SCIENCE CHINA Technological Sciences

Science China Press and Springer-Verlag Berlin Heidelberg 2014 tech.scichina.com link.springer.com

*Corresponding author (email: drchenxiao@163.com)

Special Topic: Engineering Thermophysics January 2015 Vol.58 No.1: 18 Article doi: 10.1007/s11431-014-5741-8

Numerical analysis and experimental investigation of wind turbine blades with innovative features: Structural response and

characteristics

CHEN Xiao*, QIN ZhiWen, YANG Ke, ZHAO XiaoLu & XU JianZhong

National Laboratory of Wind Turbine Blade Research & Development Center, Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China

Received September 26, 2014; accepted December 3, 2014; published online December 19, 2014

Innovative features of wind turbine blades with flatback at inboard region, thick airfoils at inboard as well as mid-span region and transversely stepped thickness in spar caps have been proposed by Institute of Engineering Thermophysics, Chinese Academy of Sciences (IET-Wind) in order to improve both aerodynamic and structural efficiency of rotor blades. To verify the proposed design concepts, this study first presented numerical analysis using finite element method to clarify the effect of flat-back on local buckling strength of the inboard region. Blade models with various loading cases, inboard configurations, and core materials were comparatively studied. Furthermore, a prototype blade incorporated with innovative features was manu-factured and tested under static bending loads to investigate its structural response and characteristics. It was found that rotor blades with flatback exhibited favorable local buckling strength at the inboard region compared with those with conventional sharp trailing edge when low-density PVC foam was used. The prototype blade showed linear behavior under extreme loads in spar caps, aft panels, shear web and flatback near the maximum chord which is usually susceptible to buckling in the blades according to traditional designs. The inboard region of the blade showed exceptional load-carrying capacity as it survived 420% extreme loads in the experiment. Through this study, potential structural advantages by applying proposed structural features to large composite blades of multi-megawatt wind turbines were addressed.

wind energy, rotor blade, flatback, local buckling, extreme loads

Citation: Chen X, Qin Z W, Yang K, et al. Numerical analysis and experimental investigation of wind turbine blades with innovative features: Structural re-sponse and characteristics. Sci China Tech Sci, 2015, 58: 18, doi: 10.1007/s11431-014-5741-8

1 Introduction

Wind energy is one of the lowest-priced renewable energy technologies available today. Installed wind energy capacity both worldwide and in China has grown exponentially over the past few years and it is expected to increase significantly in the years to come [1]. As one of the most critical compo-nents in wind turbine system, rotor blades capture kinetic energy from wind and convert it to mechanical energy,

which is eventually converted to electrical energy by gener-ators. Rotor blades are thin-walled composite structures with airfoil cross-sectional profiles. Typical construction of blade cross sections is shown in Figure 1. Spar caps of rotor blades are made of composite laminates and designed to carry primary bending moments applied to the blades, while leading panel and aft panel are made of sandwich construc-tions and designed to provide aerodynamic profiles of blade cross sections. Shear webs are also sandwich constructions and designed to support two spar caps and transfer shear forces in the blades.

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Figure 1 (Color online) Typical construction of cross sections of compo-site wind turbine blades.

Aerodynamic and structural performance of rotor blades determines the overall performance of wind turbines. Re-searchers [27] have been focusing on improving aerody-namic efficiency of rotor blades as it directly affects the power output of wind turbines and the associated cost of wind energy. Therefore, aerodynamic performance of rotor blades is usually regarded to be of primary concern in wind turbine design. Nevertheless, recent failure accidents of rotor blades have attracted researchers attention. Through a number of studies [811] it has been found that local buck-ling of large aft panels and sharp trailing edge near the maximum chord, failure of inboard region at the root transi-tion, and spar cap delamination are among major failure modes in composite rotor blades.

In order to increase structural performance of blades, sandia national laboratories (SNL) [1214] conducted a so-called blade system design study (BSDS) in which structural innovations such as flatback airfoils, thick root diameter, carbon spar cap, etc. were proposed and structural advantages of these innovations were demonstrated by ex-periments using subscale prototype blades. Meanwhile, a series of research programs have been carried out at Na-tional Laboratory of Wind Turbine Blade Research & De-velopment Center, Institute of Engineering Thermophysics, Chinese Academy of Sciences (IET-Wind) aiming to im-prove both aerodynamic and structural performance of

composite wind turbine blades. From material and structural point of view, the proposed blades were featured with (i) glass/polyester composites for lower material and manufac-turing cost compared with more commonly used glass/ epoxy composites, (ii) flatback at the inboard region and thick airfoils at both inboard and mid-span region for larger bending stiffness and strength compared with sharp trailing edge and thin airfoils used in the conventional blades, and (iii) transversely stepped thickness in spar caps for more efficient use of materials against external bending loads due to the increase of the area moment of inertia of the blade cross-section. The structural features of the proposed blades are illustrated in Figure 2. Major difference between the BSDS blade and the one proposed by IET-Wind is shown in Table 1.

Although structural advantages of the BSDS blade have been studied by SNL, the comparisons of structural perfor-mance were made between blades with different geometries, material layups, and applied loads. It should be noted that comparative study on blades with a single variable would give more conclusive information to blade designers than the one with multiple variables especially when the effect of one particular variable is of interest. Considering the flat-back at the inboard region is one of the most important in-novations proposed by both SNL and IET, this structural feature is treated as a key variable in the current study and comparative study is conducted numerically to clarify its effect on local buckling strength of the blades. Furthermore, a prototype blade with the joint use of flatback, thick air-foils and transversely stepped spar cap thickness has been manufactured and tested in order to verify the proposed design concepts with emphasis on structural response and characteristics of the blades. It is expected that more in-sights into structural performance could be gained through this study and eventually more reliable and cost-effective blade designs for wind energy utilization could be achieved.

2 Numerical modeling and analysis

In numerical modeling, a blade with structural features of flatback at the inboard region, thick airfoils at both inboard

Figure 2 (Color online) Structural features of blades proposed by IET-Wind.

Chen X, et al. Sci China Tech Sci January (2015) Vol.58 No.1 3

Table 1 Comparison of structural features between blades proposed by SNL and IET

SNL IET

Thin, large diameter root Regular root size

Thick airfoil at inboard region Thick airfoil at inboard and

mid-span region Spar cap with full length and

constant-thickness Spar cap with transversely

stepped thickness Carbon spar cap Glass spar cap

Epoxy resin Polyester resin

and mid-span regions, and transversely stepped thickness in spar caps was analyzed. Although structural advantages of many features are readily assessed according to basic struc-tural mechanics, those of flatback at the inboard region have not been thoroughly investigated by the existing studies [1214], in which different blades of multiply variables were compared with. In this study, the blade geometric con-figurations at the inboard region with flatback was deter-mined to be a key variable in the numerical modeling and other variables such as outboard geometry, material layups, and applied loads were kept the same among the blade models to be analyzed. In this study, particular focus was paid to local buckling strength of blades with different in-board configurations.

2.1 Blade geometry and material properties

Four types of blades with a total length of 10.3 m were modeled using the general finite element program Abaqus [15], and they were labeled as BS1: A baseline blade model with a conventional sharp trailing edge at the inboard region; BS2-1: A blade model with a conventional sharp trailing edge and a longer chord length at the inboard region; BS2-2: A blade model same as BS2-1 but with an additional shear web; BF: A blade model with a flatback and a same chord length as BS1, see Figure 3. All blade models had same configurations at outboard regions which dominate aerody-namic performance of the blades. Physical dimensions of aerodynamic profiles of four blade models along their spans are shown in Figure 4. Except inboard configurations, all blades had the proposed features as thick airfoils at inboard and mid-span regions and transversely stepped thickness in spar caps.

Material layups of four blades were assigned to be iden-tical when blade regions are the same. The material layups of the additional shear web in BS2-2 were the same as those of the primary shear web in other three blades. The material layups as well as the geometry of the flatback region in BF were identical to those of the additional shear web in BS2-2. Because mechanical properties of core materials signifi-cantly affect buckling strength of sandwich constructions in the blades, two different core materials, i.e., PVC foam and balsa wood, which are commonly used in wind turbine blades were selected to perform parametric study in the numerical analysis. Typical material properties of cores used in this study are shown in Table 2.

2.2 Element type and mesh density

Shell elements S4R with an offset-node formulation were used for outer blade skins and those with a conventional mid-node formulation were used for shear webs. S4R is a 4-node, quadrilateral, stress/displacement shell element with reduced integration and a large-strain formulation. A typical size of 25 mm 25 mm was used to mesh the models, before this mesh size was determined, a mesh convergence study has been conducted. It was found that when the blades were meshed with typical sizes of 35 mm35 mm, 25 mm25 mm, and 15 mm15 mm, the results of the first natural fre-quencies of the blades between 25 mm25 mm mesh and 15 mm15 mm mesh were below 2%, and the results of the first bucking eigenvalues between two meshes were below 4%, therefore, a mesh size of 25 mm25 mm was deemed sufficient. This mesh size resulted in a total number of shell elements ranging approximately from 24000 to 26000 for four blade models.

2.3 Boundary and loading conditions

Fixed boundary was applied to the blade roots. Applied loads of four blades were assumed to be identical consider-ing that outboard regions controlling aerodynamic perfor-mance of the four blades were the same. Static bending loads were applied to the blades to simulate extreme wind loads that the blades were expected to sustain in 20 years design lives according to IEC standard 61400-1 [16] and

Figure 3 (Color online) Blade geometry and cross-sections of blades considered in numerical modeling.

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Table 2 Properties of core materials used in sandwich panels

Core material Density (kg/m3)

Compressive modulus (MPa)

Shear modulus (MPa)

PVC foam 60 70 20 Balsa wood 155 4100 166

Figure 4 (Color online) Physical dimensions of the blades.

GL Guideline for the Certification of Wind Turbines [17]. There were four loading cases in terms of bending direc-tions for each blade and they are schematically shown in Figure 5 taking the blade model BF for example. The load-ing cases of Flap_max and Flap_min primarily result in compression at suction side and pressure side of the blades, respectively; while the loading cases of Edge_max and Edge_min primarily result in compression at leading edge and trailing edge of the blades.

Desired distributions of bending moments in each load-ing case were calculated by IEC and GL and they were ap-proximated by piece-wise linear fits achieved by point loads introduced at 4 and 8 m blade spans, see Figure 6. Point loads with resultants equal to the forces necessary to gener-ate bending moments were equally distributed on the spar caps responsible for loading-carrying. The use of this kind of loading introduction was also intended to simulate the actual test setups in the blade load tests which will be fur-

Figure 5 Loading directions of the blades.

ther discussed in section 3. Representatively, boundary and loading conditions of the blade model BF in the Flap_ max loading case are shown in Figure 7.

2.4 Simulation results and discussion

There were two categories of FE analysis conducted on the blade models, the one was stiffness analysis with incremen-tal loading in order to obtain the deflections and spar cap strains of the blades, and the other one was linear buckling analysis with specified loads, i.e., 100% applied loads, in order to evaluate local buckling strength of the blades under extreme wind loads. Because local buckling strength of four blades with different inboard configurations was of major interest in numerical analysis, FE results from linear buck-ling analysis is presented and discussed in this section, while FE results from the stiffness analysis were compared with and validated against those from the blade test pre-sented in section 3.

In total, 32 numerical analyses were performed to study buckling strength of blades in terms of four inboard config-urations, four loading cases and two core materials. The

Figure 6 (Color online) Bending moments applied to the blades.

Figure 7 (Color online) Boundary and loading conditions of the BF blade in the Flap_max loading case.

Chen X, et al. Sci China Tech Sci January (2015) Vol.58 No.1 5

lowest eigenvalues for linear buckling of all analyses were normalized by those obtained from the baseline blade mod-els, i.e., BS1, with the same loading cases and core materi-als. The results are summarized in Figure 8.

2.4.1 Blades with PVC foam core It can be seen that when PVC foam core materials were used, buckling strengths of the baseline blade were im-proved by new configurations introduced at the inboard regions in BS1-1, 2 and BF. The blade BF exhibited most significant improvement in buckling strength in the loading cases Edge_min and Flap_max compared with other blades. Particularly, the blade BF showed a 25% increase in buckling strength compared with its baseline blade in the loading case Edge_min. The blades BS1-1 and 2 showed preferable buckling strength in the loading case Edge_ max, while in the loading case Flap_min, only a few percentages of improvement in buckling strength were found in BS1-1, 2 and BF.

These trends are regarded to be reasonable considering the loading directions in each case. In the loading cases Edge_min and Flap_max, the trailing edge sides most susceptible to local buckling were primarily subjected to compression, the introduction of longer chord length in BS1-1, 2 and flatback in BF greatly increased local buckling resistance of inboard regions. In the loading case Edge_ max, the trailing edge sides were primarily subjected to tension, larger bending stiffness due to longer chord length in BS-1, 2 therefore exhibited more significant increase in buckling resistance than the flatback in BF. In the loading case Flap_min, the trailing edge sides were partially sub-jected to tension due to airfoil twist, the improvement of buckling strength was not significant by introducing either longer chord length or flatback. Furthermore, by examining buckling modes of each blade, it was noticed that buckling regions of the BF blade at the trailing edge sides were smaller than those of other three blades, suggesting that

Figure 8 (Color online) Normalized eigenvalue for linear buckling of different blades under various loading cases.

buckling modes were suppressed by introducing flatback at the trailing edge sides. Representatively, buckling modes of blades in the loading case Flap_max are shown in Figure 9(a).

2.4.2 Blades with balsa wood core When cored with balsa wood, the blades exhibited only slight difference, approximately within 3%, in buckling strength for different loading cases and inboard configura-tions. This can be explained by the locations of buckling modes found at the middle spans of the blades as shown in Figure 9(b). Because four blades were only different in the inboard regions, it is not surprising that buckling strength of four blades did not differ much. It should be emphasized that although balsa wood could considerably improve buck-ling strength of sandwich panels in the blades, it is much more expensive than low-density PVC foam material, meanwhile, weight penalty due to large material density is also of concern for wind turbine blades. For four blades studied in this study, their weights were increased by 5% to 6% when balsa wood was used.

3 Experimental investigation

Based on simulation results and discussion presented in section 2, it is evident that the blade with flatback showed stronger local buckling strength at the inboard regions than the blade with conventional sharp trailing edges when other variables such as outboard geometry, material layups, and applied loads were the same. Therefore, it is expected that local geometry modification from the commonly used sharp trailing edge to the flatback could be an efficient way to

Figure 9 (Color online) Buckling modes of blades in the loading case Flap_max.

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increase local buckling strength of wind turbine blades. In this section, a prototype blade BF cored with PVC foams was manufactured and tested under static bending to verify the numerical results and further examine structural charac-teristics of blades with the proposed features.

3.1 Test program

Static bending was applied to the blade which was root- fixed at a test stand. Two cranes were used to introduce loads at 4 and 8 m blade spans as shown in Figure 10.

The loads were applied in a step-wise form following a test sequence as shown in Table 3. Once a loading case was finished, the blade was rotated along its longitudinal axis to a desired position and tested in a subsequent loading case.

At each load level, applied loads were recoded by load cells mounted on cranes, deflections of the blade were measured at two loading saddle locations and at the blade tip using draw-wire displacement transducers. Blade strains were recorded by strain gauges located along the center line of spar caps, at the middle of flatback, aft panels and shear webs. Only strains longitudinal to the blade axis were measured on spar caps, while both longitudinal and trans-verse strains were measured at other locations by the corre-sponding strain gauges labeled as 0 and 90, respectively. The use of these orthogonal gauges were intended to better indicate the occurrence of local buckling at flatback, aft panels, and shear webs during the tests through changes of load-strain path directions.

The blade successfully passed 100% test loads simulating the extreme loads in the loading cases of Edge_min, Flap_min, and Edge_max. In the following loading case Flap_max, it was decided to load the blade to failure if it could survive 100% test loads. Applied loads continued to increase and when they were close to 220% test loads, the blade failed catastrophically at 6-m span. Measurements of

Figure 10 (Color online) Experimental setups for the blade test.

Table 3 Load test history of the blade

Sequence Cases Loading history

1 Edge_min 0-40%-60%-80%-100%-unloading 2 Flap_min 0-40%-60%-80%-100%-unloading 3 Edge_max 0-40%-60%-80%-100%-unloading

4 Flap_max 0-40%-60%-80%-100%-120%-

140%-160%-180%-200%- blade failure

deflections and strains at the failure load were not able to be indentified clearly due to the rapid failure of the blade, the average values of measurements taken from 200% test load step to the final failure were used to approximate structural response at the load step of 210%.

3.2 Experimental results and discussion

3.2.1 Deflection and spar cap strains Measurements of deflections and spar cap strains in each loading case were compared with the predictions from nu-merical analysis. Representatively, the results for the load-ing case Flap_max are shown in Figure 11, where results of some load steps were not shown for the sake of clarity in presentation. It can be seen that experimental measurements and numerical predictions are with good agreement. The tip deflections of the blade at the extreme loads and near the ultimate failure loads were about 1.37 and 2.56 m, respec-tively; the corresponding longitudinal strains of spar caps at 6-m span were around 3500 and 6700 at both pressure and suction side.

3.2.2 Local strains of the maximum chord section Local strain response of the inboard region of the blade with flatback is of particular interest in the experimental study. As buckling is reported as one of dominating structural re-sponse near the maximum chord of conventional blades with sharp trailing edge. The strain records of the current blade measured near the maximum chord, i.e., 2-m span, were carefully examined and they are shown in Figure 12.

It can be seen that strains measured at spar caps, aft panel, and the flatback near the maximum chord exhibited linear relation to applied loads in all loading cases up to the ex-treme loads, although slight nonlinear response was found at shear web when loads approached the extreme loads in the loading case Flap_max. This observation suggests that in general the blade has sufficient buckling resistance in these regions. Considering the loading process to the final failure loads of the blade in the Flap_max case, it is inter-esting to notice that spar cap and aft panel remained in line-ar response as shown in Figures 12(a) and (b). While shear web started to show obvious nonlinear behavior beyond the extreme loads. Longitudinal strain and transverse strain changed load paths simultaneously with the increase of ap-plied loads suggesting the occurrence of buckling at this location as shown in Figure 12(c). Similarly, the flatback near the maximum chord showed linear response up to 140% test load and local buckling started to occur after-wards as shown in Figure 12(d).

It is important to note that although local buckling was detected at shear web and flatback near the maximum chord, the corresponding buckling loads were beyond the extreme loads. Furthermore, by examining strain levels at buckling loads, it can be found that all local strains were within 200 ,

Chen X, et al. Sci China Tech Sci January (2015) Vol.58 No.1 7

Figure 11 (Color online) Comparison of blade deflections and spar cap strains. (a) Blade deflections; (b) spar cap strains.

Figure 12 (Color online) Local strain responses of the blade. (a) Spar cap strains; (b) aft panel strain; (c) shear web strains; (d) flatback strain.

which are much smaller than the failure strains of composite materials used in the blade. Therefore, the local buckling response of shear web and flatback in the loading process to the blade failure was not able to cause material failure at these locations. This observation is of significance consid-ering that when the conventional sharp trailing edge is sub-jected to buckling, it usually exhibits local buckling and the associated material failure due to large strains [10]. The flatback used in the current blade exhibited great potential to improve local buckling strength which is usually one of the weakest links in structural systems of wind turbine

blades.

3.2.3 Ultimate strength of inboard region Because the blade failure at 6-m span prohibited the as-sessment of the inboard region at the ultimate failure, it was decided to conduct an additional static bending test with an intention to fail the blade at the inboard region in the load-ing case Flap_max. This load test was achieved by using the loading saddle previously mounted at 4-m span. Pulling forces were continuously applied and monitored. When the root moment reached approximately 294 kNm which was

8 Chen X, et al. Sci China Tech Sci January (2015) Vol.58 No.1

about 420% of the root moment caused by the 100% ex-treme loads, the inboard region did not exhibit any sign of failure.

The load test was then aborted due to safety concern. Subsequently, the inboard region of the blade was visually inspected and no material failure was found. It was con-cluded that the inboard region of the blade had exceptional ultimate strength.

4 Conclusions and future work

To verify innovative structural features of wind turbine blades proposed by IET-Wind, a rotor blade with flatback, thick airfoils, and transversely stepped thickness in spar caps was numerically analyzed and experimentally investi-gated focusing on its structural response and characteristics. The following conclusions were drawn.

By comparing linear buckling strength of blades with different inboard configurations in numerical analysis, it was found that the blade with flatback showed more favora-ble buckling resistance at the inboard regions than those with the conventional sharp trailing edge when low-density PVC foam was used. However, no significant advantage of flatback was found when balsa wood was used due to buck-ling mode occurred at the mid-span where blade geometry, material layups, and applied loads were identical. In load tests, the prototype blade with the proposed features exhib-ited linear behavior under extreme loads in spar caps, aft panels, shear web and flatback near the maximum chord which is usually regarded to be susceptible to buckling. In the failure test under flapwise bending, the shear web and the flatback near the maximum chord experienced local buckling around 100% and 140% extreme loads, respec-tively, and they continued to sustain applied loads up to 220% extreme loads without any material failure. The in-board region of the blade showed exceptional ultimate strength as it survived 420% extreme loads in the experi-ment.

Followed by this study, three blades identical to the pro-totype have been manufactured and installed on a 100 kilo-watt wind turbine. A series of field tests will be carried out after an ongoing trial run period to study aerodynamic, aer-oacoustic, aeroelastic, and structural performance of rotor blades incorporated with innovative features proposed by IET-Wind. Other studies are also planned, with the objec-

tive of applying the proposed features to large rotor blades for multi-megawatt wind turbines.

This work was supported by the National Natural Science Foundation of China (Grant No. 51405468).

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