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- The energy of the oceans
Waves, a constant movement to be used05/28/2013
At sea, the force of the waves is considerable and omnipresent. Countries with well-positioned coastlines can take advantage of this energy potential and have come up with various techniques to exploit this "blue energy".
Renewable, dense energy accessible all over the world
The sea creates waves under the effect of the wind. Wave energy is a significant source of raw energy. According to the World Energy Council1, average global wave energy resources are 1.3-2 TW, equivalent to total installed electricity capacity worldwide (about 2 TW).
In France, the potential is about 40 TWh per year, about 8% of national electricity output2. In the United Kingdom, the leading country in this area, 40-50 TWh per year could be provided by wave energy, representing over 10% of the country's energy output3.
This energy resource is particularly advantageous, because:
• Waves produce 20 kW per meter of wavefront, 100 times more than solar energy collected over a surface area of 1m2;
• Wave energy can be recovered over vast areas;
• Almost all of this energy could potentially be harnessed4, with minimum losses. The efficiency of wave energy has been constantly improving over the last 20 years thanks to technological progress.
A variety of techniques...
Different systems have been dreamt up to recover the power of waves and convert it into useable energy7.
• Oscillating water columns: this technology is used to capture energy from the oscillations of water in a chamber immersed in the ocean. Changes in water levels compress air like a piston, driving a turbine connected to an alternator8. This is the principle of the LIMPET9 project, inaugurated in 2000 in Scotland, supplying 500 kW of power.
• Slack-moored floating systems: tethered to the coast or in the open sea, these facilities store the water flowing throughout the day and then drain it off, driving a turbine connected to a generator. The Wavedragon system in Denmark uses a 8,000m3 reservoir of water to drive 16 turbines10.
• Oscillating systems: waves create movement between a number of pendulor devices, arrays of boxes linked by hinged joints. Energy is recovered from the joints using a hydraulic pump to send pressurized oil into an engine that drives a generator. Pelamis in Portugal, or the "sea snake", is made up of four floating cylinders linked by hinged joints over a total length of 120 meters.
These facilities can be subject to corrosion problems, but also fragility, due to being anchored in a highly turbulent environment.
… and projects progressively emerging
Wave energy is being developed in a number of European countries and second generation technologies, like those to be used in Portugal, are already in the works.
• After the first Pelamis prototype in 2004, three machines were installed in Aguçadoura, Portugal in 2008. These machines had total output capacity of 2.25 MW, 750 kW per "snake"11. The project was shut down early in 2009, but a second generation of Pelamis is currently in the works: with a total length of 180m, these second-generation Pelamis will produce more energy at a lower cost12.
• Due to its geographical location, the United Kingdom is particularly active in the field of wave energy.
o In Scotland, the Limpet project13, with its three water columns that capture energy over a surface area of 169m2, is the only facility in the world connected to the power grid.
o The coast of Cornwall is home to the Wave Hub, the largest testing site dedicated to wave energy14.
• In France, 20km off the coast of Croisic, the SEM-REV platform (Experimental Test Site for Wave Energy) will shortly be testing new wave energy processes15. In May 2012, work started on laying the undersea cable connecting the platform to land. The first industrial tests are expected to start in 201316. SEM-REV, the first French offshore site connected to the power grid, will extend over 1km2. It is here that SBM Offshore is to test its as yet confidential wave energy power plant, called "S3"17.
Centrale Nantes is also developing a prototype, Searev, which may be tested at sea near Croisic in 201418. Searev uses a closed, sealed floater containing a wheel that acts like a pendulum19,20. Waves make the floater oscillate, causing the pendulum wheel to move and forth. Hydraulic pumps connected to the wheel charge high pressure accumulators which discharge their energy into hydraulic engines that drive electric generators.