increased energy production by optimisation of yaw...

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Optimisation of yaw control VGB PowerTech 6 l 2014

Author

Kurzfassung

Höhere Erträge von Windkraftanlagen durch Spinneranemometer

Die Windnachführung einer Windkraftanlage kann relativ einfach mit Messung der Windrichtung mithilfe einer Windfahne direkt auf der Gondel hinter dem Rotor gemessen werden. Da das Strömungsfeld auf der Gondel jedoch massiv gestört ist, fällt die Messung sehr unpräzise aus. Aus diesem Grunde ist der Betrieb von Windkraftanlagen mit signifikantem mittleren Windnachführungsfehler sowie großen Variationen um den Mittelwert ein gundsätzliches Problem, das neben Ertragsverlust auch zu erhöhten mechanischen Belastungen führt.In dem vorliegenden Beitrag wird der Produktionsverlust, verursacht durch durchschnittlich beobachtete Windnachführungsfehler, diskutiert und nachgewiesen. Diese Verluste können erheblich sein, ein Nachführungsfehler von z.B. 15° führt zu einer bis zu 6 % reduzierten Produktion.Mit dem Spinneranemometer wird ein robustes und zuverlässiges Instrument zur akkurateren Messung der Windrichtung präsentiert. Dieses Instrument wurde intensiv im Feld getestet, wobei bei zahlreichen Windkraftanlagen die Fehlausrichtung gemessen und korrigiert wurde.Der nächste Schritt in der Entwicklung des Spinneranemometers ist die Dokumentation seiner Fähigkeiten für den Nachweis von Leistungskurven. Dadurch kann der Betreiber der Windkraftanlagen die Leistung jeder Einzelanlage in jeder Windfarm zu jedem Zeitpunkt überprüfen. l

Increased energy production by optimisation of yaw controlJørgen Højstrup

Dr. Jørgen HøjstrupCEO Højstrup Wind EnergyCTO Wind Solutions Aarhus/Denmark

Introduction

Wind turbines must be aligned into the wind for optimum operational results. In a modern, large wind turbine a yaw control system measures the yaw misalignment and keeps the turbine aligned with the ever changing wind direction as perfectly as possible. This poses several challenges. First of all, the measurements of the wind direction is always performed behind the turning rotor on the nacelle of the turbine, where the flow is being highly distorted by the up-stream rotor taking energy out of the wind and the proximity of the nacelle structure. The nacelle deflects the flow such that the measured wind direction is not the correct one, and the very turbulent flow behind the rotor furthermore disturbs the meas-urement. Secondly, the yaw control mech-anism of the turbine has to follow a very wide range of timescales of fluctuations of the wind direction, based on an imperfect measurement, while at the same time try-ing to strike a balance between keeping the yaw misalignment low and not overloading the yaw motors. Because of the less than perfect measurement of wind direction, the yaw control algorithms cannot be op-timised properly, and consequently the turbine will spend a considerable amount of time running with a yaw misalignment and therefore producing less energy than it could otherwise do.Currently there are only two technologies capable of measuring the yaw misalign-ment correctly – a nacelle mounted LIDAR (laser-based remote sensing anemometer) and the spinner anemometer. It will be demonstrated that the spinner anemom-eter has some very obvious advantages re-lated to cost, data quality and robustness. In the following it will be reported on the experience made, on testing the instru-ment for measurements of yaw misalign-ment and power curve improvements.The spinner anemometer of ROMO Wind presents a much more perfect way of meas-uring the yaw misalignment and therefore makes it possible to minimise the average misalignment as well as the large variation around the average values, such that the turbine produces more energy and at the same time experiences less wear and tear (longer lifetime – less maintenance). Fur-thermore the spinner anemometer makes it possible to verify optimisation efforts

by measuring small changes in the power curve of the turbine and potentially meas-ure the absolute power curve as well – at any turbine in your wind farm, at any time, in any terrain.

Effects of yaw misalignment

In literature several examples can be found of measurements of the effects of a yaw misalignment on the power output of a wind turbine at a single wind speed, both full-scale measurements [1] as well as measurements in wind tunnels [2]. How-ever it is much more relevant to quantify the effect of a yaw misalignment on the annual energy production (AEP) of a wind turbine in a given certain wind climate. In order to calculate the reduced power curve caused by a yaw misalignment, a few simplifying assumptions will be made (F i g u r e 1). Looking at the horizontal plane, we assume that the component of the wind which will produce energy is the component perpendicular to the rotor plane. For simplicity we assume a rotor with no tilt. The wind component along the rotor plane is assumed to have no influence on the energy production.Using this simple model, we can now mod-ify the original power curve of the turbine by taking the power corresponding to the

Wind vector

Yaw error

Component that generates energy

Fig. 1. When a wind turbine is operating in a skewed airflow, only the component of the wind vector perpendicular to the ro-tor will produce energy, i.e. the energy production will be decreased relative to a properly aligned turbine.


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VGB PowerTech 6 l 2014 Optimisation of yaw control

perpendicular component and assign the magnitude of the total wind speed vector to this power. This results in a power curve moved towards higher wind speeds, result-ing in a lower energy production, (F i g -u r e 2). The energy production can now be calculated in the conventional manner by applying a wind speed distribution to the power curve and integrating over the whole distribution. Usually for standardi-zation purposes a simple one-parameter Rayleigh distribution (Weibull distribution with shape parameter = 2) will be used for this purpose. The results are shown in Ta b l e 1, where the AEP is calculated for a turbine with no yaw misalignment, and for the same turbine with a 15 degrees misalignment. The results are shown for different annual average wind speeds (6 to 8.5 m/s). The last column shows the ra-tio of the energy produced by the turbine in yaw to the energy produced with no yaw misalignment. The results show very considerable losses of AEP due to yaw misalignment of 6.9 % (6 m/s) to 4.4 % (8.5 m/s). For high annual average wind speeds the loss is relatively smallest be-cause the turbine then spends a larger pro-portion of time at maximum power output, where there is no reduction of power due to yaw misalignment. These calculations can now be repeated for a range of yaw mis-alignment angles and for different turbines.F i g u r e 3 shows the result of such calcu-lations. The coloured curves resulted from

averaging the results from turbines of dif-ferent sizes (850 to 2,000 kW), where very little variation was seen from turbine to turbine. Germanischer Lloyd/Garrad Has-san (now DNV GL) were asked to perform a general technical review of the spinner anemometer, including a check of our cal-culations of energy loss by running a more sophisticated model (the “GH Bladed” model). The DNV GL results [3] are shown as points on Figure 3. And a cosine2 curve is plotted as a convenient reference (fits well for low to moderate wind speed sites). As can be seen from these curves, turbines running with yaw misalignments will pro-duce considerably less energy than they were supposed to. Fortunately average yaw misalignment can be corrected (pro-vided of course that it can be measured correctly).Another serious effect of the yaw mis-alignment is increased fatigue load on the turbine. Sample calculations of the in-creased fatigue loads on the turbine were performed by DNV GL (then GL_GH) on a generic 2 MW turbine, showing increased fatigue loads for a range of yaw misalign-ments [5] for a number of different loads.

An example: 15 degrees yaw misalignment resulted in 110 % fatigue load for a blade root component which can be converted to a decrease in lifetime from 20 years down to 8 years (assuming that 100 % load cor-responds to 20 year lifetime), and for a hub component, 114 % fatigue load, corre-sponding to a decrease in lifetime from 20 years down to 12 years.

The spinner anemometer

There are at present only two technologies capable of measuring the yaw misalign-ment correctly – a nacelle mounted LIDAR and the spinner anemometer. It will be demonstrated that the spinner anemome-ter can do a very good job, and additionally has some very obvious advantages related to cost, data quality and robustness.

The spinner anemometer consists of three single path ultrasonic anemom-eters mounted on the spinner making measurements while the turbine operates (F i g u r e 4). Accelerometers are included such that the correct azimuth angle of the sensor in question can be determined.

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Fig. 2. The dark blue curve is the original power curve of the turbine running without yaw misalignment. The purple curve is the pow-er curve for the same turbine running with a yaw misalignment of 15 degrees, resulting in significantly less energy production.

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GL GH calc., 6 m/sGL GH calc., 7 m/sGL GH calc., 8 m/s

Fig.3. Calculated lost annual energy production as a function of yaw misalignment. The solid coloured curves (#1 to #3) signify dif-ferent annual average wind speeds. The dashed curve (#4) is a cos2 curve which is a good approximation for low wind speed sites. The coloured dots are calculations by DNV GL (formerly Garrad Hassan) using their “Bladed” model, showing very good agreement with the curves.

Tab. 1. Calculated AEP for a 15 degrees yaw misalignment corresponding to Figure 2. The first column shows the average annual wind speed (Rayleigh distribution), the second column is AEP with no misalignment, the third column is the AEP with 15 degrees misalignment. The last column is the ratio of AEPmisaligned to AEPno misalignment.

Wind speed [m/s]

V90_2.0

Energy [MWh/y]

V90_2.0 Yaw err Energy

[MWh/y]

Ratio OK/err

6.00 4,894 4,554 0.931

7.00 6,582 6,200 0.942

8.00 8,097 7,705 0.951

8.50 8,765 8,375 0.956

Fig. 4. The three ultrasonic sensors of the spinner anemometer mounted on the spinner of a turbine.


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The device was invented and patented by Professor Troels Friis Pedersen of Risø/Dan-ish Technical University. The sensors and some initial testing was described in detail in [4].The spinner anemometer was patented in 2006, and in the following years proto-types were built (by the German company METEK) and were thoroughly tested in wind tunnel and on turbines up to 3.6 MW. ROMO Wind bought the patent in 2011, and has since then worked on improving and industrialising the design as well as testing the instrument thoroughly against laser based anemometers (LIDARs), and has furthermore developed a methodology such that the spinner anemometer can be used not only for correction of yaw mis-alignment but also for documentation of the improvement of power curves follow-ing modifications of the turbine. Also oth-er more advanced functions of the sensor package are being developed. The Spinner Anemometer is now being used routinely in commercial projects where yaw mis-alignment is being measured and then cor-rected for.When the turbine is running with a yaw misalignment, each sensor of the spinner anemometer will measure a wind speed variation as a function of azimuth angle (F i g u r e 5). The magnitude of the wind

speed variation is then a measure of the yaw misalignment. The simultaneous use of three sensors opens up some possibili-ties for increased accuracy and other inter-esting options.The spinner anemometer is subject to some flow distortion because of the mounting of the sensors in direct proximity of the spin-ner. For correction of yaw errors, there is no requirement for calibration, but if you need to know the free wind speed hitting the turbine, it is necessary to make a cali-bration of the spinner anemometer, such that this flow distortion on the spinner can be can be corrected for. The flow distortion can be corrected for by simultaneous meas-urements using the spinner anemometer and a nacelle mounted LIDAR, a measure-ment mast or a similar measurement of the free wind away from the influence of the turbine.

Tests and field measurements

Soon after acquiring the patent for the spinner anemometer, full-scale testing was started. A test turbine was found (NEGMi-con NM72/2000). The spinner anemom-eter was mounted on the spinner and two forward looking LIDARs were mounted on top of the nacelle (F i g u r e 6). The two LIDARs were two different technologies, a pulsed LIDAR, the WindIris, with two hori-zontal laser beams, specifically designed for nacelle mounting, and the continuous wave Zephir LIDAR, the latter being a LI-DAR normally used for measurements of wind speed and direction vertically but for this purpose tilted 90 degrees to measure wind using the standard conical scan look-ing upstream of the turbine. This setup was chosen both for testing the two LIDARs against one another as well as testing the

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Spinner anemometer S300 „thick“ sensor 10 deg yaw Spinner anemometer S300 „thick“ sensor 20 deg yaw

Sensor 1 Sensor 2 Sensor 3

Fig. 5. Azimuthal variation of the measured velocities from the three sensors. The amplitude of the variation is used as a measure of the yaw misalignment.

Nacelle based LiDAR

AventLidar Natural Power

Spinner anemometer

ROMO Wind (patented)

Fig. 6. Instrumentation setup on the test turbine. From left to right: WindIris (pulsed LIDAR), Zephir (continuous wave LIDAR), spinner anemometer.

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SA afterEnergyMWh/y

Improvementin %

5.006.007.008.009.00

10.00

2,0373,2614,6075,9517,1968,288

2,1633,4394,8246,1897,4418,530

6.15.54.74.03.42.9

Fig. 7. Power curves measured (with spinner anemometer) before and after correction of yaw error.


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VGB PowerTech 6 l 2014 Optimisation of yaw control

spinner anemometer against the LIDARs. The spinner anemometer was calibrated using the WindIris as a reference (at 160 m upstream distance). Afterwards the results were then checked using measurements from the Zephir LIDAR.When analysing the results, we found that the test turbine happened to be operating with a yaw misalignment of approximately 14 degrees. In order to bring the yaw mis-alignment down, the vane for yaw control was then manually turned in its mounting, resulting in a yaw error after correction of approximately 3 degrees. The power curves were then measured before and after cor-rection using the spinner anemometer data (F i g u r e 7). The resulting improvement in AEP ranges from 6 % (calculated at annual average wind speed of 5 m/s) to 3 % (cal-culated at an annual average wind speed of 10 m/s). These test results followed the

theoretical expectations very closely. When checking the results using power curves measured by the Zephir, very similar re-sults were found (well within 10 % of the difference in AEP). Furthermore although the aim was not to make absolute measure-ments of power curves, the manufacturer’s power curve was measured to within ±2 % (depending on annual average wind speed). If we take into account the higher turbulence levels experienced during the measurements, the manufacturers´ power curve is approached even closer.The report of the test results had been pre-sented to DNV GL (formerly GL/GH) and they were asked to review the technology

and the analysis of the test results. Some of their main conclusions [3]:

– The spinner anemometer is capable of measuring yaw error such that this yaw error can be corrected to an insignificant level.

– ROMO’s calculations of magnitude of energy loss caused by yaw error are con-firmed by GL GH calculations using the “GH Bladed” model.

– Relative power curve measurements using the spinner anemometer can be made within 10 % accuracy (i.e. 10 % of the measured difference in AEP)

Very often when watching wind farms, one can see that the turbines are not aligned at all, which they should be since the wind direction is not expected to vary greatly across the wind farm. However, singular observations like this are not nearly as in-teresting as real life actual measurements of yaw misalignment on a large number of turbines. The spinner anemometer has now been applied in a large number of cases, and results from the first approxi-mately 100 measurements of yaw errors for customers on a variety of different tur-bines shows a significant amount (i.e. lost AEP >1 %) of yaw misalignment on about half the turbines (F i g u r e 8). The main re-sults from these measurements are shown in Ta b l e 2.

Reduce yaw error variations

When making these measurements with the spinner anemometer of yaw misalign-ment, it became clear that in addition to average yaw misalignment, there is a considerable amount of scatter of the misalignment around the average value

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Fig. 8. Measured probability of occurrence of yaw errors (approx. 100 turbines in the statistics). About half the turbines show significant yaw errors.

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Energy loss due to variation in yaw error around the average

Reduce std.dev.from 10 deg to 5 deg=> 2 % more energy

Fig. 10. Calculated relative energy output as a function of standard deviation of variations around the average value. The different coloured curves are for different static yaw error situations. From the graph it can be seen that if you can reduce the standard deviation from 10 to 5 % then another 2 % in AEP will be gained. It was assumed that the std. dev of variations is constant with wind speed and that the annual average wind speed amounts to 6.3 m/s.

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50 s Data Samples Binned average

Fig. 9. Measured yaw misalignment as a function of wind speed. The blue points are 1 minute averages. The red lines are binned results. Please note the large variation around the average value, resulting in further loss of energy.


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(F i g u r e 9). This scatter obviously causes a significant loss of energy and if the scatter can be reduced, then some of this energy can be gained back. It can be calculated that a reduction of the standard deviation of the scatter of the misalignment from 10 % to 5 % could increase the AEP by 2 % (F i g u r e 10). This decrease of the scatter could be possible because with the spinner anemometer for measurements of yaw mis-alignment, there would be a much higher quality signal to present to the controller, and therefore we can potentially construct a more efficient yawing algorithm in turn enabling the turbine to follow the wind di-rection more closely, without necessarily yawing more often.

Power curve measurements

The spinner anemometer has been in-cluded as an allowed wind speed sensor in the IEC 61400-12-2 standard for power curve measurements using nacelle an-emometry. The spinner anemometer rep-resents a much better quality wind speed measurement (including turbulence meas-urement) than the conventional nacelle anemometers and can potentially be used for power curve measurement on any tur-bine without the need for site calibration (establishing the difference in wind speed from the mast measurement position to the turbine position), which is a major dif-ficulty in complex terrain, because this has

to be performed before installation of the turbines and is a quite costly process. Ad-ditionally since the spinner anemometer measurement has a high degree of sym-metry, the problems described in the IEC standard regarding the necessity of having a site-similar nacelle anemometer calibra-tion (calibration from nacelle anemometer position to the free wind) are non-existent because the measured correction of the influence of the spinner on the measure-ment also takes into account any of the challenges regarding deviations from hori-zontal inflow which is the background for the requirements in the standard. These turbulent variations are a major problem for conventional anemometers, which will have a non-ideal angular response, leading to measurement errors depending strongly on the level of turbulence intensity.

Summary and conclusions

When carrying out the measurements it was seen that there are lots of cases with significant yaw errors that can be correct-ed resulting in significantly increased en-ergy production. The spinner anemometer has turned out to be a uniquely accurate, well tested and reliable instrument for the measurement of yaw errors. Further-more in the process of testing the spinner anemometer it became obvious that there are further possibilities to utilise the high quality measurements from the spinner

Tab. 2. Misalignment statistics corresponding to Figure 8.

The ROMO yaw misalignment statistics

Static yaw misalignment < 4 deg 4 to 8 deg 8 to 12 deg 12 to 16 deg >16 deg

Distribution 41 % 27 % 23 % 8 % 2 %

anemometer instead of using the tradi-tional nacelle anemometers for controlling the turbine. In addition to the increased energy by correcting static yaw errors, there are possibilities for correcting also dynamic yaw errors by optimising the yaw controller taking advantage of the quality of the spinner anemometer measurements both for average values as well as for tur-bulence. Finally potentially there are pos-sibilities for using the spinner anemometer for power curve measurements, already demonstrated for relative power curve measurements (improvements of turbine performance), but also possible for abso-lute power curve measurements, such that a wind turbine owner can check the perfor-mance of any of his turbines, in any wind farm at any time.

References[1] Pedersen, T.F., Gjerding, S., Ingham, P.,

Enevoldsen. P., Hansen. J.K., and Jørgensen, H.K.: Wind Turbine Power Performance Verification in Complex Terrain and Wind Farms. Risø Report R-1330 (EN).

[2] Belkheir N., Dobrev, I., Dizene, R., Massouh, F., and Khelladi, S.: Experimental study of yawed inflow around a wind turbine rotor. Proc. IMechE. Part A: J. Power and Energy, 226(5) 664-673 (2012).

[3] FalbeHansen, L., GH report: Technology re-view of the ROMO Wind Spinner Anemom-eter. Report 111789-DKHI-R-01, 12 Sept 2012.

[4] Pedersen, T.F., Sørensen, N.N., Vita, L., and Enevoldsen, P.: Optimization of Wind Tur-bine Operation by Use of Spinner Anemom-eter. Risø Report R-1654(EN), 2008.

[5] FalbeHansen, L., GH report: Fatigue load calculations for ROMO Wind to assess sensi-tivity to changes in 10-min mean yaw error. Report, 111789-UKBR-R-01, 29 Nov. 2012. l

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