Offshore Wind Energy
Commercial-scale offshore wind facilities are currently in operation
in shallow waters off the coasts of Europe, but further technology development
is needed for use in the deeper waters of the Outer Continental Shelf (OCS).
Wind is air in motion. Since the earth’s surface is made of
various land and water formations, it absorbs the sun’s radiation unevenly.
Wind is produced by the uneven heating of the earth’s surface by the sun.
Onshore, wind energy has been utilized for power generation
for more than two thousand years. In modern times, wind energy is mainly used
to generate electricity, primarily through the use of wind turbines.
Wind flows over the airfoil-shaped blades of wind turbines, causing lift (similar
to the lifting force on airplane wings), causing the turbine blades to turn.
The blades are connected to a drive shaft that turns an electric generator to
produce electricity.
Offshore Wind Energy Resources
Offshore wind turbines are being used in a number of countries
to harness the energy of the moving air over the oceans and convert it to electricity. Offshore winds tend to flow at higher speeds than onshore winds, thus allowing turbines to produce more electricity. Much of this potential energy is near major population (and energy load) centers where energy costs are high and land-based wind development opportunities are limited.
Because the potential energy produced from the wind is directly proportional to the cube of the wind speed, increased wind speeds of only a few miles per hour can produce a significantly larger amount of electricity. For instance, a turbine at a site with an average wind speed of 16 mph would produce 50% more electricity than at a site with the same turbine and average wind speeds of 14 mph.
Offshore Commercial Wind Energy Generation
Many offshore areas have ideal wind conditions for wind facilities. Denmark and the United Kingdom have installed large offshore wind facilities to take advantage of consistent winds. Today, just more than 600 MW of offshore wind energy is installed worldwide, all in shallow waters (<30 meters) off the coasts of Europe. Proposed offshore wind projects through 2010 amount to more than 11,000 MW, with about 500 MW each in the United States and Canada, and the remainder in Europe and Asia.
Commercial-scale offshore wind facilities currently are similar to the onshore wind facilities, but with modifications to prevent corrosion and protect against wave and wind interactions. Because roughly 90% of the U.S. OCS resources are over waters that are much deeper than European waters where commercial wind facilities are currently sited, new technologies are being developed (e.g., for strengthened tower foundations) to harness the wind in the harsher conditions associated with deeper waters.
Offshore Wind Energy Technology
Offshore wind facilities today are generally developed and operated as follows.
Once a suitable place for the wind facility is located, piles
are driven into the seabed. For each turbine, a support structure and a tower
to support the turbine assembly, to house the remaining plant components, and
to provide sheltered access for personnel are attached to the piles. After the
turbine (generally a three-bladed rotor connected through the
drive train to the generator) is assembled, wind direction sensors turn the
nacelle (a shell that encloses the gearbox, generator, and
blade hub) to face into the wind and maximize the amount of energy collected.
Wind moving over the blades makes them rotate around a horizontal hub connected
to a shaft inside the nacelle. This shaft, via a gearbox, powers a generator
to convert the energy into electricity.
For more information about how wind turbines work, including an animation of
a wind turbine, see How
does a Wind Turbine Work? on the National Wind Technology Center Web site.
Offshore turbines have technical needs not
required of onshore turbines due to the more demanding climatic environmental
exposure offshore. Offshore turbines look similar to those onshore
with several design modifications. These include strengthening the tower to
cope with wind-wave interactions, protecting the nacelle components from sea
air, and adding brightly colored access platforms for navigation and maintenance.
Offshore turbines are typically equipped with corrosion protection, internal
climate control, high-grade exterior paint, and built-in service cranes. To
minimize expensive servicing, offshore turbines may have automatic greasing
systems to lubricate bearings and blades and pre-heating and cooling systems
to maintain gear oil temperature within a narrow temperature range. Lightning
protection systems minimize the risk of damage from lightning strikes that occur
frequently in some locations offshore. There are also navigation and aerial
warning lights. Turbines and towers are typically painted light blue or grey
to help them blend into the sky. The lower section of the support towers may
be painted bright colors (e.g. yellow) to aid in navigation and highlight the
structures for passing vessels.
Offshore wind turbines are also bigger than
onshore turbines (to take advantage of the steadier offshore winds and economies
of scale). A typical onshore turbine installed today has a tower height of about
60 to 80 meters, and blades about 30 to 40 meters long; most offshore wind turbines
are at the top end of this range. Offshore turbines installed today are generally
between 2 and 4 MW, with tower heights greater than 200 feet and rotor diameters
of 250 to 350 feet. Turbines of up to 5 MW are being tested.
Transport of Wind-Generated Energy
Undersea collection cables connect multiple turbines in the
wind facility and transport the electricity from them to a transformer
where the combined electricity is converted to a high voltage for transmission
via undersea cables to a substation. There the electricity is connected to the
onshore electricity grid. Alternative approaches, such as using the wind to
produce hydrogen (through the hydrolysis of desalinated seawater),
which would be shipped to shore for later use, are also being investigated.
Environmental Considerations
Potential impacts on the environment that may occur during construction, operations,
and decommissioning of offshore wind facilities are highlighted below.
Marine life. Foundations can act as artificial reefs with
a resultant increase in fish populations from the new food supply. These increases
in fish population may also have stimulating effects on bird populations in
the area, which could cause collisions between birds and towers or rotors.
Migrating birds. Besides potential collisions (bird strikes),
it is possible that the birds would need to consume more energy to avoid collisions
and maintain their orientation when navigating around the turbines. Tower illumination
may also cause navigational disorientation for birds.
Interference with navigation for endangered and threatened species.
Electromagnetic fields created by the electric cables running from the turbines
and underwater noises and vibrations could affect orientation and navigational
ability.
Potential alteration of natural environments and diminution of habitats.
Underwater support pilings, anchoring devices, scour-protection materials, and
electromagnetic fields could cause a decrease in benthic communities, alter
natural environments, and possibly affect migration patterns.
Emissions. Each unit of electricity generated from the wind
that saves a unit generated from fossil fuels, which will help reduce greenhouses
gases, pollutants, and waste products that result from fossil fuel use.
Marine traffic, recreation, and other sea space uses. It
is possible that wind turbine energy plants may disrupt air traffic control
and maritime radar systems, and that facilities siting could affect recreation
and other sea space uses.
Visual impacts from towers, rotating turbine blades and navigation
and aerial warning lights.
Noise impacts from rotating turbine blades.
For More Information
Download the wind technology white paper:
The following presentation from the National Renewable Energy Laboratory also provides information on ocean-based renewable energy technologies, including wind energy technology. This presentation was shown at scoping meetings for the OCS Alternative Energy Programmatic EIS.
Links to additional information on this topic are also available on the Links page.
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