The masses of the reference and idealized blades give insight into the knowledge about the hierarchy of the design constraints for a specific blade concept.
The proposed design approach can be used by the designer to describe the hierarchy of the constraints when it is unknown. In this respect, the tendency of the blade mass reduction in Table 7-11 shows that the design constraints were removed in a gradual order towards the minimum blade mass. This means that the assumption made about the hierarchy of constraints for the reference blade was adequate. Nevertheless, care must be taken to avoid potential errors that are caused by inadequate decisions in the design process or incorrect assumptions about the hierarchy. For example, the IB No. 2bis showed that an incorrect decision, such as the selection of the reference blade as starting point instead of the IB No. 1, could cause an overestimation of the hierarchy of the tower collision constraint.
Finally, this knowledge can also be used to estimate the hierarchy of the design constraints for different blades with the same design concept when the difference between them is the change of one blade property that is not a design parameter. To illustrate this, a hypothetical example is described, which shows how this knowledge can be used to estimate the hierarchy of constraints when the blade length changes.
In Figure 7-17, the masses of four blades with a similar design concept as that of the reference blade, but with different lengths are plotted. The blades correspond to two existing blades [19], the reference blade and a prototype 90-m blade [23]. This figure also shows points of mass that represent the hierarchy of constraints (hierarchy points). In this respect, for the reference blade the points for all constraints are known while for the other blades only the points of the design driver were determined based on the discussion about design constraints in section 6.1.2.4
54
Figure 7-17 Blade mass vs. length for four blades. The hierarchy points represent the hierarchy of the design constraints. Data source: [19] and [23]
If the proposed design approach was applied to the other blades, the complete hierarchies of constraints of these blades would be defined, see Figure 7-18.
Figure 7-18 Blade mass vs. length with the inclusion of other hypothetical hierarchy points. It is worth to mention that these hierarchy points are just hypothetical.
Furthermore, the results in Figure 7-18, can be extrapolated to estimate the hierarchy of a 120-m blade which is shown in Figure 7-19.
0 5 10 15 20 25 30 35 40 45 50 0 20 40 60 80 100 120 140 Blade length [m] B la d e m a ss [ to n ] Mass Buckling Fatigue Tower Collision Yield 0 5 10 15 20 25 30 35 40 45 50 0 20 40 60 80 100 120 140 Blade length [m] B la d e m a ss [ to n ] Mass Buckling Fatigue Tower Collision Yield
55
Figure 7-19 Blade mass vs. length with the extrapolation of hierarchy points for a 120-m blade
The designer can use Figure 7-19 for the conceptual design of a 120-m blade to 1) determine on which constraint to focus, e.g. tower collision and buckling instead of fatigue and 2) identify potential improvements in the design concept of the blade.
This hypothetical example defined a potential methodology for the use of the knowledge about the hierarchy of constraints for different blades. Moreover, it showed that this knowledge could provide constructive recommendations to the upscaling of the wind turbine blade.
0 5 10 15 20 25 30 35 40 45 50 0 20 40 60 80 100 120 140 Blade length [m] B la d e m a ss [ to n ] Mass Buckling Fatigue Tower Collision Yield
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8 Conclusions
The objective of this work was to generate wind turbine design knowledge through the development of a theoretically idealized system. To do so, the concept of idealization and the design process of idealized systems were defined based on the general design theory. This design process can be applied to the whole wind turbine system or to one of its components.
A design case study of the wind turbine blade was defined to evaluate the application of this design process. The objective of this case study was the design of a blade with the minimum mass. Three types of idealizations were carried out, from which three idealized blades (IB) were obtained. The buckling constraint was idealized for the IB No. 1, the fatigue constraint idealization was added for the IB No. 2 and tower collision idealization was added for the IB No. 3. These idealized blades No.1, 2 and 3 have respectively a mass of 10.6, 9.65 and 2.85 tons. It is noticed that by idealizing the design constraints significant reductions of the blade mass were achieved.
The results of the design case study gave insight into potential knowledge for the design of a wind turbine blade with the same design concept as the reference blade. In this respect, two types of knowledge were identified: 1) about the constraint effects on the blade parameters and 2) about the hierarchy of design constraints
The magnitude of the mass difference between each idealized blade and its predecessor gave insight into the knowledge about the effect of the design constraint that was idealized between them. Particularly, the largest mass difference was found between the IB No. 1 and the reference blade that corresponded to the idealization of the buckling constraint. This indicates that there is a significant potential reduction of the mass of the reference blade, if its design concept is modified with respect to buckling. The designer can use this knowledge in the conceptual design of a blade to modify a previous blade design so better mass properties are obtained.
Furthermore the blade masses gave insight into knowledge about the hierarchy of the design constraints for this specific blade concept. In this respect, the mass tendency from one idealized blade to the next showed that the removal of the design constraints was in a gradual order towards the minimum mass. This indicates that the assumed hierarchy of constraints was adequate. However, it was found that if incorrect decisions or assumptions are made, the determined hierarchy of constraints can present errors. The designer can use this knowledge to identify which constraints require more attention and which can be ignored during the conceptual design of a blade. Finally, this knowledge can also be used to estimate the hierarchy of the design constraints for a larger blade with the same design concept as the reference blade.
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9 Discussion of the approach
The proposed design process has an interesting potential for design knowledge generation for wind turbine technology. However as this work represents the first attempt for its application, the conclusions are preliminary. In the following paragraphs, a discussion of the implications of some of the assumptions and decisions made for the present work are presented
The design case study was targeted to a single component. In this way the complexity of the design of a wind turbine system was reduced. Though the design integration of the whole system can provide more accurate results for the proposed approach and more opportunities of design improvements can be identified.
The assumed hierarchy of constraints was obtained from the literature review of other similar systems as there was no specific information about the reference blade. Even though the hierarchy was adequate, the proposed approach could have had other results if one constraint had been misplaced in the first assumed hierarchy. In this respect, if the proposed design approach is used in the conceptual design phase, the evaluation of different assumed hierarchies may provide better conclusions about the real hierarchy.
The selected design constraints consisted on five physical constraints which are the most important for wind turbine blades in literature. Nevertheless these constraints can be also subdivided to achieve more descriptive results of the knowledge about the effects of the design constraints. For example, the buckling constraint effect between the IB No. 1 and the reference blade could have been subdivided in e.g. the effect of buckling in the spar and the effect of buckling in the shell.
Additionally, it was also assumed that the removal of a design constraint does not affect the relative hierarchy of constraints. This implies that the constraints are independent from each other, which is not true since different failure mechanisms can be partially related to each other. In this respect, if it were possible to determine the effect of this assumption in the proposed approach, the results can be improved.
Other assumption with respect to the design constraints was to consider the same design driver along the entire blade component. As this assumption is applicable for few mechanical systems, the design of an idealized wind turbine can have better results if two or more design drivers are considered for different sections of the system.
Finally, the engineering models that were used for the simulation of the system behavior were kept simple, because by doing so the design process is kept also simple. This is especially useful for the conceptual design of the wind turbine. However simple models have less accuracy than complex models, hence the quantitative results of the proposed design can be affected.
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10 Recommendations
The proposed design approach is especially suitable to be applied to early iterations of the conceptual design of the wind turbine to determine the constraints that require more attention in next design iterations. Moreover, it is also suitable to be applied for the upscaling of wind turbines if there are not significant changes in the turbine design concept.
Additionally, improvements to this approach can be achieved by the following points:
• This design approach should target the integrated wind turbine system. For example, both structural and aerodynamic design can be included to the design of the idealized blade.
• The constraints can be sub-classified so the effects of the different types of a certain constraint are evaluated. For example, the fatigue constraint can be sub-classified in fatigue due to flapwise bending moment and fatigue due to edgewise bending moments. As Another example, the buckling constraints can be sub-classified in e.g. spar, shell and web buckling.
• The hierarchy of constraints can be considered as variable, instead of homogeneous over the whole system. For example, two design drivers could have been defined for the reference blade: The buckling constraint in the first blade section (<35m) and the fatigue constraint in the last blade section.
• The models used for estimating the behavior should have higher accuracy to include other failure constraints. For this purpose, FEM methods may be included to evaluate non-linear failure constraints such as buckling and fatigue.
• The specifications of the reference wind turbine need to be as detailed as possible so the inclusion of additional assumptions to the proposed design process is avoided. In this respect, if the cross section specifications of the reference blade had been available, the assumptions made about cross section geometry and materials would have not been necessary.
Furthermore, care should be taken with the implementation of this design process since its drawbacks put at risk its applicability. In this respect, the following drawbacks of this approach were identified: 1) It requires that a specific design concept is followed and hence any change in the design concept implies that the conclusions of this approach are no longer applicable, 2) it greatly depends on the models used to calculate the behavior and therefore it shares the inaccuracies of them, and 3) it only includes the major constraints of the design concept, though other minor constraints that may become important for future wind turbines are overlooked.
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A-1
Appendix A: CrossSection VBA Tool
The Crosssection VBA tool is an Excel-Visual Basic program that was developed for the design of the idealized blade. The main inputs of this tool are the cross section dimensions at each blade station and the material properties. In Figure A-1, the user interface of the tool is shown, which includes the cross section properties of a specific blade station at radius r, the material properties of the different structural elements, the activation buttons for calculation procedures and the results for the blade station.
Figure A-1 Crosssection VBA tool user interface
Module 1
The geometry procedure in Module 1 generates the cross section geometry (top graph in Figure A-1) of the blade station r from the values of the thicknesses of the structural elements and from the cross section and airfoil data.
Module 2
In this module, the ply properties of the cross section materials are selected, as well as the operational conditions of the blade. Moreover, the azimuth position and the wind speed are specified for the stresses calculation procedure.
4
5
Module 1 Module 2
3
A-2
Module 3
The first calculation procedure in Module 3 takes the cross section properties (stiffnesses, mass, center of mass, etc.) and the stresses for each blade station from Module 4 and 6 along the blade and stores them in the Distributed Section worksheet. As an example of the output of this calculation procedure, the mass density distribution along the reference blade is depicted in Figure 6-8.
Figure A-4 Mass density and cumulative mass distributions of the reference blade
The second calculation procedure in Module 3 helps determine the thicknesses of the structural elements when there are known distributions of the cross section properties along the blade. This procedure matches a specific property distribution by numerically modifying the value of the thicknesses. As explained in section 5.2, this procedure was used to define the thicknesses of the structural elements for the reference blade.
The third calculation procedure in Module 3 defines the thicknesses of the structural elements for the target idealization 3 as explained in section 6.3.1.3.
Module 4
In this module, the results of the stress calculation in Module 2 procedure are presented. The procedure calculates the stresses over the face sheets of the shell skins and UD-layers of the spar flanges. The calculation is performed with the expression for stresses in beams
= 0 75
W−
5
7W2
where 5′D and W′D are respectively the bending moments over the flap- and edgewise directions and the principal coordinates of a specific position on the cross section. In this respect, the RotorPerformance VBA tool provides the quasi-steady bending moments 5 and 5.