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Basic principles

The Wave Dragon is a floating slack-moored wave energy converter of the overtopping type. It basically consists of two wave reflectors focusing the waves towards a ramp. Behind the ramp there is a large reservoir where the water that runs up the ramp is collected and temporarily stored. The water leaves the reservoir through hydro turbines that utilise the head between the level of the reservoir and the sea level.

Main components of a Wave Dragon are:

  • Main body with a doubly curved ramp; a reinforced concrete and/or steel construction
  • Two wave reflectors in steel and/or reinforced concrete
  • Mooring system
  • Propeller turbines with permanent magnet generators

Different Wave Dragon models

The physical dimensions of a Wave Dragon will be optimised to the wave climate at the deployment site, i.e. the width of the main body, length of the wave reflectors, weight, number and size of turbines etc.

The Wave Dragon tested in Nissum Bredning is constructed to match a very humble wave climate at approximately 0.4 kW/m. This Wave Dragon is made for test purposes and is not commercially relevant with regard to power production. Key figures are shown below:

Wave Dragon key figures:

Nissum Bredning prototype

0.4 kW/m

24 kW/m

36 kW/m

48 kW/m

Weight, a combination of re-inforced concrete, ballast and steel

237 t

22,000 t

33,000 t

54,000 t

Total width and length

58 x 33 m

260 x 150 m

300 x 170 m

390 x 220 m

Wave reflector length

28 m

126 m

145 m

190 m

Height

3.6 m

16 m

17.5 m

19 m

Reservoir

55 m3

5,000 m

8,000 m3

14,000 m3

Number of low-head Kaplan turbines

7

16

16 - 20

16 - 24

Permanent magnet generators

7 x 2.3 kW

16 x 250 kW

16 - 20 x
350 - 440 kW

16 - 24 x
460 - 700 kW

Rated power/unit

20 kW

4 MW

7 MW

11 MW

Annual power production/unit

-

12 GWh/y

20 GWh/y

35 GWh/y

Water depth

6 m

> 20 m

> 25 m

> 30 m

Main body

The main body or platform is basically one large floating reservoir. To reduce rolling and pitching and to ensure an economic production of electricity Wave Dragon needs to be large and heavy. The Nissum Bredning prototype is a traditional (ship-like) steel plate construction, primarily 8 mm steel plates. The total steel weight of the main body plus the ramp is 150 t. To obtain the desired 237 tonnes total weight 87 tonnes of primarily water ballast is added!

In a 36 kW/m wave climate the main body would be 140 x 95 meter. It will be constructed in a combination of steel and reinforced concrete.
 
On the top of the Wave Dragon main body is the water reservoir. On the Nissum Bredning test-prototype there is a 55 m3 reservoir. On the 36 kW/m Wave Dragon this equals 8,000 m3.

One of the key features of Wave Dragon is that it constantly adjusts to changing wave heights by changing its own floating height. This is achieved by changing the air pressure in the open-air-chambers. A buoyancy and stability platform is mounted on the back of the platform to ensure the stability of the Wave Dragon and especially to dampen some of the pitching.

Doubly curved ramp and wave reflector

To maximise water overtopping efficiency a combination of wave reflectors and a doubly curved ramp has been designed and patented:

  • The wave reflectors significantly decrease the construction costs of an overtopping wave energy converter. The alternative would be to build a main body as wide as the Wave Dragon main body plus the wave reflectors.
  • The wave reflectors focus the wave energy towards the ramp.
  • The doubly curved ramp (elliptical + circular) increases in combination with the wave reflectors the amount of water overtopping Wave Dragon.

A wave reflector on the Nissum Bredning prototype is 27 meters long, 3.5 meters high and weighs 25 t. On a Wave Dragon built for a 36 kW/m wave climate a wave reflector would be 145 meter long and 19 meters high.

The length of the wave reflectors reduces the relative vertical forces on the shoulders and fender arrangements.

The wave reflectors are fixed to the main body by the mooring system and some additional wires. Traditional rubber fenders are placed between each wave reflector and the main body to absorb the remaining forces.

The wave reflectors are kept in position partly by the mooring system and partly by a wire arrangement.

Mooring system

The mooring system is a vital part of the Wave Dragon concept. It doesn't just moor Wave Dragon to the sea bed but is designed to interact and indeed counteract with Wave Dragon in order to reduce the forces in the mooring system and to fix the wave reflectors.

Flexible wires are used on the Nissum Bredning test prototype as the low water (6 m) would eliminate the positive effect from a slack moored system.

Propeller turbines

Propeller turbines, used in the Wave Dragon, have been used for more than hundred years in traditional hydro power plants. They are characterised by being extremely reliable and have very low maintenance costs. A propeller turbine is basically a traditional propel as shown on this picture of the Wave Dragon test turbine. This turbine has a 34 cm diameter runner designed by VeteranKaft AB and produced by Technical University of Munich. This turbine has been thoroughly tested at the university turbine test stand over a 3 years period. The test turbine has a siphon water inlet.

The test turbine and 6 new designed turbines has been instaled on the Nissum Bredning prototype. The new turbines have a cylinder gate to regulate the water inlet (either open or closed). Every single turbine has to run at full speed to be efficient. To be able to regulate the total water outlet and power production efficiently, the turbines will be started (cylinder gate opened) and stopped (gate closed) individually. Additional 3 dummy valves will be installed on the Nissum Bredning prototype to increase the flexibility and precision in the simulations. The new turbines are also designed by VeteranKraft AB and will be produced by the Austrian hydro turbine producer Kössler Ges.m.b.H.

Permanent magnet generators

Wave Dragons turbines will rotate with a variable and low speed. The most efficient way to transform this to electricity is by using permanent magnet generators. In this way no gear-box is needed, thereby reducing both losses in power and maintenance costs significantly.