The Mighty Whale

by the CADDET Japanese National Team

The World's largest offshore floating wave power device was launched in July 1998 by the Japan Marine Science and Technology Center. The full-scale prototype will be demonstrated and tested over a two-year period at the mouth of Gokasho Bay facing the Pacific Ocean.

 

The principle
The Mighty Whale converts wave energy to electricity by using oscillating columns of water to drive air turbines. As shown in the figure above, waves flowing in and out of the air chambers at the 'mouth' of the Mighty Whale make the water level in the chambers rise and fall. The water forces air into and out of the chambers through nozzles on the tops of the chambers. The resulting high-speed air-flows rotate air turbines which drive the generators.

The prototype
The Mighty Whale is 50 m long and 30 m wide and carries three air turbine generator units: one with a rated output of 50 kW + 10 kW and two of 30 kW.

After being towed to its mooring about 1.5 km from the mouth of Gokasho Bay, the Mighty Whale was anchored to the bottom of the sea (about 40 m deep) with six mooring lines; four lines on the seaward side and two on the lee side. The moorings are designed to withstand even typhoon strength wind and waves.

The Mighty Whale can be remotely controlled from on-shore. In the demonstration prototype, the energy produced is mostly used by the instruments carried on board; any surplus is used to charge a storage battery or, when this is fully charged, is used by a loading resistor. A safety valve protects the air turbines from stormy weather by shutting off the flow of air if the rotation speed of the turbines exceeds a predetermined level. So that it can be used in the future to improve water quality, the prototype is also equipped with an air compressor to provide aeration.

During the demonstration project, instruments aboard the Mighty Whale are monitoring the various factors that might affect its operation (see data in Tables 1, 2 and 3). The data are stored on board, but those relating to the safety of the Mighty Whale, and the operation of the monitoring equipment are transmitted to the on-shore control station.

Benefits
Because it has absorbed and converted most of the energy in the wave, the Mightly Whale also creates calm sea space behind it, and this feature can be utilised; for example, to make areas suitable for fish farming and water sports. The structure of the Mighty Whale itself can be used as a weather monitoring station, a temporary mooring for small vessels or a recreational fishing platform. As well as generating energy for use on-shore, the Mighty Whale can provide an intermediate energy source for aeration to improve water quality.
The Japan Marine Science & Technology Center expects this demonstration project to show great potential for this type of floating wave power device.

For more information contact the CADDET Japanese National Team in Tokyo.

 

Operating principle of floating type wave power device.

 Table 1: Design Conditions

 Steady wind speed

 (10 minute average)

 36.4 m/sec

 (1 minute average)

 42.2 m/sec

 (1 hour average)

 34.2 m/sec

 

 Current speed

 

 1.2 m/sec

 

 Wave height and period

 Max 1/3 significant height

 8.0 m

 Max 1/3 significant period

 10­15 sec

 

 Water depth

 

 40.4 m

 

 Tidal range

 High water level

 1.859 m

 Low water level

 0.029 m

Table 2: Main specifications

 Mighty Whale  Hull

 Length

 50 m

 Breadth

30 m

Depth

12 m

 Draught

8 m

Displacement

4,380 tonnes

 Light weight

 1,290 tonnes

 Air turbines

 Type

Wells

 Configuration

 Tandem

Blades

 NACA0021 (8 blades)

 Diameter

1.7 m

 Material

 Corrosion resistant Al alloy

 Weight

 480 kg (approx.)

 Max RPM

1,800 RPM

 Generators

 Type

 3-phase induction

 Rated output

 (50 kW + 10 kW) x 1; 30 kW  x 2

 Cooling

 Forced air cooled

 Operation RPM

 300­1,800 RPM

 Voltage

 AC200V, 3-phase

 Air compressor

 Type

 Air cooled oil-less

 Rated output

 7.5 kW

 Max Pressure

 7 kgf/cm2 (approx.)

 Input voltage

 AC200V

Table 3: Measured Parameters

 Environment

 Incident wave height and direction

 Wind speed and direction

 Atmospheric temperature/pressure

 Wave dissipation characteristic

 Hull dynamic response

 Hull motion

 Drift motion

 Primary energy conversion

 Water displacement in and outside air chamber

 Machinery-generated noise

 Secondary conversion

 Differential pressure, RPM, torque

 

 Safety valve

 Generator output

 Voltage, current, power

 Air compression

 Flow rate, delivery pressure, air tank pressure

 Mooring system

 Hull absolute position

 Mooring line force

 On-board power supply

 Voltage, current

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.