Friction drive may solve conventional drivetrain problems

One school of thought for wind-turbine design says getting rid of the potentially troublesome gearbox makes for more reliable machinery. Although true, the permanent magnet (PM) generators then required become heavier and more expensive. A direct-drive 1.8 MW Enercon, for example, weights 25% more than a gearbox equipped Vestas 1.8 MW. Toronto-based CWind says it has a way around the weight issue in its direct drive design that has a greater generating capacity than many existing turbines.
In the CWind design, a three-bladed rotor turns a large diameter flywheel, the surface of which turns friction wheels, each connected to a generator. Rotor and flywheel operate with the same inertia. The design is reliable because the friction drive limits the torque transmitted through the drive. The company says its turbines will eventually have ratings from 2.0 to a whopping 7.5 MW.
The design is simpler than conventional turbines, says its developer, because scaling up the power train comes by adding more generators. For example, the drive mechanism in the proposed 2 MW design uses eight 250-kW generators. Add 12 more, a larger rotor and drive wheel, and it provides a 5 MW design.
Each generator has an independent load path, so taking advantage of the variable speed lets the turbine deliver higher part-load efficiency at lower wind speeds. Compared with conventional wind turbines, the design is said to be more reliable, has lower maintenance costs, weighs less, and will better absorb wind gusts.
Using conventional industry scaling and cost-of-energy models, a wind farm of these turbines could produce significantly more energy, says the developer, at a lower cost than a similarly sized wind farm of traditional wind turbines.

The developer says design advantages include:
• No gearbox or gearbox bearings to fail, and no oil
to change
• No single point of failure
• Reduced overall cost for manufacturing, operations,
and maintenance
• Energy from gusts is stored as momentum in the
flywheel and recovered when gusts pass.

• It is impossible to overload the drive train
• Multiple generators operate at the most efficient speed.
Generators come online only with sufficient wind.
Therefore, energy production is higher than a convent-
ional unit even when running below rated power
• Independent load paths allow continuous operation at
partial load while awaiting maintenance
• Distributed load paths allow operating up to 95%
availability during faults because only the fault load
path is isolated
• The design allows maintaining, servicing, and replacing drive-system components using an internal nacelle
crane.

WPE