Despite the recent surge in wind-turbine announcements regarding direct drives, gearboxes are not going anywhere. They will remain a necessary component in many turbines, and so are pushing trends in several directions—toward lighter overall turbine weight, improved durability, and design for maintenance.
“After ten years of research and study, we know why early gearboxes did not stand up well to the duty hazards in wind turbines,” says Timken’s Hans Landis, Director, Process Industries OE & Wind Energy. “And those lessons have been disseminated throughout the gear industry.” The lessons start with a better understanding of the loads on a wind turbine gearbox. For instance, the constant pounding of the wind on a rotor and main shaft of older designs could slowly hammer away at the gearbox. Higher loads would slowly damage bearings, then gears, and eventually require a gearbox swap. More recent designs let better bearings handle loads so gearboxes need not.
Early main-shaft gearboxes on kilowatt-sized machines were off-the-shelf industrial units with minor modifications. The obvious gearbox trend is that they have gotten bigger with megawatt outputs, and each is designed for its turbine model. “There are no off-the-shelf designs today,” says Mike Carlson with Hanson Transmissions. “Each is designed for the turbine platform and in collaboration with the OEM. Inside, the slow, high-torque rotor speed is initially stepped up with a planetary stage followed by two more stages. Generally below 3 MW, the second or intermediate stage uses helical gears. But 3-MW and larger turbines need a planetary design in the second stage and a helical stage for the high-speed output.” And if OEMs continue designing larger turbines, that last stage will also use a planetary design.
Somewhat easier maintenance has also gotten attention, says Carlson. “Although not all maintenance will be possible in a crampt nacelle, more than what was previously possible will be.”
Romax, a developer of software for gearbox simulations, sees a trend of integrating the bearings that support the main shaft and rotor into the gearbox itself. This can have significant advantages and reduce overall drivetrain size. It also allows reducing gearbox stresses due to high rotor loads.
The market is seeing more integration between gearboxes and generators. This has advantages in terms of overall size, weight, and cost. In some cases, the number of bearings can be reduced in the drivetrain, and the gearbox consists of just one or two planetary gear stages with no high-speed shaft.
Several years ago, a study released by NREL showed that a combination medium-speed gearbox and PM generator would provide the lightest weight drivetrain. That may have encouraged turbine manufacturers to consider medium-speed drivetrains for large wind turbines–typically for machines of 3 MW and above. Medium-speed drivetrains use larger, more costly generators that run at lower speed but with the advantage of reducing the cost and complexity of the gearbox. Fewer gears and bearings are involved in the design – medium speed gearboxes typically consist of one or two planetary gear stages and have ratios between 1:7 and 1:35.
Moventas, a manufacturer of gears and gearboxes for wind turbines, and The Switch, a supplier of megawatt-sized permanent magnet generators and converters, now offer one to the industry. “The lower the nacelle weight, the more cost competitive the turbine,” says Moventas CEO & President Jukka Jäämaa. “Nacelle weight relates directly to foundation and construction costs of the turbine, as well as manufacturing, transport, and assembly costs in the whole supply chain.”