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Engineers Rotate the Common Wisdom of Offshore Wind Turbines

SeaTwirl explains that the powerful winds created offshore are rich in untapped renewable energy.

Powerful ocean gales enable offshore wind turbines to gather more electricity than their cousins on land. This is because the amount of energy a wind turbine can capture is proportional to the wind speed cubed.

“Engineers can expect to see wind speeds that are twice as fast over the ocean than over land. This means a wind turbine on the water can theoretically capture eight times more energy than the same wind turbine on land,” says Martin Rosander, senior structural engineer at SeaTwirl, an ANSYS Startup Program member.

However, much of this wind energy remains untapped because of the costs associated with offshore horizontal axis wind turbines. SeaTwirl, however, aims to reduce this cost with vertical axis wind turbines.

Why Traditional Offshore Wind Turbines are Expensive

Most contemporary wind turbines operate with horizontal axis blades. The blades and the turbine generators need to be large and high above the water.

The taller the turbine, the better it can capture the high winds that travel faster than those near the water. The height also optimizes the sweeping area of the blades. High wind speeds and a larger sweeping area translate into more electricity.

Rosander explains that the height and size of these parts make it challenging and expensive to install and maintain on the ocean. After all, not many ships can lift hundreds of tons over the water. As for the ships that can lift enough weight to install or replace the generator, they come with a heavy price tag.

SeaTwirl is addressing these maintenance and stabilization concerns by developing offshore wind turbines with vertical axis blades. The generators are placed underneath of vertical turbines, close to the water’s surface, making them easier to install and maintain.

Why Vertical Wind Turbines are Well-suited for Offshore Wind Generation

Comparison of a horizontal wind turbine (left) and a vertical wind turbine (right)

Vertical axis wind turbines are not as optimized as horizontal axis turbines.

A vertical axis turbine will always cause drag over a certain cross section of the wind’s direction. Horizontal axis turbine, however, are effective throughout the whole cross section of the wind’s direction.

This makes horizontal axis turbines an optimal choice in many wind power generation situations. However, vertical axis wind turbines can be a game changer when you are installing the wind farm offshore.

Blades for vertical axis wind turbines are simpler to design as their rotation is independent of wind direction. Vertical blades can also be designed to stall at wind speeds that would otherwise damage the system.

In comparison, the blades of horizontal axis wind turbines need to be rotated to the wind’s direction. They also need to be pitched to stall in unsafe wind speeds. As a result, horizontal axis wind turbines require more sensors, motors and systems to ensure the blades are always safe and facing the wind. All these moving parts increase the system’s complexity, which in turn increases maintenance and installation costs.

Vertical axis wind turbines also simplify their designs by operating with their heavy generators and gearboxes close to the water. This has two benefits. First, the vertical axis turbines can be serviced with smaller boats and equipment. Second, vertical axis turbines have a lower center of gravity, making them inherently more stable than the top-heavy horizontal axis turbines.

“If you have a higher turbine you need a bigger keel,” says Rosander. “For SeaTwirl’s vertical axis wind turbines, the keel is much smaller. You need less heavy and expensive weights at the bottom of the unit if the generator and gear box are close to the water.”

Vertical wind turbines’ simpler design and lower center of mass make them better-suited for hard-to-reach areas — like the ocean — as they are less likely to break down. Their robust design, smaller keel and easier maintenance procedures makes the energy they produce more affordable.

What Sets SeaTwirl’s Vertical Wind Turbines Apart

A diagram of SeaTwirl’s turbine in the water. The keel rotates with the blades; only the generator house is stationary.

SeaTwirl’s wind turbines are unique — even when comparing them to other vertical axis wind turbines.

For instance, most of their design rotates through the water. In fact, the only thing that is moored to the sea floor is the generator housing.

“When the turbine spins in the water it will lose energy from the extra drag, but you save money on the bearings,” says Rosander.

“Since the only static part of the system is the generator housing, the bearing doesn’t take the vertical forces from the turbine’s weight or the bending forces from the turbine’s tower. The only forces on the bearing are from the mooring system,” adds Rosander.

SeaTwirl can get away with a smaller bearing system as there are fewer forces acting on it than traditional vertical axis turbines that are stationary in the water. A smaller bearing system means a more affordable wind turbine for SeaTwirl’s customers.

“The idea is to limit the levelized cost of energy (LCOE),” says Rosander. “LCOE is equal to the total cost of the unit — from installation to maintenance and decommission — divided by the total energy produced over the unit’s lifetime.”

To reduce the LCOE, SeaTwirl limits the total cost of the unit. These costs are traditionally high for offshore turbines. As a result, reducing the cost of offshore units can be a more efficient way to improve the LCOE than increasing the turbine’s energy output.

“We can say with confidence that we can produce an offshore floating turbine with a lower LCOE than a horizontal axis offshore floating turbine,” says Rosander. “We will achieve this by saving money on maintenance, installation and by reducing the number of heavy or moving components.”

How SeaTwirl Uses Simulation to Optimize its Designs

SeaTwirl designs its turbine systems with the ANSYS platform. Simulation drives SeaTwirl’s design because — as a startup — it doesn’t have the budget to work with a lot of physical prototypes.

Computational fluid dynamics (CFD) simulation of a vertical wind turbine using ANSYS Fluent.

“It wouldn’t be feasible to design our wind turbines with only physical prototyping,” says Rosander. “There are too many unknowns. You need to do a lot of testing with wind tunnels or wave tunnels. This is costly and time consuming, so development costs get very high. ANSYS helps us avoid these costs.”

SeaTwirl uses ANSYS Fluent to assess the optimal blade size, shape and rotational speed of its turbine. SeaTwirl also simulates the turbine’s strength and braking system in ANSYS Mechanical. Currently, SeaTwirl is using simulation to assess the strength and fatigue life of its designs.

Thanks to the success of SeaTwirl’s simulations, the company plans to have a 1 MW prototype in the water come 2020. This unit will replace the 30 kW prototype which is currently in the water.

To see how simulation can help speed up your development, check out the ANSYS Startup Program.