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 Fall 2010

How It Works

Offshore Wind Power

The United States has built wind power facilities on land at a record pace in recent years. Following in the footsteps of Europe and other regions, U.S. wind developers are now looking offshore, where the winds blow stronger and more consistently. Offshore wind could more directly deliver clean energy to major coastal cities, where demand for electricity is high, without the pollution and global warming emissions associated with extracting and burning fossil fuels.

Wind turbines capture the kinetic energy of wind with their turning blades, which transfer the energy to a spinning rotor shaft that drives an electric generator. Because of the relative ease of transporting large components by sea, offshore wind turbines can be much bigger—and capture more kinetic energy—than their land-based counterparts. Offshore turbines have blades that spin in a circle up to 400 feet in diameter—twice as wide as the wingspan of a Boeing 747—and generate up to five megawatts (MW) of electricity, compared with the 240-foot span and 1.5 MW capacity common for today’s land-based turbines. Offshore turbine towers also do not need to be as tall (relative to the size of their blades) as land-based turbines (which average 250 feet), because open water is free of the vegetation and topography that create wind shear and turbulence over land.

Illustration adaped from NREL

In shallow waters (less than 100 feet deep), where almost all offshore wind projects are located, turbines can be built directly on the sea floor, with the tower anchored into a large steel tube driven 80 to 100 feet deep into the seabed (see the diagram). Turbines in deeper waters require more complex mounting structures; two recent deep-water installations in Europe use wider-base structures (such as tripods) to support their five-megawatt turbines. The electricity generated from offshore turbines passes through cables buried under the sea floor to a substation either onshore or on an offshore platform, and is then delivered to the electrical grid.  

A Promising Solution . . .

Though offshore wind power is a new development for the United States, it has been producing clean electricity for Europe since 1991. Today, nine European countries have developed more than 2,000 megawatts of offshore wind capacity; another 16 projects under construction will almost triple this capacity, and dozens more are planned. Elsewhere, China and Japan have completed their first offshore wind farms and have more projects in the pipeline.

In the United States, the National Renewable Energy Laboratory estimates that offshore wind has a technical potential three times the country’s current electricity capacity. Yet only one such project has received federal and state approval for construction (see the sidebar). Thirteen other projects along the East Coast, Gulf of Mexico, and Great Lakes are at advanced stages of permitting or development. To help coordinate the efforts of stakeholders (including project developers, regulatory agencies, and clean energy advocates) and ensure the sustainable growth of offshore wind in the United States, UCS joined with government, environmental, industry, and academic colleagues to form the U.S. Offshore Wind Collaborative in 2009.

Finally, Wind Power off Our Own Shores

The nation's first offshore wind facility clears a final hurdle to construction.

Cape Wind, a 130-turbine, 468 MW wind power project proposed five miles off the coast of Cape Code, MA, received federal approval in April 2010 after almost nine years of exhaustive reviews by state and federal agencies. The review process--which took much longer than comparable reviews for traditional coal-fired power plants--included a two-year study that examined 27 categories of potential environmental, safety, and socioeconomic impacts during construction and operation.
The vast majority of the impacts were found to be minor, negligible, or even positive. Cape Wind's developer agreed to perform several years of post-construction monitoring to assess the turbines' impact on birds, bats, and other wildlife, and to take measures to minimize these impacts.
The facility will generate enough energy, on average, to meet almost three-quarters of the demand on Cape Cod as well as the nearby islands of Martha's Vineyard and Nantucket, displacing electricity generated from dirtier fossil fuels. Because of Cape Wind's precedent-setting potential for the U.S. offshore wind industry as a whole, UCS participated in every stage of the permitting process to provide information on the economic and environmental benefits of wind power. The project can proceed once long-term financing is secured.

. . . And a Unique Set of Challenges

Like its land-based counterpart, offshore wind offers several important benefits. It generates electricity without consuming fuel or water, and produces no global warming emissions, air pollution, water pollution, or waste during operation. However, as on land, turbines can harm birds and bats, interfere with aircraft navigation, and raise social, cultural, and economic concerns for local communities. Offshore developers also need to take into account the potential impact on fish, marine mammals, and the sea floor.

Despite these concerns, studies of existing offshore wind projects have been largely positive. Observational data from the 72-turbine Nysted facility in Denmark, for example, show that birds tend to fly around, rather than through, the wind farm, even in conditions of poor visibility. Potential avian impacts can be minimized by siting turbines away from high-traffic flight paths and adjusting operations during seasonal migrations.

The other obstacle facing offshore wind development is cost. Offshore facilities often have large numbers of turbines, each of which must be actively monitored and maintained. Because of the difficulty of servicing wind farms at sea, offshore turbines involve more remote monitoring and automated systems than their land-based counterparts, but even with only a few visits to each turbine per year, operation and maintenance costs are considerably higher than those for onshore wind projects.

Deep-water projects—including the first full-scale floating wind turbine, installed off Norway in 2009—currently cost considerably more than today’s fixed-base offshore turbines, but will become more cost-competitive as developers gain experience with the technology and undertake projects with multiple turbines.

Offshore wind is still a young technology and, like any emerging energy resource, it will take time to become established. By continuing to support research and development, and building construction and maintenance experience, we can help make offshore wind a promising source of clean, reliable, carbon-free power.

Owen Westbrook is a former research fellow in the Climate and Energy Program. John Rogers is a senior analyst/advocate in the program.

To learn more about the environmental and economic benefits of wind power, visit the UCS website at