Tong W. Wind Power Generation and Wind Turbine Design

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Offshore Wind Turbine Design 385

using redundant sensors and back-up systems, also to be incorporated in the design

phase. This will ensure high availability and increased energy capture. It will offer

component cost beneﬁ ts through reduced fatigue load and the ability to mitigate or

avoid some current design driving loads.

5.4 Drivetrain and electrical system

Improving overall system efﬁ ciency is a key goal of any offshore development

effort. To obtain the lowest COE, losses must be reduced wherever possible. Large

diameter, gearless generator technologies such as so-called “direct-drive” which

uses permanent magnets generator are investigated by many companies as one

solution for reducing losses and maintenance cost related to the traditional plan-

etary gearbox-based system. Design trade-offs could be carried out for optimizing

the cost and efﬁ ciency of drive train and the converter based on the generator side

and grid-side converters location in the tower. Factors such as weight and cable

loss should be considered.

The electrical system is the critical interface between the drive train and the util-

ity network. It enables both the optimized drive-train design and the harvesting

and transfer of power to the utility in accordance with grid regulations. An opti-

mized electrical system design will result in higher system efﬁ ciency and lower

system cost. Both of which affect the COE in a positive manner.

The following issues related to optimizing the design of a multi-megawatt to

5–7 MW offshore wind turbine need to be addressed:

Design trade-offs for optimizing cost and efﬁ ciency of the drive train and •

converter: The power converter enables the electro-mechanical drive train to

operate at the optimum power factor and deliver maximum output power

through peak-power-point tracking. New circuit topologies and control, con-

verter packaging and thermal management need to be explored to reduce cost

and losses of the power converter as well as the generator. The power convert-

er’s physical location and associated electromagnetics that may include trans-

formers in the tower will be considered as a design factor to minimize cost and

losses in the cables as well as reduction of weight in the nacelle.

Electrical interconnection of wind turbines in a wind power plant: Studies need •

to focus on optimizing the electrical interface between turbines in an offshore

wind power plant. The feasibility of the medium voltage (MV) DC interconnect

system, based on cost and dynamic stability, need to be studied.

Transmission of power from offshore to onshore: Voltage level and frequency •

are factors which impact cost of cabling and converter interface at the sending

and receiving ends of electricity. MV or high voltage DC transmission is an

enabling technology for such bulk power transfers. Optimum voltage levels

as well as converter topologies need to be achieved to minimize the overall

systems cost.

Protection of electrical system: Electrical system protection from grid-faults and •

component failures is critical to system design from availability and reliability

386 Wind Power Generation and Wind Turbine Design

point of view, which directly affects the COE of an offshore plant. Advanced

means of control for handling grid-faults and low/zero-voltage ride through as

well as modular converter topologies that enable part-load/full-load operation

and optimization of ratings of the protection equipments such as circuit-breakers

along converter system need to be studied.

6 Other noteworthy innovations and improvements in technology

This section describes other key improvements that need to be achieved by the

offshore wind industry.

6.1 Assembly-line procedures

Unlike today’s construction method, i.e. building unique one-of structures, the

future offshore projects will utilize assembly-line procedures to maximize cost

control in a mass production manner.

6.2 System design of rotor with drivetrain

Current commercial wind turbines utilize a three-blade conﬁ guration. However,

two-blade designs which incorporate alternative hub structures may see a rise in

popularity because they allow turbines to reach higher rotor speed without visual or

noise constraints. Upwind conﬁ guration might be preferred as it allows less dynamic

loads and has less rhythmic noise effects. Detailed investigations need to be carried

out on the blade deﬂ ections and resonant modes under turbulent wind loading.

The WindPACT rotor study [ 18 ] was designed to explore many of these con-

ﬁ gurations and attempt to determine their impact on overall turbine operation and

COE. The study has found that several loads in the ﬁ nal two-bladed downwind

machine were higher than the corresponding loads in the baseline design. The

downwind two-bladed rotors also experience strong harmonic loading from the

tower shadow, which may excite natural frequencies. In the future offshore wind

projects detailed investigations need to be carried out on the relative advantages

and disadvantages of a two-bladed upwind with the corresponding three-bladed

version. The real beneﬁ t of two-bladed design with unconstrained tip speed is

simpliﬁ ed gearbox and more optimal direct-drive drive train. Preliminary studies

conducted by GE Global Research Center indicate that the rated speed of the wind

turbine has a large impact on the direct-drive cost. Considering aerodynamic power

only, the rated speed for a two-bladed turbine could be up to 30% higher than a

three-bladed turbine. In which case a direct-drive drive train for a two-bladed tur-

bine rated at 19.1 rpm would cost substantially less than one for a three-bladed

turbine rated at 11.7 rpm for a 5 MW system. However, energy capture will likely

to be less than an equivalent three-bladed wind turbine of the same diameter and

hub loading needs to be investigated as both tower and hub will require reinforce-

ment due to the two per rev loads. The suggestion in the WindPACT study to

Offshore Wind Turbine Design 387

combine the rotor design with drive-train conﬁ guration studies needs to be imple-

mented, which could ultimately contribute to reduce the COE.

6.3 Service model

A solid and viable offshore service model is extremely important for any success-

ful offshore wind project. The basic philosophy will evolve around a global per-

formance and product data warehouse speciﬁ cally aimed for autonomous offshore

operations.

The performance data from offshore wind power plants can be processed in the

global data warehouse, which will feed information to different areas. The operat-

ing information could be used by future product development teams or for existing

product improvements. The service information will be used for making contrac-

tual service agreements, remote monitoring and diagnostics and knowledge-based

maintenance. Customers can use the availability information to understand the

capacity factor and overall plant health.

Contractual services has a major emphasis on developing new technology tools

to support the offshore wind business. The new technology tools are aimed at

improving reliability and availability, extending parts lives and enhancing plant

performance.

7 Conclusion

Offshore wind turbines need to achieve high reliability and availability at low COE,

which is competitive to other energy sources. The chapter identiﬁ es innovative

options for new foundation concepts, construction techniques, rotor design, drive

train and electrical system while optimizing the total life cycle cost of offshore

wind power plants. Turbine design will need to incorporate best technologies and

practices from the land-based turbines while incorporating lessons learned from

ﬁ rst generation offshore pilot projects to develop a new robust turbine concept

optimized for offshore operations. Optimum turbine size will be determined for

locations and is expected to be in the multi- megawatt to 5–7 MW range suitable

for more than 25 m water depths.

References

[ 1] US wind resource map, DOE/NREL Wind powering America program, 2009,

http://www.windpoweringamerica.gov/wind_maps.asp

[2] Musial, W. & Butterﬁ eld, S., Future for offshore wind energy in the United

States (NREL/CP-500-36313), EnergyOcean 2004, Palm Beach, Florida,

June 28–29, 2004

[3] Ali, M., Zheng, D., Kothnur, V. & Grimley, R., Offshore wind energy genera-

tion in the United States – Challenges and R&D needs. Global Windpower

2004 Conference & Exhibition , Chicago, USA, 2004.

388 Wind Power Generation and Wind Turbine Design

[4] Netherlands renewable energy policy and role of offshore wind energy, 2009,

http://www.offshorewindenergy.org/txt2html.php?textfile=home/dutch_

corner.txt

[5] Rules & Guidelines 2000: IV Non-marine Technology – Regulations for the

Certiﬁ cation of (Offshore) Wind Energy Conversion Systems, Germanischer

Lloyd: Hamburg, Germany, 2000.

[6] Hasselman, K., Ross, D.B., Mueller, P. & Sell, W., A parametrical wave

prediction model. Journal of Physical Oceanography , 6 , 1976.

[7] Ewing, J.A., Some results from the joint North Sea Wave Project of the in-

terest to engineers. International Symposium on the Dynamics of Marine

Vehicles and Structures in Waves , University College, London, UK, 1974.

[8] Houmb, O.G. & Overvik, T., Parameterization of wave spectra and long term

distribution of wave height and period. BOSS ’76 Conf. , Trondheim, 1976.

[9] Rules for Certiﬁ cation of Offshore Wind Energy Conversion System,

Germanischer Lloyd, Hamburg, Germany, 1995.

[10] Anastasiou, K. & Bokaris, J., Physical and numerical study of 2D wave

breaking and nonlinear effects. Proc. of the Coastal Engineering Conf. ,

2000.

[ 11] Kriebel, D.L., Breaking waves in intermediate depths with and without cur-

rent. Proc. of the Coastal Engineering Conf. , pp. 203–213, 2000.

[12]. Seidel, M., Jacket substructures for the REpower 5M wind turbine. Conf.

Proc. European Offshore Wind 2007 , Berlin, Germany, 2007.

[ 13] Lüddecke, F., et al ., Load-Bearing behaviour of cast steel components in

offshore wind turbines under predominantly static and non-static strain.

Stahlbau , 77 (9), pp. 639–646, 2008.

[ 14] Villalobos, F., Houlsby, G. & Byrne, B., Suction caisson foundation for

offshore wind turbines, 2004, http://www-civil.eng.ox.ac.uk/people/gth/c/

c81.pdf

[15] Musial, W., Butterﬁ eld, S. & Boone, A., Feasibility of ﬂ oating platform sys-

tems for wind turbines. 23rd ASME Wind Energy Symposium , Reno, Nevada,

Jan. 5–8, 2004.

[16] Offshore Wind Turbine Foundations – Current & Future Prototypes, 2009,

http://offshorewind.net/Other_Pages/Turbine-Foundations.html

[17] de Goeij, W., Implementation of bending-torsion coupling in the design of a

wind turbine rotor blade, Master Thesis, TU Delft, 1998.

[18] Malcolm, D.J. & Hansen, A.C., WindPACT turbine rotor design study, NREL

Subcontractor Report, NREL/SR-500-32495, 2002.

[ 19] Butterﬁ eld, S., Musial, W., Jonkman, J. & Sclavounos, P., Engineering chal-

lenges for ﬂ oating offshore wind turbines. The 2005 Copenhagen Offshore

Wind Conf. , Copenhagen, Denmark, Oct. 26–28, 2005.

CHAPTER 12

New small turbine technologies

Hikaru Matsumiya

HIKARUWIND Lab. Ltd., Japan.

Small wind turbines (SWTs) have various long history and yet huge market potential

globally. Looking back last a couple of decades, various SWTs, both horizontal-axis

and vertical-axis types, have emerged, however, they have kept standing behind

in the shadows of large-scale wind turbines, as if they could not play meaningful

roles in energy production. Small turbines may be easily designed, manufactured,

tested and operated. But somehow mostly their unit costs are not only much higher

but also reliabilities are much lower when compared with those of middle to large

turbines. The truth lays in technical difﬁ culty with SWTs. Low Reynolds number

is one of the problems. Today, various new technical approaches have revealed

problems to be solved in delivering advanced SWTs to global markets with lower

cost and higher reliability.

1 Introduction

For a long time it had been thought that large wind turbines (LWTs) are global,

while small wind turbines (SWTs) are local. Actually the style of use of SWTs

is local, but all of the problems, both technical and institutional, that SWTs have

today are mostly global. This turned out during the IEA Annex XI’s Topical Expert

Meeting on “Challenges of Introducing Reliable Small Wind Turbines” in Stockholm

in September 2006. What was common consensus among the experts, engineers

and researchers are generally high cost and low reliability with SWTs in general.

SWTs could provide us more general values in electricity or in other forms. This

was the main conclusion at the expert meeting, because all of the participants had

already understood that advanced technologies were coming up.

In this chapter, attentions are mainly focused on new technologies and their

applications to SWTs, while there are plenty of types and intelligent applications

that have already come in the markets.

390 Wind Power Generation and Wind Turbine Design

A general life cycle of a wind turbine is illustrated in Fig. 1 . Today SWT

technology has evolved and is required to contribute to energy production and

environment protection much more than before. Therefore, safety, performance,

economics and environment are keen issues.

Discussions will be focused on those issues that are particular with SWTs in

their design procedure in the following sections.

1.1 Deﬁ nition of SWT

SWTs are deﬁ ned as “wind turbines with a rotor swept area smaller than 200 m

generating at a voltage below 1000 V a.c. or 1500 V d.c.” in IEC Standard [ 1 ]. If

the swept area is less than 2 m

, support system need not be included for the SWT.

This is the only one deﬁ nition accepted internationally.

However, basic understandings of SWTs may not be restricted within the

above boundaries. On the rotor size, “Mini” and “Micro” wind turbines with

much smaller rotor diameters or power outputs can be proposed. Also restriction

to electricity-producing systems is not essential. Various styles of application are

possible such as pumping system, some hybrid systems, etc.

The technically important features of SWTs which defer from LWTs are con-

sidered ﬁ rst. With high performance wind turbines of both horizontal- and

vertical-axis types, they are all driven by lift force created on the rotor blades.

Lift is everything and what could affect lift then? They are geometry of aerofoil

section, attack angle and Reynolds number. Only the last one depends on scale

of rotor. Therefore, one reasonable deﬁ nition of SWT may the rotor scales that

are liable to suffer from “ low Reynolds number problem ”.

Identification of the Purpose of Utilization

Investigation of Technical/Social Conditions

Wind Turbine

Design

External Conditions

Conceptual Design

Aerodynamic Design

Structural Design

Wind Turbine

Production

Tests and Verification

Quality Control

Operation and Maintenance

End of Use

Safety

Performance

Economics

Environment

Figure 1: Lifecycle of a wind turbine.

New Small Turbine Technologies 391

1.2 Low Reynolds number problem

Most of the aerofoil sections for wind turbines have the common weakness induced

by the low Reynolds number problem, i.e. they have a critical Reynolds number

around:

(1 2) 10

Re Re=≈−×

(1 )

If Re ≤ Re

, lift coefﬁ cients drop considerably. An example is shown in Fig. 2

for a MEL18M31 aerofoil section. Lift and drag coefﬁ cients are shown for four

Reynolds numbers, Re = 1 × 10

, 2 × 10

, 5 × 10

, and 1 × 10

. In the small attack

angle region of a < 25°, the lift coefﬁ cient for Re = 1 × 10

is quite different from

and much smaller than those for Re ≥ 2 × 10

. The similar characteristics are found

with various aerofoil sections.

What will happen with such a SWT that is often affected by low Reynolds number

problem? It would not start to rotate even if the wind speed becomes sufﬁ ciently

higher than cut-in wind speed, because of absence of pitch control mechanism and

-1.5

-1

-0.5

0.5

1.5

2.5

-100 -80 -60 -40 -20 0 20 40 60 80 100

CL, CD

Attack Angle (deg)

CL(Re=1E5) CD(Re=1E5)

CL(Re=2E5) CD(Re=2E5)

CL(Re=5E5) CD(Re=5E5)

CL(Re=1E6) CD(Re=1E6)

-0.2

-0.15

-0.1

-0.05

0.05

0.1

0.15

0.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x/c

y/c

MEL18M31

Figure 2 : A sample of low Reynolds number effect (lift coefﬁ cient for various

Reynolds number of two-dimensional MEL aerofoil section).

392 Wind Power Generation and Wind Turbine Design

due to laminar ﬂ ow separation. Then it will rapidly be accelerated with good wind,

however, the operation will very unstable when the wind is turbulent. One of the

worst cases, the brake system will soon shutdown the system to prevent the WT

from excessive rotation.

Figure 3 shows how SWTs suffer from low Reynolds number problem. Reynolds

number varies along blade axial location. However for simplicity, a representative

Reynolds number is deﬁ ned here at 75% axial location of a blade from the rotor

centre as follows:

75%

(2 )

where c

75%

is chord length of a blade at r = 0.75 R , R is rotor radius, W is relative

ﬂ ow speed to the blade aerofoil section in the cylindrical plane of rotor at r =

0.75 R , v is kinematic viscosity.

Having simply designed a non-dimensional rotor by blade element and

momentum theory (BEM theory) with tip speed 60 m/s and tip speed ratio is 6

as a typical case, one may evaluate what size of SWT shall be potentially affected

by the low Reynolds number problem by changing the rotor diameter and rota-

tional speed. In Fig. 3 the region where a low critical Reynolds number poten-

tially affect on rotor aerodynamic performance is shown below dotted line.

SWTs below several meters of diameter or rotors rotating at a low rotor speed

will tend to enter into the critical region.

Figure 4 shows some typical relations between rotor diameter and output power

for three kinds of rated wind speed: 7, 10, and 13 m/s assuming 35% of system

1,000

10,000

100,000

1,000,000

10,000,000

0.1 1 10 100 1000

Reynolds Number at 75% R

Rotor Diameter (m)

Re(100%RPM)

Re(75%RPM)

Re(50%RPM)

Re(25%RPM)

Potentially low Reynolds

number region

Figure 3: Reynolds number at 75% radius of rotor blade vs. rotor diameter at

10 m/s of wind speed and different rotor speed (100%, 75%, 50% and

25% of rated value).

New Small Turbine Technologies 393

efﬁ ciency and rotor swept area. IEC deﬁ nition of SWT is 200 m

of rotor swept

area which corresponds to 16 m of rotor diameter and to 10–100 kW of power

output depending on the rated wind speed.

Based on the above technical reason, some ﬁ ner classiﬁ cation for SWT is desir-

able. Considering both Figs 3 and 4 , an idea of sub-classiﬁ cation for small WT is

shown in Table 1 .

2 Other technical problems particular with SWTs

A non-dimensional design of wind turbines gives optimal power coefﬁ cient for a

given tip speed ratio if the rotor performance is not affected by Reynolds number.

At design point, design tip speed ratio l

is given as:

(3 )

0.01

0.1

100

1000

10000

0.01

0.1

100

1000

10000

0.1 1 10 100

Swept Area [m

]

Power [kW]

Rotor Diameter [m]

V=7 m/s

V=10 m/s

V=13 m/s

Swept Area

IEC Small WT

Figure 4: Relation between rotor diameter and power or rotor swept area.

Table 1: Sub-classiﬁ cation of SWT.

Sub-class Rotor diameter Technical feature

Micro wind turbine

D < 1.6 m*

Both size and power are very small

Mimi wind turbine

1.6 m ≤ D < 5 m

Full or small Reynolds number problem

SWT

5 m ≤ D < 16 m**

Less small Reynolds number problem

In IEC 61400-2, a rotor swept area smaller than 2 m

, which corresponds to 1.6 m in

diameter, is specially classiﬁ ed in such a way that the maximum yaw rate shall be 3 rad/s.

200(4 / ) 15.96 16.p =≈

394 Wind Power Generation and Wind Turbine Design

where R , V

and Ω

are rotor radius, design wind speed and design rotor speed,

respectively. Therefore, when design wind speed V

is ﬁ xed, the following relation

is obtained:

Ω=

(4 )

The smaller the turbine becomes, the faster the rotor rotates. This means a SWT is

a high-speed turbo-machine and a particular attention shall be paid on control and

safety even though the machine is very small. All the design issues for mechanical

safety are well described in IEC 61400-2.

Safety issue relates also to the operation and environmental conditions, because

SWTs are very easy to be built on domestic roofs, for example. For any kind of

WT, an open area with free wind ﬂ ow without obstructs nearby is the optimal for

the WT and safest for human beings.

3 Purposes of use of SWTs

Among all, the objectives of the use of SWT(s) must be identiﬁ ed ﬁ rst. Some

SWTs are very interesting as rotating toys or can be impressive artistic monuments

in public parks or in front of buildings. There are also many wind engines pumps

or mills. However, the primary important technical aspect of a SWT is its feature

as an energy producing machine.

Drag-driven WTs such as Savonius rotors are also often used. These WTs are of

high-torque/low-speed type and their efﬁ ciencies are not as high as lift-driven

WTs of low-torque/high-speed type. Among vertical-axis wind turbines (VAWTs),

Darrieus rotor is lift-driven type with an elegant shape. Straight Darrieus rotor is

also lift-driven type. If carefully designed, these lift-driven VAWTs may also have

not far less efﬁ ciency than horizontal-axis wind turbines (HAWTs). As technical

grounds are quite same between lift-driven VAWTs and HAWTs, main efforts are

made to describe the technical issues of HWATs.

Those who conduct a technical design of a SWT had better refer to IEC standard

“IEC 61400-2, Wind turbines – Part 2: Design requirements for small wind tur-

bines”, which fully describes safety philosophy and engineering integrity with all

technical requirements for SWTs.

However, due to various reasons, most of SWTs are technically and commer-

cially behind LWTs.

When IEA WIND, Topical Expert Meetings “Challenges of Introducing Reli-

able Small Wind Turbines” was held in Stockholm in fall 2006, 17 experts from 8

countries came to such a conclusion that to create a technically and commercially

reliable common market for SWTs is crucial. As a result, a new task (Task 27)

entitled “Development and Deployment of Small Wind Turbine Quality Label-

ling” was initiated in 2008 under the cooperative activities of IEA R&D WIND

(IEA Implementing Agreement for Co-operation in the Research, Development,

and Deployment of Wind Energy Systems). Considering the present situation that

although internationally accepted IEC standards relevant for SWTs industry

already exist (IEC 61400-2: 2

Ed:2006), its application is still not common and