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

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Low-pressure Hydro Turbines and Control Equipment for Wave Energy Converters Powered By Docstoc
					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 2001
                  Research funded in part by
               THE EUROPEAN COMMISSION
                    in the framework of the
                Non Nuclear Energy Programme
                          JOULE III
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 ......................................................................................................... 8
    5.3      MAXIMIZING ENERGY CAPTURE ............................................................................... 10
    5.4      ENERGY PRODUCTION ............................................................................................... 12
    5.5      HYDRO TURBINE DESIGN FOR USE IN WAVE ENERGY CONVERTERS............................ 13
    5.6      POWER TAKE-OFF AND TURBINE CONTROL. ............................................................... 15
    5.7      TRANSMISSION .......................................................................................................... 15
    5.8      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




                                                                   2
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 turbine
and 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 conventional
turbine 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 the
complete 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 restricted
freedom 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 design
identified 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 at
energetic 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.




                                              3
2 The Partnership

Löwenmark F.R.I (Prime SME proposer)
Mr. Erik Friis-Madsen
Blegdamsvej 4,
DK - 2200 Copenhagen N
Tel.: (45) 3537 0211
Fax.: (45) 3537 4537
e-mail: Loewenmark@city.dk
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 0219
Fax.: (45) 3537 4537
e-mail: wave-dragon@spok.dk
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 122
A - 3151 St. Pölten - St. Georgen
Tel.: (43)2742885272
Fax.: (43)2742884626
e-mail: office@koessler.com
Web: http://www.koessler.com
Business area: Hydro Turbine manufacturing

Mogens Balslev A/S
Mr. Henning Hoejte Hansen
Produktionsvej 2
DK - 2600 Glostrup
Tel.: (45) 72177217
Fax.: (45) 72177216
e-mail: hhh@balslev.dk
Web: http://www.balslev.dk
Business area: Consulting Engineering - Electrical Design

BeltElectric ApS
Mr. Peter Rasmussen
Knasterhovvej 21
DK - 5700 Svendborg
Tel.: (45) 6254 1331
Fax.: (45) 6254 2331
e-mail: belt@dk-online.dk
Business area: Generator technology
Eltra A.m.b.a.
Mrs. Lise Nielson


                                             4
Fjordvejen 1-11
DK-7000 Fredericia
Tel.: (45) 76224441
Fax.: (45) 76223171
e-mail: eltra@eltra.dk
Web: http://www.eltra.dk
Business area: Independent grid systems operator

Armstrong Technology Associates
Mr. Greame Mackie
Coble Dene Royal Quays
North Shield Tyne & Wear
UK - NE29 6DE
Tel.: (44) 191 257 3300
Fax.: (44) 191 257 3311
e-mail: gcm@ata.uk.com
Web: http://www.ata.uk.com/
Business area: Naval architechts

VeteranKraft AB
Mr. Evald Holmén
Gjörwellsgatan 14
SE - 112 60 Stockholm
Tel.: (46) 861 83911
Fax.: (46) 861 83911
e-mail: evaldholmen.chello.se
Business area: Hydro turbine design

Technical University Munich, Laboratory for Hydraulic Machinery
Mr. Rudolf Schilling
Arcisstrasse 21
DE - 80290 Munich
Tel.: (49) 8928916295
Fax.: (49) 8928916297
e-mail: schilling@lhm.mw.tu-muenchen.de
Web: http://www.lhm.mw.tu-muenchen.de
Business area: University

Aalborg University, Hydraulics and Coastal Engineering Laboratory
Mr. Peter Frigaard
Sohngaardsholmsvej 57
DK - 9000 Aalborg
Tel.: (45) 96358479
Fax.: (45) 98142555
e-mail: peter.frigaard@civil.auc.dk
Web: http://www.civil.auc.dk/i5/hyd/hyd_en.htm
Business area: University

University College Cork, Dept. of Applied Mathematics, Hydraulic & Maritime Research
Centre
Mr. Gareth Thomas
University College Cork


                                             5
IRE - Cork
Tel.: (353) 4219024332
Fax.: (353) 421270813
e-mail: thomasgp@ucc.ie
Web: http://www.ucc.ie/research/hmrc
Business area: University




                                       6
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 pressure
head 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 traditional
hydropower 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 amount
of 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 a
relatively large storage capacity (app. 8x107 Joules). However this is not enough to eliminate
major fluctuations in power production caused by the wave groups, as this calls for a 3 times
larger 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). The
Wave 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 concept
has 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 extensive
conducted 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 Wave
Dragon.

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.




                                                7
5 Technical description

5.1   Design of the Wave Dragon

The Wave Dragon is a floating offshore wave energy converter of the overtopping type. It
consists 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. The
wave 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 the
overtopping 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 is
foreseen.

5.2   Seaworthiness

The Wave Dragon has three parts that influence movements: the long wave reflectors that are
connected 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 investigating
different control strategies for improving overtopping.


                                               8
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 survivability
testing 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 was
generally 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 numerical
modelling 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)




                                                  9
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 the
hydraulic 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. Obviously
maximum 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 situation
in terms of energy capture.



                                                 10
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 function
of 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 to
the 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 two
things: improving the geometrical layout in order to make desired hydraulic behaviour occur
automatically for most wave situations, and devising an automatic regulation system adjusting
the 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 the
freeboard (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 system
configuration 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.




                                                 11
                              OVERTOPPING @ JONSWAP: PROTOTYPE
                                  2trim0              3trim2           3trim1
                                  3trim0              4trim2           4trim0
               80
                          3
                         m /sec
               60

               40

               20

                 0
                     6       7        8        9         10                        11
                          Wave Peak Spectral Period [Tp] sec

              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, and
with 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 the
flow 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 viability
concerns.




                                                 12
                                          Velocity in turbine outlet - model scale - 3m Hs waves
                                                            Average: 23,6 cm/s
                   50
                   45
                   40
                   35
            cm/s   30
                   25
                   20
                   15
                   10
                    5
                    0
                              241




                                    481




                                               721




                                                       961




                                                               1201




                                                                           1441




                                                                                     1681




                                                                                            1921




                                                                                                   2161




                                                                                                          2401




                                                                                                                 2641
                        1




                                                                      Seconds x 16



                   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 occurring
at 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 marine
growth 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 through
applying wave paddles installed by Aalborg University in the tail water tank of the turbine test
stand at Technical University Munich.



                                                                      13
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 heads
occurring, 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)


                                       efficiency vs. unit discharge
                        100


                          90

                    [%]
                    η     80
                                        ηh 3 blades
                          70            ηov 3 blades
                                        ηh 4 blades
                                        ηov 4 blades
                          60
                           2.5              3.0                3.5             4.0
                                                        3
                                                  Q1' [m /s]
                     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 reduce
turbine performance by up to 4.5% (30).




                                                  14
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 vanes
one 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 synchronous
generator 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 in
turbine 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 requirements
for 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 a
Wave 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 it
was 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 horizontal
motion 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 within
the 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).




                                                15
        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 test
cable 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 the
specific 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 systems
for 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 performance
can 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 increasing
the 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 bow
freeboard 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 the
reservoir 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 this
feature 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 is
expected to increase overtopping by 30-50%.



                                                  16
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 to
improve 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 €/kW
assuming 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 assumed
and 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 studies
correlated 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 been
specified to 25 years, although the main structure and turbines have expected lifetimes of 50
years, 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 substantial
improvements from mass production and learning effect are to be expected when commercial
exploitation is initiated, the feasibility figures given here rest on quite conservative
assumptions.




                                               17
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 this
significantly 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 been
achieved 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, with
the 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 €/kWh
although 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 the
overtopping 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.




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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 been
submitted 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 the
consortium.

Provided that the results from these two projects come out as expected, a first full-scale
demonstration 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 grid
connection and maintenance costs, and promising the introduction of a significant new
renewable energy technology.

Partners for carrying out the next phase in the exploitation plan have already been
incorporated into the consortium.




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9 References
(1) Armstrong Technology (1999): Wave Dragon Mooring. Preliminary study into the
    design 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, Patent
    Classification: F03B13/22, Copenhagen (In Danish)
(11) Friis-Madsen, E. (1999b): Offshore Wind-/wave-energy converter, European patent
    confirmed (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 Wave
    Energy, Report for the Danish Energy Agency, J. no. 51191/98-0017. Aalborg
    University


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(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 wave
    reflectors). 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 Craft
    internal project report. TU Munich, LHM - Munich
(28) LHM-TUM (2000b): Report from a parameter study concerning the layout, design and
    operation strategy of the water turbines to be employed in the Wave Dragon. Joule Craft
    internal project report. TU Munich, LHM - Munich
(29) LHM-TUM (2000c): Report from a parameter study concerning the choice of suitable
    turbines 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 the
    influence 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 Wave
    Dragon 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 a
    floating 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 Offshore
    Wave 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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