1 Introduction

The cost of on-land megawatt-level wind turbine blades accounts for approximately 20%-30% of the whole machine. Therefore, researching for a new type of cheaper and more environmentally-friendly blade materials is of great significance in reducing the cost of wind power generation and enhancing the competitiveness of wind power.

Large-scale wind turbine blades, currently, are usually manufactured with the polymeric composites with glass fibers^[1], which have the feature of high strength, good thermal stability, good fatigue life, shock resistance capability, and enough stiffness in blade applications. The main reinforcement material is alkali-free glass fiber (E-glass). Unsaturated polyester resin (UPR) or epoxy resin (EPR) is usually adopted as matrix^[2]. Besides, carbon fiber^[3], wood laminates^[4], and bamboo laminates^[5] are also available options to replace the E-glass as the main reinforcement material of wind turbine blades. Although the glass fiber has quite outstanding and stable chemical, physical, and mechanical properties, it is difficult to degrade and recycle, and has a very limited space for cost reduction, raising vital environmental, and costing issues. Carbon fiber has many advantages over glass fiber in terms of high strength, low density, and better mechanical properties. However, its high price limits its use in the making of blades. Finland birch wood as the main material used in wood blades, the technology of which in blade manufacturing is owned by Vestas, is very resource constrained, and thus it lacks condition for extensive application due to its less availability. On the contrary, bamboo, known for its superior mechanical properties to wood, is a resource abundant and widely-distributed plant. For example, the distribution area of bamboo is about 8 million hectares only in China, accounting for 40% of the total area in the whole world.

With the structure designed, bamboo laminates can be used as wind turbine blade materials, which are not only economical and recyclable but also suitable for various specifications of wind turbines. As the awareness of environmental protection increases, research of large wind turbine blades based on bamboo laminates has high economic and environmental values. However, research and applications of bamboo in the design of wind turbine blades are quite insufficient. Only a few articles mainly concentrating in the mechanical properties of bamboo laminates and in the design of small blades can be referenced^[6-9].

The basic characteristics and mechanical properties of bamboo-based laminates, which are taken as the major enhancement materials based on the test data, are presented in this paper. The aerodynamic-structural integrated design of a 1.5 MW large wind turbine blade is carried out. The process of the blade structure layer design is given in detail. Finally, some technical issues raised from the design of bamboo-based blade are discussed.

2 Mechanical property tests of bamboo laminates

Large-scale bamboo blades use bamboo laminates which involve a series of special~processes as the main reinforcement materials. The middle parts of bamboo aging from four to seven years are used to obtain bamboo strips, which are thin with multi-layers thickness and have good properties. By processing the bamboo strips with special compression molding techniques, bamboo laminates are obtained, which can be made in accordance with the requirements of different specifications and forms. As the primary materials to make main load-bearing components in large wind turbine blades, bamboo laminates are subject to both tensile and compression just as the glass fiber is. Its density and gel content and other physical properties, anisotropic strength and stiffness, Possion's ratio, fatigue properties, and unique miter performance must be obtained in accordance with the strict standards. To acquire these properties, multi-group bamboo pieces with the length of 500 mm-2 000 mm, the width of 5 mm-22 mm, and the thickness of 1 mm-3 mm are delicately produced, and then are pressed into bamboo laminates with the thickness of 20 mm-50 mm as prototypes in the tests for their mechanical properties.

2.1 Static mechanical performance

To obtain the static mechanical data which are key inputs for the bamboo blade design, multi-group tensile and compressing tests of the bamboo laminates are completed in strict accordance with the requirements of test standard for tensile test methods of laminated composite plates (ASTM D 3500-2003) and test standard for compressing test methods of laminated composite plates (ASTM D 3501-2005) , respectively. Partial prototypes of the bamboo laminates after tests are shown in Fig. 1.

As bamboo laminates and epoxy resin are molded into composites, their dimensionless stiffness and strength are compared with unidirectional glass fiber composite with different types of resin (see Fig. 2) , where the performance of bamboo laminate-epoxy is normalized as 1. It is apparent that the dimensionless stiffness of bamboo composites is superior to those of glass fiber-epoxy and glass fiber-polyester composites, thus ensuring less tip deflection of bamboo blade than fiber glass blade under the same load and weight condition. However, the dimensionless strength of bamboo composites is relatively lower than that of the fiber glass composites.

For the design of a large scale wind turbine blade, consideration of its stiffness and anti-buckling characteristics takes precedence over its strength. In addition, the strength of the bamboo blade can be enhanced by the composite structures formed with bamboo laminates and glass fibers under a constraint of the blade weight equivalence. Thus, it can be seen that the application of bamboo laminates in the design of large wind turbine blades is feasible.

Differing from glass fiber, the variability in shape and machinability of bamboo laminates are poor. Consequently, they should not be made too long after molding, which means that a large number of overlapping sections will exist in the production process, resulting in unique miter joint problems. As gaps are prone to exist in the overlapping sections, which leads to resin accumulation, the production process is so complex that the mechanical properties of mitered sections will decrease significantly, affecting blade overall performance. The static test is designed to obtain the main mechanical properties of the mitered sections and safety factors of process. Through multiple groups of tensile tests on miter testing prototypes and process testing prototypes to get strength values and variance, along with data statistics and conversion methods to obtain strength characteristic values of the mitered sections, it is concluded that the safety factor of miter process is between 5 and 6 by comparing with the characteristic strength of the bamboo laminates.

2.2 Fatigue properties

The fatigue property test of bamboo laminates consist of standard and mitered bamboo laminate tests. With the use of hydraulic servo fatigue testing machine of MTS, the tests are conducted according to the test method for tensile and fatigue properties of fiber reinforced plastic laminates (GB/T16779-1997) , and the sine wave control and data acquisition are accomplished through computers. Then, fatigue S-N curves of the standard and mitered bamboo laminates under alternating stress $R=-1$ are obtained, as shown in Fig. 3. It can be seen that the stress-strain modulus of bamboo laminates can still reach about 70 MPa at 200 million cycles, whereas the performance of the mitered bamboo laminates is much poorer with a drop of 60% at 200 million cycles. As miter has an obvious impact on the fatigue performance of bamboo laminates, the design of the mitered sections is particularly important.

3 Design of aerodynamic, structural, and structural refinements ofbamboo blade

Based on the mechanical property data of the above bamboo laminates, a 1.5 MW design of bamboo blades is carried out in this paper. The design adopts the type of upwind horizontal axis wind turbine, variable speed and variable pitch, and doubly-fed generator, which is the most mature type in wind energy industry. Table 1 shows the basic parameters of the rotor.

3.1 Aerodynamic-structural integrated design method

Bamboo blades are designed with a novel multi-objective optimization design method^[10-11]. Instead of achieving a single optimal solution, two closely-related but mutual conflicting design objectives are selected in this paper, which are the maximum annual energy production in an assigned wind farm and the minimum blade mass with a given structural forms of double web I-beam^[12-16]. The aerodynamic design and structural design are then coupled in the effective improved multi-objective algorithm NSGA-II to obtain the optimal solution set, namely, the Pareto optimal solution set for selecting the optimal blade design^[17].

The primary goal of blade design is to achieve the best aerodynamic performance. Annual power production is calculated using a modified blade element momentum (BEM) theory. The BEM, which is proven to be relatively reliable for wind turbine blade design, is an efficient classical theory model for the calculation of wind turbine aerodynamic performance. Besides aerodynamic optimization, the structure optimization of blade is also taken into account. Double-web structure is used for the blade, the composites made of bamboo laminates and epoxy resin are used in the blade spar, the biaxial fabric epoxy glass fiber composite is used in the inner/outer skins, and the PVC foam and balsa wood are used in the filled portion. The structural scheme is shown in Fig. 4. Ultimate loads for the bamboo blade are determined according to the design standard of GL2010^[18-19].

Determination of the constraint conditions in wind turbine blade design is a semi-empirical process. In terms of constraint of aerodynamic configuration, it is demanded that there is a gradual decrease in chord, twist angle, and thickness all the way from the root to the tip of the blade. Meanwhile, in order to satisfy the manufacturing~requirement, a group of three-order polynomials defining nine variables are adopted, and 17 design variables are also set up to effectively represent the common shape and structure.

Among the optimal solutions from the Pareto set, the solution with satisfactory aerodynamic performance is selected, and the mass is approximately 6$\times$10.3 kg, which is similar to the current commercial blades. Between the cut-in wind speed and the speed of 8.5 m/s, the rotor rational speed varies linearly, tracking the optimal rotor power coefficient which can be up to 0.49 according to the wind tunnel test results (see Fig. 5) . The rated power is achieved at a wind speed of 10.5 m/s, and pitch control is then activated to maintain a constant power output above the rated wind speed.

3.2 Structural detail designs

After the completion of the aerodynamic-structural integrated design mentioned above, the aerodynamic layout of the bamboo blade is determined, but it is still necessary to carry out detailed structural design to further improve the mechanical properties of the blade. Taking the thickness and location of strengthening layer of spar, trailing edge, leading edge, etc. on each section as variables and setting the minimum section mass of each section as optimization objective, a more optimized structural layer of the whole bamboo blade is obtained quickly. The process of structural layer of the bamboo blade is basically the same as that of the glass fiber. However, due to the special structure and properties of bamboo laminates, more process details need to be considered for bamboo blade and more factors need to be taken into account in the design process, such as overall layer range, miter joint, strengthening layer, transition from root to tip, and lapping joint in chord direction. Different parts of the bamboo blade have its unique design consideration and priority to strictly guarantee the overall stiffness, strength, connection gap, thickness variation, and operational feasibility of the blade.

The spar of the bamboo blade takes the central line as a benchmark. It has the same thickness but variable width. Theoretically, there should be no gap in the chordwise direction among bamboo laminates. Thus, the gap should strictly be controlled during the practical manufacturing process, with the error less than 2 mm. Furthermore, negative impact of the miter on the bamboo laminate properties should be considered, and thus only one miter is acceptable in every four bamboo laminates.

The spar should be designed based on parts, chordwise locations, and spanwise locations. The width distribution of the spar should be continuous without sharp changes to ensure that the blade flapwise, edgewise, and torsional stiffness distributions are continuous. The laying width of spar gradually decreases from the root to the tip of the blade, and the cylindrical section on the root should be covered by bamboo laminates. The spar structure of the bamboo blade is shown in Fig. 6.

There is also a necessity to spread unidirectional glass fiber, whose paving width should be wider than bamboo laminates, over the surface of the spar to enhance its overall strength. Therefore, the stress concentration, the interlayer adhesion between bamboo laminates and fiber, and the infusion craftwork between those two materials are the key and challenging problems. Corona treatment will roughen the surface of bamboo laminates, thus enhancing the adhesive strength of interfaces.

The laying of bamboo laminates at the blade root should be conducted in strict accordance with the size of designed bamboo laminates to ensure that the gaps within bamboo materials and between bamboo materials and the bolt are in the tolerance range. As for tailing design for blade tip, the thickness of bamboo laminates needs to decrease evenly to zero at the tip to keep continuous and avoid step response. In the laying of the spar of bamboo blade, not only the continuous change in the width of the spar must be ensured, but also the gaps between the laminates and the sandwiches must be controlled to be as small as possible. Balsa is adopted as the core material at the blade root, and PVC is used at the middle part and the tip. In addition, the thickness of overall core materials along the blade spar is varied continuously. It is necessary to pave core materials in a correct way during the lapping of core materials and bamboo laminates. Appropriate tailoring for the core materials should even be made to ensure that the gap between the laminates and core materials is in the tolerance range. If the thickness of core materials in the chordwise direction varies considerably from that of bamboo laminates, additional triangular blocks must be paved as transition to ensure that there are no sharp changes in the chordwise thickness and spanwise thickness.

The pre-embedded bolt technology is used in the blade root design. After the flange of blade root is installed, bolt sleeves are wrapped by tailored glass fibers and are closely contacted with the diversion felts at the bottom. The number of the root bolts is 54 with the size of M30 originally, but the thickness of the pre-embedded bolt component increases to about 60 mm after the addition of embedded bolt sleeves and the bolt sleeve strengthening layer. In order to ensure the connection among the blade root, flange and pitch bearing, double-layer bamboo laminates are adopted at the cylindrical section of the blade root to effectively improve the stiffness and strength.

Based on the aerodynamic-structural integrated design methods and the structural detail design method, the bamboo blade can be designed and manufactured. Figure 7 shows a 1.5 MW bamboo blade.

4 Analysis of mechanical performance of bamboo blade

At the completion of the overlaying of the bamboo laminates, glass fiber reinforcing layer on the spar and other detail designs mentioned above, a finite element model of the designed bamboo blade is established using the finite element software MSC.PATRA/NASTRAN, combined with the ultimate loads calculated from unsteady aerodynamic and structural dynamic methods according to the design standard of GL2010, to analyze the blade strength, deflection, stability, vibration characteristics, and fatigue properties.

Stress cloud and deflection of the 1.5 MW bamboo blade are provided in Fig. 8. When subjected to the ultimate loads, the maximum tip deformation of bamboo blade is 6.43 m in the flapwise direction, much lower than that of the conventional 1.5 MW glass fiber blade in commerce. Also, the stress distribution of bamboo laminates is continuous with a maximum stress of approximately 78 MPa appearing in the core material overlaps near the trailing edge, less than the allowable stress of bamboo laminates itself.

The selected dangerous points on the blade section are presented in Fig. 9. According to the DNV standard, all points are likely to be dangerous, thus need to be checked. According to a simplified form of fatigue point selection provided by the GL2010, the leading edge, the trailing edge, and the spar are chosen as the most dangerous fatigue points. In this paper, various layer sections in the blade spanwise direction are chosen, and in each section, nine dangerous points are taken into account, which are leading edge, the overlapping positions of bamboo laminates and the core materials in the blade suction side (SS) and pressure side (PS), the thickest points of bamboo laminates and the trailing edge points on the SS and PS. Fatigue damages at the nine dangerous points are calculated under nearly 300 operational conditions. Figure 10 presents the fatigue damage at the most dangerous points in various conditions. It is noted that the damage factor is less than 1, meeting the requirement of the composite fatigue check.

5 Conclusive remarks

Bamboo laminates are chosen as the major enhancement materials for blade spars, and their application in large-scale wind turbine blade design is investigated in this paper. The mechanical properties of the bamboo laminates are firstly introduced and analyzed based on the bamboo material property tests. The aerodynamic-structural integrated design of a 1.5 MW wind turbine bamboo blade is then carried out using a multi-objective optimization method. In particular, the overlaying process of the blade structure is documented in detail.

The bamboo blade is high cost-effective, environmentally-friendly, and recyclable. In spite of its high stiffness, however, it has relatively low strength, relatively poor fatigue performance, and degraded properties caused by miters compared with the conventional glass fiber blade. All of these restrict the extensive application of bamboo laminates in the design of wind turbine blades. To solve these problems, future work should be carried out mainly on two aspects. Firstly, the basic physical and mechanical properties of bamboo laminates under various environments and conditions still need to be more deeply and widely studied due to its presentation of a great dispersion. Secondly, research on comprehensive mechanical characteristics of coupling of bamboo laminates and a variety of resin, adhesive, and glass fiber and other materials commonly used in wind turbines, improvements of bamboo laminates and the formation of superior composite materials should be the key breakthroughs in blade design and their applications in future.