Block Island Wind Farm consists of five GE Haliade 150-6MW offshore turbines with enough capacity to generate 30 MW of renewable electricity from the breezes whipping over the sea’s surface. (Image: GE Reports)

The small island sits off the coast of Rhode Island between the northeastern tip of New York’s Long Island and Martha’s Vineyard, Massachusetts. There, like the archetypal New Englander, it cuts a hardy and stout profile, with chin out, to face the battering North Atlantic Ocean, the bruising wind and the weather it brings.

But a new era has dawned, and the relationship between the wind and Block Island will never be the same. In the wee hours of May 1, the diesel generators operated by the Block Island Power Company, the island’s electricity provider, fell silent.

Workers shut down the units at 5:30 a.m. and switched some 2,000 customers over to a new source.

When lights came back on moments later, they were powered by electrons streaming from Deepwater Wind’s Block Island Wind Farm. The switch made Block Island the United States’ first community to receive electricity from an offshore wind project. It should allow the power operator to save  almost a million gallons of diesel fuel every year.

The wind farm consists of five GE Haliade 150-6MW offshore turbines with enough capacity to generate 30 MW of renewable electricity from the breezes whipping over the sea’s surface here. It sits in the ocean three miles south of Block Island, which now receives its power via undersea cables connecting the wind farm to the island and the Sea2Shore cable linking the wind farm to the mainland grid in Narragansett, Rhode Island.

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Image credit: HAWE Hydraulics

The muscle that pitches wind-turbine blades can come from either a hydraulic or electric device on most turbines rated at and below 2.5 MW. But for turbines over 3 MW, the job of pitching blades more often falls to hydraulics. And hydraulics can handle more.

“Hydraulics in wind turbines usually refers to the assemblies for brake control and regulating the blade setting through pitch control,” said Bob Pettit, Corporate Technical Director at HAWE Hydraulics.

In a nutshell, the process of turning wind into electricity involves a drivetrain turned by a rotor of two or three blades and related equipment. While small turbines often have fixed rotor blades – they do not pitch – larger turbines require blades that pitch, and so are mounted on bearings.

To drive each blade to its best pitch position requires a hydraulic pump, motor, reservoir and associated equipment. For example, the pump and motor are usually mounted in the nacelle while hydraulic pistons are mounted in the hub. A hydraulic rotary joint allows passing hydraulic fluid from the stationary side to the rotating side. Pitch control then varies the pitch of the blades to maintain nearly-constant rotational speed at the generator.

Aside from wind velocity changes, the pitch of the blades may vary even during a 360-degree rotation of a single blade. Such control is necessary because wind velocity at the 12 o’clock position may be significantly different from its velocity at the 6 o’clock position, according to Afzal Ali, Director of Marketing at Deublin. Typically, the pitch of each blade varies continuously and independently.

In an emergency, hydraulic pitch control can operate without an external power supply thanks to an accumulator, a sort of hydraulic battery. Hydraulic actuation provides the shortest stopping time, a wider range of operating temperatures than alternative systems and it’s backlash free.

The rotor brake, also hydraulically activated, engages during emergency stops as well as service work when the turbine is manually shut down. Another hydraulic system, one that activates yaw brakes, is comprised of several hydraulically-activated brake calipers which act on a break disk at the top of the tower. During normal turbine operation, brake calipers are under maximum pressure to keep the nacelle facing the wind direction.

Hydraulic equipment usually includes an accumulator as a way to store energy in case of an emergency shutdown. The system is usually designed to fill the accumulator during off-demand periods, when pump flow is not allocated to system actuators. The pressurized fluid stored in the accumulator can then pitch the turbine blades to a safe position where they can stay until power is restored or the halting condition is corrected.

Researchers are considering the advantages of replacing gearboxes in some turbines with hydraulic transmissions.

“For instance, there is some compressibility in the fluid that prevents shock loading to the generator,” said University of Minnesota and Mechanical Engineering Professor Kim Stelson. “Hydraulics equipment is lighter than the conventional equipment it replaces. Also, control is somewhat simplified. Controls in a conventional turbine are based on torque, a quantity that can be difficult to measure, although it can be approximated with current. But in a hydraulic transmission, it is measured with pressure from the pump and that is more convenient.”

Other researchers are also devising ways to replace gearboxes in large turbines. While working on the development of a variable-displacement hydraulic machine in the 1980s, engineers at Artemis Intelligent Power found a way to improve hydraulic efficiency. The company replaced the mechanical valves and swash-plates of conventional variable-displacement hydraulic machines with computer-controlled high-speed solenoid-valves. This opens new markets for hydraulics because Digital Displacement machines are efficient at all load levels and have super-fast response to computer control. This development has led one Japanese turbine OEM to build a multi-MW unit with a hydraulic drive that is now working off the coast of Japan near Fukushima.

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The artist’s concept is of a 7-MW SeaAngel. The turbine is due for installation in the U.K. this year.

Hydraulic drivetrains in wind turbines present great possibilities. For one, a hydraulic pump and motor would allow getting rid of the troublesome gearbox, thereby improving reliability. With such an arrangement, the turbine rotor would turn a variable-volume pump which would drive a hydraulic motor to spin the generator. Accumulators could briefly store excess pump energy and thereby smooth out wind’s variable nature. A few companies have tested hydraulic drivetrains but abandoned the concept because of insufficient efficiency.

Engineers at Mitsubishi Heavy Industries (MHI, www.mhi-global.com) now think the system will work with the help of technology acquired from Artemis Intelligent Power (www.artemisip.com), a U.K.-based hydraulic company that developed unconventional variable-volume pumps and motors governed by computer controls.

The SeaAngel’s nacelle, being transported at Yokohama Dockyard & Machinery Works, houses the turbine’s DDT hydraulic drivetrain.

Together, the companies have designed and built the hydraulic drivetrain into a working prototype that was launched at Yokohama Dockyard & Machinery Works in Japan late last year. Referred to as the SeaAngel, the test turbine is part of a bigger project to development a hydraulic drivetrain for offshore wind turbines. The prototype is also the world’s first wind turbine with a large-scale hydraulic drivetrain.

The 2.0-MW prototype is working on a dock in Yokohama harbor.

Artemis engineers refer to their drivetrain as a Digital Displacement transmission (DDT) and say the Mitsubishi turbine uses the largest one to date. The DDT consist of a Digital Displacement pump and motor replacing a conventional gearbox. The 2.0-MW DDT prototype is based on MHI’s MWT100 gear-driven wind turbine. The hydraulic equipment will provide a 1:100 speed increase for the generator’s design speed of 1,000 rpm.

MHI says the hydraulic drivetrain offers several advantages over conventional systems. For one, it improves on efficiency and reliability by eliminating the conventional gearbox and inverter components that have been problematic. Also, the new configuration is said to improve the cost performance over conventional turbines as a result of its widely used hydraulic equipment, materials, and relatively inexpensive alternators.

Artemis’ Digital Displacement design is said to increase efficiency by providing an alternative way to build variable-displacement hydraulic pumps and motors. Controls can individually open the valves on a cylinder to change the volume pumped.

Using the concept, a 7-MW offshore wind turbine is due for installation in the U.K. this year, says MHI. Two more on floating platforms have been ordered by the Japanese government for operation off Fukushima, Japan. The new installations are slated to begin trial operations this year and August 2014. The company says a mass-produced commercial model will be targeted for market launch in 2015. WPE

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