Design and Analysis of Small-Scale Lift-Type Vertical-Axis Wind

Design and Analysis of Small-Scale Lift-Type Vertical-Axis Wind Turbine Using Composite Blade Design and Analysis of Small-Scale Lift-Type Vertical-Axis Wind Turbine Using Composite Blade Yang Zhong-Jia, Gu Yi-Zhuo*, Li Min, Li Yan-Xia, Lu Jie, and Zhang Zuo-Guang Key Laboratory of Aerospace Advanced Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University Received: 27 May 2013, Accepted: 3 March 2014 SUMMARY This paper focuses on verifying the feasibility of applying fibre reinforced polymer matrix composite to vertical axis wind turbine. A small-scale lift-type vertical axis wind turbine using glass fibre reinforced epoxy matrix sandwich composite blade was proposed. An airfoil profile of blade was designed and a composite blade was fabricated. The structure parameters of the blade were optimized by means of the finite element modeling method. The mechanical and the resonant properties of the blade were investigated. Finally, a wind turbine with the designed blade was assembled, and its generating capacity was tested. It was demonstrated that the designed composite blade and the wind turbine had excellent performance, indicating a promising technology for utilizing wind power. Keywords: Vertical axis wind turbine, Wind blade, Sandwich composite, Mechanical properties 1. INTRODUCTION Smithers Information Ltd., 2014 In recent years, renewable energies, such as wind energy, solar energy, hydro energy and geothermal energy, attract more and more attention owing to their clean and sustainable characteristics. Among these energy sources, wind power develops fast and has greatly promising application prospects 1. In the field of high-power generation connected to national electric network, large-scale horizontal axis wind turbines (HAWT) dominate in wind power. Urban areas generally have weak wind conditions and strong turbulence due to the presence of highrise buildings, so the power output of HAWT decreases in these conditions 2. In addition, visual impact, acoustic pollution, structural issues and safety problems limit the usage of large-scale wind turbines 1. Compared with largescale HAWT, small-scale vertical axis wind turbine (VAWT) has low-noise and good maintenance and works well with wind from any direction [3]. Compared with small-scale HAWT, VAWT has better designability. It has been fully proved that VAWT is more appropriate for urban applications 4-6, which is believed to be an attractive small-scale source of green energy. At present, improving the performance of VAWT is becoming a hot topic. Kim and Gharib 7 investigated the effect of an upstream flat deflector on the power output of a straight-bladed counterrotating VAWT. They found that the power output increased significantly if the turbine was properly positioned behind the deflector. Chong et al. 8 introduced an innovative device called omni-direction-guide-vane to be integrated with a VAWT for improving the wind rotor performance and minimizing the safety concern. This paper reports the design of a small-scale lift-type vertical-axis wind turbine (SLVAWT) which was a lift force driven wind turbine. It consists of three airfoil shaped blades that are attached to a rotating vertical shaft, as shown in Figure 1. The wind blows over the airfoil contours of the blade, which creates aerodynamic lift and pulls the blades along 9. The wind blade is a key component for a wind turbine, whose performance is determined by the structure and property of the blades. Brown and Brooks 10 designed and analyzed a vertical axis thermoplastic composite wind turbine blade, which qualified composites to be used for VAWT and offered an opportunity to decrease the cost of VAWT. However, thermoplastic materials generally have lower specific strength, specific modulus and creep resistance, which might decrease the lifetime of VAWT. Polymers & Polymer Composites, Vol. 22, No. 4, 2014 423

Yang Zhong-Jia, Gu Yi-Zhuo, Li Min, Li Yan-Xia, Lu Jie, and Zhang Zuo-Guang Figure 1. Schematic of SLVAWT This paper adopted thermosetting sandwich composite to fabricate the blades of SLVAWT. A model was established to simulate wind load distribution on the composite blade and to optimize its structure parameters. Then, the vibration performance of the optimized composite blade was simulated and analyzed. The results present the good properties of a smallscale vertical-axis wind turbine with the optimized composite blade. 2. EXPERIMENTAL 2.1 Materials In this paper, 0.2 mm thick plain glass fibre weave and 0.2 mm thick unidirectional glass fabrics were purchased from Nanjing Fibreglass Research & Design Institute. 129A epoxy resin and 129A curing agent were supplied by Tianjin Dasen Material Science and Technology Ltd. The blend weight ratio of the resin and curing agent was 100:38, and the curing condition was 3 h at 60 C. The mechanical properties of the cured epoxy resin are shown in Table 1. Polyurethane (PU) foam with density of 60 kg/m 3 was purchased from Beijing Bolide Trade Ltd. Based on the standards of GB/T 1452-2005, GB/T 1453-2005 and GB/T 1455-2005, tensile, compressive and shear properties of the foam were respectively measured. The mechanical properties of the foam are listed in Table 2. Table 2. Mechanical properties of PU foam Tensile strength (MPa) 0.62 Tensile modulus (MPa) 26.97 Poisson ratio 0.27 Compressive strength (MPa) 1.18 Shear strength (MPa) 0.67 Shear modulus (MPa) 16.27 2.2 Design and Preparation of Composite Blade To improve the start-up performance of SLVAWT, an asymmetric airfoil with high lift coefficient was designed, as shown in Figure 2. Asymmetric and tortuous airfoil profile provides higher starting torque, which can improve the starting performance of a vertical axis wind turbine. For the design of a wind turbine blade, the structure of blade should Figure 2. Airfoil profile of blade Figure 3. Schematic of composite blade structure be optimized to be as light as possible, with enough strength and resistance to deformation 11. Thus, we adopted a sandwich composite with high structural efficiency to fabricate wind blade. The airfoil blade illustrated in Figure 3 was prepared as follows. A certain amount of polyurethane ingredients was injected into a precast mould. After 2 h curing and foaming at 25 C, the foam core with the airfoil shape was obtained 12. Then, five layers of glass fibre fabric were laid-up on the surface of the foam core and epoxy resin with curing agent was applied to each layer of fabric to impregnate the fibres. The fabrics were laid-up in the sequence of plain/ plain/ unidirectional/ plain/ plain fabrics. The uncured sandwich-structure composite blade was put into a mould in an oven and the composite was cured at 60 C for 3 h. Finally, the blade was demoulded and was polished to improve the surface quality. 2.3 Measurement of Mechanical Properties Tensile, compressive and shear properties of the composite laminate fabricating blade skin were tested according to the standards of ASTM D 3039, ASTM D 3410, and ASTM D 3518, respectively. These data Table 1. Mechanical properties of cured 129A epoxy resin Tensile strength (MPa) 65 Tensile modulus (GPa) 3.0 Flexural strength (MPa) 110 Flexural modulus (GPa) 3.7 Compressive strength (MPa) 111 424 Polymers & Polymer Composites, Vol. 22, No. 4, 2014

Design and Analysis of Small-Scale Lift-Type Vertical-Axis Wind Turbine Using Composite Blade were used to analyze the mechanical properties of the composite blade. Figure 4. Sketch of three-point bending test for composite blade Three-point bending experiment was adopted to measure the strength and deformation ability of the composite blades. A three point bending test of the blade was carried out using an Instron 3382 universal testing machine, and the span was set at 1 m, as shown in Figure 4 and Figure 5. According to the standard of GB/T 3356-1999, the stress loaded on the blade was calculated using Equation (1): σ = 3PS 2bh 2 (1) where σ means the stress loaded on the blade, P is the force loaded on the blade, S means the span, b means the width of the blade, and h is the thickness of the blade. Figure 5. Three-point bending test for composite blade 2.4 Testing Performance of Wind Turbine This paper established an experimental device for wind turbine operation testing, as shown in Figure 6. The turbine operation data acquisition system consists of tachometer, rectifier, load, wind speed sensor, capture card and computer. The blades were assembled with petioles and generator to be a wind turbine, as shown in Figure 1. The rotation speed and generating power of the wind turbine with different wind speed were measured. Figure 6. Schematic of wind turbine operation test 3. STRUCTURAL DESIGN AND EVALUATION OF THE COMPOSITE BLADE 3.1 Simulation of Mechanical Behaviour of Blade with Finite Element Modeling Method In order to evaluate the structural reliability of the blade, we simulated its mechanical behaviour with the finite element modeling (FEM) method and predicted the failure load. Polymers & Polymer Composites, Vol. 22, No. 4, 2014 425

Yang Zhong-Jia, Gu Yi-Zhuo, Li Min, Li Yan-Xia, Lu Jie, and Zhang Zuo-Guang As the studied blade had foam sandwich-structure with laminate skins, the properties of the foam and laminate skins need to be defined. Table 2 shows the mechanical properties of the PU foam, and Table 3 shows the mechanical properties of unidirectional laminate and plain laminate. For the blade three-bending case, the load was concentrated on the width of 100 mm, and the airfoil profile outline of the blade and the supporting points were Figure 7. Load and restraint of the blade model restrained without freedom of motion, as shown in Figure 7. The FEM flow chart is shown in Figure 8. As a kind of anisotropic material, the characteristics of deformation and failure for composite are very different from those of traditional materials, such as metal and polymer. We simulated the three point bending behaviour of composite blade with three kinds of failure criteria, which were the greatest stress failure criterion, Cahill failure criterion and TsaiWu failure criterion. According to the greatest stress failure criterion, when composite suffers complex stress, the cause of failure is one of the component forces reaching the strength limit of the material. Cahill failure criterion holds the viewpoint that there are relationships among basic strengths and gives Equation (2) to describe how they influence each other: 2 σ 1 X σ 1σ 2 + σ 2 2 2 X 2 Y + τ 2 12 2 S = 1 2 (2) where X means the longitudinal strength, Y is the transverse strength and S is the shear strength. σ 1 means the longitudinal stress, σ 2 means the transverse stress and τ 12 means the shear stress. Figure 8. The procedure of finite element modeling The TsaiWu failure criterion gives equations with more terms to improve the accuracy of the mathematical model: (3) (4) (5) (6) Table 3. Measured mechanical properties of composite laminate Mechanical properties Unidirectional fabric laminate Plain weave laminate Axial modulus (GPa) 32.61 17.72 Transverse modulus (GPa) 3.14 17.72 Poisson ratio 0.16 0.25 Shear modulus (MPa) 2347 2470 Axial strength of tension (MPa) 800 310 Axial strength of compression (MPa) 360 280 Transverse strength of tension (MPa) 18 310 Transverse strength of compression (MPa) 35 280 Shearing strength (MPa) 6 10 (7) (8) (9) (10) where X, Y,S, σ 1, σ and τ have the 2 12 same meanings as those in Equation (2), and t means tensile, and c means compression. 426 Polymers & Polymer Composites, Vol. 22, No. 4, 2014

Design and Analysis of Small-Scale Lift-Type Vertical-Axis Wind Turbine Using Composite Blade A comparison of the results from the FEM method with the measured values is shown in Figure 9. The failure loads are 4374 N, 4613 N and 3503 N respectively based on TsaiWu, the greatest stress and Cahill failure criteria, while the experimental value is 4326N. These results demonstrate that the FEM method based on the TsaiWu failure criterion has higher accuracy and is used to optimize the structure of composite blade. 3.2 Design of Lay-up Structure of Blade Skin Fibre lay-up structure is critical for composite design, which significantly influences the mechanical property of composite. According to the international standard IEC61400-1, for level I wind turbine, the blades have to survive in 50 m/s gust load without any catastrophic consequences to the unit. The aerodynamic load on the blade in this case can be calculated by Equation (11), and is multiplied by safety design factor 1.5. Thus, the blade skin has to withstand the 2344 Pa load without damage, and the fibre lay-up structure of blade skin must yield a blade strength above 2344 Pa. measured by three-point bending test was 60 MPa, demonstrating that the composite blade is strong enough to survive a 50 m/s gust load. 3.3 Design of Petiole Span For the proposed wind turbine in this paper, each blade has two connections with two petioles (Figure 1) and the span affects the stress and deformation distribution on the blade 8. Therefore, in this paper the connecting position of the petiole was considered 13. The effect of petiole span (ranging from 500 mm to 1700 mm) was analyzed with the FEM method. To be convenient for calculation, the blade average wind load was set to be 1000 Pa. Figures 10 to 13 present the simulation results. Figure 9. The load-displacement curves of the composite blade under three point bending load, including the FEM results based on different failure criteria and the experimental result Figure 10. The stress distribution on the blade with different petiole spans p = v 2 /1.6 (11) where p means the aerodynamic load, and ν means the wind speed. According to the FEM result, the layup structure with two layers of plain weave fabric at 0 degree direction or one layer of plain weave and one layer of unidirectional fabric can meet the requirement for the blade strength. The maximum displacement of the former is 2.2 mm, and that of the latter is 1.86 mm, which is attributed to higher resistance to deformation using unidirectional fabric. Thus, we designed the laminate skin with the lay-up structure of plain/ plain/ unidirectional/ plain/ plain, and the flexural strength of the composite blade Polymers & Polymer Composites, Vol. 22, No. 4, 2014 427

Yang Zhong-Jia, Gu Yi-Zhuo, Li Min, Li Yan-Xia, Lu Jie, and Zhang Zuo-Guang Figure 11. The curve of the maximum stresses on the blade with different petiole spans obtained from FEM analysis It can be clearly seen that the minimum value of the maximum stress and the minimum displacement occur for 1000 mm petiole span. Thus, the blade with 1000 mm petiole span was chosen for the designed blade. 3.4 Analysis of Vibration Performance of Blade Generally, the design of wind turbine must have fatigue considerations. Several factors relating to the fatigue phenomena of wind turbine blade can be summarized as follows 14 : Figure 12. The deformation distribution on the blade with different petiole spans Figure 13. The curve of the maximum displacements on the blade with different petiole spans obtained by FEM analysis long and flexible structures vibration in its resonant mode randomness in the load spectra due to the nature of wind continuous operation under different conditions The occurrence of resonance can result in fatigue failure of the wind turbine, so the natural frequency of blades has to be different from the incentive frequency and its integer times 15. To avoid resonance, modal analysis is necessary. In this work, we made a modal analysis including the frequency and the mode of vibration, where the FEM method and the Lanczos algorithm were applied 16. Before modeling, we imposed fullyconstrains to the positions where the petioles were connected to the blade for limiting its rotational degree of freedom and displacement degree of freedom, as shown in Figure 14. The first to fifth order vibration models were calculated, and the results are shown in Table 4. As is shown in Figure 15, the rotate frequency and its integer multiples are under 250Hz, and the natural frequencies are above 75Hz. The blades will not resonate during its operating process since the difference between the natural frequency and the driving frequency is large enough. 428 Polymers & Polymer Composites, Vol. 22, No. 4, 2014

Design and Analysis of Small-Scale Lift-Type Vertical-Axis Wind Turbine Using Composite Blade Table 4. The simulation results of the blade s vibration performance Modal order time Natural frequency (Hz) Vibration model 1 84.42 brandish 2 88.51 brandish 3 88.54 brandish 4 230.64 reverse 5 240.96 reverse Figure 14. Finite element model of the blade for modal analysis and the two blue parts indicate fixed points 3.5 Performance of Wind Turbine Based on the designed blade structure, we assembled the composite blades, petioles and a 300W generator to be a wind turbine and measured the running performance. The measured results are shown in Figure 16. As is shown in Figure 16, the wind turbine starts up when the wind speed is approximately 3.2 m/s, and generates a power of 300W when the wind speed reaches 13 m/s, i.e. the rated wind speed is 13 m/s. In our previous work, we manufactured a kind of composite blade with seven-layer fabric skin weighing 3700 g, while the blade studied in this paper weighed 2800 g. The former wind turbine assembled with the same generator and heavier blades had a starting wind speed of 4.5 m/s and a rated wind speed of 18 m/s. The comparison of the two kinds of blades shows that decreasing blade weight significantly improves the starting performance and the efficiency of wind turbine. Figure 15. The Campbell chart of the blade, 1P to 4P mean the rotate frequency and its integer multiples, and the first to fifth order represent the natural frequency of the first five vibration models Figure 16. The power curve and the rotate speed curve of the wind turbine According to the international standard IEC61400-1, the wind turbine for a level-wind site should have a rated wind speed less than or equal to 15 m/s, and the wind turbine for a level-wind site should has rated wind speed less than or equal to 13 m/s. Therefore, the blade designed in this paper can reach the requirements of the level and level-wind sites, which is attributed to the low density and good mechanical properties of the composite structure. Polymers & Polymer Composites, Vol. 22, No. 4, 2014 429

Yang Zhong-Jia, Gu Yi-Zhuo, Li Min, Li Yan-Xia, Lu Jie, and Zhang Zuo-Guang 4. CONCLUSIONS This paper designed and produced SLVAWT with glass fibre-reinforced epoxy resin matrix sandwich composite blade, and studied the performances of the composite blade and the wind turbine. The experimental and simulating results prove that the optimized composite blade meets the requirements of level and level-wind sites. By means of the method of FEM, the vibration performance of the composite blade was analyzed, which indicated that the blade would not resonate during the running process. Moreover, the decreased blade weight resulting from the low density and good mechanical properties of the composite structure could significantly improve the starting performance and the efficiency of wind turbine. This work demonstrates that a thermosetting composite material is suitable to make SLVAWT blade, and it offers a new avenue for improving the performance of SLVAWT. References 1. Chong W.T., Poh S.C., Fazlizan A., et al, Journal of Central South University of Technology, 19(3) (2012) 727-732. 2. Mertens S., Kuik G. van and Bussel G. van, Journal Of Solar Energy Engineering, 125(4) (2003) 433-440. 3. Sharpe T. and Proven G., Energy and Buildings, 42(12) (2010) 2365-2375. 4. Bertényi T., Wickins C., and McIntosh S., 48th AIAA Aerospace Sciences Meeting, Orlando, Florida, USA. (2010) AIAA-2010-1376. 5. Danao L.A., Eboibi O., and Howell R., Applied Energy, 107 (2013) 403-411. 6. Edwards J.M., Angelo Danao L., and Howell R.J., Journal of Solar Energy Engineering, 2012. 134(3) (2012) pp. 11. 7. Kim, D. and Gharib M., Journal of Wind Engineering and Industrial Aerodynamics, 115 (2013) 48-52. 8. Chong W., Fazlizan A., Poh S.C., et al., Applied Energy, 112 (2013) 601-609. 9. Islam M., Ting D.S.K. and Fartaj A., Wind Engineering, 31(3) (2007) 165-196. 10. Brown K. and Brooks R., Plastics, Rubber and Composites, 39(3-5) (2010) 111-121. 11. Xudong W., Zhong Shen W., Jun Zhu W., et al., Wind Energy, 12(8) (2009) 781-803. 12. Chang R.R., Chiang T.H., Tseng, Y.C., et al., Procedia Engineering, 14 (2011) 1988-1995. 13. Jureczko M., Pawlak M., and Mężyk A., Journal of Materials Processing Technology, 167(2-3) (2005) 463-471. 14. Shokrieh M.M. and Rafiee R., Composite Structures, 74(3) (2006) 332-342. 15. Maalawi K.Y. and Negm H.M., Journal of Wind Engineering and Industrial Aerodynamics, 90(8) (2002) 961-986. 16. Dargel, P.E., Wollert A, Honecker A., et al., Physical Review B, 85(20) (2012) 205119-1-11. Acknowledgements This work was supported by funding from the National 863 Program of China [Project No. 2009AA0345] and the National 973 Program of China [Project No. 2010CB631100]. 430 Polymers & Polymer Composites, Vol. 22, No. 4, 2014