Assessing Turbulence IntensityAssessing Turbulence IntensityAssessing Turbulence Intensityand its Impact on the Structural Integrity of Wind TurbinesPO 174PO 174 and its Impact on the Structural Integrity of Wind TurbinesPO 174PO 174 and its Impact on the Structural Integrity of Wind TurbinesPO. 174PO. 174

Thomas Hahm Silva Mäusling Steffen WussowThomas Hahm, Silva Mäusling, Steffen WussowF2E Fl id & E E i i G bH & C KG H b G f2 dF2E Fluid & Energy Engineering GmbH & Co. KG, Hamburg, Germany, www.f2e.degy g g , g, y,

Abstract MethodsAbstract MethodsThe international IEC standard 61400-1 /1 2/ defines the design requirements for wind turbines and The assessment of turbulence intensity requires two steps which are the evaluation of ambientThe international IEC standard 61400 1 /1, 2/ defines the design requirements for wind turbines andclassifies different wind turbine classes in terms of reference wind speed and turbulence intensity The

The assessment of turbulence intensity requires two steps which are the evaluation of ambientturbulence intensity at the site and of additional wake generated turbulence intensity Both of theseclassifies different wind turbine classes in terms of reference wind speed and turbulence intensity. The

i t ti f th l i t t li ti b t th d t i i t titurbulence intensity at the site and of additional wake generated turbulence intensity. Both of theset b f d ith th l l ti t l “ k 2 ”intention of these classes is to cover most applications but they do not give a precise representation steps can be performed with the calculation tool “wake2e”.

of any specific site or even a specific wind farm layout, where the individual wind turbine is subject toof any specific site or even a specific wind farm layout, where the individual wind turbine is subject tothe influence of multiple wakes from upwind machines Therefore the suitability of a wind turbine in a If no or only incomplete data of ambient turbulence intensity is available, it can be calculated on thethe influence of multiple wakes from upwind machines. Therefore the suitability of a wind turbine in a

i d f h t b dy p y

basis of surface roughness or land cover data For the latter “wake2e” provides access to thewind farm has to be assessed. basis of surface roughness or land cover data. For the latter wake2e provides access to theCORINE data on land cover /4/

I l l d i d f ll i l t i th d i t t b l i t itCORINE data on land cover /4/.

In closely spaced wind farms as well as in complex terrain the design parameter turbulence intensity If the CORINE data is used, terrain sectors up to ahas become more and more important and in many cases even the crucial parameter. There is a

, pdistance of 25 km from every wind turbine will behas become more and more important and in many cases even the crucial parameter. There is a

need for reliable predictions of turbulence and its impact on the structural integrity of the wind turbinedistance of 25 km from every wind turbine will beincluded automatically Subsequently roughnessneed for reliable predictions of turbulence and its impact on the structural integrity of the wind turbine

d i it lif tiincluded automatically. Subsequently, roughness

during its life time. categories are assigned to the individual terrain

Thi t t th l l ti t l “ k 2 ” hi h i bl t i d t bi i

g gsectors. On the basis of this roughnessThis poster presents the calculation tool “wake2e” which is capable to assess wind turbines in a sectors. On the basis of this roughnessclassification “wake2e” determines the ambientspecific wind farm layout and at a specific site with respect to turbulence intensity. classification wake2e determines the ambientt b l i t iti t th it th b i f thspecific wind farm layout and at a specific site with respect to turbulence intensity. turbulence intensities at the site on the basis of theVDI and DIN standards /5, 6/. The ambient

Bases and Objectives,

turbulence intensities are ascertained for theBases and Objectives turbulence intensities are ascertained for thei di ti d i d d t h b h i htvarious directions and wind speeds at hub height.

The wake of a wind turbine is characterized by high velocity gradients enlarged turbulence and a The course of the wind speed thereby follows theThe wake of a wind turbine is characterized by high velocity gradients, enlarged turbulence and ad d i E i ll i th k i b l i t l 3 5 t di t

p yNormal Turbulence Model (NTM) of IEC /1/Current coverage of the raster data on land coverrandom meandering. Especially in the near wake region below approximately 3–5 rotor diameters Normal Turbulence Model (NTM) of IEC /1/Current coverage of the raster data on land cover

for the CLC2006 inventorywhere a sharply defined wake is still present the loads on downwind turbines can be severe. For

for the CLC2006 inventory

Th l t i t d t i f llp y p

more than 10 years we apply three-dimensional Computational Fluid Dynamics (CFD) to simulate the The complete input data is as follows:more than 10 years we apply three-dimensional Computational Fluid Dynamics (CFD) to simulate theturbulent flow behind a wind turbine These simulations could be validated with high resolutionturbulent flow behind a wind turbine. These simulations could be validated with high resolution Ambient turbulence intensity (if not based on CORINE data):measurements /3/ and give insight into the wind conditions a turbine experiences inside a wind farm.

y ( )g g p

Measurements or data of surface roughnesseasu e e ts o data o su ace oug ess

Frequency distribution of wind speed and wind directions:Frequency distribution of wind speed and wind directions:

Calculated or measured valuesCalculated or measured values

Wind farm layout and information about curtailments and shut offsWind farm layout and information about curtailments and shut-offs

Elevation data for orographically complex terrainElevation data for orographically complex terrain

Th l l ti f ff ti t b l i t it i b d th d d ib d i /7 8/ ThThe calculation of effective turbulence intensity is based on methods described in /7, 8/. Thecalculation of turbulence intensity additionally produced in the wake depends not only on distance buty y p p yon ambient turbulence intensity as well as on thrust coefficient and tip speed ratio of the wind turbineon ambient turbulence intensity as well as on thrust coefficient and tip speed ratio of the wind turbine.Specific modes of the ind t rbines like noise damped or po er red ced operating modes can th sSpecific modes of the wind turbines like noise damped or power reduced operating modes can thus

CFD simulation of the airflow around a WT of type ENERCON E-66 be included into the calculation and their effect on load reduction can be studied.ypContours of velocity magnitude (white = low wind speed, blue = high wind speed) in a horizontal cut at hub height of the WT

For a day to day assessment of the suitability of a wind turbine at a site in a wind farm the effectsFor a day to day assessment of the suitability of a wind turbine at a site in a wind farm the effectsfrom m ltiple akes are transferred into a single parameter the effecti e t rb lence intensitfrom multiple wakes are transferred into a single parameter – the effective turbulence intensity.The effective turbulence intensity is a substitute value to be applied for the whole lifespan of the windy pp pturbine It rates the load by the ambient turbulence intensity and the additional load induced by theturbine. It rates the load by the ambient turbulence intensity and the additional load induced by thewake situationwake situation.

Th t l h t th t t l i t it f i d t bi t itThere are two general approaches to assess the structural integrity of a wind turbine at a site:

D t t th t th it diti th th d f th d i f thDemonstrate that the site conditions are no more severe than those assumed for the design of thewind turbinewind turbine

Demonstrate the structural integrity for condition equal or more severe than those at the siteDemonstrate the structural integrity for condition equal or more severe than those at the site

Both of these approaches require effective turbulence intensities The following figure shows the roleBoth of these approaches require effective turbulence intensities. The following figure shows the rolef ff ti t b l i t it i th f i th t t l i t it f i d t biof effective turbulence intensity in the process of assessing the structural integrity of a wind turbine

within a wind farm.

Flow chart for the assessment process.

R lt & C l iResults & ConclusionsTh ff ti t b l i t it i l l t d di t th t t f i tifi d t h i lThe effective turbulence intensity is calculated according to the state of scientific and technicalknowledge. However, a direct validation of the results is not possible as the effective turbulenceg , pintensity is a substitute value to be applied for the whole lifespan of the wind turbine and notintensity is a substitute value to be applied for the whole lifespan of the wind turbine and notaccessible by measurement Therefore results have been compared to load calculations usingaccessible by measurement. Therefore results have been compared to load calculations usingcommon approaches for wake deficit and turbulence modeling. These calculations assumed that thepp gwind turbine operates 100% of its lifetime under half-wake conditions i e half of the rotor is exposedwind turbine operates 100% of its lifetime under half wake conditions, i.e. half of the rotor is exposedto the wake and the other half to the undisturbed wind field which should cover any loading the windto the wake and the other half to the undisturbed wind field, which should cover any loading the windt bi i i l i d f Th lt h th t d th diti l di tturbine experiences in a real wind farm. The result show, that under these conditions closer distanceswill be predicted compared to the approach used in „wake2e“.will be predicted compared to the approach used in „wake2e .

The methods to calculate ambient turbulence intensity on the basis of land cover or roughness dataThe methods to calculate ambient turbulence intensity on the basis of land cover or roughness datahave been validated in several projects in the north of Germany and show good agreement withhave been validated in several projects in the north of Germany and show good agreement withmeasurements. Few comparisons have been made outside this area. This results indicate that thepuse of turbulence structure correction parameters in accordance with /2/ is inevitable in complexuse of turbulence structure correction parameters in accordance with /2/ is inevitable in complexterrain Otherwise the results can under predict ambient turbulence especially at high wind speeds Interrain. Otherwise the results can under predict ambient turbulence especially at high wind speeds. Inthi t t it b t t t th d f f t i t f ththis context it may be necessary to treat the edges of forests in some cases as an escarpment of theterrain. Automatically generation of turbulence structure correction parameters using satellite data ofte a uto at ca y ge e at o o tu bu e ce st uctu e co ect o pa a ete s us g sate te data oterrain elevation will be implemented into “wake2e” It should be noted that in some cases of complex

Process of assessing the structural integrity of a wind turbine and the role of effective turbulence intensityterrain elevation will be implemented into wake2e . It should be noted that in some cases of complext i i ll t hill t iti th lt id bl b ti t dProcess of assessing the structural integrity of a wind turbine and the role of effective turbulence intensity. terrain especially at hill top positions the results may considerably be over estimated.

fReferencesReferences5 VDI 3783 Blatt 12; Umweltmeteorologie Physikalische Modellierung von Strömungs und Ausbreitungsvorgängen5. VDI 3783 Blatt 12; Umweltmeteorologie - Physikalische Modellierung von Strömungs- und Ausbreitungsvorgängen

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in der atmosphärischen Grenzschicht; 1999., g q ; ,

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EWEA 2011 Brussels Belgium: Europe’s Premier Wind Energy EventEWEA 2011, Brussels, Belgium: Europe s Premier Wind Energy Event