Demonstrating their worth - wave power technologies13 August 2008
The development of marine energy devices has begun in earnest worldwide. Here a few companies explain how their work is progressing
Aquamarine Power, Scotland
Aquamarine Power was established in 2005 and is developing both wave and tidal devices simultaneously. The firm’s Oyster wave energy device is the result of several years’ testing and development in partnership with Queen’s University Belfast. The first full-scale working version of this near-shore wave energy converter is now fabricated and ready for testing prior to installation and grid connection at the European Marine Energy Centre (EMEC) on the western edge of the Orkney mainland in Scotland.The principle behind Oyster is simple. The system consists of a steel oscillating wave surge converter, or pump, fitted with double acting water pistons, deployed near the shore in depths of around 10m. Each passing wave activates the pump which delivers high pressure water via a sub-sea pipeline to the shore. Onshore, high pressure water is converted to electrical power using conventional hydroelectric generators.
The peak power generated by each Oyster unit is between 300-600kw, depending on location and configuration. When deployed in multi-megawatt arrays, several near-shore pumps will feed a single onshore hydroelectric generator, attached to a single manifold pipeline. It has the capacity for building a single onshore generating plant with an installed capacity of 21MW or higher.
The company is also developing the Neptune tidal device. At 2.4MW, Neptune is one of the most powerful tidal stream devices under development, and is designed to be competitive with other sources of electricity generation in the UK market. Using largely proven technology, the device consists of two horizontal axis tidal turbines, which will be mounted on a single monopole for the commercial demonstrator. The device features bi-directional (flood and ebb) generation, and its design is heavily influenced by the use of proven components from the wind turbine industry.
Once installed, Neptune is designed to use only relatively small workboats for maintenance.
Over the last year Oyster has progressed from the wave tank to a full-scale prototype. Work continues on the Neptune tidal device.
Applied Technologies Company, Russia
Applied Technologies Company was established in Moscow in 1991. It is developing an offshore float wave electric power station (FWEPS) – an oblong, axially symmetric capsule that can float vertically on the sea surface. The main elements of the module are its mechanical oscillatory drive and electric generator. To increase the converter’s efficiency, the latter incorporates auxiliary energy storage. All units are maintained inside the sealed capsule-float. It is possible to develop both a single modular FWEPS for power up to 50kW and multi-modular plant designed for greater megawatts.
FWEPS pilot modules and assembly units are at the stage of manufacturing, adjustment and field test preparation.
AWS Ocean Energy, Scotland
It is well known that the energy absorbed by a wave power system is proportional to the swept volume of the device – the more water that is affected, the greater the energy absorbed. The majority of devices rely on buoyancy effects to generate force and achieve swept volume through ‘bobbing’, ‘nodding’ or some other form of resonant interaction with the waves. These resonant interactions tend to be limited in both amplitude and more importantly bandwidth, the latter being important as ocean waves come in all shapes and sizes.
AWS Ocean’s Archimedes Waveswing energy system, however operates in a completely different way – it is a compressible sub-sea device that expands and contracts in response to the changes in pressure exerted by a passing wave. Therefore the motion (and hence swept volume) is not limited to wave height, but is related to the change in relative pressure. In this way, the Waveswing can achieve motions considerably in excess of wave height – thus achieving a true ‘point absorber’ effect. In terms of bandwidth, the resonance of the Waveswing is dependent on an active mechanical spring rather than a passive hydrodynamic spring as is the case with a simple floating buoy. Furthermore, the correct spring constant is independent of draft – again a step forward from a floating buoy.
From its Alness base in the Scottish Highlands, AWS Ocean Energy is now developing a 250kW demonstrator unit which will be tested at the European Marine Energy Centre. This unit will have a diameter of 8m and will be moored in 50m of water via a tension leg. The crown of the floater will be submerged 9m below low tide. The interior of the device is at low pressure with the hydrostatic forces being balanced by a combination of high pressure gas springs and active hydraulics. Power generation is via hydraulically driven induction generators.
The company has backing from Shell Technology Ventures and institutional investors, plus a £2.1M (US$4.1M) grant from the Scottish Government which will assist with the construction of the demonstrator unit.
Float Incorporated, US
The Rho-Cee wave energy converter (WEC) system is a large, floating oscillating water column to be moored in deep water. The Rho-Cee is integrated with Float’s pneumatically stabilised platform (PSP) to take advantage of controllable stability, load capacity and the deck area that it provides.
The name Rho-Cee derives from the expression for the characteristic impedance of water gravity waves; the product of water mass density, r, with the length-dependent velocity of such waves, C. It is the base principle of the WEC design that its input impedance matches the characteristic impedance of the targeted waves. Impedance matching maximises the capture of wave energy; with minimum reflection.
Several restraints dictate that the input impedances of the absorber elements be quite small. This requires resonant operation of the OWCs. Hence, several water columns are tuned to frequencies, with bandwidths, that span the energetic region of the yearly average incident wave power spectrum. The normalised bandwidths govern both the resistive input impedance and the output power potential of each oscillator.
The successively tuned water columns are geometrically nested to minimise space and weight of materials – hence cost. The nested units are then repeated, endwise, to form a linear array of identical contiguous WECs in a two-dimensional configuration – one that is aligned perpendicular to the usual propagation direction of incident waves. All wave energy absorption takes place along the face length of the Rho-Cee. The company expects to deliver 60% of incident power; that is, a wave-to-wire efficiency three times that of any other extant system.
Float is currently proposing to develop a 30MW, 1250m long Rho-Cee floating breakwater for installation in Northern Iberia. The Float PSP is now being considered for the basic platform in the Urbanisme sur Mer programme of the Principality of Monaco.
Ocean Wave Energy Company, US
The Ocean Wave Energy Company was established in the US in 1978, following the invention of the first wave activated linear electrical generator (LEG). Three LEGs are co-assembled as the edge elements of a submerged tetrahedron module with upper portions housed in a common buoyancy chamber. Electricity is produced as other reciprocal LEG parts, mounted on respective shafts of large point absorber buoys, are directly driven by buoyancy and gravity forces from impinging waves. Electrical output is combined within and between modules of an interconnected array.
The company’s Ocean Wave Energy Converter is a minimal system constructed to provide low cost modules that form horizontal open web trusses. In the ocean, discrete module-to-module neutral buoyancy and damper sheets accommodate very wide planar distribution of self-supported arrays. The spaced apart floating and neutrally suspended modules thrive in dominant interference waves. One Ocean Wave Energy Converter may not be able to absorb ambient waves but other units encounter field energy while allowing regeneration with an interconnected web. Connector tolerances allow some flexure as concentrated loads are radially dispersed in the structural matrix.
The Ocean Wave Energy Web includes sensor/control networks that predict and map diffuse wave conditions in advance of particular buoys for optimal pre-tuning of submergence, power take-off loads, or module ballast. Nearly all buoyancy and gravity forces are extracted while closely following hydroface fluctuations. Scale variation of buoy volume, module height, and quantity may correspond to all range of wave conditions or power requirements.
Ocean Wave Energy Company recently purchased a manufacturing facility to perform generator control system tests. It will deploy several large Ocean Wave Energy Converter modules for an extended duration in active seas. Stakeholder negotiations are still underway with respect to installation sites in state and federal waters in the US.
Trident Energy, England
Trident Energy is currently finalising the launch of its first offshore trials and will be deploying a system off the east coast of England during the second half of 2008.
The company’s unique system, known as the direct energy conversion method (DECM), uses only one moving part which makes it significantly simple to construct, install and maintain, says the company.
Trident Energy was founded in December 2003. Hugh Peter Kelly, founder of the company, had previously invented the tubular linear motor which is an internationally established product in the automation industry. This invention led to the development of a prototype system which would convert the power of sea waves directly into electricity. An initial experiment in the late 1990s in the Atlantic off the north coast of Devon was immediately successful, generating electricity directly from the sea. This led to a study being commissioned by the Department of Trade and Industry and undertaken by Durham University in 1999, into the effectiveness of using linear generators as an alternative to hydraulics for wave energy capture.
The commercial viability of the system was confirmed following a series of wave tank tests at the New and Renewable Energy Centre (NaREC) in July 2005. Securing additional private equity and a research and development grant from the East of England Development Authority in April 2007, Trident Energy launched its offshore test project. The first stage of the project was to design and test a scale version of its offshore test platform at NaREC, which was successfully completed in July 2007. The company is now at the final stage of the project looking to deploy an offshore system during the second half of 2008.
The DECM is based around proprietary technology developed by Trident Energy and established technologies used within the oil and gas market. With only one principal moving part per converter, Trident Energy’s DECM is described as the simplest marine renewable energy generation system that exists. Electricity is generated without the use of hydraulic equipment or air compression. Furthermore, Trident Energy’s technology has a patented self-protection mechanism, which automatically retracts the floats from the sea in storm conditions storing them in protective chambers above the water line. Survivability in adverse weather conditions is key to the long term viability of any sea wave energy conversion system.
The company is currently in the final stages of preparing for the deployment of a fully functional test rig off the Suffolk coast. The year-long sea trial will gather detailed data on the performance of the new technology, demonstrating its robustness under a variety of challenging weather conditions, prior to the design and deployment of the first full-scale commercial system.
wavegen – a member of the Ocean Energy Division of voith-siemens Hydro Power Generation – uses Oscillating Water Column (OWC) technology with a Wells turbine power take-off. In 2000 Wavegen commissioned Limpet, the world’s first grid connected commercial-scale wave energy plant. This plant, located on the island of Islay off the west coast of Scotland, is used for Wavegen’s extensive turbine development programme.
wavegen has accumulated eight years of plant operation including 30,000 generating hours. Currently prototype turbines for projects in Spain and in the UK are being tested. Availability levels approaching power industry norms are been achieved.
The Spanish project involves incorporating a wave energy plant into a new outer breakwater which is being built in the small town of Mutriku in the Basque Country in Northern Spain. Wavegen is supplying the complete mechanical and electrical equipment and commissioning is scheduled for early 2009. The wave energy plant will be owned and operated by the Basque Energy Board (EVE) and is supported under the EU FP6 Programme.
In the UK, Wavegen is working with npower renewables who has submitted a planning application to the Scottish Government for the Siadar Wave Energy Project (SWEP). The project will be located in the coastal waters near Siadar, on the Isle of Lewis, off the west coast of Scotland. The scheme will consist of a breakwater type structure up to 250m in length. The 4MW plant will consist of 40 100kW turbines, with commissioning planned for late 2010.
The company says its operational experience of grid connected plant and extensive plant development programme allows it to supply reliable proven plant for incorporation into new and existing breakwater structures.