Terminator Wave Energy Devices

ex exterminator undulate goose egg Devices1.0 Executive SummaryThe offshore ocean flutter elan vital re opening, as a derivative ricochet of solar life force, has considerable potential for making a signifi r after(prenominal)t contri thoion to the alternative usable cypher supply. oscillate mightfulness doojiggers atomic number 18 generally categorized by the method used to come the cogency of the twines. They elicit alike be categorized by location and strength take-off body. The postcode line methods or run principles send packing be categorized into three main groups (1) Oscillating urine Column (OWC) (2) Overtopping Devices (OTD) (3) kink Activated Bodies (WAB) Locations ar shoreline, tightfitting shore and offshore.This report discusses just about Terminator curl vigour widgets which extend vertical to the direction of hustle travel and capture or reflect the power of the rock. These thingmabobs atomic number 18 typically oceanward or snug gle shore however, adrift(p) versions energise been intentional for offshore applications.2.0 IntroductionTraditional sources of cipher such as oil, gas, and coal ar non-re peeledable. They as well create pollution by releasing huge quantities of carbon dioxide and opposite pollutants into the atmosphere. In contrast, casts atomic number 18 a renewable source of nada that doesnt cause pollution. The zipper from vibpaces alone could supply the worlds galvanicity needs.The total power of waftures breaking on the worlds coastlines is estimated at 2 to 3 million megawatts. In some locations, the ripple energy tightness can average 65 megawatts per mile of coastline. The problem is how to accouterments curve energy efficiently and with minimal environmental, social, and scotch impacts.Ocean brandishs atomic number 18 caused by the wind as it blows across the open expanse of peeing, the gravitational pull from the sun and moon, and changes in atmospheric pressure, te rra firmaquakes etc. Waves created by the wind be the most(prenominal) common waves and the waves relevant for most wave energy technology. Wave energy transition takes advantage of the ocean waves caused in the main by the inter performance of winds with the ocean surface. Wave energy is an irregular hesitate low- oftenness energy source. They be a powerful source of energy, but are difficult to harness and convert into electricity in vainglorious quantities. The energy needs to be converted to a 60 or 50 Hertz frequency in advance it can be added to the electric utility grid.Part of the solar energy received by our planet is converted to wind energy by believes of the differential gear heating of the earth. In cover part of the wind energy is transferred to the water surface, thitherby forming waves.While the average solar energy depends on factors such as local climate and latitude, the metre of energy transferred to the waves and hence their resulting size depends on the wind drive on, the duration of the winds and the duration over which it blows. The most energetic waves on earth happen to be mingled with 30 degrees to 60 degrees latitude, in general the waves generated are stronger on the southern parts of the countries (John brook, ECOR).Wave power cheats extract energy directly from the surface bowel gesturement of ocean waves or from pressure fluctuations below the surface. Wave power varies easily in different parts of the world, and wave energy cant be attach nucleusively e precisewhere. It has been estimated that if less than 0.1% of the renewable energy available indoors the oceans could be converted into electricity, it would satisfy the present world ask for energy to a greater extent(prenominal) than basketball team ms over.A variety of technologies are available to capture the energy from waves. Wave technologies have been designed to be installed in respectable shore, offshore, and far offshore locations. Offshor e outlines are situated in deep water, typically of more than 40 meters (131 feet).Types of power take-off accommodate hydraulic ram, elastomeric water pump, pump-to-shore, hydroelectric turbine, carry turbine and e foresightfulate electric generator. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the request of capture.3.0 Type of Wave efficacy ConvertersOcean waves represent a form of renewable energy created by wind au becausetics passing over open water. legion(predicate) thingmabobs are being actual for exploiting wave energy. The energy origin methods or operating principles can be categorized into three main groups (Harris Robert E. et al.)Oscillating water Columns (OWC)Waves cause the water towboat to muster up and fall, which alternately compresses and depressurize an air editorial. The energy is extracted from the resulting oscillating air arise by using a come up turbineOvertopping Devices (OTD) Ocean wa ves are noble into a germ higher up the sea aim, which store the water. The energy is extracted by using the difference in water train amongst the artificial lake and the sea by using low head turbinesWave Activated Bodies (WAB)Waves activate the oscillatory heads of body parts of a device relational to all(prenominal) other, or of one body part relative to a fixed reference. Primarily heave, pitch and roll motions can be determine as oscillating motions whereby the energy is extracted from the relative motion of the bodies or from the motion of one body relative to its fixed reference by using typically hydraulic systems to compress oil, which is then used to drive a generator.The wave activated bodies (WABs) can be further categorized in sub-groups describing the energy extraction by the principle motion of the foul uping body (heave, pitch and roll).A variety of technologies have been proposed to capture the energy from waves based on supra extraction methods Some o f the technologies that have been the intention of recent developmental efforts and are appropriate for the offshore applications being considered are terminators, attenuators and focalize absorbers (U.S. surgical incision of the Interior, whitethorn 2006). identification number 1 Schematic drawings of WEC devices for operating principles and lede locations(Harris Robert E. et al.)The m any(prenominal) different types of wave energy converters (WECs) can be classified in to various ways depending on their horizontal size and orientation. If the size is very small compared to the typical wavelength the WEC is called a point absorber. In contrast if the size is comparable to or giantr than the typical wavelength, the WEC is cognise as line absorber, this can also be referred to as terminator or attenuator. A WEC is called terminator or attenuator if it is aligned along or conventionality to the prevailing direction of the wave poll respectively (John brook, ECOR).The relations hip between the three main classificationsPrincipal LocationOperating PrincipleDirectional distinctiveThese classifications are shown in count on 2, presenting the possible operating principles for the location and the directional typicals. At the shoreline the totally feasible operating principles are oscillating water columns and eclipseping devices, which are terminators. sort shows that at near shore and offshore, point absorber or attenuator devices can still be WABs, whilst for terminator devices all three categories of the operating principles are possible. OWCs and OTDs are static energy converters of the terminator kind. As a result their moorage has to be stiff, restraining modes of motions but allowe for adjustment towards a replicate wave approach and for tidal ranges. The station keeping overtopments for the mooring of wave activated bodies can be either static or dynamic.Figure 2 Possible operating principles for the maven location and directional characteris tic3.1 AttenuatorsAttenuators are long multi-segment be adrift structures oriented collimate to the direction of the wave travel. The differing heights of waves along the length of the device causes flexing where the segments connect, and this flexing is machine-accessible to hydraulic pumps or other converters (U.S. Department of the Interior, may 2006).3.2 Point AbsorbersPoint absorbers have a small horizontal dimension compared with the vertical dimension and utilise the rise and fall of the wave height at a single point for WEC (Harris Robert E. et al.). It is comparatively small compared to the wave length and is able to capture energy from a wave face greater than the physical dimension of the absorber (James, 2007).The dexterity of a terminator or attenuator device is linked to their trail axis being, according, parallel or orthogonal to the entry wave crest. The point absorber does not have a principal wave direction and is able to capture energy from waves arriving from any direction. As a termination the station keeping for the terminator and attenuator has to allow the unit to weathervane into the paramount wave direction, but this is not necessary for the point absorber (Harris Robert E. et al.).3.3 TerminatorsA Terminator has its principal axis parallel to the incident wave crest and terminates the wave. These devices extend perpendicular to the direction of wave travel and capture or reflect the power of the wave. The reflected and transmitted waves determine the efficiency of the device (Harris Robert E. et al.). These devices are typically installed onshore or near shore however, floating versions have been designed for offshore applications. (U.S. Department of the Interior, May 2006). There are mainly two types in Terminator WEC.3.3.1 Oscillating Water Columns (OWC)The oscillating water column (OWC) is a form of terminator in which water enters through a subsurface opening into a sleeping room with air trapped in a higher plac e it. The wave action causes the captured water column to move up and win like a piston to force the air through an opening connected to a turbine (U.S. Department of the Interior May 2006). The device consists essentially of a floating or (more usually) bottom-fixed structure, whose upper part forms an air house and whose immersed part is open to the action of the sea. The reciprocating extend of air displaced by the at heart free surface motion drives an air turbine mounted on the top of the structure.3.3.1.1 Efficiency of Oscillating Water Column (OWC)The efficiency of oscillating water column (OWC) wave energy devices are particularly affected by persist oscillations basically for two reasons.(1) Because of intrinsically unsteady (reciprocating) flow of air displaced by the oscillating water free surface.(2) Because of increasing the air flow rate, above a restore depending on, and approximately proportional to, the rotational speed of the turbine, is know to give rise to a rapid drop in the aerodynamic efficiency and in the power output of the turbine.A method which has been proposed to part circumvent this problem consists in controlling the pitch of the turbine rotor blades in order to prevent the instantaneous angle of incidence of the relative flow from prodigious the critical value above which severe stalling occurs at the rotor blades (see Gato and Falcao, 1991). Although considered technically feasible (Salter, 1993) this has never been implemented at full scale owing to mechanical difficulties. Alternately, the flow rate through the turbine can be prevented from becoming excessive by equipping the device with air valves.Two different schemes can be envisaged, in the archetypical one, the valves are mounted between the sleeping room and the atmosphere in parallel with the turbine (by-pass or relief valves, on or near the roof of the air chamber structure) and are made to open (by active or passive control) in order to prevent the overpres sure (or the under pressure) in the chamber to exceed a limit which is defined by the aerodynamic characteristics of the turbine at its instantaneous speed.In the second scheme a valve is mounted in serial publication with the turbine in the duct connecting the chamber and the atmosphere. Excessive flow rate is prevented by part closing the valve. In both schemes, the air flow through the turbine is controlled at the expense of energy diarrhea at the valves. Theoretically the two methods, if properly implemented, are equivalent from the point of view of limiting the flow rate through the turbine. even, the resulting pressure changes in the chamber are different (reduction and increase in pressure oscillations in the first and second cases, respectively). Consequently the hydrodynamic process of energy extraction from the waves is differently limited by valve physical process in the two control methods.The main purpose of this work is to analyse theoretically the performance of an OWC wave energy device when valves are used to limit the flow through the turbine. Both schemes are considered and compared a valve (or a set of valves) mounted in parallel with the turbine (by-pass or relief valve) or a valve mounted in the turbine duct. The hydrodynamic analysis is done in the time domain for regular as fountainhead as for irregular waves. The spring-like effect due to the compressibility of the air is taken into account and is discussed in some detail. Rea comeic characteristics are assumed for the turbine. Numerical results are presented for simple two-dimensional chamber geometry for whose hydrodynamic coefficients analytical expressions are known as functions of wave frequency.3.3.2 Overtopping Devices (OTD)Overtopping devices have reservoirs that are filled by impinging waves to levels above the average surrounding ocean. The released reservoir water is used to drive hydro turbines or other conversion devices. Overtopping devices have been designed and teste d for both onshore and floating offshore applications. It gathers the energy by waves overtopping into a raised reservoir, and extracting this by draining the water through low head turbines. OTD consists of three main elementsTwo wave reflectors. Attached to the central computer program these act to focus the incoming waves.The main platform. This is a floating reservoir with a doubly curved wild leek set about the incoming waves. The waves overtop the ramp which has a inconsistent crest freeboard 1 to 4 m and underneath the platform open chambers operate as an air cushion maintaining the level of the reservoir.Hydro turbines. A set of low head turbines converts the hydraulic head in the reservoir (Tedd James et al., 2005)3.3.2.1 Overtopping theoryThe theory for good example overtopping devices varies greatly from the traditional analogue systems approach used by most other WECs. A linear systems approach may be used with overtopping devices. This considers the water oscillati ng up and down the ramp as the excited body, and the crest of the ramp as a highly non-linear power take off system. However due to the non-linearities it is excessively computationally demanding to model usefully. Therefore a more physical approach is taken.Figure 4 shows the schematic of flows for the Wave Dragon. Depending on the current wave state (HS, Tp) and the crest freeboard Rc(height of the ramp crest above mean water level, MWL) of the device, water will overtop into the reservoir Qovertopping. The power gathered by the reservoir is a harvest-time of this overtopping flow, the crest freeboard and gravity. If the reservoir is over filled when a large volume is deposited in the basin there will be loss from it Qspill. To minimize this, the reservoir level h must be kept below its maximum level hR. The useful hydraulic power converted by the turbines is the product of turbine flow Qturbine, the head across them, water density and gravity (Tedd James et al., 2005).In coasta l engineering the average flow Q is converted into non dimensional form by dividing by the breadth of the device b, gravity g and the significant wave height HSIn the case of the floating OTD it has been seen that there is a dependency on the wave period. The dominant physical explanation for this is the effect of energy passing beneath the draft of the structure.Figure 6 Layout of OTD3.3.2.2 Wave Reflector Wings bingle of the most distinctive aspects of the Overtopping WEC is the long slender wings mounted to the front corners of the reservoir platform. These are designed to reflect the moving waves towards the ramp. A wider section of wave is available to be exploited with only a moderate increase in capital cost. The overtopping volume in a wave is very drug-addicted on the wave height therefore by providing only a moderate increase in height, such(prenominal) more energy can overtop the ramp.In order to choose the correct lengths, angles, and position of these wings extensive computer modelling is used. Secondary bonuses of the presence of the wave reflector wings include better weather-vaning performance to face the waves, lower peak mooring forces, and improved horizontal stability of the main platform. As the aft and rear mooring appendix points are separated further, the yaw of the platform is more stable.Therefore the device will not turn away from the predominant wave direction, and will also realign itself faster as when the wave direction changes (Tedd James et al., 2005). Lastly the reflectors wings act as stabilisers to the device. As they float under their own delicacy they counteract any list of the platform. This is important as the more horizontal the platform is kept the less water is spilt and so the more efficient the device operation.3.3.2.3 Low Head Turbines and Power TrainTurbine operating conditions in a WEC are instead different from the ones in a median(prenominal) hydro power plant. In the OTD, the turbine head range is typic ally between 1.0 and 4.0 m, which is on the lower bounds of live water turbine experience. While there are only slow and relatively small variations of flow and head in a river hydro power plant, the strong stochastic variations of the wave overtopping call for a radically different mode of operation in the OTD. The head, being a function of the significant wave height, is varying in a range as large as 14, and the sack has to be regulated indoors time intervals as short as ten seconds in order to achieve a good efficiency of the energy exploitation (Tedd James et al., 2005).On an unmanned offshore device, the environmental conditions are much rougher, and routine charge work is much more difficult to perform. finical criteria for the choice and construction of water turbines for the WEC have to be followed it is advisable to aim for constructional simplicity rather than maximum peak efficiency. Figure 6 shows the application ranges of the known turbine types in a graph of head H vs. rotational speed nq.The specific speed nq is a turbine parameter characterizing the relative speed of a turbine, therefore giving an indication of the turbines power density. Evidently, all turbine types except the Pelton and the cross flow type are to be found in a relatively take band running diagonally across the graph. Transgressing the left or lower border means that the turbine will run too slowly, hence being unnecessarily large and expensive. The right or upper border is defined by technological limits, namely material strength and the risk of infection of cavitations erosion. The Pelton and the cross-flow turbine do not quite follow these rules, as they have a runner which is running in air and is only partially loaded with a free jet of water. Thus, they have a lower specific speed and lower power density. Despite its simplicity and robustness, the cross flow turbine is not very suitable for OTD applications (Tedd James et al., 2005).Figure 7 Head range of the c ommon turbine types, Voith and Ossberger3.3.2.4 Performance in StormsSurvivability is essential, and Overtopping devices are naturally adapted to perform well in storm situations, where the wave will pass over and under the device with no potential end-stop problems.3.3.2.5 Wave PredictionPerformance of almost all wave energy converters can be improved with prediction of the incoming waves. The cost to implement would be low as the control hardware is typically in place, only the measuring system and improved control techniques need to be developed. To explain the concept behind the device a simple example can be used. If a measurement of some wavelengths ahead of the wave energy converter shows large waves passing, then at a given time later(prenominal) this energy will be incident on the device.The control of the device can then be altered quickly to extract this larger energy, e.g. by increasing hydraulic bulwark to an oscillators motion allowing more energy to be captured with in the stroke length, or by draining the reservoir of an overtopping device to allow for a large overtopping volume(Tedd James et al., 2005).The challenges are threefold to implement a system for measuring the waves approaching the ramp, to accurately transform this into usable infix for the control systems, and to construct new control strategies to make the best use of this. The standard approach for performing such deterministic sea-state prediction involves discrete frequency domain techniques. This is computationally intensive, as the two Fourier transforms must be made to convert from the time domain to the frequency domain and return to the time domain.3.4 Energy Capture and Practical LimitsThe power captured from waves by the primary mechanical conversion (before secondary conversion to electrical power) can be related to the energy in the incoming waves over a certain width. Theoretical values have been established in some cases. For a heaving axi-symmetric body the maximu m capture width is the inverse of the wave number. The capture width is often compared to the front width of the device.This width ratio can be larger than one for a point absorber with small dimensions compared to the wavelength. treacly effects reduce efficiency. For an OWC, Wang et al. (2002) found that the capture width ratio may reach a value of 3 and above at an optimum wave period. For Pelamis, Retlzler et al. (2001) found a capture width up to 2 in regular waves and around one in random seas (Specialist Committee V.4, 2006).A constant or a semi discrete array of wave energy converters acting as an absorbing wall perpendicular to the wave direction is called a terminator and its capture width equals the width of the device and is not related to the length of the incident waves.As the wave conditions are stochastic, the tuning parameters of the energy converters are compromises between the optimum values at various sea conditions. The capture width must be established for ea ch sea state. Fixed devices are subject to sea level variation according to tidal effects. This is critical for fixed oscillating water columns and fixed overtopping systems whose performances are dependent on the mean sea level. The intake of an OWC must be located at an optimised design level from the mean free surface.The height of an overtopping system is also optimised for sea states occurring at a given mean sea level. Therefore, sites with minimal tide are preferred. From this point of view floating devices are more suitable. The immersion of a floating device can also be tuned with respect to the actual sea state. For instance the Wave Dragon overtopping device is partially floating on air chambers and its draught can be modified (Specialist Committee V.4, 2006).The performance of the overtopping device is sensitive to the distribution of the overtopping rate. The more variable the overtopping flow into the reservoir, the larger the energy of the reservoir and turbines must be to achieve the same performance.4.0 Mooring RequirementsThe two major requirements for a WEC mooring are to defy the environmental and other fill ups manifold in keeping the device on station, and to be sufficiently cost effective so that the overall economics of the device remain viable. The following list shows the requirements that need to be considered for WEC moorings systems (Harris Robert E. et al.)The primary purpose of the mooring system is to maintain the floating structure on station within specified tolerances under normal operating load and extreme storm load conditions.The excursion of the device must not permit tension tons in the electrical transmission cable(s) and should allow for suitable specified clearance distances between devices in multiple generalisations.The mooring system must be sufficiently compliant to the environmental loading to reduce the forces acting on anchors, mooring lines and the device itself to a minimum unless the stiffness of the mooring itself is an active element in the wave energy conversion principle used.All components must have adequate strength, fatigue life and durability for the operational lifetime, and nautical growth and eating away need to be considered.A degree of redundancy is highly desirable for individual devices, and essential for schemes which link several devices together.The system as a entirely should be capable of haunting for 30 years or more, with replacement of particular components at no less than 5 years.The mooring must be sufficient to accommodate the tidal range at the installation location.The mooring system should allow the removal of single devices without affecting the mooring of adjacent devices.remotion of mooring lines for inspection and maintenance must be possible.The mooring must be sufficiently stiff to allow berthing for inspection and maintenance purposes.Contact between mooring lines must be avoided.The mooring should not adversely affect the efficiency of th e device, and if it is part of an active control system it must also be designed dynamically as part of the overall WEC system.Revenues from WECs, in comparison to the offshore industry, are smaller and their economics more strongly linked to the location, installation costs and down time periods. The mooring system has an important impact on the economics and it is necessary to provide, at low installation cost, a undeviating system that has little downtime and long intervals between maintenance. The suitability of design approaches from the offshore industry for WECs are ranked in Appendix I (Harris Robert E. et al.).5.0 Environmental Considerations changeover of wave energy to electrical or other usable forms of energy is generally anticipated to have limited environmental impacts. However, as with any emerging technology, the nature and extent of environmental favors remain uncertain. The impacts that would potentially occur are also very site specific, depending on physical and ecological factors that vary considerably for potential ocean sites. As large-scale prototypes and commercial facilities are developed, these factors can be expected to be more precisely defined (U.S. Department of the Interior, May 2006).The following environmental considerations require monitoring (U.S. Department of the Interior, May 2006).Visual appearance and noiseare device-specific, with considerable variability in visible freeboard height and noise extension above and below the water surface. Devices with OWCs and overtopping devices typically have the highest freeboard and are most visible. Offshore devices would require navigation hazard warning devices such as lights, break signals, radar reflectors, and contrasting solar day marker painting.However, Coast Guard requirements only require that day markers be visible for 1 nautical mile (1.8 km), and thus offshore device markings would only be seen from shore on exceptionally clear days. The air being drawn in and ex pelled in OWC devices is likely to be the largest source of above-water noise. Some subaquatic noise would occur from devices with turbines, hydraulic pumps, and other moving parts. The frequency of the noise may also be a consideration in evaluating noise impacts. step-down in wave height from wave energy converterscould be a consideration in some settings however, the impact on wave characteristics would generally only be observed 1 to 2 km away from the WEC device in the direction of the wave travel. Thus there should not be a significant onshore impact if the devices were much more than this distance from the shore. None of the devices currently being developed would harvest a large portion of the wave energy, which would leave a relatively calm surface behind the devices.It is estimated that with current projections, a large wave energy facility with a maximum density of devices would cause the reduction in waves to be on the order of 10 to 15%, and this impact would rapidly dissipate within a few kilometers, but leave a slight lessening of waves in the overall vicinity. Little information is available on the impact on sediment transport or on biological communities from a reduction in wave height offshore. An isolated impact, such as reduced wave height for recreational surfers, could maybe result.Marine habitatcould be impacted positively or negatively depending on the nature of additional submerged surfaces, above-water platforms, and changes in the seafloor. synthetic above-water surfaces could provide habitat for seals and sea lions or nesting areas for birds. Underwater surfaces of WEC devices would provide substrates for various biological systems, which could be a positive or negative complement to existing natural habitats. With some WEC devices, it may be necessary to control the growth of marine organisms on some surfaces.Toxic releasesmay be of concern related to leaks or accidental spills of liquids used in systems with working hydraulic fluids. Any impacts could be minimized through the selection of nontoxic fluids and careful monitoring, with adequate spill chemical reaction plans and secondary containment design features. apply of biocides to control growth of marine organisms may also be a source of toxic releases.Conflict with other sea space users, such as commercial shipping and fishing and recreational boating, can occur without the careful selection of sites for WEC devices. The impact can potentially be positive for recreational and commercial fisheries if the devices provide for additional biological habitats.Installation and Decommissioning Disturbances from securing the devices to the ocean floor and installation of cables may have negative impacts on marine habitats. Potential decommissioning impacts are primarily related to disturbing marine habitats that have adapted to the presence of the wave energy structures.6.0 DiscussionsA vast number of parameters bewitch (and interact with) the net power p roduction from any WECOvertopping, determined byFree-board (adjustable in Wave Dragons)Actual wave heightPhysical dimension of the converter (ramps, reflectors etc.Outlet, determined by surface of reservoirTurbine designTurbine on/off strategyMooring system, free or restricted orientation toward wavesSize of the energy converterWave climateEnergy in wave front (kW/m)Distribution of wave heightsAvailabilityTheoretical availability Reliability, maintainability, serviceabTerminator Wave Energy DevicesTerminator Wave Energy Devices1.0 Executive SummaryThe offshore ocean wave energy resource, as a derivative form of solar energy, has considerable potential for making a significant contribution to the alternative usable energy supply.Wave power devices are generally categorized by the method used to capture the energy of the waves. They can also be categorized by location and power take-off system. The energy extraction methods or operating principles can be categorized into three main gr oups (1) Oscillating water Column (OWC) (2) Overtopping Devices (OTD) (3) Wave Activated Bodies (WAB) Locations are shoreline, near shore and offshore.This report discusses about Terminator wave energy devices which extend perpendicular to the direction of wave travel and capture or reflect the power of the wave. These devices are typically onshore or near shore however, floating versions have been designed for offshore applications.2.0 IntroductionTraditional sources of energy such as oil, gas, and coal are non-renewable. They also create pollution by releasing huge quantities of carbon dioxide and other pollutants into the atmosphere. In contrast, waves are a renewable source of energy that doesnt cause pollution. The energy from waves alone could supply the worlds electricity needs.The total power of waves breaking on the worlds coastlines is estimated at 2 to 3 million megawatts. In some locations, the wave energy density can average 65 megawatts per mile of coastline. The probl em is how to harness wave energy efficiently and with minimal environmental, social, and economic impacts.Ocean waves are caused by the wind as it blows across the open expanse of water, the gravitational pull from the sun and moon, and changes in atmospheric pressure, earthquakes etc. Waves created by the wind are the most common waves and the waves relevant for most wave energy technology. Wave energy conversion takes advantage of the ocean waves caused primarily by the interaction of winds with the ocean surface. Wave energy is an irregular oscillating low-frequency energy source. They are a powerful source of energy, but are difficult to harness and convert into electricity in large quantities. The energy needs to be converted to a 60 or 50 Hertz frequency before it can be added to the electric utility grid.Part of the solar energy received by our planet is converted to wind energy through the differential heating of the earth. In turn part of the wind energy is transferred to t he water surface, thereby forming waves.While the average solar energy depends on factors such as local climate and latitude, the amount of energy transferred to the waves and hence their resulting size depends on the wind speed, the duration of the winds and the duration over which it blows. The most energetic waves on earth happen to be between 30 degrees to 60 degrees latitude, in general the waves generated are stronger on the southern parts of the countries (John brook, ECOR).Wave power devices extract energy directly from the surface motion of ocean waves or from pressure fluctuations below the surface. Wave power varies considerably in different parts of the world, and wave energy cant be harnessed effectively everywhere. It has been estimated that if less than 0.1% of the renewable energy available within the oceans could be converted into electricity, it would satisfy the present world demand for energy more than five times over.A variety of technologies are available to ca pture the energy from waves. Wave technologies have been designed to be installed in near shore, offshore, and far offshore locations. Offshore systems are situated in deep water, typically of more than 40 meters (131 feet).Types of power take-off include hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine and linear electrical generator. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture.3.0 Type of Wave Energy ConvertersOcean waves represent a form of renewable energy created by wind currents passing over open water. Many devices are being developed for exploiting wave energy. The energy extraction methods or operating principles can be categorized into three main groups (Harris Robert E. et al.)Oscillating Water Columns (OWC)Waves cause the water column to rise and fall, which alternately compresses and depressurize an air column. The energy is extracted from the resulting osc illating air flow by using a Wells turbineOvertopping Devices (OTD) Ocean waves are elevated into a reservoir above the sea level, which store the water. The energy is extracted by using the difference in water level between the reservoir and the sea by using low head turbinesWave Activated Bodies (WAB)Waves activate the oscillatory motions of body parts of a device relative to each other, or of one body part relative to a fixed reference. Primarily heave, pitch and roll motions can be identified as oscillating motions whereby the energy is extracted from the relative motion of the bodies or from the motion of one body relative to its fixed reference by using typically hydraulic systems to compress oil, which is then used to drive a generator.The wave activated bodies (WABs) can be further categorized in sub-groups describing the energy extraction by the principle motion of the floating body (heave, pitch and roll).A variety of technologies have been proposed to capture the energy f rom waves based on above extraction methods Some of the technologies that have been the target of recent developmental efforts and are appropriate for the offshore applications being considered are terminators, attenuators and point absorbers (U.S. Department of the Interior, May 2006).Figure 1 Schematic drawings of WEC devices for operating principles and principal locations(Harris Robert E. et al.)The many different types of wave energy converters (WECs) can be classified in to various ways depending on their horizontal size and orientation. If the size is very small compared to the typical wavelength the WEC is called a point absorber. In contrast if the size is comparable to or larger than the typical wavelength, the WEC is known as line absorber, this can also be referred to as terminator or attenuator. A WEC is called terminator or attenuator if it is aligned along or normal to the prevailing direction of the wave crest respectively (John brook, ECOR).The relationship between the three main classificationsPrincipal LocationOperating PrincipleDirectional CharacteristicThese classifications are shown in Figure 2, presenting the possible operating principles for the location and the directional characteristics. At the shoreline the only feasible operating principles are oscillating water columns and overtopping devices, which are terminators.Figure shows that at near shore and offshore, point absorber or attenuator devices can only be WABs, whilst for terminator devices all three categories of the operating principles are possible. OWCs and OTDs are static energy converters of the terminator kind. As a result their mooring has to be stiff, restraining modes of motions but allowing for adjustment towards a parallel wave approach and for tidal ranges. The station keeping requirements for the mooring of wave activated bodies can be either static or dynamic.Figure 2 Possible operating principles for the principal location and directional characteristic3.1 Atten uatorsAttenuators are long multi-segment floating structures oriented parallel to the direction of the wave travel. The differing heights of waves along the length of the device causes flexing where the segments connect, and this flexing is connected to hydraulic pumps or other converters (U.S. Department of the Interior, May 2006).3.2 Point AbsorbersPoint absorbers have a small horizontal dimension compared with the vertical dimension and utilize the rise and fall of the wave height at a single point for WEC (Harris Robert E. et al.). It is relatively small compared to the wave length and is able to capture energy from a wave front greater than the physical dimension of the absorber (James, 2007).The efficiency of a terminator or attenuator device is linked to their principal axis being, according, parallel or orthogonal to the incoming wave crest. The point absorber does not have a principal wave direction and is able to capture energy from waves arriving from any direction. As a consequence the station keeping for the terminator and attenuator has to allow the unit to weathervane into the predominant wave direction, but this is not necessary for the point absorber (Harris Robert E. et al.).3.3 TerminatorsA Terminator has its principal axis parallel to the incident wave crest and terminates the wave. These devices extend perpendicular to the direction of wave travel and capture or reflect the power of the wave. The reflected and transmitted waves determine the efficiency of the device (Harris Robert E. et al.). These devices are typically installed onshore or near shore however, floating versions have been designed for offshore applications. (U.S. Department of the Interior, May 2006). There are mainly two types in Terminator WEC.3.3.1 Oscillating Water Columns (OWC)The oscillating water column (OWC) is a form of terminator in which water enters through a subsurface opening into a chamber with air trapped above it. The wave action causes the captured water c olumn to move up and down like a piston to force the air through an opening connected to a turbine (U.S. Department of the Interior May 2006). The device consists essentially of a floating or (more usually) bottom-fixed structure, whose upper part forms an air chamber and whose immersed part is open to the action of the sea. The reciprocating flow of air displaced by the inside free surface motion drives an air turbine mounted on the top of the structure.3.3.1.1 Efficiency of Oscillating Water Column (OWC)The efficiency of oscillating water column (OWC) wave energy devices are particularly affected by flow oscillations basically for two reasons.(1) Because of intrinsically unsteady (reciprocating) flow of air displaced by the oscillating water free surface.(2) Because of increasing the air flow rate, above a limit depending on, and approximately proportional to, the rotational speed of the turbine, is known to give rise to a rapid drop in the aerodynamic efficiency and in the power output of the turbine.A method which has been proposed to partially circumvent this problem consists in controlling the pitch of the turbine rotor blades in order to prevent the instantaneous angle of incidence of the relative flow from exceeding the critical value above which severe stalling occurs at the rotor blades (see Gato and Falcao, 1991). Although considered technically feasible (Salter, 1993) this has never been implemented at full scale owing to mechanical difficulties. Alternately, the flow rate through the turbine can be prevented from becoming excessive by equipping the device with air valves.Two different schemes can be envisaged, in the first one, the valves are mounted between the chamber and the atmosphere in parallel with the turbine (by-pass or relief valves, on or near the roof of the air chamber structure) and are made to open (by active or passive control) in order to prevent the overpressure (or the under pressure) in the chamber to exceed a limit which is de fined by the aerodynamic characteristics of the turbine at its instantaneous speed.In the second scheme a valve is mounted in series with the turbine in the duct connecting the chamber and the atmosphere. Excessive flow rate is prevented by partially closing the valve. In both schemes, the air flow through the turbine is controlled at the expense of energy dissipation at the valves. Theoretically the two methods, if properly implemented, are equivalent from the point of view of limiting the flow rate through the turbine.However, the resulting pressure changes in the chamber are different (reduction and increase in pressure oscillations in the first and second cases, respectively). Consequently the hydrodynamic process of energy extraction from the waves is differently modified by valve operation in the two control methods.The main purpose of this work is to analyse theoretically the performance of an OWC wave energy device when valves are used to limit the flow through the turbine. Both schemes are considered and compared a valve (or a set of valves) mounted in parallel with the turbine (by-pass or relief valve) or a valve mounted in the turbine duct. The hydrodynamic analysis is done in the time domain for regular as well as for irregular waves. The spring-like effect due to the compressibility of the air is taken into account and is discussed in some detail. Realistic characteristics are assumed for the turbine. Numerical results are presented for simple two-dimensional chamber geometry for whose hydrodynamic coefficients analytical expressions are known as functions of wave frequency.3.3.2 Overtopping Devices (OTD)Overtopping devices have reservoirs that are filled by impinging waves to levels above the average surrounding ocean. The released reservoir water is used to drive hydro turbines or other conversion devices. Overtopping devices have been designed and tested for both onshore and floating offshore applications. It gathers the energy by waves overtop ping into a raised reservoir, and extracting this by draining the water through low head turbines. OTD consists of three main elementsTwo wave reflectors. Attached to the central platform these act to focus the incoming waves.The main platform. This is a floating reservoir with a doubly curved ramp facing the incoming waves. The waves overtop the ramp which has a variable crest freeboard 1 to 4 m and underneath the platform open chambers operate as an air cushion maintaining the level of the reservoir.Hydro turbines. A set of low head turbines converts the hydraulic head in the reservoir (Tedd James et al., 2005)3.3.2.1 Overtopping theoryThe theory for modeling overtopping devices varies greatly from the traditional linear systems approach used by most other WECs. A linear systems approach may be used with overtopping devices. This considers the water oscillating up and down the ramp as the excited body, and the crest of the ramp as a highly non-linear power take off system. However due to the non-linearities it is too computationally demanding to model usefully. Therefore a more physical approach is taken.Figure 4 shows the schematic of flows for the Wave Dragon. Depending on the current wave state (HS, Tp) and the crest freeboard Rc(height of the ramp crest above mean water level, MWL) of the device, water will overtop into the reservoir Qovertopping. The power gathered by the reservoir is a product of this overtopping flow, the crest freeboard and gravity. If the reservoir is over filled when a large volume is deposited in the basin there will be loss from it Qspill. To minimize this, the reservoir level h must be kept below its maximum level hR. The useful hydraulic power converted by the turbines is the product of turbine flow Qturbine, the head across them, water density and gravity (Tedd James et al., 2005).In coastal engineering the average flow Q is converted into non dimensional form by dividing by the breadth of the device b, gravity g and the signi ficant wave height HSIn the case of the floating OTD it has been seen that there is a dependency on the wave period. The dominant physical explanation for this is the effect of energy passing beneath the draft of the structure.Figure 6 Layout of OTD3.3.2.2 Wave Reflector WingsOne of the most distinctive aspects of the Overtopping WEC is the long slender wings mounted to the front corners of the reservoir platform. These are designed to reflect the oncoming waves towards the ramp. A wider section of wave is available to be exploited with only a moderate increase in capital cost. The overtopping volume in a wave is very dependent on the wave height therefore by providing only a moderate increase in height, much more energy can overtop the ramp.In order to choose the correct lengths, angles, and position of these wings extensive computer modelling is used. Secondary bonuses of the presence of the wave reflector wings include better weather-vaning performance to face the waves, lower pe ak mooring forces, and improved horizontal stability of the main platform. As the aft and rear mooring attachment points are separated further, the yaw of the platform is more stable.Therefore the device will not turn away from the predominant wave direction, and will also realign itself faster as when the wave direction changes (Tedd James et al., 2005). Lastly the reflectors wings act as stabilisers to the device. As they float under their own buoyancy they counteract any list of the platform. This is important as the more horizontal the platform is kept the less water is spilt and so the more efficient the device operation.3.3.2.3 Low Head Turbines and Power TrainTurbine operating conditions in a WEC are quite different from the ones in a normal hydro power plant. In the OTD, the turbine head range is typically between 1.0 and 4.0 m, which is on the lower bounds of existing water turbine experience. While there are only slow and relatively small variations of flow and head in a r iver hydro power plant, the strong stochastic variations of the wave overtopping call for a radically different mode of operation in the OTD. The head, being a function of the significant wave height, is varying in a range as large as 14, and the discharge has to be regulated within time intervals as short as ten seconds in order to achieve a good efficiency of the energy exploitation (Tedd James et al., 2005).On an unmanned offshore device, the environmental conditions are much rougher, and routine maintenance work is much more difficult to perform. Special criteria for the choice and construction of water turbines for the WEC have to be followed it is advisable to aim for constructional simplicity rather than maximum peak efficiency. Figure 6 shows the application ranges of the known turbine types in a graph of head H vs. rotational speed nq.The specific speed nq is a turbine parameter characterizing the relative speed of a turbine, thus giving an indication of the turbines power density. Evidently, all turbine types except the Pelton and the cross flow type are to be found in a relatively narrow band running diagonally across the graph. Transgressing the left or lower border means that the turbine will run too slowly, thus being unnecessarily large and expensive. The right or upper border is defined by technological limits, namely material strength and the danger of cavitations erosion. The Pelton and the cross-flow turbine do not quite follow these rules, as they have a runner which is running in air and is only partially loaded with a free jet of water. Thus, they have a lower specific speed and lower power density. Despite its simplicity and robustness, the cross flow turbine is not very suitable for OTD applications (Tedd James et al., 2005).Figure 7 Head range of the common turbine types, Voith and Ossberger3.3.2.4 Performance in StormsSurvivability is essential, and Overtopping devices are naturally adapted to perform well in storm situations, where t he wave will pass over and under the device with no potential end-stop problems.3.3.2.5 Wave PredictionPerformance of almost all wave energy converters can be improved with prediction of the incoming waves. The cost to implement would be low as the control hardware is typically in place, only the measuring system and improved control techniques need to be developed. To explain the concept behind the device a simple example can be used. If a measurement of some wavelengths ahead of the wave energy converter shows large waves passing, then at a given time later this energy will be incident on the device.The control of the device can then be altered quickly to extract this larger energy, e.g. by increasing hydraulic resistance to an oscillators motion allowing more energy to be captured within the stroke length, or by draining the reservoir of an overtopping device to allow for a large overtopping volume(Tedd James et al., 2005).The challenges are threefold to implement a system for me asuring the waves approaching the ramp, to accurately transform this into usable input for the control systems, and to construct new control strategies to make the best use of this. The standard approach for performing such deterministic sea-state prediction involves discrete frequency domain techniques. This is computationally intensive, as the two Fourier transforms must be made to convert from the time domain to the frequency domain and return to the time domain.3.4 Energy Capture and Practical LimitsThe power captured from waves by the primary mechanical conversion (before secondary conversion to electrical power) can be related to the energy in the incoming waves over a certain width. Theoretical values have been established in some cases. For a heaving axi-symmetric body the maximum capture width is the inverse of the wave number. The capture width is often compared to the front width of the device.This width ratio can be larger than one for a point absorber with small dimensi ons compared to the wavelength. Viscous effects reduce efficiency. For an OWC, Wang et al. (2002) found that the capture width ratio may reach a value of 3 and above at an optimum wave period. For Pelamis, Retlzler et al. (2001) found a capture width up to 2 in regular waves and around one in random seas (Specialist Committee V.4, 2006).A continuous or a semi discrete array of wave energy converters acting as an absorbing wall perpendicular to the wave direction is called a terminator and its capture width equals the width of the device and is not related to the length of the incident waves.As the wave conditions are stochastic, the tuning parameters of the energy converters are compromises between the optimum values at various sea conditions. The capture width must be established for each sea state. Fixed devices are subject to sea level variation according to tidal effects. This is critical for fixed oscillating water columns and fixed overtopping systems whose performances are de pendent on the mean sea level. The intake of an OWC must be located at an optimised design level from the mean free surface.The height of an overtopping system is also optimised for sea states occurring at a given mean sea level. Therefore, sites with minimal tide are preferred. From this point of view floating devices are more suitable. The immersion of a floating device can also be tuned with respect to the actual sea state. For instance the Wave Dragon overtopping device is partially floating on air chambers and its draught can be modified (Specialist Committee V.4, 2006).The performance of the overtopping device is sensitive to the distribution of the overtopping rate. The more variable the overtopping flow into the reservoir, the larger the capacity of the reservoir and turbines must be to achieve the same performance.4.0 Mooring RequirementsThe two major requirements for a WEC mooring are to withstand the environmental and other loadings involved in keeping the device on stati on, and to be sufficiently cost effective so that the overall economics of the device remain viable. The following list shows the requirements that need to be considered for WEC moorings systems (Harris Robert E. et al.)The primary purpose of the mooring system is to maintain the floating structure on station within specified tolerances under normal operating load and extreme storm load conditions.The excursion of the device must not permit tension loads in the electrical transmission cable(s) and should allow for suitable specified clearance distances between devices in multiple installations.The mooring system must be sufficiently compliant to the environmental loading to reduce the forces acting on anchors, mooring lines and the device itself to a minimum unless the stiffness of the mooring itself is an active element in the wave energy conversion principle used.All components must have adequate strength, fatigue life and durability for the operational lifetime, and marine growth and corrosion need to be considered.A degree of redundancy is highly desirable for individual devices, and essential for schemes which link several devices together.The system as a whole should be capable of lasting for 30 years or more, with replacement of particular components at no less than 5 years.The mooring must be sufficient to accommodate the tidal range at the installation location.The mooring system should allow the removal of single devices without affecting the mooring of adjacent devices.Removal of mooring lines for inspection and maintenance must be possible.The mooring must be sufficiently stiff to allow berthing for inspection and maintenance purposes.Contact between mooring lines must be avoided.The mooring should not adversely affect the efficiency of the device, and if it is part of an active control system it must also be designed dynamically as part of the overall WEC system.Revenues from WECs, in comparison to the offshore industry, are smaller and their econ omics more strongly linked to the location, installation costs and down time periods. The mooring system has an important impact on the economics and it is necessary to provide, at low installation cost, a reliable system that has little downtime and long intervals between maintenance. The suitability of design approaches from the offshore industry for WECs are ranked in Appendix I (Harris Robert E. et al.).5.0 Environmental ConsiderationsConversion of wave energy to electrical or other usable forms of energy is generally anticipated to have limited environmental impacts. However, as with any emerging technology, the nature and extent of environmental considerations remain uncertain. The impacts that would potentially occur are also very site specific, depending on physical and ecological factors that vary considerably for potential ocean sites. As large-scale prototypes and commercial facilities are developed, these factors can be expected to be more precisely defined (U.S. Departm ent of the Interior, May 2006).The following environmental considerations require monitoring (U.S. Department of the Interior, May 2006).Visual appearance and noiseare device-specific, with considerable variability in visible freeboard height and noise generation above and below the water surface. Devices with OWCs and overtopping devices typically have the highest freeboard and are most visible. Offshore devices would require navigation hazard warning devices such as lights, sound signals, radar reflectors, and contrasting day marker painting.However, Coast Guard requirements only require that day markers be visible for 1 nautical mile (1.8 km), and thus offshore device markings would only be seen from shore on exceptionally clear days. The air being drawn in and expelled in OWC devices is likely to be the largest source of above-water noise. Some underwater noise would occur from devices with turbines, hydraulic pumps, and other moving parts. The frequency of the noise may also be a consideration in evaluating noise impacts.Reduction in wave height from wave energy converterscould be a consideration in some settings however, the impact on wave characteristics would generally only be observed 1 to 2 km away from the WEC device in the direction of the wave travel. Thus there should not be a significant onshore impact if the devices were much more than this distance from the shore. None of the devices currently being developed would harvest a large portion of the wave energy, which would leave a relatively calm surface behind the devices.It is estimated that with current projections, a large wave energy facility with a maximum density of devices would cause the reduction in waves to be on the order of 10 to 15%, and this impact would rapidly dissipate within a few kilometers, but leave a slight lessening of waves in the overall vicinity. Little information is available on the impact on sediment transport or on biological communities from a reduction in wave hei ght offshore. An isolated impact, such as reduced wave height for recreational surfers, could possibly result.Marine habitatcould be impacted positively or negatively depending on the nature of additional submerged surfaces, above-water platforms, and changes in the seafloor. Artificial above-water surfaces could provide habitat for seals and sea lions or nesting areas for birds. Underwater surfaces of WEC devices would provide substrates for various biological systems, which could be a positive or negative complement to existing natural habitats. With some WEC devices, it may be necessary to control the growth of marine organisms on some surfaces.Toxic releasesmay be of concern related to leaks or accidental spills of liquids used in systems with working hydraulic fluids. Any impacts could be minimized through the selection of nontoxic fluids and careful monitoring, with adequate spill response plans and secondary containment design features. Use of biocides to control growth of ma rine organisms may also be a source of toxic releases.Conflict with other sea space users, such as commercial shipping and fishing and recreational boating, can occur without the careful selection of sites for WEC devices. The impact can potentially be positive for recreational and commercial fisheries if the devices provide for additional biological habitats.Installation and Decommissioning Disturbances from securing the devices to the ocean floor and installation of cables may have negative impacts on marine habitats. Potential decommissioning impacts are primarily related to disturbing marine habitats that have adapted to the presence of the wave energy structures.6.0 DiscussionsA vast number of parameters influence (and interact with) the net power production from any WECOvertopping, determined byFree-board (adjustable in Wave Dragons)Actual wave heightPhysical dimension of the converter (ramps, reflectors etc.Outlet, determined bySize of reservoirTurbine designTurbine on/off st rategyMooring system, free or restricted orientation toward wavesSize of the energy converterWave climateEnergy in wave front (kW/m)Distribution of wave heightsAvailabilityTheoretical availability Reliability, maintainability, serviceab