Instant Floating Offshore Wind Turbine: Just Remove Tower

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The floating wind industry is up to something. Instead of relying on the familiar three blades perched on a tall tower, new floating turbines are configured like strange, seagoing carousels, with blades that circle around a vertical axis. The vertical axis is the key that enables radical new design strategies, including — wait for it — a wind turbine that doesn’t have a tower at all.

The Offshore Floating Wind Advantage

The towerless wind turbine in question is a project of the U.S. Department of Energy’s Sandia National Laboratories. It didn’t just pop up out of nowhere. The lab has been researching vertical axis technology since the 1970’s.

In 2019 the Energy Department put out a call for new, cost-cutting floating wind turbine designs with the aim of cutting costs, and a team at Sandia was ready with a project called ARCUS.

The ARCUS project resulted in a 22-megawatt concept turbine that zeroes in on an opportunity that has been skipped over by first-generation floating wind turbines. The first floating turbines resemble onshore turbines, except they float. They also cost about 3-5 times more than on shore turbines, according to the Sandia team.

After running the numbers, the Sandia team realized that floating wind actually has an interesting advantage over onshore turbines. The turbine represents only 20% of the cost of a floating wind farm. The rest goes to the complex systems needed to make a floating platform workable. So, it’s worth the time and effort to examine the 80% of costs other than the turbine, and work on cutting them.

For the record, turbines account for almost half the cost of onshore wind farms, according to Sandia.

The Vertical Axis Solution

That leads to the burning question of why Sandia settled on a vertical axis configuration. That’s a good question.

Vertical axis turbines are mainly used in niche applications on a small scale. However, the technology is improving, and the applications have been expanding into the offshore area.  A number of new offshore vertical axis turbines have recently crossed the CleanTechnica radar, too.

The lead researcher on the ARCUS project, Dr. Brandon Ennis of the Wind Energy Computational Sciences Department at Sandia, told CleanTechnica that the convergence of two different factors makes the vertical axis a good fit for floating wind turbines.

One factor is the location of the drive train. A conventional horizontal axis wind turbine needs a proportionately tall tower to keep the tips of its blades from hitting the ground. The blades rotate around a horizontal axis at the top of the tower and that’s where the the energy conversion action takes place, in a structure called the nacelle.

In contrast, vertical axis turbine blades don’t sweep the ground. The turbine tower can be proportionately shorter, which means it can be lighter than a standard turbine tower. In addition, the drive train can be located at the bottom of the structure, which in this case is the floating platform. That provides more stability for turbine towers that sit on the waves, in addition to streamlining the aerodynamics.

“The longstanding advantage is they [vertical axis turbines] improve mass properties relative to horizontal axis, especially in the floating offshore area,” Ennis explained.

The other factor is the appearance of a new kind of platform in the oil and gas industry, called a tension leg platform. Ennis explained that tension leg platforms are commonly used in the offshore wind industry because they are not a good fit for conventional wind turbines with tall towers. However, they have a lower mass than other types of platforms used by the oil and gas industry, which dovetails with the aim of the ARCUS project.

“A lot of innovation in this platform type from the oil and gas industry can be brought to bear on offshore wind,” Ennis said.

Where’s The Tower?

The turbine tower vanished from the ARCUS project when the Sandia team took a closer look at their preferred vertical axis turbine, called a Darrieus turbine after its inventor. Also called an “eggbeater,” a Darrieus turbine deploys two curved, vertical blades that circle the axis like the blades on an eggbeater.

“What we found is the tower itself represents a large portion of the mass, but it doesn’t capture any of the energy,” said Ennis. “The goal is to drive down the mass and eliminate as much mass as possible not directly related to energy production.”

From that perspective, removing the tower is a no-brainer.

“In a towerless vertical axis wind turbine, the blades are bent and held in position by tension. This produces a 50% mass reduction and allows for rotor area control through tension centers, like a bow and arrow,” Ennis explained, adding that Arcus is Latin for bow.

Next Steps For The New Towerless Offshore Wind Turbine

Sandia has just posted a webpage packed with technical details about the ARCUS wind turbine project along with an economic analysis. You can also look it up under US patent 11,421,650 B2.

“ARCUS is a towerless VAWT [vertical axis wind turbine) that replaces the traditional rigid tower with pre-stressed blades and tensioned center supports, Sandia explains. The lab lists the following advantages:

• Enables a structurally efficient usage of turbine material
• Enables rotor area control for high wind speed load reduction
• Effectively enables tension-leg platforms (TLP) which have the lowest hull mass compared to alternative architectures
• Produces an estimated $55/MWh LCOE for the optimized system

Overall, the ARCUS team’s primary emphasis is on the time it takes for a new industry to reach mass-market maturity. If all goes according to plan, the ARCUS system will enable the floating offshore wind industry jump into maturity sooner rather than later.

“There is a wide range of estimates for this emerging industry based on site conditions,” the Sandia team explains, “But it is apparent that the ARCUS system enables a shifted cost timeline relative to existing technologies. The ARCUS system enables the LCOE of a mature floating offshore wind industry to be realized in the near-term, progressing the cost curve by 10 or more years.”

The impact on offshore wind costs would be significant. In a 2022 report on wind energy, the Energy Department’s National Renewable Energy Laboratory estimated the cost of conventional, fixed-bottom offshore wind turbines at $78 per megawatt, with floating wind turbines coming in at $133.

The ARCUS estimate of $55 beats both, and the Sandia team anticipates that will help attract private sector partners to take the concept to the next level.

The lab points out that the ARCUS system was developed with the industry partner FPS Engineering & Technology , which specializes in floating offshore platforms. They also partnered with the American Bureau of Shipping, which counts the marine certification, classification, and innovation among in its mission.

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Image: New design for vertical axis floating offshore wind turbine, without a tower (courtesy of Sandia National Laboratories).