Authored by:
Robert Repas
Associate Editor
robert.repas@penton.com

Everyone’s owned a pinwheel at one time or another. I remember riding in the back of my parents’ car, sticking my pinwheel out the window as we traveled down the street. The speed at which the pinwheel turned matched the velocity of the car. On backstreets with low-speed limits the pinwheel did great. But high-speed interstates were another matter: My pinwheel soon flew apart as the car accelerated to cruise speed.

Wind turbines can suffer a similar fate. Not by sticking them out the window of a car, of course, but by subjecting them to wind speeds found in thunderstorms and other weather events that exceed turbine design limits. Those conditions force turbine designers to somehow prevent the rotor speed of the turbine from exceeding its design range. Ignoring such precautions may produce a scene similar to the wind-turbine explosion videos that now populate YouTube. (One example: http://bit.ly/4FEHbZ.)

Brakes could be used to keep the rotor from turning, but they’re subject to failure or overload by the wind force on the turbine. The turbine in the example video went into a runaway condition when its brakes failed, letting the rotor freewheel.

A better technique used to combat runaway conditions like this is blade pitch control. The pitch of a wind-turbine blade describes the angle of the blade chord to the plane of rotation. It is analogous to the way the pitch of a leadscrew determines how far its load moves with each rotation of the screw. Similarly, the pitch of a turbine blade is the distance the blade would travel through the air in one rotation if it were 100% efficient. Obviously turbine blades don’t move forward. Rather, the wind pushes air past the blades as though the blades had moved forward.

Wind speed versus the desired turbine rpm determines blade pitch. There is a specific pitch angle for any given wind speed to optimize output power. Pitch angles greater or less than this value reduce power output, even to the point of zero rotation with high winds.

The hub of a wind-turbine propeller houses the pitch-control system which, according to the European Wind Energy Association, accounts for about 3% of a wind turbine’s total price. That small investment makes a difference when conditions deteriorate. In fact, many turbine makers now consider pitch control a good “insurance” policy.

When wind speed reaches 25 m/sec (50 mph) or higher, the pitch-control system fail-safes the blades in a manner that reduces wind loading and stops the turbine rotor from turning. These systems also monitor wind speed and load to set the turbine blades at the best angle needed for power output. Changes in blade pitch typically start when wind speed reaches 12 to 13 m/sec (27 to 29 mph), the point where the turbine reaches peak performance. If wind starts to exceed that level, the pitch-control systems kick in to reduce the blade angle of attack, taking a lower percentage of energy from the wind to keep the generator near 100% output without overspeeding.

Pitch-changing systems generally come in two forms: either electric or hydraulic. Rarely do makers of wind turbines use both types. According to research from Intercedent Asia, the choice of pitch-control system depends on the turbine manufacturer. In other words, the type of pitch-control system never becomes a major issue should you find yourself buying a wind turbine.

In a hydraulic system, hydraulic actuators control the pitch of all blades simultaneously. The actuator typically works against a spring that functions as a stop fail-safe upon loss of hydraulic pressure. Hydraulic systems also seem to have a longer life, more driving power for a higher speed response, and a low-maintenance backup system (the spring) in case of failure. However a major drawback is the hydraulic fluid itself. Should a seal leak, it’s possible for the blade to sling hydraulic fluid over a wide geographic area, contaminating the surrounding countryside. Additionally, hydraulic systems tend to use more energy as the hydraulic pump must run continuously to keep pressure high.

Obviously, electric pitch-control systems have no risk of leaking hydraulic fluid. They also consume less power and waste less energy. Once the actuator reaches its desired position, the actuator motor can turn off while still holding the blade at the proper pitch angle. However, electric pitch-control systems need fail-safe batteries or supercapacitors to allow for loss of primary power or control. Fail-safe batteries typically last only two to three years, and then must be replaced — not a simple task as the fail-safe batteries sit in the hub of the rotor, not in the nacelle. (The hub location assures power remains available for the pitch-control system during an emergency such as a grid loss or failure of the slip rings.) Electric systems also work better in colder climes where the oil in hydraulic systems loses viscosity as the temperature drops.

Future developments may bring a third option: a hybrid electrohydraulic system. Hybrid technology uses electricity to control blade pitch for daily operation, but uses hydraulic power to operate the fail-safe that prevents damage to the blades.

Proponents of hybrid solutions say that because pitch control relies mostly on electrical power, it mitigates the risk of leaking oil. They also contend this would lower energy costs. As hybrid pitch-control systems rely on hydraulics for fail-safe power, advocates point out the lack of need for fail-safe batteries and their corresponding maintenance.

© 2011 Penton Media, Inc.