Project and manufacturing of turbine blades - Small wind power applications Mário João de Sousa Brito

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1 Project and manufacturing of turbine blades - Small wind power applications Mário João de Sousa Brito Abstract This thesis main objective was selecting a manufacturing process, as well developing a mechanical design, for a blade to a small wind turbine, with 2,12 m diameter, and 2 kw rated power. Starting from a previous aerodynamic study in which was defined the best airfoil to this application, and was also proposed a blade geometry for a similar rated power wind turbine. The intended goal was obtaining a long lasting blade at the lowest possible cost, selecting a large quantity manufacturing process. Material selection for this part fell over aluminium alloys. Adaptation was made from the original geometry, followed by geometric modelling and mechanical design of the part, particularly concerning fatigue and corrosion. The rated power for this modified geometry was estimated. Careful survey of several manufacturing processes led to choose precision forging. Theoretical calculations were made and then a finite element analysis was also performed, heat treating were proposed, for obtaining final mechanical properties of the part. The part cost was estimated. Alternative manufacturing processes to obtain same geometry were proposed. An economic and sustainability analysis was performed to several manufacturing processes. A comparison was made with similar turbines currently at service, concluding for total viability of this project. Reference was made to overload protection systems. Keywords: Blade, part, material, forging, design, dies. Introduction : In this thesis a manufacturing process for a small wind turbine blade was selected and studied, as also a mechanical design for that same part (blade) was developed. The departure point was reference [1], retrieving a convenient blade shape from an aerodynamic study performed at [1], with further adaptation of the geometry to fit into selected manufacturing process. The selected profiles were Eppler 387. Calculations were made for both mechanical and manufacturing designs. Later, these calculations were further refined and checked through computational methods, particularly finite element analysis simulations. Resulting numerical models were validated producing small scale models of that part by a simplified version of the manufacturing process which, nevertheless, allows an accurate proportional approach to full scale process. Figure 1 small wind turbine Whisper 200 Project Motivation: In today s world energy scarcity is undeniable, as are its side effects both at economical and geostrategic levels. Nevertheless, there still remain some, largely unexploited, energy resources. This happens because those resources were uneconomical, with a few, rare, exceptions... until now. As fossil energy 1

2 prices soar to unseen levels, other energy sources become increasingly attractive; wind power is one of these, allowing spread production to local level. This power source has been in use for thousands of years, went almost forgot by mid-twentieth century, but increasingly returned since late 1970 s to a full scale expansion in early twenty-first century, but exploiting only a tiny fraction of available resource. Meanwhile small scale economical feasibility only can be achieved if equipment be rather inexpensive, as cheap as possible, without sacrificing durability (20 or more years). The main goal for this work was obtaining a small wind turbine blade, economical and reliable, intended for mass production. Figure 2 World energy 2008, production by energy source Materials and manufacturing: For this part three material classes were selected [5], [10], [11]: Polymeric, metals and composites (glass fibre reinforced) A comparison has been made between them, taking into account the topics below, - Yield stress - Fatigue stress - Corrosion resistance - Density (weight) - Manufacturing process Maximum stresses at blade sections were obtained from [1], and the highest values were: - Airfoil Ep 387: λ = 6,75 σmáx = 74 MPa - Airfoil Ep 387: λ = 7 σmáx = 76 MPa Property/material Polycarbonate PPOm AA3003- H16 AA5052- H32 AA6061- T4 AA6061- T6 Glass fibre blanket Glass fibre Yield stress (Sy) (MPa) 128 > Not defined Not defined Ultim. Stress (Su) (MPa) Fatigue stress (Sf) (MPa) 2,5 2, Young Modulus (E) (GPa) 3% <10% Strain % Not defined Not defined Density ( / ) Table 1 Selected material properties The selected materials were aluminium alloys AA5052-H32, AA6061-T6, and for later feasibility confirmation AA6061-T4. Ruling those choices was fair distribution between mechanical properties (especially yielding and 2

3 fatigue resistance), moderate density, high corrosion resistance, well developed and spread fabrication methods and relatively modest wrought material prices. Part geometric modelling: Departing from [1], the more appropriate airfoil for this part is Ep-387, in terms of aerodynamical and structural performance, with the blade bearing 74 MPa maximum stress. The selected TSR was 7 (blade design from [1] to keep typical TSR for high performance turbines), with optimized α to obtain better energy efficiency. Figure 3 Eppler 387 airfoil profile, thickness 9,06% of chord The original blade in [1] had a extremely thin tip, with 2 mm average thickness. That could bring serious problems to part fabrication and increased risks of damage and failure under service conditions. So, to improve fabrication and simultaneously strengthen this part, the last 20 cm of the blade tip were removed, accepting the penalty in energy production. The remaining turbine parameters were kept constant, without further modification from original geometrical design (ω = 695 rpm, U = 13 m/s). After modifications, blade TSR is λ = 5,6. Figure 4 Turbine dimensions [1] Figure 5 Base airfoil description Station r/r (m) radius (m) chord (m) Rotation Maximum Angle of angle (º) thickness (m) attack (º) 1 0,213 0,2663 0,12 16,69 0,0112 6, ,252 0,315 0,13 13,84 0,0119 6, ,299 0,3738 0,13 11,15 0,0116 5,68 4 0,353 0,4413 0,12 8,61 0,0106 5,68 5 0,412 0,515 0,1 6,54 0,0095 5,68 6 0,476 0,595 0,09 4,88 0,0084 5,68 7 0,542 0,6775 0,08 3,55 0,0075 5,68 8 0,608 0,76 0,08 2,48 0,0068 5,68 9 0,674 0,8425 0,07 1,64 0,0062 5, ,738 0,9225 0,06 0,96 0,0056 5, ,797 0,9963 0,06 0,42 0,0052 5, ,851 1,0638 0,05 0 0,0047 5,68 Table 2 airfoil section general characteristics Mechanical engineering design: From calculations (Glauert s theory and mechanics-statics methods [4], [6]), lift force (L) over the whole blade is equivalent to a single concentrated load of = 99,16 N, applied in x = 3

4 0,486 m. The blade tip lift force was studied by the same methods, but separately, since was considered that maximum tension would be reached at approximately ¾ of blade length. Lift force for this tip ¼ section was calculated has = 54,27 N. To obtain the maximum stresses in the blade root and also at ¾ blade length section, calculations were performed using mechanics of materials methods [7]: Factor of safety [6]: Stresses/section ( r = 0,2663 m ) ( r = 0,760 m ) Bending stress σf (MPa) 38,6 38,5 Centripetal stress σc (MPa) 4,24 4,08 Total stress σ (MPa) 42,8 42,6 Table 3 Section stresses Factor of safety ny ny (Yield) ( r = 0,2663 m ) ( r = 0,760 m ) AA5052-H32 4,32 3,38 AA6061-T6 4,76 3,72 AA6061-T4 2,55 2 Table 4 Factor of safety The resulting factors of safety are quite over dimensioned, mostly because a simplified approach was made to facilitate calculations. Also, a finite element stress study was carried, which gave the following results. Figure 6 Blade stresses (COSMOSXpress) Figure 7 Blade stresses (COSMOSXpress) The small surfaces that can be seen over the blade surface are just hardpoints for concentrated load application to allow COSMOSXpress analysis, they are not part of real blade surfaces. Properties AA5052-H32 AA6061-T4 AA6061-T6 Yield Stress (Sy) MPa , Ultimate Stress (Su) MPa Young Modulus (E) GPa Density (ρ) / Table 5 Material properties SolidWorks data base A can be seen in the figures above, the load was applied as a distribution of concentrated loads over the blade central line. That was because COSMOSXpress didn t allowed a correctly applied distributed load on the part surface, so to approach the real load as good as possible, the equivalent concentrated load for each section 4

5 between stations was applied on the middle point of that section. This method was widely used in the early days of aeronautics. The maximum stress and factor safety for this was Material MPa ny AA5052-H32 42,65 4,57211 AA6061-T6 42,65 6,44776 AA6061-T4 42,65 5,33468 Fatigue: Miner s rule Before applying Miner s rule, a more precise estimation was made for the number of cycles of load during turbine lifetime. That was made in an annual basis using Weibull s distribution with k = 2, a statistical method largely employed for wind turbine engineering design. j Nj (ano) tensão mín (σmín) tensão máx (σmáx) Figure 8 Weibull Distribution, k = 1, k = 2, k = 3 tensão alternada (σa) tensão média (σm) Nfj Nj/Nfj (dano anual) , ,4 21,4 3,284E+15 9,725E E+08 42, ,8104 0, ,4 1,001E+48 2,00E E+08 32, , ,7 21,4 2,955E+18 6,768E-11 Tabela 6 Miner s rule results Total 1,649E-10 Calculations for Miner s rule were performed for AA2024-T4 aluminium alloy, since only data for this alloy was available in [10]. To correct that deviation, a relationship was established between fatigue characteristics of AA2024-T4 and those of the selected alloys. That was achieved using ASME-elliptic fatigue analysis method for all alloys and thereafter taking proportions between each one of selected alloys and AA2024-T4: Nf/Nf2024-T4 AA5052-H32 0,887 AA6061-T6 0,645 AA6061-T4 0,773 Table 7 - nf/nf(2024-t4) relationship Each one of these values, multiplied by the number of years obtained from Miner s rule, gives values between 4,687E+09 and 5,318E+09 years. These figures are absurd, of course, but they come in line with infinite life concept for materials. Comparing them with figures given through Goodman and ASME Elliptic, conclusion can be made that this part will have a useful life of several decades, or, in some cases, over a century. Natural frequencies: Natural rotation frequency (rated power): 11,6 Hz. Rotation frequency has some proximity to bending natural frequency, but not too much, thus, no resonance is expected, nevertheless, extreme weather or system malfunction could lead rotation near 24,6 Hz, so an emergency breaking system is necessary. 5

6 Modified blade performance: After modification, the rated power was calculated by three methods, all of these converged to near of 2000 W. So rated power was set in 2 kw. Manufacturing process: For fabricating this part, the selected process was precision forging. This process is characterized by compressing material inside two forging dies (closed die type), forcing it to flow under compression stresses until cavities inside become fully filled, obtaining the part in an almost finished shape, reducing, or even avoiding subsequent machining to get designed shape. This process demands high deformation forces and low deformation speeds (0,1 ), and is normaly an hot forging process, with temperatures near the upper limits of temperature ranges for the alloys to be worked [2], [3]. The objective is minimizing pressures, enhancing material deformation, cavity filling and lowering part defects, distortions and residual stresses. Typical parameters (aluminium alloys [3]): - Low deformation speeds (0,1 ) - Hydraulic presses to ton. Speed, pressure and position tight control. - Draft angles 0 to 0,5º outside and 0,5 to 1º inside. - Heated dies, up to 430 ºC, preforms up to 480 ºC. - Die lubrication, mostly by mineral oils/graphite formulations. - Sizes up to: plan view area > 6450 cm, length > 2540 mm. The capital costs for this process are higher than those for conventional forging, or machining, but for large scale fabrication, with a great number of parts produced, this process becomes economically attractive, surpassing both former ones, and meanwhile the part mechanical and surface quality is superior when made by precision forging [3]. Chosen material: AA6061, T = 480ºC, =, [3] MPa effective strain - effective stress Total cost/piece ($1990) Cost comparison for the manufacture of an aluminum alloy 7075-T73 component [3] hand forging Convencional forging Precision forging m/m Production quantity Figure 9 effective strain effective stress Figure 10 cost comparison Heat treatments [12]: The chosen heat treatment produces a T6-type temper, setting a AA6061-T6 alloy. Precipitation hardening heat treatment [12]: - Solution heat treatment: dissolution of soluble phases, performed at 530±6ºC for 65 minutes in air. 6

7 - Quenching: development of supersaturation: immersion in water, remaining always below 38ºC. - Aging: precipitation of solute atoms at elevated temperature (artificial aging): 175±6ºC for 8h. Preforms (forging stock), part volume: 5.13E-04 - Plate: allows a shape nearer to the finished part, making forging easier. It must be obtained from wrought material by metal cutting processes, like sawing, shearing or plasma cutting. - Bar: cylindrical rod, typical industrial preform, but not the best for this part. Figure 11 Plate preform Figure 12 rod perform Process theoretical calculations Plate preform rod preform Average pressure (tool): 116,4 MPa < p < 194 MPa 131 MPa < p < 218,3 MPa Maximum forging force: 9,556 MN < Fmax < 15,927 MN 10,868 MN < Fmax < 18,130 MN (975 ton < Fmax < ton) (1.109 ton < Fmax < ton) Forging simulation Finite element method (FEM) Figure 13 upper die surface Figure 14 lower die surface 7

8 Figure 15 Upper die model surface mesh (782 elem.) Figure 16 lower die model surface mesh (2.404 elem.) Figure 17 plate preform model volume mesh (3.900 elem.) Figure 18 effective strain Figure 19 effective stress 8

9 A finite element method simulation was performed to evaluate forging process, plastic deformation and stresses inside dies, as forces needed to the process. The FEM program used was I-FORM3. Several problems occurred during simulation caused by heavy strain in the material, but nevertheless a near net shape was obtained, which allowed material flow, stress and strain visualization and also extrapolations for the process. No flash was formed, even in the most heavily strained area (blade tip region). Simulation progressed until dies were very nearly closed and then stopped due to numerical constraints. Retrieved data from this simulation for applied load is: y = 82,407x ,1x x Figure 20 load graphic x: displacement (mm), y: force (kn) Using the above equation (graphic), and after estimating 1,6 mm for complete closing of dies, the expected interval for Fmax is: F (22,6) = kn (1564,09 ton) F (22) = kn (1.125 ton) F (23) = kn (1.913 ton) These values are consistent with those formerly obtained by theoretical calculation. Quality control (inspection): Non-destructive inspection methods, (automatization possible) [13], [14]: Material quality X-ray inspection Eddy currents Hardness Dimensional inspection Artificial vision Laser coordinate-measuring Table 8 - inspection methods Final cost of the part Obtaining fabrication cost estimative was only achievable for die casting and machining [15], but, knowing cost relation between those processes and precision forging [3], an estimate was made for the part cost, and taking into account relations between processes and heat treatment cost: 12 < Part cost < 15,6 The turbine final mass (3 blades + hub + bolts/ nuts/ washers), is approximately: 5,25 Kg. Alternative fabrication processes (equal geometry) - Forging with hammers: power drop type. For AA6061 alloy. - Milled preform forging milling preform nearer to finished shape for greatly reducing forging deformation and forces. Intended for AA6061 and AA5052 alloys. - Sheet forging and forming. Intended for AA5052-H32 alloy. - Bending and welding - simplified geometry, blade axis rotation eliminated. For AA5052-H32 alloy. 9

10 Energy production and economic viability Average production: 4000 kwh/year (333 kwh/year). Electricity price (assumption): 0,10 /kwh. For a 20 years useful life: To support massification, the ideal cost would be 5.000, turbine itself represents 1,25%, allowing a broad margin to profits and equipment installation costs. This study did not take in account any subsidies or tax incentives to wind energy production. Conclusions After this study, a conclusion was made that this part is feasible, achieving the proposed goals for durability, toughness, and low cost. The chosen fabrication process is well fit for this part and allows production at a moderate rate (2 to 6 parts per minute), but with very good mechanical quality and very close to net shape. The part cost is very competitive, since the whole turbine cost is under 100, according to the made cost estimations (for production in USA/EU). But this is only achieved for large scale production owing to great capital costs for this process, so it is a full scale option, not for small production series. Bibliography [1] Brumioul, Nicolas; Evaluation on aerodynamic criteria in the design of a small wind turbine with a lifting line model, Tese de Mestrado em Engenharia Aeroespacial, IST, Janeiro de [2] Rodrigues, Jorge; Martins, Paulo; Tecnologia Mecânica Tecnologia da Deformação Plástica, Volumes I e II, Escolar Editora, 2005 [3] Kuhlman, G.W.; Forging of Aluminum Alloys, Metals Handbook, 1990 [4] Falcão, A.O.; Turbomáquinas, Secção de Folhas AEIST, 2009 [5] Smith, William F.; Príncipios de Ciência e Engenharia dos Materiais, 3ª Edição, McGraw-Hill de Portugal, 1998 [6] Shigley, Joseph E.; Mischke, Charles R.; Budynas, Richard G.; Mechanical Engineering Design International Edition, 7 th Edition, McGraw-Hill, 2004 [7] Beer, Ferdinand P.; Johnston Jr, E. Russell; DeWolf, John T.; Mecânica dos Materiais, 3ª Edição, McGraw- Hill de Portugal, 2003 [8] Dowling, Norman E.; Mechanical Behavior of Materials, 3 rd Edition, Pearson - Prentice-Hall, 2007 [9] Montalvão e Silva, Júlio; Maia, Nuno; Vibrações e Ruído Teoria, Secção de Folhas AEIST, 2007 [10] Wilkinson Steel and Metals: [11] [12] - Metals Handbook Desk Edition, ASM Edition, 2006 [13] Coutinho, Luisa; Processos de Ligação, Métodos não destrutivos de controlo da qualidade, Secção de Folhas AEIST, 2009 [14] Pinto, J. R. Caldas; Técnicas de Automação, Lidel, 2004 [15]