Introduction

Currently a huge amount of time and money is being invested in the research, development and implementation of technologies to exploit renewable energy sources; it is estimated that £95-billion was invested in new renewable capacity in 2009 alone. [1] In the Department of Energy and Climate Change's 2009 report plans were set out to raise the production of  energy from renewables to 30% of the total; some believe that electricity harnessed from wave power could be the solution to meeting these goals.

Wave formation and energy potential

Waves are formed by the sustained flow of wind above the surface of an area of water. Energy is transferred from the air to the water where it is accumulated as potential and kinetic energy.

[3] This diagram shows the generation process of wind-waves, it shows how the flow of air causes differential pressures that steepen the waves. Positive and negative areas of pressure are shown by the plus and minus symbols. The curved arrows represent the flow of air and fluid. This is known as the Jeffrey's 'sheltering' model of wave generation.

The energy possessed by a wave is affected by several factors including the wind speed and distance of open water over which they were generated. Waves move freely with little resistance in deep water and consequently once winds subside waves can easily continue for great distances, some as far as ten thousand kilometres. [2]  Because of their ability to travel unaffected there is a much larger concentration of wave activity on coastlines exposed to large expanses of water and prevailing winds.

Because of their position the west coasts of the Americas, Europe, Australia and new Zealand are potentially huge sources of this renewable energy. It is estimated that the worldwide deep water wave power resources that could be practically captured are in excess of 2TW (13% of global energy consumption); estimates also predict  15-25% of current UK demand could be fulfilled by wave energy power generation. [4][5] Some wave farming projects have already begun, or are in development, a few significant schemes are listed on the map below.

Worldwide Wave-Farm Developement

Scotland

The UK and Scottish Governments are providing significant funding for new wave farm projects; encouraging this new sector could introduce many jobs  and a lot of money into the area. In the 1980s Denmark took advantage of the new market in wind turbines encouraging the sector which is now worth more than £3 billion a year to that country. [6]

Pelamis Wave Power

The Scottish Government has invested £4 million in the construction and deployment of 4 Pelamis Wave machines. When completed this wave-farm will have the largest output capacity world-wide with an average yield of 3MW. [7]

Pelamis currently has 5 projects in development around the Scottish coast, the largest planned site off the north coast of Sutherland has the goal to generate up to 50MW of power by the end of its second phase. [8]

Plans to build a 4MW wave farm by npower renewables were approved by the Scottish Government in 2009. The station is based on Oscillating Water Column technology. [9] OWCs make use of a large column into which waves flow, these waves pressurise the air inside the column and force it through a wells turbine (a turbine which rotates in the same direction regardless of which way the air flow is coming from); the turbine in turn is connected to a generator.

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Portugal

The first ever commercial wave farm began operation 5km off the coast of Portugal on the 15th July 2008. [10] The farm makes use of the same pelamis wave machines that are being installed around the Scottish coast. The three machines are capable of generating 750kW each, coming to a total installed capacity of 2.25MW. [10] The machines were taken out of action 2 months after they were deployed due to technical problems with the joints; and are now indefinitely out of service due to their owners moving into liquidation. [11]  

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United States

The wave energy potential in the United States is estimated to be equal to 6.5% of its current total capacity. However there are currently very few projects being planned to exploit this large resource. [12]  Notably the first wave farm to be built in the US has begun construction near Oregon. The farm makes use of OPTs power buoy technology. [13] Large buoys float on the surface of the water and move with the waves; this movement is used to pump hydraulic fluid that is then used to turn a generator and produce electricity.

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England

16KM off the coast of Cornwall a wave power research project has begun with the aim of encouraging investment in the R&D of wave energy generation devices. The intention of the 'Wave Hub' project is to make it easier for developers to deploy their wave power devices without having to spend large amounts on developing infrastructure to transport the energy to the grid. [14]
The scheme consists of a 25km subsea cable connecting a 12 tonne hub to the UKs nation grid (via a controller and other power conditioning equipment at the shore). Currently the hub has a 20MW capacity. [14] Three projects have been confirmed to be implemented at the wave hub project; including OPT who are also constructing the first wave farm in the US.

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Australia and New Zealand

Just as in Europe and America the western coast of Australia and New Zealand has great potential for the production of electricity from wave energy devices. New Zealand's National Institute of Water and Atmospheric Research estimates  that the country's ocean waves carry on average 25kW per metre of coastline. [15]

The most noteworthy implementation of wave power in Australia and New Zealand is the oscillating water column generators made by Oceanlinx. This company's makes use of turbines with variable pitch blades in order to generate electricity from a bi-directional flow of air. This has efficiency advantages over the more common well's turbine approach.

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Technologies

Many different techniques have been implemented to harness energy from wave power; EMEC has categorised the variety of different devices into these 6 categories.

Attenuator Attenuator devices float on the surface of the water, they are directed straight into the waves and ride over them. This causes movements in their joints which is captured and used to generate electricity. The pelamis wave is an example of an attenuator style wave energy generator.
Point Absorber Point absorber generators normal consist of a large buoy or other buoyant container that moves in all directions absorbing energy from waves. This energy can then be taken from the buoy in a number of different ways. The power boy developer by Ocean Power Technology is an example of a point absorber generator.
Oscillating Wave Surge Converter This type of generator makes use of a paddle with a large area which is forced to swing in response to the surge of a wave. Energy is extracted from this pendulum movement of the arm. This design is not as widespread as others due to its vulnerability to damage during high seas.
Oscillating Water Column OWCs consist of a large column of air into which waves flow; these waves pressurise the air and force it through a turbine. This turbine usually has an ability to accept airflow from both directions while always spinning the same way; this allows it to generate power when the wave is entering and leaving the column. Many OWCs use a wells turbine due to its ability to accept bidirectional flow; but some designers are moving towards turbines which can change the angle of blade pitch in response to air flow.
Overtopping Device Overtopping devices use ramps and walls to capture water from waves above the mean height of the ocean. This water is stored in a reservoir and is released through turbines; as it loses its potential energy electricity is generated.
Submerged Pressure Differential Submerged pressure differential devices make use of the increasing and decreasing pressures caused during the cycle of a wave. These buoyant devices move with the wave and this movement is captured and converted into electrical energy.

References

1. Sawin, Eric Martinot and Janet. Renewables Global Status Report 2009 Update. s.l. : Renewable Energy World, 2009.

2. Brooke, John. Wave energy conversion, Volume 6 By Engineering Committee on Oceanic Resources. s.l. : Working Group on Wave Energy Conversion, 2003.

3. http://misclab.umeoce.maine.edu/boss/classes/SMS_491_2003/Week_6.htm

4. Cruz, João. Ocean wave energy: current status and future prepectives. 2008.

5. http://www.pelamiswave.com/wave-energy/the-resource

6. http://business.scotsman.com/alternativeenergysources/Scotland-seas-into-the-future.3348444.jp

8. http://www.pelamiswave.com/our-projects/farr-point-wave-farm

9. http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/7844782.stm

10. http://news.bbc.co.uk/1/hi/scotland/4563077.stm

11. http://www.greentechmedia.com/green-light/post/pelamis-wave-power-jettisons-its-ceo-rough-waters-ahead

12. http://news.bbc.co.uk/1/hi/technology/6410839.stm

13. http://www.smartplanet.com/business/blog/smart-takes/first-wave-power-farm-in-us-to-be-built-in-oregon/4537/

14. http://www.wavehub.co.uk/about_wave_hub.aspx

15. Stevens, Craig, Smith, Murray and Gorman, Richard. Ocean bounty: energy from waves and tides.s.l. : Water & Atmosphere, Vol.13, No.4., 2005.