Current affairs

Eric Russell reports on the development of tidal stream, the hitherto overlooked sustainable energy phenomenon

There is one source of renewable energy which, while it has attracted little media attention thus far, has more benefits than those currently making the headlines. It offers power which is predictable, plentiful, reliable, and available on the generating site – yet the source is silent, invisible and non-polluting.

This least researched of all energy environments is to be found in areas previously only of interest to deep-sea fishermen and submariners. Known as tidal stream, it lies in a layer between the floors of large rivers and seas, to a depth of about 20m below the surface.

The streams are created by movement of the tides, often magnified by local topographical features such as headlands and straits, in much the same way as wind funnels through narrow valleys and around hills.

The underwater flow can drive generators similar to wind turbines, but the energy is much more concentrated than it is with wind power. A typical five-knot current equates to a wind velocity of 270km/hr.

Tidal stream turbines can therefore be relatively small. A 1MW tidal turbine propeller, for example, would be less than 20m in diameter, compared to the 60m of a comparable wind turbine. Tidal turbines can also be packed more closely together than on wind farms. The power density of a tidal turbine farm is about 50-100MW/km2, compared with perhaps 10-20MW/km2 for a wind farm.

And there is plenty of energy available. Last year, the European Union identified 106 sites suitable for tidal turbines, 42 of them around the UK. One assessment says that tidal stream in the Pentland Firth could eventually deliver 15 terawatt hours of electricity a year – nearly two thirds of Scotland’s needs.

The DTI says the UK’s first large-scale wave and tidal power generation farms could be contributing to the national grid within three years under its £42 million support scheme funded from the Marine Research Deployment Fund.

The Fund helped finance the European Marine Energy Centre (EMEC), at Stromness in the Orkneys, opened in 2004 for the testing of wave energy conversion devices. This year, the centre is being extended to include tidal energy.

The new project is sited at the Fall of Warness, off the island of Eday. It comprises four test berths, each with a dedicated 11kV cable connecting back to the local grid and a fibre-optic communications link to carry control and monitoring data.

Tidal stream, it should be emphasised, is a completely different energy resource to wave energy. Wave power yields too little energy to be a significant source for countries bordering the North Sea and the Mediterranean, quite apart from the issue of the hostile surface environment, which subjects devices to the slam-loads of gale-driven waves.

Tidal stream has advantages over more established alternative energy sources. Wind power is unpredictable and unreliable – and the installations are visually intrusive. Solar power, meanwhile, is proving practical only for individual installations. Serious hydroelectric power is limited to specific locations. Biomass still has to make a major contribution to the reduction of greenhouse gases.

Of course, tidal stream has its detractors, from those who say it’s too expensive to those, on the environmental side, who fear that marine animals could be killed by tidal turbine blades. According to tidal stream experts, the latter concern is unfounded. Notwithstanding that marine life has the capacity to avoid underwater objects in the first place, the rotating turbine blades generally turn once every four seconds, so the tips travel at similar speeds to fish. This is quite different from the chopping effect of ships’ propellers.

World first

A number of projects are in development across the globe, though the world’s first tidal stream turbine was built in Britain. Seaflow, a 300kW unit sited in the Bristol Channel, was the work of Marine Current Turbines.

Joe Verdi, commercial director, said: “The installation is still exceeding expectations and energy capture has been up to 25% better than expected. It has also successfully negotiated two winters, and the results persuaded company backers to invest further. We’ve learned valuable lessons which we’ll be applying in our next project, Seagen – that should be contributing 1MW to the national grid by 2006.”

Seagen will be sited in Northern Ireland at Strangford Lough Narrows, a 400m-wide channel. The DTI is contributing £3.85m towards the £8 million total cost. The generator, mounted on a pile set in the sea bed, can be jacked up clear of the water for maintenance. Verdi says costs are now comparable to offshore wind power but further cuts are possible.

Scottish company Rotech is developing a bi-directional turbine based on the venturi principle and to be sold worldwide by Lunar Energy. The total installation will weigh in at about 1,000 tonnes, including foundations. The bi-directional venturi casing has an inlet diameter of 21m, narrowing to 14m, and a five-bladed fixed-pitch propeller of innovative blade form. This will drive a sealed hydraulic pump and an ABB 11kV alternator pack designed for four-year maintenance periods. It will be tested at the EMEC site.

The DTI has granted £2.7m to SMD Hydrovision to further develop its TidEl system. A 1MW unit will be installed at EMEC in 2006. Each unit is positively buoyant and comprises two turbines on a steel frame held down to the sea bed with steel ropes. At dead water, the turbines are angled vertically. As the tidal stream builds up, it forces the turbines to face into the stream. When maintenance is needed, the ropes are released, the unit floats to the surface and it’s towed to a servicing facility.

In Wales, Tidal Hydraulic Generators is planning a 3.5MW turbine system for the Ramsey Sound. The Sound is 50-70m deep and has a six-knot maximum flow. The DTI-supported project also has the backing of the Welsh Assembly.

The prototype device comprises a seabed-fixed frame supporting five turbines. The Glasgow-based Babtie Group is designing the units. “If the project is successful,” says David Baird, the division’s managing director, “there will be commercial potential for similar systems across Scotland.”

All over the world

Such developments are not, of course, confined to the UK. Blue Energy Canada is currently developing its Davis Hydro Turbine, whose hydrofoil blades employ a hydrodynamic lift principle, causing the turbine foils to move proportionately faster than the speed of the surrounding water.

Meanwhile, in the north Norwegian town of Hammerfest, the first commercial tidal power plant is already providing enough energy for 30 homes. The pilot scheme, set up by Hammerfest Stroem, could eventually be expanded into a park with 20 turbines.

Professor Ian Bryden, of Robert Gordon University, is concerned that research should focus on engineering systems rather than the undersea environment. “There is a huge amount still to be learnt about the shear flows, storm surges and turbulence in the tidal stream layer,” he says.

His department has been developing a software programme to model undersea environments. Tidesim uses computational fluid dynamics (CFD) technology to model the conditions tidal stream turbines could meet.

The department has also developed Sea Snail, a support system for tidal current energy devices, which uses reversible hydrofoils to produce downforce and hold generating units in place in energetic flows, where the sea bed does not allow conventional anchorage.

Dissenting voices

The alternative to a propeller-driven turbine is a hydrofoil, which oscillates when water streams over it. The motion actuates a hydraulic motor to drive the alternator. This is the principle behind the 150kW Stingray, developed by Engineering Business. However, despite having trialled the Stingray successfully, the company has put the cat among the pigeons by putting the project on hold.

The potential for tidal stream energy is certainly there, but whether it becomes mainstream, we’ll have to watch this space.

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