4 1Site Considerations

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WindEnergy


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,kilometre, the wind is hardly influenced by the

surface of the earth at all. In the lower layers of theatmos here however wind s eeds are affected bthe friction against the surface of the earth. In thewind industry one distinguishes between therou hness of the terrain, the influence from

obstacles , and the influence from the terraincontours, which is also called the orography of thearea. We shall be dealing with orography, when we

investigate so called speed up effects, i.e. tunneleffects and hill effects , later


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oug ness

In general, the more pronounced the

roughness of the earth's surface, the more.

Forests and large cities obviously slow thewind down considerably, while concrete

down a little. Water surfaces are evensmoother than concrete runways, and will

,long grass and shrubs and bushes willslow the wind down considerably.


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Roughness Classes and Roughness LengthsIn the wind industry, people usually refer toroughness classes or roughness lengths,when they evaluate wind conditions in a

landscape. A high roughness class of 3 tore ers o an scapes w many rees an

buildings, while a sea surface is inroughness class 0.

roughness class 0.5. The same applies tothe flat, open landscape to the left which

.The proper definition of roughness classesand roughness lengths may be found in the

. length is really the distance above groundlevel where the wind speed theoreticallyshould be zero.


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This graph was plotted with the wind speedcalculator on the next page. It shows you how windspeeds vary in roughness class 2 (agricultural landwith some houses and sheltering hedgerows with

some 500 m intervals), if we assume that the wind isow ng a m s a a e g o me res.

The fact that the wind profile is twisted towards alower speed as we move closer to ground level, is

.

important when designing wind turbines. If youconsider a wind turbine with a hub height of 40

,notice that the wind is blowing at 9.3 m/s when thetip of the blade is in its uppermost position, and only. .

means that the forces acting on the rotor blade whenit is in its top position are far larger than when it is inits bottom osition.


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Wind Shear Formula

v = v ref ln(z/z0 )/ln(z ref/z 0 )v = wind speed at height z above ground level.

re , . . re . ...the natural logarithm function.

z = height above ground level for the desired velocity, v.=Roughness lengths may be found in the Reference Manual.z ref = reference height, i.e. the height where we know the exact wind speed v ref .In the above exam le assume we know that the wind is blowin at 7.7 m/s at 20 m

height. We wish to know the wind speed at 60 m height. If the roughness length is0.1 m, thenv ref = 7.7z = 60z 0 = 0.1

z ref = 20 hence,v = 7.7 ln(60/0.1) / ln(20/0.1) = 9.2966 m/s

*) = The formula assumes so-called neutral atmospheric stability conditions, i.e.t at t e groun sur ace s ne t er eate nor coo e compare to t e a r

temperature. Further details may be found in the engineering handbook Guidelinesfor Design of Wind Turbines from Risoe National Laboratory and DNV.


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http://www.windpower.org/en/tour/wres/she

ar.htm


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Wind Speed Calculator

Enter your wind speed measurement inan column at the a ro riate hei ht, e. .10 metres. Then click outside the field,click Submit, or use the tab key. Theprogramme will then calculate wind speeds

for other heights. You may plot your resultsin a separate window by clicking on Plot inthe appropriate column. (If the plot windowdisappears, it is probably hidden behindthis window).


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roughness

lengthm

.

0.0002

.

0.0024

.

0.03

.

0.055

.

0.1

.

0.4

.

1.6

150m

140m

130m

120m

110m

m

90m

80m

70m

60m

50m

40m

30m

20m

m

Plot Plot Plot Plot Plot Plot Plot

13.112.11.10.10.9 .68 .413.12.11.110.10.9 .58 .312.12.111.10.10.9.418.112.12.10.10.10.9 .28 .012.11.10.10.10.19 .17 .812.11.10.10.108.97 .612.11.10.10.9 .88.817.412.11.10.10.9 .68 .67 .212.11.10.9 .89 .48.417.012.11.10.9 .69 .28 .16 .712.11.19 .89.4197.86 .411.10.9 .59.118.67 .55 .911.10.9 .18.718.27 .05 .411.110.8 .58 .17 .66 .34 .610.9 .37 .67 .16 .65 .23 .4


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Avera e wind s eeds are often available from meteorolo ical observationsmeasured at a height of 10 metres. Hub heights of modern 600 to 1,500 kW windturbines are usually 40 to 80 metres, however. The spreadsheet will calculateaverage wind speeds at different heights and roughness classes. Just enter a windspeed measured at a certain height for a given roughness class and click theSubmit button.Please note, that the results are not strictly valid if there are obstacles close to thewind turbine (or the point of meteorological measurement) at or above the specifiedhub height. ["close" means anything up to one kilometre].You should also note, that there may be inverse wind shear on hilltops because of

e e ec , .e. e w n spee may ac ua y ec ne w ncreas ng e g ur nga certain height interval above the hilltop. You should consult the European Wind

Atlas mentioned in the bibliography in the Reference Manual for further information.Take a look at the example below the table to make sure you understand how itworks, before you start entering your data. More accurate and extensive roughness

.


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An Exam le

As an example, have a look at the spreadsheet above. We have already entered 10.

declines as you approach ground level. You will also notice that it declines more

rapidly in rough terrain.

wind speed. If you look at the column with roughness class 2, you will see that windspeeds declines 10 per cent going from 100 metres to 50 metres. But the power ofthe wind declines to 0.9 3 = 0.73 i.e. b 27 er cent. From 613 to 447 W/m 2 .

If you compare the wind speeds below 100 m in roughness class 2 with roughnessclass 1, you will notice that for a given height the wind speeds are lower everywherein roughness class 2.If you have a wind turbine in roughness class 2, you may consider whether it isworthwhile to invest 15,000 USD extra to get a 60 metre tower instead of a 50 metre

tower. In the table you can see that it will give you 2.9 per cent more wind, and youcan calculate, that it will give you 9 per cent more wind energy.You can solve this problem once you have learned how the turbine electricityproduction varies with the available wind energy. We will return to that questionw en you ave earne to use t e power ens ty ca cu ator an t e w n energy

economics calculator.Now, try the calculator for yourself.


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WindShearandEscarpments

Do not Include the Altitude of Your Terrainin Wind Shear Calculations

The aerial photograph above shows a good site for wind turbines along a shorelinewith the turbines standin on a cliff which is about 10 m 30 ft. tall. It is a common

mistake to believe that in this case one may add the height of the cliff to the heightof the wind turbine tower to obtain the effective height of the wind turbine, when oneis doin wind s eed calculations, at least when the wind is comin from the sea.This is patently wrong. The cliff in the front of the picture will create turbulence , andbrake the wind even before it reaches the cliff. It is therefore not a good idea to

move the turbines closer to the cliff. That would most likely lower energy output, andcause a lower lifetime for the turbines, due to more tear and wear from theturbulence.If we had the choice, we would much rather have a nicely rounded hill in thedirection facing the sea, rather than the escarpment you see in the picture. In case

of a rounded hill, we might even experience a speed up effect, as we explain laterwhen we get to the page on the hill effect.


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TheRoughnessRose

If we have measured the wind speed exactly at hub height over a long period

exact predictions of energy production. Usually, however, we have torecalculate wind measurements made somewhere else in the area. Inractice that can be done with reat accurac exce t in cases with ver

complex terrain (i.e. very hilly, uneven terrain).Just like we use a wind rose to map the amount of wind energy coming fromdifferent directions, we use a roughness rose to describe the roughness ofthe terrain in different directions from a prospective wind turbine site.Normally, the compass is divided into 12 sectors of 30 degrees each, like in

the picture to the left, but other divisions are possible. In any case, theyshould match our wind rose, of course.For each sector we make an estimate of the roughness of the terrain, usingthe definitions from the Reference Manual section. In principle, we could thenuse the wind speed calculator on the previous page to estimate for each

sector how the average wind speed is changed by the different roughness ofthe terrain.


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Accounting for Roughness Changes Within Each Sector

sector (i.e. roughness class 0) some 400 m from the turbine site,

and 2 kilometres away we have a forested island. If west is an,

change in roughness class from 1 to 0 to 3.This requires more advanced models and software than what we

.

software to manage all our wind and turbine data, so at a futureupdate of this site we'll explain how professional wind calculationsoftware works.Meanwhile, you may look at the Links page to find the link toRisoe's WAsP model and Energy & Environmental Data's

WindPro Windows-based software.


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Accounting for Wind Obstacles

s ex reme y mpor an o accoun orlocal wind obstacles in the prevailing winddirection near the turbine (closer than 700

,

predictions about energy output. We returnto that subject after a couple of pages.


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Wind Turbine Design: Basic Load Considerations

Whether you are building wind turbines orhelicopters, you have to take the strength,

the dynamic behaviour, and the fatiguepropert es o your mater a s an t e ent reassembly into consideration.

windmill, SouthAustralia


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Extreme Loads (Forces)

Wind turbines are built to catch the wind's kinetic (motion) energy. You

rotor blades, like the old "American" windmills you have seen in theWestern movies.Turbines with man blades or ver wide blades i.e. turbines with a versolid rotor, however, will be subject to very large forces, when the windblows at a hurricane speed. (Remember, that the energy content of thewind varies with the third power (the cube) of the wind speed).

Wind turbine manufacturers have to certify that their turbines are built, sothat they can withstand extreme winds which occur, say, during 10minutes once every 50 years.To limit the influence of the extreme winds turbine manufacturerstherefore generally prefer to build turbines with a few, long, narrow

blades.n or er to ma e up or t e narrowness o t e a es ac ng t e w n ,turbine manufacturers prefer to let the turbines rotate relatively quickly.


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Fatigue Loads (Forces)

Wind turbines are subject to fluctuating winds , and hence fluctuating.

wind climate.Components which are subject to repeated bending, such as rotorblades ma eventuall develo cracks which ultimatel ma make thecomponent break. A historical example is the huge German Growianmachine (100 m rotor diameter) which had to be taken out of serviceafter less than three weeks of operation. Metal fatigue is a well known

problem in many industries. Metal is therefore generally not favoured asa material for rotor blades.When designing a wind turbine it is extremely important to calculate inadvance how the different components will vibrate, both individually, andjointly. It is also important to calculate the forces involved in each

bending or stretching of a component.s s t e su ect o structura ynam cs, w ere p ys c sts ave

developed mathematical computer models that analyse the behaviour ofan entire wind turbine.

ese mo e s are use y w n ur ne manu ac urers o es gn e r

machines safely.


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Structural D namics: An Exam le

A 50 metre tall wind turbine tower will have a tendency to swing backand forth, say, every three seconds. The frequency with which the towerosc ates ac an ort s a so nown as t e e gen requency o t etower. The eigenfrequency depends on both the height of the tower, the

thickness of its walls, the type of steel, and the weight of the nacelle andro or.Now, each time a rotor blade passes the wind shade of the tower, therotor will push slightly less against the tower.

the tower each time the tower is in one of its extreme positions, then therotor blade may either dampen or amplify (reinforce) the oscillations of

.The rotor blades themselves are also flexible, and may have a tendencyto vibrate, say, once per second. As you can see, it is very important to

turbine that does not oscillate out of control.*) A very dramatic example of structural dynamic forces at work underinfluence of the wind undam ened torsion oscillations is the famouscrash of the Tacoma Bridge close to Seattle in the United States. Youmay find a short movie clip (700 K) on the disaster on the Internet.


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Small downwind turbine (22 kW).

"coning" away from the tower.


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During World War II the

company F.L. Smidth (now acement machinery maker)

-

three-bladed wind turbines.


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In the 1950s J. Juul

became a pioneer in'

first alternating current(AC) wind turbines at

,

Denmark.


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A car enter, Christian Riisa er,however, built a small 22 kW windturbine (39K, JPEG) in his own back

yard using the Gedser Wind Turbinedesign as a point of departure. Heused inexpensive standardcomponents (e.g. an electric motor asgenerator, and car parts for gear and

mechanical brake) wherever possible.Riisager's turbine became a successw many pr va e ouse o s arounDenmark, and his success gave thepresent day Danish wind turbine

designing their own wind turbines fromaround 1980.


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During World War II the

F.L. Smidth (now a cementmachinery maker) built anumber of two- and three-bladed wind turbines.


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www.windpower.org Danishsite,containslotsofbackgroundincludingniceanimations

Manwell J.F.etalWindEnergyExplained

ISBN0471499722(coverseverythinginlectureandlotsmore)

BurtonT.etal:WindEnergyHandbook,

,

. . . . . .

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