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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
Christoph Heilmann BerlinWind GmbH, Germany
Heilmann Christoph (1) F Anke Grunwald (1) Michael Melsheimer (1)
(1) BerlinWind GmbH, Berlin, Germany

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

Dr. Christoph Heilmann has been working in the wind industry for fiveteen years. He is currently Head of the R&D department at BerlinWind GmbH, Germany. He studied mechanical engineering at the Technical University of Berlin, Germany. After finishing his doctorate in 2005, he was for five years Project Manager at Deutsche WindGuard Dynamics GmbH leading R&D projects and wind energy training courses. Since 2009 he is at BerlinWind GmbH, developing wind turbine rotor balancing and load measuring systems, as well as photometric and laser-based blade angle measuring systems. Additionally, he is involved in consulting.


Poster Download poster (8.89 MB)


Boost wind turbine yield and life expectancy through rotor blade angle correction


Independent measurements at the rotors of wind turbines (WT) without suspicion reveal that approx. three quarters exceed the tolerable design limit values for blade angle deviation (BAD) and/or mass imbalance (MI) of the type certification. The changed aerodynamics from BAD significantly lower the annual energy production (AEP). MI and BAD both increase vibration amplitudes, fatigue loads and life time consumption, so does the OPEX from damages and downtimes.


For BAD, it has to be distinguished between the absolute BAD from the design blade angle, which is mainly responsible for AEP deficits - and the relative BAD between the blades of a rotor, which is mainly responsible for increased vibration.
BerlinWind has carried out two big field studies: measuring absolute blade angle measurements of 277 WT (average 2 MW, with and without known BAD issues) and measuring relative blade angle and mass imbalance of 239 WT (without known BAD or imbalance issues). The independent photometric and vibration measurements followed proven procedures, ensuring statistically safe results. The critical limits applied for MI were WT-specific and for absolute BAD according to the WT certification guideline by DNV-GL (+/- 0.3°).
To estimate the effect of absolute BAD on the energy yield, power curves and yield from operational data were compared before and after blade angle adjustment and as well between WT with different BAD in the same farm.
The Cost-Benefit Study is based on a German wind farm with 10 2 MW WT, AEP 410T€ per WT, 20 years planned service life. Damage and repair costs are based on O&M statistics.

Main body of abstract

The field study on absolute BAD shows that only 7% of the rotors have a tolerable BAD. 50% have an intolerable absolute and relative BAD, 43% have a tolerable relative BAD but still an intolerable absolute BAD. Therefore, it is essential that the applied measuring method is capable to determine the yield-relevant absolute BAD. For about 75% (25%), the maximum absolute BAD of the rotor exceeds 0.6° (1.8°), i.e. two (six) times the limit. Evaluating the investigated 831 blades individually shows that only 21% are within the limit, 47% (32%) are adjusted intolerably towards stall (feather). Especially, if Pitch-WT operate under stall condition, heavy vibration reduces component service life by several years. The mean absolute BAD is -0.34°, the standard deviation as large as 1.9°. The extremes are -8.8° and +11.8°. Combining both field studies shows that 73% of all wind turbines are affected by intolerable Blade Angle Deviation and/ or Mass Imbalance.
Evaluating power curves shows that the mean absolute BAD per rotor of 1.0° reduces AEP by approx. 7.4 %. The financial impact of the resulting damage costs and yield loss is enormous. BerlinWind’s cost-benefit study shows that damage costs, standstill yield loss, and AEP loss from absolute BAD can add up in 20 years to about 7 Mio € for the German 20 MW wind farm. While investing only 100T€ in periodic blade angle and vibration measurements prevents theses damage and AEP-related losses.


Statistics show that only a minority of wind turbines have correctly adjusted blade angles. The large share of faulty adjustments proves that the typical production and commissioning procedures are not sufficient. Neglecting this fact will result in huge yield losses and damage costs, while conducting effective measurements and implementing corrective measures comes at a fraction of these costs.
Using proven statistically safe, independent optical blade angle measurements, blade angles can be re-adjusted to the optimum design angle from certification. Vibration measurements validate the achieved load reduction. Therefore, blade angle correction is an essential tool to significantly increase AEP and life expectancy of wind turbines, and reduce OPEX.

Learning objectives
Statistics from independent measurements show that in practice, many wind turbines exceed their certification limits for blade angle deviation.
Suitable reliable measurement methods are available.
A cost-benefit study shows that performed periodically during the entire service life, the measurements cost a fraction of the avoided damage costs and AEP loss.