Offshore Market
Sprouts Larger Wind Turbines


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Each one of the V-164 8-megawatt wind turbines is capable of supplying electricity for 7,500 average European households. Image: MHI Vestas

The wind energy market is reaching farther offshore and wind turbines are growing in capacity and stature to deal with the environment. The world’s largest—so far—is now being tested in waters off of Denmark. The V-164 8.0-MW machine sits on a 140-meter-tall tower and uses a 164-meter rotor diameter to capture a swept area about the size of three football fields. Each of the three blades is 80 meters long.

Globally, offshore wind projects are trending farther from shore into deeper waters, partly to reduce visual impact and public opposition to offshore wind farms, according to a 2013 report prepared for the U.S. Department of Energy by Navigant Consulting, Inc. That move corresponds with a trend toward larger turbine size and hub heights to take better advantage of higher wind speeds and produce more energy.

In Denmark, the 8-MW machine is being built and tested by MHI Vestas Offshore Wind, a joint venture of Denmark’s Vestas Wind Systems A/S and Japan’s Mitsubishi Heavy Industries, Ltd. Designed for a 25-year service life with an eye on reduced maintenance, the company says the higher energy output will allow developers to use fewer turbines to produce a required amount of energy.

“Operation and maintenance is always an issue at sea,” says Fort Felker, director of the National Wind Technology Center at the National Renewable Energy Laboratory in Golden, CO. “It’s more difficult so [developers] want fewer numbers.”

The DOE report expects the European market to grow rapidly over the next year to two years, pointing to 2,900 MW of capacity built in 2013, most in Germany and the U.K. Most of the 5,300 MW of world-wide offshore installations are in northwestern Europe and the North Sea.

Folker also points to economies of scale. “In the end, for instance, building 100 10-MW wind turbines is cheaper than 1,000 1-MW turbines,” he notes. “And it lowers construction costs.”

The V-164 prototype installation. Image: MHI Vestas

Market Difference

Offshore prospects in the U.S. are not nearly as robust. There are no commercial offshore installations operating but eleven projects, totaling 3,824 MW, are in advanced stages of development, most along the mid-Atlantic and Northeast coasts. DOE expects U.S. projects to use 4-MW to 5-MW machines, as opposed to earlier generations of smaller turbines used in Europe.

In Denmark, the V-164 is working at the National Test Center for Large Turbines in Østerild, in shallower water than the machine is ultimately expected to work. The nacelle and hub have very large pitch bearings and two long hydraulic pitch cylinders inside each blade mounting area. The nacelle has a width and height of 8 meters, and is 20 meters long: very large but somewhat small for this large a machine, according to reports.

To accomplish that, engineers devised a tube-shaped, compact, medium-speed drivetrain. Essentially a self-supporting structure, it includes a main shaft housing as a key structural and drivetrain element mounted on a cast main chassis.

The main shaft is supported by bearings and attached to the rotor hub in front. A flexible shaft coupling connects it to the gearbox and magnetic generator, and individual drive elements are connected through bolted flanges. Flange connections are designed to eliminate misalignment.

The new generation of offshore turbines is intended to require as little maintenance as possible. Image: MHI Vestas

 

 

 

Larger

Folker expects offshore turbines to continue to gain size, partly because of economics and also because the market—unlike that for land-based machines—is not restrained by transportation and public perception issues. “Size is very much constrained by land-based transportation issues,” he says. “Blades and other components can only be so big to be transported by truck. That’s not the case offshore.”

The DOE report also expects the trend toward larger turbines to continue, “driven by advancements in materials, design, processes, and logistics, which allow larger components to be built with lower system costs.” Folker says the current generation of large turbines will probably top out at 10 MW, and notes a number of manufacturers now offering wind turbines with 6-MW and 7-MW capacities.

One of those is from MHI Vestas, a 7-MW turbine under development that will incorporate hydraulic drive. Mitsusbishi in 2010 acquired Scottish firm Artemis Intelligent Power Ltd., which developed the new drivetrain system to replace a traditional gearbox and the need for power conversion electronics, using “highly reliable” synchronous generators. Called Digital Displacement Transmission technology, the company says it consists of Digital Displacement Pumps and Digital Displacement motors to eliminate the need for conversion electronics. The system is being tested and will be used in the firm’s 7-MW SeaAngel wind turbine.

According to the DOE report, “Hydraulic drives represent the newest (and least-tested) innovation and seek to address the challenges of increasing turbine size in a different way. In essence, the objective of the hydraulic drive design is to separate the rotational speed of the rotor from that of the generator, subsequently enabling the use of standard synchronous generators without the need for a frequency converter. If the design uses a high-voltage generator, the need for a transformer is also negated. This configuration potentially allows for the elimination of components most susceptible to failure while reducing the turbine top mass.”

The promise of higher reliability translates to lower O&M costs in the harsh offshore environment. To date, however, the design “has not experienced success in the marketplace,” notes Folker.

Operations and maintenance is always an issue at sea.

Fort Folker, director, National Wind Technology Center

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August 2014

by John Kosowatz, Senior Editor, ASME.org