L B iDOttl !d6(Q2 / & / > f o ' « - W 
1 5 
IMPROVED ROTOR DESIGN FOR A 
SMALL SCALE HORIZONTAL AXIS 
WIND TURBINE SUITABLE FOR LOW 
WIND POTENTIAL 
By 
Mahinsasa Narayana 
621 
62A-5 
This Thesis was Submitted to the Department of Mechanical Engineering 
of the University of Moratuwa, Sri Lanka in Fulfilment of the 
Requirements for the Degree of Master of Philosophy 
University of Moratuwa 
7627S 16775 
Department of Mechanical Engineering. 
Faculty of Engineering 
University of Moratruwa 
Sri Lanka 
August 2002 
7 6 2 7 5 
Declaration 
I hereby declare that this submission is my own work and that, to the best of my 
knowledge and behalf, it contains no material previously published or written by 
another person nor material, which to substantial extent, has been accepted for the 
award of any other academic qualification of a university or other institution of higher 
learning except where acknowledgment is made in the text. 
Mahinsasa Narayana 
I certified that the above statement is correct 
Supervisor 
I 
ACKNOWLEDGEMENT 
I would like to express my sincere gratitude to the supervisor Dr. A.G.T.Sugathapala, 
for his invaluable assistance, guidance, advice and encouragement throughout the 
course of this research study and without which, I would never succeed. 
My thanks are extended to the former chairman Mr. M. Victor Mendis and former 
General manager Mr. P.A.S. Fernando of the National Engineering Research & 
Development Centre of Sri Lanka for providing the required funds and facilities 
throughout the research study. 
My thanks are also extended to University of Moratuwa for providing me with the 
necessary laboratory and library facilities and I would like express thanks to the 
Energy Unit of the Ceylon Electricity Board for providing some wind data. 
I would like to take this opportunity to thank the staff of the Renewable energy 
department and workshop of the National Engineering Research & development 
centre of Sri Lanka for their invaluable assistance towards the manufacture of wind 
rotor models for this research study. 
II 
CONTENTS 
Abstract 
List of figures 
List of tables 
Nomenclature 
1. Introduction 1 
1.1 .Current global status of energy generation 1 
1.2. Wind as a source of energy 4 
1.3.Wind Energy application in Sri Lanka 6 
1.3.1. Wind energy for water pumping in Sri Lanka 6 
1.3.2. Rural electrification 7 
1.3.3. Grid connected wind turbines in Sri Lanka 9 
^ 1.4.100W NERDC wind turbine for off-grid power generation in Sri 
Lanka 9 
1.5.Objectives 11 
1.6.0utline of thesis 11 
2. Performances analysis of wind rotors 13 
2.1 .Introduction 13 
2.2.Aerodynamic behaviour of a wind rotor 14 
2.2.1. Ideal performance of a wind rotor 14 
2.2.2. Performance of a wind rotor with wake rotation effect 16 
4 2.2.3. Performance of a wind rotor with effect of wake rotation 
and blade resistance 21 
2.2.4. Vortex system of the rotor 25 
2.3.Performances of the wind rotor developed by NERD Centre 27 
Page 
VI 
VII 
IX 
XI 
3. Electrical generators used for wind turbines 
3.1.Generators used for the grid connected applications 
3.1.1 Introduction 
3.1.2 Wind turbine with asynchronous (induction) generators 
3.1.3 Wind turbine with multi-pole permanent magnet generators. 
Ill 
32 
32 
32 
32 
Page 
3.1.4 Wind turbine with synchronous generators 34 
3.2.Generators used for small scale off - grid wind turbines 35 
3.3.Performances of the permanent magnet generator developed by 
36 
NERDC 
4 3.3.1 Development of a suitable electrical generator for micro scale 
power generation 36 
3.3.2 Design parameters of PMG 37 
3.3.3 Characteristics of the permanent magnet generator 41 
4. System performance of the battery-charging wind turbine of the 
NERD centre 44 
4.1 .Introduction 44 
4.2.Combined characteristics of NERDC wind rotor and generator 44 
4.3.Performance of the system at different sites 48 
4.3.1 Design wind speed 48 
4.3.2 Energy indices of the wind turbine 49 
4.3.3 Performance of the wind turbine at a low wind potential site 49 
4.3.4 Performance of the wind turbine at a high wind potential site 52 
4.3.5 Comparison of performance of the wind turbine at two 
different sites 54 
4.4. Energy requirement of a rural community 55 
5. Design of a wind rotor suitable for low wind potential 56 
5.1 Selection of operation parameters 56 
5.2 Modification on increasing diameter together with same solidity of 
f the existing wind rotor 56 
5.3 Initial torque of the wind rotor and the permanent magnet 
generator 60 
5.4 Design of a high solidity wind rotor 61 
5.5 Perform of the high solidity wind rotor in low wind potential site 66 
6 Validation by model testing 74 
6.1 Introduction 74 
6.2 Dimensional analysis arid physical modelling 74 
6.1.1 Governing parameters 74 
IV 
6.1.2 Scale down modelr of wind rotors 
6.3 Experimental set up 
6.4 Comparison of the performance of the models of existing rotor and 
high solidity rotor 
6.4.1 Experimental investigation 
Discussion and Conclusions 
7.1 Wind rotor performance 
7.2 Energy options 
7.3 Concluding remarks: cost of energy production 
7.3.1 Cost of energy at Makewita low wird potential site 
7.3.2 Cost of energy at Mirijivila good wind potential site 
7.3.3 Cost of energy from 100W solar panel 
7.3.4 Comparison of cost of energy production and amount of 
energy generation by solar and wind power systems 
References 
Appendix I 
Appendix II 
Appendix III 
Appendix IV 
Appendix V 
Appendix VI 
V 
ABSTRACT 
The design wind speeds of most of the existing wind turbine rotors are in the range of 
6 to 15 m/s with cut-in wind speed of 3.5 m/s. The performance of such a wind 
turbine in Sri Lanka is not satisfactory, where the wind velocities are relatively low. 
This is due to low initial torque, which leads to difficulty in starting, as well as due to 
poor running efficiencies. This makes wind turbines less attractive for areas with low 
wind speeds. 
The main objectives of this study were to predict the performance of the existing 
NERDC wind turbine system and identify the main causes for its poor running 
performance at low wind speed and thereby design a rotor with improved 
performance. When improve the performance of the rotor to extract more energy from 
low wind-speeds, cut-in wind speed and design wi: d speed of wind turbine should be 
reduced. Low starting torque of wind rotors was identified as a main restriction 
against the reduction of cut-in wind speed of wind turbines. This study intends to 
analyse the aerodynamics of wind rotors theoretically and thereby introduces 
appropriate changes to the geometrical parameters of the blades. Especially, 
possibility of increase of solidity of the rotor, without effecting adversely on its 
aerodynamic efficiency was analysed. 
The blade elementary theory and the momentum theory were used to analyse the 
aerodynamic performance of rotors theoretically and these results were validated by 
wind tunnel model testing. 
The results of this study indicate that the permanent magnet generator and rotor of the 
NERDC system were not matched properly, which resulted in low overall system 
efficiency. In addition, the design parameters of the rotor were not appropriate for 
sites with low wind potential. Other finding of this study was suitable wind rotor for 
extract more energy from low wind potential, should be with higher diameter and 
higher solidity. 
LIST OF FIGURES 
1 Figure 1.1 Global primary energy requirements, 1850 - 2100 
2 Figure 1.2 Global carbon emissions from fossil fuel use, 1850 - 2100 
3 Figure 1.3 Development of the wind energy use wordwide 
4 Figure 1.4 Wind energy use by continents in percentage at the end of 
2000 
5 Figure 1.5 Basic configuration of NERDC wind turbine 
6 Figure 2.1 Control volume used for a wind turbine 
7 Figure 2.2 Flow behind the rotor with "Wake Rotation" effect 
8 Figure 2.3 Velocity diagram of a blade element 
9 Figure 2.4 Maximum power coefficient of ideal wind rotor with wake 
effect 
10 Figure 2.5 Forces diagram of a blade element 
11 Figure 2.6 Power coefficient Vs tip speed ratio (Xo), for different C/Cd 
12 Figure 2.7 Vortices due to tip of the blade 
13 Figure 2.8 Simplified vortex system of a wind rotor 
14 Figure 2.9 Existing 2-b)aded wind rotor of the NERDC 
15 Figure 2.10 Power coefficient curve of the existing NERDC wind rotor 
16 Figure 2.11 Power performance of the existing NERDC wind rotor with 
different wind speeds 
17 Figure 2.12 Torque performance of the existing NERDC wind rotor with 
different wind speeds 
18 Figure 3.1 Combine characteristics of wind rotor and induction generator 
at different wind speed 
19 Figure 3.2 Combine characteristics of wind rotor and multi-pole PMG at 
different wind speed 
20 Figure 3.3 Combine characteristics of wind rotor and synchronous 
generator 
21 Figure 3.4 Section of rotor showing magnetic circuit 
22 Figure 3.5 Rotor of the permanent magnet generator (PMG) 
23 Figure 3.6 NERDC permanent magnet generator 
24 Figure 3.7 Experimental set-up to find ou the performance of permanent 
magnet generator 
25 Figure 3.8 Characteristic performance of permanent magnet generate r 
(PMG) with 24V battery bank 
26 Figure 4.1 Generator and rotor performance curves of NERDC wind-
turbine 
27 Figure 4.2 Combine performance of the generator and the rotor 
VII 
Page 
28 Figure 4.3 100W NERDC wind turbine 47 
29 Figure 4.4 Output power characteristics of NERDC wind turbine 48 
30 Figure 4.5 Wind speed and energy distribution at Makewita, Ja-Ela, 
height 20m 50 
31 Figure 4.6 Wind speed and Energy, frequency distribution at Mirijivila, 
height 20 m 52 
32 Figure 5.1 Generator and rotor performance curves of the modified 
NERDC wind turbine (with 1:3 gear box) 57 
33 Figure 5.2 Performance of modified wind turbine 58 
34 Figure 5.3 Initial torque of modified wind rotor with different wind 
speeds 61 
35 Figure 5.4 Optimum incidence angle at the section r=0.9R=1980 mm 61 
36 Figure 5.5 Optimum incidence angle at the section r=0.4R=880 mm 64 
37 Figure 5.6 Linearized blade angles of high solidity rotor 65 
38 Figure 5.7 High-solidity 4-bladed wind rotor 66 
39 Figure 5.8 Power coefficient curve of 4-bladed high solidity wind rotor 
model 67 
40 Figure 5.9 Combine performance of 4-bladed rotor and PMG (with 1:5 
gear box) 67 
41 Figure 5.10 Initial torque of high solidity wind rotor with different wind 
speeds 68 
42 Figure 5.11 Performance of high-solidity 4-bladed wind turbine 69 
43 Figure 5.12 Overall efficiency of wind turbines 72 
44 Figure 6.1 Configuration of the set-up and air velocity data points 78 
45 Figure 6.2 Friction pulley arrangement of break dynamometer 79 
46 Figure 6.3 Wind tunnel and experimental set-up 80 
47 Figure 6.4 Power characteristics of the manufactured 2- bladed model 82 
48 Figure 6.5 Torque characteristics of the manufactured 2-bladed model 83 
49 Figure 6.6 Power characteristics of the manufactured 4-bladed model 85 
50 Figure 6.7 Torque characteristics of the manufactured 4-bladed model 85 
51 Figure 6.8 Typical rotor curves and load curves of friction pulley 86 
VIII 
y 
LIST OF TABLES 
Page 
1 Table 1.1 World Energy Use by Source in 1900 and 1997 1 
2 Table 1.2 CCVEmissions of commonly used fuels 2 
3 Table 1.3 Forecast of renewable energy electricity generation capacity 
(MWe) 4 
4 Table 1.4 Top five nations in wind energy usage at the end of year 2000 5 
5 Table 2.1 Maximum C p with Xo of an ideal wind rotor with wake effect 20 
6 Table 2.2 Geometrical parameters of existing 2-bladed wind rotor at 28 
NERDC 
7 Table 2.3 Theoretically calculated performance of the wind rotor 29 
8 Table 3.1 Stator details 38 
9 Table 3.2 Windings details 38 
10 Table 3.3 Characteristic performance of permanent magnet generator 
(PMG) with 24V battery bank 42 
11 Table 4.1 Performance of NERDC wind turbine 47 
12 Table 4.2 The energy calculation indices of NERDC wind turbine at 
Makewita site 51 
13 Table 4.3 Energy indices of high solidity wind turbine at Makewita site 51 
14 Table 4.4 The energy calculation indices of NERDC wind turbine at 
Mirijivila site 53 
15 Table 4.5 Energy indices of high solidity wind turbine at Mirijivila site 53 
16 Table 4.6 Comparison of energy indexes of Makewita and Mirivila sites 54 
17 Table 4.7 The daily energy demand for a typical rural house in Sri Lanka 55 
18 Table 4.8 Efficiency of electrical equipment used in wind power generation 55 
19 Table 5.1 Performance of modified wind turbine by increasing diameter of 
rotor together with same solidity of the existing wind rotor 58 
20 Table 5.2 The energy calculation indices of NERDC modified wind turbine 
at Makewita site 59 
21 Table 5.3 Performance comparison of existing and modified wind turbines 59 
22 Table 5.4 Selected solidity of wind rotor to obtain the required higher 
torque coefficient (Cm) 62 
IX 
23 Table 5.5 Optimum incidence angles, blade angles and chord lengths at the 
section r=0.4R and r=0.9R of the designed wind rotor 65 
24 Table 5.6 Geometrical parameters of high solidity wind rotor 65 
25 Table 5.7 Performance of high solidity wind turbine 69 
26 Table 5.8 The energy calculation indexes of high solidity 4-bladed wind 
turbines at Makewita site 70 
27 Table 5.9 Energy indices of high solidity wind turbine at Makewita site 70 
30 Table 5.10 Comparison of performance existing, modified and designed 
wind turbines 71 
31 Table 5.11 Overall efficiency r\mc % of wind turbines 71 
32 Table 6.1 Parameters of existing wind rotor Model 77 
33 Table 6.2 Parameters of high solidity wind rotor 77 
34 Table 6.3 Geometrical parameters of the 2-bladed rotor model 81 
35 Table 6.4 Geometrical parameters of the 4-bladed rotor model 81 
36 Table 6.5 Experimental Cv & C m values of 2-bladed low solidity wind rotor 82 
37 Table 6.6 Experimental C p & C m values of 4-bladed high solidity wind rotor 
model 84 
38 Table 7.1 Capital cost, cost of energy production and amount of energy 
generation by each wind turbines and 100W solar panel 92 
X 
NOMENCLATURE 
Q- Flow rate of air 
V Tip loss of the wind rotor 
a- Angle of attack 
Cfl- Angular speed of the rotor 
P- Blade angle 
r - Circulation 
p - Density of air 
u - Viscosity of air 
Q - Rotational speed of the air in the rotor wake 
Xo- Tip speed ratio 
0o - Incidence flow angle 
0g- Magnetic flux at the air gap of the generator 
K- Local speed ratio 
A - Swept area of the wind rotor 
b - No. of blades 
cd- Drag coefficient 
Ci- Lift coefficient 
Cm - Coefficient of moment 
cP- Coefficient of power 
C-Pmax" Maximum power coefficient 
Cpr - Local power coefficient 
D - Drag 
F - Axial thrust 
K d - Distribution factor 
K f - Pitch factor 
L- Lift 
1- Chord of the blade 
M - Moment 
MTOE- Million tons of oil equivalent 
N - Rotational speed of generator 
XI 
n b - Hub ratio of wind rotor 
P - Number of poles 
P<>- Energy content in the undisturbed wind 
P u - Rotor power 
R- Radius of the rotor 
ro- Hub radius 
Re- Reynolds No 
T - Torque 
V i - Undisturbed velocity of air 
v 2 - Axial velocity of air at down stream wake 
W - Velocity of wind relative to the rotor blade 
Z n - Number of conductors 
1 
XII