Abstract

ABSTRACT

Wave Power is a technology that was founded in the 70s, but which still not has reached full industrial recognition as a energy source. In this project there is a literature study on dierent concepts of Wave Energy Converters (WEC) which has been build in full scale, and there is a great variety in the concepts. There is a theory chapter where ocean waves theory, hydrodynamics and maximum power capture are presented. The modeling work was done on linear rotating blades which can be said to have the greatest potential for energy absorption .There are variety of mechanisms were developed to extract the wave energy into useful electrical energy. Here modeling of rotating blades as well as the simulation of the mechanism going to be done using catia v5 modelling software..Fabrication of that model also going to be made to compare the results with the simulation.Analysis of the rotating blades going to be analysed and and discussed for the various loads.Here modeling ,simulation ,Analysis,fabrication done using catia v5 software and Ansys software.

INTRODUCTION

SHORT HISTORY OF WAVE ENERGY MAKERSThe oil crisis in the early 70s made a demand for new energy sources, and so the research on wave energy converters(WEC) started. The researchers recognized the potential the WEC was as energy sources. The Norwegian scientists Kjell Budal and Johannes Falnes from the Norwegian University of Science and Technology NTNU (former University of Trondheim) was two of the pioneers in the research on wave energy makers. They started their cooperation in 1973 by bringing up new ideas on the area (Budal and Falnes 1985), and some of the ideas where later made into theories and physical experiments. At the same time, at the University of Edinburgh, scien- tist Stephen Salter began his rst experiments on dierent types of heaving oats(Cruz 2007).1.2 The point absorber wave energy converter

One of the WEC types is the point absorber, and it is the type of scope in this thesis. Usually axisymmetric about a vertical axis, the point absorber type of energy converter is an appealling one because is it small relative to the wavelenght of the incident waves as seen in gure 1.1, and therefore the scattering wave eld can be neglected and the forces on the body are only due to the incident wave. The potential capture width is large as the point absorbers are capable to absorb the energy from a wavefront many times the key horizontal dimension of the absorber. The point absorbers are of second generation among the WECs and they are designed to operate at a wide variety of oshore and nearshore sites where a high level of energy is available The issues of harnessing waves as viable sources of energy include the complicated nature of predicting the wave climate, as well as the diculties associated with optimizing a wave energy converter (WEC) for a particular wave climate. System parameters including inertia, center of gravity position, system draft, and submerged volume all may aect the power output of a WEC. Mass parameters such as inertia and center of gravity position can be manipulated during operation by utilizing ballast, thus aecting system power output. Because of the complex simulations that they require, testing dierent design parameters is very time consuming, which limits the number of designs which may be considered.

Hydrodynamic simulators, such as Ansys AQWA, are capable of simulating dierent WEC congurations. However, these simulators are extremely time-consuming, due to the fact that the energy and momentum equations are solved thousands (or millions) of times during one simulation [1]. Thus, it is essential that the computation time of analyzing WECs is reduced, allowing for more designs to be analyzed during the optimization process. In order reduce computational time, we developed a function approximation that maps the design conguration of a WEC to its power output (section 3.2). This reduction in computational time allows for a larger portion of the design space to be searched in a given amount of time. Given this function approximation, evolutionary algorithms can be utilized to nd the ideal ballast conguration for a WEC in much less time than with traditional hydrodynamic analysis methods. Optimizing the ballast design for a WEC is an essential component to ensuring that the WEC is cost eective. The geometry of a WEC has been dened and was developed by Columbia Power Technologies INC (Columbia Power). With this dened geometry, geometry parameters such as system draft or submerged volume are set, and cannot be changed. However, the mass parameters may be changed during operation in order to aect the power output. The power output of this particular WEC has been shown to be most sensitive to the inertia and center of gravity positions of each of its components. Installing ballast chambers in the WEC allows us to modify those values during the course of operation, which aects the power output of the device. However, there are countless possibilities for the ballast chamber conguration, so some type of search has been perform to nd an optimal ballast chamber conguration which maximizes the WEC power output. An evolutionary search over ballast chamber congurations should yield a WEC design which is superior to a ballast-free model. In this paper, we show how we developed a neural net- work function approximator that maps the mass parameters of each component of a WEC to the power output for that WEC (section 3.2). Next, a time-domain simulator was created with this function approximator, which predicted the annual energy output of a WEC given its ballast conguration. Finally an evolutionary algorithm was utilized to intelligently search through the set of potential ballast congurations, using the time-domain simulator to rank each design. This ultimately lead to the optimal ballast conguration within the

1.3 Full scale wave energy convertersSeveral concepts for wave energy converters(WEC) have reached the full-scale stage. The four main technologies are as followed(Cruz 2007, ch. 7): The Archimedes Wave Swing, the Oscillating Water Column, the Wave Dragon and the Pelamis. These are investigated in the following subsections.1.3.1 Archimedes Wave Swing (AWS)Uniquely among the concepts of WECs, the Archimedes wave swing is fully submerged. This is an advantage in case of storms and also because of its non-visibility over water. Shown in gure 1.2(a), the AWS is characterized as a point absorber type, as its diameter is small compared to wavelength. The AWS consists of an air lled chamber xed to the sea bed. The chamber, called the silo, is open on top. Surrounding the silo is an upside-down cylindrical cup, called the oater. An air lock between the silo and the oater prevents water to get in to silo. The heaving motion of the oater is created due the pressure rise and fall when wave crest directly above the device. The oscillations can be converted into electrical power by the terms of a permanent/magnet linear generator.1.3.2 Oscillating Water Columns (OWC)The Oscillating water columns are formed by a chamber which is lled with air above the water line as shown in gure 1.2(b). The water level inside the chamber rises and falls due to wave action, alternatly pressurising and rarefying the air within the chamber. Pressurized air escapes from the chamber through a turbine-generator unit producing electrical power. Secondly, when the water level falls, the air is sucked back into the chamber through the turbine-generator assembly to continue power production. The turbine- generator most often used is the Wells turbine. There are also submergded WEC.There is a reservoir where the overtopping water with higher potential energy than the surrounding water is gathered. The extraction of energy happens when the water drains back to the sea through low head hydro turbines within the reservoir. The wave dragon has dimensions of 300 metres wide and 170 metres deep, and is made of concrete. This means that it will be very stable, and therefore less vulnerable to fatigue problemsFigure 1.4 shows the Pelamis, which is a semi-submerged, articulate-structured wave energy converter composed by cylindrical sections linked by hinged joints. The Pelamis is held on station by a compliant mooring system that allows the machine to weathervane to align itself head-on to incoming waves. When waves travel along the body, the Pelamis perform an snake-like motion1.3. FULL SCALE WAVE ENERGY CONVERTERS3

(a) The Archimedes(b) The Oscillating Water Column

Wave Swing

(U.S. Department of Energy n.d.) (Renewable Energy Journal 2007)

Figure 1.2: The Archimedes Wave Swing and the Oscillating Water Column

OWCs where the chamber is on the seabed.

1.3.3 Wave Dragon

Unlike the other concepts of wave energy conversion, the wave dragon does not oscillate with the waves; it gathers wave energy passively by utilising the overtopping principle. The drawing in figure 1.3 illustrates the overtopping principles and how the captured water is drained through the turbines. The front face of the device is formed as a curved ramp. There are long reflector wings mounted to the reservoir whose purpose are to channel the waves to the ramp where the waves surge upon, as if it was a beach. Behind the crest of this ramp there is a reservoir where the overtopping water with higher potential energy than the surrounding water is gathered. The extraction of energy happens when the water drains back to the sea through low head hydro turbines within the reservoir. The wave dragon has dimensions of 300 metres wide and 170 metres deep, and is made of concrete. This means that it will be very stable, and therefore less vulnerable to fatigue problems.

1.3.4 The Pelamis

Figure 1.4 shows the Pelamis, which is a semi-submerged, articulate-structured wave energy converter composed by cylindrical sections linked by hinged joints. The Pelamis is held on station by a compliant mooring system that allows the machine to weathervane to align itself head-on to incoming waves. When waves travel along the body, the Pelamis perform an snake-like motion

Figure 1.3: The Wave Dragon

(Wave Dragon n.d.)

1.4. OUTLINE OF THE THESIS5

Around the joints. This motion is resisted by hydraulic rams that pump high-pressure oil through hydraulic motors via smoothing accumulators. These hy-draulic motors drive electrical generators to produce electricity. A pelamis wave farm can be formed by a number of devices connected to an sea bed cable through an umbilical cable which the power from all the joints is fed down.

1.4 Outline of the Thesis

Chapter 2 is the theory chapter where the ocean waves theory, mass-spring-damper dynamics, hydrodynamics and maximum power capture are pre-sented. It also contains a electrical analogue to the mechnical mass-spring-damper system.

The modeling chapter 3 presents the equations needed to model a ax-isymmetric point absorber WEC. The excitation force acting on the buoy is also presented with dierent methods for calculation. An open-loop analysis roundups the chapter.

The model is built in the graphical simulation tool Simulink and this is the subject for chapter 4 together with dierent control strategies. To be able to simulate on the model, a control strategy must be included, and I have included PID control and Model Predictive Control as strategies to be able to capture maximum power with Phase Latching.

At last there is a chapter were further work and some concluding remarks are given.

Chapter 2

Theory

This chapter will give an introduction to many fields, like ocean surface waves, the loop dynamics of a mass-spring-damper-system, the dynamics of a floating body in heave. Also the hydrodynamics of oshore devices and the energy absorption by oscillating bodies are presented. The findings in this chapter are the fundaments of chapter 3.

2.1 Ocean surface waves

An understanding of the ocean wave environment is fundamental to be able to explore the possibilities there is in capturing wave energy. This chapter will present many of the fundamental characteristics of the ocean environment.

2.1.1 Creation of waves

Waves are formed due to wind energy. In fact, the waves are a condensed form of solar energy. Wind sweeps over the earths surface because of the unevenly heating from the sun. Heated air expands and its pressure drops which in turn forces the warm air to rise. When the warm air rises, cold air is dragged under to fill the empty space, and a circulation of air is begun. This is what we call wind.

Since land heats quicker than water this leads to greater thermal pressure on the coastlines. This is therefore these areas are likely to be more exposed to rougher wind conditions than inland areas.

The rotation of the earth does also have an eect on the wind formation. The Coriollis eect causes the longitudinal flow of air to be disrupted as winds are deflected to the right of their course in the northern hemisphere and to the left in the southern hemisphere. Cells of circulating air is formed,

2.1. OCEAN SURFACE WAVES7

Three in each hemisphere. By definition, Norway lays on the border between the Polar cell and the Ferrel cell, but is regarded to be a part of the Westerlies (known as Vestavindsbeltet in Norwegian), (Meteorologisk institutt n.d.)

When wind travels over ocean water, it creates ripples on the surface. These are often called capillaries, and as they grow the wind gets better friction on the surface. This turns the ripples into chops after a period time, and the chops transform into small waves when the wind increase. Large waves and swell may then be formed if the wind continues to blow and increases in strength. The size of the waves are determined by windvelocity, wind duration and fetch(the horizontal distance the waves are able to develope on), and together these determine the total amount of energy stored in the wave.

2.1.2 Wave equations

The fundamental wave equation describes all wave behavior a is commonly known as2u= c2r2u(2.1)

t

where r2 is the Laplacian and c is the velocity of the waves propagation. The Laplacian is three-dimensional, but as the simulations is only performed

THEORY in two dimension equation 2.1 reduces to the two-dimensional wave equation

22u 2u! (Kreyzig 1999, p.583).

u= c2+(2.2)

tx2y2

If c = vp is the "phase velocity" the wave propagates with in positive and negative x-direction, then vp is obtained from the dispersion relation and is given after a long series of equations, which are beyond the scope of this thesis to reproduce, as

Gg1/2

vp =tanh(kh) =tanh(kh)(Falnes 2005, p.70).(2.3)

kk

where k = 2/ is the wave number, h is the depth of the water and g is the gravitational force.

Figure 2.2: The water moves in orbits which diminish with increasing depth. (Wikimedia 2007)

The water particles in a wave move in circular orbits, with a radius near the surface almost equal to the amplitude of the wave. This circular motion displaces water, which means that the wave transmits energy as it travels. As seen from figure 2.2 the orbits diminish with respect to increasing depth, and the radii of the orbits can be considered to be negligible at a depth of /2. Here 1 is the direction of propagation of the wave, 2 is the crest, and 3 is the through of the wave. The following equations gives the horizontal, vx, Unlike the other concepts of wave energy conversion, the wave dragon does not oscillate with the waves; it gathers wave energy passively by utilising the overtopping principle. The drawing in figure 1.3 illustrates the overtopping principles and how the captured water is drained through the turbines. The front face of the device is formed as a curved ramp. There are long reflector wings mounted to the reservoir whose purpose are to channel the waves to the ramp where the waves surge upon, as if it was a beach. Behind the crest of this ramp there is a reservoir where the overtopping water with higher potential energy than the surrounding water is gathered. The extraction of energy happens when the water drains back to the sea through low head hydro turbines within the reservoir. The wave dragon has dimensions of 300 metres wide and 170 metres deep, and is made of concrete. This means that it will be very stable, and therefore less vulnerable to fatigue problems.

which is a semi-submerged, articulate-structured wave energy converter composed by cylindrical sections linked by hinged joints. The Pelamis is held on station by a compliant mooring system that allows the machine to weathervane to align itself head-on to incoming waves. When waves travel along the body, the Pelamis perform an snake-like motion

2.1. OCEAN SURFACE WAVES9

and vertical, vz, fluid velocities, respectively.

vx = x

vz = z

where

k

=ge(kz)(f b)

=cosh(kz + kh)(f b)(2.4)

sinh(kh)

=gike0(kz)(f + b)

=isinh(kz + kh)(f + b),(2.5)

sinh(kh)

e0(kz) =de(kz)=sinh(kz + kh)= e(kz) tanh(kz + kh).(2.6)

d(kz)cosh(kh)

z is the depth below the surface. Here, k is the wave number, h is the water depth, = f + b is the complex amplitude of the wave (Falnes 2005, p.70-71), and is the complex amplitude of the velocity potential(Falnes 2005, p. 64).

2.1.3 Wave Transport of Energy

Potential energy is related by the elevation of water from the wave through the wave crest (Falnes 2005, p. 75).

1

Ep = mgh =2kh2 [J](2.7)

where m is the mass, g is the gravitation force and h is the potential height and k is the restoring force coecient applied if an object is moved a distance h from its equilibrium(Tipler and Mosca 2004, p. 434). The time-averaged potential energy per unit (horizontal) in a harmonic wave is given by the

Ep(x, y) = (g/4)|(x, y)|2 [J/m2](2.8)

where is the amplitude of the wave (Falnes 2005, p. 76). For a progressive, plane harmonic wave we have

Ep(x, y) = (g/4)|A|2 [J/m2](2.9)

Kinetic energy is given byEk =1mv2[J](2.10)

2

where m is the mass and v is the speed of the object (Tipler and Mosca 2004, p. 434). By integrating the average kinetic energy per unit volume from z = to z = 0 (Falnes 2005, p.76, eq. 4.124) the average kinetic energy per unit (horizontal) area:

0 2

Ek =2|A|2Z e2kzdz =|A|2(2.11)

222k

Using 2 = gk we obtain

Ek(x, y) = (g/4)|A|2[J/m2].(2.12)

As can be expected from the equipartition theorem, (Tipler and Mosca 2004, p. 544), the kinetic energy in a wave is equal to the potential energy in the wave. The total energy in a wave is accordingly a summation of kinetic and potential energy in the wave, and thus the time-average stored energy per unit (horizontal) area for a progressive, plane, harmonic wave is (Falnes 2005, p. 77)Etot = (g/2)|A|2 [J/m2].(2.13)

The energy per unit crest length is found by multiplying equation 2.13 by the wavelength of the wave.

EL = (g/2)|A|2(2.14)

The power per unit crest length is found based on the knowledge that all the energy in the wave is transmitted completely with each successive wave, and diving equation 2.14 by the period of the wave givesP =g|A|2[W/m](2.15)

2T

In deep water the wavelength equals = gT 2/(2) (Falnes 2005, p. 72), so the power per unit crest on deep water isP =g2H2T[W/m].(2.16)

4

GENERATING ELECTRICITY FROM WING WAVESJust like wind mills and wind turbines that generate power and electricity from the wind, scientists are now working to generate power from the sea. Stephen Wood, an assistant professor of marine and environmental systems at Florida Institute of Technology's College of Engineering is working on this technology for its advance and proper use. This technology will use Wing waves in a very efficient way to generate electricity and power from the sea. AMERICAS PREMIERE WAVE POWER FARM SETS SAILWave energy is among the impressive list of renewable energy resources that is being developed in the United States. New Jersey-based developer, Ocean Power Technologies has launched a project that features the nations first commercial wave power farm off the coast of Reedsport, Oregon. Once the project is completed, wave energy will generate power for several hundred homes in Oregon. The wave power farm operates on the wave energy that is created when a float on a buoy flows with the natural up and down movement of the waves.

AQUAMARINE POWER MAKES RIDING THE WAVES MORE INTERESTINGEvery time we turn around we are seeing something new and exciting in renewable energy forms and Aquamarine Power is the latest in a long line of energy sources to be pumped up about. Aquamarine Power is a company that is developing wave energy. This is a source of energy that has barely been tapped into and they are one of the leaders in developing it.

OSMOTIC POWER PLANT SET TO OPENWith the big push on alternative energy sources, world leaders everywhere are pushing for new power technology to create power plants that will use different resources to keep the earth greener. Her Royal Highness, The Crown Princess Mette-Marit of Norway has just made a huge step as a leader in this movement as she has announced the opening of the worlds first osmotic power plant, due to begin operations on November 24, 2009.

2 NEW & INNOVATIVE OCEAN WAVE ENERGY DEVICESOcean wave energy can be captured directly from surface waves. Blowing wind and pressure fluctuations below the surface are the main reasons for causing waves. But consistency of waves differs from one area of ocean to another. Some regions of oceans receive waves with enough uniformity and force. Ocean waves contain tremendous energy. Currently scientists and companies are considering exploiting the wave power of oceans to harness clean and green energy.

STUDYING SEA WAVES WITH RADAROff shore wind is steady and blows harder. If a country is densely populated it is hard to find open space to install wind farms. That is why there are more and more offshore wind farms in densely populated Europe where there is limited space on land and relatively large offshore areas with shallow water. Scientists of the Geesthacht GKSS Research Centre are interested in offshore winds and mechanics of sea waves. They are working on a radar system to study the behavior of the sea waves. This technology will be available for utilization on the North Sea on the FINO3 research platform. This technology will help in finding out the details of the interactions between offshore wind power machines and swells.

AGUCADOURA GENERATING POWER FOR 1,500 HOMESAs the conventional sources of energy are dwindling, scientists are continuously looking for alternative sources of energy. We are frequently reading about generation of alternative and clean energy from unconventional sources. Portugal built Agucadoura, the world's first wave farm off its coast. This wave farm has three Wave Energy Converters which are producing a total of 2.25MW.

EUROPEAN MARINE ENERGY TO TEST TIDAL POWERThe European Marine Energy Centre (EMEC) site is going to be the place where marine energy farm Aquamarine Power is going to become the first Scottish company to test both wave and tidal technologies. Aquamarine Power has reached an agreement with EMEC to place its tidal stream power device known as Neptune at the test site on the Isle of Eday. Neptune is an Edinburgh-based company.

CLEAN ENERGY FROM FLOWING WATERSThe flowing waters in the rivers and tidal waves can be a good source of alternative energy. With 70% of the earth's surface covered with water, a great amount of energy can be produced by placing turbines at strategic locations under strong currents. This method of generating electric power is called hydrokinetic power generation. In fact, plans are under way to install 875 submerged turbines inside the Niagara river. WAVE POWER IN SCOTLANDThe development of the first subsea commercial wave farm by a Scottish company took another important step forward today (Tuesday February 20th 2007) with news that Scottish wave energy company, AWS Ocean Energy Ltd. based in Alness, Ross-shire, has secured 2.128 million funding from the Scottish Executive. The funds will be used to develop and commercialise AWS' Archimedes Wave Swing, one of the few proven technologies worldwide for generating clean, renewable electricity from the ocean's waves.

OCEAN RENEWABLE ENERGY COALITIONThe Ocean Renewable Energy Coalition (OREC) was founded in 2005 by Sean O'Neill (founder of Symmetrix) and Carolyn Elefant (Law Offices of Carolyn Elefant). The mission of OREC is the advancement and commercialization of offshore renewables, including offshore wind, ocean wave, OTEC and ocean and stream based tidal and current (hydrokinetic) technologies. In 2006 OREC lobbied for federal funding for offshore renewable projects and tax incentives to stimulate private investment. OCEAN ENERGY BIONICSBioPower Systems is developing a new ocean energy technology in Australia that will use bionics to mimic natural systems in order to produce energy. Both bioSTREAM and bioWAVE technologies use biomimicry, which refers to the adaptation of biological traits in engineered systems. BioPower Systems has copied many of the beneficial traits from natural systems in the development of the new ocean energy conversion systems. The company is researching this new technology for application. Laboratory testing will be completed in 2007, and full-scale ocean-based prototypes will be tested in 2008. Commercial units are expected to reach the market by the end of 2009.

PELAMIS OFFSHORE WAVE ENERGY IN PORTUGALA Portuguese energy company called Enersis is funding a commercial wave energy project in Northern Portugal. Construction will begin at the end of October 2006. The project will use Pelamis wave generator technology (manufactured by Ocean Power Delivery) to harness energy from the ocean. After two decades of research and testing at the Lisbon Technical Institute, the first stage of this ocean energy project is intended to produce 2.25 megawatts and power homes through the nation's state-run electrical grid system. Ocean Power Delivery is considered to be the world's leading ocean energy company.

BASICS OF OCEAN WAVE ENERGYOcean wave energy is captured directly from surface waves or from pressure fluctuations below the surface. Waves are caused by the wind blowing over the surface of the ocean. In many areas of the world, the wind blows with enough consistency and force to provide continuous waves along the shoreline. Ocean waves contain tremendous energy potential. Wave power devices extract energy from the surface motion of ocean waves or from pressure fluctuations below the surface.Ocean Wave Energy ResourceWave power varies considerably in different parts of the world. Areas of the world with abundant wave power resource include the western coasts of Scotland, northern Canada, southern Africa, Australia, and the northwestern coast of the United States, particularly Alaska.Whereas wind resource potential is typically given in gigawatts (GW), wave and tidal resource potential is typically given in terawatt-hours/year (TWh/yr). The Electric Power Research Institute (EPRI) has completed a recent analysis of the U.S. wave energy resource potential, available . EPRI estimates the total wave energy resource along the outer continental shelf at 2,640 TWh/yr. That is an enormous potential, considering that just 1 TWh/yr of energy will supply around 93,850 average U.S. homes with power annually. While an abundance of wave energy is available, it cannot be fully harnessed everywhere for a variety of reasons, such as other competing uses of the ocean (i.e. shipping, commercial fishing, naval operations) or environmental concerns in sensitive areas. Therefore, it is important to consider how much resource is recoverable in a given region. EPRI estimates that the total recoverable resource along the U.S. shelf edge is 1,170 TWh/yr, which is almost one third of the 4,000 TWh of electricity used in the United States each year. The recoverable wave energy resource for each region is estimated as: West Coast 250 TWh/year East Coast 160 TWh/year Gulf of Mexico 60 TWh/year Alaska 620 TWh/year Hawaii 80 TWh/year Puerto Rico 20 TWh/yearOcean Wave Energy TechnologiesThe wave energy devices being developed and tested today are highly diverse, and a variety of technologies have been proposed to capture the energy from waves. Some of the more promising designs are undergoing demonstration testing at commercial scales. Wave technologies have been designed to be installed in the nearshore, offshore, and far offshore locations. While wave energy technologies are intended to be installed at or near the water's surface, there can be major differences in their technical concept and design. For example, they may differ in their orientation to the waves or in the manner in which they convert energy from the waves. Although wave power technologies are continuing to develop, there are four basic applications that may be suitable for deployment on the Outer Continental Shelf (OCS): point absorbers, attenuators, overtopping devices, and terminators.

Terminator devices extend perpendicular to the direction of the wave and capture or reflect the power of the wave. These devices are typically onshore or nearshore; however, floating versions have been designed for offshore applications. The oscillating water column is a form of terminator in which water enters through a subsurface opening into a chamber, trapping air above. The wave action causes the captured water column to move up and down like a piston, forcing the air though an opening connected to a turbine to generate power. These devices generally have power ratings of 500kW to 2 MW, depending on the wave climate and the device dimensions.

Attenuators are long multisegment floating structures oriented parallel to the direction of the waves. They ride the waves like a ship, extracting energy by using restraints at the bow of the device and along its length. The differing heights of waves along the length of the device causes flexing where the segments connect. The segments are connected to hydraulic pumps or other converters to generate power as the waves move across. A transformer in the nose of the unit steps up the power-to-line voltage for transmission to shore. Power is fed down an umbilical cable to a junction box in the seabed, connecting it and other machines via a common subsea cable to shore.Attenuators are typically long, multi-segment floating devices that are positioned in the direction of incoming waves. The segments flex and move at hinged joints as waves pass along, and the mechanical motion of the flexing is converted to electrical energy. The foremost example, the Pelamis device developed by the Scottish company Pelamis Wave Power, has been deployed in a variety of locations in the UK and in Portugal. Pelamis was the world's first offshore wave energy converter to be grid-connected, in 2004, and supplied electricity from the first multiple-machine wave farm at Agucadoura, Portugal to the country's Enersis in 2008. Currently the company is working with partners to build a 10 MW project off the Shetland Islands using multiple current-generation Pelamis devices. The company ScottishPower Renewables and the utility E.ON are testing Pelamis devices in Orkney currently; the former plans to install 66 such devices for a 50 MW facility off Marwick Head in Orkney. Pelamis Wave Power has initiated development of the Farr Point Wave Farm, for which they have obtained a seabed lease to develop a 15 MW farm that could eventually be expanded to 50 MW.

A point absorber is a floating structure with components that move relative to each other due to wave action (e.g., a floating buoy inside a fixed cylinder). Point absorbers often look like floating oceanographic buoys. They utilize the rise and fall of the wave height at a single point for energy conversion. The relative up and down bobbing motion caused by passing waves is used to drive electromechanical or hydraulic energy converters to generate power. Point absorbers are oscillating bodies that absorb energy from waves coming from all directions but whose output power is small due to their small size. An array of such devices is required to deliver substantial power to the grid. Several pilot projects have utilized point absorber WECs, including the PowerBuoy, developed by Ocean Power Technologies, a company with offices in the UK, Australia and the United States. The PowerBuoy will be installed off the coast of Reedsport, Oregon this spring, making it the first commercial wave power station in North America. The initial deployment of one 150-KW buoy will be extended to up to 10 buoys, for a 1.5 MW power station. Current estimates are that it will generate 4,140 MWh/year, or enough to power 375 homes. The PowerBuoy has already been tested in Scotland, Spain and Hawaii, and future large-scale projects are underway for Portland, Australia, Cornwall, UK and Coos Bay, Oregon.

Overtopping devices have reservoirs that are filled by incoming waves, causing a slight buildup of water pressure like a dam. The water is then released, and gravity causes it to flow back into the ocean. The energy of the falling water is used to turn hydro turbines to generate power. Specially built floating platforms can also create electricity by funneling waves through internal turbines and then back into the sea. Overtopping devices harness energy from incoming waves by capturing water in a central reservoir and releasing it back to the sea through a number of hydroelectric turbines. An example is the Wave Dragon project in Denmark. Over the past decade, Denmark has hosted smaller-scale Wave Dragon projects.For example, in 2003 a 1:4.5 scale prototype was deployed in Nissum Bredning in Northern Denmark. Results from these earlier deployments have led to the latest commercial-size deployment of a 1.5 MW plant, which will occur at the Danish Wave Energy Center, DanWEC, at Hanstholmn in the Danish part of the North Sea. A 4 MW plant is to follow at Ekofisk in the North Sea, with the longer-term goal of installing a fully fledged power plant of 4-11 MW with a production price of 0.04/kWh (~ 5.33 U.S. cents/kWh). Overtopping devices harness energy from incoming waves by capturing water in a central reservoir and releasing it back to the sea through a number of hydroelectric turbines. An example is the Wave Dragon project in Denmark. Over the past decade, Denmark has hosted smaller-scale Wave Dragon projects.For example, in 2003 a 1:4.5 scale prototype was deployed in Nissum Bredning in Northern Denmark. Results from these earlier deployments have led to the latest commercial-size deployment of a 1.5 MW plant, which will occur at the Danish Wave Energy Center, DanWEC, at Hanstholmn in the Danish part of the North Sea. A 4 MW plant is to follow at Ekofisk in the North Sea, with the longer-term goal of installing a fully fledged power plant of 4-11 MW with a production price of 0.04/kWh (~ 5.33 U.S. cents/kWh).

Oscillating water columns (OWCs) consist of a partially submerged chamber that contains both a water column formed by the ingression of ocean waves and an air column formed by the trapped air. A turbine located at the top of the chamber experiences the expansion and contraction of the air column as ocean waves continuously change the water levels within the chamber. The LIMPET wave energy project near the Scottish island of Islay and the Oceanlinx Port MacDonnell project, soon to be deployed in South Australia, are examples of oscillating water columns. The LIMPET plant employs a full-scale on-shore OWC with a total installed capacity of 500KW. This grid-connected project is coordinated by Queens University Belfast and supported by the European Union under the Joule III Programme. The plant has been serving the local economy since early 2000. More recently, the Australian company Oceanlinx received close to $4 million in funding from the Australian Centre for Renewable Energy to deploy a 1 MW commercial-scale OWC, which will be a concrete box-like structure located 4 km off the coastline from Port MacDonnell. Grid connection is expected in late 2013, at which point it is anticipated that the deployment may serve up to 1,000 households.

ELECTRICITY FROM OCEAN WAVE ENERGY:TECHNOLOGIES,OPPORTUNITIES AND CHALLENGES The energy from ocean waves is a largely untapped resource that could play an important role in our electricity future. It is more consistent and predictable than that of other renewable resources such as wind and solar. Although several pilot projects have been successfully deployed worldwide, and some of them are grid-connected, the economic production of electric power from wave energy remains to be demonstrated. A key path forward will be the integration of smart technologies that harness vast amounts of sensor and meteorological data to support wave farm operations.With estimates of economically recoverable wave energy resources ranging from 140 to 750 TWh/year worldwide with existing technology, energy from ocean waves is a largely untapped resource that could play an important role in our electricity future. It is more consistent and predictable than other renewable resources like wind and solar. What is more, the maximum energy density of waves (between 40 and 60 degrees latitude) is found in both hemisphereswhere the advanced industrial economies of Europe, the United States and Japan reside. A key barrier to making wave energy a reality, however, is cost. According to current estimates, the levelized cost per MWh of wave energy production is more than 1.5 times that of wind and nearly three times that of coal-based power. Wave energy is more expensive than wind energy in part because wave energy conversion is in a much earlier development phase. Looking forward, this barrier will have to be overcome for wave energy production to reach its full potential. A key to reducing costs will be predicting the characteristics of waves, which can be reliably determined days in advance. This predictability will give wave energy producerswith low operational costs and a non-polluting technologyattractive market opportunities in the near future. Many different techniques have already been proposed and tested for both on-shore, near-shore and off-shore wave energy extraction. The process of energy generation at a wave energy converter (WEC) consists of a number of steps, which include energy absorption from ocean waves by a type of energy capture mechanism, transmission of mechanical power to the generator by a power take-off mechanism and controlling power output by means of suitable power electronics or arrays of similar WECs, or both. Most current research and development efforts are focused on improving the mechanical designs of WECs energy capture and power take-off mechanisms. WECs can be categorized broadly into four groups based on their underlying operating principle:

A number of these technologies have the potential to scale up to large power projects serving hundreds of thousands of homes and possibly industries along the coast. But wave power, like other forms of renewable energy, will be a distributed resource and produce fluctuating power. As such, it too will require smart electricity grid support for reliable power delivery. Just as large-scale wind farms are coming under the purview of the smart grid, requiring information models for the control and monitoring of power injection, wave power will have to receive the same attention in the very near future. But because ocean waves are more predictable than winds and solar reception, wave energy represents somewhat lesser challenges to smart grid operations. For the same reason, wave energy may be exploitable in a somewhat greater range of grid applications, for example, voltage regulation. How can we integrate predictions of wave characteristics in making electrical control decisions? In many test sitesDanWEC at Nissum Bredning and Wave Hub, in the UK, for exampleunder-water electrical infrastructure has been built with substations, transformers and switchgear, to evacuate power from WECs in a farm and bring it to shore. Equipment choice and ratings in part determine the reliability of the electrical power produced, but the quality and quantity of power output heavily depend on how well-informed each WEC is about the nature of incoming waves.

Wave powerWave power is the transport of energy by ocean surface waves, and the capture of that energy to do useful work for example, electricity generation, water desalination, or the pumping of water (into reservoirs). Machinery able to exploit wave power is generally known as a wave energy converter (WEC).Wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave-power generation is not currently a widely employed commercial technology, although there have been attempts to use it since at least 1890.[1] In 2008, the first experimental wave farm was opened in Portugal, at the Aguadoura Wave Park.[2] The major competitor of wave power is offshore wind power.Physical concepts

When an object bobs up and down on a ripple in a pond, it experiences an elliptical trajectory.

Motion of a particle in an ocean wave.

A = At deep water. The orbital motion of fluid particles decreases rapidly with increasing depth below the surface.

B = At shallow water (ocean floor is now at B). The elliptical movement of a fluid particle flattens with decreasing depth.

1 = Propagation direction.2 = Wave crest.3 = Wave trough.

Photograph of the water particle orbits under a progressive and periodic surface gravity wave in a wave flume. The wave conditions are: mean water depth d=2.50ft (0.76m), wave height H=0.339ft (0.103m), wavelength =6.42ft (1.96m), period T=1.12s.

Waves are generated by wind passing over the surface of the sea. As long as the waves propagate slower than the wind speed just above the waves, there is an energy transfer from the wind to the waves. Both air pressure differences between the upwind and the lee side of a wave crest, as well as friction on the water surface by the wind, making the water to go into the shear stress causes the growth of the waves.[4]Wave height is determined by wind speed, the duration of time the wind has been blowing, fetch (the distance over which the wind excites the waves) and by the depth and topography of the seafloor (which can focus or disperse the energy of the waves). A given wind speed has a matching practical limit over which time or distance will not produce larger waves. When this limit has been reached the sea is said to be "fully developed".In general, larger waves are more powerful but wave power is also determined by wave speed, wavelength, and water density.Oscillatory motion is highest at the surface and diminishes exponentially with depth. However, for standing waves (clapotis) near a reflecting coast, wave energy is also present as pressure oscillations at great depth, producing microseisms.[4] These pressure fluctuations at greater depth are too small to be interesting from the point of view of wave power.The waves propagate on the ocean surface, and the wave energy is also transported horizontally with the group velocity. The mean transport rate of the wave energy through a vertical plane of unit width, parallel to a wave crest, is called the wave energy flux (or wave power, which must not be confused with the actual power generated by a wave power device).Wave power formulaIn deep water where the water depth is larger than half the wavelength, the wave energy flux is[a]

with P the wave energy flux per unit of wave-crest length, Hm0 the significant wave height, Te the wave energy period, the water density and g the acceleration by gravity. The above formula states that wave power is proportional to the wave energy period and to the square of the wave height. When the significant wave height is given in metres, and the wave period in seconds, the result is the wave power in kilowatts (kW) per metre of wavefront length.[5][6][7][8]Example: Consider moderate ocean swells, in deep water, a few km off a coastline, with a wave height of 3 m and a wave energy period of 8 seconds. Using the formula to solve for power, we get

meaning there are 36 kilowatts of power potential per meter of wave crest.In major storms, the largest waves offshore are about 15 meters high and have a period of about 15 seconds. According to the above formula, such waves carry about 1.7kW of power across each metre of wavefront.An effective wave power device captures as much as possible of the wave energy flux. As a result the waves will be of lower height in the region behind the wave power device.Wave energy and wave-energy fluxIn a sea state, the average energy density per unit area of gravity waves on the water surface is proportional to the wave height squared, according to linear wave theory [b][10]where E is the mean wave energy density per unit horizontal area (J/m2), the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy,[4] both contributing half to the wave energy density E, as can be expected from the equipartition theorem. In ocean waves, surface tension effects are negligible for wavelengths above a few decimetres.As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the wave energy flux, through a vertical plane of unit width perpendicular to the wave propagation direction, is equal to

with cg the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength , or equivalently, on the wave period T. Further, the dispersion relation is a function of the water depth h. As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths. The energy flux is with the group velocity, see Herbich, John B. (2000). Handbook of coastal engineering. McGraw-Hill Professional. p. A.117, Eq. (12). ISBN978-0-07-134402-9. The group velocity is , see the collapsed table "Properties of gravity waves on the surface of deep water, shallow water and at intermediate depth, according to linear wave theory" in the section "Wave energy and wave energy flux" below.^ Here, the factor for random waves is 116, as opposed to 18 for periodic waves as explained hereafter. For a small-amplitude sinusoidal wave with wave amplitude the wave energy density per unit horizontal area is or using the wave height for sinusoidal waves. In terms of the variance of the surface elevation the energy density is . Turning to random waves, the last formulation of the wave energy equation in terms of is also valid (Holthuijsen, 2007, p. 40), due to Parseval's theorem. Further, the significant wave height is defined as , leading to the factor 116 in the wave energy density per unit horizontal area.^ For determining the group velocity the angular frequency is considered as a function of the wavenumber k, or equivalently, the period T as a function of the wavelength .

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