Aerodynamic Optimization and Open Field Testing of a 1 kW Vertical Axis Wind Turbine
PO. ID 101
Gabriele Bedon1, Marco Raciti Castelli1, Uwe Schmidt Paulsen2, Luca Vita2, Ernesto Benini1 1University of Padua, Department of Industrial Engineering – 2DTU Wind Energy, Department of Wind Energy
Abstract
Optimization Results
The design of a Darrieus wind turbine rotor is a complex process due to the nonstationarity of the flow field inside the machine and the dynamic stall effects at rotor blades. The combination of a genetic algorithm with a Blade Element – Momentum (BEM) code can be successfully adopted to improve the aerodynamic design of existing wind turbines.
The optimization Pareto front is reported in Figure 5. The individual represented by the red cross is chosen for the following considerations. The chosen configuration presents the chord distribution shown in Figure 6.
In this work, starting from the Venco Twister 1000-T vertical-axis wind turbine (VAWT), considered as a baseline configuration, the WOMBAT (Weatherly Optimization Method for Blades of Air Turbines) algorithm is adopted to provide the optimal chord distribution for a SCSc rotor characterized by the same swept area. The optimization is performed considering two objectives: the enhancement of the power coefficient for a wind speed of 7 m/s - the wind speed that maximizes the power production probability at the test site - and the maximization of the power coefficient for a wind speed of 12 m/s - the nominal wind speed of the baseline turbine.
Fig. 5: Pareto front from WOMBAT. Fig. 6: Chord distribution for the chosen individual. The optimal chord distribution is not uniform:
An optimized three-bladed rotor is designed according to the geometrical constraints imposed by the original turbine configuration and its aerodynamic performance is investigated at the DTU Wind Energy test ground.
• The minimum value of the chord at the rotor top and bottom leads to a lower aerodynamic resistance of the blade sections where the power production is very low.
Case Study and Methodology
• An increase of the chord length is registered towards the centre of the rotor, in order to maximize the aerodynamic load where the radius is increasing, thus enhancing the power production.
The Venco Twister 1000-T, tested at DTU Wind Energy, is considered as a case study in order to proceed subsequently with the experimental activity on the optimized prototype. Venco Twister 1000-T is a 2-meter high turbine characterized by three linear twisted blades placed at 1 meter from the rotational axis. A picture of the turbine is shown in Figure 1. The test conducted at the DTU Wind Energy test site highlighted a maximum power coefficient for the total conversion of around 0.20, as shown in Figure 2.
•The successive chord reduction represents a compromise between the increase of the aerodynamic load and the decrease of the interference factor, that would reduce the flow velocity across the rotor section. The rotor power coefficient and the power production for different rotational speeds are shown in Figures 7 and 8 as a function of the free-stream wind speed.
Fig. 7: Rotor Power Coefficient.
Fig. 8: Rotor Power Production.
Rotor Prototype Fig. 1: Venco Twister 1000-T.
Fig. 2: Twister 1000-T experimental CP curve.
In the optimized configuration, a fixed thickness to chord ratio is maintained along the blade span, whereas the chord distribution is free to be optimized. The NACA 0015 profile is chosen, since it provides one of the highest performance in combination with a variable chord distribution. The electrical generator needs to be maintained in the central position, due to the fact that the bearings are not designed to be stressed with a bending moment. The so-called ”cut SCS” (SCSc) variant is therefore adopted as optimization geometry, shown in red in Figure 3.
The rotor prototype is obtained by assembling several parts, designed in order to fit the original generator geometry and preserve the blade optimized shape. Rotor blades are manufactured adopting the rapid prototyping technique. They are made of ”Proto-plus”, a material with a density of 0.59 g/cm3 and a tensile strength of 40 MPa. Every blade is subdivided into three portions, in order to make their manufacturing and final assembly easy. As can be seen from Figure 9, rotor blades are assembled with the metal reinforcement bars; epoxy resin is then sucked up from the bottom to the top, in order to fill the holes between the steel and the blade parts, providing additional consistence. The prototype, installed on the 11-meter high tower at Risø - DTU Wind Energy, test site is shown in Figure 10.
Fig. 3: SCSc vs SCS geometry.
Fig. 4: WOMBAT Algorithm.
The rotor is mounted on a permanent magnet generator equipped with eddy current brakes. The optimization procedure will therefore consider the range between 0 and 270 rpm to find the best performance. A total of 50 individuals are evaluated and evolved for 100 generations, using the WOMBAT algorithm, represented in Figure 4. The genes are considered as control point coordinates for a spline curve that describe the trend of the chord. The fitness of the profile is calculated considering: • the individual power coefficient at a wind speed equal to 7 m/s, considered as a design wind speed. In fact, for this wind speed, the energy production probability for the Risø test site is maximized. • the power production at 12 m/s, considered as the maximum production. This value is the same declared by Venco for the original Twister 1000-T turbine configuration.
Fig. 9: Connection of blade sections.
Fig. 10: Installation at DTU Wind Energy test site.
Conclusions The aerodynamic design of a commercial vertical axis wind turbine is improved by adopting an optimization algorithm combined with a Blade Element - Momentum simulation code. Two target wind speeds are selected, in order to provide an optimized configuration suitable for the same working condition as the baseline configuration. The optimized rotor configuration results to be characterized by a variable chord distribution, obtained in order to maximize the energy conversion at two design wind speeds. A complete simulation campaign is conducted, showing that rotor performances are highly increased with respect to the baseline configuration. However, an experimental test is needed in order to confirm the predictions and to estimate the aerodynamic and electrical losses, not included in the simulation model. For this reason, a prototype obtained with the rapid prototyping technique is realized and installed on an 11-meter tower at the test site of DTU Wind Energy.
EWEA 2013, Vienna, Austria: Europe’s Premier Wind Energy Event
AERODYNAMIC OPTIMIZATION AND OPEN FIELD TESTING OF A 1 KW VERTICAL-AXIS WIND TURBINE Gabriele B edon, PhD Student at the Department of Industrial E ngineering, University of Padua, Via Venezia 1, 35131 Padova,
[email protected] Marco Raciti Castelli, Research Associate at the Department of Industrial E ngineering, University of Padua, Via Venezia 1, 35131 Padova, Italy,
[email protected] Uwe Schmidt Paulsen, Senior Scientist at DTU Wind E nergy, Department of Wind Energy, Frederiksborgvej 399 Building 125, room S14 & 16, 4000 Roskilde, Denmark,
[email protected] Luca Vita, Post Doctoral Researcher at DTU Wind Energy, Department of Wind Energy, Frederiksborgvej 399 Building 125, room S14 & 16, 4000 Roskilde, Denmark, luca.
[email protected] Ernesto Benini, Associate Professor at the Department of Industrial Engineering, University of Padua, Via Venezia 1, 35131 Padova, Italy,
[email protected]
Abstract The design of a Darrieus wind turbine rotor is a complex process due to the non-stationarity of the flow field inside the machine and the dynamic stall effects at rotor blades. The combination of a genetic algorithm with a Blade Element – Momentum (BE-M) code can be successfully adopted to improve the aerodynamic design of existing wind turbines. In this work, starting from the Venco Twister 1000-T vertical-axis wind turbine (VAW T), considered as a baseline c onfiguration, the WOMBA T (W eatherly Optimization Method for Blades of Air Turbines) algorithm is adopted to provide the optimal chord distribution for a SCSc rotor characterized by the same swept area. The optimization is performed considering two objectives: the enhancement of the power coefficient for a wind speed of 7 m/s - the wind speed that maximizes the power production probability at the test site - and the maximization of the power coefficient for a wind speed of 12 m/s - the nominal wind speed of the baseline turbine. An optimized three-bladed rotor is designed according to t he geometric al c onstraints imposed by the original turbine configuration and its aerodynamic performanc e is investigated at the DTU Wind Energy test ground.
1. Introduction and Background The purpos e of this work is to increase the rot or performances in terms of power production and power coefficient of a real vertical axis wind turbine (the V enco Twister 1000-T) by changing the blade aerody namic design. The aerodynamic design optimization of a vertical axis wind turbine is a complex process due to the difficulties in obt aining fast and reliable performance estimation. Apart from expensive experimental tests, which can be c onducted on the physical models, the performances of a wind turbine can be preliminary estimated by means of numeric al methods. The most important numerical approaches available in the literature are: Computational Fluid Dy namics (CFD) codes, which are based on a numerical approximation of the Navier-Stokes equations and can provide a reliable description of the turbine behavior. However the required computational time and effort are high [1]. Codes based on the Vortex Model: this approach also allows obtaining a reliable performance
estimation [2]. Anyway it still requires a considerable computational time to c onverge, especially at low tip speed ratios, when complex vortexes are generated [3]. Codes based on the BE-M Theory: equating the blade element force on the rotor to the change in fluid momentum, a very fast prediction of the rotor performance is obtained [3–5]. On the other hand, the results provided by the algorithm do not include much additional data, including detailed air flow information. These methods can be coupled with an optimization algorithm, capable of evaluating the aerodynamic performance of the rotor geometry. Genetic algorithms have been developed to find the optimal solution in complex spaces efficiently [6,7] and are widely used for solving engineering problems: this leads to an iterative loop t hat should provide an improved design configuration. This approach was followed by several authors. Bourguet et al. [8] adopted a CFD code coupled wit h a genetic algorithm to find the optimal airfoil shape to increase both the nominal power production and the aerodynamic efficiency while reducing the weight of the blade. The optimal blade profile resulted to be very close to the NACA 0025 geometry. Carrigan et al. [9] matched an evolutionary algorithm and a bi-dimensional CFD simulation for the simulation of a H-Darrieus rot or equipped with NA CA 0015 airfoils. The optimal configuration resulted to have an increase in efficiency of around 6%, achieved with an increase of 58% in rot or blade thickness and a reduction of 40% in rotor solidity. Being the aim of this work the maximization of the rotor performanc e in terms of power production and power coefficient, a simulation tool based on the BE-M Theory is selected. This allows the evaluation of a large number of blade geometries in a limited computational time, widening the investigation space considered by the algorithm. The adopted BE-M code is developed by Raciti Castelli et al. [10] and is based on the double multiple streamt ube model developed by Strickland [3] and Paraschivoiu [4,5]. Moreover, dynamic stall models bas ed on the Gormont-Strickland and Gormont-B erg studies [11] are included, as well as a blade finite aspect ratio correction from Viterna and Corrigan [12]. This BE-M code is coupled with the genetic algorithm bas ed on the studies of Deb [13], provided as gamultiobj function in the Matlab suite, obtaining the WOMBA T algorithm, first presented by Bedon et al. [14].
2. Case Study and Methodology The V enco Twister 1000-T, tested at DTU Wind Energy [15], is considered as a case study in order to proceed subsequently with t he ex perimental activity on the optimized prototype. Venco Twister 1000-T is a 2-meter high turbine characterized by three linear twisted blades placed at 1 meter from the rotational axis. The three blades are connected by spokes wit h the generator, placed in corres pondence to the rot ational axis. A picture of the turbine is shown in Figure 1. The test conducted at the DTU Wind Energy test site highlighted a maximum power coefficient for the total conversion of around 0.20, as shown in Figure 2 [17]. The adopted blade geomet ric paramet ers are quite difficult to measure. However, it can be observed that the blade is characteriz ed by a symmet ric profile, with uniform chord and thickness along the whole length. Moreover, it is difficult to establish the influence of the spoke design (quite wide and thick) on t he power production. The numerical simulation of the Venco rotor would therefore require a consistent amount of work on several geometric parameters that need to be optimized. A more simple baseline geometry is thus adopted, nevertheless maint aining the same swept area of Twister 1000-T, equal to 3.61 m².
Fig. 1: Venco Twister 1000-T VAW T [16].
Fig. 2: Venco Twister 1000-T experimental power coefficient curve [17]. A fixed thickness to chord ratio is maintained along the blade span, whereas the chord distribution is free t o be optimized. Considering the results obtained by Bedon [18], the NA CA 0015 profile is chosen, sinc e it provides one of the highest performance in combination with a variable chord distribution. The simulation algorithm was validat ed with turbines adopting blades of Troposkien shape [19]: this design allows limiting blade inertial loads [20]. Finally, a variable chord distribution is adopted, because of the difference in blade section radius. The electrical generat or needed to be maintained in the central position, due to the fact that the bearings are not designed to be stressed with a bending moment. The so-called ”cut S CS” (SCSc) variant [21] is therefore adopted as optimization geometry. This solution is realized by cutting a 20% in SCS height, 10% at the top and 10% at the bottom extremities. In order to have a fair comparis on, the same swept area is maintained, by increasing the maximum radius. A comparison between the original SCS and the SCSc midline geometries is shown in Figure 3.
Fig. 3: Comparison bet ween the classical SCS geometry (blue line) and the proposed S CSc (red line) one. The S CSc shape requires connecting arms at bot h top and lower sections. Connecting arms are anyway provided in the original Venco configuration. The same s wept area of Twister 1000-T, 3.61 2 m , is maintained. The main geometrical parameters of the optimized rotor configuration are reported in Table 1. H [m] 1.96 R [m] 1.18 N 3 Blade Shape SCSc Table 1: Main geometric al features of the optimized rotor configuration The rotor is mount ed on a permanent magnet generator equipped wit h eddy current brakes. The maximum rot ational speed without their activation is 270 rpm [22]. The optimization procedure will therefore consider the range between 0 and 270 rpm to find the best performance. A total of 50 individuals are evaluated and evolved for 100 generations. The number of genes ng provided by the genetic algorithm is equal to half of the number of vertical divisions. The genes are considered as control point coordinates for a spline curve (ng/2 abscissas and ng/2 ordinates) that describe the trend of the chord. The gamultiobj algorit hm tends to a solution that minimizes the fit ness value. The fitness of t he profile is thus calculated considering the following expressions:
where: is the individual power coefficient at a wind speed equal to 7 m/s, considered as a design wind speed. In fact, for this wind speed, the energy production probability for the Ris ø test site is maximized [23,24].
is the power production at 12 m/s, considered as the maximum production. This value is the same declared by Venco for the original Twister 1000-T turbine configuration [15].
3. Optimization Results The WOMBA T algorithm provides a Pareto front containing several optimum individuals. The Pareto front is reported in Figure 4.
Fig. 4: Pareto front from the WOMBA T optimization algorithm. The red cross is the individual chosen for the analysis. The individual represented by the red cross is chosen for the following considerations. In fact, individuals characterized by higher power c oefficients present a sensibly lower power production, due to the curve dec reasing slope. On the other side, individuals with a slightly higher power production present a sensibly decrease in power coefficients. The chosen configuration presents the chord distribution shown in Figure 5.
Fig. 5: Chord distribution for the chosen individual. The optimal chord distribution, considering a fixed airfoil thickness/chord ratio, is therefore not uniform. The minimum value of t he chord at the rotor top and bottom leads to a lower aerodynamic
resistance of the blade sections where the power production is very low. An increase of the chord length is registered towards the centre of the rotor, in order t o maximize the aerodynamic load where the radius is increasing, thus enhancing the power production. The successive chord reduction represents a compromise between the increase of the aerodynamic load and the decrease of the interference factor, that would reduce the flow velocity across the rotor section. A complete simulation of the optimal individual is conducted, in order to have a complete overview of the rotor perform ance. The rotor power coefficient and the power production for different rotational speeds are shown in Figures 6 and 7 as a function of the free-stream wind speed.
Fig. 6: Rotor power coefficient for several angular velocities, as a function of the unperturbed wind speed.
Fig. 7: Rotor power production for several angular velocities, as a function of the free-stream wind speed. These curves represent the estimation of t he rotor performance, obt ained from the BE-M algorithm. The real performance is however reduced by the presence of the spokes and by the efficiency of the energy conversion performed by the electrical generator, the rectifier and the inverter.
4. Rotor Prototype The rotor prototype is obtained by assembling several parts , designed in order to fit the original generator geometry and preserve the blade optimized shape. In t he original configuration, the spoke connections are shifted azimut hally, in order to connect the twisted blades, as can be seen in Figure 8. On the other side, the prototype configuration provides a blade whose medium line is entirely contained in a plane. A disk, presenting the spoke links in the correct position, is therefore designed and installed on top of the electrical generator, using the existing bolts. With reference to Figure 8, the disk is represented in red, whereas the generator is reproduced in whit e color.
Fig. 8: Connecting disk (red) installed on the original Venco generat or (white). Six spokes, obtained from a 10 mm steel plate and manufactured by laser-cut in order to obtain the desired geometry, are connected to the correct links. Their width is varying between 105 mm at the generator side and 80 mm at the blade side, for a total length of 900 mm. Spokes are intended not to be in contact with the blades: six metal components, the so-called ”inblades”, are designed in order to be fixed on t he terminal portion of the spoke, as well as to be part of the blade. These components, represented in Figure 9, are obtained from a 8 mm steel plate and manufactured by laser-c ut.
Fig. 9: Connection component between the blade and the spoke (”In-blade”).
The in-blade components present one rectangular and two circular holes. In the rectangular opening, t wo 1-mm thick steel plates are inserted. These plates permit to lock the orientation of the blade with respect to the spoke. In eac h circular hole, four 1-mm diameter steel wires, providing strength to the whole assembly, are inserted. Rotor blades are manufactured adopting the rapid prototyping technique. They are made of 3 ”Proto-plus” [25], a material with a density of 0. 59 g/cm and a tensile strength of 40 MP a. E very blade is subdivided into three portions, in order to make their manufacturing and final assembly easy. As can be seen from Figure 10, rotor blades are assembled with the metal reinforcement bars; epoxy resin is then sucked up from the bottom to the top, in order to fill the holes bet ween the steel and the blade parts, providing additional consistence.
Fig. 10: Connection assembly of the in-blade components and blade sections, inserting the steel reinforcements. Steel strings are added in order to reduce the blade deformation and ensure an operational stability. Such strings have a diameter of 2 mm and do not compromise t he aerodynamic efficiency excessively. The prototype, installed on t he 11-meter high tower at Risø - DTU Wind Energy, test site is shown in Figure 11.
Fig. 11: Installation at Ris ø - DTU Wind Energy test site.
5. Conclusions The aerodynamic design of a commercial vertical axis wind turbine is improved by adopting an optimization algorithm combined with a Blade Element - Momentum simulation code. Two target wind speeds are selected, in order to provide an optimized configuration suitable for the same working condition as the baseline configuration. The optimized rotor configuration res ults to be characterized by a variable chord distribution, obtained in order to maximize the energy conversion at two design wind speeds. A complete simulation campaign is conducted, showing t hat rotor performances are highly increased with respect to the baseline configuration. However, an experimental test is needed in order to confirm the predictions and to estimate the aerodynamic and electrical losses, not included in the simulation model. For this reason, a prototype obtained with t he rapid prototyping technique is realized and installed on an 11-meter tower at the test site of DTU Wind Energy. Further work is required in order to complete the experimental campaign of measurements on the prot otype.
Nomenclature c [m] CP [-] h [m] H [m] N [-] ng [-] P [W] R [m] U∞ [m/s]
airfoil chord power coefficient blade element height rotor height blade number number of genes power production rotor maximum radius free-stream mean air velocity
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