No shortage of direct-drive wind turbine developments
Key element of a direct-drive wind turbine is the low-speed generator that eliminates the need for a gearbox from the turbine’s drivetrain (see main wind turbine technology article, August 2011 Control Engineering). A synchronous permanent magnet-type generator is most often used. Benefits include a simpler drive train, lighter weight wind turbine, and increased reliability for offshore applications.
Power-generation equipment and services provider Alstom is developing a “next-generation” 6-megawatt (MW) offshore wind turbine using direct-drive (DD) technology. Alstom and power-conversion specialist Converteam announced in mid-March 2011 their collaboration on what they call “the world’s largest DD permanent magnet (PM) generator for a wind turbine.”
Initially, Converteam will supply DD generators for two 6-MW Alstom offshore wind turbine prototypes. Reportedly lighter and more compact than prior-generation DD units, the “advanced high density” DDPM generator is designed to improve turbine drivetrain reliability. Pierre Bastid, CEO of Converteam, said, “The generator will have the largest torque of any permanent magnet generator built to date.”
The first prototype 6-MW turbine will be installed before year-end onshore near Le Carnet in France’s Loire river estuary. Prototype 2 will be installed offshore in Belgian waters in 2012. A pre-series product step is scheduled for 2013 prior to full commercialization, with full serial production expected in 2014, according to an Alstom spokesman.
Also enhancing the turbine’s reliability is trademarked “Alstom Pure Torque,” a design feature that protects the drivetrain and generator from wind-induced deflection loads. It diverts harmful stresses to the tower structure. “Only torque is transmitted to the generator, which ensures sufficient air gap is maintained between the generator rotor and stator, thereby boosting its performance and reliability,” says the company.
In June 2011, Siemens Energy installed a prototype of its latest offshore wind turbine with DD rotor technology in Hovsore, Denmark—where trial operation of the 6-MW machine has begun. SWT-6.0-120 wind turbine with 120-m rotor diameter was developed “specifically for offshore projects of the future,” according to Siemens. After an extensive commissioning and trial operation process on the first unit, Siemens plans to install more prototypes this year for further testing and validation.
Additional “pre-series” 6-MW wind turbines will be installed in 2012-13 for more tests and to optimize turbine performance. Serial production is planned for 2014, according to the company.
Earlier this year, Siemens introduced two other DD wind turbines: SWT-3.0-101 rated for 3 MW and SWT-2.3-113 rated for 2.3 MW, with rotor diameters of 101 m and 113 m, respectively. These turbines likewise feature a DD low-speed PM synchronous generator.
Not all offshore
Northern Power Systems (NPS) has entered the utility-scale DD wind turbine market, with the recent introduction of a 2.3-MW machine using a PM synchronous generator. The company is building on its long-term success with smaller (100 kW) DD wind turbines. NPS considers DD technology to be more reliable and efficient, while needing less maintenance than geared machines. Northern Power has also introduced a new 100-m diameter rotor option for the 2.3-MW turbine.
Among other Siemens Energy announcements of multiple European orders, two projects call for 3-MW gearless wind turbines (19 of 151 total onshore turbines in the total order). Thirteen SWT-3.0-101 machines will be installed at the 39-MW Dagpazari project in Turkey and six at the 18-MW Millour Hill project in the U.K.
Siemens SWT-3.0-101 direct-drive turbines will also be part of a wind project in North Dakota. Phase 2 of Minnesota Power’s Bison I project will install 15 Siemens 3-MW DD turbines rather than 17 of the 2.3-MW machines in the original project plan. DD technology has the potential to reduce maintenance time, resulting in higher turbine availability. An additional 10,000 MWh of energy per year is expected from the turbine upgrade, according to Minnesota Power.
In related news, Finnish company The Switch—a supplier of wind turbine generator and power converter packages—was selected by Prokon Energiesysteme GmbH of Germany to supply direct-drive PM generators and full-power converters in the 3-MW power range for its wind turbines.
The move was intended to provide Prokon with independence to manage the entire process from wind turbine manufacturing through to operations and maintenance, a long-held aspiration for Prokon, according to managing director Ralf Dohmann. The company develops, finances, and manages its own wind farms where the turbines will mainly be used.
The Switch believes that its optimized drivetrain packages will enable Prokon to quickly exploit preferred PM technology for reliability, maximized yields, and power quality that matches strictest international grid codes. Jukka-Pekka Mäkinen, president and CEO of The Switch, explained that the project will provide an opportunity to bring a third-generation DD PM generator with the latest technical features and manufacturability to the market.
20-MW turbine, advanced controls?
Optimism is in the air. A report from the European Union-funded UpWind project, published at the European Wind Energy Association (EWEA) 2011 conference in Brussels, suggested that 20-MW capacity wind turbines are feasible. Among UpWind project findings is that rotor diameters of around 200 meters (656 ft) would be needed for turbines of 20 MW power capability. This compares to 150 m for the largest prototype wind turbines being built today.
UpWind project findings were generally optimistic but indicated the need for various technology innovations to be in place for design, materials, and turbine operations to implement 20-MW machines. Main innovation areas identified were blades, smart adaptation to local wind conditions, wind farm layout, and controls/maintenance considerations.
For turbine blades, focus is on lowering fatigue loads to allow building longer, lighter blades. More flexible blade materials and separately controlled, two-section blades were among suggested approaches to lessen fatigue loads. For turbine controls, one suggestion was applying a nacelle-mounted LIDAR (light detection and ranging) system to sense approaching wind gusts—a capability beyond existing anemometers. LIDAR sensors would interact with the turbine control system to adjust blade and nacelle position prior to impact of wind gusts.
The report also pointed out that substantially more investment in wind energy research will be needed by the European Union and member states to ensure the innovations of UpWind and other projects can be developed and applied.
Various constraints on generator design and permanent magnet materials discussed in the August 2011 CE article on “Direct-drive wind turbines” would also apply to the development of these much higher power wind turbines and would require appropriate solutions.
Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at email@example.com
See links to related reading about wind power automation, below.