Gasch R., Twele J. (Eds.) Wind Power Plants: Fundamentals, Design, Construction and Operation

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3.1 Rotor

the flow remains attached to the profile, however larger pitch angles are required..

Fig. 3-7 shows the comparison of typical power curves for these two concepts of

power limitation. Both methods of pitching the blade – to stall or to feather - are

extensively discussed in chapters 6 and 12.

v = 8 m/s

u = const

v = 16 m/s

u = const

v = 8 m/s

u = const

v = 8 m/s

u = const

v = 16 m/s

u = const

v = 16 m/s

u = const

Fig. 3-5 Schematic view of the stall effect (limiting power at wind speed higher than rated wind

speed)

v = 8 m/s

v = 16 m/s

u = const

v = 8 m/s

v = 16 m/s

u = const

v = 16 m/s

u = const

Fig. 3-6 Schematic view of “Pitching to feather” (limiting power at wind speed higher than rated

wind speed)

3 Wind turbines - design and components

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16 18 20

Windgeschwindigkeit v in m/s

Leistung P in kW .

stall

pitch

Wind speed v [m/s]

Power P [kW]

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16 18 20

Windgeschwindigkeit v in m/s

Leistung P in kW .

stall

pitch

Wind speed v [m/s]

Power P [kW]

Fig. 3-7 Comparison of stall and pitch control: Power curves of two 600 kW wind turbines, data

from type approval test reports; e.g. in [24]

3.1.1 Rotor blade

The design of the single rotor blade is determined by the selected aerodynamic

profiles, the inner and outer geometry and as well as the chosen materials.

As it will be discussed in detail in chapter 5, the required quality of the aero-

dynamic profile depends on the chosen design tip speed ratio. Western mills (Fig.

3-3, left) only require the profile of a cambered plate, whereas wind turbines for

electricity generation require high-quality profiles with a high ratio of lift and drag

coefficient (i.e. high lift/drag ratio).

The aerodynamic profiles of the NACA44XX and NACA63XXX series are

very common, but today there are also aerodynamic profiles developed specifi-

cally for wind turbine rotors, see Fig. 3-8.

Above all, a high lift/drag ratio is important in the area of the blade tip. The

circumferential speed in the inner blade part is smaller, which leads to a smaller

local relative velocity and local speed ratio. Therefore, the profile chord length has

to be larger which allows thicker profiles to be applied in this rotor section, (cf.

chapter 5). This is helpful for the reduction of the large material’s stress, as the

highest loads are found at the blade root. Hence, different aerodynamic profiles

are used in the inner and outer part of the rotor blade. Fig. 3-8 shows a typical dis-

tribution of different profile geometries along the blade radius.

3.1 Rotor

Fig. 3-8 Different blade profiles along the radius of a blade, (EUROS)

Center of elasticity

30% c

Center of gravity

Radial line (often pitch axis)

Aerodynamic

pressure line

Airfoil cord length c

Suction side

Pressure side

BondingBonding

Belt Web

Center of elasticity

30% c 30% c

Center of gravity

Radial line (often pitch axis)

Aerodynamic

pressure line

Airfoil cord length c

Suction side

Pressure side

BondingBonding

Belt Web

Fig. 3-9 Blade section - characteristic lines of the blade dynamics

In the design calculations of rotor blades (statics, vibrations, flutter etc.) the posi-

tion of four lines, which pass radially along the blade, Fig. 3-9, is of great impor-

tance:

x The radial line is the axis of rotation of the blade pitch mechanism, resp.

the line in the blade flange centre perpendicular to the rotor shaft axis

x The elastic line (centre of elasticity) is the position of the shear centre

line in the supporting structure (approx. centre of gravity of the main

3 Wind turbines - design and components

spar). The elastic deformations of the flap- and edge-wise movements are

counted from here as well as the twist of the blade section caused by tor-

sional moments

x The line of the centres of gravity represents the points of action of forces

resulting from inertia and weight

x The pressure line consists of the points of attack of the lift and drag

forces. When the flow is attached, it is approx. 30% of the chord length.

If e.g. the stall effect occurs when exceeding the rated wind speed the

pressure line will move which may cause the rotor blade to vibrate (stall

flutter). These vibrations can be reduced by vibration dampers (contain-

ing liquids) in the blade tip. In order to provoke the stall effect along a

defined line on the blade surface and prevent it from oscillating, some

stall-regulated wind turbines are equipped with vortex generators (stall

stripes) on their blade surface.

The necessary quality of the aerodynamic profile generates requirements in rela-

tion to the production process and the applied materials of the rotor blade. The

simple profile of a Western mill, Fig. 3-3 left, is manufactured from cambered

steel plates.

The rotor blades for electricity generating wind turbines, designed with a high

tip speed ratio, Fig. 3-3 middle and right, have to meet higher demands. Their pro-

files are mostly laminated with glass fibre reinforced plastics (GFRP), and, most

recently, with carbon fibre reinforced plastics (CFRP). The latter are more expen-

sive, but their admissible material strength is up to three times higher than that of

GFRP, Fig. 3-10. Their fatigue strength also tends to be higher which is ideal for

lightweight design.

The separate moulds for the suction and pressure side of the blade (Fig. 3-11)

are covered with the woven fibre fabrics (rovings), which are then soaked with

polyester or epoxy resin. Today, this is mostly done automatically in a vacuum

process in order to reduce the adverse health effects for the workers, prevent air

bubbles which reduce the material strength and achieve a more defined material

usage. After evacuating the mould sealed with a plastic film, the resin is pumped

into the mould at defined points. Some rovings types are delivered already soaked

with resin (so-called “pre-pregs”). Moreover, some manufacturers use so-called

sandwich-constructions where balsa wood is located in between the inner and

outer rovings. In a defined heating cycle the resin is hardened, and finally the two

halves of the blade are bonded together. The spar of the GFRP blade, which pro-

vides defined geometry and material strength, is filled with foam and/or addition-

ally stiffened by GFRP ribs, webs and belts (Fig. 3-9). The final coating has to be

weather-proof and UV resistant.

Erosion protection film is attached to the leading edge to reduce the abrasive

material removal under operation. Flow controlling elements are applied on the

blades, e.g. vortex generators, to assure defined flow conditions and flow direction

despite the wind fluctuating by time and once per revolution due to the wind

profile.

3.1 Rotor

Stiffness in GPa

Tensile strength in MPa

Glass fibre

Carbon fibre

Stiffness in GPa

Tensile strength in MPa

Glass fibreGlass fibre

Carbon fibreCarbon fibre

Fig. 3-10 Material strength values of glass and carbon fibres, (EUROS)

Fig. 3-11 Blade production, separate moulds for suction side and pressure side, mould in front:

soaking of the laminate with epoxy resin by a vacuum process (NOI)

3 Wind turbines - design and components

Fig. 3-12 Connection of rotor blade at hub: left “IKEA-bolts” (Bonus), right: bolt sleeves

(Vestas)

Blade tip

activated by

centrifugal force

Pre-stressing

spring

Return cylinder

Hydraulic

pipe

Blade tip

activated by

centrifugal force

Pre-stressing

spring

Return cylinder

Hydraulic

pipe

Fig. 3-13 Rotor blade with tip brake (turning the blade tip) (LM, [6])

3.1 Rotor

The load transmission from the GFRP rotor blade to the metal flange of the hub is

a tricky issue (Fig. 3-12). For this screw connection with the hub, there are used

either sleeves for stud bolts laminated into the blade root, or the so-called “Ikea-

connection” with a cross bolt

Another design detail which needs special attention is the turnable blade tip of

stall-regulated rotors which serves as an aerodynamic brake. Activated by cen-

trifugal forces, it deploys when the rotor is over-speeding (Fig. 3-2 and 3-13). For

braking, a threaded guidance turns the blade tip by 90°, so it is nearly perpendicu-

lar to the relative velocity. Since the turnable blade tip is located at the maximum

radius, the affected ring section, and as well the resulting braking force and torque,

are very large.

The specific blade properties change with increasing size. Light weight design

is an imperative, above all for the rotor blades of the Megawatt turbines. If the

rotor blades were scaled up using the laws of similarity (cf. chapter 7), the weight

of the rotor blades would increase with the cube of the blade radius (m ~ r³). The

large weight would cause enormous bending forces and correspondingly high

stress causing problems with the material strength. Fig. 3-14 shows the blade mass

of commercial rotor blades versus the rotor diameter. The interpolation curves

have an exponent of approx. 2.2 instead of 3.0, thanks to the lightweight design.

Moreover, the diagram illustrates that when it comes to the rotor blade weight, it is

advantageous to use epoxy instead of polyester resin.

Rotor blade weight/ (kg)

Fig. 3-14 Rotor blade weight depending on rotor diameter and material, (EUROS)

3 Wind turbines - design and components

3.1.2 Hub

There are various possibilities for the design of the hub and the attachment of the

blades. Most of them were tested in the 1980s at prototype turbines of different

sizes. In the following, these variants are presented. Although the rigid hub is used

nearly exclusively for commercial wind turbines (Figs. 3-15 and 3-23). Perhaps

the other hub types will be considered again during future developments because

they help to reduce stress and thus component weight.

The rotor blade connection to the hub can be made

- rigid or

- flexible by using a hinge (flapping), see Fig. 3-16. Another special hub type

can be used at two-bladed rotors where the two blades are fixed together rigidly

and have a common hinge in the hub, called the teetering hinge. All of these three

types of blade-hub connection may be combined with a controlled pitching of the

blade around its axis for power and rotor speed limitation. Theoretically, move-

ment can also occur around the third axis at the blade-hub connection, the slewing

axis. However, this is not used in practice. Instead, in order to compensate alter-

nating loads (torque peaks) in this direction, additional components are included in

the drive train (special couplings or the elastic support of the gearbox). Fig. 3-17

gives an overview of different hub types and the stress relief achieved in the blade

root and rotor shaft.

Fig. 3-15 Nacelle with rigid hub of a three-bladed rotor (photo by company Zollern)

3.1 Rotor

Fig. 3-16 Denomination of the axes at the blade flange

The flapping hinge rotor is a characteristic of the SÜDWIND 1237 which has a

downwind rotor (Fig. 3-18). The flapping hinge at each rotor blade relieves the

blade root and the rotor shaft from all bending stress around the flapping axis.

Such bending stress results from the ‘wind pressure’ (i.e. thrust) and the three-

dimensional non-uniform wind velocity field (Fig. 3-19). At a hub with rigid blade

connection the stochastic spatial fluctuations of the wind in the rotor swept area

cause an eccentricity of the point of action of the resultant forces from the rotor

axis leading to bending stress in the rotor shaft. This is also avoided by flapping

hinges.

During operation, a balance is created between the centrifugal forces F

and the

thrust F

at the rotor, Fig. 3-21. This causes a self-adjusting of the flapping angle

and also its limitation, typically below 10°. Larger cone angles only occur if the

centrifugal forces are too small due to low rotor speed, i.e. shortly before rotor

standstill. Additional components in the rotor are therefore required - e.g. return

springs (SÜDWIND), stoppers, gear or hydraulic components - to assure the start

up of the wind turbine. These components are complicated and costly, hence the

flapping hinge principle is seldom applied to larger rotors [7].

3 Wind turbines - design and components

Blattanschluss ... starr starr und pitchend schlagend

Blade-hub

connection …

rigid

rigid with

pitch

flapping teetering

Reduces all flapwise

bending moments in the

blade root and also the

alternating bending

moments in the rotor shaft

due to the spatially

inhomogeneous wind field

High, alternating

bending loads in the

rotor shaft and

alternating flapwise

bending loads at the

blade root

Reduces in all turbine

components at strong

wind and storm the

loads due to the mean

aerodynamic forces

Reduces the alternating

bending moments in the

rotor shaft due to the

spatially inhomogeneous

wind field and reduces also

the dynamic components of

the flapwise bending

moment in the blade root

Blattanschluss ... starr starr und pitchend schlagend

Blade-hub

connection …

rigid

rigid with

pitch

flapping teetering

Reduces all flapwise

bending moments in the

blade root and also the

alternating bending

moments in the rotor shaft

due to the spatially

inhomogeneous wind field

High, alternating

bending loads in the

rotor shaft and

alternating flapwise

bending loads at the

blade root

Reduces in all turbine

components at strong

wind and storm the

loads due to the mean

aerodynamic forces

Reduces the alternating

bending moments in the

rotor shaft due to the

spatially inhomogeneous

wind field and reduces also

the dynamic components of

the flapwise bending

moment in the blade root

Fig. 3-17 Different hub design types