Matthew Lackner, UMass Amherst
Northern California has some of the strongest offshore winds in the U.S., with immense potential to produce clean energy. But it also has a problem. Its continental shelf drops off quickly, making building traditional wind turbines directly on the seafloor costly if not impossible.
Once water gets more than about 200 feet deep – roughly the height of an 18-story building – these “monopile” structures are pretty much out of the question.
A solution has emerged that’s being tested in several locations around the world: wind turbines that float.
In California, where drought has put pressure on the hydropower supply, the state is moving forward on a plan to develop the nation’s first floating offshore wind farms. On Dec. 7, 2022, the federal government auctioned off five lease areas about 20 miles off the California coast to companies with plans to develop floating wind farms. The bids were lower than recent leases off the Atlantic coast, where wind farms can be anchored to the seafloor, but still significant, together exceeding US$757 million.
So, how do floating wind farms work?
Three main ways to float a turbine
A floating wind turbine works just like other wind turbines – wind pushes on the blades, causing the rotor to turn, which drives a generator that creates electricity. But instead of having its tower embedded directly into the ground or the seafloor, a floating wind turbine sits on a platform with mooring lines, such as chains or ropes, that connect to anchors in the seabed below.
These mooring lines hold the turbine in place against the wind and keep it connected to the cable that sends its electricity back to shore.
Most of the stability is provided by the floating platform itself. The trick is to design the platform so the turbine doesn’t tip too far in strong winds or storms.
There are three main types of platforms:
- A spar buoy platform is a long hollow cylinder that extends downward from the turbine tower. It floats vertically in deep water, weighted with ballast in the bottom of the cylinder to lower its center of gravity. It’s then anchored in place, but with slack lines that allow it to move with the water to avoid damage. Spar buoys have been used by the oil and gas industry for years for offshore operations.
- Semisubmersible platforms have large floating hulls that spread out from the tower, also anchored to prevent drifting. Designers have been experimenting with multiple turbines on some of these hulls.
- Tension leg platforms have smaller platforms with taut lines running straight to the floor below. These are lighter but more vulnerable to earthquakes or tsunamis because they rely more on the mooring lines and anchors for stability.
Each platform must support the weight of the turbine and remain stable while the turbine operates. It can do this in part because the hollow platform, often made of large steel or concrete structures, provides buoyancy to support the turbine. Since some can be fully assembled in port and towed out for installation, they might be far cheaper than fixed-bottom structures, which require specialty vessels for installation on site.
Floating platforms can support wind turbines that can produce 10 megawatts or more of power – that’s similar in size to other offshore wind turbines and several times larger than the capacity of a typical onshore wind turbine you might see in a field.
Why do we need floating turbines?
Some of the strongest wind resources are away from shore in locations with hundreds of feet of water below, such as off the U.S. West Coast, the Great Lakes, the Mediterranean Sea and the coast of Japan.
The U.S. lease areas auctioned off in early December cover about 583 square miles in two regions – one off central California’s Morro Bay and the other near the Oregon state line. The water off California gets deep quickly, so any wind farm that is even a few miles from shore will require floating turbines.
Once built, wind farms in those five areas could provide about 4.6 gigawatts of clean electricity, enough to power 1.5 million homes, according to government estimates. The winning companies suggested they could produce even more power.
But getting actual wind turbines on the water will take time. The winners of the lease auction will undergo a Justice Department anti-trust review and then a long planning, permitting and environmental review process that typically takes several years.
Globally, several full-scale demonstration projects with floating wind turbines are already operating in Europe and Asia. The Hywind Scotland project became the first commercial-scale offshore floating wind farm in 2017, with five 6-megawatt turbines supported by spar buoys designed by the Norwegian energy company Equinor.
Equinor Wind US had one of the winning bids off Central California. Another winning bidder was RWE Offshore Wind Holdings. RWE operates wind farms in Europe and has three floating wind turbine demonstration projects. The other companies involved – Copenhagen Infrastructure Partners, Invenergy and Ocean Winds – have Atlantic Coast leases or existing offshore wind farms.
While floating offshore wind farms are becoming a commercial technology, there are still technical challenges that need to be solved. The platform motion may cause higher forces on the blades and tower, and more complicated and unsteady aerodynamics. Also, as water depths get very deep, the cost of the mooring lines, anchors and electrical cabling may become very high, so cheaper but still reliable technologies will be needed.
But we can expect to see more offshore turbines supported by floating structures in the near future.
This article was updated with the first lease sale.
Matthew Lackner, Professor of Mechanical Engineering, UMass Amherst
This article is republished from The Conversation under a Creative Commons license. Read the original article.
I am offering my comments as a private individual with no personal, financial or business interests in the wind power industry or their related industries involving the sale and supply of these components used to manufacture, build and install the foundation pilings or the wind turbines and turbine blades.
Offshore wind turbines whether they are mounted on monopiles driven or drilled and inserted into the seabed or built as floating wind turbines in deep water will still kill avian species as they continue to do on land based Horizontal Axis Wind Turbines/VAWT even though some still continue to deny this occurs.
An illness that has been identified that is referred to as wind turbine syndrome that has been linked to the Horizontal Axis Wind Turbine/HAWT where the vibrations created by the rotation of the large turbine blades are transmitted to the tower it is mounted on and then into nacelle and then into the tubular tower and then into the ground where this vibration is transmitted in all directions to nearby homes where the residents of these homes have reported these symptoms identified as low frequency vibration which have been identified by physicians as Wind Turbine Syndrome.
A great deal if information about Wind Turbine Syndrome has been published by scientists involved in the study of human health and physicians that have examined the individuals that live near wind turbines
and how these turbines have affected their health where previously they did not have these symptoms
classified as Wind Turbine Syndrome.
Installing these turbines over water eliminates the need to police the area under the turbine towers as the surface currents and wave action will carry away the dead birds and bats that impact the turbine blades.
There is a better, safer, less costly patented, wind tunnel tested wind turbine design.
That turbine is the Vertical Axis Wind Turbine that was designed, built, wind tunnel tested which was granted a Unites States patent to Mr. Adam Fuller.
Mr. Fullers Vertical Axis Wind Turbine is smaller and can be built for less cost and can be stacked in a tubular tower support frame to create a greater surface area exposed to the prevailing winds in all directions. This is due to the fact that the air scoops are fully exposed to any wind direction and the wind turbine will spin at very low wind speeds and creates much less measured noise or vibration at higher wind speeds as the air scoops absorb the wind and turn exposing many more air scoops to the prevailing winds or winds in any direction for that matter and create electricity at lower wind speeds.
Mr. Fullers Vertical Axis Wind Turbine presents itself as a solid object to the wind in any direction and avian species can avoid it completely.
In the case of a Horizontal Axis Wind Turbine the turbine blades do not appear as solid objects and as the bird or bat can be struck and killed by the turbine blade anywhere in the spinning diameter of the turbine blades total width and height as the Horizontal Axis Wind Turbine HAWT profile creates a clear open cylinder as the turbines blades rotate and the nacelle the turbine blades are mounted to rotates to meet the prevailing winds.
Mr. Fullers Vertical Axis Wind Turbines generator can be accessed and serviced at ground level with
unskilled labor and hand tools as his simple to use and repair design uses toothed sprockets and SAE roller chain to transmit the torque created by the spinning turbine rotor via the roller chain to the sprocket mounted on the generator head at the base of the turbine frame.
There is a better way to do this that has less total cost per kilowatt, that way is by using Mr. Fullers Vertical Axis Wind Turbine on land.