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Floating Axis Wind Turbines (FAWTs, 浮動軸型風車)

よく見かける水平軸型風車(Horizontal Axis Wind Turbine; HAWT)は、高いタワーの上にローターを載せる形式です。 このデザインは、陸上風車、着床式洋上風車で、非常に成功しています。しかし、「浮体式」の洋上風車でも同じデザインが有効でしょうか? 残念ながら、同じコンセプトは適当で無いと私たちは考えています。なぜなら、HAWTのような重心位置の高い(トップヘビー)なデザインの浮体式構造物は、海洋工学の基本に反しているからです。

背の高い浮体構造物を洋上で垂直に保とうとすると、大型の浮体が必要となります。洋上の原油や天然ガスの採掘プラントでは、大型浮体を使いますが、原油・ガスに比べて風力の値段は安いため、同じような大型浮体のコストを負担することはできません。浮体式洋上風力発電を事業化するには、考え方を変える必要があります。

海洋工学では、大重量の物体の動きも浮力で支える事ができます。それならば、同じ考えを浮体式風車のローターに使ってはどうでしょうか? これは垂直軸型風車(Vertical Axis Wind Turbine; VAWT)ならば、容易に実現でき、VAWTローターの下端を水中に入れる、Fig. 1のようなデザインになります。 このデザインでは、VAWTローターは、円筒型浮体(スパー)の上に搭載され、一体となって回転します。

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.

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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