wind turbine blades

Structural Analysis and Design of the Composite WindTurbine Blade

Wen-Hsiang Wu & Wen-Bin Young

Received: 22 November 2010 /Accepted: 28 February 2011 /Published online: 12 March 2011# Springer Science+Business Media B.V. 2011

Abstract The wind turbine blade sustains various kinds of loadings during the operationand parking state. Due to the increasing size of the wind turbine blade, it is important toarrange the composite materials in a sufficient way to reach the optimal utilization of thematerial strength. Most of the composite blades are made of glass fibers composites whilecarbon fibers are also employed in recent years. Composite materials have the advantagesof high specific strength and stress. This study develops a GUI interface to construct theblade model for the stress analysis using ANSYS. With the aid of visualization interface,the geometric model of the blade can be constructed by only a few data inputs. Based onthe numerical stress analysis of the turbine blade, a simple iterative method was proposed todesign the structure of the composite blade.

Keywords Wind turbine blade . Stress analysis . Structural design . Composite blade

1 Introduction

Glass fiber or carbon fiber reinforced polymer composites are currently the mostpopular materials used for the wind turbine blade due to their light weight and superiormechanical properties. A wind turbine blade may sustain various loading during theoperation and parking states. Due to the increasing size of the composite wind turbineblade, it is an important issue to arrange the composite materials in a sufficient way toreach the optimal utilization of the material strength. The fiber direction and thicknessdistribution of the composite are two important design parameters in the turbine bladestructure.

The outer shape of a wind turbine blade is usually determined by the consideration ofaerodynamic efficiency, and is not subjected to change in general. The structure of the

Appl Compos Mater (2012) 19:247–257DOI 10.1007/s10443-011-9193-z

W.-H. Wu :W.-B. Young (*)Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, Taiwan,Republic of Chinae-mail: [email protected]

composite blade is composed of the composite skins bonded together with a spar or foamcore. The design problem thus becomes the determination of the fiber orientation andthickness distribution on the composite skin. There are many reports that address the designof composite plate with simple geometry. However, limited studies are reported in thestructural design of composite with the complex blade geometry. Giguère [1] divided theblade into several sections and determined the stiffness of each section as the basis formaterial distribution. Tangle [2] pointed out that composite material could be used at thewind turbine blade to efficiently sustain the complex loading for increasing size of blade.Bechly [3] reported an early work on the finite analysis of the composite wind turbineblade. Kong [4] conducted structural design of the composite wind turbine blade undervarious loads and fatigue life. Jensen [5] tested a full-scale 34 m composite wind turbineblade to failure under flap-wise loading. Ovalization of the load carrying box girder wasmeasured in the full-scale test and simulated in non-linear FE-calculations. Maheri [6]developed an aero-structure code to predict the performance of a horizontal axis windturbine with adaptive blades.

For structural optimization, Veers et al. [7] addresses the issues in design andfabrication of the wind turbine blade. Minimization of the blade weight was the designgoal in their study. It was also reported that suitable design of the fiber directions andusage of unidirectional carbon fiber to replace the glass fiber can reduce the total weightof the turbine blade, while maintaining the same strength and stiffness [8]. Optimizationof the wind turbine blade structure was reported by Jureczko [9] using the modifiedgenetic algorithm. Berry [10] introduced the fabrication processes of a large size windturbine blade using mixed glass and carbon fibers to reduce the total weight. Lund [11]designed the laminated multi-material composite shell structure by maximizing thebuckling load factor.

This study develops a GUI interface to construct the blade model for the stressanalysis using ANSYS. With the aid of visualization interface, the geometric model ofthe blade can be constructed by only a few data inputs. Based on the numerical stressanalysis of the turbine blade, a simple iterative method was proposed to determine thethickness distribution of the composite skin of the blade structure. The thicknessdistribution of the composite skin is selected based on the concept of uniform loading.In other word, with the applied wind loading during the parking state, the turbine bladewill be designed to have the same safety factor in each station by applying the failurecriterion of the maximum principal stress.

2 Construction of Wind Turbine Blade Model

Stress analysis of the wind turbine blade must be conducted in order to determine the stressfield for the failure criterion. A program with a graphical user interface based on Matlab isdeveloped to facilitate the construction of the geometric model of the blade. The programwill generate the geometry data that can be imported to the ANSYS for following finiteelement meshing and stress analysis. Figure 1 shows the graphical interface of the program.With the input of airfoil geometry, skin thickness, chord length, pitch angle, distance to thecenter of each station, the blade geometry can be generated.

The mechanical properties of the composite material, including Young’s modulus,Poisson ratio, fiber stacking layers and angles, can also be designated in the interfaceprogram. The geometric model can be output as a text file and imported to ANSYS. Thefinal geometry of a sample turbine blade is as shown in Fig. 2.

248 Appl Compos Mater (2012) 19:247–257

3 Structural Loading

The wind turbine blade is subjected to complex loading induced by the incoming windforce and the dynamic force. In some situations, the wind turbine blade may experienceextreme wind speed caused by severe weather conditions, such as a hurricane ortornado. The control system of the wind turbine is assumed to be able to pitch theblades in a feathered position (the parking position). The design of blade must be ableto withstand the extreme wind speed without damage or failure at this position. Thisstudy follows the IEC 61400–1 wind turbine design/safety standard to have an extreme

Fig. 1 The graphical user interface for constrcuting the blade geometry model

Fig. 2 The wind turbine blademodel in ANSYS after importingthe data generated from the GUIprogram

Appl Compos Mater (2012) 19:247–257 249

wind speed of 59.2 m/s at the parking state as a design guide for the thicknessdistribution of the blade structure.

Three methods are sued to approximate the wind loading at the extreme wind speed atthe parking position. Blade element moment (BEM) theory [12] is able to yield goodpreliminary predictions of wind load. It provides the necessary means to predict theaerodynamic forces and moments acting on a turbine blade. The blade element momenttheory divides the entire blade to several elements and calculates the wind load at eachelement. Figure 3 shows the cross section of a blade element under loading at the parkingposition. The blade is subjected to an extreme wind speed, U, and the resulting lift and dragforces are dFL and dFD. The lift and drag forces can be defined as

dFL ¼ CL1

2rU2cdr ð1Þ

dFD ¼ CD1

2rU 2cdr ð2Þ

where CL and CD are the lift and drag coefficients, ρ is the air density, c is the cord length ofthe air foil, and dr is the length of the blade element.

The second method is the simulation using FAST. FAST [13] is a structural-response,horizontal-axis wind turbine (HAWT) specific code originally developed by Oregon StateUniversity and the University of Utah for the National Wind Technology Center (NWTC).It uses the University of Utah’s AeroDyn aerodynamic subroutine package for calculatingaerodynamic forces.

A direct analysis of the flow field around the wind turbine blade was also performed toderive the wind load on the blade using ANSYS CFX. For a given extreme wind speed, thereactive pressure distribution on the blade can be calculated. Figure 4 show the stream linesof the flow field around the blade, where the wind is along the y direction and x-z plane isthe rotational plane of the blade.

Figures 5 and 6 show the resulting lift and drag force distributions on a 3.5 m long bladecalculated by the three methods. The force distributions are similar to each other. Theresulting wind loads are higher by using FAST and lower using CFX. The calculationmodel of FAST and CFX are generally more close to the real case, but requiring more initial

T

U

dFD

dFL

Fig. 3 The cross section of ablade element under loading atthe parking position


data and calculations. The method of BEM is simple and provides the wind load withreasonable accuracy. It will be the most efficient way to calculate the wind load for thestress analysis.

4 Thickness Distribution in Blade Structure

In the preliminary design of the thickness distribution for the blade skins, the blade can besimplified into two kinds of cross sections as shown in Fig. 7 [1]. The root part can besimplified as the cylindrical shape and the airfoil of the blade can be simplified as parallelplates. The thickness and chord length of the root are tsH and cH. For the airfoil part withchord length, c, the maximum thickness, t, and blade skin thickness, tsA, the simplifiedparallel plates will have skin thickness tsA, length 0.4c, and distance between plates, 0.9 t asshown in Fig. 7(b). With the simplified geometry, the moment of inertia of the cross sectioncan be approximated by the following equation.

IðrÞ ¼ MðrÞtðrÞ=2spðrÞ ð3Þ

Fig. 4 The stream lines of the flow field around the blade under a extreme wind speed


where M(r) is the applied moment load at each blade element calculated from the windloads, r is the distance from the center of the blade element to the rotational center, t(r) isthe maximum thickness of each blade element, and σp(r) is the material strength. If theblade element is designed to be subjected to the stress of the material strength, the

Fig. 5 The lift forces calculatedby the BEM, FAST and CFX

Fig. 6 The drag forces calculatedby the BEM, FAST and CFX


corresponding blade thickness can be determined by Eq. 3. Therefore, the skin thicknesscan be approximated as

tsH ¼ IH

pðc3H8 Þð4Þ

tsAðrÞ ¼ IðrÞ0:162ct2

ð5Þ

where IH is the moment of inertia for the root section and I(r) is for the sections with airfoilshape.

The composite material used in this study is the woven glass fiber reinforced epoxy. Thewoven fiber composite is considered as an isotropic material that has an ultimate strength,85 MPa. If a 3.5 m long wind turbine blade is divided into 11 blade elements, the initiallyestimated thickness distribution by using Eqs. 4 and 5 is listed in Table 1. In order to avoidproblems in the fabrication, a constraint is imposed on the thickness distribution that theblade skin has a minimum thickness of 1 mm. The thickness of the spar used in this study is10 mm. Other constraints caused by the fabrication process and other factors are notconsidered in the current study. Also, the design is based on the failure criterion of thematerials; the deflection of the blade is not constrained during the optimization design. Inthe current case, the deflection of the blade increases from 140 mm to 160 mm for theinitial and optimized designs.

The total weight of the initially designed blade is 30.375 kg. Under the load induced bythe extreme wind speed (59.5 m/s), the maximum deformation at the tip is 140.2 mm andthe corresponding maximum principal stress is shown in Fig. 8. If the failure criterion of themaximum principal stress is employed for the composite blade, the corresponding safetyfactor for each blade element is also listed in Table 1. For a design goal of safety factorequaling to 2, the elements 2 and 3 are considered to be not safe enough.

To determine the thickness distribution of the composite blade skin with efficientmaterial utilization, a simple way is to have about the same safety factor for all the bladeelements. In the initial design, the safety factor of elements 2 and 3 is lower than 2, andsome elements have the safety factor much larger than this. Therefore, modification of the

Fig. 7 The simplified cross sections for the blade a clindrical shape for the root b parallel plates for theblade


Table 1 Initially estimated thickness distribution for blade skin

Baseline

Blade element 01 02 03 04 05 06 07 08 09 10 11

Thickness (mm) 9.68 5.34 4.8 4.72 4.53 3.93 3.16 2.13 1.13 1 1

Max. principalstress (MPa)

34.5 57.1 49.4 38.0 30.6 30.5 25.8 23.7 20.4 21.2 18.7

Safety factor 2.46 1.49 1.72 2.23 2.78 2.79 3.30 3.58 4.17 4.01 4.53

Total weight 30.38 kg Total deformation 140.2 mm

(a)

(b)

Fig. 8 The maximum principal stress of the initially designed blade a The leading side b the back side


thickness distribution according to the safety factor value of each element can beconstructed based on the following equation.

tnþ1 ¼ SFdesign

SFbaselinetn ð6Þ

Table 2 The thickness distribution for blade skin after 3 iterations

Baseline

Blade element 01 02 03 04 05 06 07 08 09 10 11

Thickness 9.68 5.34 4.8 4.72 4.53 3.93 3.16 2.13 1.13 1 1

Max principal stress 34.5 57.1 49.4E 38.0 30.6 30.5 25.8 23.7 20.4 21.2 18.7

Factor 2.46 1.49 1.72 2.23 2.78 2.79 3.30 3.58 4.17 4.01 4.53

Total weight=30.38 kg Deformation 140.2

Iteration 1

Blade element 01 02 03 04 05 06 07 08 09 10 11

Thickness modified 7.87 7.17 5.58 4.22 3.26 2.82 1.92 1.19 1 1 1

MPSa after modified 40.2 39.9 42.4 41.1 39.6 39.9E 37.7 34.7 23.2 24.5 22.1

Factor after modified 2.12 2.13 2.00 2.07 2.15 2.13 2.26 2.45 3.66 3.47 3.85


Iteration 2

Blade element 01 02 03 04 05 06 07 08 09 10 11

Thickness modified 7.44 6.74 5.57 4.09 3.04 2.65 1.7 1 1 1 1

MPS after modified 41.8 41.9 42.8 42.2 41.7 42.0 41.0 38.2 23.8 25.3 23.0



Iteration 3

Blade element 01 02 03 04 05 06 07 08 09 10 11

Thickness modified 7.33 6.65 5.61 4.06 2.98 2.62 1.64 1 1 1 1

MPS after modified 42.3 42.4 42.6 42.5 42.3 42.4 41.9 38.3 23.8 25.4 23.1



aMPS: maximum principal stress

Fig. 9 The thickness distribu-tions of the composite balde skinfor both the initial and finaldesigns


where tn and tn+1 are the original and modified thicknesses of each element, SFdesign is thedesign goal of the safety factor, and SFbaseline is the safety factor calculated based on thestress analysis.

Based on Eq. 6, the final thickness distribution of the blade skin is listed in Table 2 after3 iterations to have a safety factor 2 for each element. After 3 iterations, the total weight ofthe blade converges to a value with the accuracy of one decimal. The final design of theblade has a total weight of 27.8 kg and the tip deformation is about 160.0 mm. Thethickness distributions of the composite blade skin for both the initial and final design areshown in Fig. 9. The thicknesses of most elements are reduced while those of the elements2 and 3 are increased to enhance the strength at those areas. The corresponding maximumprincipal stresses are shown in Fig. 10. A uniform stress loading on the blade can beachieved with the final design. Near the tip of the blade, the stress loading is lower than thatat other areas because of the use of higher thickness (the limit thickness constraint) thannecessary.

In the study, the design safety factor is equal to two. If a higher safety factor is desired,the total weight will increase because of the increase of the composite skin thickness. Basedon requirement of the design, the thickness distribution of a composite blade skin can bedetermined by the simple iterations using Eq. 6. Figure 11 shows the thickness distributionsof the composite blade skin for designs with different safety factors. In case a higher safetyfactor is desired due to other considerations, thicker composite skin is required for the bladestructure.

Fig. 11 The thickness distribu-tions of the composite blade skinfor designs with different safetyfactors

Fig. 10 The distributionsof maximum principal stress ofthe composite balde skin for boththe initial and final designs


5 Conclusions

Awind turbine blade geometry constructing interface is developed in the study to facilitatethe stress analysis using ANSYS. Three different methods, BEM, FAST, CFX, areemployed to estimate the wind load at the extreme wind speed while the blade is at theparking position. The BEM is shown to be an efficient method in calculations of the windload with certain accuracy. Based on the maximum principal stress failure criterion, thecomposite blade skin can be determined under a specified safety factor. A uniform stressloading on the blade can be derived with this design methodology.

References

1. Giguère, P., Selig, M.S.: Blade design trade-offs using low-lift airfoils for stall-regulated HAWTs.National Renewable Energy Laboratory (1999)

2. Tangler, J.L.: The evolution of rotor and blade design. National Renewable Energy Laboratory (2000)3. Bechly, M.E., Clausen, P.D.: Structural design of a composite wind turbine blade using finite element

analysis. Comput. Struct. 63, 639–646 (1997)4. Kong, C., Bang, J., Sugiyama, Y.: Structural investigation of composite wind turbine blade considering

various load cases and fatigue life. Energy 30, 2101–2114 (2005)5. Jensen, F.M., Falzon, B.G., Ankersen, J., Stang, H.: Structural testing and numerical simulation of a 34

m composite wind turbine blade. Compos. Struct. 76, 52–61 (2006)6. Maheri, A., Noroozi, S., Toomer, C.A., Vinney, J.: WTAB, a computer program for predicting the

performance of horizontal axis wind turbines with adaptive blades. Renew. Energy 31, 1673–1685(2006)

7. Veers, P.S., Ashwill, T.D., Sutherland, H.J., Laird, D.L., Lobitz, D.W., Griffin, D.A., Mandell, J.F.,Musial, W.D., Jackson, K., Zuteck, M., Miravete, A., Tsai, S.W., Richmond, J.L.: Trends in the design,manufacture and evaluation of wind turbine blades. Wind Energy 6, 245–259 (2003)

8. Locke, J., Valencia, U.: Design studies for twist-coupled wind turbine blades. Wichita State University,National Institute for Aviation Research, Wichita, Kansas SAND2004-0522 (2004)

9. Jureczko, M., Pawlak, M., Mezyk, A.: Optimisation of wind turbine blades. J. Mater. Process. Technol.167, 463–471 (2005)

10. Berry, D.S.: CX-100 Manufacturing final project report. TPI Composites, Inc. SAND2007-6065 (2007)11. Lund, E.: Buckling topology optimization of laminated multi-material composite shell structures.

Compos. Struct. 91, 158–167 (2009)12. Manwell, J.F., McGowan, J.G., Rogers, A.L.: Wind Energy Explained. John Wiley & Sons, LTD, (2002)13. Jonkman, J.M., Jr. M.L.B.: FAST User’s Guide (2005)


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