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.
It 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.
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.
Like a dam
The 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.
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.
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 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.
Mature turbine technology
Wave 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.
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
- 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.
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.
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.