their tools and workspace.

Casting of the stator and the rotor was done successfully, and the parts were

trimmed and cleaned.

Failure analysis was carried out on parts which were found to be under

excessive stress, and the design was found to be safe from all angles. The

blades will require re-tightening periodically while the wood stabilizes, for the

first few months of operation.

This project was undertaken to learn about and implement a small scale

renewable energy system. It is the hope of the authors that this will prove

fruitful now and in the future.

10.2 Future Projects based on this Platform

The proposed design for the Stand Alone Micro Wind Electric Generator is

intended to serve as a base for further projects and research. It is envisioned

that this project will create a Wind Laboratory, for the exploration of renew-

able energy. The proposed design in this project uses a tilting tower, which will

enable two students to raise and lower the turbine for alterations frequently

and safely. Some suggestions are given below.

10.2.1 Doubling the Magnetic Flux

In the current configuration, the PMG has one rotor with magnets, while the

other is blank. By placing magnets on the other rotor as well, the magnetic

flux linkage will be increased many-fold. The magnets must be arranged on

the second rotor to be opposite the ones on the existing rotor, and attracting

poles should be arranged so that the flux is pulled through the stator.

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10.2.2 Creating a New Stator

The stator of the permanent magnet generator can be replaced by another

stator with increased number of turns per coil. The emf induced in the coil

is directly proportional to the number of turns of the coil. Hence the output

voltage of the permanent magnet alternator can be increased by increasing the

number of turns per coil.

A good research project would be to create a number of stators with differ-

ent windings, and researching the wind turbine characteristics with different

stators.

The amperage output from the stator coils can be varied by replacing the

existing stator with a new one having stator coils of different gauges. A coil

wound from #18 copper conductor will have higher current carrying capacity

than a #21 copper conductor. At the same time, a stator incorporating more

turns of a higher guage number will attain operating voltages sooner.

10.2.3 Researching Different Blades

The blades used in this project were optimized for the mean windspeed, and di-

mensioned using Schmitz dimensioning. There are many other types of blades,

and many other profiles that can be used. A good project would be to test dif-

ferent types and sizes of blades and gain practical knowledge of the differences

in blade designs.

10.2.4 Creating a Maximum Power Point Tracker

The MPPT is a setup used to determine the optimum power that can be fed

to the load at a given power available in the wind. The load tries to absorb

maximum power it can take from the generator at all points of time. When

wind speeds are too low the load tries to absorb all the available power. This

makes the blades attached to the rotor stop rotating. This is called ‘stalling’.

In such situations the load should be cut off from the generator. Hence load

should be given to the generator only when the blades rotate above a given

speed. Above the cut off speed the load can be given to the generator till the

67


speed reaches a point where the load cannot take any further voltage. When

the generated voltage exceeds the maximum voltage that can be fed to the

load the supply is cut off and given to a dummy load. When the wind speeds

go dangerously high, the terminals all the three phases of the generator should

be shorted so that the blades stop rotating.

10.2.5 Implementing Active Pitch Control

The blade hub has been manufactured to allow the future inclusion of active

pitch control. A simple servo mechanism and required linkages can be inserted

to pitch the blades based on the rpm of the turbine. Power could be supplied

to the electronics using a small stationary magnet and a simple coil on the

rotor.

Alternatively, centrifugal force based pitch control could also be imple-

mented. The design is very versatile.

10.2.6 Using a Permanent Magnet DC Motor as a Generator

The design being modular, only one new part would need to be fabricated to

accomodate the permanent magnet DC motor, or any other similar generator

with a shaft. In the current design, the rear hub and spindle assembly of the

Maruti Esteem is used. To transfer the power to a spinning shaft, the front

hub assembly (drive hub) would be required, which will accomodate the blades

and blade hub in the existing configuration.

10.2.7 Investigation into the Furling Tail

The proposed design for this project incorporates a furling tail which has been

designed using rules-of-thumb, as given in section 6.4.1. Future students could

investigate the properties of the tail, experiment by varying the weight and

other dimensions, and develop emperical or analytical models. Such research

has been carried out by the National Renewable Energy Laboratories (NREL)

in the USA, and the analytical model is downloadable as software from their

68


website. A correlation between the observed results and the existing software

model would be a worthy project.

10.2.8 System Modelling in Simulink

Simulink (or a PyLab-Works, an open source alternative to Matlab + Labview)

can be used to create a mathematical model of the system. Such mathematical

models are useful for analyzing performance parameters, testing innovations

and modifications, and for adapting the design to different scenarios.

System modelling is a growing field, and is used in increasingly more diverse

and widespread ways.

10.3 A Final Word

The state of our home planet, our only planet, is such that man’s influence on

globally sustaining systems can no longer be ignored: we are biting deep the

hand that feeds us. Through our greed, magnified by the economics of scale

and the legalized exploitation called globalization, our mistakes now have huge

consequences. It remains only to be seen whether we will wake up, or will carry

our greed to the impending grave of our (and many other) species.

In the future, the focus of production will be on locally sourced and locally

consumed goods and services, if we as a human race are to learn to live with

Nature. We must learn to serve our fellow travellers on this spaceship Earth;

the plans and animals alike.

The use of small scale power generation with renewable and clean sources

like wind is becoming more widespread. The establishment of such power

generation will remove communities from the energy gridlock, will encourage

energy savings, and will reduce fossil fuel usage. All of this will help our

continued stay on Blue Planet Earth.

69

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A. BILL OF MATERIALS & COST BREAKDOWN

Item Specification QuantityStainless steel all thread 12mm dia 3mStainless steel nuts 12mm 40Locking nuts 12Locking washers 12Mild steel plates 6mm thick, 150mm radius 2Stainless steel bolt 8mm dia, 4cm long 6Steel band 5mm thick, 30 mm wide, 1.3m long 1Insulated copper wire 19 gauge 1 kgNeodymium magnets 2”× 1”× 1/2” 10Epoxy resin 3.5 kgFiber glass 800 gmSilica Powder 800gmNuts 8mm 36Washers 8mm 72Galvanized iron pipe 6”6m 1Galvanized iron pipe 5”6m 1Plywood 18mm 2× 3 feetPlywood 12mm 3× 4 feetPlywood 6mm 2× 3 feetWood beams 2.5”4”80cm 7Epoxy Paint Rust Proofing, Grey 1 litreThinner 1 litreGrease White 500gm

Table A.1: Bill of Materials

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Item CostEsteem Hub — Second hand 350Esteem Hub — New 1,250Wood for Blades 1,700Plywood for Moulds 315Stainless Steel Hardware 2,900Mild Steel Materials 1,400# 19 Gauge Copper Wire 550Neodymium Magnets 6,000Polyester Resin and Fibre Mat 800Linseed Oil 105Epoxy Paint and Thinner 300Grease 80Calk 140Workshop Fees 3,000Carpenter’s Fees 3,000Miscellanous Expenses 417Total 22,307

Table A.2: Cost Breakdown. At the time of printing, the yaw weldment, tower,tail, and anchor had not yet been created, and so are not included in this costbreakdown.

73

B. DERIVATION OF EQUATIONS RELATED TO

BLADE DIMENSIONING

In this appendix, the equations used for Schmitz dimensioning of the blades,

and Betz limit, are derived, along with a few other formulæ that were found

useful in the course of this project.

Figure B.1 shows the velocity diagrams and angles that are referred to

below.

Figure B.1: The complete velocity diagram, used for Schmitz dimensioning ofthe rotor. In this diagram, αA is the angle of attack, φ is the apparent windangle, v is wind speed, w is apparent wind speed, ur is the velocity of the bladeat this radius, and β is the blade twist from the horizontal.

74


B.1 Power in the Wind

The power available in a fluid flow (the wind) is equal to the rate of flow of

kinetic energy:

KE =1

2mv2

∴ P = E =1

2mv2

now, m = ρAv

=⇒ P =1

2ρAv3

For a system which extracts power from a flow, the power extracted comes

from the reduction in KE, as the fluid flows past the system. The freestream

velocity (v1) is slowed to the downstream velocity v3 after passing through the

wind turbine. This is shown in figure B.2.

Figure B.2: Wind flowing through the wind turbine.

75


Eex =1

2m(v2

1 − v23)

where, m = ρAv2

Assuming, v2 =v1 + v3

2(Averaging)

∴ Eex =1

2ρA

v1 + v3

2(v2

1 − v23)

Which becomes P =1

2ρAv3

1

1

2

[1 +

v3

v1

][1−

(v3

v1

)2]

The value of v3v1

varies with the design; in fact, it is a property of the design,

and remains constant over a range of wind speeds. However, it is not convenient

to accurately calculate or physically measure the value of v3. Therefore, the

term 12

[1 + v3

v1

] [1−

(v3v1

)2]

is represented as a coefficient of performance for

the blades, CP . Therefore, the equation for power extracted from the wind

turbine becomes:

P =1

2ρAv3

1CP (B.1)

B.2 The Betz Limit

Not all the power available in a fluid flow can be extracted. If it was, then

the downstream velocity would be reduced to zero, which means that the wind

entering the system would have nowhere to go, which is a physically impossible

situation.

The Betz limit is the theoretical maximum power that can be extracted

from the wind.

If we plot the CP versus v3v1

, we get the curve shown in figure B.3. This curve

clearly shows that the maximum power extractable is 59.26% of the available

power. Most wind turbines do not come close to this efficiency. Common

efficiencies for any wind turbine range around 20–40%.

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Figure B.3: CP versus v3v1

, showing the best possible value of CP .

B.3 Schmitz Dimensioning Formulæ

To dimension the blade, it is necessary to find the blade twist, and chord

for each section. This is facilitated by deriving relations for these that are

functions of the radial distance from the center of the hub.

Using figure B.1, the velocity of the blade (ur) is found to be given by the

following relation.

ur = rω +∆u

2

Also, the angle of apparent wind to the plane of rotation (φ) can be seen

to be:

φ = 90− γ

Where γ is the angle of apparent wind to axis of rotation.

In the wind velocity diagram, |w1| 6= |w3| due to viscosity and friction losses

as the wind passes through the turbine blades.

77


Analysing for a circular ring element of one blade:

dL = ∆wdm

dm = 2πρv2rdr

dP = dmω

= durω

= dL sin(φ)rω

= ∆wdm sin(phi)rω

Using elementary geometry, it is found that:

w = w1 cos(φ1 − φ)

v2 = w sin(φ) = w1 cos(φ1 − φ) sin(φ)

∴

dP = rωdm∆w sin(phi)

= r2ωρ2πdrw21 sin [2(φ1 − φ)] sin2(φ)

∴ P varies with φ

dP

dφ= 0 =⇒ Maximum Power

These results are used to substitute into other formulæ in this derivation.

The Chord Length

The purpose of Schmitz dimensioning is to maximize the power available from

the blade element. To do this, the equation for elementary power is differen-

tiated and set to zero.

dP

dφ= (r2ωρ2πdrw2

1)−2 cos [2(φ1 − φ)] sin2(φ) + 2 sin [2(φ1 − φ)] sin(φ)

= 0

=⇒ dPdφ

= sin(φ) sin(2φ1 − 3φ) = 0

This means that either:

sin(φ) = 0 =⇒ φ = 0,π

2(ridiculous), or

sin(2φ1 − 3φ) = 0 =⇒ φ =2

3φ1

78


Using this result for φ,

dL = dm∆w

= ρ2πrdrw21 cos(φ1 − φ) sin(φ)× 2w1 sin(φ1 − φ)

= ρ2πrdrw21 sin2

(φ1

3

)cos2

(φ1

3

) (B.2)

From Airfoil theory, the lift dL is generated by a blade of chord cT in

accordance with the equation:

dL =ρ

2w2cTdrCL (B.3)

Equating equations (B.2) and (B.3), we get:

cTCL cos2(φ3

)2

= r2π sin [2(φ1 − φ)] sin(φ)

Thus the formula for chord based on sectional distance from the center of

the hub and the coefficient lift of the section’s airfoil profile:

=⇒ cT (r) =16πr

CLsin2

(φ1

3

)(B.4)

The Blade Twist

Again looking at figure B.1, the following relations are observed:

tan(φ1) =v1

ur=v1

ωr

=⇒ tan(φ1) =λDr

1R

=r

λDR

∴ φ1 = tan−1

(R

λDr

)The blade twist β can also be related as:

β = φ− αA

=2

3φ1 − αA

∴ β =2

3tan−1

(R

λDr

)− αA

(B.5)

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B.4 Blade Tip Velocity w.r.t. Windspeed

Figure B.1 shows the various angles and speeds that the blade experiences

while rotating. The velocity of the blade ur is a function of the radius and the

freestream wind velocity, assuming that the tip speed ratio is maintained. This

assumption is valid when the wind turbine is properly loaded, which should be

100% of the time.

ur =λDr

R× v (B.6)

B.5 Rotor RPM w.r.t. Windspeed

The rotor RPM is directly related to the windspeed via the Tip Speed Ratio.

Note that the wind turbine will not magically spin at an rpm which will main-

tain the TSR. The wind turbine, with no load, will spin at approximatly twice

λD, the design TSR. When it is properly loaded by a tuned MPPT, however,

the wind turbine will slow to the design TSR, which will correspond to the

optimum power conditions.

The analysis and design of the wind turbine blades are carried out assuming

that it is loaded properly, and is rotating at the design TSR.

λD =Tip Velocity (vt)

Incoming Windspeed (v)

∴ vt = λDv

also, vt = Rω

ω =N × 2π

60

=⇒ vt = RN × 2π

60

=⇒ λDv = RRPM × 2π

60

∴ N =60λD2πR

× v (B.7)

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B.6 Reynould’s Number w.r.t. Windspeed and Radius

For using PyXFOIL in viscous mode to analyze the airfoil profile, we must

supply the Reynould’s number. This equation is useful to calculate it:

Re =vc

ν

v = λDv1r

R

∴ Re =λDv1rc

Rν

Since our wind turbine will be relatively close to sea level, we can take ν =

νs = 1.33× 10−5. Thus we get:

Re =λDv1rc

R× 1.33× 10−5(B.8)

B.7 Mach Number w.r.t. Windspeed and Radius

PyXFOIL also accepts the Mach number, if available, for additional consider-

ation, in the viscous solution.

MA =v√

1.4RgT

To avoid confusion with the radius R, the Gas Constant has been represented

as Rg. It’s value is Rg = 289. Temperature has been assumed to be constant

at 300K. Since v = λDv1rR

, this gives us:

MA =λDv1r

R× 347.189(B.9)

B.8 Summary

Equations (B.4) and (B.5) are the main equations used in Schmitz dimension-

ing. By selecting an appropriate angle of attack and obtaining the correspond-

ing coefficient of lift for each airfoil section, the required chord and blade twist

can be calculated for maximum power conditions.

All the equations contained in this chapter have been found to be indis-

pensable in the course of this project work.

81

C. THE MH100 FAMILY OF AIRFOILS

C.1 Introduction

The MH100 family of airfoils was developed by Mr. Martin Hepperle, and

are distributed for non-commercial purposes via his website, http://www.

mh-aerotools.de/airfoils/index.htm. These airfoils were developed specif-

ically for the purpose of creating a stall-controlled wind turbine, and were

adopted for the same purpose in this project.

The airfoils were analyzed in XFOIL, for conditions prevailing at each sec-

tion.

Figure C.1: The five airfoils of the MH100 family superimposed using thecalculated values of chord and twist.

82


C.2 MH102

The MH102 airfoil is meant to be used at the blade root. This analysis was

carried out with Re = 65915 and Ma = 0.0126.

The results selected αA = 1.7, corresponding to CL = 0.5148 and LD

=

15.52.

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C.3 MH104

The MH104 airfoil is meant to be used at a position 40% of the radius from

the hub. This analysis was carried out with Re = 142375 and Ma = 0.0126.


=

45.28.

84


C.4 MH106




=

51.68.

85


C.5 MH108




=

38.41.

86


C.6 MH110

The MH110 airfoil is meant to be used at the tip of the blade. This analysis

was carried out with Re = 116105 and Ma = 0.091.

The results selected αA = 9, corresponding to CL = 0.9638 and LD

= 43.39.

87

D. DRAFTS OF MECHANICAL PARTS

The following pages contain drafts which were used as blueprints for the

construction of the Wind Turbine. Most of the drawings were generated in

Pro-Engineer using the PDF export option, and were remastered in Inkscape

to fit in this report. The plots of the Airfoils were created using PlotFoil, an

open source package written by Shamim P. Mohamed, licensed under the GNU

General Public License.

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Figure D.1: The draft of the profiles used in the blade.

89


Figure D.2: Draft showing the end view of the blades, and the placement ofairfoil sections.

90


Figure D.3: The Blade Rotor, to which the blades are attached. Note thatspace has been provided to allow for future inclusion of active pitch control.

91


Figure D.4: The Magnetic Rotor disk. Two such disks were made.

92


Figure D.5: The PMG Stator. This is a resin casting with copper coils con-tained inside it.

93


Figure D.6: The rear wheel hub of a Maruti Suzuki Esteem. This hub waschosen as the basis for our design.

94


Figure D.7: The Spindle Weldment. The spindle is from hardened steel, a partof the Maruti Suzuki Esteem rear hub assembly.

95


Figure D.8: The Yaw Mount Weldment. The smaller inclined rod is for thetail to swing on.

96


Figure D.9: The Furling Tail.

97


Figure D.10: The Tower. The tower is designed to tilt down on the fulcrumpin.

98


Figure D.11: The Ground Anchor for the Tower. One side is open to enablethe tower to tilt down.

99


Figure D.12: The Fulcrum Pin. This pin must bear the entire weight of thestructure, and the vertical component of the tension in the guy wires.

100

Stand Alone Micro Wind Electric Generator

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