Self-Start Performance Evaluation in Darrieus-Type Vertical Axis Wind Turbines: Methodology and Computational Tool Applied to Symmetrical Airfoils

Transcription

1 Self-Start Performance Evaluation in Darrieu-Type Vertical Axi Wind Turbine: Methodology and Computational Tool Applied to Symmetrical Airfoil N.C. Batita 1, R. Melício 1, J.C.O. Matia 1,2 and J.P.S. Catalão 1,3 1 Univerity of Beira Interior, 2 Centre for Aeropace Science and Technologie R. Fonte do Lameiro, 62-1 Covilhã (Portugal) Phone: , Fax: nelon.batita@gmail.com, rui.melicio@di.ubi.pt, matia@ubi.pt 3 Center for Innovation in Electrical and Energy Engineering, Intituto Superior Técnico Av. Rovico Pai 1, Libon (Portugal) Phone: , Fax: catalao@ubi.pt Abtract. An increaed interet in vertical axi wind turbine ha been timulated by the rapid growth of wind power generation and by the need for a marter electrical grid with a decentralized energy generation, epecially in the urban area. The lift type vertical axi wind turbine (Darrieu wind turbine) performance ediction i a very complex tak, ince it blade move around the rotor axi in a three dimenional aerodynamic environment that lead to everal flow phenomena, uch a, dynamic tall, flow eparation, flow wake deformation and their natural inability to elf-tart. Thee iue can be overcome with the ue of everal more or le complex olution, being one of them the development of a blade ofile capable of making the wind turbine elf-tart. Thi paper focue on eenting a new methodology for fat development of new blade ofile, for elf-tart capable Darrieu wind turbine, which i a complex and time-conuming tak. Key word Vertical axi wind turbine, blade ofile, elf-tart, eure coefficient. 1. Introduction The increaing cot of foil fuel and the different agreement among the indutrialized countrie with the aim of reducing CO 2 emiion ha driven the renewable ource in an increaed acceptance for energy oduction. The wind energy ytem have been conidered a one of the mot cot effective of all the currently exploited renewable ource, o the demand and invetment in wind energy ytem ha increaed in the lat decade. Several tudie have been conducted to model, imulate [1] and characterize [2] the wind behaviour to timulate the acceptance of the wind energy in the market, by offering tool to help and eae the enterie I&D. The invetment in wind energy for the 27 EU Member State i expected to grow in the next 2 year reaching almot 2 billion in 23, with 6% of that invetment in offhore ytem [3]. In the pat for Portugal alone, the wind power goal foreeen for 21 wa etablihed by the government a 375 MW. The value of 375 MW already reeented about 25% of the total intalled capacity [4]. Neverthele, thi value ha recently been raied to 51 MW, by the mot recent governmental goal for the wind ector. A the penetration level of wind power increae into the power ytem, the overall performance of the electric grid will increaingly be affected by the characteritic of wind turbine. One of the major concern related to the high penetration level of the wind turbine i the impact on power ytem tability and power quality [5]. The decentralized energy generation i an important olution in a marter electrical grid with a growing acceptance for the urban area. Alo, the increaing need for more environmentally utainable houing and the new European norm regulating thi iue, have contributed for the omotion of wind energy ytem in building. In urban area the wind i very turbulent and untable with fat change in direction and velocity. In thee environment the vertical axi wind turbine (VAWT) have everal advantage over horizontal axi wind turbine (HAWT). Several olution have been eented to overcome the Darrieu type VAWT inability to elftart: ue of a guide-vane [6], uing a hybrid configuration of a Savoniu VAWT (drag type wind turbine) and a Darrieu VAWT (lift type wind turbine) [7]-[9], ue of mechanical ytem to optimize the blade pitch [1], [11], ue of blade that change their form during operation [12], [13], or a pecific blade ofile capable of offering elf-tart capabilitie to the wind turbine without extra component [14], [15]. Our paper eent a methodology and computational tool for fat development of a pecific blade ofile capable of offering elf-tart capabilitie to the VAWT without extra component. The VAWT in order to elf-tart relying only on the blade ofile, without the help of extra component and external power, mut take advantage of the drag force caued by the wind on the blade when the turbine i in a topped poition, without comomiing the wind turbine performance at high tip peed ratio (TSR) RE&PQJ, Vol.1, No.9, May 211

2 If poible the lift force hould be ued in cooperation with the drag force to induce the elf-tart capabilitie of the wind turbine, epecially when the turbine i topped and the wind flow tart to achieve higher velocitie. So, it i eential to tudy the blade ofile behaviour in relation to the wind when the wind turbine i topped. One difficulty arie here, ince the blade may be at any given poition around the rotor, implying the need to tudy the blade ofile at any angular poition from º to 36º. In the eviou ituation, the dynamic tall behaviour, air flow eparation and any other aerodynamic diturbance mut be taken in conideration [16], [17]. To tudy thee aerodynamic diturbance, require a high computational oceing time, which lead to a time conuming ituation not adviable in the firt tep of the development tudie. Hence, the methodology that i opoed for the development of new blade ofile i only uitable for fat analyi when there i the need to compare everal blade ofile olution to tart retricting and eliminating different deign. It i important not to forget the analyi of the different apect of the wind flow diturbance acting on the wind turbine in a more advanced tage of the tudy. Thi paper i organized a follow. Section II eent the methodology and computational tool ued for the development of the elf-tart ofile capabilitie of the new blade ofile analyi. Section III eent the output data for everal known blade ofile. Finally, concluding remark are given in Section IV. 2. Methodology and Computational Tool A. Methodology To tudy the blade ofile deign modification and the implication that thoe modification bring to the wind turbine performance, a cloe relation between the urface of the blade and the wind flow mut be created. In thi methodology the eure coefficient C i going to be ued, which i a dimenionle number that decribe the relative eure throughout a flow field and i intimately correlated to the flow velocity, and can be calculated at any point of the flow field. To tudy the C around the blade ofile urface, firt there i the need to divided it into egment a hown in Figure 1, and then to calculate the C in each egment. The maller the egment the more accurate will be the reulting analyi. y/c, x/c Fig. 1. Segmented NACA2 blade ofile. The relation between the blade ofile egment and the C i hown in Figure 2. To tudy a blade deign performance, capable of offering the VAWT the ability to elf-tart, there i the need to tudy the wind flow behaviour around the airfoil when the wind turbine i in a topped poition. In thi ituation, the wind flow may encounter the blade ofile at any given azimuthal poition from º to 36º. Accordingly, the dynamic tall behaviour, air flow eparation and any other aerodynamic diturbance mut be taken in conideration [14]-[18]. To tudy thee aerodynamic diturbance, require a high computational oceing time, which lead to a time conuming ituation not adviable in the firt tep of the development tudie. So, a mentioned eviouly, the methodology that i eent here to demontrate the development of new VAWT blade ofile i only uitable for fat analyi when there i the need to compare everal blade ofile olution to tart retricting and eliminating different deign. The opoed methodology i alo ueful when aociated to a blade deign oftware application, giving a fat view on the blade performance when any modification i made to the ofile deign. Moreover, it i very important not to forget the analyi of the different apect of the wind flow diturbance acting on the wind turbine. Fig. 2. Preure coefficient C acting on the blade ofile urface. Figure 2 how the point i and i + 1 of the egment of length in the blade ofile urface and their correponding Carteian coordinate in the x and y axi. In the triangle formed by egment in relation to the x and y axi, o reeent the oppoite ide and a reeent the adjacent ide and their length are given by a = x i+1 x i (1) o = yi+ 1 y o = yi y i i+ 1 upper urface lower urface (2) RE&PQJ, Vol.1, No.9, May 211

3 When o i poitive it mean that the urface egment i oriented in the direction to the wind turbine rotation, while when o i negative the egment i oriented in the oppoite direction. The blade ofile egment length i given by 2 2 = a + o (3) The egment angle β in relation to the blade chord line (the x axi) i given by ( o a) β = arctan (4) By having the C exerted in each egment of the blade, there i the need to get the C contribution to the tangential force T and the C contribution to the normal force N, which are hown in Fig. 3. B. Computational Tool The aerodynamic behaviour and performance data for different blade ofile i not alway available, and in the majority of the cae i incomplete. Thi data i very hard to obtain and it i a very time-conuming tak. Several Computational Fluid Dynamic (CFD) tool are commonly ued to generate the aerodynamic performance data needed. The computational tool eented in thi paper i the JavaFoil [19], which i a fat oceing computational tool with a validated good accuracy. Thi tool i able to analye different blade ofile of any configuration, offering all kind of different output data, for intance: velocity and eure coefficient ditribution along the blade chord; lift, drag and momentum coefficient; flow field with eure vector; flow tream line and eure ditribution along the fluid flow; boundary layer evaluation card; polar card evaluation with the relation between lift and drag coefficient and with the angle of attack variation; and other information. The JavaFoil i alo able to evaluate multi-foil and ground effect configuration. One important feature of JavaFoil i the ability to ave all the analyi and output data to a file. Adding the lat feature to it integrated cripting module, it i poible to automate the computational ocee and to complement the information need with other tool. Fig. 3. Preure coefficient tangential force T and normal force C and it contribution to the N acting on the blade ofile urface. A hown in Figure 3, the angle ϕ i the C angle in relation to the blade chord line and i given by ϕ = 18 º 9º β (5) The C contribution to the tangential force T and to the normal force N can be exeed a in (6) and (7). Thee contribution mut be multiplied by the blade ofile egment length and are given by T T N N = C = C = C co = C co in in when o when o < upper urface lower urface The equation (6) and (7) how the relation between the C, T, N, the angle ϕ and the egment length. (6) (7) In the theoretical background of the tool, it ue everal method for airfoil analyi, mainly divided in two main area 1) Potential Flow Analyi: which i done by a panel method with a linear varying vorticity ditribution baed in the XFOIL code. Thi method i ued to calculate the velocity ditribution along the urface of the airfoil 2) Boundary Layer Analyi: which analye the upper and lower urface of the airfoil with different equation, tarting with the panel method and performing everal calculation baed in [2], in a called integral boundary layer method. Depending on the Reynold number and other parameter, the tool give u the ability to chooe different analyi method and configuration offering more flexibility to the computational oceing. C. Uing the Methodology and the Computational Tool To obtain the C vector along the blade ofile urface with the JavaFoil tool, after uploading the ofile coordinate, in the velocity area the maximum and minimum angle and it increment mut be et RE&PQJ, Vol.1, No.9, May 211

4 By electing the eure coefficient parameter for evaluation and analying the airfoil, it i poible to ave the oceed data to a file with the x and y coordinate and the correponding C vector, needed for the method explained in the eviou ection of thi paper. In a normal operation of a VAWT, the variation of eure and wind peed have little influence in the wind denity, o the wind flow can be treated a being incomeible. Hence, it thi aumed that: when C at one point i null the eure at that point i the ame a the eure of the unditurbed wind flow; when C i equal to one, that point i a tagnation point, meaning that the flow velocity at that point i null (relevant when optimizing the drag force); when C i negative in the point of tudy, the wind i moving at a higher peed than in the unditurbed wind flow (relevant when optimizing the lift force). When tudying the elf-tart capabilitie of a VAWT, it i very important to tudy the drag effect exerted in the turbine blade. By getting the C with value between one and null, it i poible to ee the blade ofile egment that are uffering from drag force. By relating thee value with the negative or poitive value of o, it i poible to ee the drag force contribution to the forward movement of the blade or to the oppoite direction, repectively. 3. Airfoil Data Evaluation For the airfoil data evaluation by applying the methodology and computational tool eented in eviou ection, the ymmetrical NACA where elected, namely: NACA12, NACA18, NACA2, NACA25, NACA3. The NACA12 and NACA18 are the claical blade ofile ued in the VAWT. Thee ofile are conidered to have low elf-tart capabilitie. The thicker NACA2 blade ofile can be commonly found in the traight-bladed Darrieu wind turbine. The thicker blade how a better performance of elf-tart [14]. By evaluating everal ymmetrical blade ofile with an aerodynamic multi-criteria hape optimization, reference [21] conidered the NACA25 to have an optimal hape deign. The NACA3 by having a thicker blade ofile i cloer to a elf-tart capacity nature. Thi ticker blade lead to an increae drag at high tip peed ratio leading to a decreae of performance. All five blade ofile are hown in Figure 4. y/c NACA25 NACA18 NACA12 NACA2 NACA x/c Fig. 4. NACA12, NACA18, NACA2, NACA25 and NACA3 blade ofile. In order to apply the opoed methodology, the eure coefficient need to be calculated around the blade ofile. For the data evaluation eented here, the C i calculated for all egment around the blade ofile for any given angle between º and 36º. The JavaFoil tool offer the eure coefficient evaluation aociated with the x and y coordinate. Thi evaluation can be automatically done to the entire 36 º at the ame time in the velocity area. By applying the equation (1) and (2) to the given x and y coordinate, the oppoite ide o and the adjacent ide a are obtained. By applying equation (3), the length of the airfoil urface expoed to the wind force i obtained. With the equation (4) and (5), the C angle in relation to the blade chord line ϕ i obtained. With the data calculated eviouly, it i now poible to determine the C contribution to the tangential force T and the C contribution to the normal force N. Thee force are related to the actual tangential and normal force reponible for the blade movement, by the eure coefficient. The C contribution to the tangential force T, and the C contribution to the normal force N, for the choen NACA airfoil are hown in Figure 5 and 6, repectively. C contribution to Tangential Force NACA12 NACA18 NACA2 NACA25 NACA Angle Fig. 5. C contribution to the tangential force T RE&PQJ, Vol.1, No.9, May 211

5 4 NACA12 NACA18 NACA2 NACA25 NACA3 4. Concluion C contribution to Normal Force Angle Fig. 6. C contribution to the normal force N. In Figure 5 it can be een that the ticker the blade the higher the eure coefficient contribution to the forward movement of the wind turbine blade (contribution to the tangential force). The NACA3 eent 26% better performance than NACA12. In Figure 6 it can be een that the airfoil NACA12 eent the mot deirable behavior. The maller the axial exerted force the maller the needed blade/arm connection reinforcement. When the wind turbine i in a topped poition the drag force have a coniderable contribution to the elf-tart of the wind turbine. The eure coefficient i alo ued to tudy the drag contribution to the forward movement of the wind turbine blade. In an incomeible flow, when the eure coefficient reache value between one and null, that i a tagnation point. The tudy of thee value that contribute to the forward movement of the wind turbine blade are hown in Figure 7. Drag contribution to T NACA12 NACA18 NACA2 NACA25 NACA Angle Fig. 7. Drag contribution to the forward movement of the wind turbine blade T. In Figure 7 it can be een that the ticker the blade the higher the drag contribution to the forward movement of the wind turbine blade. In the NACA25 the drag force contributing to the tangential force are 11% higher than in NACA12, and in the NACA3 the force are 15% higher than in NACA12. Thee data are in good agreement with the tudie eented in [14] and [15]. Indeed, the ticker blade are able to ovide the wind turbine the elf-tart capabilitie, while the thinner blade wind turbine ha mot likely unable to elf-tart. Thi paper focue on the analyi of the behaviour of the flow around a VAWT blade airfoil when it i in a topped poition, in order to boot the development of new blade ofile capable of offering the lift type VAWT the ability to elf-tart. A new methodology i introduced in cooperation with the JavaFoil computational tool, to tudy thee phenomena, a a fat apoach for comparing the everal blade ofile deign modification and enhancement in new airfoil development. To eent the data output, everal blade ofile were tudied and eented in thi paper. The NACA3 airfoil eent the bet performance. Thi blade eent the bet C contribution to the tangential force and how the bet drag force contribution to the forward movement of the wind turbine blade. The NACA12 how the lowet C contribution to the normal force. Reference [1] R. Melício, V.M.F. Mende, and J.P.S. Catalão, Modelling and Simulation of a Wind Energy Sytem with Fractional Controller, in: ICREPQ 1, vol. 2, paper 261, Granada, Spain, March 21. [2] F. S. Correia, L. C. Gonçalve, and L. C. Pire, Characterization of wind energy potential and availability at Beira Interior (Portugal), in: ICREPQ 1, vol. 1, paper 686, Granada, Spain, March 21. [3] European Wind Energy Aociation, The economic of wind energy, EWEA 29. [4] A. Etanqueiro, R. Catro, P. Flore, J. Ricardo, M. Pinto, R. Rodrigue, and J. Peça Lope, How to epare a power ytem for 15% wind energy penetration: the Portuguee cae tudy, Wind Energy, vol. 11, no. 1, pp , Jan.-Feb. 28. [5] R. Melício, V.M.F. Mende, and J.P.S. Catalão, Fractional-order control and imulation of wind turbine with full-power converter, in: MELECON 21, pp , Valletta, Malta, A. 21. [6] M. Taka, H. Kuma, T. Maeda, Y. Kamada, M. Oki, and A. Minoda. A traight-bladed vertical axi wind turbine with a directed guide vane row-effect of guide vane geometry on the performance, J. Therm. Sci. 29; 18(1):54-57 [7] R. Gupta, A. Biwa, K.K. Sharma. Comparative tudy of a three-bucket Savoniu rotor with a combined threebucket Savoniu-three-bladed Darrieu rotor, Renew. Energy 28; 33(9): [8] Md. Jahangir Alam, M.T. Iqbal. Deign and development of hybrid vertical axi turbine, in: CCECE '9, pp , St. John', NL, 3-6 May 29. [9] T. Wakui, Y. Tanzawa, T. Hahizume, T. Nagao. Hybrid configuration of Darrieu and Savonieu rotor for tandalone wind turbine-generator ytem, IEEJ Tran. Electr. Electron. Eng. 24; 124-B(2): [1] P. Cooper and O. Kennedy. Development and analyi of a novel vertical axi wind turbine, in: Proc. of 42nd Annual Conf. of the Autralian and New Zealand Solar Energy Soc. 24, Perth, Autralia, 1-3 December 24. [11] I. Parachivoiu, O. Trifu, F. Saeed. H-Darrieu wind turbine with blade pitch control, Int. J. of Rotating Machinery 29, 29:1 7. [12] J. DeCote, A. Smith, D. White, D. Berkven, and J. Crawford, Self-tarting Darrieu wind turbine, Deign Project Mech. 42. Dalhouie Univerity, Hallifax, Canada, Ail RE&PQJ, Vol.1, No.9, May 211

6 [13] P. Bhatta, M. A. Paluzek, J. B. Mueller, Individual blade pitch and camber control for vertical axi wind turbine, in: WWEC28, Kington, Canada, 24 June 28. [14] B. K. Kirke, Evaluation of elf-tarting vertical axi wind turbine for tand-alone application. PhD Thei, School of Engineering, Griffith Univerity, Autralia, Ail [15] R. Dominy, P. Lunt, A. Bickerdyke, and J. Dominy, Selftarting capability of a Darrieu turbine, J. of Power and Energy 27, 2221(1): [16] C. S. Ferreira, G. van Kuik, G. van Buel, F. Scarano, Viualization by PIV of dynamic tall on a vertical axi wind turbine, Experiment in Fluid 28; 46(1): [17] C.J.S. Ferreira, G. Buel, F. Scarano, G. Kuik, PIV viualization of dynamic tall VAWT and blade load determination, In Proc. 46th AIAA, Reno, Nevada, USA, 7-1 Jan. 28. [18] C.J.S. Ferreira, A. Zuijlen, H. Bijl, G. Buel, and G. van Kuik. Simulating dynamic tall in a two-dimenional vertical-axi wind turbine: verification and validation with particle image velocity data, Wind Energy 21; 13(1):1 17. [19] M. Hepperle, JavaFoil Analiy of Airfoil. [2] R. Eppler, D. Somer. A Computer Program for the Deign and Analyi of Low-Speed Airfoil, NASA TM- 821, 198. [21] R. Bourguet, G. Martinat, G. Harran, and M. Braza, Aerodynamic multi-criteria hape optimization of VAWT blade ofile by vicou apoach, Wind Energy 27: RE&PQJ, Vol.1, No.9, May 211