Tong W. Wind Power Generation and Wind Turbine Design

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Intelligent Wind Power Unit with Tandem Wind Rotors 355

the maximum output coefﬁ cient at each D

is denoted by C

max

, where P is the

output, A is the rotational area of the front wind rotor, r is the ambient air density,

V is the wind velocity, N is the rotational speed [subscript F, R, T: the front, the

rear and the relative rotational speeds, N

= N

− N

(the rotational direction of N

is positive), subscript ‘opt’ means the rotational speed giving C

max

and L is the

dimensionless axial distance between both wind rotors [= l / d

]. Besides, the blade

setting angles b

and b

of these wind rotors were optimized [ 14 , 15 ]. With the

increase of the rear rotor diameter, that is D

becomes larger, the rotational speed

of the front wind rotor N

Fopt

becomes slower, with almost the same speed, regard-

less of the blade proﬁ les. The front blade proﬁ les, however, affects markedly the

rotational speed of the rear wind rotor N

Ropt

, with the decrease of D

. The front

blade G with the reasonable twist makes the rear wind rotor speed N

Ropt

compara-

tively faster at the smaller D

(see Tandem Wind Rotor GE), in comparison with

the front blade E without the twist. That may be caused from the difference of

the ﬂ ow condition at the rear wind rotor inlet, namely, the front wind rotor outlet.

That is, the two-dimensional front blade E has unacceptable ﬂ ow separation whose

scale is large at the smaller radius due to the poor and negative angle of attack, and

0.1

0.2

0.3

0.4

0.4 0.6 0.8 1 1.2

Pmax

L = 0.08

V = 10 m/s

-2000

2000

4000

opt

min

-1

Ropt

Topt

Fopt

0.1

0.2

0.3

Tandem Wind Rotor EE

Tandem Wind Rotor GE

PmaxR

= b

= 11 deg.

= 10 deg. b

= 16 deg.

Figure 33 : Effect of the diameter ratio on the performance.

356 Wind Power Generation and Wind Turbine Design

the separation disturbs the smoothly rotation of the rear wind rotor. Such rotational

speeds resultantly contribute the relative rotational speed N

Topt

The maximum output C

max

is larger with increment of D

and has the maxi-

mum value where D

is about 1.0, and C

, max

of Tandem Wind Rotor GE com-

posed of the twisted blades is higher than that of Tandem Wind Rotor EE. As

recognized in Fig. 33 , the maximum output coefﬁ cient C

max

is affected mainly by

the output of the rear wind rotor C

maxR

Figure 34 shows the rotational behaviours of the tandem wind rotors. The rota-

tional directions are changed drastically at about D

= 0.84, in the smaller N

That is, the rear wind rotor rotates in the same direction as the front wind rotor,

while D

is less than 0.84. On the contrary, the front wind rotor rotates unfortu-

nately in the same direction as the rear wind rotor, while D

is more than 0.84.

Such rotational behaviour of the front wind rotor is not acceptable as for the

proposed intelligent wind power unit because the front wind rotor must keep the

rotational direction as expected in Chapter 3 (see Fig. 7 ).

The maximum output coefﬁ cient and the rotational behaviours against D

are

independent to the front blade proﬁ les, as recognized in Figs 33 and 34 . Therefore,

-2000

-1000

1000

2000

3000

1.12

1.0

0.84

0.71

0.51

, N

min

-1

Tandem Wind Rotor EE

V = 10 m/s L = 0.08

= 11 deg. b

= 11 deg.

0 500 1000 1500 2000 2500 3000 3500 4000

0.51

0.71

0.84

1.0

-2000

-1000

1000

2000

3000

min

-1

V = 10 m/s L = 0.08

Tandem Wind Rotor GE

= 10 deg.

= 16 deg.

, N

min

-1

Figure 34 : Effect of the diameter ratio on the rotational behaviours of the tandem

wind rotors.

Intelligent Wind Power Unit with Tandem Wind Rotors 357

-1500

1500

3000

opt

min

-1

Fopt

Topt

Ropt

V= 10 m/s

0.1

0.2

0.3

0.4

Tandem Wind Rotor GE

Tandem Wind Rotor EE

0 0.1 0.2 0.3 0.4

Pmax

L (= l / d

)

=11 deg.

=0.71

=10 deg. b

=16 deg. D

=0.84

Figure 35 : Effect of the axial distance between front and rear wind rotors on the

performances.

the desirable diameter ratio getting the sufﬁ cient output and playing the expected

rotational behaviour is in close to D

= 0.84.

6.3 Optimum axial distance between front and rear wind rotors

Figure 35 shows the effect of the axial distance between the front and the rear wind

rotor, measured at the twist centre of the blade, on the performances, where D

Tandem Wind Rotor GE is 0.84 and D

of Tandem Wind Rotor EE is 0.71. The

rotational speed of the front wind rotor N

Fopt

becomes slower with the decrease of

L because of the ﬂ ow interaction between both wind rotors, regardless of the blade

proﬁ le. The rotational speed of the front wind rotor with blade G is slightly slower

than that of the front blade E, but the front blade G makes the rear wind rotor

speed faster. Resultantly, the relative rotational speed N

Topt

of Tandem Wind Rotor

GE is slightly faster than that of Tandem Wind Rotor EE. Such characteristics

are scarcely affected by the diameter ratio D

. The maximum output coefﬁ cient

max

is higher at the smaller L because the rear wind rotor can get effectively the

wind energy as recognized in Fig. 36 .

Besides, C

max

of Tandem Wind Rotor GE is markedly higher than that of Tandem

Wind Rotor EE because GE has not only the optimum but also the desirable

blade proﬁ le.

These data suggest that the rear wind rotor should be set as close as possible to

the front wind rotor, taking the bending moment and the vibration of the blade into

account.

358 Wind Power Generation and Wind Turbine Design

6.4 Characteristics of the tandem wind rotors

Figure 37 shows the output coefﬁ cient C

against the relative tip speed ratio l

[=(relative tip speed)/(wind velocity)], in comparison with C

of Single Wind

Rotor G. The relative tip speed ratio giving the maximum output of Tandem

Wind Rotor GE is faster than that of Single Wind Rotor G, but the output coef-

ﬁ cient of Tandem Wind Rotor GE is unacceptable. The latter is caused by the rear

wind rotor without the twist. Then, the rear baled E was modiﬁ ed to the blade G

(see Fig. 31 ) designed so as to get the desirable angle of attack ( a = 11°) irrespective

of the radial position taking account of the ﬂ ow conditions at the rear wind rotor

inlet. The maximum output coefﬁ cient can be improved successfully but the rela-

tive tip speed ratio l

may be unacceptable as shown in Fig. 38 . That is, the blade

proﬁ les must be optimized more as the tandem wind rotors.

(a) L=0.08

(b) L=0.12

Figure 36 : Flow through the tandem wind rotors.

0.1

0.2

0.3

0.4

0.5

0246810

Single Wind Rotor G :

= 0 deg.

Tandem Wind Rotor GE :

= 10 deg.

L = 0.08

V = 10 m/s

= 0.84

= 16 deg.

Figure 37 : Output of tandem wind rotor GE.

Intelligent Wind Power Unit with Tandem Wind Rotors 359

Figure 39 shows the performances in the generating mode against the wind veloc-

ity, where the rated wind speed was set at V = 10 m/s and T is the rotational torque.

The rotational speeds of the front and the rear wind rotors were adjusted at the best

relative tip speed ratio giving the maximum C

in the slower wind speed, and adjusted

so as to keep the output constant in the rated operation. The output is controlled well

with the both wind rotor speeds in place of the traditional mechanism. The abnor-

mally high rotation of the front wind rotor N

can also be suppressed well by the

rotation of the rear wind rotor N

in the blowing mode, at the faster wind velocity.

In the prototype connected to the grid system, the rotational speeds are

adjusted easily by the double rotational armature type doubly fed generator with

the secondary feed.

7 Conclusion

Intelligent wind power unit, which is composed of the tandem wind rotors and

the double rotational armature type generator without the traditional stator,

was proposed.

The performances of the preliminary model tandem wind rotors were investigated

experimentally, as the ﬁ rst step to optimize the proﬁ les of the unit. The desirable

proﬁ les as the tandem wind rotors are that the diameter ratio is D

= (rear wind

rotor)/(frond wind rotor) = 0.84 and the axial distance between both wind rotors

should be set as short as possible.

The authors are carrying on this research project to be put this wind power unit

to practical use. The optimum proﬁ les will be presented near future.

Some parts of this research project were co-sponsored by Fukuoka Industry,

Science & Technology Foundation, JFE 21st Century Foundation, Research Project:

Grant-in-aid for Scientiﬁ c Research (c) (2) in Japan, Japan Science and Technology

Agency, Iwatani Naoji Memorial Foundation and Research Project: Grant-in-aid for

JSPS fellow.

0246810

Single Wind Rotor G :

= 0 deg.

Tandem Wind Rotor GG :

= 5 deg.

0.1

0.2

0.3

0.4

0.5

V = 10 m/s

= 0.84

L = 0.08

= 15 deg.

Figure 38: Output of tandem wind rotor GG.

360 Wind Power Generation and Wind Turbine Design

References

[1] Soerensen, H.C., Larsen, J.H., Olsen, F.A., Svenson, J. & Hansen, S.R.,

Middelgrunden 40 MW offshore wind farm: a prestudy for the Danish

offshore 750 MW wind program. Proc. of 10th International Offshore and

Polar Engineering Conference , 1 , pp. 484–491, 2000.

[2] David A. Sper., Wind Turbine Technology , ASME Press: New York, p. 93,

1994.

[3] Ushiyama, I., Shimota, T. & Miura, Y., An experimental study of the two staged

wind turbines. Proc. of World Renewable Energy Conference , pp. 909–912,

1996.

4 8 12 16 20

P W

V m/s

L = 0.08

= 0.84

Tandem Wind Rotor GG

= 5 deg. b

= 15 deg.

0.1

0.2

0.3

T Nm

-2000

-1000

1000

2000

3000

, N

min

-1

Rated wind velocity

(10 m/s)

Figure 39 : Performance of the tandem rotors in the generating mode.

Intelligent Wind Power Unit with Tandem Wind Rotors 361

[4] Jang, T.J. & Heo, H.K., Study on the development of wind power system

using mutually opposite rotation of dual rotors, Proc. of the Renewable

Energy 2008 , CD-ROM O-WE-015, 2008.

[ 5] Appa, K., Counter rotating wind turbine system. Energy Innovations Small

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of 30kW counter-rotating wind turbine system. Renewable Energy , 30 (5),

pp. 631–644, 2005.

[ 7] Shen, W.Z., Zakkam, V.A.K., Sørensen, J.N. & Appa, K., Analysis of counter-

rotating wind turbines. Journal of Physics : Conference Series , 75 (012003),

9 pp., 2007.

[8] Kanemoto, T. & Galal, A.M., Development of intelligent wind turbine gen-

erator with tandem wind rotors and double rotational armatures (1st report,

Superior Operation of Tandem Wind Rotors). JSME International Journal ,

Ser. B, 49 (2), pp. 450–457, 2006.

[9] Muller, S., Deicke, M. & De Doncker, R.W., Adjustable speed generator

for wind turbines based on doubly-fed induction machines and 4-quadrant

IGBT converter linked to the rotor. Conf. Record of the 2000 IEEE , 4 ,

pp. 2249–2254, 2000.

[10] Performance and geometry of airfoils supply system (PEGASUS), online,

http://riodb.ibase.aist.jp/db060/index.html

[ 11] Kaya, Y., New Energy Thesaurus (in Japanese), Kogyo Chosakai Publishing:

Tokyo, p. 434, 2002.

[12] Ushiyama, I., Wind Turbine Technology (in Japanese), Morikita Publishing:

Tokyo, pp. 87–107, 2002.

[ 13] Kanemoto, T. & Galal, A.M., Development of intelligent wind turbine gen-

erator with tandem wind rotors and double rotational armatures. JSME Inter-

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CHAPTER 11

Offshore wind turbine design

Danian Zheng

& Sumit Bose

Infrastructure Energy, General Electric Company, USA.

Global Research Centre, General Electric Company, USA.

Offshore wind plants have become a real option in the EU countries when

onshore wind plants quickly occupy the prime land for wind energy generation.

This chapter reviews the state of the art and some of the technical challenges

anticipated during the development of U.S. offshore wind power plants in the

U.S. It highlights challenges that are unique to the coastal U.S., e.g. hurri-

canes, in addition to the general challenges for the offshore wind industry,

irrespective of geographical location. It will be shown that experience from the

oil and gas industry is not completely applicable to the wind industry, although

it certainly should be leveraged and modiﬁ ed to ﬁ t the unique needs of offshore

wind. Finally, some of the research needed to further address these challenges

are outlined.

1 Introduction

Offshore wind presents a tremendous opportunity for the United States and Europe.

Onshore wind plants often face land use disputes, noise, and visual impact obsta-

cles. Moving wind plants offshore not only mitigates these problems, but provides

other important advantages including:

Availability of large continuous areas suitable for major wind plants •

Higher wind speeds, which generally increase with distance from the shore •

Less turbulence, which allows turbines to harvest energy more effectively •

Reduced fatigue loads on the turbine •

Lower wind shear allowing shorter towers •

364 Wind Power Generation and Wind Turbine Design

The United States enjoys signiﬁ cant onshore and offshore resources. While

onshore plants are more cost effective to develop than offshore ones, some of the

onshore sites in the U.S. have the following disadvantages:

They are away from the population centers in the U.S. More than half of the •

United States population lives at or near shore.

Similarly (and perhaps as a consequence of 1 above), these onshore resources •

are away from the grid connections.

They are mostly in areas which do not have signiﬁ cant electricity demand thus •

depressing electricity prices in those areas and upsetting the economical struc-

ture for wind developers.

On the other hand, some regions of New England and the Mid-Atlantic states

have some of the highest electricity prices, thus making wind and other (generally

more expensive) renewable energy sources more cost effective. East coast states

also have high land usage preventing the development of onshore wind plants or

even the development of fossil-based onshore power generation stations. These

states also have readily available grid connectivity close to the shore where most

of the population is located.

While onshore wind plants are well understood, offshore installations present unique

challenges to the wind industry, particularly as coastal communities demand them to

be placed further off shore. These challenges are discussed in the following sections.

2 Offshore resource potential

Signiﬁ cant offshore wind energy resources are available in the United States. Figure 1

shows the U.S. wind power resource with superb wind power availability around

the coastal areas. Table 1 shows the offshore wind resources along U.S. coast.

Looking at Fig. 1 and Table 1 , one could argue the United States enjoys signiﬁ cant

wind resources inland, and much of these resources are in mid-western states with

limited land use or visibility issues. However, while development of these inland

areas is more cost effective than offshore plants, they are unlikely to be fully

developed for several reasons:

Distance from population centers – more than half of the U.S. population lives •

at or near a coastline. Developing midwestern sites results in signiﬁ cant energy

transportation costs.

Distance from grid – similarly, available inland resources are not near grid •

connections.

Low local demand – available midwestern sites primarily fall in areas which do •

not have signiﬁ cant electricity demand, thus depressing electricity prices in

those areas.

On the other hand, it is clear that both New England and the mid-Atlantic regions

have signiﬁ cantly more offshore wind resource than is available on land. In addition,

New England and the mid-Atlantic states have the highest electricity prices, thus

making wind and other renewable energy sources more cost effective. East coast

states also have higher land usage, which tends to prevent development of inland