Marine Energy Devices
Marine Current Turbines General Marine Current Device Description
Water currents offer an analogous energy resource to wind. This background knowledge led to the development of underwater windmills now popularly known as Marine Current Turbines (MCT). MCT’s extract the kinetic energy from the large volume of flowing sea water and by the help of generators convert them into electrical energy.
Main component of a marine current turbine
A marine current turbine is usually a turbine rotor (15 to 20 meters in diameter) that is set normal to the flow direction and mounted on wing – like extensions from either side of tabular steel monopile some 3 meters in diameter which is set into a hole drilled into the seabed from a jack- up barge.
Marine Current Turbines can either be a horizontal axis (propeller or axial flow) type or a vertical axis (darrius or cross flow) type. The fixed structure types like the one described in the paragraph above are used for shallow water installations usually 20 – 30m deep whiles floating design types with mooring systems are suitable for both shallow and deep water installations usually to depth of 50meters and more.
Unlike wind turbines, rotational speeds of marine current turbines are lower but owing to the higher density of sea water (approximately 8times that of air) the overall power outputs are higher.
Another similarity with wind turbines is the number of turbine blades. The number of MCT blades vary but those under development currently either 2 or 3 for the best trade offs between cost and maximum energy extraction.
Marine current turbines do not work in isolation to generated electricity; they are couple to a generator set. However because the desired output speed to the generator must be high, the low speed of the rotating turbine shaft is converted to the desired speed by a gearbox. The generator then converts the shaft’s rotational energy to electric energy and this is then transmitted onshore via under water cables to a transformer and then on into the grid.
Marine Current Turbine Types
2 bladed – 2 rotor Turbine Type By Marine Current Turbine Limited
he uniqueness of Marine Current Turbine Limited’s turbine type lies in its maintenance technology. The design is patented and is such that the turbines can be installed and maintained completely without the need for any major underwater works.
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3 - Bladed Turbine Type by Hammerfest Strom AS in Norway
The 3 – bladed design by Hammerfest Strom AS is similar to the 2 – bladed type described above by Marine Current Turbine Limited. Their design also incorporates a modular design that allows critical components to be lifted out of the water for maintenance and repair.
Ducted Marine Turbine Type by Lunar Energy
In addition to the use of a 3 – bladed technology, the Rotech Tidal Turbine by Lunar Energy has its rotor ducted. The Venturi shaped duct accelerates the fluid through the turbine, increasing the energy that can be captured by the turbine blades for a given diameter. This design also consists of a foundation from a large concrete plinth that acts as a foundation anchor thus reducing seabed preparation.
References:
Seaflow marine current turbine type: www.marineturbines.com/projects.htm
3 – Bladed marine current turbine type: www.e-tidevannsenergi.com/index.htm
Ducted marine current turbine type: www.lunarenergy.co.uk/technology-description.htm
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Scottish Marine Energy Devices under Development
Pelamis Wave Energy Converter
The Pelamis is being developed by Ocean Power Delivery based in Edinburgh. The pelamis is a semi – submerged, articulated structure composed of cylindrical sections linked by hinged joints. The wave – induced motion of these joints is resisted by hydraulic rams, which pump high – pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. Power from the joints is fed down a single umbilical cable to a junction on a sea bed. Several devices can be connected together and linked to shore through a single seabed cable.
Specifications:
Total Device Length: 120m
Device Diameter: 3.5m
Centerline Device Spacing: 150m (2-3 Rows stacked)
Structural Steel Weight: 380tons
Rated Power: 750kW
Water Depth: >50m
Power Take Off: Hydraulic using bio-degradable fluids
References:
Ocean Power Delivery Limited
www.oceanpd.com/Pelamis/default.html
www.oceanpd.com/PDFS/E21%20EPRI%20Assessment.pdf
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LIMPET
Wavegen, an Inverness based company is the developer of the Limpet which is the acronym for Land Installed Marine Powered Energy Transformer. Limpet is a shoreline wave energy converter sited on the Island of Islay, off Scotland’s west coast and uses the principle of an oscillating water column (OWC).
The Limpet unit has an inclined oscillating water column that couples with the surge dominated wave filed adjacent to the shore. The typical water depth to the OWC is seven meters. The design of the air chamber is important to maximize the capture of wave energy and conversion to pneumatic power. The turbines are carefully matched to the air chamber to maximize power output.
The performance has been optimized for annual wave intensities of between 15 and 25Kw/m. The water column feed a pair of counter – rotating turbines of which drives a 250kW generator giving a rating of 500kW.
Reference:
Limpet description
www.wavegen.co.uk/what_we_offer_limpet_islay.htm
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