Wave Energy

What is Wave Energy

Ocean wave energy can be converted into electricity. But wave energy is completely different to tidal power. To understand ocean wave energy readers need to consider what a ‘wave’ actually is. An ‘ocean wave’ is not made of water any more than a sound wave is ‘made of’ air.

Raw wave energy

Raw wave energy

There is no difference between a sound wave and the waves, which we see on the ocean. Wind causes friction between the air and the water molecules so transferring its energy into the sea and the result is ‘wave’ energy, which ripples through the water. The same effect is achieved by blowing on a bowl of water. The energy expended in blowing is transferred into the water. The water, like air, is merely the medium through which the wave travels. Ocean waves do not travel across the ocean in the form of horizontally moving water. The water only moves up and down. It is the wave energy itself, which moves. The ocean wave itself is composed entirely of wave energy and the amount,of wave energy moving around in the world’s oceans is enormous.

The key to understanding what wave energy is and the uses to which it can put, lies in understanding the nature of ‘energy’ itself. ‘Energy’ is not ‘generated’ here on Earth. It is merely converted from some other form of energy already present on Earth or continuously arriving in the form of solar energy. Wave energy is a concentrated form of energy transferred from the wind. And before the energy left the wind to become wave energy it had a previous existence in the form of solar energy which generated the wind itself, or in some instances in the form of geothermal energy which had been released suddenly as a result of an underwater earthquake causing a sudden rush of wave energy in the form of a Tsunami.

What is a wave

Ocean wave energy is an example of mechanical energy transfer. A ‘wave’ is an oscillation or disturbance that travels through a medium, accompanied by a transfer of energy. In the case of ocean wave energy the medium is water. And the wave energy is transferred from one location to another with little or no permanent displacement of the medium through which it travels. Think of the executive toy where the ball hits one side. With no apparent effect on the balls in the middle the ball at the other end lifts up. That is an example of energy transferred through the middle balls in the form of a shock wave and coming out the other side.

Tidal energy is different from wave energy insofar as the source of the wave energy is wind or a geothermal shock. Tides however represent a transfer of gravitational energy through the kinetic process of lifting and dropping the entire ocean. Ultimately however all energy and all the matter which is interchangeable with energy comes from the same original source, the moment of creation of the universe. In what form the energy existed before that, or how it came into being out of nothing (even time and space did not exist), is as much a philosophical or religious question as a scientific one. Physicists are nowhere near finding an answer.

Harnessing wave energy

Wave Energy used to generate electricity

Wave Energy used to generate electricity

The earliest use of wave energy from the ocean was in France in 1799 but by 1973 there were 340 patents registered in the UK alone. In small-scale tests in 1974 an invention called ‘Salter’s Duck’ achieved 90% wave energy conversion rates and 81% efficiency. Owing to the controversy over climate change and oil becoming more expensive again in recent years wave energy has attracted renewed interest. Harnessing ocean wave energy does not involve constructions like today’s tidal power plants, which capture the power of water moving at the coast. Ocean wave energy is harnessed by placing buoys on the water surface, which rise up and down as the ocean moves up and down under them and that motion can be transferred elsewhere for the wave energy to be converted into a usable form.

Wave energy conversion devices are categorised by the method they use to capture the energy, by the ‘power take off system’ they use, and their location. Wave energy capture systems use the vertical motion of the waves to extract the energy from them. Once the wave energy is captured at its source the wave energy has be carried to the point where it is used. The UK is well endowed with wave energy. Owing to its seas’ exposure to westerly winds and owing to seasonal variations in wind speeds the UK’s wave energy is more readily available in the winter at the peak of highest demand because wave energy is most readily available when the winds are strong. The longer and stronger that the wind blows across the surface of the sea the more wind energy is stored in the form of waves. The wave energy stored in a wave is proportional to the ‘wave height squared’, This formula has dramatic implications for the amount of wave energy stored in bigger waves. A 100 foot wave has 10,000 times more wave energy than a one foot wave.

Hydrolic rams converting energy

Hydrolic rams converting energy

Wind blowing over the sea surface creates a wave. Energy is transferred from the wind into the water in the form of a wave. The amount of wave energy stored in the wave is determined by its size, the speed it’s travelling and water density. Density is important. Compare the wave energy transported by a, light foam across a bowl compared with wave energy transported by a thick viscous liquid. The energy required to form the latter wave is greater and so the wave energy transported is greater too. Wave energy is most apparent on the water surface where the wave energy is imparted into the water medium by the wind. It reduces at depth. The ‘wave energy’’ which travels horizontally across the water and its transport rate along a vertical plane is called the wave energy ‘flux’. Sometimes the term ‘wave power’ is used but that can cause confusion with the term ‘wave power’ used to describe the output of wave power devices after conversion of the wave energy into electricity. The two concepts are different.

Wave Energy for the Environment

Wave energy is potentially the most environmentally friendly source of electricity. It is clean renewable, and is available across the entire globe. The ready availability of wave energy has a partial correlation with absence of solar potential. And the amount of wave energy arriving in any location can be anticipated days in advance by examining weather reports. Owing to the fact that the wave has travelled long distances the energy arriving is often out of sync with the wind and solar energy available that day in the locations the wave reaches. Wave energy can therefore help to overcome variations in output of renewable energy systems across the world. Waves dissipate the energy of the wind across the globe and have a better consistency of delivery than solar and direct wind electricity generation.