Description of the wave energy converter Wave Dragon

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1. Overtopping Devices
These devices generate energy by exploiting the potential energy in the elevated water near
the crest of waves. This is achieved by allowing this water to overspill into a form of
reservoir and then allowing it to flow back into the sea via a low-head turbine. This simple
concept, which could theoretically utilize conventional technology, needs to be refined in
order to overcome several challenges (in addition to the ones facing all wave energy devices),
e.g.:
• Just generating energy from feeding the crests of waves into a turbine would cause
great variations in flow through the turbine, leading to low efficiency and, probably,
problems with turbines passing ‘slugs’ of water.
• The device has to take the near-crest water from a wide range of wave heights, so an
inability to accommodate such changes in water elevation would result in low overall
efficiency. This challenge is also exacerbated when one adds in tidal range as well.
• The device has to respond minimally to the waves in order to ensure that it presents an
optimal profile to the waves to they can spill into the reservoir and to reduce the
movement of the water within the reservoir.
1.1 TAPCHAN
Outline Description
The Tapered Channel wave device or TAPCHAN consists of three main elements: tapered
channel, reservoir and power station (Figure 1). The outermost part of the tapered channel,
the collector, is designed to concentrate the incoming sea waves into a narrower width.
These concentrated waves are then directed to the converter, which is a near horizontal,
evenly-tapering channel whose height corresponds to the top of the reservoir. As waves
move up the converter, the taper promotes an increase in wave height such that, when the
crest of the wave is higher than the edge of the converter, water pours into the reservoir. The
shape of the converter is matched to the local wave climate. The reservoir collects and stores
the overflow from individual waves so as to buffer variations in water flow through the
turbine (which generates electricity) back to the sea. During periods of low wave activity
when no water enters the reservoir, flow to the turbine is cut off automatically. The preferred
method for constructing the reservoir is by damming up a pre-existing bay (rather than
expensive digging out of a lagoon) or a pre-existing natural depression behind the shoreline.
Whichever method is used, the reservoir should be between 3m and 8m above mean sea
level.
History of Testing
Only one TAPCHAN has ever been built (in 1985) in a naturally occurring reservoir at
Toftestallen in Øygarden near outside Bergen, Norway (Figure 2) by Norwave A.S. in
Excerpt from a non published book by Tom Thorpe - https://www.linkedin.com/in/thomaswthorpe/

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conjunction with several engineering companies. It had a maximum width of 40 m and a
length of 90 m and the surface area of the reservoir for this scheme is 8,500 m2
, giving a
storage capacity of several minutes. Power was generated by allowing the water to flow back
to the sea from the elevated reservoir through a vertical Kaplan turbine with a 3m nominal
head driving a 350 kW asynchronous generator. The plant ran successfully for several years
before the collector was destroyed during an attempt to reshape it and so improve its
collection efficiency.
Figure 1 Outline of a TAPCHAN
Copyright G. Boyle, Renewable Energy: Power for a Sustainable Future, OUP, 1996
Figure 2 The TAPCHAN at Toftestallen, Norway
Copyright SINTEF (2008) Image from Google Earth
Future Plans
Suitable locations for TAPCHAN systems must have: consistent waves, with a good average
wave energy and a tidal range of less than 1m, suitable coastal features including deep water

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near to shore and a suitable location for a reservoir. Therefore, TAPCHAN systems are not
suitable for many coastal regions.
The original design team had plans for a similar construction at Golta in Norway, whilst other
teams have studied sites in Bali and Java (Indonesia). Despite considerable promise, for
various reasons these schemes have never been implemented.
Technical Status
This device uses mature technology, which has been proven in a typical location. Therefore,
it is at TRL9, having successfully tested a full-scale prototype.
1.2 Wave Dragon
Outline Description
The Wave Dragon is a floating overtopping device designed for water depths of 20 m and
deeper (Figure 3). The central, front face of the device is a doubly-curved ramp, up which
incoming waves surge (as on a beach). Behind the crest of the ramp lies a reservoir that
gathers the water that overtops the ramp (Figure 4). Energy is extracted as the water captured
in the reservoir is allowed to drain back to the sea through a number of low-head hydro
turbines located within the reservoir. The amount of energy captured by the device is
increased by long reflector wings mounted either side of the reservoir, which channel the
waves towards the central ramp. The device is slack moored for and aft to allow it to turn
into the principal wave direction. An outline summary of the Wave Dragon proposed for
various wave climates is shown in Table 1, with device showing an average wave-to-wire
efficiency of 21%. The device has been tested at 1:4.5 scales in Nissum Brending and a
commercial-size demonstration scheme is planned for the south west coast of Wales.
Structure
The preliminary dimensions for the first full-scale demonstration device are large (Figure 5):
• Distance between tips of wings = 300m
• Wing length = 145m
• Length (tip of wing to rear of central housing) = 170m
• Height = 17m

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These large dimensions ensure that the device extends over at least one wavelength and so it
should even out the wave forces on the device. This aspect of the design is intended to
ensure that it has a minimal response to the waves and that only a relatively light mooring is
needed. The structural weight of the device is also large (33,000t) which again reduces
movement. This weight comprises 31,000t concrete, 2,000t steel (possibly other materials in
the future).
Figure 3 The 1:4.5 Scale Wave Dragon at Nissum Brending
Photograph courtesy of Wave Dragon.
Figure 4 Principle of Overtopping on the Wave Dragon
Diagram courtesy of Wave Dragon.

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Table 1 Summary of Key Aspects of the Wave Dragon
Wave climate: 12 kW/m 24 kW/m 36 kW/m 48 kW/m
Weight: 6,500 22,000 tons 33,000 tons 54,000 tons
Width 170 m 260 m 300 m 390 m
Length: 96 m 150 m 170 m 220 m
Wave reflector length: 84 m 126 m 145 m 190 m
Height: 12 m 16 m 16.8 m 18.1 m
Number of turbines: 8 16 16-20 16-24
Generator type: Permanent magnet synchronous generators
Generator rating: 8 x 185 kW 16 x 250 kW 16-20 x 350 kW 16-24 x 500 kW
Device dated dower: 1.5 MW 4 MW 7 MW 12 MW
Water depth: >15 m >20 m >25 m >30 m
Annual output 4 GWh 12 GWh 20 GWh 35 GWh
Mooring type: Single Point Mooring with 5 - 8 legs
Table courtesy of Wave Dragon.
There are ballast compartments within all sections that allow the device to be raised and
lowered with respect to MSL to ensure that the central section maintains its optimum height
above mean sea level for wave capture (1-4.5m), with a draught of 11-14m. The ballasting is
achieved by pumping seawater in and out of these compartments.
The structure is designed for a lifetime of 50+ years like offshore concrete oil and gas
platforms in accordance with DNV recommendations.
Mooring
The device is designed to operate in water depths of 20 m and deeper. With the overall
loading on the device being reduced, it can use a simple and relatively light catenary
mooring, with six anchors in front of the device and a single rear mooring to restrain rotation
about front mooring (Figure 5). The anchors are gravity based and filled with “natural
material” of ~ 8,000t weight, which are easy to install. This system allows the devices to be
moored independently in a staggered line when implemented as a wave farm (Figure 5).
Mechanical and Electrical Plant
The low-head turbines are a variable speed, propeller type and were specifically designed for
a variation in pressure head of 1:4 by Kössler GmbH (Austria), Veteran Kraft (Sweden) and
the Technical University of Munich, where it was also tested in their renowned turbine test
stand. The turbines have fixed guide vanes and propeller blades, which makes them very

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robust and minimises maintenance costs. The variable speed operation allows higher
efficiency levels to be attained. There are a large number of turbines (8-24, dependent of the
size of the device and the wave climate at the site), which enable the device to cope with
large variations in water ingress to the reservoir (dependent on the sea state) without losing
efficiency (by changing the number of turbines operational). The hydraulic efficiency of the
turbine is around 91 % in the relevant head and flow ranges.
A grill screen is fitted around the turbines to prevent marine debris from damaging the
turbines (experience on the Nissum Bredning device shows that fish are not washed up to the
reservoir).
Each of the low-head hydro turbines is directly coupled to a 350kW permanent magnet
synchronous generator. In order to achieve a high efficiency throughout the wide head range,
each turbine is operated at variable speed (between 50 and 270 rpm), so standard frequency
inverter-controllers are used, giving a total rating of 7MW for the demonstrator device, the
largest of any single wave energy device.
The device is tuned to a particular sea state by changing its height above MSL; this is
achieved by blowing/removing air from compartments beneath the reservoir. As the wave
power levels increase, the water depth in reservoir increases so more turbines are switched.
A comprehensive software package has been devised, controlling the overtopping and the
operation of the turbines, which has allowed optimum turbine-operating strategies to be
developed. It has been found that for maximum energy production the turbines need to be
switched on and off very frequently and two solutions have been proposed: a hydraulically-
operated, cylinder gate upstream of the guide vanes and a siphon intake and both are being
investigated.
Operation and Maintenance
The Wave Dragon M&E plant is designed to be inspected and maintained at sea. Having
multiple turbines and generators means that each has a lower weight, which permits easier
removal and installation during operation. In addition, this provides multiple redundancy, so
maintenance and repair can be carried out without loss of output. It is intended that O&M
personnel will work onboard the device (on the central section, well out of the water) and
experience little movement of the device. If these design features are achieved, they should
enable work on an the device without shutdown, so the planned refurbishment of 4-5 turbines
annually (during the summer for better weather windows) and any unplanned maintenance
would be possible.

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Figure 5 The Outline Design and Mooring for the Demonstration Wave Dragon

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Device Developer’s Statement
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.
Worlds first floating grid connected wave energy device was the Wave Dragon prototype
deployed in Nissum Bredning, Denmark in 2003. Long term testing has been carried out to
determine system performance; i.e. availability and power production in different sea states,
and the efficiency targets have been verified.
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
History of the Device
The concept was invented by Erik Friis-Madsen 1986. A 1:50 scale was tested in a wave
tank as part of the Danish Wave Energy Programme between 1998-1999 at Aalborg
University and later at University College Cork. This enabled the developers to model both
the overtopping (so as to optimise energy capture by modifying crest freeboard height,
number of turbines operating) and the reflector wings (to improve capture efficiency for
specific locations). These (and the later tests at AAU and Nissum Brending) resulted in
improvements in weather-vaning, lower peak mooring forces and improved horizontal
stability of the main platform.
A 1:4.5 scale device has been under test in inland sea of Nissum Bredning from 2003 for a
total of more than 20,000 hours operational experience. The later device despite being a
scale model is impressively large measuring nearly 60m wide and weighing over 200t; this
size matched the size of waves produced at Nissum Bredning to produce results that could be
correctly scaled to a full size device operating in real sea conditions. This testing has allowed
this prototype to progress to the point where it was producing electricity 80% of the time,
whilst proving its stability and power-production potential. There have been a few
component failures and also structural failures in the main mooring line (defect force
transducer and loss of pin bolts in shackles), which led to the device being washed up on the
shore (although this caused little structural damage, demonstrating the device’s robustness).
These have been investigated and technical solutions found.

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Future Plans
UK
Wave Dragon is developing a 7 MW device for deployment in 2012 in the Irish Sea, about 4
km off the South West Wales coast close to Milford Haven. This commercial-size device is
intended to be tested for 3-5 years, after which it will be removed from the site (and deployed
elsewhere as part of a wave farm) and the site decommissioned. Hence, the device is best
looked on as a commercial prototype system that is expected to produce information useful in
follow-on commercial schemes, in particular:
• The best way both to construct a Wave Dragon of such a large physical size (how to
make the structure and the facilities needed) and deploy it.
• Finalise the development of the power takeoff and control systems
• Develop and demonstrate an O&M scheme and so operate a full size wave energy
device for a sufficient length of time to prove its reliability.
• Establish the socio-economic impact of Wave Dragon such as job creation, life cycle
assessment and environmental impact related to a MW-size wave energy device.
As such, this device is being designed as an experimental platform and will be on-site for 3-5
years after which it will be transported to another location to form part of an 11 device wave
farm. All devices will have a projected service life of ~25+ years for the turbines and ~50+
years for the structure.
Many of the studies required before deployment at sea (e.g. environmental impact, public
acceptability, etc.) have already been carried out to ensure the feasibility of Wave Dragon
power plants.
The scheme has been supported by the Welsh Development Agency with a €7.4 million grant
from the Welsh Assembly Government as an Objective One project. The project is also
supported by the EC 6th
Framework programme with a €2.4 million grant for research related
to the Welsh Demonstrator project.
Portugal
Wave Dragon has formed a project-development company, TecDragon - Tecnologia da
Energia das Ondas SA, in cooperation with a group of Portuguese and German investors in
order to develop a 50 MW wave farm in Portuguese waters, starting deployment in 2013.
Technical Status
Having proven the device concept at a small scale in a ‘marine’ environment, with plans for
demonstrating the device in its final form within the next 1-2 years, the Wave Dragon is at
Technical Readiness Level 7. In the normal way of presenting the status of wave energy
devices, the Wave Dragon is ready for deployment of its first commercial scale prototype.

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Contact Details
The 1:4.5 scale model and the future device for Portugal have been developed by:
Wave Dragon ApS
Blegdamsvej 4
DK-2200 Copenhagen N
Denmark
Phone: + 45 3536 0219
Fax: +45 3537 4537
Email: info@wavedragon.net
Web: http://www.wavedragon.net
The device for the Welsh coast is being developed in conjunction with:
Wave Dragon Wales Ltd
Gwern-y-Fran
Maerdy
Corwen
Denbighshire
LL21 9PB
UK
Phone: +44 7968 060483
Email: iain@wavedragon.co.uk (Iain Russell, UK Manager)
Web: http://www.wavedragon.co.uk
1.3 The Seawave Slot-cone Generator (SSG)