Encyclopedia of Energy || Wave and Tidal Energy Conversion

Wave and TidalEnergy Conversion

GEORGE LEMONISCentre for Renewable Energy Sources

Attica, Greece

1. Introduction

2. Wave Energy Conversion

3. Tidal Energy Conversion

4. Conclusions

Glossary

fetch Distance over which wave-inducing wind–sea inter-action takes place.

high tide Highest water level during a tidal period.linear wave Theoretical wave form of sinusoidal shape;

describes offshore wave conditions with satisfactoryaccuracy.

low tide Lowest water level during a tidal period.lunar tide Ocean tide induced by the gravitational forces

of the moon.neap tide Lowest low tide during a lunar month.solar tide Ocean tide induced by the gravitational forces of

the sun.spring tide Highest high tide during a lunar month; period

of revolution of the moon around the earth (B29.5days).

tidal range Difference in water level between consecutivehigh and low tides.

This article outlines the state of the art of exploita-tion of two renewable energy sources that areavailable in abundance in the world’s oceans: oceanwave energy and tidal energy. It is evident thathumanity could cover a significant part of itselectricity demand from these energy sources. Thepotential global wave energy contribution to theelectricity market is estimated to be on the order of2000 TWh/year, approximately 10% of the worldelectricity consumption. The global tidal rangeenergy potential is estimated to be on the order of3000 GW, with approximately 1000 GW(B3800 TWh/year) being available at comparably

shallow waters. Tidal current energy conversiontechnologies, which have started to be investigatedduring the recent past, are predicted to supply up to48 TWh/year from sites around Europe. Further-more, other large tidal current resources are still tobe explored worldwide. Although research anddevelopment on ocean energy exploitation is underway in several countries around the world, oceanenergy conversion technologies have not yet pro-gressed to the point of massive power generation.This is due in part to the often rough andunpredictable conditions under which these technol-ogies have to operate. However, considerable pro-gress has been made during the past decade invarious engineering fields associated with oceanenergy conversion. Recent advances indicate thatsome technologies could meet the goal of powerproduction at the commercial level by or before2010. This article outlines the nature and worldwideresource of ocean wave and tidal energy, explains thebasic conversion concepts, and points out theadvantages and disadvantages of ocean energyconversion. The presentation of the ocean energyconversion development status is confined to briefdescriptions of the most advanced technologies.

1. INTRODUCTION

Worldwide energy consumption is predicted to riseconsiderably over the coming decades. Many coun-tries in the world, constantly reminded that tradi-tional methods of energy production are contributingto serious environmental problems and in view oftheir commitment to the Kyoto Protocol, have seenthe urgent need for pollution-free power generation.The energy sector has been forced into a renovatingprocess that sees an opening toward renewableenergy. In the dynamic evolution of the renewable

Encyclopedia of Energy, Volume 6. r 2004 Elsevier Inc. All rights reserved. 385

energy industry, an ocean energy industry is emer-ging. Although the technology is relatively new andcurrently not economically competitive with moremature technologies (e.g., wind energy), the interestin ocean energy conversion from governments andindustry is steadily increasing. An important featureof ocean energy sources is their high density, anddensity is highest among the renewables.

The oceans, occupying nearly three-quarters of theworld’s surface and the atmosphere above, interceptmost of the energy from the sun—approximately80,000 TW. This energy appears in the oceans in avariety of forms, for example, as waves generated bywind blowing over the ocean surface and as currentsdriven by the wind or caused by thermal gradients.

In addition, the oceans are influenced by thegravitational interactions in the planetary system ofthe sun, the earth, and the moon that represents anextremely large reservoir of kinetic energy. Thisreservoir is tapped slowly, but continuously andpredictably, to provide the energy to maintain thetidal system in the oceans.

Various other sources of renewable energy areavailable in the oceans, for example, the vast thermalpotential between the upper warm and the deep coldocean water layers and the density gradients betweenwater layers of different salinity.

Recent technological advances indicate that oceanwave energy and tidal energy conversion technolo-gies could be developed on the level of commercialpower production in the foreseeable future. Thisarticle describes the main features of these renewableenergy sources and of the state of the art and theperspectives of their use.

2. WAVE ENERGY CONVERSION

2.1 Wave Energy: Physics and Resources

Among different types of ocean waves, wind-generated waves have the highest energy concentra-tion. Wind waves are derived from the winds as theyblow across the oceans (Fig. 1). This energy transferprovides natural storage of wind energy in the waternear the free surface. Once created, wind waves cantravel thousands of kilometers with little energy lossunless they encounter head winds. Nearer the coast-line, the wave energy intensity decreases due tointeraction with the seabed. Energy dissipation nearshore can be compensated by natural phenomenasuch as refraction and reflection, leading to energyconcentration (‘‘hot spots’’).

Ocean waves contain two forms of energy: thekinetic energy of the water particles that generallyfollow circular paths (the radius of which decreaseswith depth) and the potential energy of elevatedwater particles (Fig. 2). On the average, the kineticenergy in a linear wave equals its potential energy.The energy flux in a wave, the wave power, isproportional to the square of the amplitude and tothe period of the motion. The average power in long-period (B7–10 s), large-amplitude (B2 m) wavescommonly exceeds 40 to 50 kW per meter width ofoncoming waves.

Like most forms of renewables, wave energy isunevenly distributed over the globe. Increased waveactivity is found between the latitudes of B30 andB601 in both hemispheres, induced by the prevailingwestern winds (‘‘westerlies’’) blowing in these regions(Fig. 3). Particularly great resources are located alongthe western European coast, off the coasts of Canadaand the United States, and along the southern coastsof Australia and South America.

Conservative estimates of the Commission of theEuropean Communities predict a wave energy

Fetch

Seabed

Windgeneration

Wavepropagation

Wind−Sea interactionunder the influence of

gravity

FIGURE 1 Wind-induced wave generation.

Kinetic energy Potential energy

FIGURE 2 Wave energy components.

386 Wave and Tidal Energy Conversion

market in the medium term (B2010) of approxi-mately 5.5 TWh/year. However, the potential world-wide wave energy contribution is estimated to be onthe order of 2000 TWh/year, approximately 10% ofthe world electricity consumption.

2.2 Wave Energy Conversion Techniques

In contrast to other renewable energy sources, thenumber of concepts for wave energy conversion isvery large. Although more than 1000 wave energyconversion techniques are patented worldwide, theapparent large number of concepts for wave energyconverters can be classified into a few basic types.The followings concepts are widely adopted:

� The oscillating water column that consists of apartially submerged hollow structure open to theseabed below the water line (Fig. 4). The heavemotion of the sea surface alternatively pressurizesand depressurizes the air inside the structure,generating a reciprocating flow through a ‘‘Wells’’turbine installed beneath the roof of the device. Thistype of turbine is capable of maintaining constantdirection of revolution despite the direction of theairflow passing through it.

� Overtopping devices that collect the water ofincident waves to drive one or more low headturbines (Fig. 5).

� Heaving devices (floating or submerged) thatprovide a heave motion that is converted by mecha-nical and/or hydraulic systems in linear or rotationalmotion for driving electrical generators (Fig. 6).

� Pitching devices that consist of a number offloating bodies, hinged together across their beams(Fig. 7). The relative motions between the floating

6040

30

20 20

2020

4040

4030

70

10

1020

20

20

15

30 30

20

6070

100

2020

2030

40 50

10

30

3050

70

FIGURE 3 Global wave power distribution in kilowatts per meter of crest width.

Seabed

Generator

Wavedirection

Concretestructure

Air column

Airflow Wellsturbine

FIGURE 4 Operating concept of an oscillating water column.

Seabed

Buoyancy

Generator

Reservoir

KaplanturbineWave

direction

FIGURE 5 Operating concept of an overtopping device

(floating type).

Wave and Tidal Energy Conversion 387

bodies are used to pump high-pressure oil throughhydraulic motors that drive electrical generators.

� Surging devices that exploit the horizontalparticle velocity in a wave to drive a deflector or togenerate a pumping effect of a flexible bag facing thewave front (Fig. 8).

2.3 Advantages and Disadvantages ofWave Energy Conversion

It is important to appreciate the difficulties facingwave power developments, the most important of

which are the following:

� Because of irregularity in wave amplitude,phase, and direction, it is difficult to obtainmaximum efficiency of a device over the entire rangeof excitation frequencies.

SeabedSeabed

Wavedirection

Pressurizedair

Wavedirection

BA

Buoy

FIGURE 6 Operating concepts of heaving devices: (A) Archimedes Wave Swing concept, adapted from Rademakers et al.(1998), and (B) floating buoy converter.

Seabed

Buoyantsegments

Wavedirection

FIGURE 7 Operating concept of a pitching device (Pelamis

concept). From Yemm (1999).

Seabed

Wavedirection

Flap

FIGURE 8 Operating concept of a surging device.


� The structural loading in the event of extremeweather conditions, such as hurricanes, may be ashigh as 100 times the average loading.

� The coupling of the irregular slow motion(frequency B0.1 Hz) of a wave to electrical generatorstypically requires B500 times greater frequency.

Obviously, the design of a wave power converter hasto be highly sophisticated to be operationallyefficient and reliable, on the one hand, and econom-ically feasible, on the other. As with all renewables,the available resource and variability at the installa-tion site have to be determined first. The precedingconstraints imply comparably high constructioncosts and possibly reduced survivability. Thesefactors, together with misinformation and lack ofunderstanding of wave energy by the industry,government, and public, have often slowed downwave energy development.

One of the important advantages of wave energyconversion is its environmental compatibility giventhat wave energy conversion technologies are gen-erally considered to be free of polluting gasemissions. The abundant resource and the high-energy fluxes in the waves prescribe, at appropriatedesign of the devices, economically viable energyproduction. Other advantages of wave energyinclude its natural seasonal variability, which followsthe electricity demand in temperate climates, and theintroduction of synchronous generators for reactivepower control. The negligible demand of land use isan important aspect, followed by the current trendsof offshore wind energy exploitation. As for mostrenewables, the in situ exploitation of wave energyimplies diversification of employment and security ofenergy supply in remote regions. Furthermore, thelarge-scale implementation of wave power technol-ogies will stimulate declining industries (e.g., ship-yards) and promote job creation in small andmedium-size enterprises.

2.4 Wave Energy ConversionDevelopment Status

Wave energy conversion is being investigated in anumber of countries, particularly in the EuropeanUnion member states, China, India, Japan, Russia,and the United States. Although the first patentcertificate on wave energy conversion was issued in1799, the intensive research and development(R&D) study of wave energy conversion began afterthe dramatic increase in oil prices in 1973.

During the past 5 years or so, there has been aresurgent interest in wave energy, especially inEurope. Nascent wave energy companies have beenhighly involved in the development of new waveenergy schemes such as the Pelamis, the ArchimedesWave Swing, or the Limpet. Currently, the world-installed capacity is approximately 2 MW, mainlyfrom pilot and demonstration projects.

The electricity generating costs from wave energyconverters have shown significant improvementduring the past 20 years or so and have reached anaverage price of approximately 0.08 h/kWh at adiscount rate of 8%. Compared with the averageelectricity price in the European Union (B0.04 h/kWh), the electricity price produced from waveenergy is still high, but it is forecasted to decreasefurther with the development of the technologies.

Although early programs for R&D on waveenergy considered designs of several megawattsoutput power, more recent designs are rated atpower levels ranging from a few kilowatts up to 2to 4 MW. Massive power production can be achievedby interconnection of large numbers of devices.

The amount of ongoing development work onwave energy technologies is very large and cannot bedescribed adequately in a single article such as this.In the framework of this article, the promising waveenergy device developments are described briefly.More detailed reviews can be found in the extensiveliterature in this area.

The Archimedes Wave Swing, developed byTeamwork Technology BV in The Netherlands,consists of a hollow pressurized structure, the upperpart of which is initiated to heave motions by theperiodic changing of hydrostatic pressure beneath awave. A 2-MW pilot plant has been constructed andwas waiting to be deployed off the Portuguese coastduring the spring of 2003.

The Energetech Oscillating Water Column wasdeveloped by Energetech Ltd. in Australia. Thedevice uses a novel variable-pitch turbine and aparabolic wall to focus the wave energy on the devicecollector. This scheme has received a power purchaseagreement with the local utility at Port Kembla,80 km south of Sydney, for a 500-kW plant.

The European Pilot Plant on Pico Island in theAzores is a 400-kW oscillating water column devel-oped by a European team coordinated by theInstituto Superior Tecnico (Portugal), which consistsof six Portuguese partners and two partners from theUnited Kingdom and Ireland. The plant was designedas full-scale testing facility. It is fully automated andsupplies a sizable part of the island’s energy demand.


The Floating Wave Power Vessel is a floatingovertopping device for offshore operation developedby Sea Power International in Sweden. It consists of afloating basin supported by ballast tanks in foursections. A patented anchor system allows theorientation of the vessel to the most energetic wavedirection. A pilot plant was developed and deployedduring the 1980s near Stockholm, and the companyhas signed contracts in various countries for com-mercial deployment of the technology.

The LIMPET is a 500-kW oscillating watercolumn developed by the Queen’s University ofBelfast and Wavegen Ltd. in the United Kingdom.A 75-kW prototype was constructed on the island ofIslay in Scotland in 1991. The LIMPET is thesuccessor of this prototype, intended to addressmany of the issues currently hindering the full-scaledeployment of oscillating water column devices.

The McCabe Wave Pump consists of threerectangular steel pontoons that are hinged togetheracross their beam. The bow of the fore pontoon isslack moored, and two more slack moorings areattached part of the way down the aft pontoon. Thisallows the system to vary its alignment so as to headinto the oncoming seas. A 40-m long prototype wasdeployed in 1996 off the coast of Kilbaha in Ireland.

The Mighty Wale is an offshore device, based onthe oscillating water column concept, that wasdeveloped by the Japan Marine Science & Technol-ogy Center. A 120-kW prototype with three oscillat-ing water columns in a row has been operating1.5 km off Nansei Town at 40 m depth since 1998.The mooring system is designed to withstand wind–wave conditions resulting from a ‘‘once in 50 years’’storm.

The OPT Wave Energy Converter, developed byOcean Power Technology in the United States,consists of a 2- to 5-m diameter buoy-type cylinderclosed at the top and open to the sea at the bottom. Ahydraulic ram is positioned between the top of theshell and a highly buoyant steel float containedwithin the shell. The relative motion of the shell tothe buoyant float activates a hydraulic system topump oil at high pressure to a generator. Extensivetests on a large scale in the eastern Atlantic have beenconcluded, and as of this writing, the first commer-cial schemes were about to be built in Australia andin the Pacific. The individual converters are rated atbetween 20 and 50 kW, intended to meet multimega-watt demands using arrays.

The Pelamis device is a semisubmerged articulatedstructure composed of cylindrical sections linked byhinged joints. The wave-induced motion of these

joints is resisted by hydraulic rams that pump high-pressure oil through hydraulic motors via smoothingaccumulators. A 130-m long and 3.5-m diameterdevice rated at 375 kW is being developed by OceanPower Delivery Ltd.–OPD in Scotland.

The Pendulor is a surging-type converter consist-ing of a rectangular box that is open to the sea at oneend. A pendulum flap is hinged over this opening, sothat the actions of the waves cause it to swing backand forth. This motion is then used to power ahydraulic pump and generator. Several schemes(Z5 kW) have been built in Japan, and there areplans to develop a larger plant.

The Point Absorber Wave Energy Converter,developed by Ramb�ll in Denmark, consists of afloat connected to a suction cup anchor by apolyester rope. The relative motion between the floaton the sea surface and the seabed structure activatesa piston pump (actuator) inserted between the ropeand the float. A 1:10 scale model was tested at sea atthe Danish test site ‘‘Nissum Bredning’’ over a periodof 3 months, and a 1:4 scale model with a 2.5-mdiameter is currently being developed for open seatesting.

The concept of the Salter Duck was introduced in1974 by S. Salter. An important feature of this deviceis the capability of converting both the kinetic andpotential energies of the wave, thereby achieving veryhigh absorption efficiencies (theoretically 490%).The system has undergone considerable developmentsince 1983 and was redesigned in 1993. The currentdesign is characterized by high availability andoverall efficiency and low energy production costs.

The Wave Dragon is an offshore wave energyconverter of the overtopping type developed by agroup of companies coordinated by Wave DragonApS in Denmark. It uses a patented wave reflectordesign to focus the wave toward a ramp and to fill ahigher level reservoir. Electricity is produced by a setof low-head Kaplan turbines. The scheme has beentested in the laboratory on a 1:50 scale and on a1:3.5 scale model turbine. A 57-m wide prototypeweighing 261 tons on a 1:4.5 scale rated in full scaleto 4 MW is being deployed in Nissum Bredning inDenmark.

3. TIDAL ENERGY CONVERSION

3.1 Tidal Energy: Physics and Resources

Tidal energy conversion techniques exploit thenatural rise and fall of the level of the oceans caused


principally by the interaction of the gravitationalfields in the planetary system of the earth, the sun,and the moon. The main periods of these tides arediurnal at approximately 24 h and semidiurnal at12 h 25 min. This motion is influenced by thepositions of the three planets with respect to eachother, and these positions vary during the year.Spring tides occur when the tide-generating forces ofthe sun and moon are acting in the same directions.In this situation, the lunar tide is superimposed to thesolar tide (Fig. 9). Some coastlines, particularlyestuaries, accentuate this effect, creating tidal rangesof up to approximately 17 m. Neap tides occur whenthe tide-generating forces of the sun and moon areacting at right angles to each other.

The vertical water movements associated with therise and fall of the tides are accompanied by roughlyhorizontal water motions termed ‘‘tidal currents.’’Therefore, a distinction should be made betweentidal range energy (the potential energy of the waterat high tide) and tidal current energy (the kineticenergy of the water particles in horizontal directionin a tide).

Tidal currents have the same periodicities as dothe vertical oscillations and so are predictable, butthey tend to follow an elliptical path and do notnormally involve a simple to-and-fro motion. Wheretidal currents are channeled through constrainingtopography, such as in straits between islands, inshallows between open seas, and around the ends ofheadlands, very high water particle velocities canoccur. These relatively rapid tidal currents typicallyhave peak velocities during spring tides in theneighborhood of 2 to 3 ms�1 or more.

Currents are also generated by the winds as wellas temperature and salinity differences. The term‘‘marine currents,’’ often found in literature, encom-passes several types of ocean currents. Wind-drivencurrents affect the water at the top of the oceans,from the surface down to approximately 600 to800 m. Currents caused by thermal gradients andsalinity differences are normally slow deep watercurrents that begin in the icy waters around the northpolar ice. Wind-driven currents appear to be lesssuitable for power generation than do tidal currentsbecause the former are generally slower. Moreover,tidal currents usually exhibit their maximum speed incomparably shallow waters accessible for largeengineering works.

The global tidal range energy potential is estimatedto be 3000 GW, with approximately 1000 GW beingavailable in comparably shallow waters. However, thetidal energy resource that could realistically bedeveloped (tidal ranges of 4.5–5.0 m or higher) isconfined to a few regions with exceptional tidal ranges(Fig. 10). Within the European Union, France and theUnited Kingdom have sufficiently high tidal ranges ofup to approximately 10 m. Beyond the EuropeanUnion, Canada, the Commonwealth of IndependentStates (CIS), Argentina, Western Australia, and Koreahave potentially interesting sites that have beeninvestigated periodically.

Recent studies indicate that marine currents havethe potential to supply a significant fraction of futureelectricity needs. Tecnomare SpA and IT Power Ltd.studied 106 locations in European waters that werepredicted to be suitable for current energy exploita-tion. The potential of these sites resulted in an

Sun

Thirdquarter

Firstquarter

Half moon New moon

29.5 days

Earth

Moon

Moonorbit

Solartide

Lunartide

Earthorbit

FIGURE 9 Interaction of lunar and solar tides during a lunar month.


installed capacity of marine current turbines of morethan 12,000 MW. Locations with particularly in-tense currents are found around the British Islesand Ireland, between the Channel Islands andFrance, in the Straits of Messina between Italy andSicily, and in various channels between the Greekislands in the Aegean. Other large marine currentresources can be found in regions such as SoutheastAsia, both the east and west coasts of Canada, andmany other places around the globe that requirefurther investigation.

3.2 Tidal Energy Conversion Techniques

3.2.1 Tidal Range EnergyThe technology required to convert tidal rangeenergy into electricity is very similar to the technol-ogy used in traditional hydroelectric power plants.The first requirement is a dam or ‘‘barrage’’ across atidal bay or an estuary. Building dams is an expensiveprocess. Therefore, the best tidal sites are thosewhere a bay has a narrow opening, thereby reducingthe necessary dam length. At certain points along thedam, gates and turbines are installed. When there isan adequate difference in the elevation of the wateron the different sides of the barrage, the gates areopened. This ‘‘hydrostatic head’’ that is createdcauses water to flow through the turbines, turningan electric generator to produce electricity (Fig. 11).

Electricity can be generated by water flowingboth into and out of a basin. Because there are twohigh tides and two low tides each day, electricalgeneration from tidal power plants is characterizedby periods of maximum generation every 12 h, withno electricity generation at the 6-h mark in between.Alternatively, the turbines can be used as pumps topump extra water into the basin behind the barrageduring periods of low electricity demand. Thiswater can then be released during the day periods

with the greatest electricity demand, thereby allow-ing the tidal plant to function with some of thecharacteristics of a ‘‘pumped storage’’ hydroelectricfacility.

A recently proposed technology to exploit tidalrange energy, termed ‘‘offshore tidal conversion,’’relies on the use of impoundment structures placedoffshore on shallow flats with large tidal ranges.Rather than blocking an estuary, impoundmentstructures would be completely independent of theshoreline. Such structures, which could be built ofeconomical rubble mound construction materials,would resolve many of the environmental andeconomic problems of shoreline tidal barrages.

3.2.2 Tidal Current EnergyTidal currents can be harnessed using technologiessimilar to those used for wind energy conversion, thatis, turbines of horizontal (Fig. 12) or vertical (Fig. 13)axis (‘‘cross-flow’’ turbines). Because the density ofwater is some 850 times higher than that of air, thepower intensity in water currents is significantlyhigher than that in airflows. Consequently, a watercurrent turbine can be built considerably smaller thanan equivalent-powered wind turbine.

A novel technique recently developed by Engineer-ing Business Ltd. in the United Kingdom uses ahydrofoil, which has its attack angle relative to theonset water flow varied by a simple mechanism(Fig. 14). This causes the supporting arm to oscillate,which in turn forces hydraulic cylinders to extendand retract. This produces high-pressure oil, which isused to drive a generator. A more detailed descriptionof the technique is provided in the next section.

Other techniques for tidal current energy conver-sion have been proposed in the past, for example, thegeneration of vortices by submerged delta wings todrive horizontal axis turbines.

10

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6

6

5

5

8

7

7

7

FIGURE 10 Regions of exceptional average tidal range. Seabed

Turbine

SeaBarrageBasin

Sluicegate

FIGURE 11 Operating concept of a tidal barrage.


3.3 Advantages and Disadvantages ofTidal Energy Conversion

3.3.1 Tidal Range EnergyTidal barrage technology is considered mature, butas with all large civil engineering projects, therewould be a series of technical and environmentalrisks to address. The main environmental risks areassociated with the changes in water levels thatwould modify currents and sediment transport anddeposit. However, there are regional developmentbenefits as well. For example, the La Rance plant inFrance, the only commercial tidal range conversionscheme so far, includes a road crossing linking twopreviously isolated communities and has allowedfurther development of the distribution network forraw materials and developed products.

Tidal barrage projects normally require highcapital investment at the outset and so have relativelylong construction periods and low load factors,

leading to long payback periods. As a consequence,the electricity cost is highly sensitive to the discountrate used. Therefore, access to suitable funding is aserious problem and is unlikely to be achievedwithout public intervention.

Offshore tidal technology is expected to beassociated with less environmental damage than aretraditional shoreline barrages. Furthermore, offshoretidal schemes are predicted to be more economicalthan tidal barrages. The structures would be built ina similar way to rubble mound breakwaters usingconventional inexpensive materials such as sand,loose rock, and gravel. Their load factors would behigher than those of barrages. In contrast tobarrages, which must generate primarily in onedirection (on the ebb tide) to minimize progressivedisruption of the intertidal zone, offshore tidalconverters would be free to use both the ebb andflood tides for generation.

FIGURE 12 Horizontal axis tidal current turbine. Reprinted

with the permission of Marine Turbines Ltd.

FIGURE 13 Vertical axis tidal current turbine (floating plat-

form). Reprinted with the permission of Ponte di Archimede nello

Stretto di Messina SpA.

FIGURE 14 Stingray generator. Reprinted with the permissionof Engineering Business Ltd.


3.3.2 Tidal Current EnergySeveral types of tidal current conversion devices,particularly fully submerged devices, are subject tothe corrosive effects of seawater. This leads to highconstruction costs because it becomes necessary to usecorrosion-resistant materials. In addition, maintenancebecomes difficult because divers are needed to accesssubmerged machinery. Although placing the generatorabove water can minimize the need for divers,maintenance costs would still be higher than, forexample, in wind turbines. The installation of largeschemes could also be confronted with major technicalproblems, safe and economical methods of deploy-ment, and recovery requiring further investigation.

One specific advantage of tidal current conversiontechnologies is their limited environmental impact.This is considered negligible because their operationis not associated with polluting gas emissions. Theirinstallation requires only minimal land use, and fullysubmerged devices will not affect their surroundingsoptically or acoustically. Their effects on flora andfauna have not been studied extensively, but it isunlikely that they would be of significance. Incontrast to atmospheric airflows, the availability oftidal currents can be predicted very accuratelybecause their motion will be tuned with the localtidal conditions. The power density from flowingwater is much higher than that from most otherforms of renewable energy. Thus, marine currentconverters could be built considerably smaller thanequivalent machines, for example, those for windenergy conversion. Finally, submerged marine cur-rent converters are considered to operate in a safeenvironment. It is evident that disturbances causedby extreme weather conditions are significantlyattenuated to the depths of approximately 20 to30 m where the devices will normally operate.

3.4 Tidal Energy ConversionDevelopment Status

3.4.1 Tidal Range EnergyThe first large-scale, commercial tidal range energyplant was built on the La Rance estuary in Franceduring the 1960s and has now operated successfullyfor more than 30 years. The La Rance station is stillthe only industrial-sized tidal power station in theworld. Its 240-MW power is roughly one-fifth that ofan EDF (Electricite de France) nuclear reactor and ismore than 10 times that of the largest other tidalstations in the world. The barrage was designed fortwo-way operation. Today, it is operated nearlyexclusively in ebb generation mode. The net annual

energy yield of the 10 horizontal Kaplan turbinesaverages 544 GWh.

The good performance of La Rance has resulted inexamination of several other projects in France,particularly in La Somme and Saint-Brieux bays, inBrest roadstead, along the Cotentin peninsula westcoast, and a 200-square-kilometer area betweenGrandville, Chausey islands, and Coutainville. Theseprojects were finally abandoned due to their veryhigh investment costs and concerns over ecologicalimpact. The Russians built a small 400-kW devicenear Murmansk that was later followed by a 17.4-MW experimental device built by the Canadians atAnnapolis. A series of small plants have beeninstalled in China. In the United Kingdom, a seriesof industrial consortia in collaboration with thegovernment have investigated the prospects for tidalenergy on the Severn, Mersey, and a number ofsmaller estuaries. None of these schemes hasprogressed to full-scale development.

The comparably high generation costs and longpayback periods of tidal range schemes imply thatwithin deregulated electricity markets, which arebased on private investment, tidal energy is unlikelyto be commercially developed if the kilowatt-hourprice does not become competitive with cost-effectiverenewable energies.

The generation costs of offshore tidal energy areexpected to be lower than those of tidal barrageschemes. Offshore tidal energy is currently beinginvestigated in the United Kingdom. Three projects arein development, with two of them having a capacity of30 MW and one having a capacity of 432 MW.

3.4.2 Tidal Current EnergyTidal current technology is in its infancy. However,recent developments in this area open up prospectsfor commercial deployment of some schemes in thenear future. The economical viability of theseschemes has not yet been proven, with the estimatedproduction costs typically ranging from approxi-mately 0.08 to 0.35 h/kWh. However, it is antici-pated that the production costs will decrease as thetechnologies advance.

Currently, different pilot plants are in operation orabout to be installed, mainly in Europe. The devicesrely on the horizontal or vertical axis turbineconcepts and on the previously outlined oscillatinghydrofoil technique. The most promising schemesare described in what follows.

The SEAFLOW device (Fig. 12), developed byMarine Current Turbines Ltd. in the United King-dom, consists of an axial flow rotor of approximately


15 m in diameter, rated at approximately 300 kW ina current velocity of roughly 2.0 to 2.5 ms�1. Thesystem will be installed on a mono-pile, socketed tothe seabed at a depth of approximately 30 m. Thisproject is expected to lead directly to a second phaseinvolving the development of 5 to 10 larger turbines,approximately 1 MW each. The SEAFLOW device isthe successor of a 15-kW prototype with a 3.5-mrotor diameter that was tested successfully in 1994.

A similar device developed by SINTEF EnergyResearch in Norway is aimed to be the first tidalcurrent converter that produces power for the grid.The turbine has a 25-m high tower with a 20-mdiameter rotor, although the tower is capable ofsupporting a 30-m rotor. The turbine design allowsthe pitch of the blades to be changed to optimizepower transfer from the tidal currents. When the tideturns, the blades are turned 1801 and the direction ofrotation is reversed so that both current directionscan be used. The prototype was expected to undergofurther development until 2004. If the pilot projectproduces the results that its owners are seeking, theyowners will proceed with the installation of a 20-turbine plant in the Kvalsund Strait.

The ENERMAR device (Fig. 13), developed byPonte di Archimede nello Stretto di Messina SpA inItaly, employs a patented vertical axis rotor termed‘‘Kobold’’ connected to a synchronous generator.The rotor has an outer diameter of 6 m and consistsof three blades with a span of 5 m each and a chordlength of 0.4 m. An important feature of the device isthat the direction of rotation of the rotor isindependent of the current direction. The system ismounted on a floating platform 10 m in diameter. In2002, a prototype was deployed in the Strait ofMessina 150 m offshore at a depth of 20 m. Testsperformed on the prototype indicate that the turbineproduces approximately 20 kW of power at acurrent speed of 1.8 m/s, indicating a global systemefficiency of 23%. In a current of 3.2 m/s, 150 kW isexpected.

The Stingray Generator (Fig. 14), developed atEngineering Business Ltd. in the United Kingdom,uses a novel technique patented by the companyduring the late 1990s. The device produces electricityusing the oscillatory movement of hydroplanesdriven by flowing water. The hydroplanes rise andfall as the angle of each hydroplane is altered at theend of each stroke. A hydraulic cylinder is connectedto the hydroplane arm, and the up-and-down move-ment generates high-pressure oil that is used to drivea hydraulic motor, which in turn drives an electricgenerator. A full-scale prototype rated at 150 kW

was deployed in September 2002 at Yell Sound at adepth of 36 m and has since been in operation. Asimultaneous program, aiming to start installation ofa 5-MW demonstration farm with connection to thelocal distribution network, was expected to com-mence in the summer of 2004.

4. CONCLUSIONS

The sea is a colossal reservoir of energy ofparticularly high density—the highest among therenewables. The use of this energy resource couldcover a significant part of the electricity demandworldwide. Currently, the exploitation of two formsof ocean energy, namely ocean wave energy and tidalenergy, is considered prospective for massive powergeneration in the near future.

Although R&D on ocean energy conversion isunder way in several countries around the world,ocean energy technologies have not yet been devel-oped on an industrial level. However, considerableprogress has been made in this sector over the pastdecade or so in Asia, Australia, Europe, and NorthAmerica. For some technologies, this has resulted inapproaching commercialization, whereas other tech-nologies require further R&D.

Wave energy conversion is characterized by a largenumber of techniques. The environmental impact ofmost of them is considered negligible. Differentschemes, mainly oscillating water columns andheaving, pitching, and overtopping devices, couldmeet the goal of electricity production on acommercial scale by 2010.

The concepts for tidal energy conversion can beclassified into techniques converting the potentialenergy of the tides and techniques exploiting thekinetic energy of tidal currents. The traditionaltechnique of exploiting the tidal head by using damsbuilt across bays or estuaries still remains costly,having a major environmental impact. However,offshore impoundment structures could resolve manyof the problems of shoreline tidal range energyconversion. The progress in tidal current conversionover the past years has been considerable. Recently,developed techniques are being considered environ-mentally compatible and are predicted to becomeeconomically viable in the medium term.

The potential for improvement of the technoeco-nomical performance of ocean power conversiontechnologies is large. After their integration and theirpenetration into the world electricity market, oceanenergy conversion technologies are expected to make


substantial contributions to the achievement of awide range of objectives of environmental, social,and economic policies in many countries aroundthe world, providing superior services to theirpopulations.

SEE ALSO THEFOLLOWING ARTICLES

Hydropower Resources � Hydropower Technology �

Ocean, Energy Flows in � Ocean Thermal Energy �

Tidal Energy � Wind Energy Technology, Environ-mental Impacts of � Wind Resource Base

Further Reading

Clement, A. H., McCullen, P., Falcao, A., Fiorentino, A., Gardner,

F., Hammarlund, K., Lemonis, G., Lewis, T., Nielsen, K.,

Petroncini, S., Pontes, M-T., Schild, P., Sjostrom, B. O.,

S�rensen, H. C., and Thorpe, T. (2002). Wave energy inEurope: Current status and perspectives. Renewable SustainableEnergy Rev. 6, 405–431.

Fraenkel, P. L., Clutterbuck, P., Stjernstrom, B., and Bard, J.

(1998). ‘‘Seaflow: Preparing for the world’s first pilot project for

the exploitation of marine currents at a commercial scale.’’

Paper presented at the Third European Wave Energy Con-

ference, Patras, Greece.Lewis, T. (1985). ‘‘Wave Energy: Evaluation for C.E.C.’’

EUR9827EN.

Petroncini, S. (2000). Introducing wave energy into the renewable

energy marketplace. Master’s thesis, University of Edinburgh.Rademakers, L. W. M. M., van Schie, R. G., Schuttema, R.,

Vriesema, B., and Gardner, F. (1998). ‘‘Physical model testing

for characterizing the AWS.’’ Paper presented at the ThirdEuropean Wave Energy Conference, Patras, Greece.

Ross, D. (1995). ‘‘Power from the Waves.’’ Oxford University

Press, Oxford, UK.

Salter, S. H. (1989). World progress in wave energy—1988. Intl. J.Ambient Energy 10(1).

Tecnomare SpA & IT Power Ltd. (1995). ‘‘Marine Currents

Energy Extraction.’’ EUR16683EN.

Thorpe, T. W. (1992). ‘‘A Review of Wave Energy.’’ ETSU-R-72.Thorpe, T. W. (1999). An overview of wave energy technologies:

Status, performance, and costs. Paper presented at IMECHE

seminar, ‘‘Wave Power: Moving towards Commercial Viabili-

ty,’’ London.Tidal Electric. (2002). Power comes in. Intl. Water Power Dam

Constr., 24–27.

Yemm, R. (1999). The history and status of the Pelamis WaveEnergy Converter. Paper presented at IMECHE seminar, ‘‘Wave

Power: Moving towards Commercial Viability,’’ London.


Documents

Encyclopedia of Energy || Wave and Tidal Energy Conversion