The tide is rising for a new generation of maritime sea-power. Tidal energy schemes harness gravitational effects of the moon and sun to generate sustainable energy. Jon Herbert looks at the current and proposed massive energy infrastructure projects around the UK coast.

Taking power from the predictable ebb and flow of the tides makes sense. Tidal technology should be extremely attractive. However, putting it into wide practice has taken a long time. With some of the world’s greatest tidal ranges, the UK is now taking a serious lead. This should also generate substantial supply chain opportunities.

In his March 2015 budget statement, Chancellor George Osborn announced that negotiations are due to start soon examining whether creating a subsidised cost structure is viable for energy generated from a proposed tidal lagoon that would be built off the Swansea coast. If so, construction will involve creating a six-mile long seawall stretching up to two miles out to sea at a cost of £1 billion. The wall will house turbines that could power up to 120,000 homes for 120 years.

World’s largest

If the proposal made by the green energy company, Tidal Lagoon Power (TLP), goes ahead, it will become the world’s largest tidal project and the first involving the construction of a man-made lagoon. The project currently forms part of the UK’s National Infrastructure Plan.

TLP’s full proposals actually include a £6 billion series of six lagoons around the Welsh coast (Cardiff, Newport and Colwyn Bay), in Somerset (Bridgwater Bay) and in Cumbria. Jointly, they will see 14 miles of seawall being built in total.

TLP’s new projects aside, the UK is already home to another of the world’s largest tidal schemes — in the Pentland Firth between the northern coast of Scotland and Orkney.

Good news for once

The good news is that even without controversial offshore wind farms, the UK is almost uniquely endowed with abundant marine energy around its shores, in coastal waters and further out to sea.

Energy and Climate Change Secretary Ed Davey is enthusiastic. He explained recently that: “Tidal lagoons could provide 8% of our electricity needs, replacing foreign fossil fuels with clean, reliable homegrown electricity and creating fantastic economic opportunities.”

However, that is not the only form of tidal power.

Costs once again

Lagoon power faces a major problem, ie costs, particularly during construction. Everything hinges on the upcoming negotiations. Mr Davey says that talks with his department will determine whether a guaranteed price for power generated by the lagoon will be “affordable and value for money”. He also notes the project is still subject to a planning decision.

If that particular log-jam can be broken, and subsequent commercial streams brought on shore at a much lower cost basis as TLP anticipates, the UK could lead the world with a revolutionary energy technology.

The Swansea costs would be subsidised by electricity bill payers under the Government’s scheme to encourage new forms of homegrown, low carbon power generation. TLP is reported to be keen for £168 per MW hour for electricity in Swansea to be reduced to £90–95 per MW hour for power from a second and more efficient lagoon proposed in Cardiff.

The £90 figure is said to compare favourably with £92.50 from the planned Hinkley nuclear station. In a broader perspective, that figure is made more attractive by the proposed 120-year life of the tidal lagoon, which involves much lower technical and financial risks than nuclear power.

Pent up power

A Severn barrage has been discussed for years. The Swansea scheme that is now on the table is not a barrage across the entire estuary — that has been linked to major environmental and wildlife problems — but a lagoon on one side of it. Understanding how the mass movement of water can be harnessed to extract energy is important.

Tides can be used in two ways. Lagoon projects use of the potential energy (gravity) of the large masses of water lifted by the tides. This is possible at relatively few sites around the world where the maximum and minimum levels of the tide — tidal range — are substantial.

Incoming tides will fill lagoons that act very much like damns. Pent up water falls under gravity, turning a series of sub-sea turbines. Unlike more fickle wind power, tides — and their height — are completely predictable and regular.

It also means that power can be taken from two incoming and two outgoing tides within every 24 hours. The potential of lagoon power is said to be so great that excess energy could be stored and made available to displace more environmentally unfriendly carbon-based power sources.

More moon power

Meanwhile, the main alternative to making use of the gravitational bonus that the moon (and sun) provide in creating the tides is tidal stream. This makes use of the kinetic energy of fast moving current. It is particularly suitable in tidal races between main lands and islands.

January 2015 saw the beginning of MeyGen Phase 1a, the first construction phase of the MeyGen Tidal Energy Project. Located in the Inner Sound of the Pentland Firth, it will be the world’s first multi-turbine tidal stream energy project with a capacity of 6MW from four 1.5MW turbines.

The tidal turbines, which will be held in place on the seafloor by a heavy base, will each have three blades on a horizontal axis with an 18 metre diameter. At the Lowest Astronomical Tide (LAT), they will be at least 8 metres beneath the sea surface.

A 4.4kV subsea cable will bring power from each turbine onshore to a Power Conversion Centre at the Ness of Quoys, Canisbay. The Centre will house converters, control equipment, transformers and switchgear to both manage the turbines and allow electricity to be exported to the national grid from 2016 onwards.

Phase 1a will run for 25 years before being decommissioned, with an option to re-energise the project. MeyGen plans to follow up this success with further phases on the Inner Sound lease site until the full planned 398MW capacity is in place.

The full tidal project could then power nearly 175,000 homes. Atlantis is the majority MeyGen stakeholder and developer of robust turbines designed to operate continuously in a robust and often turbulent sub-sea environment. The Crown Estate and Scottish Enterprise are backing the project.

Slow history

Tidal power has been slow to come to reality. The first tidal power station was built at La Rance in France from 1960 to 1966 and has 240MW installed capacity.

The world’s first commercial-scale and grid-connected tidal stream generator was the SeaGen S 1.2MW tidal energy convertor installed in Strangford Lough, Northern Ireland, in 2008 with a five-year licence. Following extensive environmental studies and a wide post installation environmental monitoring programme, the licence has been extended until 2018. Since 2008, it has generated more than 8GWh of electricity.

The world’s first marine energy test facility was set up in Orkney in 2003 to help develop the wave and tidal energy industry. Since then, the European Marine Energy Centre (EMEC) has worked on more wave and tidal energy innovations on one site under varied real sea conditions than any other organisation in the world.

It includes a grid-connected tidal test site at the Fall of Warness off the island of Eday in a narrow channel that concentrates tides reaching eight knots (four metres per second) between the Atlantic and the North Sea. Eight different technologies are currently being tested.

Who rules the waves?

The third main way of taking energy out of the seas and oceans is to harness the power of the sun through the generation of winds that in turn create waves.

The potential for wave power dwarfs tidal power. But again, there are problems. These are greater than those faced by tidal power because of the extremes of the operating environment.

Many ingenious systems have looked at taking power from the oscillating movement of waves. Relatively few are being taken forward. A major challenge is designing physical machinery that can withstand the terrific forces created by storm waves or the freak giant waves that are now known to occur regularly out at sea.

Risks have put off investors. But there is little doubt that the endless power of waves can be captured, even if commercialisation is being delayed.

In Spain, the 300kW Mutriku wave project has operated since 2011 but this is a rare exception. The Pelamis project in northern Scotland showed great promise since 2004, generating 250MW/h of electricity but went into administration late in 2014. Funding is a problem for other projects. However, four key technologies are being tested at EMEC.

The general conclusion is that tide and waves have tremendous green potential but might struggle in the short term to compete against the more established technologies of the wind and solar industries.

But can time and tide wait until we are ready?

First published by Croner-i on 21 April 2015