The competitiveness of Ocean Wave Energy devices has been reaching

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The competitiveness of Ocean Wave Energy devices has been reaching Powered By Docstoc
          Frank Neumann, IST-MARETEC, Pavilhão Turbomáquinas 2º andar
                    Av. Rovisco Pais 1, 1049-001 Lisboa/Portugal
    Tel. +351 21 841 7536 ; FAX +351 21 841 7398 ; e-mail

By nature, coastal waters are sensitive areas with strongly competing uses. On the one
hand, the population density in countries with significant coastline is often notably
higher in the littoral zone than inland. Consequently, this brings along a concentration
of infrastructures and industrial activities, leading to severe interventions in the natural
coastline. On the other hand, this area is traditionally of high importance for leisure and
tourism purposes, and not at last a particular ecosystem. This requires protecting and
maintaining this zone, mainly with respect to biochemical pollution, but also in what
concerns the erection of coastal structures. Typically sheltering transport infrastructure
or land reclamation, coastal structures represent not only a visual impact but also an
intervention in the natural hydrodynamic processes. Therefore, efforts should be made
to obtain the desired function by minimal intervention, which can represent the most
economical solution, as well.
Perforated caisson breakwaters can represent such a solution, as they combine the
advantages of traditional rubble-mound and vertical caisson breakwaters. These can be
summarised as low reflection values, effective material consumption and high
prefabrication grade and offering berthing facilities.
Another important advantage of these structures can be their suitability to integrate
ocean wave energy caissons, due to the similar dimensions, materials and fabrication
demands. To transform and utilise the energy of ocean waves can be regarded as a
superior engineering solution than simply dissipating or reflecting it. Due to the present
strive of many countries to strongly increase the renewable energy share of the
electricity production, an intensive use of coastal and near-shore waters will be
inevitable and ocean wave energy utilisation is one potential contributor of the resulting
energy mix. Therefore, it is important to find reasonable ways of implementing this
renewable energy form.
To date, the Oscillating Water Column (OWC) can be considered as the wave power
technology that is closest to maturity, due to intensive research and development efforts
in various countries for over 20 years. At present stage of development, the integration
of OWC caissons in vertical (perforated) breakwaters can therefore be an important step
to overcome the premature status of wave energy utilisation.
The integration of wave energy utilisation into port and coastal protection structures is a
logical consequence of socio-economical and environmental considerations on both
breakwater construction and shoreline wave energy plants. The massive intervention
into the natural coastline by building large breakwaters gains significant ecological
acceptance by producing clean energy and reducing reflections. Further, the presently
too expensive wave energy extraction can be reduced to an economically competitive
value. The present article outlines the approach for preliminary design of Oscillating
Water Column (OWC) breakwater integration, largely based on the design methodology
for the OWC pilot plant on the Island of Pico/Azores (Falcão, 2000). The synthesis of
different case study results suggests that by such a combination, the demonstration
status of OWC technology can be overcome soon and wave energy extracting
breakwaters can be a realistic alternative to the traditional coastal protection schemes in
near future.

                  WHY CLEAN ENERGY FROM THE SEA?
With the conscience of the sea being a sensitive ecosystem that is being highly exploited
and strained by human activities already, any additional exploration should be justified
well on the background of sustainability. Despite the size and the water masses of the
world‟s oceans, it is evident that its resources are limited and that only a moderated and
diversified exploration is acceptable in the medium to long term. The extraction and
conversion of mechanical energy stored in ocean waves has practically not been used to
date, and apart from its large potential in general, which will be discussed below, there
are several specific applications in which its utilisation is an attractive alternative to
other technologies. In particular, fresh-water or oxygen pumping (e.g. for aquiculture),
desalination and hydrogen production are suitable fields for the application of ocean
wave energy conversion, but the main field of past and present research and
development concentrates on electricity production. This may be mainly due to the
strive to contribute notably to the world-wide energy supply, which is most realistic via
electricity production. However, the aforementioned uses of wave energy can be
established by minor modifications or even simpler equipment than for electricity
The increasing regional and international interest of enforcing renewable energies is not
only a necessity imposed by the exhaustive cultivation of fossil fuels but also a much
more elegant and sustainable solution in terms of engineering and economical
considerations. In the Kyoto protocol, the European countries obliged themselves to
increase their electricity production through renewable energies from 6% to 12% by
2010, which means much more than a doubling of the installed capacity, due to the
annual growth of production. The growth of the largest contributors among the
„classical‟ renewable energy sources, namely hydroelectricity and onshore wind energy,
is very limited due to the extensive exploration of those technologies so far. A further
expansion of large-scale hydroelectricity and wind farms onshore are extremely
controvertible and cannot be considered to be the main factor of growth in the future,
whereas other „classical‟ renewable energy technologies have not proven their ability to
contribute significantly to the energy supply yet.
Concerning the capacity of wave energy, it should be distinguished, similar to
conventional hydropower or other renewable energy sources, between the gross
potential of the resource, the technically explorable potential, the economically
reasonable potential and the ecologically acceptable potential. While the gross potential
of wave energy worldwide is enormous, the economically reasonable and ecologically
acceptable potential are subject to local circumstances and will be several degrees of
magnitude smaller. Still, careful estimations rate this potential world-wide to a similar
range as that of present large-scale hydroelectric production (Weiss, 2001).

                     OCEAN WAVE ENERGY – VIABLE?
Following the oil-crisis in the early-mid seventies, there was a strong involvement in
wave energy research and development between 1974 and 1985 (for overview, see e.g.
Graw, 1995). However, the expectations were very high and the time for developing
viable solutions was short. Subsequently, various full-scale pilot projects failed and
ocean wave energy lost its credibility, leading to the minimisation of financial
commitment to this technology branch. Nevertheless, several extraction technologies
have reached acceptable grades of technical maturity (prototypes, pilot plants) both for
onshore or offshore and small or large-scale application. In India and Japan,
combinations of OWC and breakwater were realised and in Great Britain, a test station
on the Isle of Islay/ Outer Hebrides was successfully tested and succeeded by a next
generation plant (Heath, 2000). At present, there are 3 plants in Europe and 5 planned to
be installed in Europe, Asia e Australia, of which 3 contracts were signed within the
„Scottish Renewable Order‟. The growing interest mainly in Great Britain and Denmark
in supporting wave energy, and the fact that 40 of the 131 participants of the last
European wave energy conference came from the industry, are not the only indicators
that there is ocean wave energy as a technology branch has reached a „pre-competitive‟
stage. Recently, an implementation agreement of the International Energy Agency
(IEA), which had been known to be rather reluctant in supporting new renewable energy
technologies, was signed (Pontes, 2000).

Being a baseline for the presented work, the pilot plant on the island of Pico/Azores was
planned, developed and built over several years by the wave energy research group of
IST (Instituto Superior Técnico) and INETI (Instituto Nacional de Engenharia e
Tecnologia Industrial). The plant is of the Oscillating Water Column (OWC) type (see
figures 1 and 2), which is the most common wave energy technology. Several pilot and
test projects have existed world-wide so far and considerable research and development
work has been dedicated to OWC technology.
The rated power of the Pico plant is 400 kW, which is estimated to deliver around 0,5
GWh per year, a value that is a notable contribution to the island‟s electricity
consumption (Falcão, 2000; Weiss, 2001). When comparing to the island‟s minimum
electricity consumption, the plant is capable of providing one third of the needs that are
otherwise covered by Diesel generators. The regional utility EDA (Electricidade dos
Açores) took over the plant ownership after erection.

Being conceived as a test plant rather than a commercial project, there have been
several unpleasant, but valuable experiences during the construction and assembling
period, which caused notable delays in the final commissioning of the plant. The
technically most important conclusions with respect to next generation plants may be
the following:

    Structure items that require in-situ works should be avoided (such as in-situ
     reinforced concrete works and items that require extensive diving works)
    Civil engineering structure including construction process signs for highest cost
     partition (estimated 2/3 to 3/4 of project capital) and most serious problems
                     => significant improvements required
    In early development stage, plants at remote sites are not recommendable
     (experienced contractors with modern facilities are required to ensure sufficient
     quality         => e.g. caisson construction with floating docks)

 Figure 1: a) Cross-section of the pilot plant of Pico/Azores (left; Falcão, 2000) and b)
              Working principle of an OWC and the Wells turbine (right)

 Figure 2: a) Side-view (left) and b) Back-view (right) of the pilot plant of Pico/Azores
                                     (Falcão, 2000)

The motivation to integrate wave energy utilisation into port and coastal protection
structures is a logical consequence of socio-economical and environmental
considerations on both breakwater construction and shoreline wave energy plants. The
massive intervention into the natural coastline by building large breakwaters gain
significant ecological acceptance by producing clean energy and reducing reflections.
Further, the presently too expensive wave energy extraction can be reduced to an
economically competitive value. Such a combination was already traced in India and
Japan, (Thiruvenkatasami and Neelamani, 1997; Takahashi, 1988) and has been a
principal research objective at IST-MARETEC. Backed by the first operational
experience to be obtained by the pilot plant on the Island of Pico/Azores, it is expected
to overcome the demonstration status of OWC technology soon by integrating wave
energy caissons into breakwaters. On the other hand, it can certainly not be expected
that this particular combination will contribute significantly to regional or world-wide
electricity production, due to the limited amount of potential sites for such projects.
However, apart from being an ideal starting point to establish OWC technology and
ocean wave energy in general, such combined breakwater structures can be considered a
superior engineering solution when compared to rubble-mound or classical vertical
breakwaters. This is mainly due to the conversion of energy instead of simply resisting
or dissipating it and to the multiple use of a single structure, as sketched in figure 3.

  Figure 3: Conceptual cross-section of a multi-purpose breakwater incorporating an
                          OWC power plant (Graw, 2001)
It is evident that aspects like the integration of clean energy extraction in breakwater
structures and the double use of a single structure have notable advantages leading to a
better economic viability and a higher ecologic acceptability of both, breakwater
structures and wave energy caissons. With respect to the latter, it is further crucial to
mention the use of advanced construction methods for caissons (floating docks etc.) as a
factor of quality and economy enhancement. The prospect of applying the experience
gained in the Pico project and potentially integrating a next generation device into a
breakwater in Portugal, were the driving forces for the work presented in this paper.

Apart from performance and reliability enhancement of the electromechanical power-
take-off equipment, which are not within the focus of this paper (for reference, see e.g.
Falcão, 2000), the analysis of a multidisciplinary problem including resource
assessment, site choice and characterisation, hydrodynamic modelling, productivity
estimation, structural layout, construction process and economical considerations are
topics of recent and ongoing work. The hydrodynamic performance is modelled by the
3D numerical code Aquadyn, which had been adopted for OWC (Brito-Melo et al.,
2000). An analytical wave-to-wire model created in the same context closes the gap to
the estimation of annual production, which is required for economical considerations.
This approach has been applied both in the cases of Pico and LIMPET (next generation
shoreline OWC on the Isle of Islay/Scotland, see Heath et al., 2000) power plants.
Resource Assessment and Site Choice and Characterisation
The first steps that played a major role for the planning and design of the Pico plant may
partly be considered as less relevant for a breakwater integration, which is due to the
following reasons. Firstly, the wave energy resource offshore and near-shore has been
focus of intensive development works and presently there exists a European Atlas for
the wave energy resource, as well as a Portuguese near-shore atlas (Pontes et al., 2000).
Moreover, wave propagation and sea state prediction can presently be considered as
standard tasks due to the developments mainly in the field of numerical models in the
recent years. The aspect of site choice will be very restrained by the existence of
breakwater sites available for OWC integration and the site characterisation will be
largely accomplished within the „regular‟ breakwater project.

                       HYDRODYNAMIC MODELLING
Apart from wave tank experiments that were compulsory for the Pico project and will
be recommendable for upcoming OWC plants, an important tool for the hydrodynamic
design of wave energy caissons is the numerical modelling. In order to simulate the
OWC performance, the 3D Boundary Element Method (BEM) code AQUADYN based
on linear theory was adapted for OWC devices (Brito-Melo et al., 2000). The adaptation
was validated against published data obtained with the commercial code WAMIT and
wave tank experiment results for the Pico pilot plant and later also applied on several
case study situations, among them the Pico and LIMPET OWC plants.


   Figure 4: a) Ground plan of the site of the Pico plant (left; Falcão, 2000) and b)
Visualisation of numerical model including the bathymetry (r.; Brito-Melo et al., 2000)
By means of the numerical code, it is possible to determine the hydrodynamic
parameters of damping and added mass associated with the pressure radiation problem
inside the pneumatic chamber of the OWC system. The numerical code further allows to
determine the diffraction and radiation transfer functions in the frequency domain (see
figure 5). The superposition of the diffraction and radiation problem is assumed to be
linear, which has shown to be an acceptable assumption when compared to model tests
(Brito-Melo et al., 2000).
                600                                                                              0,07

                                              isolated OWC device                                                                              onshore OWC device
                                              onshore OWC device                                                                               isolated OWC device

                                                                           |H R| (m 3s-1Pa -1)
 |HD| (m2s-1)

                                               Diffraction problem                               0,04                                                Radiation problem


                100                                                                              0,01

                 0                                                                                 0
                      0,0   0,5       1,0        1,5       2,0       2,5                            0,0              0,5          1,0               1,5        2,0       2,5

                                   Frequency (rad/s)                                                                          Frequency (rad/s)

  Figure 5: a) Diffraction (left) and b) Radiation (right) response function for the Pico
                 plant in the frequency domain (Brito-Melo et al., 2000)

In order to obtain useful statements about the amount of energy to be expected, the
transfer functions obtained with the numerical program have to be transferred into the
time domain and related to the context of plant operation. This is done by the simulation
of the complete conversion chain of wave to electrical energy (wave-to-wire model),
which is based on the linear wave theory and is performed in the time-domain, as
sketched in figure 6.

    q = volume flow (subscripts: t = turbine; v = valve;
                                                                                                  q t (t ) + q v (t ) = q d (t ) + q r (t ) -
                                                                                                                                                          V0   dp
    d = diffraction; r = radiation);
    t = time; V0 = initial chamber volume                                                                                                                 g P a dt
    g = specific weight of water;
    Pa = athmospheric pressure ; p = pressure                                                                 N                                 t
                                                                                                  qd (t ) =   å an cos(nwt + a n ) qr (t ) = ò hr (t - τ) p¢(τ)dτ
    a = wave amplitude; w = wave frequency; hr = time
                                                                                                              n =1
    domain radiation response function;                                                                                                        -¥

    t = time delay; HD = frequency domain diffraction
    response function; Si = wave spectrum; IFT =                                                  an = 2 H D (w ) S i (w )w
                                                                                                                                        hr ( t ) = IFT [H R ( w )]
    Inverse Fourier Transform; HR = frequency domain
    radiation response function                                                                                             AQUADYN-OWC

                Figure 6: Organisation chart of the wave-to-wire-model (Brito-Melo et al., 2000)

                                  WAVE LOADS AND STRUCTURAL ASPECTS
By nature, wave energy devices are placed in regions of high incident wave power,
which is normally related to very rough sea states that have to be considered for the
survivability design. In coastal engineering, the lack of commonly accepted approaches
to estimate the impact loads on structures caused by breaking waves has been a
persisting and often cited problem (e.g. Allsop/Vicinanza, 1996). Example calculations
using the probably most common approaches in practice for one of the case study
situations resulted in a design load estimate spreading from 20 MN to above 100
MN, which makes an economical, but safe design very difficult. Recent developments,
of which the most notable may be the MAST-PROVERBS project of the European
Union and the new Coastal Engineering Manual (CEM) of the US army Corps of
Engineering, are expected to offer an improved approach to this field. However, at
present this difficulty still exists and is amplified by the fact that an OWC caisson is not
a common coastal structure. The critical moment of wave impact and the typical
stability demands for breakwater caissons are sketched in figure 7.

                        vn                     Sliding:
                                                      m * (G0 - Fu )
                                          l                               h
                                               gS =                                0

                                   b                       Fh
d >b >0
d = curvature angle            d               Overturning:
b = attacking angle
l = wave front height                                 G0 * eG - Fu * eu
v n = hitting speed
                                          0    go =
                                                           Fh * eF

 Figure 7: a) Impulsive impact condition as interpreted by Takahashi (1996) (left) and
 b) Most commonly regarded failure modes for breakw. caissons (e.g. Takahashi, 1996)
Apart from the difficulty of obtaining reasonable estimates for the breaking wave
impacts, the interpretation of the impact loads on the caisson walls is problematic. Due
to the particular geometry of an OWC chamber and the fact that neither maximum
pressures nor their dimensions and spatial distribution can be described satisfactorily at
present, the local resistance of the cellular caisson walls directly facing the wave
impacts may be critical. Whereas „regular‟ caissons are usually completely filled by
sand, gravel/rock or concrete, which provide a structural support for the front wall, this
structural element is not supported in case of OWC caissons due to the void caused by
the air chamber. This aspect of “local impact loading” should be regarded for stability
design and there is no design methodology known that takes such a specific case into

                             CONSTRUCTION PROCESS
Including the aspects of construction process and constraints implied by the breakwater
structure in an early design phase is considered of notable importance for this project.
This is because the OWC breakwater integration is rather a technology implementation
project than basic research, i.e. the separate aspects of energy capture, conversion and
transportation and breakwater caisson construction are existing engineering fields. The
synchronisation of the interfaces of these tasks will be a decisive factor for the
acceptance of such a concept, mainly due to economical considerations. On the one
hand, as many as possible potential sites have to be found and on the other hand the
OWC caisson construction has to be integrated as much as possible in the “regular”
breakwater construction process. Due to the amount of potential project sites, initially
the integration of OWC caisson construction into rubble-mound repair works was
studied, revealing several unfavourable factors of such an approach. Firstly, complex
loading conditions that are not fully understood could occur at the interfaces between
rubble-mound and caisson structure. Further, embedding the caisson into the breakwater
body could result in weakening the cross-section of the breakwater and additional works
would be required in order to place the caisson inside. The conclusion was that there
exist very few synergy effects for such a combination.
Significantly more advantageous is a “seamless” integration into caisson breakwaters.
For these structures, a floating dock or equivalent equipment is provided by the
breakwater construction, which does not only enhance the structure quality but is also
economically advantageous. Further, the seabed preparation works required for the
OWC caisson are equivalent to the ones for the other caissons. The fact that the OWC
replaces a breakwater section moreover enables an OWC construction at marginal costs.
                              ECONOMIC ASPECTS
As indicated in the previous section, the project aims at providing a convincing practical
solution of combining wave energy production with breakwater structures. It is
therefore required to make realistic balances of expenses and profits to be expected by
the OWC integration. For most potential sites in Portugal, an annual production of 800
MWh and an electricity „buyback tariff“ of 6 Euro-cents are indicative values at
present. Assuming operation and maintenance costs of 4% of the project capital and a
depreciation period of 20 years with an annual interest rate of 4%, a maximum project
capital of 425 kEURO could be spent to be commercially viable. At present, the
required project capital for a stand-alone OWC plant with such an annual production is
estimated to be around twice that value.

The studies indicate that at the present stage of development, the seamless integration of
OWC devices into vertical breakwaters is a promising option for the Iberian Atlantic
coastline within the next years. Assuming that by integrating the OWC caisson
seamlessly into the breakwater, around one third of the project capital can be saved, and
applying a moderate learning effect, it is expected to be required a total subsidy volume
for this structure type between 600 kEuro and 1400 kEuro in an optimistic and
pessimistic estimate, respectively. Following the same learning curve, this subsidy
volume could be re-gained after 30 to 120 plants in operation, respectively.

As the indicative values of the preceding section show, the combination of wave energy
extraction and caisson breakwaters appears a promising solution towards sustainable
port structures. On the other hand, a combination of OWC and rubble-mound
breakwaters is not particularly attractive at present stage of development. Generally, the
OWC as a breakwater element is subject to additional stability demands that are not
considered by standard design procedures. Therefore, higher structure costs than for
classical structures with the same weight have to be calculated. An improved
understanding of breaking wave forces is crucial for a more economic design.
As a result of the presented work, presently the particularly advantageous wave energy
integration into perforated breakwaters is research focus at IST-MARETEC in the
forthcoming 3 years. These low-reflective structures gain interest in Europe and are
considered as more intelligent engineering approaches than vertical caissons, since they
minimise both reflections and wave forces by dissipating wave energy in a chamber
between a perforated front wall and the massive caisson wall. Beside a more complex
design procedure, the dimensions for perforated caissons are typically larger and the
structural expenses higher than for “regular” caissons. Due to its characteristics, the
perforated breakwater offers a very favourable situation for wave energy integration.

This work has been sponsored by the “Marie Curie Fellowship” contract EVK3-2000-
55015 within the “EESD – Energy, Environment and Sustainable Development”
programme of the European Union.
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