Wave

The energy in ocean waves comes from the winds, and this in turn comes from the sun. The energy given to the oceans in this way has been estimated to be 2.7 TW per year. The power transmitted by a wave varies as its period and the square of its height. In mid-Atlantic this reaches about 90 kW per metre of wavefront, for waves with a height of about 4 m and a period of about 10 s, but falls to less than 20 kW per metre on the coast due to friction with the seabed and by its breaking motion (Heath 2005). This implies that devices out at sea are able to tap more energy, but they are more vulnerable than devices on the coast that can tap less power. The energy in waves is quoted in kW per metre because any device installed at a certain distance from the shore would absorb the wave energy and prevent any further devices nearer the shore from obtaining any more energy.

Many devices have been proposed to convert wave energy into useable form, but so far none of them has operated commercially on a large scale. Inevitably the devices have to be very large and therefore costly, and they are subject to corrosion by sea water and buffeting by the waves. The £3.5M wave power generator Osprey 1 weighing 8000 tons sank in shallow water off the Scottish coast in a freak summer storm. It was designed to produce 2 MW of electricity, but never operated. More recently, a much publicised device, costing over a million pounds, is designed to produce 75 kW, enough to power 25 three-bar electric fires. An oscillating water column wave power device designed to produce 100 to 500 kW was built in Norway. It was expected to produce electricity at 3 to 4 pence per kW based on a lifetime of twenty-five years, but after operating for three years it was destroyed by a storm (Ross 1995; Heath 2005).

Another device, known as LIMPET, 'consists of three inclined concrete shafts that dip beneath the surface of the ocean. A submerged opening at the foot of each shaft lets in water, allowing waves to alternately push water into and pull water out of the shaft. This motion in turn draws air into and out of the top of the shaft, powering a turbine'. The energy conversion efficiency was estimated to be 48% to give an average power input of 202 kW. In practice the mechanical and electrical energy losses reduced the mean average output to 21 kW (MacKay 2008). Since 2000 this device has been supplying power to the local electricity grid.

A wave power generator device, called Pelamis, is being tested off the Orkney Islands. It 'consists of four floating cylinders, each about 30 m long and 3.5 m in diameter, connected by hinged joints. The waves cause the cylinders to move relative to each other and this motion is used to pump high-pressure oil through hydraulic motors, which drive electric motors'. It is designed to withstand the powerful waves that are expected in the open sea. Another similar device is Salter's duck.

In Portugal, a Dutch company has installed a device that contains a volume of trapped air below sea level; as the waves pass over it the air is compressed and generates energy. There are many other wave power devices being made worldwide.

All these wave energy devices are presently uneconomic, but their designers are confident that the costs will eventually come down to acceptable levels. There are very few statistics available. Some Governments provide subsidies for wave power; for example the Portuguese government pays 16 p/kWh for wave-generated electricity. As this is about six times the current price it is clear that wave power has a long way to go before it becomes a practical proposition.

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