Generating power from taming the tide

The rise and fall of the tides is an obvious and reliable source for renewable energy. But is exploiting it economically feasible? Simon Wilson reports.

Tidal power: is the sea the future for energy?

It could well be part of it, especially in Britain a country blessed with a long coastline and unusually high tidal ranges (the vertical gap between low and high tide), which make tidal energy easier to harness. And unlike wind and solar power, the rise and the fall of the ocean is reliable and predictable. The most obvious site for a tidal barrage a dam-like structure that captures the energy from tidal forces is across the Severn Estuary, which has the second highest tidal ranges in the world.

A succession of feasibility studies in recent decades has ruled out a Severn barrage on economic and environmental grounds, but the question will no doubt be revisited as new technologies evolve. Meanwhile, Britain is already home to potential world leaders in both tidal stream and tidal lagoon technology the first off the north coast of Scotland and the second in south Wales.

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What is tidal stream energy?

It's energy generated by turbines placed on the seabed to capture the kinetic energy of moving water. Atlantis, an Australian company listed on the London Stock Exchange's Aim market, is currently building the world's largest tidal-stream project, MeyGen, in the Pentland Firth between the Scottish mainland and Orkney a stretch of ocean famed for its treacherously fast-flowing tides.

In a major landmark for the UK renewables industry, it installed its first four turbines on the seabed in September, and delivered its first power to the grid in November. "Psychologically, this is the unleashing," says Atlantis boss Timothy Cornelius. "It was a great story before; now it is an infrastructure project."

How does it work?

Four massive turbines with an initial output capacity of 6 MW make up the first stage of the MeyGen project. Each turbine resembling a bulked-up wind turbine, about 15 metres high and weighing almost 200 tonnes sits on a heavy foundation structure resting on the seabed, secured by its own weight. Tidal currents rotate the rotor blades, which power a generator to create electricity, in the same way as wind power.

Underwater cables then carry the electricity to an onshore substation (in this case at Ness of Quoys), and the substation transfers it into the national grid. Phase one of the Atlantis project cost around £51m, and the firm plans to spend nearly £500m on expanding MeyGen and building other projects off the Scottish coast over the next two years. The ultimate goal is to boost the project's capacity to 398 MW, delivered from 269 turbines enough energy to power 175,000 houses.

Will this be remotely cost-effective?

According to John Ponton, a former professor of engineering at the University of Edinburgh who is sceptical about the long-term cost-effectiveness of renewables, it is too early to say for sure but he reckons probably not. Ponton argues that tidal energy is at least a more plausible source of reliable electricity than wind or solar. Nevertheless, he says that to provide 2GW of effective supply would require 5GW of installed turbines, or 2,500 of the largest currently available.

And he calculates that the Scottish government's grant of £23m to MeyGen for up to 6 MW suggests that to provide the same effective capacity as Hinckley Point C nuclear station would require 8,250 turbines at a cost of £22bn. That compares to £18bn for Hinckley, with a projected lifespan of 60 years, as opposed to 25 for the tidal turbines.

What about the tidal lagoon route?

Tidal lagoons are a new technology, and if the prototype lagoon planned for Swansea Bay in south Wales gets the final go-ahead from government, it will be the first of its kind in the world. The £1.3bn plan is for a six-mile U-shaped seawall enclosing part of the bay to create a lagoon. The wall will have 16 underwater bidirectional turbines built into it. That's enough to generate 350 MW of power, or enough to supply 155,000 houses with electricity equivalent to 90% of Swansea's domestic energy use, or 11% of Wales's overall needs.

If all goes to plan, the idea would then be to build a further five, larger, lagoons at Cardiff, Newport and Bridgwater Bay in Somerset and at Colwyn Bay in north Wales and in west Cumbria. The Cardiff scheme would cost £9bn and have a capacity of 3,000 MW, compared with the projected 3,200 MW from Hinckley Point C.

Will there be big subsidies?

Large-scale public investment will be needed for this kind of huge infrastructure project. But proponents say the sums involved are comparable with, or cheaper than, nuclear and other renewable options. For example, measured over its planned 120-year lifespan, the support required to make the Swansea scheme viable would cost taxpayers £25.78 per megawatt-hour of electricity. That's around the same as the level for nuclear plants. For the proposed scheme in Cardiff, which is much bigger, the cost would fall to £7.80 lower than for any other form of renewable energy.

Who else is worth watching?

In Shetland last summer, Nova Innovation deployed whatit said was the world's first fully operational array of powerturbines in the Bluemull Sound, between the islands ofUnst and Yell. The scheme is on a smaller scale than thePentland Firth scheme, with five turbines creating 500kWof power for the local grid. Off Brittany, a French firm,OpenHydro, is attempting a more powerful 1MW array.

In Northern Ireland, DP Energy says it is in the advancedstages of developing a 10 MW tidal farm in Fair Head as thefirst stage in a 100 MW project. The firm's wider portfolioof planned projects adds up to 300 MW. In Cornwall,meanwhile, another Australian firm, Carnegie WaveEnergy, has launched a project that will use innovativesubmerged underwater buoys, tethered to the seabed16 miles offshore, to harness the energy of the ocean as itrises and falls.