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

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3.6 Characteristic wind turbine data

112

References

[1] The editors are grateful for the photos and figures provided by wind turbine manufac-

turers and suppliers.

[2] Körber F., Besel G. (MAN), Reinhold H. (HEW): Messprogramm an der 3MW-

Windkraftanlage GROWIAN (Test program on the 3 MW wind turbine GROWIAN),

Report on the research project 03E-4512-A of the German Federal Ministry of Tech-

nology, München, Hamburg 1988

[3] According to information of Sydkraft; Malmö (S) 1991

[4] N.N.: What we have learnt about wind power at Maglarp and Näsudden, Statens ener-

gieverk, Stockholm (S) 1990

[5] Wachsmuth R. (MBB): Rotorblatt in Faserverbundbauweise für Windkraftanlage

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energy 1990 of the German Federal Ministry of Technology, KFA Jülich (Ed.) 1990

[6] Hau E.: Windkraftanlagen - Grundlagen, Technik, Einsatz, Wirtschaftlichkeit (Wind

power plants – Basics, technology, application and economics), Springer-Verlag,

Berlin, Heidelberg, New York

1988 / 2003

[7] Maurer J.: Wind turbinen mit Schlaggelenkrotoren, Baugrenzen und dynamisches

Verhalten (Wind turbines with a flapping hinge rotor, design limits and dynamic

characteristics), PhD thesis, TU Berlin; VDI Reports, series 11, No. 173, Düsseldorf

1992

[8] Wortmann F.X. (Institut für Aerodynamik und Gasdynamik der Uni Stuttgart): Neue

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[9] Person M.: Zur Dynamik von Wind turbinen mit Gelenkflügeln - Stabilität und

erzwungene Schwingungen von Ein- und Mehrflüglern (On the dynamics of wind

turbines with flapping hinge blades – stability and forced vibrations of one and multi-

bladed turbines), PhD thesis at the Institut für Luft- und Raumfahrt of TU Berlin, VDI

Fortschritt-Berichte (Progress reports) Series 11, No. 104, Düsseldorf 1988

[10] Franke, J.B.: Erarbeitung eines Konzeptes zur Berechnung der Lebensdauer von

Getriebeverzahnungen unter Berücksichtigung der Betriebszustände von

Windenergieanlagen (A concept for the lifetime calculation of the gear tooth system

considering the operating conditions in wind turbines), TU Berlin/Germanischer

Lloyd WindEnergie GmbH, 2004

[11] DUBBEL, Beitz W. und Küttner K.-H.: Taschenbuch für den Ingenieur (Handbook of

Mechanical engineering), Springer-Verlag Berlin, Heidelberg, New York, 2002

[12] Deutsches Institut für Normung: DIN 3990, Tragfähigkeitsberechnung von Stirnrädern

(Calculation of load capacity of spur gears), Beuth-Verlag, Berlin, 1987

[13] International Organization of Standardisation (ISO): Calculation of load capacity of

spur and helical gears, ISO 6336, Genf, 1996

[14] Boiger, P.: Die Aerogear-Baureihe soll für mehr Sicherheit und Ruhe bei Getrieben

sorgen (The Aerogear product line will improve safety and reduce noise of gearboxes),

Windkraft Journal, 2002

[15] Germanischer Lloyd WindEnergie GmbH: Richtlinien für die Zertifizierung von

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1993 to 2003

[16] Haibach, E.: Betriebsfestigkeit, Verfahren und Daten zur Bauteilberechnung (Fatigue

analysis, procedure and data for component desig ), VDI-Verlag, 1989

[17] Schlecht, B., Schulze, T., Demtröder, J.: Modelle zur Triebstrangsimulation von Multi-

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3 Wind turbines - design and components

113

wind turbines), Tagungsband Dresdener Maschinenelemente Kolloquium 2003, S.

351-362, Verlagsgruppe Mainz, Aachen, 2003

[18] Thörnblad, P.: Gears for Wind Power Plants, 2nd Int. Symposium on Wind Energy

Systems, Amsterdam, 1978

[19] N.N.: Betriebsanleitung für die Allgaier Windkraftanlage (Operating manual for the

Allgaier wind turbine) System Dr. Hütter, Type WE 10/G6, Uhingen

[20] Institut für Solare Energieversorgungstechnik e. V. (ISET): EU-Projekt NEW

ICETOOL (deutsches Teilprojekt), http://www.iset.uni-kassel.de/icetool/

[21] BINE Informationsdienst: Blitzschutz für Windenergieanlagen (Lightning protection

for wind turbines), Projektinfo 12/00, Fachinformationszentrum Karlsruhe,

http://bine.info

[22] Fördergesellschaft Windenergie e.V. und Bundesministerium für Wirtschaft und

Technologie (Hrsg.): Blitzschutz von Windenergieanlagen(Lightning protection for

wind turbines), Abschlussbericht, BMWi-Forschungsvorhaben 0329732, Juli 2000

[23] Deutsches Windenergie-Institut (DEWI): Studie zur aktuellen Kostensituation 2002

der Windenergienutzung in Deutschland (Study on the costs of wind energy utilization

in 2002 ), Wilhelmshaven 2002

[24] Germanischer Lloyd WindEnergie GmbH: Richtlinie für die Zertifizierung von

Condition Monitoring Systemen für Windenergieanlagen (Guideline for the

Certification of Condition Monitoring Systems for Wind Turbines), Edition 2003

[25] Heilmann, C., Liersch, J., Melsheimer, M.: Rotorunwuchten sind vermeidbar (Rotor

unbalance is avoidable), Sonne, Wind und Wärme 4/2006

[26] Huß G. (MAN): Modifizierung des Anlagenkonzepts WKA-60 im Hinblick auf eine

Leistungssteigerung, Landaufstellung und eine Verbesserung der Wirtschaftlichkeit,

(Report on the concept modifications of the WKA-60 wind turbine concerning

performance improvement, onshore installation and improvement of cost-

effectiveness) Report on the research project 032-8824-A in the status report wind

energy 1990 of the German Federal Ministry of Technology, KFA Jülich (Ed.) 1990

[27] Bundesverband WindEnergie e.V., BWE Service GmbH (Hrsg.): Windenergie 2005

Marktübersicht (Wind energy market 2005), and previous editions starting 1996

[28] Erneuerbare-Energien-Gesetz (EEG) der Bundesregierung (German Renewable

Energy Sources Act (EEG)), 2000, 2003, 2004 http://www.erneuerbare-energien.de/

[29] Körber, F.: Baureife Unterlagen für GROWIAN (Documentation for the construction

of GROWIAN), Final report on the research project of the German Federal Ministry of

Technology, München, 1979

R. Gasch and J. Twele (eds.), Wind Power Plants: Fundamentals, Design, Construction 114

and Operation, DOI 10.1007/978-3-642-22938-1_4, © Springer-Verlag Berlin Heidelberg 2012

4 The wind

4.1 Origins of the wind

4.1.1 Global wind systems

The global atmosphere can be considered as a thermal engine in which the air

masses are transported due to different thermal potentials. This thermal engine is

since it exists in the atmosphere in all the three states: vapour, droplets and ice. So,

its latent heat when changing the state of aggregate from one phase to another is

the dominant influence on the weather. The earth is a sphere, so getting from the

equator closer to the poles the total irradiation by the sun declines more and more.

Consequently, there is excess energy in the atmosphere in the equatorial zones and

a deficit in the area of the poles. To equalize this imbalance, heat is transported by

the air flow from the equator to the southern and northern hemisphere. This is

done by the air mass exchange of the global wind systems.

Additionally, the Coriolis force from the rotation of the earth deflects the flow

direction of the global winds. The global wind systems are shown in Fig. 4-1.

In each hemisphere there are three different zones: the tropical latitudes, the

mid-latitudes and the polar latitudes. The tropical latitudes (Inter-Tropical Con-

vergence Zone, ITCZ) on each side of the equator are between the low-pressure

belt (tropical doldrums belt), and the subtropical high-pressure belt. This region is

also called the Horse Latitudes and is found along the 30° latitude. Within this re-

gion forms the Hadley circulation (also Hadley cell): hot tropical air rises at the

equator and flows in the higher atmospheric layers towards the poles while it is

deflected more and more to the east due to the rotation of the earth. Around 30°

latitude the air descends and flows back to the equator in the lower atmospheric

layers causing the Trade Winds. The two Trade Wind systems meet in the ITCZ,

closing the loop of the Hadley circulation.

The mid-latitudes (i.e. the Ferrel cell) are limited in the direction of the poles

by the sub-polar through which is found along 60° latitude. Within the mid-

latitudes the cyclonic western winds (the prevailing Westerlies) dominate. The

polar zone is governed by the widely stretching low-pressure area situated above

the polar region, with mainly eastward air flows.

4 The wind

115

Fig. 4-1 Global wind circulation, black arrows: flow near the ground [2]

4.1.2 Geostrophic Wind

High above the ground, the friction of the ground has no influence on the air flow.

This undisturbed flow is called the geostrophic wind; the pressure gradient force

and the Coriolis force are in balance, Fig. 4-2. The gradient force results from the

air pressure differences and points always towards the low-pressure region. If due

to the pressure difference air mass moves, e.g., northwards along a Meridian, it is

deflected by the Coriolis force to the right. If it moves from north to south, it is de-

flected to the left. So a balance between the two forces is found only if the

geostrophic wind moves along the lines of constant pressure, the isobars. Hence,

on the Northern hemisphere the low-pressure region is found always to the left of

the geostrophic wind and the high-pressure to its right. On the southern hemi-

sphere it works the other way round.

4.1 Origins of the wind

116

Low pressure area

1000

1050

1100 hPa

Fig. 4-2 Geostrophic wind G and isobars (on northern hemisphere) [2]

The geostrophic wind can be calculated from the pressure gradient above a region

and is often equivalent to the wind above the boundary layer measured by

radiosondes. More details are found in the literature, e.g. [5].

4.1.3 Local wind systems

Apart from the global winds there are also local wind systems which origin from

potential differences. Again temperature differences are the driving cause. The

most important of these local wind systems are the sea-land breeze and the moun-

tain-valley breeze.

Sea-land circulation

The sea-land breeze is a diurnal wind system at the sea shore emerging under

sunny weather conditions with a high temperature difference between the land and

the water. In a weaker form it also occurs at the shore of bigger inland lakes. Dur-

ing the day, the onshore ground heats up stronger than the water. Correspondingly,

the air above shows a temperature difference causing a pressure gradient from the

sea to the land. The resulting “sea breeze” is the flow of cool, humid air to the

shore, Fig. 4-3. High above the ground, there is a return flow from the land to the

sea. In the late afternoon, the land is cooling down faster than the sea, so the flow

conditions reverse. This causes the seawards pointing land breeze. Due to the fric-

tion at the ground it is weaker than the sea breeze.

4 The wind

117

Mountain-valley circulation

In the mountains, there are also thermal circulations from the superposition of two

flow systems: the slope winds and the mountain-valley winds. High-pressure con-

ditions with a high irradiation are required to form an intense circulation as long as

there is no disturbing influence from the large-scale wind flow. After sunrise the

slopes of the mountain are strongly heated by the sun, so are the air masses closely

above it. Its thermal lift produces the rising slope wind. Later in course of the

morning this flow is followed by the valley wind, an upwards flow through

the valley. This flow replaces from below the air rising on the slopes. At night it is

the other way round: due to the accelerated cooling of the ground, the air close

to the slopes cools down faster than the air in free atmosphere at the same altitude.

The gravitational force drives the falling downhill winds which meet in the valley

and cause the mountain winds directed to the valley’s exit, Fig. 4-4.

Fig. 4-3 Scheme of the sea-land breeze, example: day [2]

Fig. 4-4 Mountain-valley breeze (white: slope winds, black: valley wind). Left: valley wind at

noon; right: mountain wind at night [10]

4.2 Atmospheric boundary layer

118

4.2 Atmospheric boundary layer

The lowest layer of the atmosphere is a turbulent layer called the atmospheric

boundary layer. The air flow in this layer is influenced by the friction at the

ground, the orography, the topography and as well by the vertical distribution of

temperature and pressure. The geostrophic winds above the boundary layer are not

influenced by the friction at the ground.

Thus, between this undisturbed geostrophic winds and the ground there is the

layer where the wind speed is varying strongly with the height above ground. The

friction at the surface roughness takes energy from the air flow causing a vertical

gradient of the wind speed. This leads to a turbulent exchange of momentum and

air masses with the air flowing in higher layers above the ground.

Fig. 4-5 Scheme of the atmospheric boundary layer

Wind turbines have to operate within this atmospheric boundary layer. The prop-

erties and the intensity of this air flow determine the amount of extractable energy

and also the loads on the wind turbines. The height of the atmospheric boundary

layer varies depending on the roughness of the ground, the vertical temperature

profile and wind speed. In a clear night it may extend only approx. 100 m above

the ground. On a warm summer day with low wind speeds it may reach even up to

2,000 m. The assumed mean height of the atmospheric boundary layer is around

1,000 m.

4 The wind

119

4.2.1 Surface boundary layer

The air mass flowing directly above the ground is called surface boundary layer

(or Prandtl layer). Its height is often given as a fixed value of approx. 10% of the

atmospheric boundary layer’s height. In reality, it varies e.g. depending on the ver-

tical temperature profile.

The variation of the wind speed with the height above ground is called the ver-

tical wind profile (also called wind shear or height profile). Knowing its shape is

very important for the determination of the energy yield of a wind turbine. More-

over, the vertical wind profile produces particular loads on the rotor and the entire

structure of the wind turbine. It is influenced by the surface roughness, the nature

of the terrain, the topography and also the vertical temperature profile. This ther-

mal stratification is characterized by the three states of atmospheric stability: sta-

ble, neutral and unstable, Fig. 4-6.

If the thermal stratification of the atmosphere is unstable, the air close to the

ground is warmer than the air above it. This situation is typical for the summer

time when the sun heats up strongly the ground. The air close to the ground heat

up as well, so its density reduces and it rises up. This causes a strong vertical mass

transfer with increased turbulence. The stronger vertical mixing under unstable

atmospheric conditions leads to a small gradient of the wind speed with increasing

height above ground.

If the thermal stratification is stable, the temperature of the air close to the

ground is smaller than in the layers above it. This situation is typical for the winter

time when the ground cooled down strongly and correspondingly the air density

close to the ground is higher than in the higher layers. This stable equilibrium

leads to a small vertical mass transfer, turbulence is suppressed. So there is a

strong gradient of the wind profile since the

vertical differences in the (horizontal)

wind speed are not equalized. In a stable atmosphere occur under certain circum-

stances also significant changes of the wind direction with increasing height.

If the atmosphere is neutral (neutrally stable) the surface boundary layer is nei-

ther heated up nor a cooled down resulting in an adiabatic temperature profile. The

air temperature decreases by approx. 1°C per 100 m height. This situation often

occurs at high wind speeds. In this case, the vertical wind profile depends no

longer on the thermal mixing but only on the surface friction.

The influence of the temperature on the vertical wind profile is shown in Fig.

4-10.

4.2 Atmospheric boundary layer

120

Fig. 4-6 Vertical temperature profiles: Change of temperature T with height z

4.2.2 Vertical wind profile

For the wind in a neutrally stable atmosphere above an idealised absolutely flat

and unlimited landscape with a uniform roughness length there are analytic

descriptions of the increase of the wind speed v (symbol V in the IEC standards)

with the height z in the surface boundary layer.

Sometimes (e.g. also in [20]), the mean vertical wind profile is described by the

Hellmann power law:

)(

, (4.1)

where v(z

) resp. v(z

) is the wind speed in the height z

resp. z

The power law exponent (also called empirical wind shear exponent) is

= 0.14 for normal conditions [1]. But it varies with the height z, the roughness,

the atmospheric stratification and the nature of the terrain and orography. There-

fore, a measured power law exponent is valid only for the individual site and the

used measuring heights z

and z

. Its application to other heights is not permitted.

Hence, the application of equation (4.1) is limited.

4 The wind

121

A description of the mean vertical wind profile for a neutrally stable atmosphere,

based on the boundary layer physics of Prandtl, is the logarithmic wind profile

which considers also the surface roughness:

ln)(

. (4.2)

The logarithmic wind profile depends on several parameters: the shear stress ve-

locity u

, the height z above ground, the roughness length z

and the Kármán con-

stant which is usually assumed to be

| 0.4.

For a known roughness length at a site the application of equation (4.2) to two

different heights gives the following very useful equation

 



1122

zvzv

. (4.3)

Having measured a wind speed v

in the height z

(measuring mast) this equation

allows to calculate the wind speed v

in the height z

(e.g. hub height) if the

roughness length z

is given, table 4.1 shows typical values. The roughness length

is a measure for the surface structure; the influence of z

on the vertical wind

profile is shown in Fig. 4-7.

Table 4.1 Roughness length z

for different types of terrain

Type of terrain z

in m

Calm water 0.0001 – 0.001

Farm land 0.03

Heather with few bushes and trees 0.1

forest 0.3 – 1.6

suburb, flat building 1.5

City centers 2.0

Having erected a measuring mast which records the wind speed at several heights

of the wind profile, the roughness length z

may be determined for the close area

around it. Assuming two measured wind speeds v

) and v

), and using

ln (z

/ z

)= ln z

- ln z

the transformation of equation (4.3) gives

lnlnlnln zz



. (4.4)