SELECTION AND PERFORMANCE GENERATOR COUPLING TO VERTICAL AXIS WIND TURBINES TO URBAN APPLICATIONS

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1 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium SELECTION AND PERFORMANCE GENERATOR COUPLING TO VERTICAL AXIS WIND TURBINES TO URBAN APPLICATIONS Jorge A. Villar Alé, Damien Vincent; Gabriel da S. Simioni, Felipe Kroll, Lucas M. Esperança, Pedro R. Neto. Wind Energy Center (CE-EÓLICA), Pontificial Catholic University of Rio Grande do Sul (PUCRS) Av. Ipiranga Porto Alegre (RS) Brazil - ce-eolica@pucrs.br ABSTRACT: The Vertical axis wind turbines (VAWT), possess different features which favor an urban application. The market of this kind of wind turbine provides conventional machines like the machine with straight or curved blade type Darrieus or Savonius machine. Currently there are new designed concepts, between these models, whose the purpose is to create machines with better aerodynamics performances and energy production. The current study show two result lines (i) Test of vertical axis wind turbines rotor models in wind tunnel for an urban application,. (ii) Analyze and selection of electrical generator performance coupling to different model of rotor tested. In the last procedure, are evaluated the potential of permanent magnets generator of axial flux: (a) Generator made in Brazil, (b) Prototype of generator designed in Chile, (c) Commercial Chinese generator. For the current study, the alternatives of wind turbine s rotor are: (a) Conventional H rotor VAWT, b) Conventional Savonius rotor, (c) New model of VAWT. The results show alternatives, advantages and disadvantages of different studied configurations. The aerodynamics performances of power wind turbines rotors are obtained with the result in wind tunnel and with the theoretical process which uses the Double Multiple Streamtube model (DMST). Key words: Generators, wind turbines, test in wind tunnel. INTRODUCTION In the market, there is a significant increase in use of vertical axis wind turbines (VAWT) for small urban applications [1]. Fig.1 shows a commercial VAWT and its respective power curve. The turbine has 75 W of nominal power, a diameter D=1.5 m, and a height H=1.5m. Moreover, there is a market for electric generators for applications in small wind turbines. Fig. shows an example of commercial generator of 5 W, suitable to use of VAWT Wind Velocity (m/s) Source [ Fig 1: VAWT 75 Watts D=1,5m H=1, GL PMG 5 A Rotation Speed (rpm) Source [ ] Fig : Commercial small electric generator. To design a small wind rotor, it is important to check what the operative conditions of rotor are when it is coupled with a commercial electrical generator. How will be the power output of the turbine for different wind speeds? What is the best size of rotor for a given commercial generator? In this paper is present a procedure for direct coupling turbine/generator, trying to optimize the area swept by maximizing the power required. It is considered like reference a vertical axis wind rotor, which the geometry is similar to commercial (Fig. 1), called turbine H: the aerodynamic performance results are obtained from CE- EÓLICA laboratory test []. The prototype has D=1m and H=1m.To study the coupling is used the above generator (Fig. ), called in this paper: Generator GL.

2 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium Menet [3] presents a study and basic equations for coupling a rotor Savoinus and a car alternator. This paper presents the operational conditions between a prototype of VAWT and a commercial generator. After a method is established to optimize the area for a maximum power; and finally a rotor area is determined for a specific wanted power. We consider as basic parameters the equation of power curve of electric generator function to rotation and the equation of power curve of turbine, also function to rotation. Each of these curves can be adjusted by a specific polynomial equation and can realize same equality when we don t consider the mechanical losses (Fig. 3). The procedures described here are detailed in a work of academic exchange [4]. Fig 3: Coupling method Fig. 4 shows a coupling procedure, of a wind rotor with a commercial generator, obtaining a power curve function to wind velocity. Fig 4: Integration between power curves of turbine and generators Fig.5a shows that the proposed method looks for an optimized area of rotor to generate a maximum power. Fig.5b shows that this procedure reveals that more than one rotor can satisfy the power required. (a) Optimized area to maximum power (b) Area to specific power required. Fig 5: Optimized area to maximum power and area to specific power required.

3 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium 1. COUPLING METHOD An electrical generator is characterized by power curve in function to angular velocity. This curve can be approximate by a polynomial equation like: P = a. ω + a. ω + a (1.1) gen 1 In the Eq. (1.1), 1 P gen is the power developed by the generator, ω is the angular velocity of rotor and a, a, a are constants characterizing the generator. Fig. 6 shows a polynomial approximation of generator GL. It should be noted that the method remains valid with using of other equations to adjust the power curve. 8 7 P =,8n -,5n.4.35 Cp = -,3335.λ² +,768.λ -, Rotation Speed (rpm) Fig 6: Power curve of Generator GL Power Coeficient Cp Tip Speed Ratio Source : [ ] Fig 7: Small VAWT (H =D=1m) test in laboratory. A vertical axis wind turbine (VAWT) is characterized by the power coefficient in function to tip speed ratio. This curve can be approximate by the polynomial equation: Cp = b. λ + b. λ + b 1 (1.) b In the Eq. (1.), Cp is the power coefficient; λ is the Tip Speed Ratio (TSR) and, b1, b are constants which characterize the wind turbine power coefficient. The TSR is given by: U ω. R λ = = V V (1.3) In the Eq. (1.3), λ is the Tip Speed Ratio, U is the peripheral velocity of rotor, V is the freestream velocity, ω is the angular velocity of rotor, and R is the radius of turbine. The Fig.7 shows a curve of power coefficient of a small VAWT [1] denominate turbine H and corresponding to the polynomial equation. The power of turbine is given by: 1 Ptur. Cp. A. ρ. V = (1.4) 3 In the Eq. (1.4), Ptur is the potential developed by the turbine, Cp is the power coefficient, A is the area of the turbine, ρ is the air density, and V is the freestream velocity. So, from the three last equations, the power of turbine is defined by:

4 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium 1 Ptur =.( b. λ + b. λ + b ). A. ρ. V 3 1 (1.5) 1 ω. R ω. R 3 Ptur =. b. + b1. + b. A. ρ. V V V (1.6) The equation of power is now known in function to fixed characteristics of wind, like freestream velocity; and size of vertical axis wind turbine. Without considering mechanical losses, the turbine-generator coupling can give as: P gen P = (1.7) tur With this equation: the functioning point of coupling can be determinate, and so the power values of turbine can be known for different wind freestream velocity, as it shows in Fig V = 6 m/s V = 8 m/s V = 1 m/s V = 1 m/s V = 14 m/s Generator GL Rotation (rpm) Fig 8: Power curve of small VAWT (H = D) and Generator GL Considering the approach of Eq. (1.7), and using the equations Eq. (1.1) and Eq. (1.6) is found: 3 3 ( b R V H ρ a ) ω ( b1 R H ρ V a1 ) ω ( b R H ρ V a ) = (1.9) Where A=RH and H is the turbine height. This equation allows, for a given swept area, finding the ω = f ( V ) relation between the angular velocity of rotor and the freestream velocity:. Determined ω, we can use Eq. (1.6) to find the power output (using a specific generator). Fig.9 shows an example of the power obtained for different wind speed for using a generator GL and a defined rotor with D=1.5m and H=1.5m (A=.5m) Wind Velocity (m/s) Fig 9: Power of coupling small VAWT (D=1.5 m ;H=D) and a Generator GL

5 Using Eq.1.6 and considering a turbine with H =. k. R EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium. OPTIMAL POWER COUPLING ω. R ω. R Ptur =. b. + b1. + b. k. R.. V V V 3 ρ (.1) Where k is a constant which defines the relation between the radius and the height of turbine; and ω has been determined in relation to the freestream velocity V in using the Eq. (1.9). The power of the VAWT depends directly to the radius of turbine. But the evolution of the power is not linear in relation to radius value (Fig. 1). 8 Turbine D =.9 m 7 6 Turbine D = 1.5 m Turbine D = 1.1 m Generator GL Rotation (rpm) Fig 1: Power curve of small VAWT (H=D) to different rotor diameters (V = 1 m/s) The method searches for the greatest power of rotor and its respective area where: A R H R k R k R =.. =..(.. ) = 4.. and: R A 4. k = (.) The principle of the method, to find the optimal size of coupling, is to say that the power value increases to a maximum, before to decrease asymptotically. Fig. 11 shows the procedure that enables to optimize the rotor area to maximize the power in using a rotor characterized by H=D and V=1m/s Maximum Area (m²) Fig 11: Optimized area to maximum power output wind turbine (V= 1 m/s and H = D).

6 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium Fig.11 shows for a freestream velocity of 1 m/s, the maximum power which could be developed is 3 W with a diameter D = 1.1 m. Fig. 1 shows as example the power curve to optimal size (D=1.1m) and to other two rotor diameters (D=.9m and D=1.5m). We can conclude that for a wind speed of 1m/s the best option is a rotor of D=1.1m and H=1.1m. 6 Turbine D =.9 m 5 Turbine D = 1.5 m Turbine Dopt = 1.1 m Velocity (m/s) Fig 1: Power curve of coupling for different rotor diameter (H = D) This study also evaluated the performance of the coupling from other two electrical generators: one named Generator CHL and another named Generator BR. The generator BR is manufactured in Brazil by Enersud and used in small horizontal axis wind turbine [5]. The Generator CHL is a prototype in development in Chile with preliminary performance results obtain in the laboratory [6]. As shown in Fig.13 and Fig.14 the Chinese Generator (GL) has best performances when it is coupled to the rotor studied below. The Generators BR and the Generator CHL can be used to VAWT, however it should be a study specific to their optimization looking for a suitable rotor size. Fig. 13 and Fig.14 show also what would be the performance of the rotor when it operates at maximum power. This is achieved using the maximum power coefficient. 7 6 VAWT (D=1,1m H=1,1m) 6 m/s 8 m/s 1 m/s 1 m/s 5 Generator GL Generatorr BR Generator CHL m/s Power with Cp(max) 14m/s rotation (rpm) Fig 13: Power of coupling small VAWT using three electrical generators.

7 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium Generator GL Generator BR VAWT (D=1,1m H=1,1m) Generator GL Generator CHL Generator BR Power with Cp(max) Generator CHL Velocity (m/s) Fig 14: Power curves coupling small VAWT using three electrical generators. 3. RESULTS USING COMMERCIAL ROTOR To validate the method, a comparison between the results and the manufacturer s characteristics has been done. Using the manufacturer's power curve (Fig.1) determined the power coefficient (Eq. (1.4)): Cp. ρ. AV. P tur 3 = (3.1) To determine the Tip Speed Ratio (Eq.1.3) the angular velocity is fixed to the nominal angular velocity and using the turbine diameter given by the manufacturer. Fig.15 shows the power coefficient obtained from the manufacturer's power curve using this procedure..3.5 Power Coeficient Cp Cp = -.15.λ λ Tip Speed Ratio Fig 15 : Power coefficient of commercial small VAWT. Fig.16 shows the coupling results of the commercial VAWT and the generator GL. We observe that the curve is very close to the original manufacturer power.

8 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium Commercial coupling D=1.5 m Optimal coupling: generator GL and turbine H: D = 1.5 m Wind Velocity (m/s) Fig: 16: Power curve from manufacturer and Generator GL. 4. RESULTS USING A SAVONIUS ROTORS To continue the study, we compare two coupling using the Generator GL with Savonius rotors: One called rotor Savonius JP (A savonius rotor with experimental results [7.] whose the curve was ajuste by specific equation of rotor Savonius [8]), and other rotor tested in laboratory in this paper: named Savonius BR [9] [1] whose methodology is presented in Ref. [11]. Fig. 17 shows the curve of power of each rotor. Fig.18 shows the results of procedure to optimal power coupling using these rotors. It is observed that for Savonius rotor the performance of generator GL is low, especially when used with Savonius BR. Indeed, the rotor operates at low rotation and the power coefficient is very low. For a nominal wind speed of 1m/s, it provides about 1W using the model Savoinus JP and around 6W with the Savonius BR to approximately the same area swept. It could be noted that the tests conducted in CE-EÓLICA for the Savonius rotor should be retried to check the adequacy of the results obtained in the wind tunnel. An option to optimize this coupling would be working with a transmission system with pulleys to increase the speed of the turbine rotor shaft. However this solution increases the mechanical transmission losses. Savonius BR D = 86 mm H = 86 mm e = 8 mm Power Coefficient Cp Savonius JP Savonius BR (7 m/s) Savonius BR (8.5 m/s) Savonius BR (11 m/s) Polynomial (1m/s) Tip Speed Ratio TSR Fig 17: Power coefficient - Savonius rotors. 3 5 Savonius BR D = 1.17 m Savonius JP D = 1.1 m Wind Velocity (m/s), Fig 18: Optimal coupling Savonius (V = 1 m/s) using Generator GL

9 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium 5. SPECIFIC COUPLING PROCEDURE The next method permits to determine the size of the rotor from the power wanted (not necessarily the maximum power) with the aid of Eq. (1.1) and (1.6). The area using is A=RH.; with H =. k. R, where k defines the relation between the radius and the height of turbine. ( ρ ω ) ( 1 ρ ω) ( ρ ) P k b V R k b V R k b V R tur = + + (5.1) The final equation is only function to rotor radius. = c. R + c. R + c. R + c (5.) The process is: First we adopt a required power by the turbine. We use the Eq. (1.1) to determine the rotation velocity. Finally via the Eq. (5.1) we obtain the rotor radius which enables to furnish the required power. The Ferrari s method and the Cardan s method [1] can solve this equation. As shown on Fig. 19 there could be more than one only solution to the problem. In this case the required power is 5 W, and it is so necessary to have a rotor with A=.74m (D=.86m) or a rotor with A=1.96m (D=1.4m). Fig. shows a power curve results for this two alternatives. It is observed that the rotor with larger area allows a slight increase in power for speeds lower than 1 m/s Maximum Area (m²) Fig 19: Area to maximum power output with Generator GL ( V = 1 m/s and H=D) Turbine D = 1.4 m Turbine D =.86 m Wind Velocity (m/s) Fig : Sizing coupling to 5 W ( V = 1 m/s) using Generator GL

10 EWEA 11 - Europe s Premier Wind Energy Event March 11, Brussels, Belgium 6. CONCLUSION The work presents the operational conditions between a prototype of VAWT and a commercial generator. A method is established to optimize the area for a maximum power; and the rotor area is also determined for a required power. We used the basic equations of electric generator and turbine power curve without considering the mechanical losses. The methodology allows us to study the performances of a wind rotor coupled to an electric generator by checking which the size is appropriate by optimizing the rotor area The work analyzes a power curve obtained in laboratory for a turbine type H and a recent wind tunnel test of Savonius rotor. The result of the power curve of Savonius rotor obtained in wind tunnel results was lower than those found in reference. Thus the new tests will be conducted to verify the performance of the rotor. We have used three electric generators respectively called GL, CHL and BR. The results show that the Generator GL has best performances when it is coupled to the rotor type H previously studied. The Generators BR and Generator CHL can be used to VAWT, however it should be a specific study to their optimization looking for a suitable rotor size. For type H turbine we can conclude that for a wind speed of 1m/s the best option is a rotor of D=1.1 m and H=1.1 m if we use a Generator GL. In the initial proposal of the work, it was expected to theoretical results of Cp to VAWT rotor under analysis using the Double Multiple Streamtube model (DMST) [8]. It could be noted that it was not possible to use the conventional DMST to model this rotor. The rotor has high solidity and low TSR and the results in iterative process did not allow converged solution. This work will continue in the near future with new tests of wind rotors well as learning more about optimization procedures for direct coupling between electrical generators and VAWT which can be used in urban areas. REFERENCES [1] Eriksson S., Bernhoff H., Leijon M. Evaluation of different turbine concepts for wind power Renewable and Sustainable Energy Reviews No , (8). [] Alé J.A.V, Petry M.R., Garcia B.S., Simioni G., Konzen G.; Performance Evaluation Of The Next Generation Of Small Vertical Axis Wind Turbine European Wind Energy Conference & Exhibition (7). [3] Menet J.L., Adouble-step Savonius Rotor for Local Production of Electricity: a Design Study; Renewable Energy 9, (4). [4] Vincent D. Report: Analytic and experimental studies of vertical axis wind turbines, and generator coupling.. Academic advisor Alé J.A.V. PUCRS University, FENG. 1; [5] Alé, J. A.V., Development of vertical axis wind turbine, Projeto Finep FNDCT/CT-ENERG. Internal Reports (in Portuguese). PUCRS University, CE-EÓLICA (4). [6] Jara W., Martin A.,. Tapia J. A; Axial Flux PM Machine for Low Wind Power Generation RF ICEM 1- XIX International Conference on Electrical Machines. Rome, Italy, Sep. (1). [7] Hayashi T., Li Y., Y. Hara and K Suzuki. Wind Tunnel Tests on a Three-stage Out-phase Savonius Rotor European Wind Energy Conference & Exhibition (4). [8] Paraschivoiu, I., Wind Turbine Design With Emphasis on Darrieus Concept, Ecole polytechnique de Montreal. Book. (), [9] Simioni G., Vincent D.; Kroll, F., L. M. Esperança, Neto. P. R. Savonius Testing Wind Tunnel. Internal Report (in Portuguese). PUCRS University, CE-EÓLICA (1). [11] Rangel P.N. Report : Estudo e teste de rotores eolicos de eixo vertical. Academic advisor Alé J.A.V. PUCRS University, FENG. 1;. (in Portuguese) [1] Alé, J. A. V., Simioni G. da S., Chagas Filho J. G. A., Procedures Laboratory For Small Wind Turbines Testing. European Wind Energy Conference & Exhibition April 1, Warsaw, Poland. [1] Nickalls, R. W. D. "A new approach to solving the cubic: Cardan's solution revealed", The Mathematical Gazette 77 (48): (1993).