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New wave-energy technology

Wave energy is developing rapidly, but perhaps not quite so rapidly as tidal-current systems – in part since it’s harder to develop devices that can extract energy from the chaotic multi-vectored energy pattern that exists at the interface of the sea and the air, than from the smooth laminar tidal flows further down.

However, there are already some clear winners more or less fully developed, like the UK’s Pelamis wave snake, the near shore Oyster hinged wave flap and Wavegens oscillating water column (OWC) system. In addition, Australian company Oceanlinx has developed a variant of the OWC concept and there are many buoy type systems, like that developed by US company OPT. But there are also a host of less well known and sometimes novel ideas emerging and being tested. For example, Wave device developer Wello Oy is testing a 500 kW device at EMEC on the Orkneys. Called the Penguin, it is a floating asymmetric vessel which houses an eccentric rotating mass, mounted on a vertical shaft.

A perhaps simpler approach is to use wave motion to power hydraulic pistons, as with the Sea Dog pump developed by Independent Natural Resources Inc in the USA www.inri.us. (http://www.inri.us) In the UK Ecotricity is backing a bicycle pump-like ‘Searaser’ piston device, which, similarly, pumps sea water to shore, possibly up into a reservoir on a hill, so that electricity can be generated via a turbine when required. They may test a prototype soon off Falmouth: www.ecotricity.co.uk/our-green-energy/our-green-electricity/and-the-sea/searaser

Floats or buoys of various designs are clearly still popular ways to extract energy from the rise and fall of the sea. Some systems, like Clearpowers Wavebob, can be tuned to match different wave frequencies. Perhaps less familiar is the CETO system, which has an array of submerged buoys tethered to seabed pump units. The buoys move in harmony with the passing waves, driving the pumps which pressurise water that is delivered ashore via a pipeline, to drive hydroelectric turbines. The high-pressure water can also be used to supply a reverse osmosis desalination plant, replacing electrically driven pumps usually required for such plants. More at: www.carnegiewave.com/

Somewhat similar is Atmocean’s Wave Energy Sequestration Technology (WEST) system which consists basically of small buoys connected to one another over a stretch of ocean. They are planning to install 10 to 20 units 60 miles off the coast of New Jersey.http://atmocean.com/

A more complex version has been developed by 40southenergy, with a part submerged unit, at a depth between 15 and 25 meters (depending on model type and site) called ‘Lower Member’, and one or more parts submerged at a depth between 1 and 12 meters (depending on sea state) called ‘Upper Members’. The relative motion between Upper Members and Lower Member is converted directly into electricity. A full scale 100 kW prototype, the D100t, has been in the water since Aug 2010. www.40southenergy.com

The conventional OWC concept has also be revisited: Dresser-Rand working with Cranfield University have developed a variable radius turbine called HydroAir, which is said to be more efficient and flexible than the normal two-way Wells turbines used in OWC. www.dresser-rand.com/products/hydroair/

Some devices make use of fixed platforms, with wave energy absorbers underneath, like Buldra system developed by Fred Olsen. Australian company, AquaGen Technologies has come up with a SurgeDriv system, which has a series of floats linked with tension cabling via the seabed and then to a generator on a platform above the sea surface, thus keeping as much of the infrastructure out of the water as possible. The floats can be retracted below the surface to ride out storms. It has a 1.5kW demonstration system at the Lorne Pier in Victoria. www.aquagen.com.au

Portuguese company ‘Sea For Life’ has developed a ‘gravitational wave-energy absorber’, WEGA. It has an articulated suspended body, semi-submerged in the water attached to a mount structure via a rotary head, which allows it to adapt to the direction of the waves: so it oscillates in an elliptical orbit. Power is extracted via an hydraulic cylinder, which pushes high pressure fluid through an accumulator and a motor, to drive a generator. Multiple devices can be placed on a single mount structure. The hydraulic motor and electric generator are on top of the mount structure, which protects them from the elements and enables easy access for maintenance. www.seaforlife.com/

Fully submerged systems also have their attractions – they can follow the circular motion of the waves under the surface. US Air Force Academy researchers in Colorado Springs have demonstrated that submerged energy converters can harness up to 99% of the kinetic energy inherent in an ocean wave. Dr. Stefan Siegel has developed a fully submerged cyclodial wave-energy conversion device, with funding from the National Science Foundation.

Ideas for smoothing out the energy absorbed from waves are also being explored. For example, the floating Danish Waveplane has a series of slots designed to catch waves at different heights, the captured flows then being used to create a vortex to drive a turbine. In the UK, Ecotricity are backing the Snapper linear motor wave unit invented by Prof. Ed Spooner at Edinburgh University. It has magnetically tripped springs storing burst of energy. The project is being co-ordinated by Narec with an EC FP7 grant.

Finally, Danish company Floating Power Plant is developing a 10 MW commercial version of their Poseidan prototype hybrid wave/wind device. Poseidon is basically a floating, anchored, platform, which can accommodate both wave-energy converters and wind turbines. It is claimed that it can achieve an efficiency in transforming inherent wave energy to electricity of 35%, and should be able generate 28 GWh per year if located in the Portuguese part of the Atlantic Ocean. A 37 meter wide 25 metre long 350 tonne model was tested at the Vindeby wind park, off Lolland coast in 2008. The wave system is based on a hydraulic power take-off system, using a double function piston pump to transform the energy from the wave into water pressure that is then sent through a turbine, thus generating electricity. www.floatingpowerplant.com

The wave-energy field is clearly still is a state of creative flux, with many rival ideas under test – the above is just a sample. It will be interesting to see which pan out.

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