Floating Offshore Wind Holds Promise for Vertical-Axis Turbines

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
minmizing 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 Strangberg 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 (LCOE) 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 LCOE 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.”

Source: FS – GreenTech Media
Floating Offshore Wind Holds Promise for Vertical-Axis Turbines