improving accuracy of nacelle-anemometer-based site calibration

1

IMPROVING ACCURACY OFNACELLEANEMOMETERBASED SITE CALIBRATION

THROUGH CFD MODELLINGAndrea Vignaroli

Windsim User MeetingTønsberg, 17/06/2008

Hello Everyone! Someone of you recognize me, but I will introduce myself forthe others: my name is Andrea Vignaroli and i work at the wind energy team ofthe technical research centre of Finland or Vee Tee Tee as you prefer .Unfortunately I couldn’t join you, but, men … we use computers to solve navierstokes equations, so … why not to use it to virtually participate at windsimusermeeting 2008. So, here I am!I will show you very shortly what I did togetherwith some colleagues during the last months. I will refer only to the methodbecause results are confidential, but somehow it’s good not to go into details asi’m not there with you to answer any questions. So, let’s start… and see you atthe conclusions.

2

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

2

WHAT

•Part of project between VTT, Harakosan Europe (turbine manufacturer)and Halfmex (windfarm owner)

•Power curve measurement of Harakosan Z72 WTG (2MW), turbine n.2at Högsåra windfarm

•Site calibration is required for the Högsåra site according to the IECstandard 61400121 requirements of terrain variation

•The site calibration procedure in Högsåra deviates from the proceduredescribed in the standard 61400121

Guess Why? ;)

In the first month of 2008 we had a project regarding a site calibration inHogsara, an island in the southern finland archipelago. The site calibration isrequired for the Högsåra site according to the IEC 61400121 requirements ofterrain variation but it deviates from the procedure described in the standard.Guess Why?

3


3

WHERE

Yes! Good guess! The turbine is already there. Actually the windfarm is alreadythere. 3 Harakosan Z72 with a nominal power of 2 MW. The photo shows a viewfrom the mast to approximately 0 degree. The one in the photo is the wind turbineunder the power performance test and for which the site calibration is performedThe measurement sectors according to the standards, placements of the turbinesand meteorological mast can be seen in the figure on the right. The sector from240° to 280° was selected as a final measurement sector, because therequirements of minimum amount of data are fulfilled and therefore theproduction losses due to the site calibration measurements are minimized

4


4

HOW Measurements

Here´s the anemometer equipped boom we installed. The turbine is programmed to bealways 90° to the wind during the site calibation measurements

+

To get the wind speed measurements as close to the hub height as possible andkeeping the turbine capable to production the site calibration anemometer ismounted on the roof of nacelle with boom pointing backwards. This is tominimize nacelle and blades effect to the wind speed. The turbine is activelyyawed perpendicular to the wind direction with blades pitched at 0°. The turbineis remote controlled in order to choose easily when measure or when produceenergy accordingly to the wind conditions. The wind speed is recorded by a ThiesFirst Class anemometer, the same mounted on the 73m lattice mast in the centreof the windfarm.

5


5

HOW ModellingWind & Turbulence

Profiles

WINDSIM FLUENT

Then we used CFD to investigate the influence of the nacelle and blades on thewindflow at anemometer position. Windsim was used to calculate speed andturbulence profile for a wind blowing form west in order to have boundaryconditions for a microscale simulation to be performed with fluent

6


6

HOW –Modelling –Some Windsim 4.7 settings

Forest Height 15m

Forest Porosity 0.3

C1 0.002

C2 0.003

Forest cells along Z 2

Direction 270°

Height of Boundary Layer 480 m

Speed ABL 7.9 m/s

Turbulence model Standard ke

As you have seen on the third slide, the surroundings of the turbine andmeteorological mast are covered by forest. So, it was the perfect situation to testthe forest model in windsim. You can see the settings i used for it , while theother settings in the terrain module are quite standards. I ran the windfieldmodule, as i said, only for one direction and for one wind speed and here you cansee my settings for it. I assumed that all the variations given by different windconditions are inside the uncertainty level estimated for the whole site calibrationprocess.

7


7

Validation ofWindsim

simulation atmast position

The results given by this simulation were compared to the measured valuescollected before the three turbines were installed. Both average wind speedprofile and a vertical profile of turbulence intensity for a wind coming from270º±15º were compared. The modelled results extracted at mast position showvery small deviations from the measurements especially the ones that come fromthe top anemometer. How did i reach it? Using what is scientifically called “amanual iteration process”. Maybe in the future this can be automatized inwindsim.

8


8

HOW –Modelling –FLUENTSpeed, TKE, EP profilesextracted at T2 position

Inlet boundary conditions

A 3D CAD detailed model of the Harakosan Z72 wind turbine was realized basedon technical specifications provided by the manufacturer. A cubic volume wasdesigned and meshed around the turbine. The geometry of the model was chosenin order to reproduce an atmospheric flow and avoid as much as possibleblockage effect on the solution. Then wind speed vertical profile , turbulentkinetic energy profile and turbulent dissipation rate profile were extracted inwindsim at turbine position every 10 m up to 360 m and and applied in fluent asinlet conditions. An aerodynamic roughness length of 0.5 m was applied to theground wall. Lateral and top boundaries were treated as frictionless wall. Outletwas treated as outflow. No roughness was applied to the surface of the obstaclebecause the first node near the surface is too far for a good description of theboundary layer in the interaction flowsurface.For this reason the simulations took into account only the effects in the flow fieldgiven by the shape of the obstacle. The 3D model of the turbine was positioned inthe same conditions supposed to happen during the site calibration. The bladesare pitched to standstill position and the yaw is turned 90º to the wind. Onedeviation is that in the reality the rotor is slowly turning because the activation ofthe rotor brake was not possible during the measurements . In order to take intoaccount this effect three simulations were performed with different rotor position.

9


9

POSTPROCESSING

•FlowCorrectionFactor = MeasuredSpeedUp –NacelleSpeedUp•Flow vertical inclination at nacelleanemomter

•Uncertainties(based on RisoeR1209 document)

•Reliability assessment of theentire modelling process throughmeasured data

After that it was finally possible to collect all the information about nacelle andblades influence and in the meanwhile to return back to the method described inIEC61400 standard.Then an analysis to assess the reliability of the simulations was done. The aim ofit was to show that the simulations performed are representative of the differentwind conditions that can be encountered during the acquisition of valid data forsite calibration purpose and that the results achieved can be applied to themeasurement.In the end let’s talk about uncertainty. Because of insufficient amount of datafrom simulation in order to approach the estimation of uncertainties in CFDsimulations, with a statistical method, the problem of uncertainties wasapproached with an estimation based on the experience and what already exist inliterature . The uncertainties from the normal site calibration procedure and fromthe CFD simulations were combined together using the equation suggested in thestandards and expanding it to take account the uncertainty of modelling. Webelieve that numerical uncertainty can be compared to the uncertainties fromanemometer calibration and data acquisition system.

10


10

CONCLUSIONS

FUTURE PLANS•To investigate how the influence of nacelle and theterrain combine regarding the flow vertical inclination•To investigate the effect of boom vibrations on theresults (error or uncertainty?)•Sensitivity analysis to implement a Monte Carlo basedmethod for CFD uncertainty assessment

•A site calibration after theinstallation of the wind turbinewas performed•CFD modelling for calculatingwind and turbulence profiles andnacelle influence was used•A hard work and a lot ofassumptions were needed

Here i’m again… in conclusion we performed a site calibration after theinstallation of the wind turbine and CFD resulted to be the only way to calculatewind profile, turbulence profile and nacelle influence. The entire project wasdefinitely very challenging, and we put a lot of effort on it. Despite, a lot ofassumptions were needed to reach the final result. Of course improvements arepossible … as you can see from the future plans … That’s all folks… how wasthe sailboat trip? What? No wind? I don’t believe you… . Anyway… i hope youenjoyed this alternative way of sharing with you my last issues concerningwindsim and CFD modelling. i‘m looking forward to be physically present nexttime. Have a nice user meeting conclusion. Moi Moi!

11


11

THANK YOU!

KIITOS!

TAKK!

GRAZIE!

For info and questions:

Documents

improving accuracy of nacelle-anemometer-based site calibration