DEVELOPMENT OF SAFE VERTICAL AXIS WIND …iawe.org/Proceedings/7APCWE/M3B_1.pdfDevelopment of Safe Vertical Axis Wind Turbine for Over Speed Rotation, November 8-12, 2009, Taipei,

DEVELOPMENT OF SAFE VERTICAL AXIS WIND TURBINE

FOR OVER SPEED ROTATION

Minoru Noda1, 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, [email protected] 2 Professor, Institute of Technology and Science, The University of Tokushima

2-1 Minami-Josanjima, Tokushima, Japan, [email protected] 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, 2009, Taipei, Taiwan

The Seventh Asia Pacific Conference on Wind Engineering

November 8-12, 2009, 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 0.9 m and 0.45 m, respectively.

The shapes of the wing were two types as shown in Figure 2. One was NACA0012 as

a symmetric shape, and the other was defined by mapping it as the centerline of NACA0012

is corresponding to the orbit of the wing. Both wing cord length, B, were 0.1125 m. Therefore,

the solidity ratio of the SW-VAWT, σ, was 0.12.

In this study, the characteristics of the SW-VAWT were investigated by the relation

between the power generation efficiency, Cp, and the tip speed ratio, β. Cp 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.

RLU

nT

RLU

Tn

C p⋅

=

⋅

=3

3 3022

160

2

ρ

π

ρ

π (1)

where, ρ and U are the air density (kg/m3) and wind speed (m/s). β is given by n, R, and U as

follows.

U

nR

U

Rn

30

602

ππ

β == (2)

In this test, to change β, U was changed keeping n to 200 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=0.45m

L=0.9m

Load cell

DC servo motor

Turbine

(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, 2009, Taipei, Taiwan

Result and Discussion

Figure 3 shows the relations between Cp and β measured with changing φ for each

wing shape. From the Figure 3 (a), Cp becomes the maximum value, that is about 30%, in β=3

or β=3.5, when φ=0°. However, when φ change to +2° or –2°, the maximum Cp decreased

greatly. Moreover, the maximum Cp did not appear and Cp was negative for all β during φ >

+2° or φ < -2°.

x/B

y/B

0.0 0.2 0.4 0.6 0.8 1.0-0.2

-0.1

0.0

0.1

0.2

(a) Type A (Symmetric shape)

x/B

y/B

0.0 0.2 0.4 0.6 0.8 1.0-0.2

-0.1

0.0

0.1

0.2

(b) Type B (Asymmetric shape)

Figure 2: Shapes of the Straight Wing of the SW-VAWT

Cp(%)φ(deg.)

-6-4-2 0+2+4+6

β0 1 2 3 4 5

-20

0

20

40

60


Cp(%)φ(deg.)

-6-4-2 0+2+4+6

β0 1 2 3 4 5

-20

0

20

40

60


Figure 3: β-Cp Curve Changed by Pitching Angle of the Wing

Figure 3 (b) shows that Cp becomes the maximum value in β=3 or β=3.5 when φ=-2°

and φ=4°, and becomes the almost 0 or negative during φ= -6° or φ > 0°. These results indicate that Cp 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


the coil springs was adjusted for the pitching angle of the wing to begin to change at the

rotational speed of about 210 rpm.

To investigate the effect of this system on the performance of the SW-VAWT, β-Cp

curve was measured for some rotation speed conditions. The wind speed, U, was changed to

change the β for each constant n, 150 rpm, 200 rpm, 205 rpm, 210 rpm, 215 rpm and 220 rpm.

The initial pitching angle of the wing was set to 0° 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.


The β-Cp curves measured by this test were shown in Figure 5. Figure 5 (a) indicates

that β-Cp curves were almost the same with that of the case of the fixed pitching angle during

n < 205 rpm, and the maximum Cp began to reduce in n=210 rpm, and Cp completely changed

to negative for all region of β in n > 215 rpm.

In Figure 5 (b), it is found that β-Cp curves were also almost the same with that of the

fixed pitching angle during n < 205 rpm. In n=205 rpm, the Cp began to reduce. Moreover, Cp

became almost 0 or negative in n > 215 rpm.

As the results of these tests, it is clarified that the Cp 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.

Coil spring

Linked rod

Support arm

Stopper

Linked rod

Support arm

Figure 4: Turbine with the Developed Autonomous Control System

Cp(%)

β

n(rpm)150200205210215220

1 2 3 4 5-20

0

20

40

60


Cp(%)

β

n(rpm)150200205210215220

1 2 3 4 5-20

0

20

40

60


Figure 5: β-Cp Curves for Each Rotation Speed (Turbine with Autonomous Control System)


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 NACA0020. The wing length and the chord

length of the wing were 1.8 m and 0.3 m, respectively. The radius of this turbine was 1.5 m.

Therefore the solidity ratio of this turbine was 0.095. 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, F0, was set to 53 N (case 1), 174 N (case 2), 233 N (case 3), and about 620 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 φ30 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, β, and the power generation

efficiency, Cp, 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 620 N (case 4) in this test.

450

450

1800

2900

Unit : mm

300

R 150

0

(a) Front view (b) Plane view

Figure 6: General Views of the Prototype of the New SW-VAWT

movableplate

movableplate

movableplate

linkedrodlinked

rod

linked rod

spring to adjustmax. rotation speed

spring toadjust max.rotation speed

spring to adjustmax. rotation speed

base plate

(a) Normal condition

center ofmovable plate

(b) Rotation speed saving condition

Figure 7: Autonomous Control System of the New SW-VAWT


(a) the new SW-VAWT

(b) Autonomous control system

Photo 1: Autonomous Control System of the New SW-VAWT


The results of the field test were shown in Figures 8, 9 and 10. 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.

00limit FAn = (3)

where A0 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.

Max. wind speed (m/s)

Max. rotation speed (rpm)

case 1case 2case 3case 4

0 5 10 15 20

50

100

150

200

(a) Max. wind speed v. s. Max. rotation speed

Initial tension of spring (N)

Lim

it rotation speed (rpm)

Instantaneous values1 min. mean values

0 50 100 150 200 250

20

40

60

80

100

(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


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 10 shows the relation between the tip speed ratio, β, and the wind power

efficiency, Cp, in natural wind. In this figure, the Cp became maximum, that is about 15% -

20%, in β=2.5 approximatery. This result indicates that the generation control rule should be

improved more.

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7

n(r

pm

)

U(m/s)

Data

(a) Mean wind speed v. s. Rotation speed

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7

W(W

)

U(m/s)

Data

(b) Mean wind speed v. s. Generated power

Figure 9: Relation among Rotation Speed, Generated Power and Wind Speed

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

CP

β

Dataf(x)=−0.007276x4×(x−3.20)

Figure 10: Relation between β and Cp 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.

Documents

DEVELOPMENT OF SAFE VERTICAL AXIS WIND …iawe.org/Proceedings/7APCWE/M3B_1.pdfDevelopment of Safe Vertical Axis Wind Turbine for Over Speed Rotation, November 8-12, 2009, Taipei,