Janet Wood presents a guide to the wave and tidal power designs soon to be tested in UK seas
UK INDUSTRY has felt for many years that it ‘lost the lead’ on wind power: it was at the forefront of development in the 1950s, but it is currently mainly a component supplier for wind projects. It is determined to remain at the forefront of wave and tidal exploitation, and the UK government has been willing to help support two new facilities that are intended to shift technology from the research phase to development and ultimately application.
The first centre is at Blyth on the coast of northeast England, close to the UK’s first offshore wind farm. It will house the UK’s most sophisticated wave simulation tank.
While onshore development takes place in northeast England, Scotland’s Western Isles will be home to offshore work in seas that are ultimately likely to have wave and tidal generators in commercial operation. Stromness in Orkney is to house Marine Energy Test Centre, funded by a £400,000 (US$621,193) grant from the Scottish Executive and Highlands and Islands Enterprise. And while research and development stations are being set up at Blyth and on Orkney, several wave and tidal current prototypes are due to be installed around the UK this year.
Limpet and Osprey
Two similar devices developed by Wavegen, known as Limpet and Osprey, use a partially submerged shell. As the water enters or leaves the shell, the level of water in the chamber rises or falls in sympathy. A column of air, contained above the water level, is alternately compressed and decompressed by this movement to generate an alternating stream of high velocity air. The air passes through a Wells turbine, which turns in the same direction regardless of which way the air is flowing across the turbine blades. The turbine is simple: it has no pitching blades or gearbox.
The Limpet version of the technology is sited on the shoreline. A 75kW demonstration device was in successful operation for 10 years and is now decommissioned. A larger version using two 250kW generators – known as the Limpet 500 – has been operational since the end of 2000.
The Osprey 2000 is Wavegen’s nearshore version of the oscillating water column technology, which rests directly on the seabed and is designed to operate in the nearshore environment in a nominal mean water depth of 15m. Rated at 2MW it is expected to feed into an existing grid.
In May 2002, Wavegen won a £2.3M (US$3.6M) grant from the UK’s Department of Trade and Industry (DTI) to support the development and demonstration of offshore floating devices, which will be sited off hte west coast of Scotland.
Marine Current Turbines
The tidal stream generators under development by Marine Current Turbines function similarly to windmills. They will be installed in areas with high tidal current velocities, which the company notes have the advantage of being ‘as predictable as the tides that cause them, unlike wind or wave energy’.
The technology under development consists of twin axial flow rotors 15m to 20m in diameter, each driving a generator via a gearbox. The twin power units of each system are mounted on wing-like extensions either side of a tubular steel monopile some 3m in diameter which is set into a hole drilled into the seabed from a jack-up barge. The company has dealt with the problem of maintaining undersea turbines by a hoist system: the turbines will be lifted clear of the water to enable maintenance to be carried out from surface vessels.
The submerged turbines, which will generally be rated from 600 to 1000kW, will be grouped in arrays or ‘farms’ under the sea, at places with high currents. Compared to wind turbines, marine current turbines of a given power rating are smaller and can be packed closer together, so the company says they involve negligible land use or other environmental impact.
Marine Current Turbines Ltd now plans to design, build and install its first grid-connected marine current turbine, rated at 300kW. The turbine is due to be installed at Lynmouth off the North Devon coast.
The company plans to test and evaluate the experimental unit, in order to assist with the development of a commercial model rated at about 700kW.
In the second phase of its work, Marine Current Turbines aims to install a 4MW array of 700kW by summer 2004.
Stingray is being developed by the Engineering Business, a company which provides equipment and services to offshore businesses including submarine cabling, and the oil and gas industry.
Stingray consists of a hydroplane which moves in an approaching tidal water stream. This causes the supporting arm to oscillate, which in turn forces hydraulic cylinders to extend and retract. The high pressures are used to drive a generator. Following a feasibility study on the design which began in August 2001, the UK DTI awarded the company a £1.6M (US$2.5M) grant which will allow a demonstration project to be carried out and the company plans to install its demonstration plant before the third quarter of 2002.
The site chosen for the project is at Yell Sound, where a current meter installed on the seabed for a month showed a peak Spring tide velocity in excess of 5 knots. The Stingray design that will be installed is 24m high, using a hydroplane some 15m across. It is rated at 150kW and is expected to remain in place for a year before being decommissioned.
On 7 August 2002 Stingray was tested at its operational site at Yell Sound off the Scottish coast. Stingray was ‘dunk’ tested – it was fixed to anchors on the sea bed and stability tests were carried out. It was then raised again and returned to shore to carry out tests on its control and hydraulic systems.
Stingray will have one further ‘dunk’ test before it is finally deployed at Yell Sound at a depth of around 30m.
ocean-power-delivery‘s (OPD) Pelamis system is described by the company as 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 all the joints is fed down a single umbilical cable to a junction on the sea bed. Several devices can be connected and linked to shore through a single seabed cable.
A typical full-scale Pelamis machine would be 150m long and 3.5m in diameter and have an output of 750kW, the company says. In March 2002, OPD secured £6M (US$9.3M) funding from an international consortium of venture capital companies that included Norsk Hydro Technology Ventures, 3i and Zurich-based Sustainable Asset Management. The first full-scale pre-production prototype is due for fabrication and installation later this year. It will be tested at the proposed UK Marine Energy Test Centre in Orkney.
OPD also recently came to an agreement with BC Hydro, the Canadian West Coast utility, to carry out a full feasibility study for a 2MW scheme for installation off Vancouver Island during 2003.
A new device to generate power from wave energy is under development by Devon-based ORECon, a spin-off company from the University of Plymouth’s department of mechanical and marine engineering.
The ORECon design uses the same oscillating water column principle employed in the Stingray shore-based device. An open-based cylindrical chamber is hung vertically in the water; as the wave compresses the captured air in the column it pushes it through a turbine to create electricity. However the OREC version uses six oscillating water columns, so the designers claim it can be tuned to get the maximum energy from a variety of different waves.
In recent tests, individual columns generated between 100W and 800W of power but using multiple columns increased this to between 500W and 2000W. The tests were carried out on a device sized at one-sixth of the likely final version, which would probably be around 30m wide and weigh 200t. It would generate 1.5MW and several would be deployed together to reduce deployment, infrastructure and maintenance costs. The ORECon device joins five others recently tested in seas around the UK. Further development will probably be carried out at the new Orkney test centre.
Offshore Wave Energy
Offshore Wave Energy (OWEL) has developed and patented a new concept of wave energy device which traps and compresses the air in successive wave troughs. The compressed air is accumulated in a reservoir and is then used to drive a turbine. The devices are robust and designed to be installed on floating platforms, mored offshore in sea areas where energetic wave spectra are experienced.
OWEL has completed a successful feasibility study with the aid of a UK government SMART award, and are now seeking investors to take the project through to a development phases.
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|How much power is there?|
| Wave and tidal streams hold tremendous energy potential – but tapping that power and getting it to shore calls for significant engineering development. That means estimates of the useable energy from these sources vary widely.
In Scotland, for example, Professor Ian Bryden, based at Robert Gordon University’s Centre for Environmental Engineering and Sustainable Energy, put some preliminary estimates on the energy available from the North Sea. With the caution that estimates could vary widely depending on the assumptions made, he said his own estimate put the North Sea’s annual potential at 18-25TWh of power from wave resources and 40-50TWh from tidal currents, along with 60-400TWh from offshore wind.
Bryden was cautious about how much of that energy could be accessed with existing technology. The Pentland Firth, he said by example, might provide 400MW using existing technology (in the development phase).
He assessed its practical maximum at 4 GW, but looking to the future he said that with new technology and new structures to export the power, ultimately some 16GW might be retrievable.