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Low-pressure Hydro Turbines and Control Equipment for Wave Energy Converters (Wave Dragon)

Hans Chr. Soerensen

& Rune Hansen

EMU

Contract: JOR3-CT98-7027

Final Publishable Report

June 2001Research funded in part by

THE EUROPEAN COMMISSION

in the framework of the Non Nuclear Energy Programme

JOULE III



2

1 ABSTRACT...................................................................................................................... 3

2 THE PARTNERSHIP...................................................................................................... 4

3 INTRODUCTION............................................................................................................ 7

4 PROJECT OBJECTIVES............................................................................................... 7

5 TECHNICAL DESCRIPTION....................................................................................... 8

5.1 DESIGN OF THE WAVE DRAGON .................................................................................. 8

5.2 SEAWORTHINESS ......................................................................................................... 85.3 MAXIMIZING E NERGY CAPTURE ............................................................................... 10

5.4 E NERGY PRODUCTION ............................................................................................... 12

5.5 HYDRO TURBINE DESIGN FOR USE IN WAVE ENERGY CONVERTERS............................ 135.6 POWER TAKE-OFF AND TURBINE CONTROL. ............................................................... 15

5.7 TRANSMISSION .......................................................................................................... 155.8 OPTIMISATION........................................................................................................... 16

5.9 FEASIBILITY .............................................................................................................. 17

6 RESULTS........................................................................................................................ 18

7 CONCLUSION............................................................................................................... 18

8 EXPLOITATION PLANS AND ANTICIPATED BENEFIT ................................... 19

9 REFERENCES............................................................................................................... 20

Research funded in part by

THE EUROPEAN COMMISSION

in the framework of the

Non Nuclear Energy Programme

JOULE III



3

1 Abstract

The Wave Dragon is a 4 MW floating offshore wave energy converter of the overtopping

type. Through performing tests on a scale 1:50 model of the Wave Dragon, real-time

overtopping time series were provided. These allowed the development of a feasible turbineand regulation strategy for handling the varying heads and flows occurring in the reservoir. A

model turbine with a runner diameter of 340 mm was designed, and tested in a conventionalturbine test stand. The results revealed very high efficiencies (91.3% peak efficiency), and

more importantly a very flat performance curve yielding high turbine efficiency for thecomplete range of heads available at the Wave Dragon. A suitable power take-off and grid

connection system was developed, addressing power quality issues, as well as more practical

issues of flexible cabling solutions. It was concluded that feasible solutions to the technical barriers envisioned prior to the project had been found. Also means for improving the

overtopping characteristics of the device were put forward.

The feasibility of the Wave Dragon at original 1st

generation design was investigated and key performance figures were given as net annual power production of 5.1-3.1 GWh/year, 2,775-3,150 €/kW in construction costs and a power production price of 0.19-0.27 €/kWh. The

figures includes availability losses, all losses in the power train, and losses from restrictedfreedom of movement for two of the scenarios, with a wave energy potential of 16 and 24

kW/m wave front respectively. Significant scope for improvement, especially from enhanced

overtopping from improved design, mass production and learning effects were also identified.

Through implementing the known technical improvements to the Wave Dragon designidentified through the project an annual net power production of 8.9 GWh/year and a

production price of 0.12 €/kWh is foreseen for a 24 kW/m wave potential. With additional

technical improvements, mass production benefits and learning effects allowing a power production of 10.7 GWH/year at a price of 0.08 €/kW by 2010. Long-term targets are a power

production of 20.3 GWh/year at a price of 0.04 €/kWh by 2016 where deployment atenergetic Atlantic Sea locations with a 36 kW/m wave climate is foreseen. Such wave climate

can be found relatively close to most parts of the European Atlantic coast.



4

2 The Partnership

Löwenmark F.R.I (Prime SME proposer)

Mr. Erik Friis-Madsen

Blegdamsvej 4,DK - 2200 Copenhagen N

Tel.: (45) 3537 0211Fax.: (45) 3537 4537

e-mail: [email protected] Business area: Consulting Engineering, energy systems

EMU (Now SPOK ApS, Project coordinator)Mr. Hans Christian Soerensen

Blegdamsvej 4, 1.tv.DK - 2200 Copenhagen N

Tel.: (45) 3536 0219Fax.: (45) 3537 4537e-mail: [email protected]

Web: http://www. spok.dk Business area: Project management, development of renewable energy projects

Kössler Ges.m.b.H.Mr. Werner Panhauser

St. Georgner Hauptstrasse 122A - 3151 St. Pölten - St. Georgen

Tel.: (43)2742885272

Fax.: (43)2742884626e-mail: [email protected]

Web: http://www.koessler.comBusiness area: Hydro Turbine manufacturing

Mogens Balslev A/SMr. Henning Hoejte Hansen

Produktionsvej 2DK - 2600 Glostrup

Tel.: (45) 72177217

Fax.: (45) 72177216e-mail: [email protected]

Web: http://www.balslev.dk Business area: Consulting Engineering - Electrical Design

BeltElectric ApSMr. Peter Rasmussen

Knasterhovvej 21DK - 5700 Svendborg

Tel.: (45) 6254 1331

Fax.: (45) 6254 2331

e-mail: [email protected] Business area: Generator technologyEltra A.m.b.a.

Mrs. Lise Nielson



5

Fjordvejen 1-11

DK-7000 Fredericia

Tel.: (45) 76224441Fax.: (45) 76223171

e-mail: [email protected]

Web: http://www.eltra.dk Business area: Independent grid systems operator

Armstrong Technology Associates

Mr. Greame MackieCoble Dene Royal Quays

North Shield Tyne & Wear

UK - NE29 6DETel.: (44) 191 257 3300

Fax.: (44) 191 257 3311e-mail: [email protected]

Web: http://www.ata.uk.com/Business area: Naval architechts

VeteranKraft ABMr. Evald Holmén

Gjörwellsgatan 14

SE - 112 60 StockholmTel.: (46) 861 83911

Fax.: (46) 861 83911e-mail: evaldholmen.chello.se

Business area: Hydro turbine design

Technical University Munich, Laboratory for Hydraulic Machinery

Mr. Rudolf SchillingArcisstrasse 21

DE - 80290 Munich

Tel.: (49) 8928916295Fax.: (49) 8928916297

e-mail: [email protected]: http://www.lhm.mw.tu-muenchen.deBusiness area: University

Aalborg University, Hydraulics and Coastal Engineering Laboratory

Mr. Peter Frigaard

Sohngaardsholmsvej 57DK - 9000 Aalborg

Tel.: (45) 96358479Fax.: (45) 98142555

e-mail: [email protected]

Web: http://www.civil.auc.dk/i5/hyd/hyd_en.htmBusiness area: University

University College Cork, Dept. of Applied Mathematics, Hydraulic & Maritime ResearchCentre

Mr. Gareth ThomasUniversity College Cork



6

IRE - Cork

Tel.: (353) 4219024332

Fax.: (353) 421270813e-mail: [email protected]

Web: http://www.ucc.ie/research/hmrc

Business area: University



7

3 Introduction

The Wave Dragon is a 4MW offshore wave energy converter of the overtopping type. It

consists of two wave reflectors focusing the waves towards a ramp, a reservoir for collecting

the water overtopping the ramp, and a number of hydro turbines for converting the pressurehead into power. Wave Dragon is invented by Mr. Friis-Madsen, Löwenmark Consulting

Engineers, and is covered by international patent (10), (11).

Turbines of the Kaplan/propeller type have been in commercial use for decades in traditionalhydropower plants. They can even be utilized in streams that only give very low head for the

turbines. In order to minimize fluctuations in the power production, a large reservoir is

needed, and this is traditionally achieved by constructing a dam at the site. Even if the amountof storage is very low, rapid fluctuations of head and flow do not occur in the traditional small

hydro power plant (20).

Offshore wave energy converters can by sake of nature only store a limited amount of water.In comparison with other offshore wave energy converter concepts the Wave Dragon has arelatively large storage capacity (app. 8x10

7Joules). However this is not enough to eliminate

major fluctuations in power production caused by the wave groups, as this calls for a 3 timeslarger storage capacity. Accommodating existing turbine and control system technologies to

this situation will lead to significant reductions in capital costs and power losses of the

turbines and all subsequent elements of the power train to the grid on land.

The development of the Wave Dragon began in 1987 and is described in (24) and (23). TheWave Dragon belongs to the type of wave energy converters called 'overtopping devices'. The

state of the art within this segment is that the overtopping systems are functioning, which was

proved by the Norwegian shore-based device TAPCHAN. The Swedish Sea Power concepthas been tested in scale 1:3 in Cattegat in the early 1990's but with no reported performance

statistics. The test series carried out during the current project is therefore the most extensiveconducted on this device type in the world to date.

4 Project Objectives

The primary objective of the project was to establish a feasible turbine strategy including

control equipment, suitable for handling the low and variable heads occurring at the WaveDragon.

This could be established through providing data on the real-time inflow to the reservoir,through performing wave tank tests on a scale 1:50 model of the Wave Dragon. With these

data the turbine strategy could be decided, and a model turbine designed and tested verifying performance for the low- and varying heads and flows posed by the measured inflow data.

Also turbine control could be simulated.

Secondary objectives included investigations of all other Wave Dragon major component

parts such as power take-off, grid connection systems, mooring systems and structural layout.On the basis of these investigations the feasibility of the Wave Dragon was re-evaluated.



8

5 Technical description

5.1 Design of the Wave Dragon

The Wave Dragon is a floating offshore wave energy converter of the overtopping type. Itconsists of two wave reflectors focusing the waves towards a ramp where they run up and

overtop into a reservoir. The hydraulic head in the reservoir is converted into power by

leading the water back into the sea through a number of Kaplan propeller hydro turbines. Thewave reflector principle in combination with the doubly curved ramp profile is subject to

international patents (10), (11).

Figure 1The basic functioning of the Wave Dragon

Compared to competing devices the ramp design and wave reflectors, which improve the

overtopping by app. 70% (41), (36) make the Wave Dragon state of the art within theovertopping devices. The Wave Dragon is regarded to be one of the leading offshore wave

energy converters also due to the low-head hydro turbines developed during this project,which are considered beyond state of the art. The basic layout and function of the Wave

Dragon are given in figure 1.

The Wave Dragon is envisaged to be deployed in farms of up to 200 units, minimising grid

connection and maintenance costs. Large-scale deployment of farms of e.g. 600 MW isforeseen.

5.2 Seaworthiness

The Wave Dragon has three parts that influence movements: the long wave reflectors that areconnected to the main structure, the water in the reservoir, and the pressured air buoyancy

system. These features make the Wave Dragon an unconventional vessel. Furthermore, the purpose of the Wave Dragon is to maximise overtopping, whereas the purpose of most other

marine structures is to minimize it, indicating that the theoretical knowledge base was limited(21).

An extensive number of test trials on a scale 1:50 model has been performed (41), (40), (33), (34), (35), (14) at Aalborg University, Hydraulics & Coastal Engineering Laboratory and

University College Cork, Hydraulics and Maritime Research Centre with the purpose of improving the understanding of the hydraulic behaviour of the Wave Dragon. The following

three categories of tests were executed: tests establishing survivability during extreme seas,

tests establishing hydraulic performance during operation conditions, and tests investigatingdifferent control strategies for improving overtopping.



9

Survivability

The survivability of the Wave Dragon has been tested for a 100-year storm situation in the

North Sea with a significant wave height of 10 m and a wave period of 14.1 s. The general

survivability has been evaluated, and forces in the mooring system as well as the interface between the wave reflectors and the main structure were measured. The force measurements

points are given in figure 2 below, and a picture of the scale 1:50 model during survivabilitytesting can also be seen.

Figure 2 Force measurements points and the Scale 1:50 model during testing. Source: (14)

The results of the initial survivability tests revealed that the Wave Dragon survivability wasgenerally satisfactory, but the initial tests revealed that pitch and heave motions were

undesirably high (34). The model geometry was then modified on the basis of numericalmodelling studies (8), (37) giving significantly reduced forces and motions as it can be seen

from figures 3 and 4.

Figure 3 Extreme mooring force before and after modifications to

model layout. Source: (35)



10

Based on the test series on the modified model, it was concluded that all measured forces

were found within the tolerance levels, and furthermore the vessel demonstrated good

seaworthiness, and had a tendency to stabilise in extreme seas (35), (14), (41).

A number of survivability tests were also passed, where the model was subjected to various

types of damages in the mooring system (14).

Suitable mooring systems for full-scale deployment as well as a planned scale 1:4 research project was carried out by Naval architects from Armstrong Technology, and suitable slack

mooring systems were identified (1), (3). Also preliminary stability calculations were performed with satisfactory results (2).

Hydraulic response

Apart from the survivability during extreme conditions, the general hydraulic performance(movements) is also of critical importance for forces in the system as well as for the ability to

catch overtopping. The desirable hydraulic performance is one of minimal motions.

Figure 4 Heave, surge and pitch motions before and after

modifications to the test model. Source: (35)

The modifications performed on the model generally reduced motions considerably, as it can be seen from figure 4, but this did not increase overtopping significantly during the next test

series performed at Aalborg University.

5.3 Maximizing Energy Capture

The Wave Dragon captures energy through the waves overtopping into the reservoir and thehydraulic head being converted into power through a number of hydro turbines. The task of

maximising the energy capture in terms of overtopping is far from a trivial one. Obviouslymaximum overtopping occurs when the crest freeboard height is minimal. But this situation is

not optimal in terms of energy content, as the water is collected at a very low head. This

indicates that there is an optimal crest freeboard height corresponding to each wave situationin terms of energy capture.



11

From this static consideration, a number of dynamic considerations are introduced,

complicating matters significantly. First of all, the weight of the water in the reservoir causes

submersion of the vessel. This means that the actual ramp crest freeboard height is a functionof the water level in the reservoir.

Secondly, the pitch motions of the vessel obviously introduce varying crest freeboard.

And finally, the fact that the centre of gravity of the water in the reservoir is located further tothe aft than the centre of gravity for the vessel itself means that the water level in the reservoir

actually affects vessel motions as well. The effect of this is depicted in figure 5 below,showing significantly higher motions for the bow part than the stern.

Figure 5 Non-dimensional maximum amplitude of oscillations at bow

and aft. Source: (35)

Maximising the energy capture for the Wave Dragon leaves ample scope for regulation and

optimisation. As the above-described mechanisms each account for decreased energy

collection compared to optimum, the task of maximising energy capture amounts to twothings: improving the geometrical layout in order to make desired hydraulic behaviour occur

automatically for most wave situations, and devising an automatic regulation system adjustingthe system settings in accordance with the given sea state, taking the hydraulic behaviour of

the Wave Dragon into account.

Obtaining the first of these has been one of the main results from the current project, with test

results clearly indicating behavioural characteristics of the Wave Dragon should be improved(41), and theoretical studies revealing how it can be improved (21), (25), (32), (41). On this

background the Wave Dragon geometrical layout was modified on a number of points

towards the end of the project, incorporating the findings into the geometrical layout. One

especially interesting outcome of the test trials was the discovery that trimming of thefreeboard (lowering of the initial ramp freeboard height relative to the stern freeboard height)yielded significantly increased overtopping (41), as it can be seen from Figure 8.

The explanation for this is primarily that the filling of the reservoir results in the model 'tilting backwards' due to the movement of the centre of gravity caused by the water, and the model

moving back to a horizontal position when the reservoir empties. Obviously a systemconfiguration where the initial crest freeboard height is a little lower than the stern freeboard,

leaving the model in a horizontal position when the reservoir is filled, has a significant

potential for improving the overall energy capture. This mechanism could be called the'shovel effect' where the Wave Dragon heads into the waves, getting the overtopping at a low

freeboard, and 'lifting' it up, when the reservoir is filled.



12

Figure 8 The effect of trim on performance in 1 - 5 m m significant

wave height for long crested waves. XtrimY’ means that the

freeboard of the stern is X m, and the bow is lowered by Y m.

Source: (41).

Obtaining the second has been achieved in devising a fairly simple mathematical model for regulating the crest freeboard to changing sea states. This model uses data on the outflow

through the turbines to adjust the freeboard height, through adjusting the air pressure in the

buoyancy chambers. Simulation studies have revealed this model to be remarkably stable, andwith a very good predictability as well (13), (12).

5.4 Energy production

As the issue of maximising the energy capture is far from being a trivial matter, the task of

optimising the conversion of the energy has also paved new scientific ground.

First of all the hydraulic head at the Wave Dragon is 1-3.5 m, which is at the very edge of current hydro turbine experience. Secondly, significant variations in the hydraulic head will

occur frequently despite the relatively large size of the Wave Dragon reservoir (15), (16), (17), (27), (20). An example of the variations in flow through the reservoir on a wave-by-wave basis is given in Figure 6. In conventional hydro power plants the head variations

occurring are handled through adjusting guide vanes and runner blade angles, correcting theflow to the available head. However, this solution is only viable when the head variations are

relatively small and infrequent. With the frequent head variations occurring at the Wave

Dragon, adjustable guide vanes and runner blades were considered unfeasible due to viabilityconcerns.

OVERTOPPING @ JONSWAP: PROTOTYPE

0

20

40

60

80

6 7 8 9 10 11Wave Peak Spectral Period [Tp] sec

m3/sec

2trim0 3trim2 3trim1

3trim0 4trim2 4trim0



13

Velocity in turbine outlet - model scale - 3m H s waves

Average: 23,6 cm/s

0

5

10

15

20

25

30

35

40

45

50

1

2 4 1

4 8 1

7 2 1

9 6 1

1 2 0 1

1 4 4 1

1 6 8 1

1 9 2 1

2 1 6 1

2 4 0 1

2 6 4 1

Seconds x 16

c m / s

Figure 6 Example of real-time overtopping test data series. Scale 1:50

values. Source: (41)

When designing a hydro turbine, the turbine is optimised for a given design point with a given

head and flow. The turbine efficiency decreases rapidly when the operating situation is far removed from the design point, and accommodating hydro turbines to the operation

environment at the Wave Dragon therefore called for new approaches (15), (16), (17), (27), (20).

5.5 Hydro turbine design for use in wave energy converters.

A model turbine (D=340 mm) was designed by VeteranKraft AB and Technical University

Munich, Laboratory for Hydraulic Equipment and Machinery with two different runners (3-

vs. 4-bladed) and tested at a conventional turbine test stand for the heads and flows occurringat the Wave Dragon. The model turbine was tested for a wide range of heads, with a

corresponding set of guide vane and runner blade angles. Also two different intake structures

(siphon vs. cylinder gate) were tested, and a test series establishing the effect of marinegrowth on turbine performance was carried out.

Figure 7 The model turbine at the test stand at TU Munich

Finally a 'wave test' series was carried out, establishing whether pressure waves propagating

beneath the turbines did influence turbine performance. This last test was carried out throughapplying wave paddles installed by Aalborg University in the tail water tank of the turbine test

stand at Technical University Munich.



14

The tests gave very good results (19) with peak hydraulic efficiencies of 90.6% and 91.3% for

the two runner designs. Even more encouraging was the fact that especially the 3-bladed

runner design gave a very flat performance curve, with high efficiencies for all headsoccurring, as it can be seen from figure 8 and 9 below.

Figure 8 Hill chart of the 4-bladed runner in conjunction with the cylinder

gate intake. Turbine hydraulic efficiencies as function of guide vane

angle, runner blade angle, outflow volume and rotational speed.

Source: (19)

6 0

7 0

8 0

9 0

1 0 0

2 . 5 3 . 0 3 . 5 4 . 0

η h 3 b l a d e s

η o v 3 b l a d e s η h

4 b l a d e s η o v

4 b l a d e s

Q 1 ' [ m

3 / s ]

η

[%]

e f f i c i e n c y v s . u n i t d i s c h a r g e

Figure 9Turbine efficiency vs. unit discharge for 3- and 4-bladed

runner. Source: (19)

Also the tests revealed that pressure waves propagating beneath the turbines did not affect

turbine performance (18), whereas marine growth inside the turbine outlet could reduceturbine performance by up to 4.5% (30).



15

5.6 Power take-off and turbine control.

At hydropower plants the power take-off system conventionally consists of generators with a

fixed speed determined by the frequency of the local grid. This is a simple and feasible

solution, given the fact that variations in hydraulic head can be coped with through varying

guide vanes and runner blade angles. Also the fact that the head is never far from the design point of the turbine, means that the hydro turbines will operate at high efficiencies at all times.As discussed above this is not the case at hand with the Wave Dragon.

To maintain high efficiency for the Wave Dragon with fixed runner blades and guide vanesone obvious solution is to apply a generator allowing the turbines to be operated at variable

speed (16), (27), (28), (29). In the present study a permanently magnetised synchronousgenerator equipped with frequency converter, developed for the wind turbine industry,

appeared very suitable for the job (4), (26). Despite the fact that losses in such a system are

somewhat higher than in a conventional asynchronous solution, it was found that the gains inturbine efficiency outweighed this. Furthermore, the price and losses of this technology are

rapidly decreasing these years, making it a very competitive solution within the time frame of the Wave Dragon development itself.

Also it was found that this type of system was capable of fulfilling the minimum requirementsfor power quality posed by a Danish independent grid systems operator to offshore wind

farms (6).

5.7 Transmission

The general layout of the power transmission system to shore was evaluated in cooperation

with the independent Danish grid system operator Eltra. The systems were evaluated for aWave Dragon farm of 200 MW installed power (50 units), as a certain farm size is called for in order to make the grid connection costs for offshore deployments feasible. The grid system

was evaluated for location near shore (25 km) and far offshore (100 km), and requirements for

the internal farm system were specified.

Internal farm grid

One technical issue, which at the outset was considered especially puzzling, was whether itwas possible to find a cable sufficiently flexible to handle the motions of the Wave Dragon.Specifically the issue of weathervaning was considered to be of a decisive importance, as

other movements could be partly compensated through connecting the cable near the center of gravity.

Consultations with an offshore cable company with considerable experience in the design of dynamic special cables for oil and gas field revealed that this problem is solvable.

Today’s technology within double cross-armored submarine cables allows a horizontalmotion of ± 45°. There is a good probability of larger horizontal motion, but this would

require close analysis, simulations and perhaps tests and hence would not be possible withinthe scope of this project. Furthermore, in order to do this, the cable structure, cross section,

etc. would have to be determined in every detail (6).



16

Figure 10 Transmission line - 100 km between offshore substation and shore. Source: (6)

Laboratory fatigue bending tests have been conducted for this type of cable subjecting a testcable to as many as 4 million bends. These tests demonstrate that it is possible to design

dynamic submarine cables with a lifetime of up to 50 years in terms of fatigue.

For the grid connection design, the minimum requirements stated for Danish offshore wind

farms were taken as a starting point (5). The exact layout of the system would depend on thespecific location, as well as a technical economical design optimisation study, which should

be carried out in the planning phase of any large project. The general layout of two systemsfor 25 vs. 100 km offshore location was put forward, with the main difference being the

critical cable length, determining whether an AC or a DC connection should be applied.

5.8 Optimisation

The project results revealed that the energy capture in terms of overtopping is largely

dependent on the hydraulic behaviour of the model. Optimising the hydraulic performancecan be achieved through efficient control and regulation of the system, but also through

improving the design. As the test series indicated a number of operation strategies increasingthe overtopping, the Wave Dragon geometrical layout was modified in order to obtain the

desired hydraulic performance. The main issues to be optimised were: trim, ramp profile,

wave reflector settings and pitch movement reduction (32), (9).

Regarding trim, test series indicated significantly improved overtopping when the initial bowfreeboard setting was set below the stern freeboard height (41). Whereas the centre of gravity

of the Wave Dragon lies almost in the centre, the centre of gravity of the water in the reservoir

lies somewhat astern. This means that the Wave Dragon will have a tendency to 'tilt' when thereservoir fills. With the improved geometry, the model is trimmed when the reservoir is

empty and moves towards a horizontal position as the reservoir fills. The discovery of thisfeature is a significant result, and is expected to yield significantly improved overtopping.

A Ph.D. study carried out at Aalborg University (21) has suggested that the ramp profile

applied on the Scale 1:50 model was too steep. The same study concluded the ramp profile of the new layout to be the best among 28 tested profiles. Implementation of this result alone isexpected to increase overtopping by 30-50%.



17

The test series performed during this project have suggested that the opening angle between

the wave reflectors was set too high (41), a fact that has also been verified through numerical

simulations (25). Optimising wave reflector set-up is also foreseen to improve performance.

As the test model utilised in this project was a 1st

generation floating laboratory model,

implementation of the results from the project into a 2nd

generation layout is expected toimprove performance significantly.

5.9 Feasibility

The feasibility of the Wave Dragon at its current state of development was investigated and

key performance figures were calculated to a construction price of 2,775-3,150 €/kWassuming single unit manufacturing, depending on the distance to shore. The annual net

power production was estimated to 3.1-5.1 GWh/year, for different deployment options(16/24 kW/m energy potential, 25/100 km offshore deployment, and loss from restricted

freedom of movement vs. free movement allowed). Availability of 95% has been assumedand all losses in the power train included. The production price was calculated to 0.19-0.27 €/kWh (31) depending on the scenario.

The feasibility estimates were based on construction cost estimates provided by the project

partners and external suppliers for the Wave Dragon geometry specified at the initiation of the

project. Annual power production has been calculated from a number of parameter studiescorrelated to model test results (9). The figures are given for the North Sea with an energy

potential of 16 kW/m (25 km offshore) and 24 kW (100 km offshore) per meter wave front.All losses in the Wave Dragon and grid system to shore have been included, and an

availability of 95% has been assumed. Losses from applying a mooring system limiting the

freedom of movement to +/- 45° have been included. The Wave Dragon lifetime has beenspecified to 25 years, although the main structure and turbines have expected lifetimes of 50years, and the interest rate applied was 5% p.a.

As the current project did not involve detail design analysis for a specific site, the feasibility

figures given above are still to be considered rough estimates. Nevertheless, as significant

means for improving the overtopping characteristics have been identified, and substantialimprovements from mass production and learning effect are to be expected when commercial

exploitation is initiated, the feasibility figures given here rest on quite conservativeassumptions.



18

6 Results

The main project results consist in the following:

The survivability of the Wave Dragon has been verified.

The overtopping characteristics have been established and means for improving thissignificantly have been identified, and the results from the test series have been incorporated

into a new geometrical layout.

A very fast operating on/off hydro turbine capable of handling the low and frequently varying

heads and flows has been designed, with a peak efficiency of over 90%. This has beenachieved without utilising variable runner blades and guide vanes. This is probably the

highest efficiency found in the energy conversion process of any wave energy converter, withthe possible exception of wave energy converters with direct power take-off.

A feasible control and regulation system has been developed (13), and a very promising power take-off system has been specified.

The grid system layout has been determined, and it has been concluded that the system will

allow the Wave Dragon to fulfil minimum requirements posed by the grid system operators to

Danish offshore wind farms (5), (6).

The feasibility of the Wave Dragon including all losses has been given to 0.19-0.27 €/kWhalthough the assumptions behind this figure are still connected with some uncertainty, as

detailed site-specific design optimisation has not been a part of the current project. The figure

concerns offshore North Sea conditions, but significantly more energetic sites can be found,e.g. near the U.K. and Irish west coast. Also the construction costs were estimated for single

unit fabrication and significant cost reductions are expected from serial production. Target production price for the first full-scale prototype (2006) is 0.12 €/kWh, with a long-term

target (2016) of 0.04 €/kWh in a 36 kW/m wave climate.

7 Conclusion

A very feasible turbine and turbine control strategy for use in wave energy converters of theovertopping type have been developed. This allows the initiation of a scale 1:4 real-sea test

programme as a natural intermediate step between the laboratory testing performed during the present project and a full-scale offshore deployment for commercial exploitation.

On the basis of the present project it can be concluded that the technical risks identified prior to the project have all been resolved, allowing the development process to progress into field-

testing.

The feasibility of the Wave Dragon is still considered such that the technology can be price

competitive to e.g. offshore wind power in 10-15 years.



19

8 Exploitation Plans and Anticipated Benefit

The next phases in the exploitation have already been initiated, with a scale 1:50 test program

for the design modifications performed on basis of the results from the current project being

carried out. This is supported from the Danish Wave Energy Program (32).

An application for a scale 1:4 real-sea test program to be carried out in 2002-2004 has beensubmitted to the EU ENERGIE programme, and new partners with the necessary skills for

carrying out the construction works during this phase have been incorporated into theconsortium.

Provided that the results from these two projects come out as expected, a first full-scaledemonstration plant is foreseen deployed in 2006, marking the initiation of commercial

exploitation.

The Wave Dragon is envisaged to be deployed in farms of 50-200 units, minimising gridconnection and maintenance costs, and promising the introduction of a significant newrenewable energy technology.

Partners for carrying out the next phase in the exploitation plan have already been

incorporated into the consortium.



20

9 References

(1) Armstrong Technology (1999): Wave Dragon Mooring. Preliminary study into thedesign of a catenary mooring for the Wave Dragon. Joule Craft report. Armstrong

Technology - Newcastle

(2) Armstrong Technology (2000a): Wave Dragon Prototype - Basic Structural Design.Joule Craft report. Armstrong Technology - Newcastle

(3) Armstrong Technology (2000b): Wave Dragon prototype - Preliminary Mooring Design.Joule Craft internal report. Armstrong Technology - Newcastle

(4) BeltElectric (2000): Wave Dragon Generator. Joule Craft internal project report.BeltElectric ApS - Svendborg

(5) Elsamprojekt (2000a): Grid connection of Wave Dragon - Basic specifications. Joule

Craft Project report. Elsamprojekt (Tech-wise)- Fredericia.(6) Elsamprojekt (2000b): Grid Connection of the Wave Dragon. Joule Craft Project report.

Elsamprojekt (Tech-wise)- Fredericia.(7) EMU (2000): Minutes of Work group meeting for Task 6 - Power generation & grid

connection. Copenhagen 9. November 2000(8) Frigaard, P., Lauridsen, H. & Andreasen, M. (1999): Minimising pitch movement of the

Wave Dragon. Danish Wave Energy Program report- Aalborg University

(9) Friis-Madsen, E. & Hansen, R. (2001): Wave Dragon overtopping and annual power production. Summary Report. Joule Craft internal report. Löwenmark - Copenhagen

(10) Friis-Madsen, E. (1999a): Anlæg til udvinding af vind-/bølgeenergi på åbent hav (plant

for extraction of wind-/waveenergy at open sea), Danish patent No. PR 173018, PatentClassification: F03B13/22, Copenhagen (In Danish)

(11) Friis-Madsen, E. (1999b): Offshore Wind-/wave-energy converter, European patentconfirmed (application no. 95 923 202.6 -2315), Patent Classification: F03B13/22

(12) Hald, T. & Frigaard, P. (2000): Strategy for Regulating the WD Free Board. Joule Craft

project report. Aalborg University(13) Hald, T. & Friis-Madsen, E. (2001): Strategy for regulating the crest free board of a

floating wave energy converter. Proceedings of the MAREC 2001 conference in Newcastle, March 2001

(14) Hald, T. (2001): Behaviour of the Wave Dragon in Extreme Waves. Summary report.

Joule Craft & Danish Wave Energy Program report. Aalborg University(15) Holmén, E. (1999a): Report from a study of possible performance data for turbines to

be installed on Wave Dragon. Joule Craft project report. VeteranKraft AB - Stockholm.(16) Holmén, E. (1999b): Report from a study of performance data for speed-regulated

turbines compared with fixed speed turbines to be installed on Wave Dragon. Joule Craft

project report. VeteranKraft AB - Stockholm.(17) Holmén, E. (1999c): Report from a study of turbine layouts to be installed on Wave

Dragon. Joule Craft project report. VeteranKraft AB - Stockholm.(18) Keller, J. et. al. (2000): The influence of wave pressure fluctuations on the performance

of the Wave Dragon turbines. Results of an experimental investigation on a model

turbine. Joule Craft project report. TU Munich, LHM & Aalborg University - Munich(19) Knapp, W. & Keller, J. (2000): Model tests on different turbines designed for the Wave

Dragon. Joule Craft and Danish Wave Energy Program report. TU Munich, LHM -Munich

(20) Knapp, W., Holmén, E. & Schilling, R. (2000): Considerations for Water Turbines to

be used in Wave Energy Converters. Proceedings of the 4th European Wave Energy

Conference in Aalborg December 2000.(21) Kofoed, J. P. (2000) Optimization of Overtopping Ramps for Utilization of WaveEnergy, Report for the Danish Energy Agency, J. no. 51191/98-0017. Aalborg

University



(22) Kofoed, J. P. and Burcharth, H. F. (2000) Experimental verification of an empirical

model for time variation of overtopping discharge, Proceedings of the 4th European

Wave Energy Conference in Aalborg December 2000.(23) Kofoed, J.P. et. al. (1998): Wave Dragon - A slack moored wave energy converter.

Proceedings of the Third European Wave Energy Conference in Patras, Greece October

1998(24) Kofoed, J.P. et. al. (2000): Development of the Wave Energy Converter - Wave

Dragon. Proceedings of the ISOPE 2000 conference in Seattle, May 2000(25) Kramer, M. & Frigaard, P. (2001): Wave Dragon - Numeriske beregninger af armenes

effektivitet. (Wave Dragon - Numerical calculations of the efficiency of the wavereflectors). Aalborg University (In Danish)

(26) Krogsgaard, J. (2000): Lowhead hydropower, variable speed and frequency converter,

Joule Craft project report. Jorgen Krogsgaard Consult ApS - Kastrup.(27) LHM-TUM (2000a): Report from a study on the layout and design of a simple on-off

propeller turbine to be used for power generation in the Wave Dragon Joule Craftinternal project report. TU Munich, LHM - Munich

(28) LHM-TUM (2000b): Report from a parameter study concerning the layout, design andoperation strategy of the water turbines to be employed in the Wave Dragon. Joule Craftinternal project report. TU Munich, LHM - Munich

(29) LHM-TUM (2000c): Report from a parameter study concerning the choice of suitableturbines to be used in the Wave Dragon. Joule Craft internal project report. TU Munich,

LHM - Munich

(30) LHM-TUM (2001a): Report from an experimental investigation concerning theinfluence of marine growth in the turbine draft tubes of the Wave Dragon. Wave Dragon

Joule Craft and Danish Wave Energy Program report. TU Munich, LHM - Munich(31) Löwenmark & EMU (2000): Feasibility of the Wave Dragon. Joule Craft internal

report. Löwenmark & EMU - Copenhagen

(32) Löwenmark et. al. (2000) Reconstruction of an Existing Scale 1:50 Model of the WaveDragon and Sequential Tests of Changes to the Model Geometry and Mass Distribution

Parameters. Test proposal accepted by the Danish Energy Agency. (In Danish)(33) Martinelli, L. & Frigaard, P. (1999a): Example of overtopping time series, Wave

Dragon. Joule Craft internal project report. Aalborg University

(34) Martinelli, L. & Frigaard, P. (1999b): The Wave Dragon: 3D overtopping tests on afloating model. Joule Craft & Danish Wave Energy Program report. Aalborg University

(35) Martinelli, L. & Frigaard, P. (1999c): The Wave Dragon: tests on a modified model.Joule Craft & Danish Wave Energy Program report. Aalborg University

(36) Nielsen, A. & Kofoed, J.P. (1997): The Wave Dragon. M. Sc. graduate report in civil

engineering. Aalborg University.(37) Soerensen, H. C. & Friis-Madsen, E. (1999): Wave Dragon - Tests to evaluate

hydraulic response. Danish Wave Energy Program Phase A report. EMU - Copenhagen

(38) Soerensen, H.C. et. al. (2000): Wave Dragon - Now ready for real sea. Proceedings of the 4th European Wave Energy Conference in Aalborg December 2000.

(39) SPOK ApS et. al. (2001): Real Sea Testing of a Scale 1:4 Model of a Floating OffshoreWave Energy Converter (Wave Dragon). Application submitted to the EU ENERGIE

programme EESD-1999-5.2.5, Call Identifier: 2000/C 303/10 (Confidential)

(40) UCC/HMRC (2000a): Data Acquisition & Validation - Wave Dragon Contract JOR3-CT98-7027. Joule Craft internal project report. HMRC - Cork.

(41) UCC/HMRC (2000b): Data Analysis & Summary. Wave Dragon Contract JOR3-

CT98-7027. Joule Craft internal project report. HMRC - Cork.

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