How are hydraulics used in wind turbines?

The wind-energy industry makes good use of hydraulics, in particular how well it combines power density and durability for the muscle needed to pitch turbine blades that easily weigh two to three tons. In recent years, however, most utility-scale wind turbines for land installations have turned to electromechanical systems for this function. Electromechanical systems typically allow better packaging efficiencies, plug-and-play electronic control, and a lesser likelihood of fluid leaks. Over the last few years, several design trends are reshaping how turbine designers apply hydraulics.

Take the worldwide offshore-wind industry, for instance. The trend is to place the larger machines offshore, 6 and 7-MW giants with 8 to 10-MW units in the works. The rotor on a recent 6 MW Siemens turbine weighs 25 tons. As the machines are built larger, designers must grapple with the increased loads required to pitch longer, heavier blades. To make matters worse, the harsh environment offshore makes longer service intervals the norm. Such conditions greatly reduce the plusses of a traditional electromechanical system for blade pitching.

One solution to the problem, from Parker Hannifin, combines the best attributes of a hydraulic system (power density and durability) with the best attributes of an electromechanical system (package size and plug-and-play controls) into a hybrid system, referred to as an electro-hydrostatic actuator (EHA).

An EHA system drives a linear (hydraulic cylinder) or rotary (hydraulic motor) actuator from a hydraulic pump coupled to an ac motor, similar to those used in high-precision industrial applications. The motor is controlled by a plug-and-play interface, familiar to the industry. Another benefit to the system is a directed oil flow and discharge design that eliminates inefficiencies, leak-points, and packaging challenges common to traditional hydraulic systems, thereby significantly reducing system size and complexity.

“An EHA approach eliminates the Achilles heel of a traditional electromechanical, blade-pitch control architecture,” says Dheeraj Choudhary with Parker Hannifin’s renewable energy business. “Traditional EM-systems use large banks of batteries to provide necessary backup power for emergency stops, but these could fail due to environmental conditions and charge-discharge cycling. An EHA approach uses the simple and effective hydraulic accumulators to provide backup power. Accumulators have excellent power density and are immune to environmental conditions and charge-discharge cycling.”
Another hydraulic trend has been to proactively manage oil contamination and preventive maintenance on fluid power systems to help keep turbines in peak operating condition. “Old-school maintenance programs of replacing old parts with new ones at regularly scheduled intervals is being quickly succeeded by preventive maintenance programs that extend part life, supplemented with replacement-part strategies that offer equal or better parts at a lower cost,” says Ken Rohr with Applied Industrial Technologies.

This last trend has been forming in fits and starts but it may have finally found traction thanks to hydraulic motors and pumps with variable valve timing, a feature made possible by electronic controls. It deals with the hydraulic drivetrain, an idea tried and abandoned by some hydraulic equipment companies. The concept is to let the turbine rotor drive a hydraulic pump that drives hydraulic motors that turn one or more generators. The great advantage of hydraulics is it gets rid of troublesome gearboxes that have failed too easily even in relatively benign locations on land. One 3-MW hydraulic-drive unit is going to sea to test the idea. WPE