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Waves, constant movement to be used

11/30/2010

At sea, the force of the waves is considerable and omnipresent. Their theoretical efficiency far exceeds that of wind energy. However, not every country has the same potential. Those with the best exposure have developed many wave energy recovery techniques.

Waves.
© Idé

Renewable, dense energy accessible all over the world

The sea creates waves under the effect of the wind. The amount of energy generated by the waves is low (1 W/m²/year, 200 times less than direct solar energy). However, this energy could potentially be recovered over vast areas of sea.

Using collectors, it is theoretically possible to obtain 50 kW of power per meter, 20km from coastlines. However, collectors located nearer coastlines produce less energy, with output just 20 kW/m 1km from the coast.

Average global wave energy resources are 1.3-2 TW, equivalent to total installed electricity capacity worldwide.

Wave energy is a significant source of raw energy. It is one of the densest
(or energy-rich) renewable energy sources. 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).
Although present on the coasts of all oceans worldwide, some places have more wave energy than others, such as the North Atlantic, where power is 45 kW/m, and particularly so off the coasts of Britain and Ireland. In France, its potential is about 40 TWh/year².

Expected efficiency from wave energy is far higher than for wind power for example. However, there are disadvantages in using wave energy:

   • Corrosion of submerged equipment

   • Weakness issues relating to the anchoring of items at sea and to the need for mobile mechanical systems in a highly turbulent environment

   • Environmental issues - onshore plants can disfigure the landscape, while offshore plants can interfere with sea traffic and fauna.

A variety of techniques...

There are several types of wave energy recovery systems³.

   • Onshore oscillating water columns capture the incoming waves. The water enters a chamber where it compresses the air taken in when the sea retreats. The compressed air drives a turbine which in turn drives an electric generator.

   • Submerged oscillating water columns are underwater buoys that rise and fall, tossed around by the waves. Anchored to the seabed, their movement activates a piston, draws seawater into a turbine or compresses air or oil that then drives an engine and which in turn drives an electric generator.

   • Tapered channel systems funnel waves into a channel that gets progressively narrower, causing the waves to rise and spill over the channel walls into a reservoir, gradually filling it up. The water in the reservoir then flows back to the sea through a turbine, generating electricity in the process. The reservoir can be located onshore or offshore on a floating, slack-moored platform.

    • Pendulor devices are arrays of boxes linked by articulated hinges that bob up and down on the waves. Energy is recovered through the joints between each box using pistons that activate pressurized oil pumps.

... for many projects

The development of wave energy technology has been ongoing for over 20 years. As yet little used in France, it is being developed in a number of European countries that are already using second-generation offshore technology, in Scotland and Portugal for example. With efficiency constantly improving, these projects are close to becoming profitable.

• In 1999, Scotland launched Pelamis^4-5-6, a sea snake made up of four floating cylinders
3.5 meters in diameter linked by hinged joints over a total length of 150 meters. Each joint has a hydraulic pump. The movement of the waves is used to pump pressurized oil towards a motor that drives a generator. The initial project provided for a facility of 30-40 Pelamis distributed over a square kilometer of ocean and supplying electricity to 20,000 homes.

• Still in Scotland, the Limpet project⁷ on the Isle of Islay has been tested since 2000.
It uses onshore oscillating water column (OWC) technology and has capacity of 500 kW.

• The Wave Dragon⁸ prototype was launched in Wales in 2007. This uses floating, slack-moored platform technology and has capacity of 7 MW. Waves push up a ramp and spill over into a reservoir which then empties, driving a turbine in the process. A mooring system holds the device perpendicular to the direction of the waves. This sea dragon extends over 200 meters and weighs about 30,000 tons. Long-term testing is essential to ensure the safety of the mooring system.

• Other oscillating water column or oscillating float systems are being developed in Portugal, Spain, Brittany and England.

France is developing wave energy recovery technology

True. In France, the Searev^9-10-11-12 project was launched in 2003 at Ecole centrale in Nantes. This is a second-generation offshore system that uses a closed, sealed floater containing a wheel that acts like a pendulum.
The lower half of the wheel, which has a diameter of 9 meters, is weighted with concrete. Waves make the floater oscillate, causing the wheel to move back and forth. The relative movement of the floater and the wheel activates a hydroelectric system that converts mechanical energy into electricity. Hydraulic pumps connected to the pendulum wheel charge high pressure accumulators which discharge their energy into hydraulic engines that drive electric generators. An underwater cable conveys the electricity to land.
24 meters long and weighing 1000 tons, one Searev unit will have capacity of 500 kW. Over time, a wave farm will be built, comprising dozens of modules moored to the seabed 30-50m below, 5-10km offshore.