Waves are generated by the wind as it blows across the sea surface. Energy is transferred from the wind to the waves.
Waves travel vast distances across oceans at great speed. The longer and stronger the wind blows over the sea surface, the higher, longer, faster and more powerful the sea is. Wave energy has the potential to be one of the most environmentally benign forms of electricity generation. It is a clean and renewable energy source and its potential is huge. Waves effectively average out the wind that generates them over large areas which results in a high level of consistency compared to wind or solar. Only on very few days per year are waves too weak to generate electricity.
Example of Wave Technology system – the Pelamis
The Pelamis is an offshore wave energy converter that uses the motion of waves to generate electricity. The machine operates in water depths greater than 50m and is typically installed 2-10km from the coast. The machine is rated at 750kW with a target capacity factor of 25-40 per cent, depending on the conditions at the chosen project site. On average one machine will provide sufficient power to meet the annual electricity demand of approximately 500 homes.
How the Pelamis works
The Pelamis machine is made up of five tube sections linked by universal joints which allow flexing in two directions. The machine floats semi-submerged on the surface of the water and inherently faces into the direction of the waves. As waves pass down the length of the machine and the sections bend in the water, the movement is converted into electricity via hydraulic power take-off systems housed inside each joint of the machine tubes, and power is transmitted to shore using standard subsea cables and equipment.
Wave energy is sometimes confused with tidal energy, which is quite different. Tidal energy is a form of hydropower that converts the energy of the tides into electricity or other useful forms of power. The tide is created by the gravitational effect of the sun and the moon on the earth causing cyclical movement of the seas.
- Tidal barrage systems involve the trapping of water at high tide, most commonly at a point near the mouth of a river estuary or a bay, followed by the controlled release of the water back to the sea through turbines working on essentially the same basis as an upstream hydroelectric dam.
- Tidal stream turbines (see diagram) harness the power of both the inflows and outflows of tidal energy. The tide flows through the stationary turbines, causing them to turn using the same principal as a wind turbine. There are various different designs of tidal stream turbines such as seabed standing and surface floating designs.
While tidal barrage technology is a firmly established technology, tidal stream technology is in its infancy. Recently however, the world’s first commercial scale tidal power generating device was installed in Strangford Lough, Northern Ireland. A UK company have designed and developed a tidal stream device known as SeaGen. The device is currently in the commissioning and testing phase. It is estimated that the 1.2 MW tidal stream device will be capable of providing enough clean electricity to power 1,000 homes.