Green technology: Inside an advanced wind turbine

Back in the ancient times of 10 or 15 years ago, it wasn’t unusual for wind turbines to generate power in the range of tens or hundreds of kilowatts. Compared to the power levels found on the utility grid, this puny generating capacity almost made individual turbines an afterthought. Utilities didn’t have to worry much about grid operations being interrupted by problems at a wind farm.

But wind farms are getting bigger. The next generation of multi-megawatt- scale turbines start to look less like quaint curiosities and more like regular power plants. That also means the wind turbines now coming off the drawing boards must have the same kind of reliability and safeguards as conventional power plants. Their economics must be in the same ballpark as well.

The latter point is important because since about 2002, the cost of wind-powered electricity has not declined. To that end, wind-turbine designers are striving to come up with designs that are both more economical and reliable.

Enter Clipper Windpower and its Liberty turbine. The Liberty sports innovations that include a distributed generation drivetrain design (dubbed the Quantum Drive) and high-efficiency, permanent- magnet generators. These pair up with a sophisticated control system that adjusts both rotor blades and generator operation to squeeze power even out of light breezes.

The Liberty design also incorporates ideas aimed at simplifying the maintenance of wind farms. For example, many of its components are light enough to be winched up its tower rather than be manhandled by a mobile crane.

Checking out the Drivetrain
The meat and potatoes of a wind turbine are its rotor, gearbox, and generator. Liberty’s major drivetrain components illustrate some of the directions in which windturbine development is progressing today.

The gearbox cranks up the rotational speed that the wind rotor provides. The traditional way of doing this has been through a planetary gearbox with three or four stages of rpm step-up. The gearbox output shaft turns the shaft of a generator. The generator connects to the utility grid through conversion circuits that change the ac it generates to dc, and finally to ac synchronized with the grid frequency. A transformer then boosts the output to about 30 kV for insertion onto the grid.

Traditional wind turbines configured this way have drawbacks that become apparent in machines big enough to generate more than about a megawatt. Generators and gearboxes on this scale are large and heavy. Problems in either of these two mechanisms can take the wind turbine off-line, potentially for a long time. Moreover, windfarm operators often must bring in a crane to take heavy generators and gearboxes out of the wind-turbine nacelle for maintenance.

One way the Liberty wind turbine addresses these problems is by using its gearbox to drive four smaller generators rather than one big one. The generators are light enough to be raised and lowered with a hoist built into the turbine structure. And a problem with a single generator doesn’t take the whole wind turbine off the grid.

The gearbox driving the generators has a special patented design with features aimed at minimizing the need for maintenance. Unlike those in most wind turbines, it is not in a planetary configuration. Instead, the input shaft from the turbine blades couples to a pair of 65-in.-diameter bull gears. Four double-helical pinions engage the bull gears. On the other end of each pinion shaft is an intermediate gear.

The intermediate gears are configured so they engage four singlehelix pinions. Each pinion engages two adjacent intermediate gears.

Thus the gearbox has two stages. The pair of bull gears and double-helix pinions make up the first stage. The intermediate gears engaging the output pinions on the output shafts comprise the second.

Helical gears promote smooth meshing. Clipper uses the double-helix pinions in the first stage for their ability to cancel out internally generated thrust loads. But the helices on the pinions are set at two different angles. The exact rationale for using two different helix angles is proprietary. All Clipper will say is that due to the double helix, the net thrust on the intermediate shaft assembly is zero. Thus, there is no need for a thrust bearing on the intermediate shaft. The highspeed shaft has a net thrust so it is equipped with a thrust bearing.