Long relegated to the margins of the wind industry, vertical-axis wind turbines (VAWTs) could get a new lease of life on floating platforms.

Swedish firm SeaTwirl has been making waves recently with its floating offshore VAWT design, including the announcement this week that it has secured a patent in China, expected to be the world's largest offshore wind market within a few years.

The patent, also approved recently the U.S. according to SeaTwirl, is for a design that would allow the generator and bearing housing to be replaced just above the water surface by boat, cutting the cost of installation and maintenance and minimizing downtime.

Most of the delicate equipment in standard horizontal-axis offshore wind turbines is high above the water, making repairs more difficult and dangerous.

Last month SeaTwirl said it had won the backing of offshore logistics company NorSea and Belgium's Colruyt Group for a SEK 70 million ($7 million) project to build a 1-megawatt prototype of its floating VAWT. The plan is to build the S2 prototype in 2020, most likely in Norway, SeaTwirl CEO Gabriel Strängberg told GTM.

SeaTwirl’s spate of announcements comes after research by the U.S.-based Sandia National Laboratories last year found there could be significant potential for VAWTs to cut the cost of offshore wind on floating platforms.

The five-year, $4.1 million study found VAWTs could potentially slash the cost of floating offshore wind turbines, installation and maintenance by doing away with the need for gearboxes, high-speed shafts, yaw systems and nacelles, which are all subject to faults.

But the main attraction of VAWTs compared to horizontal-axis wind turbines (HAWTs) in an offshore context is that they could potentially work with cheaper floating platforms.

“For floating offshore wind, the platform is the single largest contributor to the levelized cost of electricity,” research lead Dr. Brandon Ennis, of Sandia’s wind energy technologies department, told GTM.

“If you have a slightly more expensive rotor but a platform cost that is substantially reduced, that could be a system benefit.”

Although still in its infancy, floating offshore wind is expected to take off in the 2020s as its costs come down and many of the best sites for offshore wind farms in shallower waters are used up.

Potential advantages over standard turbines

A big challenge for HAWTs on floating platforms is that many of the heavier components of the turbine, such as the drivetrain and the generator, are high above the water.

This creates a large overturning moment that the floating platform has to stabilize, usually by adding mass to the substructure. With VAWTs such as the Darrieus design studied at Sandia, the heavy components would all sit at the base of the turbine.

This would not only contribute to its stability but also make it easier and cheaper to carry out maintenance and repairs. 

Another advantage of VAWTs is that, unlike HAWTs, they are insensitive to wind veer, where wind changes directioin n with height.

Finally, according to Quest Floating Wind Energy, a specialist analysis firm, VAWTs could help overcome wake effects associated with HAWT-based wind farms.

“The industry is looking at ever-bigger turbines,” said Erik Rijkers, Quest’s director of market development and strategy. “The GE 12-megawatt turbine spans 220 meters. This means floaters need to be spaced some 1.5 kilometers [apart], which results in large cabling costs.”

In contrast, he said, studies in France suggest that two VAWTs placed on a single floater would actually improve each other's performance, reducing cabling needs and making the technology a good bet for cramped environments such as in lakes.

Crucially, too, while larger HAWTs make things increasingly difficult for floating foundations, in the case of VAWTs an increase in size could improve performance and cost efficiency. “Offshore, you get an improved efficiency for upscaling,” said Ennis. 

Sandia calculated that in the long term the levelized cost of energy for a VAWT on a floating platform could drop to $110 per megawatt-hour, although the research body did not carry out a like-for-like comparison with HAWTs.

According to one study from 2017, the levelized cost of energy for floating offshore wind with current HAWT technology is from around $180 per megawatt-hour upwards.

Skepticism persists

The increasing cost competitiveness of VAWTs at larger size sounds like a major selling point for the technology, but Ennis admitted it could be a stumbling block in practice.

At small scale, VAWTs have historically not performed as well as HAWTs, which has prevented them from being used for large-scale power generation.

This means any company wishing to build VAWTs for offshore use would have to find backers willing to take a on a significant development risk of going big.

Bruno Geschier, chief sales and marketing officer at the floating foundation manufacturer Ideol, remains skeptical of the concept. “It would take decades and billions to have a 12-to-15-megawatt vertical-axis turbine ready for commercial deployment,” he said.

“Horizontal-axis [turbines] are already working at such sizes and have a return on experience and industrial fabrication capacity no one could match.”