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Two blades - Condor Wind's 5MW offshore turbine

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.

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
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

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 Ja­kub­owski, 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. 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 current 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 DarWinD 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 fatigue-related 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:

  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 gradually 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 yaw-control technology features and test results, Silvestro Caruso says "Analysis and tests clearly showed that nacelle yaw moments introduced by the wind of a two-bladed, 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 large-scale 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 yaw-control 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 yaw-control 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 S 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 ratio, 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-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 rated 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.

Safety

Regarding operational safety, the Condor 5 will be equipped with two independent 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.

Other two-bladers

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 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-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.

This article originally appeared in Wind Stats.

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