Advanced Wind Turbine Program Next Generation Turbine ... - NREL

2.3.1.3 Flexible Downwind Rotor Concepts
Wind turbines featuring downwind rotors with flexible blades have been designed, built, and
tested by a number of researchers and turbine manufacturers in the past, with varying degrees of
success. Some were technically unsuccessful, and none have proved commercially successful.
The reasons are varied. Some were probably overly ambitious to the degree of flexibility attempted.
Some may have been affected by the number of technical innovations introduced simultaneously
in the turbine design, which challenged the ability of engineering modeling and analysis
at the time. Inadequate modeling led to unforeseen dynamic problems during testing and
proved insurmountable.
From this experience, the utility-scale wind industry has, during the course of nearly a quartercentury
of progress, converged on the three-bladed, upwind rotor configuration. Every utilityscale
wind turbine currently manufactured either in the U.S. or in Europe, features this design.
As noted at the beginning of Section 2, GE believes there are market advantages to the threebladed
upwind rotor configuration and decided, from the outset of the NGT project, that the
NGT design should favor this concept.
Nevertheless, a downwind rotor featuring flexible rotor blades represents one means of overcoming
the most significant challenge to implementing more flexible rotor blades in an upwind rotor
configuration–accommodating blade tip tower clearance requirements. In order to halve the
stiffness of the blade in an upwind rotor arrangement, approximately 2.5% of energy capture is
sacrificed by incorporating forward coning sufficient to ensure adequate tip tower clearance,
while approximately 1.75% is sacrificed through use of uptilt. These represent fairly significant
losses that offset a substantial portion of the COE savings resulting from the use of more flexible
rotor blades.
GE decided to commission a study of the downwind rotor concept to determine whether introducing
flexible rotor blades downwind could produce savings in COE sufficient to justify further
pursuit of this concept. The study was quite broad, examining a number of variations on the
downwind flexible rotor concept, including:
• Degree of rotor blade stiffness
• Number of blades
• Hub type (i.e., rigid, teetered, or hinged/flapping)
• Tower height and type (free standing vs. guyed)
• Rotor solidity and tip speed.
Various rotor configurations were examined. The 3-bladed, upwind, rigid hub configuration was
used as a baseline and the other configurations were compared to that case. The “hinged” hub
configuration modeled blades independently hinged flapwise at their roots. The “soft” blade had
a flapwise stiffness of 0.5 and a mass of 0.67 of the corresponding regular blade properties. The
same blade was used on both the two- and the three-bladed rotors. In order to make the rotors
equivalent and to have the same performance curve, the rotor speed of the three-bladed version
was reduced from 42 rpm to 35.5 rpm.
To obtain information on both the peak loads and the fatigue loads, the study concentrated on
two operating conditions: normal operation in a “standard” turbulence with a mean wind speed
of 18 m/s, and the extreme coherent gust with direction change (ECD) at rated power. The simulations
of 10-minute records of normal operation were carried out four times, each with a different
random seed.
29