Introduction, How much power can you get from a wave, Current Technology, Buoyant Moored Device, Hinged Contour Device, Oscillating Water Column (OWC), Economics,Social Implications, Environmental Aspects, Conclusions
Waves are a result of the effects of wind on the oceans and seas. This wind originates from the major influx of energy to this planet: solar energy from the sun. The energy contained within waves is around the world is huge; in some places values of 70MW/km of wave front are experienced. In theory it could then be said that huge generating stations could be built which would capture all this energy and supply all or most of our needs. But there are many factors affecting this kind of deployment becoming a reality.
Waves are not as consistent as the tide and therefore there is a definite problem with matching supply and demand. This is one of the main reasons that Wave power has so far been restricted to small scale schemes, no large scale commercial plant is in action.
Identifying areas of suitable wave height is something that has to be done before deployment can start. The highest concentration of wind power is found in the windiest areas, which are mainly between latitudes 40 and 60 in both northern and southern hemispheres. We are lucky in the Scotland to have an abundance of Wave Energy available, mainly on the west coast. The following diagram shows the variation in wave height around Britain during the Christmas Eve, 50 year storm of 1999. The technology must be able to withstand the freak wave heights that can be experienced, in rough and remote locations where access can be difficult.
There are three main categories that wave power can be split into, these are Near Shore, At Shore and Off Shore. There are obvious environmental and social considerations to go with both of these conditions.
Near Shore operations have to consider the aesthetic influence they will have on what could be a picturesque area, they also will have a definite impact on shipping and marine life but again this will be no greater than current offshore installations. It has been suggested that a distance of 12 miles from shore is the distance within which a device is said to be near shore.
The issues discussed previously will also obviously be experienced by off shore wave installations. It has been suggested that a depth greater than 50m will constitute an offshore device.
On shore wave power will have a marked effect on the area it is deployed. There are ways of incorporating it into existing structures to minimise the effect, such as harbour walls.
Linear wave theory assumes that the motion of the water past a point is sinusoidal. The period (T) for one wave to pass this point can be expressed by:
Where, l = wavelength (m)
g = gravity = 9.81
The power contained in the wave can be expressed in terms of the length of the wave (kW/m). This is given by the following equation:
Where, a = Wave amplitude (m)
According to the DTI, there are three types of wave energy collector. These are:
· Buoyant Moored Device
· Hinged Contour Device
· Oscillating Water Column
This type of device floats on the surface of the water or below it. It is moored to the seabed by either a taught or loose mooring system. One example of this type of device will be discussed, the Edinburgh or Salter Duck. The Duck team is led by Professor Salter at Edinburgh University.
The Duck is shown in the figure below. Ducks work by independently rotating about a long linkage; this maintains its stability by out spanning wave crests. The front edge of the duck matches the wave particle motion. In moderate seas, the more cylindrical back portion creates no stern waves but when the weather is bad these parts shed energy through wave making to the rear. The device requires a depth of at least 80 metres and uses a system of weights and floats to give almost constant tension in the mooring cables.
Sourced: www.fujita.com ã1996 Ramage
This type of device follows the motion of the waves; it creates power using the motion at the joints. It is commonly moored slackly to hold it in place. One example of this type of device is the Pelamis WEC that is being developed by Ocean Power Delivery.
As the Pelamis moves with the waves, the motion is resisted at the joints by hydraulic rams that pump high-pressure oil through hydraulic motors via smoothing accumulators. These motors are used to drive generators to create power. It has been said that a 750kW device would be 150m long and 3.5m in diameter and comprise five sections.
Sourced: Wave energy: Technology transfer and generic R + D recommendations, DTI Pub/URN 01/799
This method of generating power from the tide works by using a column of water as a piston to pump air and drive a turbine to generate power. This type of device can be fixed to the seabed or installed on shore.
In Scotland, the Government awarded three wave energy projects under the Scottish Renewables Obligation. Only one of these projects has been realised and is generating power in Scotland as this pack is being written, this is the LIMPET 500 on the Island of Islay of the west coast, enabling the Island to take a step towards becoming self sufficient in renewable energy. For more information see the LIMPET Case Study.
How the LIMPET 500 works
At present, the main stumbling block to deployment of wave energy devices is funding. The Government has a very important role to play if this industry is to be given the chance to fulfil its potential. The capital costs are the problem, as it is hard to get companies to invest in technologies that have not yet been completely proved. Similar to other forms of renewable energy sources such as wind and solar, the fuel is free for the complete lifetime of the scheme.
Wave devices that are on-shore have social implications for the surrounding area. They can be integrated within harbour walls, which can affect shipping and cause noise pollution. They can create employment in the area and attract visitors.
Offshore devices have an effect on navigation and consultation with affected bodies must be undertaken. The experiences of other offshore industries, such as oil, should aid this part of planning for wave devices.
There can be environmental impacts resulting from wave powered devices. But, like other renewables, these impacts must be compared to the effects of fossil or nuclear generation.
Devices that are on-shore can have environmental benefits, such as helping to reduce the erosion of the landscape. Any devices off shore can have an effect on the aquatic life in that area but this again is very site specific and hard to predict. But anchoring systems can become almost like artificial reefs, creating a place for new colonisation.
Wave power has a potential to play an important part in the long-term goal of utilising renewable energy in Scotland. The deployment of the LIMPET 500 has brought recognition to the technology available in Scotland. This interest will stimulate the growth of the industry allowing other technologies to advance and realise their potential. Until they become economically viable and more competitive with other renewables such as wind, it is more likely that wave powered generation will supply islands or small communities within Scotland.
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