Development of two-bladed offshore wind turbine - Condor wind ...

Vol. 24, No. 2
Spring 2011
ISSN 0903-5648
Editor
Bill Canter
Assistant Editor
Jack Jackson
Contributing Editors
Eize de Vries (the Netherlands)
Birger T. Madsen (Denmark)
David Milborrow (UK)
Drew Robb (USA)
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Development of two-bladed
offshore wind turbine
by eize d e vries
Condor Wind Energy, based in the UK,
is at an advanced stage of developing
a two-bladed, 5 MW offshore wind
turbine aimed at substantially driving
down costs of energy compared to current
three-bladed designs.
Conceptually, the Condor 5 is an offshore
dedicated, medium-speed, geared
wind turbine. But, despite several conventional
drive system elements, the
overall design concept radically differs
from conventional three-bladed,
geared as well as direct drive offshore
equivalents.
The Condor 5 builds upon an initial
1.5 MW, two-bladed, variable speed
Gamma 60 research turbine, whose
product development originates from
the late 1980s in Italy. A link between the
two turbine developments is via Condor’s
co-founder and managing director,
Martin Ja kub owski, beginning back
in 2004 when he founded floating wind
turbine system developer Blue H.
Netherlands headquartered, three
years later Blue H acquired the exclusive
worldwide offshore application rights to
the Gamma 60 technology from US wind
turbine and helicopter pioneer Glidden
Doman, a Condor co-founder. A noted
pioneer, patent holder and aeronautics
engineer, Doman earlier served with
NASA exploring structural dynamics
of very large wind turbines. Doman was
also Boeing’s Mod 2 proposal manager
and system design manager at Hamilton
Standard’s 4 MW, WTS-4 turbine
program.
Radical design
An artist’s impression (next page) of the
Condor 5’s nacelle shows a distinct circular
helicopter-hoisting platform positioned
on top of the center of the nacelle.
In This Issue
The design arrangement aims at allowing
easy delivery of service personnel—
after the turbine has been brought to a
stationary mode with both rotor blades
in horizontal position. From a structural
point of view, a centrally positioned
hoisting-platform on top is likely easier
to integrate with the nacelle structure as
compared to a common rear-mounted
platform arrangement.
Human error
With European Commission financial
support, Italy’s AERITALIA together
with industrial partners designed the
Gamma 60 prototype, which was commissioned
during May 1991 at Alta
Nurra, Sardinia, Italy. According to
Jakubowski, the turbine functioned well
for about four years, but due to human
error during a 1995 storm it was damaged.
It was subsequently repaired and
put back into operation up until 1997
when the program was terminated.
Initially Blue H’s aim was to develop
in-house and up-scale the Gamma 60 together
with its floating foundation technology
in a parallel track. Turbine size
was envisaged to grow from 1.5 MW
to 2.2 MW and then again to about 3.5
MW, together with increments in corresponding
rotor sizes. However, in 2010
Jakubowski and his Blue H partners decided
to functionally split the two business
activities and founded a separate
company for offshore turbine development.
“The UK Energy Technologies
Institute, a public-private partnership
between the UK government and six
large industries, responded positively to
our offshore turbine development project
proposal. But as a precondition they
demanded a power rating increase to at
least 5 MW, which resulted in the cur-
Development of two-bladed offshore wind turbine 1
Wind technology: What’s working and what’s not 4
Improved wind station profits through excellence in O&M 5
Wind turbine data summary tables 7
Wind turbine performance summary 7
Production from individual wind plant 9
International wind energy directory 95

ent Condor offshore turbine size,” says
Jakubowski.
Italian nuclear mechanical engineer
Silvestro Caruso was from the start involved
and a driving force behind the
Gamma 60’s product development, building,
testing and optimizing. More than a
decade later he became engineering director
for the Condor 5 development team.
Most other Condor development team
members are also Italian engineers who
had worked in different technical capacities
on the Gamma 60 project. Product development
activities are conducted from
offices and other facilities in the port of
Genoa, Italy.
Issues
Onshore, two-bladed wind turbines during
the past thirty years have always
played a minority role in the wind industry
for a number of reasons, but particularly
due to aesthetic acceptance and
aerodynamic noise reasons. The latter due
to the fact that two-bladed rotors must
rotate substantially faster compared to
three-bladed rotors with the same diameter
unless very wide and, therefore, heavy
blades are being applied. But for offshore
application aerodynamic noise in most
situations is hardly an issue.
One advantage of offshore applications
is that a nacelle with a two-bladed rotor
combinations can be transported fully
preassembled and pre-tested on a ship’s
deck to a wind farm construction site. After
arrival, the assemblies can be hoisted
on top of a installed tower in a single, time-
and cost-saving operation.
The fact that two-bladed turbines are
by nature aerodynamically unbalanced
provides a major design challenge. A
well-known solution is to eliminate or at
least minimize the high structural loads
resulting from bending moments during
operation by attaching the rotor blades to
a flexible structure with limited pivoting
capability. This is called a teeter hub. However,
in the past rapid wear of the pivoting
mechanism and resulting vibrations were
reasons for their infamous premature failure.
These and other operational lifetime
issues have hampered commercial application
opportunities of two-bladed turbines,
including teeter hub concepts.
Teeter hub technology is, in practice,
only suitable for two-bladed rotors according
experts. Applying the technology
to three-bladed wind turbines would add
substantially to system complexity, resulting
in higher system—and likely installation—upkeep
costs.
Limited movement
Like the Gamma 60 turbine, the Condor
5 features a teeter hinge with both blades
being rigidly connected to a shared central
rotor hub. This assembly in turn is
attached to the main shaft by means of a
shaft with two double elastomeric-type
teeter elements. These patent pending
hinges allow limited rotor teeter angle
movement during operation of about two
degrees—positive and negative—relative
to the rotor’s central position during rotation.
The Condor 5’s rotor diameter is 120
meters, which is an average size in the
5 MW offshore turbine model segment
which typically varies between 115–128
meters.
What is really striking about Condor’s
technical specifications is a 20.2 rpm rated
rotor running speed, which corresponds
to a maximum rotor blade tip speed of
127 m/s. By comparison, most 5 – 6 MW
class offshore turbines have a rated tip
speed of about 84 – 90 m/s. One known
exception is the new 5 MW, XEMC Dar-
WinD offshore turbine with its 115-meter
rotor diameter, which features a 108 m/s
rated tip speed.
Premature rotor blade airfoil surface
erosion and high centrifugal forces are
often named as technical challenges for
rotors turning at very high tip speeds. Silvestro
Caruso is said to be aware of these
arguments but in his own view explains:
“Erosion protection is manageable, and
we plan to apply special anti-abrasive
coatings to erosion sensitive rotor blade
surface areas. Most critical with the Condor
design are extreme loads; but fatiguerelated
loads and centrifugal forces are
not an issue.”
A added benefit of a high rotor speed
for a given power rating is that the matching
rotor and main shaft torque is relatively
low. Silvestro Caruso claims that
the Condor 5’s main shaft torque is about
60% of the value for a 5 MW, three-bladed
rigid rotor.
Active yaw-control
Another unusual Gamma 60 feature that
has re-emerged in the Condor 5 is active
yaw-control, a system that fulfills two distinct
but separate control functions:
Artist’s concept of the Condor 5’s nacelle shows a distinct circular helicopter-hoisting platform positioned
on top of the center of the nacelle in line with the tower.
Apart from Condor, at
least two other companies
are developing offshore
turbines based upon a
two-bladed rotor concept.
One is 2-B Energy of the
Netherlands, and the second
is aerodyn company
SCD of Germany. Both
companies are developing
or have developed a
Other two-bladers
two-bladed, downwind
turbine of, respectively,
6 MW and 6.5 MW. The
latter SCD technology
has been licensed to Ming
Yang of China.
The only likely mature
and commercially viable
two-bladed onshore
turbines today are two
models originally kW-
1) to keep the rotor facing the wind
below rated speed;
2) power output control above rated
wind speed by adjusting the rotor angle
relative to the wind direction as a function
of the wind speed.
The functional basis of active yaw-control
is that a yaw motor activated system
gradually turns the rotor out of the wind
when wind speed increases. A full rotor
circle that initially faces the wind perpendicular
to the prevailing direction at low
and medium wind speeds, thereby gradu-
size developed by former
Lagerwey of the Netherlands
in the 1980s and
early 1990s. These original
models of 80 kW and
250 kW each have been
continuously upgraded
and are now being manufactured
by the current
Dutch owner and wind
turbine supplier, WES.
2 WindStats Report • Spring 2011 • Vol. 24, No. 2

ally changes into an eclipse, seen from the
wind direction. There are two components
to this: the “normal” to the rotor disk,
which gives “active” power and the cross
flow, which causes flapping moments in
the higher wind speed range. The resulting
reduction of aerodynamic efficiency
is not an issue, because at higher wind
speeds the available power is higher than
what the turbine can actually extract. The
reason being that the wind’s inclined angle
of blade airfoil attack from the prevailing
wind direction becomes less favorable and
thus less effective.
The inclined rotor actively turns in
the opposite direction again once the
wind calms down. Finally, during stationary
safe-mode position, the wind blows
against the smallest projected rotor plane
parallel to the prevailing wind direction.
In parked condition the Condor 5 can
survive up to 70 m/s wind speeds, whereby
the blades in horizontal position are
being kept aligned to the prevailing wind
direction. Under these conditions the mechanical
brake is closed and the rotor is
mechanically locked as well.
Yaw moments
While elaborating further on active yawcontrol
technology features and test results,
Silvestro Caruso says “Analysis and
tests clearly showed that nacelle yaw moments
introduced by the wind of a twobladed,
teeter rotor are only about 15%
compared to these values for an equivalent
three-bladed rigid rotor. This value
is about 20% for both the hub pitch and
yaw moments.”
Condor 5’s maximum yaw torque during
yaw system acceleration and maximum
yaw speeds in the range of 10 degrees
per second is well below 2500 kNm,
with a further reduction expected. More
important for a turbine of this size, only
three yaw motors are required; but only
two of which are needed at a time. The
third motor provides system redundancy,
he added.
Active yaw-control as applied in largescale
Condor wind turbines heavily contrasts
with today’s state-of-the-art pitch
controlled wind turbines. With the latter
technology, the full rotor circle continues
facing the wind and the turn-able blade’s
pitch angle is continuously varied for controlling
power output.
Yaw-control as an output regulating
principle bears some resemblance to mechanical
control systems known as “folding
tail” output control technology, commonly
applied in small wind turbines up
to about 10 – 12 kW power-ratings. The
main difference is that a majority of these
small size turbines are free yawing.
Regarding the choice made between
an upwind and downwind rotor concept,
Silvestro Caruso says “The active yawcontrol
principle will work both upwind
and downwind, but during Gamma 60
testing we found that an upwind rotor
with active yawing is aerodynamically
more stable. A second added benefit is
that no stall effect occurs. We therefore decided
for upwind in a combination with a
soft hydraulically dampened active yawcontrol
system.”
Design drivers
Considering the typically much stronger
winds and therefore higher loads endured
by large offshore wind turbines compared
to onshore equivalents of similar size, Silvestro
Caruso points at three main Condor
design drivers:
1) to minimize peak and cyclic aerodynamic
loads before they can impact
critical drive system and other main components;
2) protecting critical components from
harmful forces and moments;
3) incorporating a built-in capability
for reducing the effects of extreme aerodynamic
hurricane-type loads on the rotor.
The Condor 5 has been designed for
IEC 5 class wind sites with up to 12 – 13
m/s and perhaps even higher mean wind
speeds and hurricane conditions. These
high wind speed plus hurricane risk conditions
do occur at potential offshore sites
in Chinese waters, said Silvestro Caruso.
The “conventional” non-integrated
Condor geared drive system comprises a
main shaft supported by two main bearings,
a so-called “two and a half-stage”
planetary gearbox with i = 1:35 step-up ra-
tio, an intermediate shaft and generator.
The two main bearings have been
purpose-designed with horizontally split
housings with a removable upper section
for easy assembly or disassembly and
component exchange.
Great effort has also been taken to protect
the custom-designed and modular-
The Condor 5 features a teeter hinge with both blades being rigidly connected to a shared central rotor hub. This
assembly is attached to the main shaft by means of a shaft with two double elastomeric-type teeter elements.
designed gearbox from unwanted rotor
introduced loads and moments entering.
The main shaft and gearbox are connected
by means of gear coupling, which is integrated
within the gearbox’s low-speed input
side. A function of a gear coupling is to
absorb slight shaft misalignment, and this
serves to protect the gearbox against unwanted
rotor bending moments entering
bearings and gear systems. A flexible joint
at the high-speed side serves as a second
additional gearbox protection measure.
Separate removal
The gearbox will be manufactured by an
experienced German gearbox producer
and enables full on-board disassembly
and separate removal of the second medium-speed
gear stage. In addition, the
first gear stage upper housing cover can
be removed for major inspection and/or
repair or retrofit actions if necessary. These
combined gearbox design features allow
even major repairs to be conducted inside
the nacelle without having to replace the
entire unit. Such operations are known to
be very costly and time-consuming and
typically necessitate the employment of
a jack-up crane vessel.
The 3.3 kV, 8–pole, induction generator
(4-poles is a semi wind industry standard)
comes with an air-air heat exchanger
and full power electronic converter. The
WindStats Report • Spring 2011 • Vol. 24, No. 2 3
development o f t w o-bladed o f f s h o r e w i n d t u r b i n e

ated speed is 700 rpm. “For the generator
and converter development we cooperate
closely with Ansaldo of Italy, which is a
specialist technology developer and supplier
of both key electrical—power electronic
components. The combination with
a state-of-the-art, full power, electronic
converter ensures smooth grid connection
and disconnection and also offers excellent
grid quality behavior,” says Caruso.
All non-integrated main drive system
components are attached to a stiffly-welded,
fabricated steel bedplate or main chassis
that comes in a single piece.
Saftey
Regarding operational safety, the Condor
5 will be equipped with two indepen-
Condor 5 main specifications
Rated power 5 MW
Rotor diameter 120 m
Rotor rated speed 20.2 rpm
No. of blades 2
Cut-in wind speed 3.4 m/s
Cut-out wind speed 25 m/s
Rated wind speed 12 m/s
Survival wind speed 70 m/s
(IEC 61400 Class I)
Top head mass 259 tonnes, nacelle
+ rotor (target 239T)
Source: Condor Wind Energy, 4-2011
dent braking systems, a “soft” mechanical
brake, located on the low speed main
shaft near the rear main bearing and the
active yaw-control. In case of yaw failure,
for instance in a slewing bearing, the turbine
can still be brought to a full stop with
the mechanical brake.
The Condor 5 prototype is planned to
be operating in 2013 and should be fully
certified by Germanischer Lloyd in 2015.
Jakubowski believes that a combination 5
MW size and Condor two-bladed technology
is well suited for future large-scale series
production. He is also convinced that
the Condor 5 will offer a cost-effective, optimized
solution for many of the world’s
future offshore wind projects.
The prototype is partly financed by the
GEOMA project, which in turn is funded
by the Italian Ministry for Economic Development.
4 WindStats Report • Spring 2011 • Vol. 24, No. 2