Delft University of Technology
FACULTY MECHANICAL, MARITIME AND MATERIALS ENGINEERING
Department Marine and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl
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Specialization: Transport Engineering and Logistics
Report number: 2016.TEL.8026
Title:
Inter-relationship between
fracture failure of FRC and
structural collapse of (FRC made)
wind turbine blade
Author:
J.D. Hockers
Title (in Dutch) De relatie tussen falen door breuk van FRC en het instorten van (FRC gemaakte) windturbine blad
Assignment: literature
Confidential: no
Supervisor: dr.ir. X. Jiang
Delft University of Technology Department of Marine and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl
Student: Assignment type: Literature
Supervisor: Dr. X. Jiang Report number: 2015.TEL.xxxx
Specialization: TEL Confidential:
Creditpoints (EC): 10
Subject: Inter-relationship between fracture failure of FRC and structural collapse of (FRC made) wind turbine blade
Fibre-reinforced composite (FRC) has been widely used for construction of thin- walled engineering structures, such as the wind turbine blade, the riser pipeline of deep water drilling, the fuselage of commercial airliner, and the rotor blade of helicopter. Compared with traditional materials, the FRC generally has merits of high stiffness, high strength to weight ratio, long fatigue life, strong resistance against corrosion, etc. However, the fundamental knowledge on FRC is rather limited in contrast to which on steel or alloy. For instance, there have been many records on structural collapse of wind turbine blade. But the inter-relationship between such a structural collapse and failure of FRC is not clear. Among various failure modes of FRC, fracture has been recognized as a dominant failure mode of FRC.
This literature assignment aims to make an overview of the development of theories and approaches deployed for the research on the inter-relationship between structural collapse of (FRC made) wind turbine blade and fracture failure of FRC. The following aspects are required to be illustrated in the report:
• Development of wind turbine industry, including material, dimension, design, assessment, maintenance, relevant rules /standards etc.
• Main types of structural collapse / failure of wind turbine blades.
• Material property of FRC and the state of art of research on fracture mechanism of FRC. • Available theories and approaches (analytical, numerical, experimental methods etc.)
deployed to identify the reasons of those structural failures/ collapse.
• Possible inter-relationship between structural failure and fracture of FRC based on your review.
• The feasibility of structural optimization of wind turbine blade based on conclusion on the inter-relationship between structural failure and fracture of FRC ( what has been done, to be done and should be done related to structural optimization of wind turbine blade.)
This report should be arranged in such a way that all data is structurally presented in graphs, tables, and lists with belonging descriptions and explanations in text.
The report should comply with the guidelines of the section. Details can be found on the website. If you would like to know more about the assignment, you may contact with Dr. X Jiang through x.jiang@tudelft.nl.
The Supervisor, X. Jiang
I
NTER
-
REL ATIONSHIP BETWEEN FRACTURE
FAILURE OF
FRC
AND STRUCTURAL
COLL APSE OF
(FRC
MADE
)
WIND TURBINE
BL ADE
by
J.D Hockers
in partial fulfillment of the requirements for the degree of
Master of Science
in Mechanical Engineering
at the Delft University of Technology
J.D Hockers: 1519018 Supervisor: dr. ir. X. Jiang
S
UMMARY
The demand for clean energy is rapidly growing and the use of wind turbines is a good solutions. Because of the growing demand the wind turbines are becoming larger and therefore the blades of the turbines as well. Specific design requirements are needed for such structures and the choose of material plays an important role. Fiber reinforced composites are known for their high stiffness, high strength to weight ratio, long fatigue life, strong resistance against corrosion etc but the knowledge about fiber reinforced composites is limited compared to well known materials such as alloy or steel. It happens that complete wind turbine blades col-lapse, but why this happens is not enough clarified. Therefore, the inter-relationship between the structural collapse of the wind turbine blade and the failure of the fiber reinforced composite material that is used is also unclear. It is desired to know if there is a possible inter-relationship and to obtain insight on this subject. Since the wind energy sector is rapidly growing and the forecasts are that this will continue due to emission free power sector targets. The most common used way to generate electricity from wind energy is by using horizontal axis wind turbines (HAWT), as there are also vertical axis wind turbines (VAWT) but they are not commonly used. The standards used for the wind turbines are from the IEC 61400 standards, which are divided into multiple standards where each standard describes its own subject about the wind turbine. It has become clear that wind turbines keep increasing in size and the material choice for the turbines depends on the desired properties (ratio for strength/weight and stiffness/weight) and costs. As this report focus on the blade materials, the materials used are fiber reinforced composites. They are with polymeric matrices of vinyl ester, polyester or epoxy and reinforcement like carbon, glass fiber or aramid fiber. Most common blades are made with glass fibers, but bigger blades nowadays consist of carbon fibers although it is more expensive. For the assessment of a wind project there are different steps: initial assessment with known data (wind maps etc.), detailed site characterization with the help of wind data recording towers or computer models and long-term validation of the data. Looking at the main concerns for a wind turbine design as well as the general design of a wind turbine blade, there are multiple methods for the clearance between the tower and the blades, which are: Coning of the blades, tilting of shaft axis, curved blades and the overhangdistance. Maintenance of a wind turbine system involves yearly inspection of the ehole wind turbine system
and the help of condition monitoring systems can aid in planned maintenance. Lubrication of the different components of the wind turbine is very important as the wind turbine consist of many moving parts which are under high stresses and loads.
The material of wind turbine blades are fiber reinforced composites that consists of a polymer or a ceramic fibers and a matrix. The main types of fibers are glass, carbon and aramid. Furthermore, the advantages of the fiber reinforced composites are a high strength/stiffness ratio and light weight, designable natural fre-quency and good anti-vibration, "easy" molding process, the sensitivity of imperfections of fiber reinforced composites with high fatigue, good weather and corrosion resistance and simple maintenance. Also the dif-ferent configurations of the fibers can be woven fabrics and knitted fabrics. Woven and knitted fabrics can have also different structures. For woven fabrics there are the general woven, 3-directional and 3-dimensional fabric. The knitted fabric can be categorized into two groups, namely warp knitted and weft knitted. Warp knitted fabric is made of a group or more groups of parallel yarns that are simultaneously fed through the warp knitting machine needle and for weft knitting, the yarns on the loom are in such configuration that it is forming coil rows. The coil rows are then woven in weft into weft knitted fabric.
It has become clear that failure of wind turbine blades unfortunately happens. Each wind turbine has a different operational profile, mainly due to its location. The environment variables differ greatly between different locations and this can lead to different wind speeds, humidity, temperature, exposure to the sun, turbulence intensity, rain, lightning etc. Therefore, the causes for wind turbine blade failure also differ. The main reasons for failure are: wind gusts, ice onto the blades, lightning strikes, faulty manufacturing, erosion and eventually fatigue. After the reasons became clear, the effects on the material is shown, with matrix and fiber-matrix interface cracking due to tension, compression and combined loads. Furthermore fiber failure and debonding and failure of composite laminates and the delamination fractures can occur. There are 7 different types of damages on material level for the wind turbine blades. Namely, type 1: skin/adhesive
debonding, type 2: adhesive joint failure, type 3: sandwich debonding, type 4: delamination, type 5: splitting along fibers, type 6: buckling induced skin/adhesive debonding (special case of type 1) and type 7: cracks in gelcoat.
The blades are one of the most critical components of a wind turbine and they have to be tested and in-spected to ensure their desired specifications. On different scale levels can tests be done, namely: full-scale,
subcomponent level, material level. In which the material level can be divided from the macro-scale up to
micro-scale. The mechanical properties of the wind turbine blade are obtained by, static tests, fatigue tests and modal testing. With static tests the ultimate strength is obtained, fatigue tests are used to know the dura-bility of the object and modal testing is performed to know the modal properties, such as damping and natural frequencies. Non-destructive testing (NDT) methods to test the wind turbine blades are: visual inspection,
ultrasonic and sonic NDT, optical NDT, electromagnetic NDT, optical thermography, non-optical thermogra-phy and radiographic NDT. There are multiple analytical methods to calculate and make predictions of wind
turbine blade properties: the momentum theory (with optimal conditions), the blade element momentum
theory (BEM)(non-optimal conditions). The properties of the composite stiffness and the laminate can be
calculated but are estimations. Also there are numerical methods to verify designs or predict failure of the blade, but the most common method is the finite element method. The blade is usually modeled out of shell elements and for the thicker parts it is modeled with brick elements. The finite element method enables to find approximate solutions for larger mathematical problems by dividing the bigger problem into smaller ones. The analytical and numerical methods are methods to aid in the design process of the wind turbine blade and the results of the different methods needs to be tested with the help of experiments as discussed before.
From this it became clear that there is a possible inter-relationship between fracture failure of fiber rein-forced composites (FRC) and structural collapse of FRC made wind turbine blades. The inter-relationship is as a continues circle, where the causes for damage to a wind turbine blade (wind gusts, ice accumulation, lightning, faulty manufacturing, erosion, fatigue) results in one of the 7 types of damages known for fiber rein-forced composite wind turbine blades skin/adhesive debonding, adhesive joint failure, sandwich debonding, delamination, splitting along fibers, buckling induced skin/adhesive debonding, cracks in gelcoat). The frac-ture failure of the material (fiber reinforced composites) of the blade causes instabilities in the blade which make the blade "weaker" (lower specifications than the initial specifications). To complete the circle, the causes for damage to a wind turbine blade can increase the damages until structural collapse occurs. For optimizing the blade designs, analytical and numerical methods are used in combination with the exper-iments onto the blades but to get more insight of the inter-relationship between fracture failure of FRC and structural collapse of FRC made wind turbine blades and therefore optimizing the wind turbine blade to avoid structural collapse of the wind turbine blade is recommended to introduce standards and guidelines to iden-tify the initiations and propagations of damage for the critical design details, as most of the time the failure happens in those details. So designs are not only made with knowledge of the material bulk properties. Also more research on microscopic level to get more insight of damage accumulation is recommended instead of only know (macroscopic) failure predictions that are used for the blade designs. Further recommendation is integration of monitoring techniques, where the loads onto the blade are monitored and implemented in a micromechanical numerical model to make residual life predictions based on the actual damage state of the blade, instead of the assumed load history.
L
IST OF
F
IGURES
2.1 Total installed wind capacity by mid-2015. . . 4
2.2 Total installed wind capacity by the end of 2015 . . . 4
2.3 Top 10 new installed capacity January-December 2015. . . 5
2.4 Market forecast for 2016-2020. . . 5
2.5 Examples of utilization of wind energy . . . 6
2.6 Main types of wind turbines. . . 7
2.7 Main components of a wind turbine. . . 8
2.8 Global turbine capacities, rotor diameters and hub heights over time . . . 9
2.9 Transportation of onshore and offshore wind turbine towers. . . 17
2.10 Precast plant for precast concrete segments (left) and horizontal reinforcement loops of a pre-cast concrete segment (right) . . . 18
2.11 Young’s modulus versus density . . . 18
2.12 Wind map of the United States. . . 20
2.13 Different types of rotor configurations. . . 21
2.14 Different methods to ensure blade to tower clearance. . . 22
2.15 Blade design: a - single shear web; b - double shear webs; c - external geometry.. . . 22
3.1 Structure of general woven fabric. . . 27
3.2 Structure of 3-directional woven fabric. . . 27
3.3 Structure of 3-dimensional woven fabric. . . 28
3.4 Different types of structures of warp knitted fabric. . . 28
3.5 Types of structures of weft knitted fabric. . . 29
4.1 Wind induced blade failure. . . 32
4.2 Problems of ice on wind turbine blades.. . . 32
4.3 Blade failures because of lightning strikes. . . 33
4.4 Example of leading edge erosion on a wind turbine blade. . . 33
4.5 An example of blade failure due to fatigue. . . 34
4.6 Sketch of lamina with local triaxial stress state represented. . . 34
4.7 Illustration of crack formation under tension normal to fibers. . . 35
4.8 Example of crack due to transverse compression. . . 35
4.9 Cracks formed in matrix planes inclined to the fiber under the in-plane shear stress. . . 35
4.10 Micromechanical stages identified for the interfiber failure under tension. . . 36
4.11 Damage development scenarios after break formation. . . 37
4.12 Schematic of final failure of composite due to damage events under increased tensional loads. . 37
4.13 Schematic and an optical micrograph illustrating fiber microbuckling in a laminate. . . 38
4.14 Different modes of crack tip deformations. . . 38
4.15 Damage types on the downwind skin due to compressive loads. (sketch) . . . 40
4.16 Damage at the leading edge (Type 2). . . 40
4.17 Damage of type 5 and 7 to the downwind skin at the leading edge due to compressive load. . . . 41
4.18 Damage of type 3 and 5 on the inside of the downwind skin. . . 41
4.19 Damages to the main spar outer surface.(sketch) . . . 42
4.20 Damage type 3, 5 and 7 in the outer-face of the main spar facing the leading edge. . . 42
4.21 Damage to the internal surface of the main spar.(sketch). . . 43
4.22 Damage of type 4 and 5 onto the main spar.(sketch) . . . 43
4.23 Damage of type 1 and 4 on the downwind skin and main spar flange. . . 44
4.24 Damage of type 3 on the main spar web sandwich. . . 44
5.1 The verification escalade for a wind turbine blade. . . 48 v
5.2 Multi-scale characterization of a wind turbine blade. . . 49
5.3 Static test of a wind turbine blade in flap-wise mode (Full-scale).. . . 50
5.4 Resonant oscillation fatigue testing of a wind turbine blade.. . . 51
5.5 "Sky-workers" inspecting or repairing a wind turbine blade.. . . 52
5.6 Ultrasound echo at a bonding area of a wind turbine blade. . . 52
5.7 Comparison of (top) bulk wave ultrasonic testing and (bottom) guided wave ultrasonic testing. 53 5.8 Schematic of laser ultrasonic detection of an internal defect. . . 54
5.9 Measurement principle of local resonance spectroscopy.. . . 54
5.10 Principle of an acoustic emission measurement test. . . 55
5.11 Schematic of a 3-D digital image correlation system. . . 55
5.12 principle of out-of-plane displacement measurement system for shearography . . . 56
5.13 The principle of eddy current testing. . . 56
5.14 An infrared image of an airplane.. . . 57
5.15 The principle of line scanning thermography. . . 58
5.16 Annular axial stream tube of air crossing a wind turbine rotor for different axial induction factors. 61 5.17 Resultant relative velocity at a blade element. . . 63
5.18 Coordinate systems for the blade, laminate and ply.. . . 64
L
IST OF
T
ABLES
2.1 List of standards for wind turbine systems . . . 16
2.2 Comparison between different fiber materials.. . . 19
3.1 Properties of different fibers. . . 27
5.1 Summary and comparison for NDT methods used for wind turbine inspections. . . 60
C
ONTENTS
Summary iii
List of Figures v
List of Tables vii
1 Introduction 1
2 Wind energy 3
2.1 Importance of wind energy . . . 3
2.2 Global status of wind power in 2015. . . 3
2.3 Market developments. . . 5
2.3.1 Climate . . . 6
2.3.2 Cratering prices . . . 6
2.3.3 United States market stability . . . 6
2.4 Utilization of wind energy. . . 6
2.5 Wind turbine . . . 7
2.5.1 Types of wind turbines. . . 7
2.5.2 General components of a wind turbine . . . 8
2.6 Standards for wind turbines. . . 10
2.7 Material. . . 16
2.7.1 Tower material. . . 17
2.7.2 Blade material . . . 18
2.8 Wind resource assessment . . . 19
2.8.1 Initial assessment . . . 20
2.8.2 Detailed site characterization . . . 20
2.8.3 Long-Term validation of data . . . 21
2.9 Design of wind turbine . . . 21
2.9.1 Blade to tower clearance. . . 21
2.9.2 Wind turbine blade design. . . 22
2.10Maintenance . . . 23
2.10.1 Lubrication . . . 23
2.10.2 Component repairs & replacement . . . 23
2.11Chapter summary. . . 23
3 Fiber reinforced composites 25 3.1 Definition of fiber reinforced composites. . . 25
3.2 Advantages of fiber reinforced composites . . . 25
3.3 Types of fiber material . . . 26
3.3.1 Glass fiber . . . 26 3.3.2 Carbon fiber. . . 26 3.3.3 Aramid fiber . . . 26 3.4 Fiber configuration . . . 27 3.4.1 Woven fabric. . . 27 3.4.2 Knitted fabric . . . 28 3.5 Chapter summary. . . 29
4 Failures of wind turbine blades 31 4.1 Operational profile of wind turbines . . . 31
4.2 Causes of wind turbine blade failure . . . 31
4.2.1 Wind induced blade failure . . . 32
4.2.2 Blade failure due to ice. . . 32 ix
4.2.3 Blade failure due to lightning strikes. . . 32
4.2.4 Blade failure due to faulty manufacturing . . . 33
4.2.5 Blade failure due to erosion . . . 33
4.2.6 Blade failure due to fatigue. . . 34
4.3 Fracture mechanisms of fiber reinforced composites. . . 34
4.3.1 Matrix and fiber-matrix interface cracking. . . 34
4.3.2 Compression failure of composite laminates. . . 38
4.3.3 Delamination fractures . . . 38
4.4 Types of blade damages. . . 38
4.5 Chapter summary. . . 45
5 Testing & inspecting 47 5.1 Purpose of blade testing. . . 47
5.2 The verification escalade . . . 47
5.2.1 Full-scale testing. . . 47
5.2.2 Subcomponent testing. . . 48
5.2.3 Material (coupon) testing . . . 49
5.3 Mechanical property testing . . . 49
5.3.1 Static testing. . . 50
5.3.2 Fatigue testing. . . 50
5.3.3 Modal testing . . . 51
5.4 Non-destructive testing methods. . . 51
5.4.1 Visual inspection. . . 51
5.4.2 Ultrasonic and sonic NDT . . . 52
5.4.3 Optical NDT . . . 55 5.4.4 Electromagnetic NDT . . . 56 5.4.5 Optical thermography. . . 57 5.4.6 Non-optical thermography . . . 58 5.4.7 Radiographic NDT. . . 59 5.4.8 NDT summary. . . 59 5.5 Analytical methods . . . 61 5.5.1 Momentum theory. . . 61
5.5.2 Blade element momentum theory. . . 63
5.5.3 Composite stiffness properties. . . 64
5.5.4 Laminate properties. . . 65
5.6 Numerical method . . . 67
5.7 Chapter summary. . . 67
6 Conclusions & Recommendations 69 6.1 Conclusions. . . 69
6.2 Recommendations . . . 69
1
I
NTRODUCTION
It is all over the news, the climate is changing, the fossil energy resources are decreasing and the world needs to acknowledge it and do something about it. Also the population of the world keeps growing and the demand for electrical energy keeps increasing. In order to fulfill the demands of electrical energy and at the same time not polluting the environment, wind turbines can play an important role. Nowadays people can see wind turbines all over the world, but this type of "clean" energy is not the main resource for it. Still electrical energy is generated with polluting methods such as coal power plants. Wind turbines are considered as a fairly new method for generating electrical energy and it is rapidly growing. Because this type of power generating is fairly new and rapidly growing, there are still things unknown about it.
Due to the rapidly growing demand of clean energy, the wind turbines becomes larger. Therefore, the blades of the wind turbines are also rapidly becoming larger. Specific design requirements are needed for such struc-tures and the choose of material plays an important role. Fiber reinforced composites are known for their high stiffness, high strength to weight ratio, long fatigue life, strong resistance against corrosion etc but the knowledge about fiber reinforced composites is limited compared to well known materials such as alloy or steel. It happens that complete wind turbine blades collapse, but why this happens is not enough clarified. Therefore, the inter-relationship between the structural collapse of the wind turbine blade and the failure of the fiber reinforced composites material that is used is also unclear. This literature assignment is to make an overview of what is known about the structural collapse of wind turbine blades and fracture failure of fiber reinforced composites and the possible inter-relationship between them.
The structure of this report will mainly have a top-down perspective. Firstly the wind energy industry will be introduced where some different subjects will be highlighted such as the global status of wind energy, the market developments and the utilization of it. From there on the focus will be on the wind turbines, where the different types, the design and the material that are being used will be the main topics. After this it is know which material are used for the wind turbine blades and therefore the focus will go on the material (fiber reinforced composites) of the wind turbine blades. Here the fiber reinforced composites are outlined and what different types of fibers there are and their configurations. Now it is know what material is used for the wind turbine blades and the types of failure of the blades will be investigated and why this can happen. After this the methods for identifying failures by testing and inspecting of the turbine blades and material will be showed and finally from all of this the possible inter-relationship between the structural failure of the blades and fracture of the material will be discussed and recommendations about further research will be given.
2
W
IND ENERGY
In this chapter different subjects that plays an important role in the wind energy will be outlined. First, the importance of wind energy will be discussed and then the global know facts about the wind power from last year (2015). The market developments regarding wind energy are discussed and as well the utilization of wind energy. After this, the types of wind turbines and general components of a wind turbine will be clear and the standards needed for the wind turbine will be outlined. Further on the wind resource assessment, materials, maintenance of wind turbines and design considerations for the wind turbine will be explained.
2.1.
IMPORTANCE OF WIND ENERGY
The energy demand is worldwide rapidly increasing and continuing the extensive use of conventional en-ergy sources is polluting the environment. This causes global warming and therefore the development of alternative energy sources has becomes a necessity. Fossil energy sources are now the most conventional en-ergy sources being used and those resources are declining, which make the development of other renewable energy sources also important. Wind energy and other renewable energy sources are clean and viable alter-natives to fossil fuels. Extensive availability and low operating costs make wind one of the most effective and advantageous renewable energy sources[1].
There are long term targets for a 100% free emissions free power sector by the year 2050, which making use pf wind energy resources more interesting as wind energy is a clean energy resource. This will be discussed even more further on in this report.
So making use of wind energy resources already have become and still is a good alternative to using fossil energy resources. As the wind energy is considerably a new type of energy resource, it is still an industry where lot of progress is expected and this will be further outlined in this report.
2.2.
GLOBAL STATUS OF WIND POWER IN
2015
The market for wind installations have been rapidly growing the last years and the market keeps growing. Even the growth rates keeps exceeding the expected growth rates from the last years for this market. By the end of June 2015 the worldwide wind capacity reached 393 GW, out of which 21,7 GW were added in the first six months of 2015. In comparison with previous years, this increase is substantially higher. In the first half of 2014 and 2013 were respectively 17,6 GW and 13,9 GW added. By mid-2015 4% of the world’s electricity demand could be generated by all wind turbines installed worldwide.[2]
Looking at the figures of total installed wind capacity of the world at mid-2015(Figure 2.1) and at the end of 2015(Figure 2.2), it clearly shows that even the growth rates keeps increasing more than expected. Fig-ure 2.1shows that the expected growth rate from mid-2015 till the end of 2015 is 9%, butFigure 2.2shows that this growth rate even became larger. The figures inFigure 2.2even have some estimations and includes all installed wind capacity connected and not-connected to the grid.
Figure 2.1: Total installed wind capacity by mid-2015.[2]
Figure 2.2: Total installed wind capacity by the end of 2015.[3]
For 2015 the new global wind power had a cumulative market growth of more than 17%. China made it really possible for this new record annual growth by installing 30 GW of new installations. The total global installed wind power for 2015 was around 63 GW, which represented an annual market growth of 22%.Figure 2.3shows the top 10 countries of the new installed capacity for the year 2015.
Asia is still leading the global markets with China retaining the top spot since 2009 for the largest overall market for wind power. Europe is in the second spot and North America is placed third and closing the gap with Europe. A trend has developed and keeps continuing for the future, namely that the majority of the global wind installations were outside of the Organization for Economic Co-operation and Development (OECD). This trend started since 2010 and for the years after it continued with an exception for 2012[4].
2.3.MARKET DEVELOPMENTS 5
Figure 2.3: Top 10 new installed capacity January-December 2015.[4]
2.3.
MARKET DEVELOPMENTS
The Global Wind Report 2015[5] of Global Wind Energy Council (GWEC) shows the future market develop-ments. There are three big trends that are expected to drive growth in the medium term:
• Climate
• Cratering prizes
• US Market stability
Figure 2.4shows the market forecast for 2016-2020 and the three big trends that make this possible are dis-cussed.
2.3.1.
C
LIMATEIn December 2015 there was the Twenty-first session of the Conference of the Parties (COP21) at the United Nations Climate Change conference (UNFCCC) in Paris. Here the negotiations led to a positive outcome that was not expected. 186 countries that gathered in Paris adopted the long term targets for a 100% emissions free power sector by 2050. Wind and solar energy are leading the pack for renewable energy and this means a power sector almost completely supplied by these energy sources.
2.3.2.
C
RATERING PRICESFor the last years the United States market have very low wind prices and also other countries such as Egypt, Peru, Morocco and elsewhere. They are generating low prices,those prices are in the vicinity ofe40/MWh or below. For Morocco it is even belowe30/MWh. It is not certain if this trend will continue but it is expected that it will, because the costs of both wind and solar technology have decreased enormously in recent years. The pressure of lowering prices will continue but that is not the only reason for this trend. Also in some regions there are excellent wind resources and this contributes to the trend.
2.3.3.
U
NITEDS
TATES MARKET STABILITYThe main federal policy support for wind energy in the United states (US), namely the Production Tax Credit (PTC) has been phased out and a long term extension of it is signed into law by the US Congress and the President. This means the US wind industry will have a long period of policy stability, that will contribute to the implications of markets even beyond the US market.
Although the enormous growth of the last two years, it is not expected that this will continue at the same rate. China made this enormous growth possible and this was not expected regarding the future forecasts at the time. Now, the future forecasts are that China will not again will be responsible for such an unexpected growth.
2.4.
UTILIZATION OF WIND ENERGY
There are different ways to make use of wind energy, such as to generate mechanical power or electricity from the kinetic energy of the wind. The mechanical power can be used for specific tasks such as pumping water or grinding grain. An example of making use of mechanical power from wind energy are the Dutch windmills that were used for pumping water (figure 2.5a).
(a) Dutch windmills used for pumping water.[6] (b) Most common type of wind turbine for generating electricity.[7]
Figure 2.5: Examples of utilization of wind energy
It is not common anymore to use the wind energy directly for mechanical power as we now have electrical systems to do the same work. But as already mentioned is it possible to generate electricity from the wind energy. The system used for this process is the common used wind turbine. The most common used type of wind turbine is depicted in figure 2.5b.
2.5.WIND TURBINE 7
In this report will the focus be on the wind turbines that generate electricity from wind energy as it is the common way for the utilization of wind energy nowadays.
2.5.
WIND TURBINE
For generating electricity from wind energy, the most important thing for this is the wind turbine. The tech-nology for the wind energy industry keeps improving and this results in significant improvement in efficiency and generated power output by the wind turbines[8]. There are even developments that enable gearless tur-bine designs by using new power electronic devices and the developments of specific generators[9][10]. One of the objectives of wind turbine research is to reduce the costs and maximize the wind capture as also to decrease the amount of failures and increasing the safety of wind turbine use. Therefore, the design of the wind turbine and it size are crucial aspects. The trend is that the turbines keeps getting bigger and this resulted in different ratings for the wind turbines. The ratings for the turbines ranges from several kilowatts to megawatts[11]. This also means for bigger wind turbines that the diameter of the turbines get bigger, so longer blades that are in contact with more wind in a larger area in order to produce more energy.
2.5.1.
T
YPES OF WIND TURBINESThere are two modern main types of wind turbines: Vertical axes wind turbine (VAWT) and Horizontal axes
wind turbine (HAWT). An example of a HAWT and a VAWT is showed inFigure 2.6.
(a) An example of a HAWT.[12] (b) An example of a VAWT.[13] Figure 2.6: Main types of wind turbines.
HAWT
A horizontal axes wind turbine has the main rotor shaft and generator at the top of a tower, and in order to generate electricity it must be pointed into the wind. The rotor shaft and generator are in line with the wind direction and parallel to the ground. They are usually positioned upwind, which means that the rotor and blades are, compared to the supporting tower, closer to where the wind is coming from. So a HAWT needs a system that rotates the turbine in the correct direction.
Most of the HAWT’s that are being used commercially have 3 blades and have a high efficiency and good reliability. The HAWT’s keeps increasing in size and a downside is that tip speeds of the blades becomes higher, because of the larger blades, which results in more noise.
VAWT
Vertical axes wind turbines have the main rotor shaft in a vertical position, so perpendicular to the ground. The advantage for this is that the VAWT does not need to be pointed into the wind to be effective. Another advantage is that the generator and gearbox can be placed near the ground, this enables accessibility for maintenance.
A downside of the VAWT’s is because they are installed close to the ground, this results in less exposure to the wind, so less power output. However, they can be easily installed in urban areas like on rooftops, because they are smaller and produce less noise than HAWT’s. It also have been proven to be much more efficient on rooftops (also high in the mountains) in that way[14][15].
For this report, the main focus will be on the HAWT type, because this type is the most common used and also most conducted research is available about this type.
2.5.2.
G
ENERAL COMPONENTS OF A WIND TURBINEA wind turbine consists of different components, but the typical components of the turbine are: a rotor,
blades, a tower, a generator and a gearbox (if it is not with a direct drive). InFigure 2.7are the main compo-nents of a wind turbine displayed. It shows a HAWT as it is the most used type of wind turbine globally for now.
Figure 2.7: Main components of a wind turbine.[16]
Figure 2.7shows more than only the earlier discussed main components such as the yaw drive, yaw motor
and brake. A short description of the main components will be given.
ROTOR
AsFigure 2.7shows clearly the rotor, it can be seen that the rotor consist of blades. Those blades can be very large and recent trends shows that the rotors and blades keeps increasing in size. Overall, most turbines consists of three blades, but turbines of two blades are also functional.Figure 2.8shows the average size of the rotor over time and it clearly shows the trend for upscaling the size of the wind turbine and therefore also the size of the blades. Figure 2.8is for offshore wind turbines, but the trend is the same for onshore wind turbines, namely upscaling the size. The average rating for the offshore wind turbine are now around the 4,5∼5 MW with an average rotor diameter of 120 m. The rotor also consists a so called pitch drive, this pitch drive can rotate each blade more into to wind or less. This is needed to reduce the wind forces onto the blades in high wind conditions. So the pitch drive makes sure that the wind turbine rotates in a desired operating range[17].
2.5.WIND TURBINE 9
Figure 2.8: Global turbine capacities, rotor diameters and hub heights over time.[18]
NACELLE
The nacelle is the main housing of the most important technical parts of the wind turbine. At the top of the wind turbine is the nacelle located and it contains the gearbox, rotor shaft, generator and other elec-trical/technical components. The nacelle is attached to the rotor and can rotate with respect to the wind direction such that the wind turbine can have maximum profit of the wind.
GENERATOR
In order to get electrical energy, the generator converts the mechanical energy, from the rotating blades to the rotor shaft, into electrical power. There are many different types of generators such as induction (asyn-chronous) generators and synchronous generators. Each wind turbine can use a different type of generator such it is suited for its designated purpose.
GEARBOX
The rotor of the wind turbine usually rotates at a speed of less than 50 rpm1, but most generators converts the mechanical energy of the rotor to electrical energy at a much higher speed, 1000∼3600 rpm. So the gear-box is necessary to convert the low speed of the rotor into much higher speed needed for the generator to operate[19].
TOWER/FOUNDATION
On top of the tower are the nacelle and rotor located. At higher altitudes the quantity of the wind and speed becomes also higher. Therefore, the tower is needed to place the rotor at high altitudes to make sure the wind turbine can capture more wind energy as there is more and more consistent wind at higher altitudes.
Figure 2.7also shows some other parts that contribute for a good operating wind turbine. The anemometer measures the wind speed, so the controller can operate the desired pitch of the blades as the wind vane (not showed inFigure 2.7) gives a signal from what the wind direction is so the yaw drive can turn the rotor into the desired direction.
2.6.
STANDARDS FOR WIND TURBINES
There are standards set in order to have safe working wind turbines. The designers of the wind turbine sys-tems must follow the standards and guidelines. In this section a short overview will be given from the most important standards and guidelines for the wind turbines.
The International Electrotechnical Commission (IEC) Committee 88 [20] prepares standards that deal with test procedures for wind turbine generator systems, safety and measurement techniques. They have pro-duced standards for design requirements, measurements of mechanical loads, acoustic noise measurement techniques, and communications for monitoring and control of wind power plants.
A description of the different standards for wind turbines will be given.
IEC 61400-1
This part of IEC 61400 specifies essential design requirements to ensure the engineering integrity of wind tur-bines. Its purpose is to provide an appropriate level of protection against damage from all hazards during the planned lifetime. This standard is concerned with all subsystems of wind turbines such as control and pro-tection mechanisms, internal electrical systems, mechanical systems and support structures. This standard applies to wind turbines of all sizes and for especially small wind turbines mayIEC 61400-2be applied[21].
IEC 61400-2
This part of IEC 61400 deals with safety philosophy, quality assurance, and engineering integrity and specifies requirements for the safety of Small Wind Turbines (SWTs) including design, installation, maintenance and operation under specified external conditions. Its purpose is to provide the appropriate level of protection against damage from hazards from these systems during their planned lifetime. This part of IEC 61400 is con-cerned with all subsystems of SWT such as protection mechanisms, internal electrical systems, mechanical systems, support structures, foundations and the electrical interconnection with the load. While this part of IEC 61400 is similar toIEC 61400-1, it does simplify and make significant changes in order to be applicable to small turbines. This part of IEC 61400 applies to wind turbines with a rotor swept area smaller than 200 m2, generating at a voltage below 1000 V a.c. or 1500 V d.c[22].
IEC 61400-3
This part of IEC 61400 specifies additional requirements for assessment of the external conditions at an off-shore wind turbine site and it specifies essential design requirements to ensure the engineering integrity of offshore wind turbines. Its purpose is to provide an appropriate level of protection against damage from all hazards during the planned lifetime.
This standard focuses on the engineering integrity of the structural components of an offshore wind turbine but is also concerned with subsystems such as control and protection mechanisms, internal electrical sys-tems and mechanical syssys-tems. A wind turbine shall be considered as an offshore wind turbine if the support structure is subject to hydrodynamic loading. The design requirements specified in this standard are not necessarily sufficient to ensure the engineering integrity of floating offshore wind turbines.
This standard is fully consistent with the requirements ofIEC 61400-1. The safety level of the offshore wind turbine designed according to this standard shall be at or exceed the level inherent inIEC 61400-1. In some clauses, where a comprehensive statement of requirements aids clarity, replication of text from IEC 61400-1 is included[23].
IEC 61400-4
This part of the IEC 61400 series is applicable to enclosed speed increasing gearboxes for horizontal axis wind turbine drive trains with a power rating in excess of 500 kW. This standard applies to wind turbines installed onshore or offshore. This International Standard provides guidance on the analysis of the wind turbine loads in relation to the design of the gear and gearbox elements. The gearing elements covered by this standard include such gears as spur, helical or double helical and their combinations in parallel and epicyclic arrangements in the main power path. This standard does not apply to power take off gears (PTO).
2.6.STANDARDS FOR WIND TURBINES 11
The standard is based on gearbox designs using rolling element bearings. Use of plain bearings is permissible under this standard, but the use and rating of them is not covered.
Also included is guidance on the engineering of shafts, shaft hub interfaces, bearings and the gear case struc-ture in the development of a fully integrated design that meets the rigours of the operating conditions. Lu-brication of the transmission is covered along with prototype and production testing. Finally, guidance is provided on the operation and maintenance of the gearbox[24].
IEC 61400-11
This part of IEC 61400 presents measurement procedures that enable noise emissions of a wind turbine to be characterized. This involves using measurement methods appropriate to noise emission assessment at locations close to the machine, in order to avoid errors due to sound propagation, but far away enough to allow for the finite source size. The procedures described are different in some respects from those that would be adopted for noise assessment in community noise studies. They are intended to facilitate characterization of wind turbine noise with respect to a range of wind speeds and directions. Standardization of measurement procedures will also facilitate comparisons between different wind turbines.
The procedures present methodologies that will enable the noise emissions of a single wind turbine to be characterized in a consistent and accurate manner. These procedures include the following:
• location of acoustic measurement positions.
• requirements for the acquisition of acoustic, meteorological, and associated wind turbine operational data.
• analysis of the data obtained and the content for the data report.
• definition of specific acoustic emission parameters, and associated descriptors which are used for mak-ing environmental assessments.
This International Standard is not restricted to wind turbines of a particular size or type. The procedures described in this standard allow for the thorough description of the noise emission from a wind turbine[25].
IEC 61400-12-1
This part of IEC 61400 specifies a procedure for measuring the power performance characteristics of a single wind turbine and applies to the testing of wind turbines of all types and sizes connected to the electrical power network. In addition, this standard describes a procedure to be used to determine the power performance characteristics of small wind turbines (as defined inIEC 61400-2) when connected to either the electric power network or a battery bank. The procedure can be used for performance evaluation of specific turbines at specific locations, but equally the methodology can be used to make generic comparisons between different turbine models or different turbine settings.
The wind turbine power performance characteristics are determined by the measured power curve and the estimated annual energy production (AEP). The measured power curve is determined by collecting simul-taneous measurements of wind speed and power output at the test site for a period that is long enough to establish a statistically significant database over a range of wind speeds and under varying wind and atmo-spheric conditions. The AEP is calculated by applying the measured power curve to reference wind speed frequency distributions, assuming 100% availability.
The standard describes a measurement methodology that requires the measured power curve and derived energy production figures to be supplemented by an assessment of uncertainty sources and their combined effects[26].
IEC 61400-12-2
This part of IEC 61400-12 specifies a procedure for verifying the power performance characteristics of a single electricity-producing, horizontal axis wind turbine, which is not considered to be a small wind turbine per
IEC 61400-2. It is expected that this standard will be used when the specific operational or contractual spec-ifications may not comply with the requirements set forth inIEC 61400-12-1. The procedure can be used for
power performance evaluation of specific turbines at specific locations, but equally the methodology can be used to make generic comparisons between different turbine models or different turbine settings.
The wind turbine power performance characterized by the measured power curve and the estimated AEP based on nacelle-measured wind speed will be affected by the turbine rotor (i.e. speeded up or slowed down wind speed). The nacelle-measured wind speed shall be corrected for this flow distortion effect. Procedures for determining that correction will be included in the methodology. InIEC 61400-12-1, an anemometer is located on a meteorological tower that is located between two and four rotor diameters upwind of the test turbine. This location allows direct measurement of the ‘free’ wind with minimum interference from the test turbine’s rotor. In thisIEC 61400-12-2procedure, the anemometer is located on or near the test turbine’s nacelle. In this location, the anemometer is measuring wind speed that is strongly affected by the test tur-bine’s rotor and the nacelle. This procedure includes methods for determining and applying appropriate corrections for this interference. However, it should be noted that these corrections inherently increase the measurement uncertainty compared to a properly-configured test conducted in accordance withIEC 61400-12-1.
ThisIEC 61400-12-2standard describes how to characterize a wind turbine’s power performance in terms of a measured power curve and the estimated AEP. The measured power curve is determined by collecting simultaneous measurements of nacelle-measured wind speed and power output for a period that is long enough to establish a statistically significant database over a range of wind speeds and under varying wind and atmospheric conditions. In order to accurately measure the power curve, the nacelle-measured wind speed is adjusted using a transfer function to estimate the free stream wind speed. The procedure to measure and validate such a transfer function is presented herein. The AEP is calculated by applying the measured power curve to the reference wind speed frequency distributions, assuming 100% availability. The procedure also provides guidance on determination of measurement uncertainty including assessment of uncertainty sources and recommendations for combining them into uncertainties in reported power and AEP[27].
IEC 61400-13
This part of the IEC 61400 describes the measurement of fundamental structural loads on wind turbines for the purpose of the load simulation model validation. The standard prescribes the requirements and recom-mendations for site selection, signal selection, data acquisition, calibration, data verification, measurement load cases, capture matrix, post-processing, uncertainty determination and reporting. Informative annexes are also provided to improve understanding of testing methods.
The methods described in this standard can also be used for mechanical loads measurements for other pur-poses such as obtaining a measured statistical representation of loads, direct measurements of the design loads, safety and function testing, or measurement of component loads. If these methods are used for an alternative objective or used for an unconventional wind turbine design, the required signals, measurement load cases, capture matrix, and post processing methods should be evaluated and if needed adjusted to fit the objective.
These methods are intended for onshore electricity-generating, horizontal-axis wind turbines (HAWTs) with rotor swept areas of larger than 200 m2. However, the methods described may be applicable to other wind turbines (for example, small wind turbines, ducted wind turbines, vertical axis wind turbines)[28].
IEC 61400-14
This part of IEC 61400 gives guidelines for declaring the apparent sound power level and tonality of a batch of wind turbines. The measurement procedures for apparent sound power level and tonality are defined in
IEC 61400-11[29].
IEC 61400-21
This part of IEC 61400 includes:
• definition and specification of the quantities to be determined for characterizing the power quality of a grid connected wind turbine.
2.6.STANDARDS FOR WIND TURBINES 13
• measurement procedures for quantifying the characteristics.
• procedures for assessing compliance with power quality requirements, including estimation of the power quality expected from the wind turbine type when deployed at a specific site, possibly in groups. The measurement procedures are valid for single wind turbines with a three-phase grid connection, and as long as the wind turbine is not operated to actively control the frequency or voltage at any location in the network.
The measured characteristics are valid for the specific configuration of the assessed wind turbine only. Other configurations, including altered control parameters that cause the wind turbine to behave differently with respect to power quality, require separate assessment.
The measurement procedures are designed to be as non-site-specific as possible, so that power quality char-acteristics measured at for example a test site can be considered valid also at other sites[30].
IEC 61400-22
This International Standard defines rules and procedures for a certification system for wind turbines (WT) that comprises both type certification and certification of wind turbine projects installed on land or off-shore. This system specifies rules for procedures and management for carrying out conformity evaluation of WT and wind farms, with respect to specific standards and other technical requirements, relating to safety, reliability, performance, testing and interaction with electrical power networks. It provides:
• definitions of the elements in a wind turbine certification process.
• procedures for conformity evaluation in a wind turbine certification system.
• procedures for conformity surveillance.
• rules for the documentation that is to be supplied by an applicant for the conformity evaluation.
• requirements for certification and inspection bodies and testing laboratories.
The rules and procedures are not limited to WT of any particular size or type. However, special rules and pro-cedures apply for small wind turbines (SWT). Some elements of certification are mandatory, whilst provision is specifically made for others to be optional. For type certification, the document describes procedures re-lating to conformity testing, design, manufacture, and the plans for transportation, erection, installation and maintenance. The procedures deal with the assessment of loads and safety, testing, characteristics measuments and surveillance of manufacturing. For project certification, the document describes procedures re-lating to the assessment that particular wind turbines and support structure/foundation designs in a project are appropriate for the application and relating to transportation, installation, commissioning, operation and maintenance. The procedures deal with assessment in accordance with all modules in this document, such as the site conditions, the design of site-specific components and surveillance of manufacturing, transporta-tion, installation and operation.
The purpose of the rules and procedures is to provide a common basis for certification of wind turbines and wind turbine projects, including a basis for acceptance of operating bodies (for example: certification bodies, inspection bodies and testing laboratories) and mutual recognition of certificates[31].
IEC 61400-23
This part of IEC 61400 defines the requirements for full-scale structural testing of wind turbine blades and for the interpretation and evaluation of achieved test results. The standard focuses on aspects of testing related to an evaluation of the integrity of the blade, for use by manufacturers and third party investigators.
The following tests are considered in this standard:
• static load tests.
• fatigue tests.
• tests determining other blade properties.
The purpose of the tests is to confirm to an acceptable level of probability that the whole population of a blade type fulfills the design assumptions.
It is assumed that the data required to define the parameters of the tests are available and based on the standard for design requirements for wind turbines such asIEC 61400-1or equivalent. Design loads and blade material data are considered starting points for establishing and evaluating the test loads. The evaluation of the design loads with respect to the actual loads on the wind turbines is outside the scope of this standard[32].
IEC 61400-24
This International Standard applies to lightning protection of wind turbine generators and wind power sys-tems. Normative references are made to generic standards for lightning protection, low-voltage systems and high-voltage systems for machinery and installations and electromagnetic compatibility (EMC).
This standard defines the lightning environment for wind turbines and application of the environment for risk assessment for the wind turbine. It defines requirements for protection of blades, other structural com-ponents and electrical and control systems against both direct and indirect effects of lightning. Test methods to validate compliance are recommended.
Guidance on the use of applicable lightning protection, industrial electrical and EMC standards including earthing is provided as well regarding personal safety and damage statistics and reporting guidelines[33].
IEC 61400-25 (PARTS1-6)
The focus of the IEC 61400-25 series is on the communications between wind power plant components such as wind turbines and actors such as SCADA Systems(supervisory control and data acquisition). Internal com-munication within wind power plant components is beyond the scope of the IEC 61400-25 series.
The IEC 61400-25 series is designed for a communication environment supported by a client-server model. Three areas are defined, that are modeled separately to ensure the scalability of implementations:
1. wind power plant information models. 2. information exchange model.
3. mapping of these two models to a standard communication profile.
The wind power plant information model and the information exchange model, viewed together, constitute an interface between client and server. In this conjunction, the wind power plant information model serves as an interpretation frame for accessible wind power plant data. The wind power plant information model is used by the server to offer the client a uniform, component-oriented view of the wind power plant data. The information exchange model reflects the whole active functionality of the server. The IEC 61400-25 series en-ables connectivity between a heterogeneous combination of client and servers from different manufacturers and suppliers.
The IEC 61400-25 series defines a server with the following aspects:
• information provided by a wind power plant component, for example, ‘wind turbine rotor speed’ or ‘total power production of a certain time interval’ is modeled and made available for access. The infor-mation modeled in the IEC 61400-25 series is defined in IEC 61400-25-2.
• services to exchange values of the modeled information defined in IEC 61400-25-3.
• mapping to a communication profile, providing a protocol stack to carry the exchanged values from the modeled information (IEC 61400-25-4).
The IEC 61400-25 series only defines how to model the information, information exchange and mapping to specific communication protocols. The IEC 61400-25 series excludes a definition of how and where to imple-ment the communication interface, the application program interface and impleimple-mentation recommenda-tions. However, the objective of the IEC 61400-25 series is that the information associated with a single wind power plant component (such as a wind turbine) is accessible through a corresponding logical device[34].
2.6.STANDARDS FOR WIND TURBINES 15
IEC 61400-26-1
This part of IEC 61400 defines generic information categories to which fractions of time can be assigned for a wind turbine generating system (WTGS) considering internal and external conditions based on fraction of time and specifying the following[35]:
• generic information categories of a WTGS considering availability and other performance indicators.
• information category priority in order to discriminate between concurrent categories.
• entry and exit point for each information category in order to allocate designation of time
• informative annexes including:
– examples of optional information categories.
– examples of algorithms for reporting availability and performance indicators.
– examples of application scenarios.
IEC 61400-26-2
This part of IEC 61400 provides a framework from which production-based performance indicators of a WTGS (wind turbine generator system) can be derived. It unambiguously describes how data is categorized and provides examples of how the data can be used to derive performance indicators.
The approach of this part of IEC 61400 is to expand the time allocation model, introduced inIEC 61400-26-1, with two additional layers for recording of the actual energy production and potential energy production associated with the concurrent time allocation.
It is not the intention of this Technical Specification to define how production-based availability shall be calculated and also not the intention to form the basis for power curve performance measurements, which is the objective ofIEC 61400-12-1[36].
This document also includes informative annexes with:
• examples of determination of lost production.
• examples of algorithms for production-based indicators.
• examples of other performance indicators.
• examples of application scenarios.
IEC 61400-27
IEC 61400-27 defines standard electrical simulation models for wind turbines and wind power plants. The specified models are time domain positive sequence simulation models, intended to be used in power system and grid stability analyses. The models are applicable for dynamic simulations of short term stability in power systems. IEC 61400-27 includes procedures for validation of the specified electrical simulation models. The validation procedure for IEC 61400-27 is based on tests specified inIEC 61400-21.
IEC 61400-27 consists of two parts with the following scope:
• IEC 61400-27-1 specifies dynamic simulation models for generic wind turbine topologies/ concepts / configurations on the market. IEC 61400-27-1 defines the generic terms and parameters with the purpose of specifying the electrical characteristics of a wind turbine at the connection terminals. The models are described in a modular way which can be applied for future wind turbine concepts. The dynamic simulation models refer to the wind turbine terminals. The validation procedure specified in IEC 61400-27-1 focuses on theIEC 61400-21tests for response to voltage dips, reference point changes and grid protection.
• IEC 61400-27-2 specifies dynamic simulation models for the generic wind power plant topologies / configurations on the market including wind power plant control and auxiliary equipment. In addition IEC 61400-27-2 specifies a method to create models for future wind power plant configurations. The wind power plant models are based on the wind turbine models specified in IEC 61400-27-1.
The electrical simulation models specified in IEC 61400-27 are independent of any software simulation tool[37]. For a clear and short overview of the different standards and descriptions canTable 2.1be used.
Table 2.1: List of standards for wind turbine systems Standard Description
IEC 61400-1 [21] Wind turbine design requirements
IEC 61400-2 [22] Design requirements for small wind turbines IEC 61400-3 [23] Design requirements for offshore wind turbines
IEC 61400-4 [24] Design requirements for gearboxes (HAWT)
IEC 61400-11 [25] Acoustic noise measurement techniques
IEC 61400-12-1 [26] Power performance measurements of electricity producing wind turbines IEC 61400-12-2 [27] Power performance of electricity-producing wind turbines based on nacelle
anemometry
IEC 61400-13 [28] Measurement of mechanical loads
IEC 61400-14 [29] Declaration of apparent sound power level and tonality values
IEC 61400-21 [30] Measurement and assessment of power quality characteristics of grid con-nected wind turbines
IEC 61400-22 [31] Conformity testing and certification IEC 61400-23 [32] Full-scale structural testing of rotor blades
IEC 61400-24 [33] Lightning protection
IEC 61400-25 (parts 1-6) [34]
Communications for monitoring and control of wind power plants IEC 61400-26-1 [35] Time-based availability for wind turbine generating systems IEC 61400-26-2 [36] Production-based availability for wind turbines
IEC 61400-27 [37] Electrical simulation models - Wind turbines
2.7.
M
ATERIAL
As Njiri and Söffker [38] states, for determining the overall costs, performance, and structural load endurance in wind turbine systems, good material considerations have to be made for the various components of a wind turbine. The combination of the location of the turbine and the dimensions strongly affects the choice of which materials should be used. At each location the wind turbines are subjected to different operation conditions (offshore, onshore, location related wind profiles and properties) so each wind turbine can be subjected to different load cases and environment conditions. This is important to take into account for the choice of material, because the material must withstand the loads that it will endure and the possible corrosion of the material that can occur. Also the dimensions of the wind turbine influence the choice of material. Different loads are applied onto the wind turbine of different dimensions and also the weight of the structures can be different. To achieve the right properties of the structures and desired costs, it is needed to make material considerations.
The main focus is keep reducing the cost of producing wind power, so development of the material and tech-nologies should be for reducing the cost in terms of long term benefits.
ONSHORE OROFFSHORE
There is a difference for the material choice and properties of onshore and offshore wind turbines. Special attention is needed for offshore turbines, because it is more subjected to harsh environmental conditions, such as higher wind velocities and more susceptible to corrosion due to the exposure to the saline marine environment[39]. Due to this, the turbine must be made of material that have high corrosion resistance, high fatigue strength and high mechanical strength. As for onshore wind turbines, they don’t have the saline marine environment (if they are not installed close to the sea) and therefore the corrosion will be lesser. In order to protect the wind turbines against the saline marine environment, special coatings can be used and as mentioned this will not be needed for onshore wind turbines. The choice of materials also depends on the dimensions of the wind turbines. Offshore wind turbines are commonly larger than onshore wind turbines,
2.7.MATERIAL 17
because here is enough space to build and there are much less environment issues, such as noise "pollution". The bigger the wind turbines thus also the blades, the higher tip speeds of the blades, which results in more noise. For onshore wind turbines the noise pollution need to be taken into account more, because of the environment of the wind turbine (residential areas etc).
The main focus for the material used for a wind turbine lies within the tower and the rotor blades. As the wind turbine consists of mainly the tower and the rotor blades. As the main scope will be about the blades, the materials used for the tower will be discussed briefly.
2.7.1.
T
OWER MATERIALThe tower of a wind turbine can be constructed out of different types of materials but usually they are con-structed out of steel and/or concrete. For the investment of a turbine, about 30% accounts to the tower[40], but this will become even more due to technically developments. The main challenge for the design of the tower, and therefore the selection of material is the logistics part. This is different for onshore and offshore turbines, because of the different types of transportation.
For onshore turbines, there are maximum diameter limits due to the transportation limits. Such as depicted inFigure 2.9, the limit is for example a bridge. The environment is mainly the limit factor for the trans-portation. McKenna et al. [41] says that this transportation limit is about 4.5 m. This limit is to low for an economical design as the diameter at the tower base is larger than 6 m. Hybrid towers can be used to solve this problem as they are constructed with concrete bottom sections but the installation will be more complex this way.
(a) Transport of an offshore wind turbine.[42] (b) Tower diameters are limited by transportation constraints such as bridge heights.[43]
Figure 2.9: Transportation of onshore and offshore wind turbine towers.
For offshore wind turbines the logistics is a much lesser problem. There are no bridges or turns that give problems for transporting the wind turbines. It can be seen inFigure 2.9that multiple wind turbines can be transported on one ship. Some ships can even provide the hook up and commissioning of the wind turbines. For building large wind turbine towers there are different types of material that can be used. Steel/concrete composite and fiber-reinforced composite are nowadays used for these large towers[44]. Each material has its own advantages and disadvantages, the best choice to select the material just depends on the criteria such as location, costs, and lifetime.
CONSTRUCTION TYPES
It is know that most common wind turbines have conical steel towers. They are fabricated without stiffeners and with the trend of upscaling the wind turbines, there is a need for a greater structural strength and stiffness to withstand the forces applied on the wind turbine. This can lead to larger tower cross section and wall thickness of the tower. As mentioned before this will lead to problems for transportation and therefore new designs are needed or different materials[45].
This is why the tower made of concrete makes concrete a comparative material as the trend keeps to increase the generating capacity of wind turbines. Concrete is durable, need less maintenance and is reliable[39]. In comparison with steel towers, concrete has more flexibility in construction and design, better transportation possibilities, better dynamic response and also less maintenance needed.
The concrete towers are in the spotlight through hybrid towers. These are build in parts, the bottom part is built in concrete and the upper part is built as a tubular steel section(Figure 2.10). For bigger multi-megawatt class wind turbines are the hybrid towers a very economical solution. The concrete shafts of these towers are made of in-situ or precast concrete and they can be internal or external prestressed[46].
Figure 2.10: Precast plant for precast concrete segments (left) and horizontal reinforcement loops of a precast concrete segment (right).[46]
2.7.2.
B
LADE MATERIALBlades are one of the most critical components of a wind turbine. They are mainly responsible for the effi-ciency of the captured energy by the wind turbine and an overall breakdown of the system can occur if there is a failure of a blade. That is why the blades need high bending stiffness properties[47]. Also the blades account for approximately 15-20% of the investment costs of the turbine. For the choice of material of the blades, there are many different composite materials been used. Composite materials are used because they have low costs, have high ratios for strength/weight and stiffness/weight[48].
