The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 29, Taipei, Taiwan DEVELOPMENT OF SAFE VERTICAL AXIS WIND TURBINE FOR OVER SPEED ROTATION Minoru Noda 1, Fumiaki Nagao 2 and Akira Shinomiya 3 1 Associate Professor, Institute of Technology and Science, The University of Tokushima 2-1 Minami-Josanjima, Tokushima, Japan, tarda@ce.tokushima-u.ac.jp 2 Professor, Institute of Technology and Science, The University of Tokushima 2-1 Minami-Josanjima, Tokushima, Japan, fumi@ce.tokushima-u.ac.jp 3 Graduate student, Graduate school for Intelligent Structures and Mechanics Systems Engineering, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima, Japan ABSTRACT SW-VAWT (Straight Wing Vertical Axis Wind Turbine) generally includes an over speed rotation problem. To inhibit this problem, a revolution speed of wind turbine is controlled by a mechanical break system or an electric-magnetic break system. In this study, to inhibit the over speed rotation problem, a new SW-VAWT, which can be controlled autonomously by changing the wing pitching angle under the action of the centrifugal force, was developed through some wind tunnel tests and field tests. As the results of this study, it was found that the aerodynamic autonomous control system of the developed wind turbine works very well and this system will bring safer and cheaper wind turbine. KEYWORDS: VERTICAL AXIS WIND TURBINE, AUTONOMOUS CONTROL SYSTEM, OVER SPEED ROTATION Introduction The concern for the earth environment rises and large-scale wind power plants increase rapidly all over the world. Micro-scale wind turbines, such as propeller type wind turbines and vertical axis wind turbines, also increase rapidly. There are two types of vertical axis wind turbine, a drag type wind turbine and a lift type wind turbine. In this paper, the latter type with straight wings is called as SW-VAWT (Straight Wing Vertical Axis Wind Turbine). SW-VAWT can generate a strong torque with a high rotation speed so that it is suitable for wind power generation. However it is well known that the control of SW-VAWT is difficult because the methods to control VAWT are usually a mechanical break system by friction or a magnetic break system by a generator load, and SW-VAWT has danger that it is easy to fall into the over speed rotation state and to collapse its blade by the centrifugal force under the worst condition of the breakdown of its control system. Therefore it is necessary to secure safety to the over speed rotation problem of SW-VAWT before SW-VAWT spreads widely. In this study, a new SW-VAWT, which has never fault into the over speed rotation state achived a very simple mechanism and can generate the electric power under strong wind condition, was developed. To develop this SW-VAWT, the effect of the pitch angle of its wing blade on the power generation efficiency was investigated by wind tunnel test, a model of a new SW-VAWT installed a autonomous control system using the centrifugal force was tested to measure the relation between the power generation efficiency and its rotation speed,
The Seventh Asia Pacific Conference on Wind Engineering November 8-12, 29, Taipei, Taiwan and the field test of the prototype of SW-VAWT installed the autonomous control system was carried out. Effects of Pitching Angle of Wing on Wind Turbine Performance Configuration of Wind Tunnel Test The model of SW-VAWT with three wings is shown in Figure 1. The wing length, L, and radius of the rotation orbit of the wing, R, were.9 m and.45 m, respectively. The shapes of the wing were two types as shown in Figure 2. One was NACA12 as a symmetric shape, and the other was defined by mapping it as the centerline of NACA12 is corresponding to the orbit of the wing. Both wing cord length, B, were.1125 m. Therefore, the solidity ratio of the SW-VAWT, σ, was.12. In this study, the characteristics of the SW-VAWT were investigated by the relation between the power generation efficiency, C p, and the tip speed ratio,. C p is given from the torque, T (Nm), and the rotation speed, n (rpm), which are measured by a load cell set below the DC servo motor driving the SW-VAWT model, as following formula. n 2π T nt C p = 6 π = (1) 1 3 3 ρu 2RL 3ρU RL 2 where, ρ and U are the air density (kg/m 3 ) and wind speed (m/s). is given by n, R, and U as follows. n 2π R 6 πnr = = (2) U 3U In this test, to change, U was changed keeping n to 2 rpm. The pitching angle of the wing, φ, which was defined as the angle between the centerline of the wing and the tangent direction at the middle of the chord of the wing, was changed from 6 to +6. The positive φ means that the leading edge of the wing moves to the outside of the orbit of the wing. The model of SW-VAWT was tested by the wind tunnel whose test section was 1.5 m wide and 1.5 m height. R=.45m L=.9m Turbine DC servo motor Load cell (a) plane view (b) front view Figure 1: General Views of SW-VAWT Model for Wind Tunnel Tests
Development of Safe Vertical Axis Wind Turbine for Over Speed Rotation, November 8-12, 29, Taipei, Taiwan Result and Discussion Figure 3 shows the relations between C p and measured with changing φ for each wing shape. From the Figure 3 (a), C p becomes the maximum value, that is about 3%, in =3 or =3.5, when φ=. However, when φ change to +2 or 2, the maximum C p decreased greatly. Moreover, the maximum C p did not appear and C p was negative for all during φ > +2 or φ < -2..2.1 y/b. -.1 -.2..2.4.6.8 1. x/b (a) Type A (Symmetric shape).2.1 y/b. -.1 -.2..2.4.6.8 1. x/b (b) Type B (Asymmetric shape) Figure 2: Shapes of the Straight Wing of the SW-VAWT C p (%) 6 4 2 φ(deg.) -6-4 -2 +2 +4 +6 C p (%) 6 4 2 φ(deg.) -6-4 -2 +2 +4 +6-2 1 2 3 4 5 (a) Type A (Symmetric shape) -2 1 2 3 4 5 (b) Type B (Asymmetric shape) Figure 3: -C p Curve Changed by Pitching Angle of the Wing Figure 3 (b) shows that C p becomes the maximum value in =3 or =3.5 when φ=-2 and φ=4, and becomes the almost or negative during φ= -6 or φ >. These results indicate that C p is very sensitive to changing φ in spite of the shape of the wing, and it is easy to reduce the aerodynamic torque by changing the pitching angle of the wing. It is clarified that it only has to change the pitching angle of the wing a little to prevent the SW-VAWT from falling into the over speed rotation, when the rotation speed exceeds the upper limit. Development of Autonomous Control System for Over Speed Rotation Configuration of Wind Tunnel Test In this study, the centrifugal force was used to change the pitching angle of the wing when rotation speed of the SW-VAWT reached to the limit rotation speed. The developed autonomous control system is shown in Figure 4. This system consists of the main arm supporting the wing, the linkage system keeping the same pitching angle for three wings, and coil springs to adjust the rotation speed beginning to change the pitching angle of the wing by its tension. In this test, the tension of
Development of Safe Vertical Axis Wind Turbine for Over Speed Rotation, November 8-12, 29, Taipei, Taiwan the coil springs was adjusted for the pitching angle of the wing to begin to change at the rotational speed of about 21 rpm. To investigate the effect of this system on the performance of the SW-VAWT, -C p curve was measured for some rotation speed conditions. The wind speed, U, was changed to change the for each constant n, 15 rpm, 2 rpm, 25 rpm, 21 rpm, 215 rpm and 22 rpm. The initial pitching angle of the wing was set to and 2 for Type-A wing and for Type-B wing respectively. The size of the model of the SW-VAWT and the configuration of the wind tunnel test were the same with those of the previous test for the fixed pitching angle of the wing. Result and Discussion The -C p curves measured by this test were shown in Figure 5. Figure 5 (a) indicates that -C p curves were almost the same with that of the case of the fixed pitching angle during n < 25 rpm, and the maximum C p began to reduce in n=21 rpm, and C p completely changed to negative for all region of in n > 215 rpm. In Figure 5 (b), it is found that -C p curves were also almost the same with that of the fixed pitching angle during n < 25 rpm. In n=25 rpm, the C p began to reduce. Moreover, C p became almost or negative in n > 215 rpm. As the results of these tests, it is clarified that the C p begins to decrease rapidly when the rotation speed of the SW-VAWT exceeds the limit value decided by the tension of the coil springs, and the rotation speed reduces till the limit value surely and autonomously, regardless of the shape of the wing. Linked rod Support arm Coil spring Support arm Stopper Linked rod Figure 4: Turbine with the Developed Autonomous Control System C p (%) 6 4 2 n(rpm) 15 2 25 21 215 22 C p (%) 6 4 2 n(rpm) 15 2 25 21 215 22-2 1 2 3 4 5-2 1 2 3 4 5 (a) Type A (Symmetric shape) (b) Type B (Asymmetric shape) Figure 5: -C p Curves for Each Rotation Speed (Turbine with Autonomous Control System)
Development of Safe Vertical Axis Wind Turbine for Over Speed Rotation, November 8-12, 29, Taipei, Taiwan Field Test of SW-VAWT installed Autonomous Control System for Over Speed Rotation Field Test Configuration To test the autonomous control system for the over speed rotation in natural wind, a prototype of SW-VAWT installed this system was developed as shown in Figure 6. This prototype had three wings whose shape was NACA2. The wing length and the chord length of the wing were 1.8 m and.3 m, respectively. The radius of this turbine was 1.5 m. Therefore the solidity ratio of this turbine was.95. The main arms consisted of trussed pipes. The linkage system for the autonomous control system was changed a little to secure the space for the main shaft of this turbine as shown in Figure 7. The initial tension of coil springs, F, was set to 53 N (case 1), 174 N (case 2), 233 N (case 3), and about 62 N (case 4). This prototype was set up in the rooftop in the building of 5 stories, whose height was about 18 m. Photo 1 shows the new SW-VAWT and the autonomous control system for the over speed rotation. Although this turbine was jointed to 2 kw coreless electric generator by φ3 mm stainless pipe, this generator was not made to function to make the worst situation that the uncontrolled SW-VAWT fault into the over speed rotation state, to test the autonomous control system. To investigate the relation between the tip speed ratio,, and the power generation efficiency, C p, the installed electric generator was made to function with the PWM (Pulse Width Modulation) power generation controller. The tension of the coil springs was set to about 62 N (case 4) in this test. 29 45 45 18 R 15 3 Unit : mm (a) Front view (b) Plane view Figure 6: General Views of the Prototype of the New SW-VAWT linked rod movable plate spring to adjust max. rotation speed center of movable plate spring to adjust max. rotation speed base plate movable plate linked rod movable plate linked rod spring to adjust max. rotation speed (a) Normal condition (b) Rotation speed saving condition Figure 7: Autonomous Control System of the New SW-VAWT
Development of Safe Vertical Axis Wind Turbine for Over Speed Rotation, November 8-12, 29, Taipei, Taiwan (a) the new SW-VAWT (b) Autonomous control system Photo 1: Autonomous Control System of the New SW-VAWT 2 15 case 1 case 2 case 3 case 4 1 5 5 1 15 Max. wind speed (m/s) 2 (a) Max. wind speed v. s. Max. rotation speed Limit rotation speed (rpm) Max. rotation speed (rpm) Result and Discussion The results of the field test were shown in Figures 8, 9 and 1. Figure 8 (a) indicates the relation between the maximum wind speed and the maximum rotation speed for 1-minute period. From this figure, it is found that the upper limit of the rotation speed exists for each case, and the upper limit of the rotation speed increases with the initial tension of springs. Figure 8 (b) shows the relation between the initial tension of the coil springs and the upper limit of the rotation speed. The solid line and the dashed line were fitting lines for the instantaneous maximum values and for 1-minute mean values respectively. These curves were given by following formula. nlimit = A F (3) where A is a coefficient decided by the mass of the linkage system and of the wings. Figure 8 (b) indicates that the upper limit of the rotation speed was decided by the centrifugal force because the initial tension of the coil springs changes in proportion with squared rotation speed. 1 8 6 4 2 Instantaneous values 1 min. mean values 5 1 15 2 25 Initial tension of spring (N) (b) Initial tension v. s. Max. rotation speed Figure 8: Relations among Rotation Speed, Initial Tension of Coil Springs and Wind Speed Figure 9 shows the relation among the mean wind speed, the rotation speed and the generated power in natural wind. Figure 9 (a) indicates the relation between 1-minute mean wind speed and 1-minute rotation speed. The various relation between the wind speed and the rotation speed because the observation period was very short and some rules of generator
Development of Safe Vertical Axis Wind Turbine for Over Speed Rotation, November 8-12, 29, Taipei, Taiwan control were examined. Moreover, the autonomous control system did not work because the wind speed did not become so high during this observation. Therefore, there is not the upper limit rotation speed. Figure 9 (b) shows the relation between 1-minute mean wind speed and 1-minute mean generated power. The many relations between the wind speed and the generated power exsists because the some generation control rules was tested during the short observation period. This figure indicates that the generation starts from the wind speed of 2 m/s or 3 m/s. Figure 1 shows the relation between the tip speed ratio,, and the wind power efficiency, C p, in natural wind. In this figure, the C p became maximum, that is about 15% - 2%, in =2.5 approximatery. This result indicates that the generation control rule should be improved more. 12 Data 7 Data 1 6 8 5 n(rpm) 6 W(W) 4 3 4 2 2 1 1 2 3 4 5 6 7 U(m/s) 1 2 3 4 5 6 7 U(m/s) (a) Mean wind speed v. s. Rotation speed (b) Mean wind speed v. s. Generated power Figure 9: Relation among Rotation Speed, Generated Power and Wind Speed.3.25 Data f(x)=.7276x 4 (x 3.2).2 CP.15.1.5...5 1. 1.5 2. 2.5 3. 3.5 4. Figure 1: Relation between and C p in Natural Wind Conclusion As the results of this study, a safe SW-VAWT for the over speed rotation was developed. It is a very simple mechanism and easy to decide the upper limit of the rotation speed, and not necessary to make the cut-out wind speed. Therefore, this SW-VAWT can continue generating electric power under extremely strong wind. Moreover, it is able to set up this SW-VAWT positively in an extremely windy site, such as the rooftop of tall buildings, the top of mountains, the Polar Regions and so on.