- Wave Energy Converter

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Wave Dragon - Wave Energy Converter

Wave Dragon is a floating, slack-moored energy converter of the overtopping type that can be deployed in a single unit or in arrays of Wave Dragon units in groups resulting in a power plant with a capacity comparable to traditional fossil based power plants.

The first prototype connected to the grid is currently deployed in Nissum Bredning, Denmark. Long term testing is carried out to determine system performance; i.e. availability and power production in different sea states. The energy absorption performance stated at this website has now been independently verified and focus will now be on power production optimisation.

These tests will lead to a multi-MW deployment in 2009.

Three good reasons...

  1. The Wave Dragon concept combines existing, mature offshore and hydro turbine technology in a novel way
  2. Wave Dragon is the only wave energy converter technology under development that can be freely up-scaled
  3. Due to its size service, maintenance and even major repair works can be carried out at sea leading to low O&M cost relatively to other concepts

Learn about the Wave Dragon principles in this section. Study the Wave Dragon specifications or read about wave energy.

Principles

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The basic idea of the Wave Dragon wave energy converter is to use well-known and well-proven principles from traditional hydro power plants in an offshore floating platform.

t is really very simple: The Wave Dragon overtopping device elevates ocean waves to a reservoir above sea level where water is let out through a number of turbines and in this way transformed into electricity, i.e. a three-step energy conversion:

Overtopping (absorption) -> Storage (reservoir) -> power-take-off (low-head hydro turbines).

Simple and robust …
Wave energy converters often make use of either mechanical motion or fluid pressure and there are numerous techniques for achieving it, e.g. oscillating water/air columns, hinged rafts or gyroscopic/hydraulic devices. Wave Dragon does not have any conversion but uses the energy in the water directly.

Wave Dragon is a very simple construction and has only one kind of moving parts: the turbines. This is essential for any device bound for operating offshore where the extreme forces and fouling etc. seriously affect any moving parts.

… complex
And yet Wave Dragon represents a very complex design where large efforts have been spent on design, modelling and testing in order to:

  • Optimize overtopping
  • Refine hydraulic response: anti-pitching and anti-ruling, buoyancy etc.
  • Reduce (the effect of) forces on wave reflectors, mooring system etc.
  • Reduce construction costs, maintenance and running costs
  • Essentially produce as much electricity as possible at the lowest possible costs - and in an environmental friendly and reliable way

Floating and stationary

Stationary: First one has to imagine the Wave Dragon moored (like a ship) on relatively deep water, i.e. more than 25 m and preferably +40 m to take advantage of the ocean waves before they lose energy as they reach the coastal area. This is in contrast to many known wave energy converters that are either built into the shoreline or fixed on the seabed at shallow water.

Floating: Secondly one has to realise that Wave Dragon is a floating device that is designed to stay as stationary as possible. It doesn't convert wave to energy by popping up and down or by some parts being moved by the motion of the waves. It simply utilizes the potential energy in the water that overtops it.

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Like a dam

ImageThe water overtopping Wave Dragon is stored temporarily in a large reservoir creating a head, i.e. the difference between the 'normal' level of the water surface and the water surface in the reservoir. This water is let out of the Wave Dragon reservoir through several turbines thus generating electricity like in hydro power plants.

Overtopping

To most offshore devices extensive effort is done to avoid overtopping. With its doubly curved ramp and wave reflectors Wave Dragon is deliberately designed to maximize the amount of water that overtops as the waves reach it.

The Wave Dragon ramp can be compared to a beach. When the wave reaches a beach it changes its nature. The Wave Dragon ramp is very short and relative steep to minimise the loss of energy that takes place each time a wave reaches the beach. The wave changes its geometry and elevates, too. The special elliptical shape of the ramp optimizes this effect, and model testing has shown that overtopping increases significantly.

Wave reflecting wings

As the waves reach the reflectors they elevate and reflect towards the ramp increasing the amount of overtopping water thereby increasing the possible energy output. The doubly curved ramp as well as the wave reflectors have been patented.

Adjustable floating height

Wave Dragon is constructed with open-air chambers where a pressurized air system makes the floating height of the Wave Dragon adjustable. This is used to adjust to varying wave heights as overtopping efficiency depends on choosing the right ramp height.

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Real sea experience

Physically there is quite a complex relation between the wave height, the geometry of the ramp and wave reflectors, the floating height of the Wave Dragon and - most importantly - the amount of water overtopping and storing in the reservoir. Many simulations and model tests have been carried out but achieving more reliable and refined results will only be possible by performing the real sea testing presently undergoing.

Power generation

Power generation on the Wave Dragon is based on the potential energy in the water that overtops the ramp and is temporarily stored in the reservoir. This reservoir contains approximately 8,000 m3 water that has to be let out through the turbines in between two waves.

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Mature turbine technology

ImageWave Dragon is equipped with a series of hydro turbines which individually starts and stops in order to facilitate as smooth an electricity production as possible. Wave Dragon uses traditional hydro propeller turbines with fixed gate vanes, which is a mature and well proven technology that has been used in hydro power plants for more than 80 years.
A special, small sized and low headed turbine has been developed for possible use in the Wave Dragon. The photo shows this Kaplan turbine during the testing at the Technical University of Munich. The turbine uses a siphon inlet whereas other turbines to be installed will be equipped with a cylinder gate to start and stop water inlet to the turbine.

New generator technology

The rotation of the hydro turbines is transformed to electricity via a Permanent Magnet Generator on each turbine. The PMG generators are chosen in order to avoid the switchgear used with an asynchronous generator.

Parameters

A vast number of parameters influence (and interact with) the net power production:

  • Overtopping, determined by
    • Free-board (adjustable in Wave Dragons)
    • Actual wave height
    • Physical dimension of the converter (ramps, reflectors etc.
  • Outlet, determined by
    • Size of reservoir
    • Turbine design
    • Turbine on/off strategy
  • Mooring system, free or restricted orientation toward waves
  • Size of the energy converter
  • Wave climate
    • Energy in wave front (kW/m)
    • Distribution of wave heights
  • Availability
    • Theoretical availability; Reliability, maintainability, serviceability
    • Accessibility on the site
    • Maintenance strategy

Operating in an Extreme Environment

In the wave energy inventing and development process one has to pay close attention to the fact that a wave energy converter operates in an extreme offshore environment. Any development of wave energy converting principles - no matter how appealing or intuitively interesting they seem to be - is bound to fail if they do not confront the following problems:

  • Exposure of the structure to the extreme action of waves and wind
  • Fouling that seriously influences any structure and obstructs moving parts
  • Marine debris like fishing nets, plastics, containers, oil etc.

Operating in an extreme environment has been a key focus in designing Wave Dragon.

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Wave Dragon has been designed with the turbines as the only moving parts. This is essential; not only to reduce maintenance costs but also to minimize the harming effects of marine growth (fouling) and floating objects in the ocean (like debris).

The energy in wave motions is extremely powerful. This is exactly what is exploited in a wave energy converter but it is also a constant threat to any structure and mechanic device.

These critical aspects have been addressed in designing Wave Dragon:

  • A slack-moored system traditionally used for mooring ships will be used to absorb forces from the Wave Dragon rig resulting from the exposure to waves and wind.
  • To reduce the resulting forces on the mooring system and on the wave reflectors a special wiring arrangement has been designed and tested. The basic idea is to make the forces from one wave crest on the main platform counteract with the forces resulting from the following wave crest.
  • Extreme waves will not be a problem. Model tests have shown that high waves simply run over the rig.
  • Extreme wind will not be a major problem as Wave Dragon floats relatively low and is not exposed to the wind. Typhoons etc. will be handled by lowering the rig to just above sea level.

Standard components

To reduce maintenance costs Wave Dragon is mostly constructed by using standard materials and components. In addition, individual turbine units will be replaced for maintenance on a regular schedule (like in aircraft maintenance). This will lower handling costs and ensure a high availability and low loss of production from non-functioning turbine units.

Specifications

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:

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.

Wave Energy

Wave energy - a concentrated form of solar energy

Winds generated by the differential heating of the earth pass over the open bodies of water, transferring some of their energy to the water in the form of waves. This energy transfer results in a concentration of the energy involved: the initial solar power level of about 1 kW/m2 is concentrated to an average wave power level of 70kW/m of crest length. This figure rises to an average of 170 kW/m of crest length during the winter, and to more than 1 MW/m during storms.

The potential world-wide wave energy contribution to the production of electricity is estimated by IEA (International Energy Agency) to be between 10 and 50% of the world’s yearly electricity demand of 15,000 TWh dependent of the expected load factor and wave regime. A recent study by the DTI and Carbon Trust in UK is stating some 200,000 MW installed wave and tidal energy power by 2050 which with a load factor of 0.35 is resulting in a power production of 6 TWh/y. Independent of the different estimates the potential for a pollution free generation of energy is enormous.

Waves
Waves are caused by a number of forces, i.e. wind, gravitational pull from the sun and moon, changes in atmospheric pressure, earthquakes etc. Waves created by wind are the most common waves and the waves relevant for the Wave Dragon technology.

Wind generation
As the wind blows across the water surface air molecules from the wind interact with the water molecules they touch. This force between the air and water stretches the water's surface, resulting in small ripples, known as capillary waves. Capillary waves create more water surface increasing the friction between water and wind. This adds more energy, which increases the size of the waves, making them larger and larger.

When the winds slow down or stop, the waves continue their journey, gradually but very slowly losing their energy. Waves may travel thousands of kilometres before rolling ashore. This predictability of waves is one of the advantages of wave energy as a renewable energy source.

Movement of energy
An ocean wave in deep water appears to be a massive moving object - a crest of water travelling across the sea surface but to understand wave energy it is important to realize that this is not the case. An ocean wave is the movement of energy but the water is not moving; in the ocean - where waves move the water's surface up and down - the water is not moving towards the shore. So, an ocean wave does not represent a flow of water; it represents a flow of motion or energy from its origin to its break up. This break up may occur in the middle of the ocean or against the coast.

Circles
The water molecules in an ocean wave move in circles. The behaviour of waves depends largely on the relations between a wave's size and the depth of water through which it is moving. The movement of a water molecule changes from circular to ellipsoidal as a wave approaches the coast and water depths decrease. Eventually when the wave rolls up on a beach - and when most of us observe waves - the movement is mostly horizontal. When talking of ocean wave, and a potential deployment of Wave Dragon, the influence of water depth is negligible.

Wave Energy
Ocean waves are - as mentioned above - essential movement of energy. Waves consist of two kinds of energy.

  • The individual water molecules are moving steadily and rather slowly in a circular way, and this energy - kinetic energy - can be utilized in different kinds of wave energy converters, either directly via some kind of propeller or indirectly by oscillating water columns wave energy converters
  • In its circular movement the individual water molecules are elevated above the still-water line and thus represent a potential energy

Both types of energies are utilized by Wave Dragon, where this water is stored in an 'above-sea-level' reservoir and led out through the turbines (in principle the same technique as used in traditional hydro power plants).

Wave energy potential
The energy in ocean waves is huge. Even that fraction of the wave energy that is potentially or technically exploitable is very large compared to world electricity consumption today.

Several analysis have been done to estimate these figures. To quote just one source World Energy Council 2001 Survey that stated the 'potential exploitable wave energy' resources worldwide to be 2TW and for European waters the resource was estimated to be able to cover more than 50% of the total power consumption.

The energy potential in waves differs widely around the world. Normally wave energy is measured in kW/m, i.e. kW pr. meter wave. The following map illustration roughly depicts the average energy in ocean waves.

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One Wave Dragon unit will produce electricity corresponding to:

  • in a 24kW/m wave climate = 12 GWh/year
  • in a 36kW/m wave climate = 20 GWh/year
  • in a 48kW/m wave climate = 35 GWh/year
  • in a 60kW/m wave climate = 43 GWh/year
  • in a 72kW/m wave climate = 52 GWh/year

The Wave Dragon technology is unique in the sense that there is no up-ward restriction from wave heights or wave lengths on its size; this gives a promising perspective for profitable power production.

Environment

Wave Dragon and the environment

Wave Dragon is a clean power generation technology even compared to other renewables:

  • Very low visibility; Wave Dragon can be compared to a moored ship and will have a maximum height above mean sea level of 7 meters
  • Modest 'footprint' on seabed from anchor block and the power cable duct
  • No noise
  • No risk of spill; all hydraulic oils have been replaced with water hydraulics, no toxic antifouling is used

Potential environmental impacts and mitigating measures

Below are listed construction, operating and decommissioning activities associated with the Wave Dragon technology. For each activity potential environmental impacts are stated as well as the associated mitigation measures.

Various studies and guides for offshore wind farms have been used in compiling this list for a Wave Dragon farm.

Wave Dragon
Wave Dragon
Blegdamsvej 4
Copenhagen N 2200

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