Short Cuts for:

Marine Current

There are a range of differing designs that have been developed to extract energy from marine currents. Many are still at the concept phase, while others are at the stage of full scale testing and initial commercial deployment.

The technology

Marine currents (‘tidal streams’) are generated by both tidal and non-tidal forces which are generally diffuse but can become concentrated in certain coastal areas. The UK is fortunate in that there are significant marine current resources around the coastline of the UK.The drivers for this resource are mainly gravitational and can be accurately predicted many months, or years, in advance. This means that the issue of intermittency is not a problem and this will allow for the easier integration of generated power into the national grid system.

There are a number of available methods utilised by differing technologies to extract energy from marine currents with the most developed being:

Horizontal Axis turbines - These devices utilise a design similar in principle to that used by wind energy, with turbines being set normal to the direction of flow. However water is ~1,000 times denser than air and these properties allow for far greater economies of scale compared against wind turbines. This means that turbine diameters can be much smaller and still achieve high load factors. Seagen (Marine Current Turbines Fig 1) and Tidel (SMD hydrovision) are two of the leading designs utilising this technology

 

 

Hydroplane - For this technology the attack angle of the hydroplane(s) are set relative to the water current, the resulting lift and drag forces will cause the hydroplanes and arm to oscillate vertically. This movement can be used to drive a hydraulic motor. The stingray (Fig 2) concept (manufactured by the Engineering Business) is currently the only developed hydroplane device. It is designed to be fully submerged and remain in place on the seabed by means of a gravity base system involving ballast and earth anchors

Applications

At this stage there are a number of full scale demonstrator projects that are undergoing field testing. It is expected that full scale commercial deployment will be a reality before 2010. The high installation costs and effort of many current designs means that larger arrays will be feasible due to economies of scale.

 

Wave

There are a range of differing designs that have been developed to extract energy from wave resources. Many technologies are at the early stages in development and streamlining is likely to occur over time with fewer designs taken onto commercial deployment.

The technology

Wave energy is often thought of as a derived form of solar energy as waves are formed by wind passing over the water, and wind in turn is generated by variations in pressure caused by the differential heating of the earth’s surface. The energy of a wave will depend on the wind speed, amount of time for which it blows and the distance over which the wave can travel (fetch)

The areas of highest average wave energy tend to be where storms are common are known as the temperate zones. The UK has one of the most energetic wave climates on the planet, due to its position within the Northern temperate zone and much of the British Atlantic coast is open to the prevailing south-westerly winds and considerable fetch of the Atlantic Ocean.

In shallower areas energy is lost due to friction with the seabed and the waves breaking. Data taken from the Atlantic coast of Scotland shows that deep water waves have power levels of around 60-70kW/m, while at the shoreline this falls to 15-20kW/m. The most promising areas for commercial use are therefore those sites with the highest power levels (deepest waters), which are also close to shore (increased accessibility). Sites of this nature have been noted off the SW coast of England and Northern and western coasts of Scotland.

There is an abundant potential resource open to the UK and companies have developed several different methods to utilise this energy. These include technologies that harness wave power on the shoreline, near the shore or offshore:

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  Fig 3: OWC
  Shoreline Oscillating Wave Column (OWC - Fig 3): This consists of a partially hollow semi-submerged (collector) structure which is open below the water-line. Waves will cause the height of the water level to rise and thus force out air (at pressure) through an air turbine. The limpet is a design by Wavegen and a working demonstrator is operational on Islay
  
  
  
  

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  Fig 4. Wavedragon
  Nearshore Overtopping device (Fig 4): This design stores water from surging waves in a raised reservoir (above sea level). The potential energy is then extracted as this stored water is returned to the sea using multiple low-head variable speed turbines, similar shape to those seen in hydro schemes
  
  
  
  
  
  

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  Fig 5: Pelamis
  Offshore pitching/heaving devices: These generate power from the oscillating action caused by wave movement. These devices can be fully, or semi-submerged and include a range of design types. The most advanced concepts (in terms of stage in development) are the Pelamis (Fig 5) (constructed by Ocean Power Delivery), Archimedes Wave Swing and Aquabuoy

 

Applications

At this stage there are a number of full scale demonstrator and pre commercial projects underway. The first full scale commercial deployment will be the Pelamis array in Portugal. The high installation costs and effort of many current designs means that most applications will be large scale commercial designs with limited scope for smaller scale projects at this stage.