Tong W. Wind Power Generation and Wind Turbine Design

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Intelligent Wind Power Unit with Tandem Wind Rotors 335

of the rated output 3 kW (see Fig. 3 , [ 5 ]). The each wind rotor rotates and the

armature as usual, while the mechanical efﬁ ciency is 0.8. The numerical simula-

tion showed that the prototype with the tandem wind rotors can get nearly 85% of

the effective wind power, which is about two times higher than that of a practical

single rotor system. The ﬁ eld tests suggested that the rear wind rotor can generate

the additional output from 25 to 40% on the same stream tube and the efﬁ ciency is

high at the comparatively lower rotational speed.

Continuously, Jung et al. has proposed the tandem wind rotors consisted of the

front wind rotor with the diameter of 5.5 m, called the auxiliary rotor by them, and

the rear wind rotor with the diameter of 11 m, called the main rotor by them [ 6 ].

Wind

Generator

Anemometer

Revolution

counter

Slide resistor

Revolution

sensor

Revolution

sensor

VoltmeterAmmeter

Figure 1 : Model unit with tandem wind rotors [ 3 ].

0 200 400 600 800 1000 1200

Output W

Rotational speed min

-1

V = 10 m/s

V = 8 m/s

V = 6 m/s

Figure 2 : Output of tandem wind rotors [ 3 ].

336 Wind Power Generation and Wind Turbine Design

The tandem wind rotors drive the armature of the traditional generator through the

gearbox as shown in Fig. 4 . The ﬂ ows and the performances were predicted

numerically, by the quasi-steady strip theory with the experimental wake model

obtained in the wind tunnel, and were compared with the experiments in the full

scale. The paper showed the optimum arrangement of the tandem wind rotors and

the maximum output coefﬁ cient reaches as high as 0.5 in the full-scale test.

Furthermore, Shen et al. compared the energy production of the counter-rotating

type with that of the shingle rotating type, in the numerical simulation at the actual

velocity ﬁ eld in the island of Sprogo, Denmark [ 7 ]. The tandem wind rotors, whose

diameters are 20.5 m, consist of three blades in the commercial Nordtank 500 kW

rotor. The outputs were predicted by the commercial Navier-Stokes code Ellip-

Sys3D, and Fig. 5 shows the predicted annual energy productions (AEP).

Tandem wind rotors

Generator

Gearbox

Figure 4 : The 30 kW CRWT system [ 6 ].

Figure 3: The 6 kW prototype wind turbine [ 5 ].

Intelligent Wind Power Unit with Tandem Wind Rotors 337

The tandem wind rotors produce more energy than the single wind rotor, regardless

of the wind velocity. The annual production at the wind velocity 5 m/s and over is

2966 MWh for the tandem wind rotors, while 2066 MWh for the single wind rotor.

That is, the annual energy can be increased about 43.5%, as compared with the

single wind rotor type.

It is very important to notice that the works of these previous tandem wind

rotors quite differ from the works of the wind rotors which are boarded on intelli-

gent wind power unit proposed by the authors. The proposed tandem wind rotors

are in cooperation with the double rotational armature type generator.

3 S uperior operation of intelligent wind power unit

Many researchers/engineers have proposed the tandem wind rotors to increase the

output, as reviewed just before. Intelligent wind power unit proposed by the authors

has the promising advantages that the output is not only increased but also controlled

in response to the wind circumstances without accessories such as the gearbox, the

pitch control mechanism, the braking system and so on. That is, the operation of

the tandem wind rotors quite differ from the conventional wind rotors, and is in

cooperation with the new type generator with double rotational armatures [ 8 ].

In the upwind type unit as shown in Fig. 6 , the large-sized front wind rotor and the

small-sized rear wind rotor drive the inner and the outer armatures in the peculiar

generator, respectively, while the rotational torque is counter-balanced in both arma-

tures. As for the tandem wind rotors, the rotational torque of the front wind rotor must

be the same, in the opposite direction, as that of the rear wind rotor. The rotational

direction and speed of both rotors/armatures, however, are free and adjusted

automatically/smartly in response to the wind circumstances, as follows (see Fig. 7 ).

-1

Single Rotor

Electric energy kWh

0 5 10 15 20 25

Wind Velocity m/s

x 10

(a) AEP of sin

le wind rotor (b) AEP of the CRWT

Counter Rotor

0 5 10 15 20 25

Wind Velocity m/s

Figure 5 : AEP of CRWT and single wind rotor located in the isolated island.

338 Wind Power Generation and Wind Turbine Design

Front wind rotor

Rear wind rotor

Outer armature

Wind direction

Inner armature

Figure 6 : Intelligent wind power unit (upwind type).

Rated wind velocity

Wind velocity

Output

Rotational speed

of front wind rotor

Rotational speed

of rear wind roto

Cut-in wind velocity

Rotational speed, Output

Figure 7 : Superior operation of the tandem wind rotors.

Both wind rotors start to rotate in the counter directions at the low wind velocity, 1.

namely the cut-in wind velocity.

The increase of the wind velocity makes the rotational speed of both wind 2.

rotors increase, while the rotational speed of the rear wind rotor is faster than

that of the front wind rotor because of its small size.

Intelligent Wind Power Unit with Tandem Wind Rotors 339

The rear wind rotor reaches the maximum rotational speed at the rated wind 3.

velocity. Under the rated wind velocity, both wind rotors work at the best

efﬁ ciencies. Over the rated wind velocity, the rated operation mode starts,

where the output is kept constant regardless of the wind velocity.

With more increment of the wind velocity, the rear wind rotor decreases its 4.

rotational speed gradually, stops, then it begins to rotate in the same direction

of the front wind rotor, so as to its rotational torque coincides with the larger

rotational torque of the front wind rotor. When the rear wind rotor rotates in the

same direction to the front wind rotor, the ﬂ ow in the blowing mode suppresses

the abnormal rotation of the front wind rotor.

The above operating conditions enable successfully not only to increase the

output but also to guarantee the quality of the electric power, namely power supply

frequency at the rated operation without the traditional brake and/or the pitch con-

trol mechanisms. That is, the relative rotational speed between the front and the

rear wind rotors affects directly the quality of the electric power. The power supply

frequency can be kept constant when the wind rotors are designed so as to keep the

relative rotational speed constant. The output can also be kept constant when the

wind rotors are designed so as to keep the value, determined by the relative rota-

tional speed and torque, constant. Moreover, the counter-rotation under the rated

wind velocity makes the output higher in the poor wind circumstances, as reviewed

before. That is a remarkable advantage for area which has no acceptable wind

circumstance in the power generation.

4 Preparation of double rotational armature type generator

4.1 Double-fed induction generator with double rotational armatures

The doubly fed induction generator is installed in the large-scale wind power unit

for the grid-connected electric power system [ 9 ]. Figure 8 shows the system dia-

gram where the tandem wind rotors rotate the double rotational armature type gen-

erator. It is not necessary to control the wind rotor speeds, namely the rotational

frequency f

, mechanically because the revolving electromagnetic ﬁ eld is induced

from the grid-connected electricity and its speed can be adjusted freely by the

frequency f

modulation (the inverter) for the inner armature, namely the second-

ary feed. Besides, the system output P and voltage E can also be adjusted with the

input P

and voltage E

for the secondary feed. That is, it is easy to keep the output,

the frequency and the voltage constant as required in the grid system.

Figure 9 shows the trial model of the double rotational armature type doubly fed

induction generator. The rated output is P = 1.2 kW at the synchronous rotational

speed of 900 min

− 1

, while the output voltage is E = 200 V, the current is I = 3.5 A,

the frequency is f = 60 Hz and the number of the pole is 8.

Figure 10 shows the bench tests for the trial model. The work of the tandem

wind rotors was replaced by the motor. The motor shaft was connected to the shaft

of the inner armature while the shaft of the outer armature was kept stationary

340 Wind Power Generation and Wind Turbine Design

(1) (2) (3)

(4)

(5)

(6)

(7)

(1). Generator

(2). Torque meter

(3). Motor

(4). Power meter

(5). Tachometer

(6). Inverter

(7). External load

(Bulb)

Figure 10 : Bench test for the induction generator.

P: Input / output

f: Frequency

E: Voltage

Subscripts

1: Primary feed

2: Secondary feed

3: Wind rotors

P = P

- P

F = f

+ f

E = E

Grid

= f

+ f

, f

, E

Filter

, f

Bidirectional

Inverter

Double armature type

induction generator

Tandem wind rotor

Figure 8: System diagram of double rotational armature type double-fed induction

generator.

Figure 9: Trial model of the double rotational armature type double-fed induc-

tion generator.

Intelligent Wind Power Unit with Tandem Wind Rotors 341

because the electric characteristics depend only on the relative rotational speed

between both armatures. The rotational speed of the motor which represents the

relative speed (in this case, rotational speed of inner armature) was controlled by

the inverter. Another inverter and the transformer were used to control the input

frequency f

and voltage E

of the secondary feed of the generator. The input to the

generator, namely the output of the tandem wind rotors, was estimated with the

rotational speed and torque. The output was measured by the power meter and

consumed through electric bulbs (lamps).

In the bench test, the output frequency f = f

was kept constant at 60 Hz, and the

output voltage E = E

was also kept constant at 200 V, regardless of the external

bulb load P

bulb

Figure 11 shows the characteristics in keeping the output voltage E

and fre-

quency f

constant as described just above, where N

is the relative rotational speed

(= N

− N

, N

and N

are the outer and the inner rotational speed, and give the

positive value in the N

direction). The input frequency f

is in inverse proportion

to the relative rotational speed N

irrespective of the external load P

bulb

, though the

phase must be changed at the higher relative speed than the synchronous speed

900 min

− 1

. The input voltage E

depends not only on N

but also on P

bulb

, namely

the output P

. The input P

comes to be negative at the higher rotational speed than

the synchronous speed 900 min

− 1

, as shown in Fig. 12 . That is, P

changes fruitfully

from the input to the output, and the net output from the generator is P = P

− P

The output and/or the input are given in Fig. 13 , for instance, when the tandem

wind rotors with the relative rotational speed N

and the rotational torque T against

100

200

300

0 200 400 600 800 1000 1200 1400

, E

min

-1

-30

, f

90 W

450 W

940 W

1200 W

1650 W

bulb

, E

Figure 11: Effect of the external load and the relative rotational speed on the

input frequency f

and voltage E

342 Wind Power Generation and Wind Turbine Design

the wind speed V as shown in Fig. 14 are prepared and the voltage E

and the

frequency f

are controlled at E

= 200 V and f

= 60 Hz as shown in Fig. 15 .

4.2 Synchronous generator with double rotational armatures

The model synchronous generator (3-phase, 4-pole, permanent magnet, AC gen-

erator with double rotational armatures) shown in Fig. 16 , was prepared. The rated

output P = 1 kW at N

= 1500 min

− 1

, while the induced frequency f = 50 Hz, and

the induced voltage E = 100 V.

Figure 17 shows the bench tests for the trial model. The armatures were driven

by two isolated motors in place of the tandem wind rotors, to conﬁ rm experimen-

tally the rotational torque of the inner and the outer armatures respectively.

The shaft of the motor was directly connected to the shaft of the inner armature,

-500

500

1000

1500

2000

90 W

450 W

940 W

1200 W

1650 W

0 300 600 900 1200

min

-1

bulb

Figure 12 : Effect of the external load and the relative rotational speed on the

input P

-300

300

600

900

1200

036912

, P

, P W

V m/s

= 200 V

= 60 Hz

Figure 13: Output/input against the wind velocity.

Intelligent Wind Power Unit with Tandem Wind Rotors 343

300

600

900

1200

1500

036912

min

-1

T N.m

V m/s

Figure 14: The relative rotational speed and torque of the tandem wind rotor

against the wind velocity.

-50

100

150

200

250

300

036912

, f

Hz , E

, E

V m/s

= 200 V, f

= 60 Hz

Figure 15 : Control of the frequency f

and voltage E

against the wind velocity.

while the rotation of another separate motor is transmitted to the outer armature

through the timing belt.

Figure 18 shows the relation between the outer armature rotational torque T

and the inner armature rotational torque T

, in keeping the rotational speed of the

outer armature constant at N

= 750 min

− 1

, while changing the rotational speed of

the inner armature N

at the various external loads. These rotational torques do not

include the mechanical torques such as the bearings in the generator, and T

given by the absolute value though the direction of T

is against T

. It is obvious

344 Wind Power Generation and Wind Turbine Design

(5). Inverter

(6). Power meter

(7). Timing belt

(8). External load

(Bulb)

(1). Generator

(2). Motor

(3). Torque meter

(4). Tachometer

(8)

(5)

(2)

(3)

(4)

(7)

(6)

(1)

Figure 17 : Bench test for the synchronous generator.

02468101214

-50

-150

-250

-350

-450

-550

-650

-750

-850

N.m

= 750 min

-1

min

-1

Figure 18 : Relation between the torque of the inner and outer armatures.

Permanent magnet

Coil winding

Figure 16: Trial model of the double rotational armature type synchronous

generator.