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

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3.4 Tower and foundation

102

3.4.2 Foundation

Wind turbines have mostly a block-shaped foundation made from concrete blocks

Fig. 3-59. The flat centre foundation of self-supporting towers is dimensioned to

prevent the turbine from tilting over (avoiding a gaping joint). In contrast to that,

the guyed masts have a separated foundation design, (Figs. 3-57, 3-58 and 3-60).

The main foundation of the mast prevents it from sinking into the soil (due to the

weight and the vertical components of the tensile forces) whereas the anchoring

foundations of the guying absorb the tensile forces. At lattice towers, the separated

foundations have to bear a combination of this loading.

The basis of the flat foundation design is a soil expertise which proves that at

the chosen site the soil provides at least the minimum bearing capacity assumed in

course of the wind turbine design.

If the ground is very soft (e.g. the marsh lands at the North Sea coast) there is

the requirement of an additional expensive pile foundation. Fig. 3-59 shows a

typical flat foundation during its construction. Before the placing of the concrete it

is important to install the cable feedthrough and the grounding. The foundation of

a 600 kW wind turbine required e.g. approx. 165 m³ of concrete and about 25 tons

of steel reinforcement.

Fig. 3-59 Flat foundation

3 Wind turbines - design and components

103

Fig. 3-60 Foundation for guyed tubular steel pole

3.5 Assembly and Production

The nacelles of wind turbines are mostly assembled in the factory and transported

as one unit to the site. In the factory, cranes of different capacity are needed for

moving the components (nacelle frame, gearbox, generator, etc.), e.g. overhead

cranes or jib cranes at the individual assembly areas. Fig. 3-61 shows a workshop

of the company ENERCON. The maximum crane capacity, mostly an overhead

crane, is required for loading the completed nacelle on the truck. Fig. 3-62 gives

an overview of the nacelle and rotor mass of the different wind turbine sizes. The

comparison of the actual curves with the theoretical curve obtained by scaling up

the mass according to the similarity laws shows that with the increasing turbine

size the lightweight design achieved a significant mass reduction.

3.5 Assembly and Production

104

Fig. 3-61 Nacelle assembly shop at company ENERCON

For smaller wind turbines, the hub may already be flanged to the rotor shaft in the

workshop, Fig. 3-23, and transported together with the nacelle. But the hubs of

Multi-MW wind turbines are very heavy and big, so they have to be transported

separately. On site, the hub is assembled with the three rotor blades at the ground

and then flanged to the rotor shaft of the already mounted nacelle, Fig. 3-55.

Nevertheless, before the delivery, there is mostly a complete system test of the

drive train including the hub with pitch system on a test stand in the workshop.

The in-house production depth (vertical integration) depends strongly on the

manufacturer. The two main corporate philosophies are a high or low in-house

production depth, see table 3.5.

3 Wind turbines - design and components

105

100

0 500 1000 1500 2000 2500

Rated power in kW

Rotor and nacelle mass in t

stall

pitch

Rotor mass

Nacelle mass

Nacelle mass according to

rules of similarity

100

0 500 1000 1500 2000 2500

Stall

Pitch

100

0 500 1000 1500 2000 2500

Rated power in kW

Rotor and nacelle mass in t

stall

pitch

Rotor mass

Nacelle mass

100

0 500 1000 1500 2000 2500

Rated power in kW

Rotor and nacelle mass in t

stall

pitch

Rotor mass

Nacelle mass

Nacelle mass according to

rules of similarity

100

0 500 1000 1500 2000 2500

Stall

Pitch

100

0 500 1000 1500 2000 2500

Stall

Pitch

Fig. 3-62 Rotor- and nacelle mass versus size

Smaller manufacturers with a small market share tend more to a low in-house pro-

duction depth. In-house production depth also grows in tandem with increasing

company size. A specialty of the production of Multi-MW wind turbines is that

most of the manufacturers do not have their own foundries, even if their in-house

production depth is high, something which may turn out to be a bottle-neck. Only

a few foundries are able to produce components with a weight of up to 100 t per

single casting.

Table 3.5 Comparison of high and low in-house production depth

High in-house production depth Low in-house production depth

Advantage Disadvantage Advantage Disadvantage

Low dependency

on suppliers

High demand for

capital

Low demand for

capital

High dependency

on suppliers con-

cerning costs, qual-

ity and adherence

to schedules

High value added

High risk of fluctu-

ating production

activity

High flexibility Low value added

Easy quality con-

trol

Licensed manu-

facturing with lo-

cal suppliers

3.6 Characteristic wind turbine data

106

3.6 Characteristic wind turbine data

This section compares characteristic data of different commercial wind turbines

available in the past 10 years in Germany [27]. The data base comprises approx.

300 wind turbine types with a rated power above 30 kW for grid-connected opera-

tion. In the diagrams, the discussed characteristic data is mostly plotted against the

rated power or the rotor diameter since these two attributes characterize best the

wind turbine size.

Wind turbine size has developed rapidly in the past 20 years, as discussed in

chapter 1. Fig. 3-63 shows the growth of grid-connected wind turbines in these

two decades. The increase in the rated power by the factor of 10 is an exceptional

success in the area of mechanical engineering. In the past, a comparable stimu-

lated development occurred only in the fields of computer and information tech-

nology.

Since the 1990s, rotor diameter has grown by a factor of 8, the hub height has

increased by a factor of 5.

Looking at the rated power versus the rotor diameter in Fig. 3-64, site-related

influences have to be taken into account. The wind turbine should have its best-

efficiency point close to the wind speed of maximum energy density, cf. chapter 4.

Wind turbines designed for coastal sites with a higher mean wind speed already

achieve the same rated power with a smaller swept rotor area which results in a

higher area-specific power (ratio of rated power and swept rotor area) of up to

520 W/m² producing the upper limiting curve. The lower limiting curve of

290 W/m² represents the design with a bigger rotor suitable for inland sites with a

lower mean wind speed. For example, the 2 MW wind turbine MM82 of the com-

pany REpower with a rotor diameter of 82 m, designed for inland sites has an

area-specific capacity of 378 W/m², whereas its sister machine MM70 with the

same rated power but designed for the windy coastal sites reaches 520 W/m

due

to the smaller rotor of 70 m.

3 Wind turbines - design and components

107

1.000

2.000

3.000

4.000

5.000

1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Nennleistung in kW .

100

125

Rotordurchmesser in m .

Nennleistung

Durchmesser

D = 15 m

P = 55 kW

H = 25 m

D = 20 m

P = 75 kW

H = 30 m

D = 30 m

P = 300 kW

H = 40 m

D = 45 m

P = 600 kW

H = 60 m

D = 70 m

P = 1.500 kW

H = 80 m

D = 125 m

P = 5.000 kW

H = 120 m

Rated power in kW

Rotor diameter in m

Diameter

Power

1.000

2.000

3.000

4.000

5.000

1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Nennleistung in kW .

100

125

Rotordurchmesser in m .

Nennleistung

Durchmesser

D = 15 m

P = 55 kW

H = 25 m

D = 20 m

P = 75 kW

H = 30 m

D = 30 m

P = 300 kW

H = 40 m

D = 45 m

P = 600 kW

H = 60 m

D = 70 m

P = 1.500 kW

H = 80 m

D = 125 m

P = 5.000 kW

H = 120 m

1.000

2.000

3.000

4.000

5.000

1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Nennleistung in kW .

100

125

Rotordurchmesser in m .

Nennleistung

Durchmesser

D = 15 m

P = 55 kW

H = 25 m

D = 20 m

P = 75 kW

H = 30 m

D = 30 m

P = 300 kW

H = 40 m

D = 45 m

P = 600 kW

H = 60 m

D = 70 m

P = 1.500 kW

H = 80 m

D = 125 m

P = 5.000 kW

H = 120 m

D = 15 m

P = 55 kW

H = 25 m

D = 20 m

P = 75 kW

H = 30 m

D = 30 m

P = 300 kW

H = 40 m

D = 45 m

P = 600 kW

H = 60 m

D = 70 m

P = 1.500 kW

H = 80 m

D = 125 m

P = 5.000 kW

H = 120 m

Rated power in kW

Rotor diameter in m

Diameter

Power

Fig. 3-63 Size and power of commercially produced wind turbines over time (cf. Fig. 1-1)

500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

5.000

0 20406080100120

Rotordurchmesser in m

Nennleistung in kW

0 20 40 60 80 100 120

Seewind 22/110

REpower MM70

290 W/m

520 W/m

REpower MM82

E70, 2,3 MW

581 W/m

5000

4500

4000

3500

3000

2500

2000

1500

1000

500

Rated power in kW

Rotor diameter in m

0 20 40 60 80 100 120

Seewind 22/110

REpower MM70

290 W/m

520 W/m

REpower MM82

E70, 2,3 MW

581 W/m

5000

4500

4000

3500

3000

2500

2000

1500

1000

500

Rated power in kW

Rotor diameter in m

Fig. 3-64 Rated power versus rotor diameter

3.6 Characteristic wind turbine data

108

Another characteristic number of the wind turbines which changed with the

increasing turbine size are the mass of nacelle and rotor. As discussed in relation

to Fig. 3-62 already, lightweight design reduced the required mass compared to

the values calculated according to the similarity laws. In addition, Fig. 3-65 shows

the area-specific nacelle mass, i.e. the ratio of nacelle mass and swept rotor area.

A temporal resolution of the data reveals that with the years the gradient of the

regression line was reduced. It can be concluded that due to the gained know-how

and the further development of the (computer-aided) design methods a specific

weight reduction was attained despite the growing wind turbine size. Nevertheless,

the values spread widely, because most of the manufacturers supply for the same

rated power a “coastal type” with a smaller and a “inland type ” with a bigger

rotor diameter, as mentioned before.

Fig. 3-66 shows the mass-specific torque (ratio of rotor torque and nacelle

mass) versus the rotor diameter used to discuss the increasing power density of the

wind turbines. The torque is calculated from rated power and maximum rotor

speed. With larger rotor diameters, it increases not only due to the growing rated

power but also due to smaller rotor speed (limiting criterion from maximum blade

tip speed). There is a significant increase of the specific torque with the years; in

1996 the range covered 5-10 Nm/kg whereas in 2002 it reached 15-20 Nm/kg for

the MW class wind turbines of more than 60 m rotor diameter. This shows that the

stress gets closer to the admissible material strength, the design is more stress-

optimized (instead of high safety factors in the beginning) and also that new

materials are applied. Moreover, the power density increases if generators and

gearboxes are more compact because of e.g. better cooling and advanced power

electronics. The wind turbines are becoming (specifically) lighter and more cost-

effective.

0 20406080100120

Rotordurchmesser in m

spezifische Gondelmasse in kg /

1996

1999

2002

0 20406080100120

Rotordurchmesser in m

spezifische Gondelmasse in kg /

1996

1999

2002

Rotor diameter in m

Specific nacelle mass in kg/ m²

0 20406080100120

Rotordurchmesser in m

spezifische Gondelmasse in kg /

1996

1999

2002

0 20406080100120

Rotordurchmesser in m

spezifische Gondelmasse in kg /

1996

1999

2002

Rotor diameter in m

Specific nacelle mass in kg/ m²

Fig. 3-65 Specific nacelle mass versus rotor diameter

3 Wind turbines - design and components

109

0 20 40 60 80 100 120 140

Rotordurchmesser in m

spezifisches Drehmoment in Nm/kg

1996

1999

2002

Linear (2002)

Linear (1996)

Rotor diameter in m

Specific torque in Nm/kg

0 20 40 60 80 100 120 140

Rotordurchmesser in m

spezifisches Drehmoment in Nm/kg

1996

1999

2002

Linear (2002)

Linear (1996)

Rotor diameter in m

Specific torque in Nm/kg

Fig. 3-66 Specific torque versus rotor diameter

The energy yield of wind turbines is basically site-specific. In order to get compa-

rable values, the reference yield is introduced and defined by the wind conditions

specified in the German Renewable Energy Sources Act (EEG) from April 2002

as follows [28]:

x Mean wind speed in 30 m height: 5.5 m/s

x Roughness length z

: 0.1 m

x Wind frequency distribution according to Rayleigh: k = 2

The law establishes a virtual average German site with approx. 1,700 annual full

load hours as the basis for wind turbine comparison. This is equivalent e.g. to

moderately windy sites in the German federal state of Brandenburg. Fig. 3-67

shows that with the increasing turbine size (i.e. with the years) the area-specific

reference yield (ratio of reference yield and swept rotor area) grew by more than

50%, from approx. 600 kWh

/m² to approx. 1000 kWh

/m². The spreading of the

values depends not only on turbine type and manufacturer but also per type on the

different available hub heights, see section 3.4. The achieved increase is mainly

due to the growing hub heights providing better wind conditions for the turbine

operation.

3.6 Characteristic wind turbine data

110

200

400

600

800

1.000

1.200

0 500 1.000 1.500 2.000 2.500 3.000

Nennleistung in kW

Flächenertrag in kWh / m

Coastal sites (smaller rotors)

Inner land sites (bigger rotors)

0 500 1000 1500 2000 2500 3000

Rated power in kW

1200

1000

800

600

400

200

Specific reference yield in kWh

/m²

200

400

600

800

1.000

1.200

0 500 1.000 1.500 2.000 2.500 3.000

Nennleistung in kW

Flächenertrag in kWh / m

Coastal sites (smaller rotors)

Inner land sites (bigger rotors)

0 500 1000 1500 2000 2500 3000

Rated power in kW

1200

1000

800

600

400

200

Specific reference yield in kWh

/m²

Coastal sites (smaller rotors)Coastal sites (smaller rotors)

Inner land sites (bigger rotors)Inner land sites (bigger rotors)

0 500 1000 1500 2000 2500 3000

Rated power in kW

1200

1000

800

600

400

200

Specific reference yield in kWh

/m²

Fig. 3-67 Area specific annual reference yield versus rated power

Considering in Fig. 3-68 the best efficiency of the wind turbines, i.e. the power

coefficient c

P.max

, drawn from the power curves measured for type approval, a

wide spreading of the values can be observed. The reason for this is the broad

operating range of wind turbines. Hence, the manufacturers optimize the entire

machine for a wide range of relatively high efficiency. This is needed especially

for inland sites in order to maximize the annual energy yield: on one hand the very

frequent weak winds have to be harvested efficiently, but on the other a good effi-

ciency should also bring a high yield share from rarer strong winds (P~v³!).

The final consideration, Fig. 3-69 discusses available commercial wind turbines

concerning the two classical concepts of power limitation: stall and pitch. It shows

a clear tendency to depart from a domination of stall-controlled turbines (without

blade pitching) until the mid 1990s to a preference for pitch-controlled wind tur-

bines evident in today’s MW class turbines. In Germany, hardly any stall wind

turbines have been recently installed, mainly due to strict grid codes, cf. chapter

14. In the first half of 2005, e.g. only pitch-controlled wind turbines were erected,

of which 10% had an active stall control.

But in other countries with a different market situation and more restricted

transport and erection conditions, there is a high demand for the robust and well-

proven stall turbines. Even the company ENERCON, a classical manufacturer of

gearless pitch-controlled wind turbines, now tests a 20 kW prototype with power

limitation by stall. And also the 5 kW wind turbine Aerosmart 5, Fig. 3-57 left, re-

cently developed for stand-alone systems and developing countries, has a down-

wind rotor with stall control.

3 Wind turbines - design and components

111

0,1

0,2

0,3

0,4

0,5

0,6

0 500 1000 1500 2000 2500

Nennleistung in kW

Bestwirkungsgrad c

P.max

0,6

0,5

0,4

0,3

0,2

0,1

Maximum power coefficient c

P.max

0 500 1000 1500 2000 2500

Rated power in kW

0,1

0,2

0,3

0,4

0,5

0,6

0 500 1000 1500 2000 2500

Nennleistung in kW

Bestwirkungsgrad c

P.max

0,6

0,5

0,4

0,3

0,2

0,1

Maximum power coefficient c

P.max

0 500 1000 1500 2000 2500

Rated power in kW

Fig. 3-68 Maximum power coefficient c

P.max

of a wind turbine versus rated power (from power

curve measurements)

Number of available W. T. types

1991 1993 1995 1997 1999 2001 2003 2005

Rated power > 150 kW

Pitch

Stall

Number of available W. T. types

1991 1993 1995 1997 1999 2001 2003 2005

Rated power > 150 kW

Pitch

Stall

Fig. 3-69 Number of available stall and pitch wind turbine types on the German market [27]