DESIGN AND POWER CHARACTERIZATION OF A SMALL WIND TURBINE MODEL IN PARTIAL LOAD

REGION

by

Abdulkarim Abdulrazek

A thesis submitted in partial fulfillment of the requirements for the degree of

Master of Science in Engineering Renewable Energy and Energy Efficiency for the MENA

Region (REMENA)

University of Kassel and Cairo University

Feb 2012

Supervised by: Prof. Dr. sc. techn. Dirk Dahlhaus, Faculty of Electrical Engineering/Computer Science, University of Kassel. Prof. Dr. Joachim Peinke, Faculty of Physics, Carl von Ossietzky University Oldenburg. Dr. Michael Hölling, Faculty of Physics, Carl von Ossietzky University Oldenburg. Dr. Basman El-Hadidi, Aerospace Engineering, Cairo University.

i

ABSTRACT

In this thesis a small wind turbine model was constructed with variable speed and pitch

control in order to investigate the effect of this control strategy on wind turbines by

characterizing its design and output power.

The small wind turbine model components were mentioned with specifications and

functions in addition to description of the LabView program.

The model was experimented in the wind tunnel exists in Oldenburg University where a

testing procedure was followed. The procedure included the way how to measure the

output voltages and currents from the generator at different wind speeds and how wiring

should be connected.

This fruitful work resulted in drawing the power and c – λ curves. Discussions of the

obtained results were made and several factors were listed. These factors, such as friction,

affected the model’s performance. These discoveries helped in proposing useful

modifications for the system in order to enhance it.

ii

TABLE OF CONTENT

Abstract .............................................................................................................................. i

Table of Content ................................................................................................................ ii

List of Figures ................................................................................................................... iv

List of Tables ...................................................................................................................... v

Acknowledgement ........................................................................................................... vi

Nomenclature ................................................................................................................. vii

Introduction ...................................................................................................................... 1

Basic Theory ...................................................................................................................... 5

Design and Setup............................................................................................................... 8

Rotor .............................................................................................................................. 9

Blades ............................................................................................................................ 9

Generator .................................................................................................................... 10

Stepper Motor ............................................................................................................. 11

Blade Pitch Mechanism ............................................................................................... 12

Main Circuit ................................................................................................................. 14

Connections ............................................................................................................. 15

Shunt Resistor ......................................................................................................... 15

Variable resistor ...................................................................................................... 15

Analog to Digital Converter (ADC) ........................................................................... 16

LABVIEW program ................................................................................................... 16

General Specifications ................................................................................................. 17

Experimental Setup ..................................................................................................... 19

Characterization .............................................................................................................. 21

Procedure .................................................................................................................... 21

Optimum Pitch Angle .................................................................................................. 25

Power Curve ................................................................................................................ 27

Cp – λ curve ................................................................................................................. 28

Limitation Factors ........................................................................................................ 30

Friction .................................................................................................................... 30

Small Modifications ................................................................................................. 31

Generator Losses ..................................................................................................... 31

iii

Manufacturing and Installation of blades ............................................................... 32

Angle of Attack (AoA) .............................................................................................. 32

Conclusion and Future Improvement ............................................................................. 36

Future Improvements ................................................................................................. 36

Appendix A ...................................................................................................................... 38

Power coefficient and resistance at different wind speeds ........................................ 38

Power coefficient and pitch angle at different wind speeds ...................................... 39

Appendix B ...................................................................................................................... 40

Appendix C ...................................................................................................................... 41

Voltage Divider Circuit ................................................................................................ 41

Design Formulas .......................................................................................................... 41

Bibliography .................................................................................................................... 42

iv

LIST OF FIGURES

Figure 1.1- Total installed capacity of wind energy worldwide from 2000 to 2011* in

megawatts [1] ................................................................................................................... 1

Figure 1.2- Number of jobs in the wind energy industry worldwide from 2005 to 2012

[4] ...................................................................................................................................... 2

Figure 2.1- Power coefficient for different wind turbine types [25] ................................. 5

Figure 2.2- Ideal Power Curve ........................................................................................... 7

Figure 3.1- Small wind turbine model's assembly ............................................................. 8

Figure 3.2- Blade dimension .............................................................................................. 9

Figure 3.3- Stepper Motor Driver .................................................................................... 12

Figure 3.4- Pitch mechanism components ...................................................................... 13

Figure 3.5- Blade pitch control mechanism .................................................................... 13

Figure 3.6- Blade pitch angle position ............................................................................. 14

Figure 3.7- Tailoring the blade ........................................................................................ 14

Figure 3.8- Variable Resistor ........................................................................................... 16

Figure 3.9- The small wind turbine model ...................................................................... 17

Figure 3.10- Wind tunnel test section with wind turbine model .................................... 19

Figure 3.11- Connection Diagram ................................................................................... 20

Figure 4.1- The relation between power coefficient, pitch angle and resistance .......... 24

Figure 4.2- 9m/s contour plot shows the location of sections ....................................... 25

Figure 4.3- Section A-A .................................................................................................... 26

Figure 4.4- Section B-B .................................................................................................... 26

Figure 4.5- Power Curve .................................................................................................. 27

Figure 4.6- The rotational frequency (ω) varies with the resistor .................................. 28

Figure 4.7- Cp – λ Curve .................................................................................................. 29

Figure 4.8- The relation between V-ω on the left hand side and I-ω on the right hand

side .................................................................................................................................. 29

Figure 4.9- Contour plot for wind speeds 4 and 5 m/s ................................................... 30

Figure 4.10- Cp – λ Curve, comparison between 1st and 2nd setups ............................. 31

Figure 4.11- Cp- AoA curve at the tip of the blade ......................................................... 32

Figure 4.12- Cp- AoA curve at the base of the blade ...................................................... 33

Figure 4.13- Cp- AoA curve at the middle of the blade ................................................... 33

Figure 4.14- Velocity Triangle acting on airfoil ............................................................... 34

Figure 4.15- AoA and Blade Length for the current and proposed blades ..................... 35

Figure A.1- Power coefficient and resistor at different wind speeds ............................. 38

Figure A.2- Power coefficient and pitch angle at different wind speeds ........................ 39

Figure B.1- LabView Front Panel ..................................................................................... 40

Figure C.1- Voltage divider circuit ................................................................................... 41

v

LIST OF TABLES

Table 3.1- Generator Specification [16] .......................................................................... 10

Table 3.2- Generator's Encoder Specification [17] ......................................................... 10

Table 3.3- Stepper Motor Specification [18] ................................................................... 11

Table 3.4- Stepper Motor's Encoder Specification [19] .................................................. 11

Table 3.5- Driver Specification [20] ................................................................................. 11

Table 3.6- Small Wind Turbine Model General Specification ......................................... 18

Table 4.1- Operation methodology at 9m/s ................................................................... 21

Table 4.2- Operation methodology at 4m/s ................................................................... 22

Table 4.3- Results Template ............................................................................................ 22

Table 4.4- Cp max at different wind speeds.................................................................... 26

vi

ACKNOWLEDGEMENT

The author wishes to express sincere appreciation to Prof. Dr. Joachim Peinke to give him

the chance to be here in wonderful team work environment and his supervisor Doctor

Michael Hölling for his fruitful assistance and main support in the accomplishing this thesis

and building up his knowledge in wind energy. In addition, special thanks to Engineer

Agnieszka Parniak who was the active and prime mover of the project in the design and

construction processes. Thanks also to the members of ForWind for their valuable input.

Moreover, he would like to thank Dr. Basman El-Hadidi for his support and guide and to

Prof. Dr. Dirk Dahlhaus on his efforts to sustain his master program.

Furthermore, the author wishes the best and very special thanks for his parents and

brothers to give him the chance and support to be here in Germany doing his master thesis

and improving his educational and career levels. He also thanks his colleagues in the master

program for the joyful time that he spent with them especially for Engineer Rana Hanfy

and Engineer Fadi Abdulhadi who was his dynamic partner in this project and big

supportive.

vii

NOMENCLATURE

Abbreviations

NI National Instruments

ADC Analog to Digital Converter

VS-VP Variable Speed Variable Pitch

VS- FP Variable Speed Fixed Pitch

FS- VP Fixed Speed Variable Pitch

FS- FP Fixed Speed Fixed Pitch

GDP Growth Domestic Product

PWM Pulse Width Modulated

Symbols

ur Rotational Speed in m/s

ures Resultant Wind Speed in m/s

u1 Incoming Wind Speed in m/s

u2 Wind Velocity at rotor plane in m/s

Greek Letters

ρ Air Density

α Angle of Attack

β Angle of Relative Wind

γ Pitch Angle

Ω Ohms

λ Tip Speed Ratio

ω Rotational Frequency

1

CHAPTER 1

INTRODUCTION

The wind energy market worldwide has grown in hasty ascending manner where

Figure 1.1 shows in 2011 the installed capacity of 236GW was reached, with an

annual average growth of 1.2% [1].

This wind energy share was supported with legislations and governmental

encouragements illustrated by emphasizes on implementing their mandatory targets

by 2020 which boosted investments towards wind energy production.

Several countries approved plans to reach certain targets in the wind energy share in

2020. As an example: Egypt plans to reach 12% of its electricity production from

wind, China is building two wind turbines every one hour and expects to install

230GW by 2020 [2], Brazil announced 1,100 MW of wind energy [3] and EU has a

target of 14% of Europe’s total electricity production from wind energy [4], etc.

These plans ensure huge job opportunities for thousands of employees in wind

energy industry as illustrated in Figure 1.2.

Figure 1.1- Total installed capacity of wind energy worldwide from 2000 to 2011* in megawatts [1]

2

However, the governmental strategic plans were not only dedicated for installing wind

turbines but also in the increase of research and development investments. In

Europe, the research and development (R&D) share for wind energy will reach 3% of

its growth domestic product (GDP) in 2020 [5]. This indicates the importance of

R&D in wind energy sector.

Although the share of investment in wind energy research is increasing, researchers

are facing many constrains that limit wind energy development. Finance is considered

one of the important constrains in wind energy development which wind turbines

installation cost between 900€/kW to 1150€/kW [6]. Moreover, the installation of

such wind turbines in atmospheric conditions produces difficulties in defining the

boundary conditions of incoming flow caused by the nonexistence of precise sensors

that can define these conditions and adding weather and operational conditions.

Despite these constrains, scientists and researchers seek for better research

environment to accomplish their research. The existence of laboratories that can

generate realistic conditions by providing wind tunnel, active and passive grid,

sensors, etc. helped in improving the wind energy field. A small wind turbine models

were constructed and tested in the wind tunnel in a controllable incoming air flow

conditions for research purposes.

Figure 1.2- Number of jobs in the wind energy industry worldwide from 2005 to 2012 [4]

3

The ongoing wind tunnel experiments focus on load characterization for small wind

turbines which shows their performance and specifies the operating conditions.

Likewise, the research group of John Hopkins University in the United States has

built a small wind turbine with 12cm rotor diameter [7]. It focused on measuring the

extracted mechanical power rather than the electrical output power due to losses in

the generator. Although it was a fruitful study, they did not focus on the quality of

their blades and thus ignoring the efficient usage of aerodynamic forces.

Another study was done to examine the effect of turbulence in wake of small wind

turbine models [8]. The purpose of this experiment is to improve understanding of

transport momentum and kinetic energy of boundary layer with wind turbines. The

used models had a rotor diameter of 12cm with low blade quality without control

systems like pitch or load control.

The department of Aerospace Engineering at the Iowa State University in United

States of America carried out another experiment exploring the effect of dynamic

loads and evolution of turbulent flow in the wake of wind turbines [9]. They had a

3bladed- rotor of 254mm in diameter and a hub height of 225mm.

In Germany, Technische Universität Berlin has developed a small wind turbine of

70cm in diameter with load and pitch control [10]. The pitch control is done manually

in order to adjust the angle of attack and the load control by the means of variable

resistor. Also, they took extra attention to produce a well designed blade.

All the laboratories experiments on small wind turbine models do not account all

factors that help increasing the extracted power from wind, such as pitch control, yaw

control, etc.

As an initiative and to be one of the pioneers in providing new techniques in wind

energy research, a small wind turbine model with pitch and variable load controls

which are known as variable pitch and variable speed control strategy (VS-VP) was

constructed. This project considered to be unique of its kind in small sizes of 50cm

rotor diameter. It opens opportunities to study, characterize, and investigate, etc. in

more advanced and realistic manners what occurs to real wind turbines.

4

This project focuses on “Design and Power Characterization of a Small Wind

Turbine Model in Partial Load Region” and “Design and Characterization of a Small

Wind Turbine Model equipped with a Pitching System” [11]. This part of the project

covers the design and output power characterization of this model. Through the

thesis, the design phase of wind turbine components will be discussed. The

experimental setup will be mentioned and then interpreting the results.

5

CHAPTER 2

BASIC THEORY

The major theoretical principles to which any wind turbine is restricted are the main

task of this chapter.

The role of wind turbines is to extract energy from wind and convert it to electrical

energy. This extraction is subjected to certain limitations represented by Betz’s limit

which is the maximum energy possible to convert kinetic energy into mechanical

energy without any losses [12]. An ideal wind turbine has a maximum power

coefficient (,) of 16/27. The theoretical limit cannot be exceeded and this

caused by the aerodynamic losses due to conversion of angular momentum, tip and

drag [12]. It is clear in Figure 2.1, that the power coefficients of all wind turbine types

are located under the theoretical power coefficient.

Figure 2.1- Power coefficient for different wind turbine types [25]

6

Accordingly, the power extracted from wind can be expressed as follows:

1

2

(2.1)

where:

ρ = density of air in kg/m3,

A = rotor swept area in m2 = πR,

u = incoming wind speed in m/s,

c !" = maximum power coefficient.

The extracted power is then converted to electrical power defined as:

#$

(2.2)

where:

P = electrical power in W,

V = voltage in V,

I = current in A.

This power can be drawn at different wind speeds as shown in Figure 2.2. This curve

is called the ideal power curve which is divided into two zones. In the first zone, the

partial load zone, the maximum power coefficient (c !" ) is tracked by controlling

the rotational frequency of the generator at fixed optimum blade pitch angle. In

theory, c !" should be constant at different wind speeds [12]. So, the purpose of

controlling the rotational frequency is to maintain a constant (c !" ). The second

zone, the power control zone, used to keep the rated power constant by altering the

pitch angle. The reason for this, is to protect the generator from exceeding its rated

power and thus from failure. This control strategy is called variable speed- variable

pitch (VS-VP).

7

The speed control in the partial load zone might attempt to maintain a constant tip

speed ratio (λ) and thus maximum power coefficient at different wind speeds [12].

The tip speed ratio is defined as the relation between rotational frequency (ω) of the

rotor at the tip and the incoming wind velocity:

% &'

(2.3)

where:

% = tip speed ratio,

& = rotational frequency in rd/s,

' = blade radius in m,

= incoming wind speed in m/s.

To achieve the control goal, the generator’s power output is connected to a variable

resistive load that controls ω and thus tries to keep λ constant. The change in variable

resistor causes changes for the current in the generator and thus in the torque which

is proportional to the current. If the current increases the torque (T) in the generator

will increase causing the rotor to slow down. Otherwise, the rotor speed will increase.

Therefore, the control system should track the optimum values of both variables T

and ω to obtain c !" by the means of variable resistor.

Figure 2.2- Ideal Power Curve

8

CHAPTER 3

DESIGN AND SETUP

The mentioned components in this section were designed, including force analysis

and CAD drawings, and manufactured in the workshop at the University of

Oldenburg. Additional standard components from various suppliers were used.

Moreover, the components of the model declaring its function and specification were

mentioned. Figure 3.1 shows the model’s components assembly.

SLIDER LINKS COUPLING

GENERATOR

BLADE

CONNECTOR

STEPPER MOTOR

MAIN SHAFT

Figure 3.1- Small wind turbine model's assembly

9

Rotor

The rotor of 0.577m in diameter is considered an important part of the wind turbine

where the extracted power from wind is converted into mechanical power in terms of

torque (T) and rotational frequency (ω).

Blades

The small wind turbine is a three bladed system. It is 0.25m length and 0.032m chord

at base, see Figure 3.2.

The length was chosen according to the dimensions of the available wind tunnel with

outlet area of 1x0.8 m2. The blades are manufactured by a rapid prototyping machine

and designed in according to MEXICO project [13] with a change in the airfoil

profile near the base. The blades are made of Resin type VeroBlue, an acrylic-based

photopolymer, which is classified as one of the plastic families [14].

The purpose of using plastic is its ability to be easily formed, shaped, casted and

pressed. However, VeroBlue is durable and degrades very slowly as all kinds of

plastics. Another reason for using plastic is its suitability of being used for the

prototyping process [15]. Rigidity is the main property of VeroBlue which makes it

resistible to applied forces.

Figure 3.2- Blade dimension

0.2m

10

Generator

The wind turbine generator was selected according to few conditions. These

conditions included the design rated wind speed and of values 12m/s and 0.4

respectively. The assumption of was based on rule of thumb. Since the model

is not an ideal wind turbine, should be lower than Betz limit. In addition, the

model is subjected to losses, due to tip and drag, and to friction in the generator and

other components. Thus, the value of was considered accordingly. Inserting

these values in the extracted power formula at rated wind speed,

1

2

(1.1)

where the air density (ρ) is 1.225 kg/m3 and the rotor swept area (A) is 0.26 m2, the

obtained rated power of 110W. Despite of this calculation and assumption, a

generator with maximum power output of 217W was chosen in order to prepare the

turbine for future experiments with different components.

The generator is a DC- micromotor [16] that has the following specifications:

Manufacturer FAULHABER Type 3863H048CR

Nominal Voltage 48 V Maximum Output Power 217 W

Maximum Rotational Speed 8000 rpm Table 3.1- Generator Specification [16]

The generator is provided with an encoder that converts the generator’s rotational

frequency to a digital format. The encoder is of type IE2 – 1024, [17]. Further

specifications are given in Table 3.2.

Manufacturer FAULHABER Type IE2 – 1024

Supply Voltage 4.5...5.5 V DC Lines per Revolution 1024

Table 3.2- Generator's Encoder Specification [17]

Generally, the encoder has different functions but it was used as an indicator for the

rotational frequency.

11

Stepper Motor

The stepper motor is used for driving the pitching mechanism. It was selected based

on moment analysis, see [11]. The stepper motor was bought from FAULHABER

with electronic driver and an encoder. The specifications of these products are

shown in the tables below:

Manufacturer FAULHABER Type AM2224-R3-AV-4.8

Nominal Voltage 3 V DC Steps per Revolution 24

Step Angle 15° Holding Torque 22mNm

Table 3.3- Stepper Motor Specification [18]

Manufacturer FAULHABER Type PE22-120

Supply Voltage 4.5...5.5 V DC Lines per Revolution 120

Table 3.4- Stepper Motor's Encoder Specification [19]

Manufacturer FAULHABER Type ADCMM1S

Power Supply Voltage Min. 10 V DC Max. 28 V DC

Table 3.5- Driver Specification [20]

The stepper motor has 24 steps per revolution where each full step corresponds to

15º step angle.

Likewise, the encoder is a FAULHABER product used not only for indication and

control of direction and rotational velocity, but also positioning. Thus, these acts are

applied to tailor the blade pitch angle.

The electronic driver has two tasks, first it supplies the stepper motor with power and

secondly it receives commands from the control system.

12

It is a basic driver which requires clock and direction signals [21]. The clock is needed

to generate pulse-width modulated (PWM) signals for driving the motor and

controlling its rotational frequency. The direction signal sets the direction of rotation,

i.e. clockwise or counter-clockwise.

As recommended from manufacturers catalogue [21 p. 7], the current was set at

position switch A and 12 V was supplied for energizing the poles [11].

Blade Pitch Mechanism

The blade pitch mechanism is used to change the blade pitch angle either in partial

load zone or in power control zone. This mechanism consists of a slider, links and

connectors. It is connected to the stepper motor on one side, which again is fixed to a

threaded rail to maintain a linear motion. The slider moves on the main shaft

forwards and backwards by the means of the stepper motor. The other side of the

mechanism holds the blades. Figure 3.4 shows the assembly of these components.

Figure 3.3- Stepper Motor Driver

13

Its only function is transferring the linear motion from the stepper motor to

rotational motion in the blades. When the motor rotates by 24 steps, the slider moves

forwards or backwards by 0.5mm causes a change in blade position. This is observed

in Figure 3.5 where the blade moves from position 1 to position 2 by moving the

slider.

Figure 3.5- Blade pitch control mechanism

Figure 3.4- Pitch mechanism components

1 2

14

Since the system is limited to a certain distance [11], the blades were fixed at a blade

pitch angle of 20° from the axis of rotation with the naked eye. Figure 3.6 shows the

blade position when it was fixed to the blade pitch mechanism. As a result, the

mechanism tailors the blade in range of pitch angle 20° to 30º as shown in Figure 3.7.

Furthermore, a force analysis was done for this system to make sure that the selected

stepper motor can maintain the motion of this mechanism [11].

Main Circuit

The main circuit consists of electrical and electronic circuits which it controls,

monitors and measures data from the system. This circuit regulates the generator’s

rotational speed, pitches the blades and measures the outputs in voltage (V) and

current (I). It consists of shunt resistor, variable resistor, DC generator, generator’s

encoder, stepper motor, stepper motor’s encoder and driver, analog to digital

convertor (ADC), and a computer with LABVIEW program as a controller and data

acquisition.

Axis of Rotation Chord Line

Figure 3.7- Tailoring the blade Figure 3.6- Blade pitch angle position

15

Connections

The circuit diagram, Figure 3.11, explains the path of connections. These connections

are divided into 5 types:

Positive power cables in red

Negative power cables in blue

Ground cables in black

Control cables in green

USB cables in brown

Shunt Resistor

The shunt resistor is used to measure the voltage drop across the resistor and then

calculate the current according to:

∆# $')

(1.2)

* $

∆#

') (1.3)

where:

∆V = voltage drop across the shunt resistor,

R+ = shunt resistor which is 6mΩ in this case,

I = current in the circuit.

The shunt resistor has 6mΩ resistance, maximum voltage drop of 0.15V and

maximum current of 25A [22]. The shunt resistor had been chosen with a small

resistive value for high precision of readings.

Variable resistor

The variable resistor varies the generator’s load and hence controls the generator’s

rotational frequency; therefore the output voltage and current of the generator. It has

a maximum resistance of 85Ω with a wiper that varies from 0 to 85Ω. It was divided

into steps with a step size of 5Ω marked by the means of a multi-meter. Figure 3.8

shows how the values of the resistor were divided.

16

The total numbers of positions are 17 steps. The expected error in this process is 10%

and the wiper was changed manually.

Analog to Digital Converter (ADC)

The role of ADC is to convert analog signals into digital for further processing on a

computer. In this project, the used ADCs were NI USB 6211 and NI USB 6008.

These instruments are products of National Instruments [23; 24]. The first differ

from the second by the resolution and number of channels. The NI USB 6211 has a

resolution of 16 bits plus counters while the NI USB 6008 of 12bits.

LABVIEW program

By the means of LabView, a program was implemented to read and log the input data

and change the blade pitch angle. The data were stored in text files on the computer.

The program’s front panel is shown in Appendix B

The program is divided into two sections, one for the generator and the other for the

stepper motor. For the generator, this program calculates the output power, rpm and

current of the generator. The current is calculated from the measured voltage drop

readings across the shunt and the rpm is measured in terms of frequency which is the

frequency of the encoder. In order to convert it to rpm, the below equation was used:

Figure 3.8- Variable Resistor

17

,-.

/ 0 60

3

(1.4)

where:

,-. = rotation per minute,

/ = frequency of the encoder in Hz,

3 = lines per revolution.

Likewise, the power is the product of the calculated current from the voltage drop

across the shunt and the measured generator’s voltage output. All these values are

mean values of 1000 samples at a sampling rate of 100Hz.

For the stepper motor, the used program controls the direction of the motor,

rotational velocity and indicates its position. Its rotational velocity is controlled by

generating PWM. The change of the width of this signal affects the rotational velocity.

The direction is controlled by Boolean control switch [11].

In order to read these signals, this program was connected to ADCs by USB

connections to the computer. Then, the connections are spread according to

connection diagram, see Figure 3.11.

General Specifications

The small wind turbine model is presented in Figure 3.9 and the following table

(Table 3.6) lists its specifications.

Figure 3.9- The small wind turbine model

18

Specifications

Rotor

Rotor Diameter 0.577m

Hub Diameter 0.077m

Hub Length 0.064m

Hub Height 0.45m

Number of Blades 3

Blades

Blades Span 0.25m

Base Chord 0.036m

Material Resin VeroBlue

Nacelle

Type Tubular

Diameter 0.077m

Length 0.188m

Material PVC

Tower

Type Tubular

Diameter 0.032m

Height 0.412m

Material PVC

Base

Type Square

Length 0.2m

Width 0.2m

Thickness 0.025m

Material PVC

Table 3.6- Small Wind Turbine Model General Specification

19

Experimental Setup

The experiment took place in the wind tunnel in Carl von Ossietzky University of

Oldenburg where a 1x0.8 m2 area wind tunnel exists. It allows for velocities ranging

from 0 to 50m/s. The model was set in front of the wind tunnel’s nozzle, in an open

test section. The wind tunnel has a control screen used for setting the speed of the

incoming flow served with indicator to measure the output flow (u1) from the nozzle.

The small wind turbine model was fixed on a table in front of the nozzle’s wind

tunnel like illustrated in Figure 3.10.

Figure 3.10- Wind tunnel test section with wind turbine model

20

The cables going out from the model, as seen in the above Figure 3.10, are connected

to the main circuit. Figure 3.11 shows how the wind turbine components are

connected to the main circuit. NI USB 6008 was used for measuring the generator’s

output voltage and NI USB 6211 for measuring the shunt resistor voltage drop. The

other two NI USB 6211 were used for the blade pitch mechanism. One of them was

for changing the pitch angle and the other for position indication. The generator’s

encoder and stepper motor driver were connected to a voltage supply.

Figure 3.11- Connection Diagram

21

CHAPTER 4

CHARACTERIZATION

Procedure

To achieve the goal, characterizing the small wind turbine power output, the

generator’s output voltage, voltage drop across the shunt resistor and the rotational

frequency of the generator’s encoder were measured.

The wind tunnel was operated in wind speed range of 4 to 9 m/s with ∆u1= 1 m/s.

At each wind speed, the required data were measured at different resistor loads (R)

and pitch angles (γ). Table 4.1 lists the pitch angle range from 20 to 30° with the

resistor load from 0 to 50Ω at 9m/s. This means for each pitch angle, the resistor was

changed 10 steps at each wind speed.

Same procedure was repeated for the mentioned wind speed range except for 4m/s

where Table 4.2 shows the range of R was set from 5 to 85Ω. The mean values of the

measured data were logged every 10 seconds into a text file.

Wind Speed (m/s) Pitch Angle (⁰⁰⁰⁰)

20 5 10 15 20 25 30 35 40 45 50

21 5 10 15 20 25 30 35 40 45 50

22 5 10 15 20 25 30 35 40 45 50

23 5 10 15 20 25 30 35 40 45 50

24 5 10 15 20 25 30 35 40 45 50

25 5 10 15 20 25 30 35 40 45 50

26 5 10 15 20 25 30 35 40 45 50

27 5 10 15 20 25 30 35 40 45 50

28 5 10 15 20 25 30 35 40 45 50

29 5 10 15 20 25 30 35 40 45 50

30 5 10 15 20 25 30 35 40 45 50

9

Resistor (Ω)

Table 4.1- Operation methodology at 9m/s

22

Table 4.3 shows an example of results template taken at 9m/s wind speed and 20º

pitch angle.

Wind Speed (m/s) Pitch Angle (⁰⁰⁰⁰)

20 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

21 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

22 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

23 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

24 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

25 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

26 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

27 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

28 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

29 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

30 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

4

Resistor (Ω)

Table 4.2- Operation methodology at 4m/s

Pitch Angle (⁰) 20

Wind Speed (m/s) Resistor (Ω) Frequency (Hz) RPM VGenerator VShunt Current (A) Power (W)

5 10714.13 632.13 2.70 0.00303 0.51 1.36

10 16051.38 947.03 5.06 0.00294 0.49 2.47

15 17734.92 1046.36 6.07 0.00232 0.39 2.34

20 18534.66 1093.54 6.65 0.00189 0.31 2.09

25 19052.76 1124.11 7.00 0.00161 0.27 1.88

30 19543.53 1153.07 7.36 0.00140 0.23 1.72

35 19819.69 1169.36 7.53 0.00125 0.21 1.57

40 19959.95 1177.64 7.69 0.00109 0.18 1.39

45 20124.65 1187.35 7.83 0.00099 0.16 1.29

50 20315.69 1198.63 7.92 0.00087 0.15 1.15

9

Table 4.3- Results Template

23

After measuring and analyzing, the blades were fixed at the optimum blade pitch

angle and the load was varied to obtain maximum power coefficient at wind speeds

from 4 to 9 m/s to produce the power curve where measurements were taken at

these points. Then the wind tunnel was operated in the range from 9.5 to 14 m/s with

∆u1= 0.5 m/s at a constant resistor load and measurements were taken in order to

maintain a constant rated power by rotating the blades.

A contour plots presenting resistor load, pitch angle and c at different wind speeds

were plotted as shown in Figure 4.1 to determine . The yellow spots indicate

highest values of c and the grid white dots are the measured points. Even though

the measured points were sparse, it was possible to locate zones in which is

located. A detailed discussion about these plots will be covered later in this chapter.

24

Figure 4.1- The relation between power coefficient, pitch angle and resistance

25

Optimum Pitch Angle

As mentioned before, the blades were fixed at optimum pitch angle in the partial zone

to track c !". To identify this angle, sections were made at different locations of the

contour plots (Figure 4.1). Sections A-A and B-B for 9m/s plot were taken as an

example like in Figure 4.2.

After this cut in the 9m/s plot, it clarified that the optimum pitch angle is γ = 21°and

optimum load R = 10Ω at 99.44rpm where c !" = 0.022, (see Figure 4.3 and

Figure 4.4).

Figure 4.2- 9m/s contour plot shows the location of sections

B

B

A A

26

Figure 4.3 shows the behavior of c with pitch angle where it starts to increase until it

reaches the maximum and then decreases slightly. Likewise in Figure 4.4, the c

increase with the increase of the resistor until it reaches the maximum power

coefficient where it decreases afterwards.

Several sections were made for different locations at all wind speeds to determine the

optimum pitch angle which resulted in one pitch angle of γ = 21°. The curves that

presents both with all wind velocities and sections are shown in Appendix A. The

results for these sections are listed in Table 4.4.

Wind Speed [m/s] Pitch Angle [º] Resistor [Ω] 56789

4 21 50 0.009 5 21 30 0.015 6 21 20 0.018 7 21 15 0.020 8 21 15 0.021 9 21 10 0.022

Table 4.4- Cp max at different wind speeds

Figure 4.3- Section A-A Figure 4.4- Section B-B

27

Power Curve

According to the obtained results shown in Table 4.4, the blades were set at pitch

angle of γ = 21° and then adjusting the resistance to the optimum obtained values

corresponding to its wind speed. This was used to track c !" in the partial load zone

of the power curve. Figure 4.5 demonstrates the fruitful harvest from characterizing

the small wind turbine model power curve and the black dots present the measured

data. In the control zone, the pitch angle was altered as wind velocity increased above

the rated velocity to maintain an approximately constant rated power, Figure 4.5

The conditions that the small wind turbine model works on were attained. The

model’s cut-in wind speed was 4m/s at which it started to rotate. The rated wind

speed was at 9 m/s at which the rated power of 2.5W was determined.

Figure 4.5- Power Curve

28

Cp – λ curve

Previously it was mentioned, while analyzing Figure 4.2, that the resistive load

corresponding to c !" differ from one wind speed to another, hence a question

arises in mind; why is the variable load affecting c?

According to the theory, the power coefficient (c ) is a function of λ which depends

on the rotational frequency (ω) and incoming wind velocity. Suppose that the wind

speed is constant, and the varying parameter in this case is the rotational frequency

(ω), so, by the change of ω due to change of load, the tip speed ratio will

correspondingly changes. Figure 4.6 demonstrates the relation between variable loads

and rotational frequency at different wind speeds.

In spite of these plots, the role of incoming wind speed is summarized by shifting the

operational region of ω. Therefore, the change in resistive load causes the change in ω

and thus the tip speed ratio changes.

The change in tip speed ratio causes the power coefficient (c) to change as shown in

Figure 4.7. The figure illustrates the behavior of c with tip speed ratio in the wind

speed range from 4 to 9m/s. The figure also shows that c !" at each wind speed

gives different tip speed ratio which is opposite to what mentioned in theory. This is

because of several factors will be mentioned later.

Figure 4.6- The rotational frequency (ω) varies with the resistor

29

In addition, the rotational frequency variations change the generator’s output power

presented in change of both voltage (V) and current (I). Figure 4.8 shows the relation

between ω and both voltage (V) and current (I).

Figure 4.7- Cp – λ Curve

Figure 4.8- The relation between V-ω on the left hand side and I-ω on the right hand side

30

Limitation Factors

Together Figure 4.5, Figure 4.7 and Table 4.4 show both c !" and output power are

low compared to the design values. Also they show that the maximum power

coefficient and tip speed ratio are not equal in all wind speeds although they are close

in values.

Several factors played an important role in limiting the small wind turbine model’s

performance. This is realized in different points.

Friction

It was witnessed, in the incoming low wind velocities 4 and 5 m/s, large dark areas

determine the plot, see Figure 4.9, and this occurred due to friction.

In low wind speeds, the incoming flow had difficulties to overcome the friction in the

generator and bearings. This effect decreases with the increase of wind speed. Also,

eccentric oscillations in the rotation were visible.

Figure 4.9- Contour plot for wind speeds 4 and 5 m/s

31

Small Modifications

Moreover, small modifications in the model will enhance its performance and the

difference was observed. The comparison between two setups of the model is shown

by means of plots in Figure 4.10.

One setup represents the first time when the model was assembled labeled as “1st

Setup” in the plot whereas the second setup, labeled as “2nd Setup”, represents some

modifications applied on the model exemplified by adding lubrication, modify the

alignment of the main shaft and make it more stable and adding more range to the

pitching system. It is clearly presented that c was increased in the second setup.

Generator Losses

As information from the generator’s manufacturer, FAULHABER, the selected DC-

micromotor if used as a generator, will operate on a 40% of its efficiency. Besides the

experiment that was done by John Hopkins University [7], mentioned that its

generator was operating with efficiency between 40 to 50%. There generator was also

selected from FAULHABER. But there are more losses in the generator due to

copper, iron and mechanical losses. These losses might be the cause of such low

power.

Figure 4.10- Cp – λ Curve, comparison between 1st and 2nd setups

32

Manufacturing and Installation of blades

The manufacturing precision of blades can help in enhancing or affecting the model’s

performance. Since the blades are not 100% precise in shape and each one differs

from the other, it could contribute to lower the output power. In addition, the blades

were fitted manually with small alignment (blade pitch angle) differences.

Angle of Attack (AoA)

In the previous sections, the parameters that affected the system were studied and the

reasons why c !" is low and differs from wind speed to other were discussed. But

more questions flirted in mind; are λ and angle of attack the same all over the blade?

Is this could be a reason why the power outputs are low?

To answer these questions, some calculations were made for the angle of attack

depending on the obtained measurements. Surprisingly, the angle of attack was

different all through the blade and with wind speeds. A curve was drawn shows the

angle of attack with c at the tip of the blade, see Figure 4.11. The angle of attack is

not constant and varies by ∆AoA = 1º as an average variation based on the measured

data. The maximum power coefficient ( c !") becomes closer to the red line with

the increase in wind speeds. This also indicates the presence of friction in the system.

Figure 4.11- Cp- AoA curve at the tip of the blade

33

This change in angle of attack was caused by the change in rotational frequency (ω)

due to the change in the resistor. Similar curves were drawn but at two different

locations in the blade to see if there is also difference in angle of attack along the

blade. The drawn curves are at distances from the rotor center of 0.077m and 0.15m

and are shown in Figure 4.12 and Figure 4.13 respectively. Likewise Figure 4.11, the

maximum power coefficient ( c !") becomes closer to the red line with increase in

wind speed in Figure 4.12 and Figure 4.13.

Figure 4.12- Cp- AoA curve at the base of the blade

Figure 4.13- Cp- AoA curve at the middle of the blade

34

The difference in the three plots is in the power coefficient where c is higher near

the rotor center. This is normal because it is going away from the tip losses.

In order to have a constant angle of attack for all wind speeds, calculations were done,

depending on measured % = 3.3, to find new angle of attacks and then new twist

angles for the blade. As an example, Figure 4.14 shows the act of different wind

velocities (u2), marked with orange, on an airfoil. The resultant wind velocity (ures), the

green line, is a result of the rotational velocity (ur), the violet line, and wind velocities

(u2). It is shown that the angle of resultant wind (β) is constant for all applied wind

speeds and pitch angle (γ) is constant too as it is set to γ = 21º. Thus the angle of

attack (α) is mostly constant over some range of wind speeds.

Accordingly, Figure 4.15 was drawn to show the twist angles in the current and

proposed blades. The curve plotted in green is for the current blade while the

proposed one is plotted in blue. This modification could improve the system since the

angle of attack is shifted to higher values compared to the current one.

Figure 4.14- Velocity Triangle acting on airfoil

35

The aim of this twist is to maintain a constant angle of attack all over the blade and

thus improve its efficiency.

Figure 4.15- AoA and Blade Length for the current and proposed blades

36

CHAPTER 5

CONCLUSION AND FUTURE IMPROVEMENT

Although many problems were faced that influenced the small wind turbine model, it

had generated a power curve similar to the form of the ideal power curve where the

maximum output power is 2.5W at 9 m/s wind velocity with a maximum power

coefficient of 0.022.

Unfortunately, a constant c !" could not be achieved and therefore a constant λ;<=

which affected the system’s act due to several factors mentioned previously. While a

new design approach for the blades depending on obtained readings could enhance

the performance of the model.

At last, this small wind turbine model has proved its capability to achieve realistic

results and be subjected to realistic conditions, turbulent flow as an example, which

makes it sustainable for better research methods and improvements in wind energy

research.

Future Improvements

As a step forward in continuing this project, one can work on several points that

improve this project.

The range of output voltage was between 0 and 10V while in future this range would

shift to higher ranges. In this case, a voltage divider should be implemented to the

main circuit, a brief calculation and schematic drawing of it is found in the Appendix

C. This will allow the NI USB 6211 to read the input voltage.

The implementation of a controller program by means of LabView would improve

the act of the wind turbine model. This program should consist of feedback loops

and control commands for the variable resistor which should be replaced by digital

one. In addition, a modified pitch control program should be implemented.

37

Since the c !" was low due to friction and losses, it is recommended, before taking

any action, to measure the torque acting in the system by installing strain gauges and

pressing rod and then calculate the mechanical power. As the research team in

Oldenburg University are working on similar projects and have the expertise, it would

be helpful for installation of such measuring equipment. Accordingly, proposed

solutions would be supported with detailed information and thus accurate decisions

could be taken.

Moreover, the maximum tip speed ratio obtained was 3.3 depending on the

measurements. A modification could be done on the blades to match these

measurements in order to attain a constant angle of attack all over the blade at

different wind speeds. In order to apply these changes to the blades, a certain design

procedure should be followed. Depending on the measured tip speed ratio, the twist

angle of the blade should be changed according to angle of attack and its

corresponding distance from the rotor center (r) and then the chord length with r.

Still, there are more and more things to improve in this young small wind turbine

model.

38

APPENDIX A

Power coefficient and resistance at different wind speeds

Figure A.1- Power coefficient and resistor at different wind speeds

39

Power coefficient and pitch angle at different wind speeds

Figure A.2- Power coefficient and pitch angle at different wind speeds

40

APPENDIX B

The program’s front panel was divided into several booths:

Graphical Indicator (monitor input data graphically)

Numerical Indicator (monitor input data numerically)

Channel Parameters

Timing Parameters

File Path

Figure B.1- LabView Front Panel

41

APPENDIX C

Voltage Divider Circuit

Design Formulas

To calculate the generator’s voltage the following formulas were used:

#> #?

' @ '

'A (C.1)

and the current

$> $? 1 @

'B @ ')

' @ 'A (C.2)

Figure C.1- Voltage divider circuit

42

BIBLIOGRAPHY

[1] Windindustrie in Deutschland – Exportbilanz und Inlandsmarkt. s.l. : Verband

Deutscher Maschinen- und Anlagenbau, July 2011.

[2] Li Junfeng, Shi Pengfei, Gao Hu. China Wind Power Outlook. s.l. : Chinese

Renewable Energy Industries Association, Global Wind Energy Council,

Greenpeace, 2010.

[3] Global Wind Energy Council (GWEC). Global Wind Energy Council (GWEC).

[Online] Mar 2010. [Cited: Feb 6, 2012.]

http://www.gwec.net/fileadmin/documents/Publications/Global_Wind_2007_rep

ort/GWEC_Global_Wind_2009_Report_LOWRES_15th.%20Apr..pdf.

[4] European Wind Energy Association (EWEA). European Wind Energy Association.

[Online] 2011. [Cited: Feb 6, 2012.] http:/www.ewea.org/index.php?id=195.

[5] European Wind Energy Association (EWEA). European Wind Energy Association.

[Online] 2011. [Cited: Feb 6, 2012.] http://www.ewea.org/index.php?id=198.

[6] European Wind Energy Association (EWEA). European Wind Energy Association

(EWEA). [Online]

http://www.ewea.org/fileadmin/ewea_documents/documents/publications/WET

F/Facts_Volume_2.pdf.

[7] Kang, Hynug Suk and Meneveau, Charles. Direct mechanical torque sensor for

model wind turbines. MEASUREMENT SCIENCE AND TECHNOLOGY.

2010, 21.

[8] Raúl Bayoán Cal, José Lebrón, Luciano Castillo, Hyung Suk Kang, and Charles

Meneveau. Experimental study of the horizontally averaged flow structure in a

model wind-turbine array boundary layer. J. Renewable Sustainable Energy. 2, 2010,

013106.

[9] An Experimental Investigation on the Wake Characteristics of a Wind Turbine in

an Atmospheric Boundary Layer Wind. Zifeng Yang, Partha Sarkar and Hui Hu.

Honolulu, Hawaii : American Institute of Aeronautics and Astronautics, Inc., 2011.

[10] Wallmann, Staffan and Janke, David. Konstruktion und Aufbau einer

Messeinrichtung für Modelluntersuchungen an WEA-Rotoren mit dem

Hochlaufversuch. Berlin : Technische Universität Berlin, 2008.

[11] Abdulhadi, Fadi. Design and Characterization of a Small Wind Turbine Model

equipped with a Pitching System. Kassel : Kassel University, Oldenburg University

and Cairo University, 2012.

43

[12] J. F. Manwell, J. G. McGowan and A. L. Rogers. WIND ENERGY; Theory,

Design and Application. United Kingdom : John Wiley & Sons Ltd, 2009.

[13] Schepers, J.G. and Snel, H. Model Experiments in Controlled Conditions. s.l. :

ECN Wind Energy, 2007.

[14] RTC or Rapid Technologies & Consulting. Rapid Technologies & Consulting.

[Online] 2003. [Cited: Feb 2, 2012.] http://www.rtc-

germany.com/objet/o_materialien.html.

[15] RTC or Rapid Technologies & Consulting. Rapid Technologies & Consulting.

[Online] 2003. [Cited: Feb 2, 2012.] http://www.rtc-

germany.com/objet/o_anwendung_01.html.

[16] DR. FRITZ FAULHABER GMBH & CO.KG. FAULHABER. [Online] 2011.

[Cited: Dec 23, 2011.]

http://www.faulhaber.com/uploadpk/EN_3863_CR_DFF.pdf.

[17] DR. FRITZ FAULHABER GMBH & CO.KG. FAULHABER. [Online] 2011.

[Cited: Sep 23, 2011.] http://www.faulhaber.com/uploadpk/EN_IE2-

1024_DFF.pdf.

[18] DR. FRITZ FAULHABER GMBH & CO.KG. FAULHABER. [Online] 2011.

[Cited: Dec 23, 2011.]

http://www.faulhaber.com/uploadpk/EN_AM2224R3_PCS.pdf.

[19] DR. FRITZ FAULHABER GMBH & CO.KG. FAULHABER. [Online] 2011.

[Cited: Sep 23, 2011.] http://www.faulhaber.com/uploadpk/EN_PE22-

120_PCS.pdf.

[20] DR. FRITZ FAULHABER GMBH & CO.KG. FAULHABER. [Online] 2011.

[Cited: Sep 20, 2011.]

http://www.faulhaber.com/uploadpk/EN_ADxxMxx_PCS.pdf.

[21] DR. FRITZ FAULHABER GMBH & CO.KG. FAULHABER. [Online] [Cited:

Feb 14, 2012.]

http://www.faulhaber.com/uploadpk/EN_ADCMMxS_im_PCS.pdf.

[22] Mercateo. Mercateo. [Online] 2011.

http://www.mercateo.com/pdf/Schuricht/hiShunt-250A_dat_d.pdf.

[23] National Instruments Corporation. National Instruments. [Online] May 2008.

[Cited: Dec 23, 2011.] http://www.ni.com/pdf/manuals/371303l.pdf.

[24] National Instruments Corporation. National Instruments. [Online] Apr 2009.

[Cited: Dec 23, 2011.] http://www.ni.com/pdf/manuals/371932f.pdf.

[25] Hau, E. Wind Turbines: Fundamentals, Technologies, Application, Economics.

Germany : Springer, 2006.

Declaration for the Master’s Thesis

I hereby affirm that the master thesis at hand is my own written work and that I

have used no other sources and aids others than those indicated. Only the sources

cited have been used. Those parts which are direct quotes or paraphrases are

identified as such.

Kassel, on 27 February 2012 __________________ (Signature)