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Floating Axis Wind Turbines (FAWTs)

The concept of Horizontal Axis Wind Turbine (HAWT) mounted on a tall tower has been quite successful in onshore and bottom-fixed-offshore wind turbine application. Is it also effective in floating offshore wind turbines?
Our answer is negative because such a top-heavy floating structure is simply against the basics of ocean engineering. We know keeping the upright position of tall floating structure is expensive for the required large float system. Unlike offshore oil&gas platform, large float system should be avoided because selling price of wind is much cheaper than oil&gas. We must think differently.

In ocean engineering, we use buoyancy for supporting heavy moving objects. Why don't you use the same idea in the floating offshore wind turbine? It is possible for Vertical Axis Wind Turbine (VAWT) rotor because its lower end of turbine shaft can be in water as shown in Fig. 1. In this design, the rotor is mounted on a cylindrical float (spar buoy) and they rotate as one body.

Floating Axis Wind Turbine animation (small)
Fig. 1: Floating Axis Wind Turbine concept (click for enlarge or open mp4 movie)

Because performance of VAWT is less sensitive* to skewed flow than HAWT, we don't have to keep the fixed upright position of turbine axis. Tilt angle is passively adjustable to wind force and the righting moment of spar buoy will balance with the thrust force (horizontal component of aerodynamic force on the rotor). It will reduce the supporting structure of turbine. In addition, the large tilt angle reduces the size of spar because righting moment of spar increases with tilt angle.

(*) Small VAWTs are often installed on the rooftop of buildings where wind direction is not horizontal.

FAWT does not require high capacity power take-off (PTO) unit because it uses multiple small PTO modules as shown in Fig. 2. The mechanism of PTO is similar to that of regenerating brake system of electric vehicles. Total cost of these units will be lower than a single high capacity PTO since heavy weight of the rotor is supported by buoyancy and not on the PTO units. The small size of each PTO unit does not require any crane ships for its offshore installation and maintenance.

Vertical is the path to the light-weight. Light-weight leads to higher economy.

Power take-off (PTO) system
Fig. 2: Power take-off module of FAWT

There are two variations of FAWT. One is small FAWT with blade folding mechanism for easy transportation (Fig. 3 left). It can be used for emergency power supply in large scale hazards like earthquake and tsunami. Also, it will be the normal power supply in islands or in offshore platform of deep-sea mining. The second variation is the twin turbine configuration for large FAWTs shown in Fig. 3 (right). In this design, two VAWTs of different rotating direction are bridged for cancelling the reaction torque of generators each other. It will reduce the total cost of mooring system.

Small foldable FAWT and large twin FAWT
Fig. 3: Mobile turbine with folding blades and Large twin-turbine configuration

If you have an experience of small boats in lakes or rivers, you were taught "Do not stand up on the small boat". It is also true in floating offshore wind turbine. Although our concepts look strange for you, we are redesigning the floating wind turbine following the very basics of ocean engineering.

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