Wave Anatomy

Analysing A Wave To Find The Optimum Locations And Design

Project Context

The west coast of Scotland is home to some of the most powerful and consistent wave generating potential in the world. Despite 11,800Km of coastline and the possibility to produce between 40-70KWh per meter, we do not currently use even 1% of this free powerful resource, yet we continue to develop fossil fuel power stations.

 Aqua Power Technologies Wave Anatomy

 

Wave Overview

In order to generate power from waves, it is important to know the anatomy of waves, where they occur and their frequency. The key information to acquire, in order to start designing a method of harnessing wave power, is knowing which part of the wave generates the power and where the optimum wave can be found. In essence, as the wind blows over the water, small ripples on the water surface allow the wind some purchase to push against; from there the wave grows, in order to become ‘chop’. As the wind increases and continues to blow, the waves start to become medium in size and stature and, if the increase in wind persists, waves become large and powerful. The energy in a wave originates in the wind and is transferred to the wave when the wind blows over the water’s surface. In order for the energy to transfer from the wind into a wave, the following attributes are required.

• The distance that the wind blows over open water, commonly known as the ‘fetch’.

• The length of time the wind blows over open water.

• The speed of the wind blowing over the open water.

 

A Waves Necessity For Wind

Surrounding the Earth are areas of high pressure and areas of low pressure in the atmosphere. Areas of high pressure are constantly attempting to fill the areas of low pressure. This transition is what causes the production of wind. The movement of high pressure air into low pressure areas causes the air to move, thus creating wind. Essentially, the location for a wave generation site not only depends on waves but more importantly wind. Without wind, there are no waves.

Low pressure areas are usually created along the boundaries between warm and cold air, where a small amount of atmosphere has been removed from a region. Low pressure can be further enhanced if an air column above it is warmed by condensation of water vapour, for instance in large rain or snow systems. This warming in turn causes the air layer to expand upward and outward, removing some of the air from the column. This extraction of air in the column creates low pressure, which is subsequently filled by high pressure. This movement of high to low pressure creates the wind for waves to form.

A wave becomes more powerful as the wind blows harder, for a longer time, over a greater distance. All three attributes are needed to create powerful waves. If one attribute is not apparent, the wave formation will not occur to its full potential. Naturally, the largest waves occur in vast open expanses of water, where these three effects cumulate. As waves grow in size, the distance between individual waves (referred to as wavelength) becomes greater; this signifies that the waves have stored energy deeper into the body of water. If more energy is transferred deeper into the water, the waves have a better ability to sustain the energy over greater distances.

 

Wave Geometry Analysis

Measuring the wave length allows the wave’s size and potential energy to be revealed. To measure a wave, the time between successive wave crests is taken from a stationary point. This is commonly referred to as measuring the swell period. The direction a wave travels is determined by the direction in which the wind and swell is traveling. A change in wind direction over well-formed waves creates wave decay, which, for the purpose of this project, would not be desirable. Naturally, as the wind blows into the wave, it acts as a brake and essentially dampens the wave, reducing any energy that was initially in the wave.

If a wave has a period swell of less than 14 seconds between wave crests, it is considered susceptible to wave decay from opposing winds. These short period swell waves also create a much steeper wave compared to the larger period swells. In direct comparison, long period swells, which are greater than 14 seconds, travel with more energy below the water surface and have a lower wave height profile. This formation allows larger swell period waves to easily pass through opposing winds and tides with very little effect. Swells travel as a group of waves called ‘wave trains’ and, as the swell moves forward, the wave at the front of the wave train slows down and drops to the rear of the group, whilst the other waves move forward one position. This method of moving forward helps to conserve the energy within the swell and also allows the waves to travel great distances. The speed of a wave train is another aspect that has to be taken into consideration when attempting to harness the energy within waves. Since the wave train can be likened to a conveyor belt constantly moving forwards, the speed of the individual waves in the wave train travel twice as fast as the wave train.

Usually in front of the wave train sits the forerunners, which are the initial long period waves that travel faster than the main wave train. Usually forerunners are pulses of energy, with 18-20 second periods between pulses. The forerunner waves themselves are relatively small and only rise a few inches above the main body of water. It should be noted that the further a wave train travels, the more the distance between the forerunner and wave train increases. Since these forerunners have a relatively small height between their peak and trough, the potential energy harness is low. Although not terribly useful for energy generation, these forerunners allow marine forecasters the opportunity to discover new swells before the main wave train arrives. The depth of water that the waves are in also plays a part in the formation of the wave train. The depth at which the wave begins to feel the ocean floor is half the distance between wave crests. Wave length and swell period are directly related and therefore become useful when calculating the exact depth at which the wave will be affected by the ocean floor.

Naturally, in shallower water, longer period swells are affected by the ocean floor much more than shorter swell period wave trains. As a result, long-period swells are considered to be 14 seconds or more and short period swells up to 14 seconds or less. Long period swells usually originate hundreds of miles out at sea and the short period swells can be generated by the locally generated winds. Since the short period swells are smaller, they decay at a much faster rate and do not have the energy to travel as far as long period waves. As a wave train approaches land, the ocean floor starts to rise and, depending on the length of the swell period, the waves will feel the ocean floor at different distances from land. This is where shoaling starts to affect the waves and increases the wave height before crashing and decaying on to land at a rapid rate. As a wave nears land, the lower parts of the wave start to drag on the ocean floor and in turn slow the wave down. The resultant effect is an increase in the height of the wave.

Finally, the longer the swell period, the more energy the wave has under the water to push the wave higher. For example, a 3 foot wave with a 10 second swell period might generate a 4 foot breaking wave. In comparison, a 3 foot wave with a 20 second swell period can transform to a 15 foot wave as it breaks on the shore. As a wave moves into shallower water, the wave forces its energy upwards creating unstable waves that have great height. Alongside the altered wave height, the speed of the wave train is slowed down, and the waves in the train bunch up together. This type of wave does hold energy, but not to the same extent as the long period swell waves out in deeper water.