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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Advanced rotor technologies' taking place on Tuesday, 11 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Leonardo Bergami DTU Wind Energy, Denmark
Leonardo Bergami (1) F P Helge A. Madsen (1) Flemming Rasmussen (1)
(1) DTU Wind Energy, Roskilde, Denmark

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Leonardo graduated B.Sc. Energy Engineer at Politecnico di Milano, and M.Sc. in Wind Energy at the Technical University of Denmark in 2008. In July 2013 he graduated Ph.D. at DTU Wind Energy, with a thesis on Adaptive Trailing Edge Flaps for Active Load Alleviation in a Smart Rotor configuration; the thesis was awarded by EWEA with the Excellent Young Wind Doctor Award. Leonardo is currently working as postdoctoral researcher in the aeroelastic design section at DTU Wind Energy, and participating in national and european projects.


A two-bladed teetering hub configuration for the DTU 10 MW RWT: loads considerations


As the size of wind turbine rotors continuously grows, the need for innovative solutions that would yield to lighter rotor configurations becomes more urgent. Traditional wind turbine designs have favored the classic three-bladed upwind rotor configuration. The paper presents instead a concept study on an alternative downwind two-bladed rotor configuration with a teetering hub. The configuration allows saving the weight and material corresponding to one blade, but implies several complications: increased power losses due to tip effects, increased aerodynamic loads and load unbalance on the rotor, increased loads due to interaction with the tower frequency.


This preliminary concept study is based on a wind turbine model representative of next generation multi-MW wind turbines: the DTU 10-MW Reference Wind Turbine (RWT). To keep overall aerodynamic performances similar to the reference wind turbine, the two-bladed rotor configuration maintains the same rotor solidity ratio, hence the blade chord is increased by a factor of 1.5.
The paper focuses on the load implications of the two-bladed configuration, and investigates the interaction between the 2P (double rotational frequency) and the tower stiffness, as well as the potential load reduction from a teetering hub solution.

Main body of abstract

The two-bladed rotor configuration for the DTU 10-MW reference wind turbine allows to reduce the rotor weight by about 20 %, but has several implications. The turbine power output, in spite of a constant solidity, is in fact reduced due to higher tip losses effects; a reduction of about 4 % below rated conditions is reported.
Another major problem of the two-bladed configuration is the prevailing forcing frequency of 2P lying close to the first tower modes, thus causing a significant and undesired amplification of tower loads and vibrations: tower bottom loads four times larger than the reference ones are reported. The effects on tower fatigue damage equivalent loads (DEL) for different tower stiffness configurations are investigated, and an alternative tower stiffness distribution is proposed.
The DEL loads on the blades are increased by the larger blade surface, and the fatigue DEL on the shaft and drivetrain bearings are nearly three times higher than in the reference three-bladed rotor case due to the aerodynamic forces unbalance on the rotor operating in sheared wind flow. A teetering hub configuration reduces the loads on the blades root, and significantly on the shaft. An investigation is carried out considering different teetering hinge stiffness so to pinpoint a compromise between load reduction and teetering angle variations.


Two bladed rotor configurations for multi-MW wind turbines provide a lower rotor weight alternative to traditional three-bladed configurations, but aspects such as increased power tip losses (4 %), and increased load variations should be also evaluated in the design process.
The study indicates that a re-design of the tower stiffness towards a more compliant structure (tower mode frequency 1-1.2 times the rotational frequency) is necessary to avoid significant load amplifications from the 2P forcing frequency. Furthermore the applications of a teetering hub solution have a potential for significant alleviations of the shaft, bearings, and blade damage equivalent loads.

Learning objectives
The delegates will gain further insight on:
• Alternative two-bladed rotor configuration for multi-MW turbines
• Advantages and problematic of a two-bladed rotor
• Load implications from tower stiffness variations in relation to the rotational forcing frequency
• Load alleviation potential of a teetering hub configuration for different maximum teetering angle ranges.