Dr. Daniel M. Saban, PE, SMIEE/Chief Technology Officer/Danotek Motion Technologies/www.Danotek.com
Wind-turbine manufacturers are looking for a drivetrain that delivers high efficiencies at part load, increased availability, and with a simplified grid-tie that protects the generator from grid-side disturbances. The designs should also be applicable to direct-drive, medium speed, and high-speed systems. Many wind turbine OEMs already know the answer lies in a rapidly maturing line of permanent magnet (PM) generators, those with great potential to enhance the financial performance of onshore and offshore projects.
A common viewpoint is that PM generators are applicable only to offshore direct-drive wind turbines, where their use removes the potentially troublesome gearbox. However, PM generators offer so many compelling advantages they are now being designed into drivetrains with gearboxes to operate across a wide range of output speeds.
Double-fed induction generators
Double-fed induction generators (DFIGs) have given the wind industry many years of excellent service. But the industry is changing. The number of undeveloped Class I (ideal) wind sites diminishes with each passing year while project developers are demanding improvements in plant reliability and availability. New market entrants bring increased competitive pressures between the wind-turbine-generator OEMs. Similarly, the cost of power converters has decreased significantly so that the comparative gain of a partial-rated converter is evaporating. In such an evolving situation, PM generators deliver a wide range of performance advantages to wind-farm developers and owner-operators that can make a significant and positive difference in project economics.
For example, PM generators are incredibly flexible. Their design can be customized to optimize performance of the complete wind-turbine drivetrain. Speed, efficiency, weight, and cost are four fundamental, inter-dependent factors that can be varied to meet the wind turbine OEM’s objectives. The PM generator designer can trade magnet, stator copper, and stator-lamination properties and weights to hit required goals without need for the turbine OEM to accept a suboptimal, off-the-shelf, standard generator. The marginally higher capital costs are paid back many times during the operating life of the wind turbine. The table highlights a few PM generator system advantages over a DFIG system.
PM generators and rare earth magnets
The magnets on a PM generator’s rotor do, of course, use rare earth metals. Rapidly increasing magnet pricing over the past 12 months caused more than passing consternation within generator manufacturers and wind-turbine OEMs. Fortunately the peak magnet prices experienced this summer are already in decline with reductions of ~30% evident at the time of this writing. Note that magnets used within PM generators typically contain ~30% Nd (neodymium) by weight, with most of the remaining weight made up of low-cost ferrous materials.
Current economic uncertainties have curtailed demand for rare earth metals, but of greater significance is the emergence of new rare earth supplies in the US, Asia, and Australia, and the partial relaxation of export restrictions within China. All indications are that magnet prices will continue to decline, the major unknown being the level at which magnet magnet pricing will stabilize,.
High speed to direct drives
PM generators are flexible. They let wind-turbine designers consider drivetrains with a wide range of topologies from conventional modular gearbox and generator arrangements to fully integrated (or hybrid) systems assembled within a single enclosure, and even direct drive systems that work without a gearbox. Each topology has merits.
A conventional high-speed drivetrain with fully independent, or modular, gearbox and PM generator appears in Modular drivetrain arrangement.
This topology provides the turbine OEM with maximum flexibility in selecting suppliers of the gearbox and generator because they are both fully independent. It does however produce a least-compact arrangement.
By integrating some drivetrain components, the overall length and weight can be significantly reduced. This is accomplished by combining the main bearing and gearbox for a medium or high speed shaft design, or by close coupling the gearbox and generator as shown in Integrated drivetrain arrangement.
In a close-coupled gearbox and generator, the generator is overhung from the non-drive end of the gearbox. This arrangement, often found on medium-speed drivetrains with single or two-stage gearboxes, minimizes drivetrain length. The relatively simple physical connection between the gearbox and generator housing retains the turbine OEM’s flexibility to select independent gearbox and generator suppliers.
Improvements in gearbox technology, design, and durability now let medium-speed wind-turbine drivetrains with single or two-stage gearboxes achieve reliabilities once thought only possible with direct drives. PM generators designed to operate at speeds from about 125 to 500 rpm are significantly smaller and lighter than direct-drive equivalents.
For example, a 3-MW PM generator operating at 400 rpm has a diameter less than 2.5 m and weighs little more than 10 tons. A direct drive of similar capacity has a diameter of over 5 m. Today, medium-speed systems appear to be a preferred solution of wind turbine OEMs seeking the best balance between reliability, efficiency, size, weight, and cost.
Fully-integrated (or hybrid) drivetrains
The most compact medium-speed drivetrain topology features a fully integrated gearbox and PM generator with gears mounted inside the generator rotor. This arrangement couples suppliers for the gearbox and PM generator, and removes the flexibility of the wind-turbine OEM to independently select its preferred gearbox and PM generator suppliers. The degree of integration, however, typically involves more complex remedial work in the event of unscheduled maintenance.
By removing need for a gearbox and coupling the PM generator directly to the turbine rotor’s shaft, direct-drive systems provide the simplest drivetrain, which ought to deliver the highest reliability. However, this is difficult to achieve in practice.
With increasing wind-turbine outputs, a performance advantage can be negated by the extremely large and heavy PM generator required to handle rated torque at relatively slow (10 to 15 rpm) rotational speeds. To illustrate, consider two 3 MW PM generators. The direct drive PM generator’s diameter is more than twice that of a medium speed (400 rpm) generator, and its weight is five times that of a more compact medium-speed machine. As outputs increase, size and weight differentials grow. A PM generator in the 8 MW class can have a diameter exceeding 7m and a weight exceeding 90 tons. As generator weight increases (approximately proportional to the square of its diameter) slower speeds add considerable cost to the generator and to the turbine’s structure including the tower.
A close coupled, medium-speed arrangement offers perhaps the best compromise: reduced drivetrain length and weight, high reliability and efficiency, and an opportunity to mix-and-match major component from a range of suppliers.
Only a small fraction of the wind turbines in operation today use PM generators. But with the superior efficiencies, higher reliabilities, and lower costs arising from greater market penetration, PM generators are set to become a major part of tomorrow’s wind turbine drivetrains. WPE
What’s available now
In a growing field of PM generator system providers, Danotek is the only U.S. based supplier currently developing and manufacturing highly efficient energy conversion systems in the 600 kW to 8 MW range for the wind energy and industrial markets. The company’s approach has attracted investments from energy venture capitalists including Khosla Ventures, CMEA Capital, and GE Energy Financial Services. Patents are pending on many aspects of the company’s PM generator and power electronics products for a wide range of new and existing applications, including wind energy, power generation, and variable-speed propulsion systems and accessory drives for electric and hybrid-electric vehicle markets. A few differentiators include PM generator designs with extremely low cogging and torque ripple, and a wide operating speed range that enables a lower speed cut-in for the turbine that increases energy capture, especially at sites with frequent low wind speeds. The company’s PM generator designs can be customized to accommodate either liquid or air cooling with minimal design changes. This lets the turbine OEMs design nacelles without major changes in the overall drivetrain assembly and thus reduce cost. The advantages of optimizing each PM generator to meet the needs of turbine OEMs gives partners the opportunity to influence the generator arrangement and customize it to meet the goals of the turbine program.
1. Financial Times, June 19th, 2011: “Rare earth prices soar as China stocks up.”
2. Wall Street Journal, September 22, 2011: “Rare-Earths Demand Eases, Sapping Prices”