Exploring New Technologies for Extreme-Scale Turbines
Larger turbines – beyond today’s multi-megawatt onshore and offshore machines – are one of the most attractive options for reducing the cost of wind energy. Continued technology scale-up to rated capacities of 10MW and beyond requires novel concepts for overcoming the fundamental limitations of today’s turbine designs and materials, including structural constraints of drive-train components. This article explores how magnetic gearbox technologies could provide solutions.
By Luis Cerezo, Technical Executive, EPRI, USA
Basic Science
Magnetic gears rely on field forces, rather than physical contact between gear teeth, to achieve high ratios at greatly reduced mass and size relative to mechanical gearboxes used in today’s wind turbines. Magnetic gearing promises to alleviate constraints to increasing wind turbine ratings above 10MW while also increasing efficiency, improving reliability and reducing levelised cost of energy.
Technology Overview
Wind turbine transmissions, housed within the nacelle at the top of the tower, connect the rotor shaft with the generator. Multi-stage mechanical gearboxes are commonly employed to convert slow, high-torque rotational energy into higher-speed rotation of more than 1000 RPM required by conventional electromagnetic generators. These gearboxes are large, complex, costly and prone to premature failure. Coupling a magnetic gearbox with a conventional generator represents a potential breakthrough technology for reducing the cost and mass of drive-train components in future wind turbines without a drastic departure from the familiar architecture of today. Another competing technology is embodied in a direct-drive turbine, which relies on a massive and costly generator spinning at low speeds and containing many more magnetic pole pairs. Hydraulic drive-trains represent another emerging technology to compete with conventional gears. as well.
Conventional Technology Limitations
Over the past two decades, improving gearbox reliability has been a major emphasis of wind energy research and development (R&D). Drive-train components have been susceptible to materials degradation, wear, and failure especially under long term dynamic loading from wind gusts, which impose unequal forces and torque spikes thorugh the drive-train. Components in physical contact – bearings and gears – are particularly vulnerable to reduced lifetime and premature failure.
New Technology
Large magnetic gears are not yet commercially available, but the concept is being proven at small scale. Paired magnetically coupled devices attached to the rotor render a mechanical ratio between the driving and driven components, with no physical contact between them. High rotational speeds are achieved without the need for multiple gear stages, precision-engineered gear teeth, or continuous lubrication.
How it Works
The magnetic gearbox eliminates the potential for wear-related damage and failure to the gears themselves and eliminates the need for oil-based lubricants between the gear contact surfaces. The basic technology of magnetic gears is known from the ‘Power-Transmitting Device’, awarded US Patent 687,292 in 1901. The concept has been developed further through patents issued from 1940 onwards associated with magnetic gears and magnetic transmissions.
In magnetic gears, rotating ‘cogs’ act as magnets, with tightly controlled air gaps between them. Slower, higher-torque cogs have more magnetic poles. As poles alternate across the gap between elements, the gear ratio determines rotation or translational movement between input and output. A pure magnetic coupling has a gear ratio of unity. Magnetic gearboxes can achieve the very high ratios and rotational speeds required for wind power production from conventional generators. In addition, their capability to slip without damage under overload conditions mitigates an important failure mode.
Applying the Technology
For wind turbine applications, the three basic magnetic gear configurations shown in Figure 1 have been considered:
- Radial or cycloid magnetic gears utilise concentric low- and high-speed rotors, separated by a bar stator and made up of alternate positive and negative magnet segments. The torque transmitted is a function of flux links between the larger outer ring and smaller inner ring. Gear ratio is defined by the pole number of the driving ring over that of the driven ring.
- Trans-rotary gears use magnets in a helical arrangement, like that of screw threads. The number of poles, analogous to the number of starts in a threaded rod, is determined by magnet width and pitch. Helixes on the rotor and translator have the same pitch, and the latter slides over the former. The relationship between the rotor’s angular speed and the translator’s linear speed determines the gear ratio.
- Axial gears consist of two discs containing alternate positive and negative magnets with a modulator disc in between and gear ratio defined by pole number. However, as the size of the magnetic areas become smaller, such as when packing in more pole pairs to achieve higher gear ratios or reduce disc sizing, radial-flux leakage creates a point of diminishing returns. Manufacturing challenges, plus huge axial forces on bearings at the main rotor shaft, represent barriers to further development of this configuration for wind turbine applications.
Value to the Industry
Magnetic gearbox technology could help eliminate barriers to the development of future onshore and offshore wind turbines with capacity of 10MW and above by alleviating physical size and mass constraints that drive cost into the continued upscaling of drive-train components. This innovation is also expected to reduce levelised cost of wind generation by improving gearbox efficiency and reliability. As an alternative to multi-stage mechanical gearboxes, magnetic gearing can achieve the high differentials and rotation rates required by conventional electromagnetic generators but at greatly reduced mass. The size advantage also exists over direct-drive designs, which require ever-larger generators to transform low-speed torque into bulk power generation. Lighter and smaller magnetic gearboxes will help downsize nacelles, decreasing gravitational loading and thus enabling taller towers to reach greater wind resources.
Other Advantages
Magnetic gearing also promises performance advantages over conventional gearboxes, based largely on the lack of physical contact between driving and driven surfaces. Substantial reductions in friction losses will enable higher energy conversion efficiency – particularly at partial loads – as well as power generation at reduced wind speeds. Avoided contact and wear are expected to allow step-change improvements in gearbox reliability, both directly and through significant reductions in the vibrations and pulsations leading to additional damage and failure modes. Lubricant-free operation will further reduce maintenance requirements. Reduced noise levels may improve public acceptance. As an alternative to mechanical and hydraulic transmissions, the technology also appears suited for wave energy production, low-head hydro, boiler feedwater pump and other power industry applications.
State of the Technology
Technology developers around the world are pursuing magnetic gearboxes. The Electric Power Research Institute (EPRI) is monitoring early stage wind energy technologies through collaborative research, development and demonstration projects and has found potential for significant performance advantages over conventional mechanical gearing. According to modelling by magnetic technology developer American Maglev (AMT, Inc.), a magnetic gear coupled with a conventional generator optimised for wind turbine applications could replace an electromechanical system eight times larger. . In Europe, under the MAGDRIVE research programme, two magnetic gear prototypes were designed and tested. One can operate at extremely low temperatures under a vacuum – conditions experienced in space. The second is intended for ambient-temperature applications, from food processing to energy production. A spin-off company, MAG SOAR, claims to have achieved a world-record gear ratio of 44:1 during 2016. Other developers are touting early commercial products, generally for small-scale uses.
Multiple Projects Under Way
Researchers at the University of Setif-1 in Algeria have developed and modelled a novel drive-train topology for future offshore turbines integrating a magnetic gear with a permanent magnet synchronous generator. The Advanced Electrical Machines & Power Electronics Lab at Texas A&M University has developed and tested sub-kilowatt magnetic gear prototypes in axial, radial and trans-rotary configurations and assessed their possible use in wind turbines. Another leading US research centre, the Laboratory for Electromechanical Conversion & Control at the University of North Carolina at Charlotte (UNCC), received 2014 funding from the DOE and the US National Science Foundation focusing on the development of magnetic gears for ocean and wind energy applications. A prototype vertical-axis wind turbine with magnetic gearing has been demonstrated through a UNCC student design project. In 2015, DOE awarded funding to a public–private collaborative involving Georgia Tech focused on developing a magnetic gear platform for low-impact hydro drive-trains.
Next Steps
Magnetic gears integrated in extreme-scale onshore and offshore wind turbines are projected to achieve technology readiness level 8 (TRL8) in about eight to ten years. The next R&D milestone is to develop and test prototype magnetic gears integrated with turbine rotors and generators in the laboratory and then under field conditions to understand dynamic behaviour and assess magnetic interactions with other components inside the nacelle.
Scale-up testing and refinement in the range of 100 to 250kW will be required to define the best configurations for mechanical and magnetic coupling and to validate performance, efficiency and reliability. This will help in refining estimates of future capital, operations and maintenance costs, relative to other drive-train configurations for future wind turbines, as well as in identifying key sensitivities. Pilot testing at scales of 2MW or greater will be needed to optimise full- and partial-load efficiencies and evaluate long-term reliability.
- EPRI (2013). Technology Insights Brief: Hydraulic Drive Trains for Wind Turbines. Palo Alto, CA: 1026869.
- Armstrong, C.G. (1901). “Power-Transmitting Device.” US Patent Office. Accessible at US Patent #687,292.
- Faus, H.T. (1940). “Magnet Gearing.” US Patent #2,243,555.
- US Patent #3,301,091. US Patent #3,382,386. US Patent #3,645,650. US Patent #5,013,949.
- AMT. “Magnetic Gearbox: Enhancing Power Generation.” Accessible at http://american-maglev.com/maggear/.
- UNCC (2016). “Revolutionary Wind Turbine Magnetic Gear.” Accessible at http://isl.uncc.edu/revolutionary-wind-turbine-magnetic-gear.
Biography of the Author
Luis Cerezo is a Technical Executive in EPRI's Wind Power Program. Cerezo has developed and managed renewables power projects and scouting innovative wind technologies for over ten years. He holds masters degrees in electrical engineering and in business administration.
