Generation of design knowledge from the development of a theoretically idealized wind turbine. Z. H. Zamora Guevara

Transcription

1 Generation of design knowledge from the development of a theoretically idealized wind turbine Z. H. Zamora Guevara

2

3 Generation of design knowledge from the development of a theoretically idealized wind turbine Thesis, submitted in partial fulfillment of the requirements for the MSc. degree of the Program of Sustainable Energy Technology Zeus H. Zamora Guevara September, 2010 Graduation committee Prof.dr. G.J.W van Bussel Ir. M. Zaayer Ir. T. Ashuri Prof.dr. M.J. de Vries Delft University of technology Faculty of Aerospace Engineering Wind Energy Research Group Eindhoven University of Technology Faculty of Industrial Engineering & Innovation Sciences i

4 ii

5 Abstract Wind turbines have significantly evolved in the last three decades, though they still need significant improvements to become more competitive in the energy market. These improvements depend to a great extent on the development of wind turbine technology. In this respect, the design of an idealized wind turbine may provide a potential quantitative insight into further improvement in the design of these systems. Therefore, the objective of this work is to generate design knowledge for wind turbine technology through the development of a theoretically idealized wind turbine. To do so, the concept and the design process of this idealized wind turbine are defined based on the general design theory. Moreover, a design case study is implemented as a demonstration of how the proposed design process is applied. The blade was selected for the case study because it is the most representative component of wind turbines and represents several challenges for the development of the future wind turbine industry. The results of this case study give insight into potential design knowledge generated from the proposed design process. Finally, some recommendations about how the designer can use this knowledge for the improvement of wind turbine design are given. iii

6 iv

7 Table of contents 1 INTRODUCTION BACKGROUND INFORMATION RESEARCH OBJECTIVE REPORT STRUCTURE CONCEPT OF THE IDEALIZED WIND TURBINE DESIGN PROCESS OF THE IDEALIZED TURBINE STEPS OF THE DESIGN PROCESS Analysis Idealization Synthesis Simulation Evaluation APPLICATION OF THE DESIGN PROCESS GENERAL INFORMATION OF THE WIND TURBINE BLADE DESCRIPTION OF THE BLADE REVIEW OF BLADE DESIGN PERSPECTIVES THE REFERENCE WIND TURBINE OVERALL SYSTEM REFERENCE BLADE DESIGN OF THE IDEALIZED WIND TURBINE BLADE STEP 1: ANALYSIS Analysis of the blade component Design criteria STEP 2: IDEALIZATION Target idealization No. I: Buckling Target idealization No. II: Fatigue + buckling Target idealization No. III: Tower collision + fatigue + buckling STEP 3: SYNTHESIS Determination of the design parameters Calculation of the properties that depend on the design parameters STEP 4: SIMULATION Simulation of loads upon the blade Simulation of the behavior with respect to the design constraints STEP 5: EVALUATION v

8 7 RESULTS AND DISCUSSION IDEALIZED BLADE DESIGNS Idealized Blade No. 1 with buckling idealization Idealized Blade No. 2 with the inclusion of fatigue idealization Idealized Blade No. 2bis with the inclusion of fatigue idealization Idealized Blade No. 3 with the inclusion of tower collision idealization COMPARISON OF THE BLADES DISCUSSION OF SIMULATION RESULTS Knowledge about the constraint effects on the blade parameters Knowledge about the hierarchy of design constraint CONCLUSIONS DISCUSSION OF THE APPROACH RECOMMENDATIONS REFERENCES APPENDIX A: CROSSSECTION VBA TOOL... 1 APPENDIX B: ROTORPERFORMANCE VBA TOOL... 1 vi

9 1 Introduction 1.1 Background information Wind energy applications from the beginning of the 20th century to the early 1980 s were characterized by an intermittent and relative slow development [1-2]. However they, especially wind power turbines, have significantly evolved in the last three decades. For example, the size and power output have been increased from small 20 kw turbines for domestic use to large 5 MW turbines for offshore applications as it is shown in Figure 1-1. Moreover, the market contribution of wind power accounts for half of the new installed capacity in some developed countries despite of its cost price per kwh is still relatively high without incentives and funding [3]. However, wind power, especially offshore, needs further development to become as competitive as regular energy sources in the same conditions [4]. Figure 1-1 Development of wind turbine size, Source: LM Glasfiber presented by [5] The current and future development of offshore wind turbine technology depends to a great extent on cost effective design. This represents several technical challenges that need to be solved in an economical way by the designer. In this respect, there are three main technological trends that may help the designer overcome these challenges [6]: 1. Upscaling of turbine size 2. Integrated design of the wind turbine 3. Increase of the energy conversion efficiency These trends are inherently interrelated and hence further improvements and solutions for offshore wind turbine design should be based on the three of them. 1

10 It is desired to know how much margin there is for further improvement of wind turbines. For this purpose, the design knowledge is of foremost importance since its development can represent important progress for the wind energy technology. Zaaijer [4] presented a review of the design knowledge development for offshore wind energy technology, of which two conclusions are relevant for the present work: a) new design solutions are currently based to a large extent on qualitative analysis, intuition and expectation and b) future development should assess the potential of new ideas. In this respect, the design of an idealized turbine may provide a potential quantitative insight into further improvement of wind turbine technology. This idealized wind turbine is an imaginary turbine for which specific design constraints were removed from its design process, see chapter Research objective The main objective is to generate design knowledge for wind turbine technology by means of the development of a theoretically idealized wind turbine. This knowledge will be generated based on the comparison between the behavior and properties of the idealized turbine and those of an existing turbine. 1.3 Report structure The present report consists of 8 chapters. In chapter 2 and chapter 3, the concept and the design process of the idealized system are respectively described based on general design theory. Attention is paid to the effect of the idealization on design and the possibilities of the design knowledge generation from this design approach. In this respect, a design case study is defined for the application of the proposed design process. The following chapters deal with the design case study: In chapter 4, the objective component for the case study, the blade, is described and a review of the design perspectives of this component is presented. Chapter 5 consists of the specifications of the reference wind turbine and its blade, on which the definition of the idealized system is based. In chapter 6, the demonstration of how the design process is applied to the case study is presented. The steps of the proposed design process are addressed and the assumptions made for each of them are explained. Chapter 7 includes the results from the case study and moreover the potential of the design knowledge generation is assessed by the comparison of the idealized system with the reference system and between each other. The conclusions of the present work are presented in chapter 8. Chapter 9 consists of a discussion of the implication of the decisions made for the proposed approach. Finally, the recommendations of the present report are given in chapter 10. 2

11 2 Concept of the idealized wind turbine In the present work, the idealization of wind turbines is defined as the removal of specific design constraints from the design process of these systems. In other words, some design constrains of the wind turbine are simply assumed to be absent and hence the design process is no longer concerned with them. Therefore, the idealized turbine is the output of this design process with removed constraints. The purpose of the proposed idealization is to release the design decisions from the typical constraints and analyze the effects of this on the design of a wind turbine. After idealization, the values of the design parameters are no longer restricted and can be defined so that the idealized turbine has the opportunity to reach a better performance than that of a non-idealized turbine. This idealization may be better explained with the following figures, in which a hypothetical twodimensional design space of a system is depicted. g5 g1 g2 g3 g4 Figure 2-1 Hypothetical two-dimensional design space of a system. Source: [7], modified This design space corresponds to a system with design parameters and. The feasible region is the area defined by the design constraints (ground lines) where a design parameter point, is considered acceptable for the system. Additionally, the elliptical contours represent lines of equal performance. In this particularly case, it is considered that the performance improves as the contour size decreases, i.e. the smallest contour represents the best absolute performance. For a detailed explanation of the design space, refer to [7]. It can be seen from Figure 2-1 that the best absolute performance is outside the feasible region, hence it cannot be reached by any system with an acceptable parameter point,. The task of the designer is to obtain the feasible combination of parameters with the best possible performance. Therefore, the designer choice of parameters is usually close to the constraint limits of the feasible region. As a consequence, the region with best absolute performance remains unexplored. 3

12 The proposed idealization allows removing one or more of the design constraints as exemplified in Figure 2-2, where the constraint has been removed. g5 g1 g2 g3 Figure 2-2 Hypothetical two-dimensional design space of an idealized system with one-constraint removal. Source: [7], modified After the removal of the design constraint, the region with best absolute performance can be included into the design process. Thus the specifications for a idealized system with the best absolute performance can be obtained. Finally, the proposed idealization is an abstraction that intends to replace the real design conditions. For example, the idealized design space in Figure 2-2 replaces the design space in Figure 2-1. In this respect the proposed idealization is similar to simplification in physical modeling (see [8]). However the idealized turbine is different from a simplified turbine because the simplification is aimed to get quantitative results of the system performance [9]; while the proposed idealization focuses on the removal of constraints and the effect of this removal on the turbine performance and properties. 4

13 3 Design process of the idealized turbine A design problem is addressed by a methodological process, which structures the path between the problem and its solution. The design process is viewed as a set of steps with iterative feedback between each other. The boundaries, information flow, number of steps and level of detail per step vary from one approach to other [10-13]. To illustrate the mechanical design process, two approaches are presented in Figure 3-1 as well as the relationship between each other. The first is the basic design cycle [12], which begins when the function has been determined since this is the departure point of the designer. This cycle of five steps is iteratively applied till the desired function fulfillment and detail level have been reached. The second is the design process by Norton [10], which begins with a need and ends with a manufactured device. It consists of ten steps with feedback between one another that are classified into three main phases. Function Identification of the need Background research Goal statement Definition Requirement Analysis Analysis criteria Analysis criteria Criteria Task specifications Synthesis Provisional Design Simulation Expected properties Synthesis Synthesis Simulation Simulation Synthesis Analysis of solution Selection Design Conceptual Value of the design Evaluation Decision Evaluation Decision Evaluation Decision Detailed design Prototyping and testing Production Detailed Design Approved design Figure 3-1 Basic Design Cycle [12] to the left and the Norton s Design Process [10] to the right. The dashed arrows represent the points at which both approaches can be related. Source: [14] The design process of an idealized turbine as performed in this project is based on the basic design cycle, but an extra step is included. Therefore, the process is divided into five general steps: Analysis, Idealization, Synthesis, Simulation and Evaluation of the design. These are in turn sub-divided in tasks that are specific to the design object (integrated system or turbine component). In the following paragraphs, the general steps are described (also see Figure 3-2): 5

14 3.1 Steps of the design process Analysis According to [13], the main principle of design is form follows function. This function is basically what the system should do. Therefore, this step consists of the analysis of the system functions and their relationships with the system properties, behavior, constraints and involved natural phenomena. The final output of this analysis is the design criteria, which are a set of design objectives, design parameters, established properties and design constraints that the design must be focused on. These concepts are explained below: The design objective is the ultimate requirement that the idealized system must meet. It can be expressed in terms of system properties (e.g. high strength with low mass) or performance (e.g. maximum energy conversion) The design parameters are system properties independent from each other that can be modified in the design process to reach the design objective (e.g. rotor diameter for wind turbines) The established properties are system properties that cannot vary in the design process. These properties include every system property except the design parameters and the properties that depend on these parameters. The design constraints are natural phenomena or forced rules that restrain the value of the design parameters (e.g. fatigue failure or investment top). For the purpose of this work, three types of constraints are distinguished: 1) Physical constraints that are related to the physical phenomena involved in the fulfillment of the functions; 2) process constraints that are related to the transportability, installation capacity and manufacturability of the system; and 3) cost constraints. The design process of idealized turbines is targeted to the first type of constraints; because they inherently belong to the system and do not depend on other systems or actors. Additionally, the design criteria may also include the hierarchy of the design constraints, which give the order of the constraints based on their effect on the system design (see next step). Basically, the hierarchy defines which constraints are deign drivers Idealization The idealization step deals with the removal of the design constraints that were identified in the last step. The removal of constraints should follow a logic flow based on the hierarchy of constraints in the path to reduce the distance between the feasible and absolute best performances. This can be explained with the example of the hypothetical design space of the previous section. In Figure 2-2, the constraint g4 was removed and as a consequence the idealized system can reach the region with the absolute best performance. If the constraints or had been removed instead, the distance to the best absolute performance would have been remained the same. Therefore the constraint has the highest hierarchy. 6

15 The hierarchy of design constraints is for most practical applications unknown. Therefore, the removal of constraints can be accomplished by two approaches. 1) The hierarchy of constraints for the objective system is assumed to follow a known hierarchy for a similar system in literature. 2) One constraint at a time is assumed to become the design driver when the other constraints are idealized. Finally, in the following steps, the assumptions of both approaches must be tested to evaluate its suitability. System function Analysis Criteria Idealization Idealization type Synthesis Idealized design Simulation Expected behavior Evaluation Desired design? 1,2 n idealized designs Further Idealization? Figure 3-2 Diagram flow of the design process of an idealized system Synthesis The goal of this step is to determine the values of the design parameters, as well as the other properties that depend on them. Hence the system is fully defined and its behavior can be simulated in the following step. The determination of parameters should be based on the knowledge of the physical phenomena involved and the relationships between parameters and behavior that were identified in the first step of the process. This knowledge can be found in analytical models, empirical formulas and qualitative considerations, which can be used to give suitable values for the design parameters. Moreover, the specifications of an existing system can also give guidance to this step. 7

16 3.1.4 Simulation In the simulation step, the behavior of the idealized system is simulated once the system has been defined. The simulated behavior is the quantitative and qualitative description of the fulfillment of the functions that are related to the design constraints. The results of this step are usually obtained with design tools and software packages, which are based on engineering models. Moreover, these models should be selected based on their applicability to the properties of the idealized system Evaluation The goal of this step is to determine whether the idealized design has followed the design criteria or not. Therefore, the main task of this step is the evaluation of the compliance of the idealized system with the remaining design constraints. Additionally, the comparison between the idealized system and a reference system or between idealized systems can also be included to this step. This may serve to identify the potential design knowledge generated from the design process of idealized systems. Furthermore, two decisions are carried out after the five steps have been taken. The first decision is directly related to the output of the evaluation step: if the design has complied with the design criteria, the output of the process are the specifications of the idealized system; if otherwise, the process returns to the definition step as seen in Figure 3-2. The second decision is related to the remaining constraints. If other types of idealization want to be assessed, the process should return to the idealization step. 3.2 Application of the design process The application of the present design process should provide knowledge to the current wind turbine design by means of constructive recommendations. Particularly, it is expected that these recommendations come from the conclusions of the comparison between the idealized system and a reference system. In chapter 6, a case study of how the design process is applied is presented. The scope of this case study was set to a single component of the wind turbine (the blade, see chapter 4) and few design parameters since the design of the whole wind turbine system is very complex and beyond the scope of this work. Furthermore, the potential of the design knowledge generation was assessed by the comparison of the idealized system with a selected reference system. This reference system is described in chapter 5. 8

17 4 General information of the wind turbine blade The blade has been selected as the target of the idealization-based design because it is the most representative component of wind turbines. This makes of the blade design the primary target of research and development [15]. Besides, the blade design represents several challenges for the development of the future wind turbine industry. 4.1 Description of the blade The blade is a long body (Figure 4-1), which is mounted in cantilever on the rotor hub. Its main characteristic is the airfoil shape of the transverse area, which varies along the blade span in airfoil type, twist angle, chord and thickness (see e.g. [1]). Blades are usually made of glass fiber-reinforced composites (GFRP), though for very large blades, GFRP composites have been combined with carbon fiber-reinforced composites (CFRP) [5]. Figure 4-1 LM 61.5 P2 Wind turbine blade. Source: REpower The basic design layout of the blade cross section is shown in Figure 4-2. This layout consists of three structural elements: Shell skins, spar flanges and webs [16]. Two spar flanges aligned to the shell skins and two vertical webs constitute the load carrying box. The downwind and upwind shell skins are glued to the spar flanges by adhesive layers as well as to each other along the leading and trailing edges. 9

18 Spar flange Webs Shell skin Figure 4-2 Basic blade cross section layout. Source: [5] The common material distribution in the cross section is as follows: The shells are made of sandwich laminates with thin composite face sheets in a triax or biax lay-up and a polymeric or balsa core. The spar flanges are formed by thick monolithic composite laminates with mostly UD-layers and a small amount of angle ply-layers. Finally, the webs consist of sandwich laminates with composites face sheets in a biax lay-up (see [5]). 4.2 Review of blade design perspectives The current technological trends of the wind turbine design (see section 1.1) have a significant impact in the traditional design of blades. Particularly, upscaling of turbine size may have the most complex effect on the blade design. In this respect, 90-m long blades are envisaged within the next years [5]. Furthermore the blade cost with respect to the overall system cost is also affected by this trend: Veers [15] explained that the growth in the blade length and turbine size tends to make the blades a larger proportion of the total system cost. This statement has been proven with the increase of the blade cost portion of the total turbine cost from 10-15% in 2001 [15] to 15-20% in 2007 [17]. The changes on the design of wind turbine blades are not only aimed to improve performance and manufacturability but also to reduce the overall turbine cost. As at this time the blade cost share is small (around 15-20%), hence the reduction of COE through improvements in blade cost is limited. Nevertheless, if a 10% 20% decrease in loading is achieved by an innovative blade design, significant cost savings can be obtained for other components such as the tower and the drive train [15]. The blade mass can serve as an illustration of the design development with respect to the upscaling of blade length. In the Figure 4-3, the mass of existing wind turbines are plotted versus the blade length. It can be seen that the mass distribution of blades up to 40 meters can be related by a power trend line [18]. This fact presumably implies that these blades share similar design concepts (see [19]). Moreover, longer blades do not follow the same trend line and hence some of them are significantly lighter than it was expected by the extrapolation of the power trend line. This presumably indicates that significant improvements of the design concepts of these blades have been achieved. However, these improvements may not be sufficient for even longer future blades. 10

19 Figure 4-3 Development in blade mass versus length. Symbols indicate different manufacturers and processing technologies. Source: [18] The blade design process in common practice is usually divided in two stages[1]: Aerodynamic design and structural design. The first defines the blade external geometry and properties to maximize the annual energy yield. The second determines the materials and cross section characteristics so the blade resists its complex operative loading conditions. The blade evolution requires that this common practice of design changes. This implies that the blade design process must integrate both aerodynamic and structural concerns as the structure of the blade becomes a more dominant design criterion [15]. On one hand, the aerodynamic design development is mainly focused on the improvement of inboard airfoil profiles with higher thickness-to-chord ratios (see [20]) that can provide good structural properties at low weight and cost [15]. Additionally, other innovative airfoil prototypes for larger blades, e.g. flatback airfoils, are being evaluated [21]. On the other hand, the structural design development is focused on stiffness, density and load resistance. The blade stiffness requires to be optimized to obtain low mass, long life and low cost [1, 18]. This optimization can be achieved by 1) including new materials, 2) using modified cross section layouts or 3) combining both previous points. Carbon fiber-reinforced composites are a successful example of the inclusion of new materials [21-22]. This material has proven a significant contribution due to its capacity to provide cost effective weight reductions and increased stiffness for very long blades [15]. Other materials such as S-glass and zebrawood have been evaluated though further analysis is needed [21] Regarding modified cross section layouts, the cross section with internal shear webs in Figure 4-4 is widely used (see [1]). In this layout, the box carrying structure was replaced by two shear webs that join the aerodynamic shell skins. In addition, the shell skins are reinforced in the section where the shear webs are placed. This reinforced section is called spar cap. Other modifications of the cross section layout are for example the inclusion of sandwich materials in the spar flanges [5, 23] or the use of hatstiffened panels for the shell skins [24]. 11

20 Figure 4-4 Blade cross section layout with internal shear webs. Source: [5] The structural design also faces an additional challenge: the structural design drivers are expected to change as wind turbine blades become larger (see [15, 23, 25]). As for example, the effect of fatigue due to gravitational forces on the cross section design will increase for longer blades. This is further discussed in section Finally, an adequate structural design is not enough to avoid all risks at the production phase. Inevitably, some structural details (joints, bonds, ply drops, etc.), material properties and characteristics of the manufacturing process are not fully understood before the fabrication stage [15]. For this reason, wind turbine blades must be subjected to a full-scale test [16], which is very expensive [15]. However, unexpected failures have occurred during these tests, indicating that the present design tools are insufficient [16]. Current research efforts are aimed to reduce 1) uncertainties related to models for strength and lifetime predictions and 2) material variations (see [16]) so the need of full-scale testing decreases. 12

21 5 The reference wind turbine 5.1 Overall system The REpower 5M wind turbine [26] (Figure 5-1), which is among the largest offshore wind turbine systems that are currently commercially used, has been chosen as the reference turbine in this work. Figure 5-1 REpower Beatrice Project. Source: REpower As there is not enough published information available about the REpower 5M system the description of the reference turbine was mainly based on the NREL offshore 5-MW baseline wind turbine specifications issued by the National Renewable Energy Laboratory (NREL) [27]. These specifications are principally based on the REpower 5M system and on the available specifications from the conceptual models used in the DOWEC project [28]. In Table 5-1, the gross characteristics of the reference turbine are presented. Table 5-1 Gross characteristics for the Reference Turbine. Source: [27] Rating Rotor Orientation, Configuration Control Drive train Rotor, Hub Diameter Hub Height Cut-In, Rated, Cut-Out Wind Speed Cut-In, Rated Rotor Speed Rated Tip Speed Overhang, Shaft Tilt, Precone Tower clearance Rotor, Nacelle, Tower Mass 5 MW Upwind, 3 Blades Variable Speed, Collective Pitch High Speed, Multiple-Stage Gearbox 126 m, 3 m 90 m 3 m/s, 11.4 m/s, 25 m/s 6.9 rpm, 12.1 rpm 80 m/s 5 m, 5 o, 2.5 o 10.5 m 110, 240, 350 tons 13

22 The operational conditions for the reference turbine are taken from the definition of the IEC class I-C for offshore wind turbines [29]. Thus the wind climate has an average wind speed at the hub of 10 [m/s] and a reference turbulence intensity of Reference blade The reference wind turbine blade is a pre-twisted LM-Glasfiber blade [26] with a length of 61.5 meters. According to [27], eight different profiles are distributed along the reference blade to enhance aerodynamic performance: Two cylinder-alike profiles near the root region and 6 airfoils towards the tip (DU 00-W-401, DU 00-W-350, DU 97-W-300, DU 91-W2-250, DU 93-W-210 and NACA64 A17). The chord and the twist angle distributions of the blade (Figure 5-2) are taken directly from the NREL turbine whereas the thickness was determined based on the thickness distribution of the LMH64-5 blade [30] Chord [m] pre-twist [o] Thickness [m] r [m] Figure 5-2 Reference blade s chord, thickness and the twist angle distributions The main structural properties (flapwise stiffness, edgewise stiffness and mass distributions) of the NREL turbine blade are also defined in [27]. However, the cross section geometry and materials of the blade are not available since this information is confidential and cannot be found in any published work. Therefore the reference turbine cross section specifications were estimated with the help of the Crosssection Excel-VBA tool, see Appendix A. The cross section layout of the reference turbine (Figure 5-3) was assumed to follow the basic cross section layout (section 4.1). I.e. the aerodynamic profile is formed by thin shells, which are internally supported by a load carrying box. This in turn consists of two structural elements: the spar flanges and the webs. 14

23 0,2 0,15 0,1 Shell Spar flange Web 0,05 0 0,4-0,05 0,3 0,2 0,1 0-0,1-0,2-0,3-0,4-0,5-0,6-0,7-0,1-0,15-0,2 Figure 5-3 DU 93-W-210 cross section model at 39.7-m span of the reference blade. Note that the thicknesses have been exaggerated, the dimensions are normalized to the chord length and the horizontal coordinate is in reverse order The positions of the webs with respect the chord line were defined by following the recommendation of [31] for a 5MW turbine. The positions of the webs at the blade root are 10% and 90% of the chord length with respect to the leading edge. From the root to the 13.7-m blade station, the positions of the webs vary linearly till they reach their final values, which are maintained towards the tip. These final values are 15% and 50% of the chord length with respect to the leading edge, which correspond to 22.5% and -12.5% of the chord length with respect to the pitch axis as seen in Figure 5-3. The assumptions on materials and their properties of the reference blade are mainly based on the works of Brondsted [18], Lund [32], Thomsen [5] and Griffin [22]. The shell is made of composite sandwich laminates. These laminates have a core of balsa wood and two outer face sheets of glass fiber with 50% fiber volume fraction and a biax lay-up (±45 o orientation). The spar flange consists of glass fiber composite laminates with 40% fiber volume fraction and a lay-up of 80% UD-layers and 20% angle-ply layers. Finally, the web is made of thin glass fiber composite laminates with 50% fiber volume fraction and a biax lay-up (±45 o orientation). The properties of the GFRP composite laminates are presented in Table 5-2. These properties were determined with expressions for composites material properties from [1, 18, 33]. Specifically, the Young s modulus of the GFRP UD-layer was calculated from = + 1 (5-1) where is the volumetric fiber content and and are the young moduli of the fiber and matrix materials. In addition, the fiber orientation considerations in [33] were included to estimate the young modulus of the GFRP layer with ±45 o fiber orientation. 15

24 Table 5-2 Properties of the GRPF layers Fiber content 50% 40% orientation E [GPa] G [GPa] Density [kg/m 3 ] [MPa] 0 o ±45 o o ±45 o Based on the GFRP layer properties, the ply properties of the cross section structural elements, shown in Table 5-3, were estimated. Structural element Table 5-3 Material ply properties of the cross section structural elements Materials E [GPa] G [GPa] Density [kg/m 3 ] Shell GFRP /Balsa /GFRP Spar GFRP ±45 o and Web GFRP±45 o Having chosen the cross section layout and the material properties, the thicknesses of the shell, spars and webs of the reference blade were estimated by iteratively matching the thickness-based structural blade properties to those of the NREL turbine blade with the Crosssection Excel-VBA tool. Since the estimated cross section model of the reference turbine is based on assumptions, the match of blade properties is not perfect. Hence the thickness distributions of the reference blade corresponded to those that gave 1) a close approximation for stiffness 2) a small blade mass difference and 3) manufacturable thickness distributions (i.e. smooth surfaces). In Figure 5-4 and Figure 5-5, the flap- and edgewise stiffness distributions of the reference and the NREL blade are depicted Reference Flap-stiffness NREL Flap-stiffness Stiffnes [GPa] r [m] Figure 5-4 Flapwise stiffness of the reference and NREL turbines 16

25 Stiffnes [GPa] Reference Edge-stiffness NREL Edge-stiffness r [m] Figure 5-5 Edgewise stiffness of the reference and NREL blades The reference thickness distributions along the blade length are shown in Figure 5-6. It can be seen that the web thickness was considered proportional to the spar thickness. The proportionality constants was defined as 0.46 based in [31]. With these cross section properties, the final mass of the reference blade is [ton] that is 0.2% heavier than the NREL turbine blade mass Shell Spar Web Thickness [m] r [m] Figure 5-6 Shell, spar and web thicknesses of the reference blade 17

26 6 Design of the idealized wind turbine blade 6.1 Step 1: Analysis This step consisted of the analysis of the blade component with respect to its functions, properties and the involved natural phenomena. From this analysis the design criteria were defined Analysis of the blade component The blade component belongs to the wind turbine system as a part of the turbine rotor subsystem. It is a long beam that is characterized by the airfoil shape of its transverse sections. In the wind turbine rotor, three blades are positioned on the plane of rotation in such a way that their longitudinal axes coincide at the rotor center with 120 degrees between one another, see Figure 4-1. The boundaries of the blade consist of the bolted connection to the pitch control mechanism and the external surfaces that are in contact with the wind flow. The involved natural phenomena and functions of the blade, as stated by [13], are determined from the logical flow of energy (including loads), material and information through the blade boundaries and the effects of these flows on the blade. Additionally, the blade properties are determined form the evaluation of existing blades and similar structures. In the following sections, the natural phenomena function and properties of the blade are described. The description is based on [14], where these were identified for the wind turbine and its components by a reverse engineering approach. This approach consists on determining the basic design principles of a system by evaluating its structure and operation Flows and natural phenomena Wind with inherent kinetic energy flows into the blades, it is modified and then flows out (see Figure 6-1). The modification of the wind induces a non uniform pressure distribution upon rotor surfaces. In addition, other physical effects such as viscous effects, induced turbulence, aeroelastic effects, tip-losses, dynamic stall and wake effects (rotation and turbulence) are induced by the blades. The characteristics of the pressure distribution and the physical effects are mainly based on blade features (geometry and motion) and on wind flow properties. Wind flow with kinetic energy Modified wind flow with reduced kinetic energy Blades Pitching torque and rotation Resultant timevariant loads Figure 6-1 Blade flow diagram. Horizontal arrows represent energy and load flow 18

27 The combination of the pressure distribution and viscous effects leads to aerodynamic forces (lift and drag) that act upon the blade surfaces. It is worth to mention that these loads are variable with time as well as the wind flow (see [2]). The aerodynamic loads lead to flap- and edgewise bending moments along the blade. Moreover, the blade is also subjected to gravity and transient loads. The latter are external loads that occur when the turbine is subjected to transient operational conditions, such as braking, pitching and yawing loads [34]. Additionally, the loads induce dynamic response on the blade structure: e.g. blade rotation and resonant-induced vibrations. Finally, the resultant of all loads is transmitted to the pitch mechanism. The loads on the blade generate compression and tension stresses in the blade materials. The magnitude and time variation of these stresses can affect the blade micro- and macrostructure and be responsible of failure and undesired effects such as aerodynamic profile distortion, plastic deformation, fracture, crack propagation, fatigue, buckling instability and strength degradation Blade functions From the previous analysis of the flows and natural phenomena in the blade, two primary functions were identified: 1. Generation of aerodynamic loads from the kinetic energy of the wind (wind energy extraction), which represent the first step towards electricity generation in the wind turbine. 2. Transmission of aerodynamic loads to the blade root so they can be further transmitted and converted by other turbine components. Additionally, there are secondary functions that are directed to the fulfillment quality of the primary functions and the avoidance of undesired effects: a) Conservation of the initial blade geometry b) Prevention of failure. Finally, there are other tertiary functions that are not related to the flows and natural phenomena but to the blade lifecycle, e.g. manufacturing and transport, and to other functions of the whole wind turbine system, e.g. braking the rotor motion. The first primary function mainly depends on the aerodynamic design, while the second primary function and the secondary functions mainly depend on the structural design. Function a is related to both primary functions. On one hand, it establishes the need of structural integrity during the transmission of loads and on the other hand, it is required to avoid distortion of the aerodynamic profile during operation. Finally, Function b states that the blade should have enough resistance against failure. 19

28 Blade properties In Table 6-1, the main properties of the blade are presented. These properties were determined in [14]. Additionally, the relationships between functions and properties and the role that these properties play for the function fulfillment are also indicated in Table 6-1. Properties classification Geometric Structural Aerodynamic Material Others Table 6-1 Main blade properties Functions 1 2 a b Others Length P N N N Airfoil type (e.g. thick, cambered,etc.) P/N P/N Blade shape (e.g. chord and twist angle) P/N Cross section layout and dimensions P/N P/N P/N Mass P/N P/N Stiffness P P P Strenght P Natural frequencies P/N P/N Lift C L P Drag C D N N Torsional moment C M N N Others (e.g. stall characteristics) P/N P/N Strength with respect to density P P P Fatigue properties Others (e.g. corrosion resistance) P P/N Pitch with respect to swept area P/N P Rotational motion, etc Manufacturability Transportability, etc. P = the property plays a positive role i.e. contributes to the function N = the property plays a negative role i.e. opposes the function P/N = the property can either play a negative or positive role depending on its value P/N P P 20

29 6.1.2 Design criteria Based on the functions, natural phenomena and properties of the blade, the design criteria for the design of the idealized blade have been selected: Design objective The present design of the idealized blade is focused on the blade structure, because, as indicated in section 4.2, the structural design criterion is becoming dominant for the development of wind turbine blades. In this respect, a reduction of the blade mass helps decrease the blade loads, which has a positive effect on the design and cost of the blade and other turbine components [15]. Therefore, the design objective is to develop the blade structure for which the gravity loads are minimized. In other words the design objective is the development of an idealized blade with minimum mass. Additionally, after each idealization, the obtained idealized blade must be lighter than previous blades. Otherwise, the purpose of idealization (see chapter 2) is not fulfilled and hence the assumptions of the hierarchy of constraints need to be modified Design parameters As the design objective, the design parameters selection is also focused on structural design considerations. This means that the design parameters are selected from the properties that deal with the structural-related functions of the blade (i.e. functions 2, a and b, see Table 6-1). In this respect, mass, stiffness and natural frequency are the main structural properties of the blade. However these properties are not independent between each other, but dependent on the materials and the blade cross section. The properties of the cross section and the materials are suitable to be selected as design parameters since 1) the blade mass can be determined with them and 2) they are independent from each other. However, it is desired to have few design parameters to reduce the complexity of the design. Therefore, the design parameters are defined as the thicknesses of the three cross section structural elements. i.e. spar flange, web and shell Established properties and conditions Every blade property that is independent from the design parameters needs to be established, e.g. the blade shape, cross section layout and material type. In this work, the values of the established properties are the same as those of the reference blade (section 5.2) Design constraints For the present work, design constraints are selected from the physical constraints of the blade (section 3.1.1). As explained before these physical constraints deal with the natural phenomena and undesired effects that boost blade failure and hence are directly related to function b (blade resistance to avoid failure). If the blade design does not respect all of the physical constraints, the fulfillment of the blade function is put at risk. Therefore these constraints drive to a large extent the design of the blade. 21

30 In the following paragraphs the design constraints are described, which have been selected based on their relationships with the design parameters Buckling Buckling is a structural failure mode characterized by the change of the deformation behavior of a structure under compressive loads (see [35]). Buckling, also referred as failure due to elastic instability [24], occurs when a structural element largely deforms even though the compressive load has not reached the ultimate compressive strength of the material. If the deformations due to buckling are large enough, other failure modes are boosted e.g. fracture (Figure 6-2) Structures whose cross section dimensions are much smaller than its length (e.g. thin-walled structures) are likely to experience buckling. In this respect, the design of lighter blades has pushed the material utilization to the limit and consequently the blade structure is becoming thin-walled [32]. Buckling in the blade can be either local or global [36]: in one hand local buckling implies a large change in the deformation in specific parts of the blade (see Figure 6-2). In the other hand global buckling implies a large change in the deformation of the blade as a whole. Figure 6-2 Buckling collapse of a 34-m blade during full-scale test. Source: [37] Buckling has an important effect on the blade design. Experiences and numerical analysis of blades as long as the reference blade have shown that local buckling is by far the governing failure mode for the monolithic laminate design of the main spar flanges [5] (see also [16] and [23]). This type of buckling is also the main failure mode for the shell skins towards the trailing edge [32, 36]. It is worth to mention that buckling is strongly dependent on imperfections, which affect to a great extent the buckling resistance of the blade structure [16] Fatigue The fatigue is a failure mode characterized by the propagation of micro-structure discontinuities (microcracks) in the material [38] due to the application of time varying loads. These loads are smaller than the ultimate strength of the material. For a detail explanation of fatigue, refer to [33, 38-39]. 22

31 A fatigue cycle propagates the cracks step by step, hence each step implies the generation of a certain amount of damage in the structure. Thus when the structure fails (e.g. Figure 6-3), the damage is considered complete. Figure 6-3 Fatigue failure of a 20-m blade in the Conisholme wind farm (2009). Source: telegraph.co.uk Blade design must pay attention to fatigue since the blades 1) are relatively slender and flexible, 2) operate under complex loading conditions, 3) are subjected to vibration and resonance, and 4) must have long life in the order of years [40-41]. For many years, the fatigue constraint has been considered predominant for the design of wind turbine blades, i.e. the blades were completely driven by the fatigue constraint (see [40-42]). This was based on experience and comparisons with similar technologies for aeronautical applications: e.g. the blade must ensure several orders of magnitude more fatigue cycles than aircraft wings [41] (see Figure 6-4). Figure 6-4 Schematic S-N diagram for various fatigue critical structures. For the explanation of the diagram refer to [39]. Source: [41] based on [40] 23

32 The predominance of fatigue in the design of blades was fully appropriate to blades with lengths below 40 meters (see [40]). For blades with lengths above 40 meters, the fatigue is still an important constraint but its effect on the design of the blade structure has been reduced [32]. In this respect the fatigue due to flapwise bending moments had the largest effect on the blade design, while the fatigue due to edgewise bending moments had a very small effect. Furthermore, it is expected that for even longer blades, the blade becomes less fatigue critical due to the increase of the effect of the tower collision constraint [15]. However, as the blades get larger, the edgewise bending moment is likely to have the major effect with respect to fatigue due to the increase of gravity loads [25]. This implies that fatigue due to edgewise bending moments can then have a large effect on the blade design and even become a design driver Tower collision Inevitably, blade deflections occur under loading conditions. The deflections that depend on the flapwise bending moment are the largest. These deflections represent a risk for wind turbines with upwind configuration because if the blade deflection is larger than the tower clearance, the blade hits the tower (Figure 6-5). a) b) Figure 6-5 Nordtank turbine a) after the first blade hit the tower and b) when the turbine failed. Source: youtube.com The tower collision constraint is expected to have a larger effect on the design of very long blades, and even to become a design driver [15]. Additionally, changes in the cross section layout can also increase the effect of this constraint on the design of blades. For example, Jensen [23] and Thomsen [5] showed that the tower collision constraint becomes a design driver when sandwich instead of monolithic composite laminates (Figure 6-6) are used in the spar flanges to increase buckling resistance. 24

33 Figure 6-6 Design change proposed in Jensen. Source: [43] Compressive yield and ultimate strength The compressive yield and ultimate strength are intrinsic material properties that are dependent on the strength of the material molecular bonds. Figure 6-7 shows a schematic stress-strain curve of a fiberreinforced composite, in which the yield and ultimate strength limits are depicted: end of stage 1 and stage 2 respectively. Figure 6-7 Schematic stress-strain curves for brittle fiber, ductile matrix and fiber-reinforced composite that are exposed to an uniaxial load applied in the directions of alignment. Source: [33] Yield failure occurs when the material is loaded till it loses its elastic behavior and starts plastic deformation [33]. This means that the blade structure cannot return to its original geometry after being loaded. Ultimate failure is what is known as fracture. This failure mode is characterized of the separation of a continuous body into two or more pieces in response to imposed loads [33]. This design constraint is focused on the yield failure since it is already detrimental to the fulfillment of function a of the blade. However, for pitch-controlled blades as well as for blades above 10 meters, yield failure has been found to be a minor constraint Resonance Resonance is not a failure mode but a phenomenon of the blade dynamics when the frequency of the excitation is in the range of the blade natural frequencies. It causes the blade to have stresses and deflection much larger than those that occurred under other excitation frequencies [44]. This effect of resonance affects negatively the blade design against other failure modes. E.g. 1) a large resonant- 25