In depth with the TIGER tidal stream project

Renewable energy in the shape of wind and solar power provides an increasing share of the UK’s energy mix, reaching 40% in the third quarter of last year. So far, however, tidal power has not made significant inroads into the market.

Over the next three years, the Anglo-French Tidal Stream Industry Energiser Project, or TIGER, is aiming to demonstrate that tidal stream energy is a maturing technology which, with a revenue support mechanism to allow significant deployment, could reduce its costs to a competitive level with nuclear power.

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Last autumn the €46.8m (£38.9m) project was awarded €28m (£23.3m) from the European Interreg France (Channel) England programme to install up to 8MW of generating capacity at a number of sites around the south coast of England and the north of France. The award makes it the biggest ever Interreg project.

The programme seeks to gain experience in installation, operation and maintenance of tidal power, demonstrate cooperation with an Anglo-French supply chain, and disseminate the results to help accelerate the commercialisation of tidal stream energy.

Tidal stream energy generates electricity from tidal currents using structures resembling underwater wind turbines, as opposed to tidal range energy projects such as La Rance in France and the proposed [scrapped] Swansea tidal lagoon, which use dams or barrages to impound water and then control its release through turbines.

The TIGER consortium was assembled and is being project managed by the Offshore Renewable Energy Catapult, which has a regional office in Hayle, Cornwall. ORE Catapult sector lead for wave and tidal energy Simon Cheeseman explained that the withdrawal of government subsidies in 2018 left tidal power deployment in limbo. “The UK is at the forefront of wave and tidal technology development, but in the UK there’s no revenue support mechanism to enable developers to provide competitively priced electricity,” he said. “Tidal power has to compete in Contracts for Difference auctions but it’s not competitive because it hasn’t got the scale.”

Renewable UK has been lobbying for the reinstatement of some sort of revenue subsidy. In 2018, ORE Catapult published a report, Tidal Stream and Wave Energy Cost Reduction and Industrial Benefit, which set out how tidal power could meet three tests for the viability of new forms of renewable energy set out in the government’s clean growth strategy. These are that it should have a clear cost reduction pathway; that the UK could develop world-leading technology in a global market; and that it would reduce carbon dioxide emissions.

The report argued that costs would be driven down by “learning through doing” – gaining genuine experience of installation and operation; and from economies of scale; standardisation of components; working with the supply chain to reduce costs; and innovation offering improved performance. This would provide increased confidence in the technology to bring down the cost of capital and insurance.

The learning through doing aspect is important. Because tidal power equipment has to be installed, operated and maintained in the sea in areas where currents are strong, “the costs, risks and difficulties rise exponentially”, said Cheeseman. Though test rigs can be set up on land, eventually systems need to be tested in the water.

The report also made the point that the investment in the sector would bring an economic boost to coastal communities where traditional industries such as fishing had declined.

“In the report, we were able to demonstrate a clear cost reduction pathway, once we reached 1GW of deployment, to take tidal stream energy to £90/MWh, which would be competitive with nuclear,” he said,

Being competitive with nuclear could be significant. Tidal power is unique among renewables in being predictable. Tides and current flows are published in advance. “As a developer, if you put a turbine in the English Channel, you can calculate with a high degree of accuracy what you will get out of it,” Cheeseman said.

The cost reduction pathway has been widely acknowledged, and TIGER was developed as a way to demonstrate it. In France, the sector faced similar difficulties and there was scope for synergies. In the UK, councils and Local Enterprise Partnerships around the south and east coasts of England as far as Norfolk were interested in investing in the technology. ORE Catapult became involved to pull together a consortium (see box) and continued as project manager.

The UK side is structured around the Marine Energy Council, which brings together all the leading tidal and wave power developers, and as part of TIGER is hoping to create a French equivalent.

The overall objective of the project is to provide evidence that tidal stream energy can take the first steps along the cost reduction curve; to develop a UK/France supply chain, and to work with it to identify critical areas where cost reductions are necessary. These include blade design and manufacture; pitch control systems; subsea cables and connectors; installation methods; and improvements in overall system reliability.

Interreg is keen to see installations at different sites, both tidal channels and headlands, and with different types of turbine. Accordingly, the project will demonstrate floating devices as well as large and small turbines fixed to the seabed, and more novel devices such as that developed by Swedish company Minesto, which resembles a small aircraft or glider with a propeller/turbine. Tethered to the seabed, the glider travels in a figure-of-eight pattern, pushing the turbine through the water at a relative speed faster than the underwater current. This allows it to operate in low tidal flows where a horizontal axis turbine would be inefficient.

Because the Interreg programme is due to end in March 2023, the sites chosen had to be already operational, or close to gaining consent.

In the UK the Ramsey Sound site, off Pembrokeshire in Wales, is being revitalised. It was the site for Tidal Energy Ltd and was a fully-fledged generating site until its Delta Stream turbine developed problems and the company went into administration. As part of the project, TIGER will be removing the turbine and examining the reasons for its failure.

The site will be operated by Cambrian Offshore South West; a new turbine of potentially up to 1MW will be installed but a decision has yet to be made about its type.

Also in the UK, potential sites in the Solent region are being examined.

In France, Paimpol-Bréhat is an EDF Hydro owned open-sea test site where a turbine was installed and tested last year by Hydroquest. The site will be upgraded to accept a broader range of turbine technology. It is expected that a 100kW Minesto glider will be installed as part of the TIGER project.

At a new site in Morbihan, Brittany, French turbine developer Sabella working with Morbihan Energie will install and operate two 250kW turbines. Sabella regularly installs turbines for testing on the French island of Ushant.

At Raz Blanchard, north of the Channel Islands, Hydroquest aims to complete consenting for a 10MW pilot turbine farm. Normandie Hydrolliene, a joint venture between Edinburgh-based SIMEC Atlantis Energy and the regional development agency AD Normandie, will also aim to complete consenting for their 1.5MW horizontal-axis three-bladed turbine as the first phase of a commercial tidal project of between 5 and 20MW. Atlantis’s MeyGen project in Orkney went operational in 2018. Hydroquest will install a vertical blade turbine, allowing comparison of installation, maintenance, power generated and cost of operation.

The UK’s Orbital Marine Power, which has been testing its turbines at the European Marine Energy Centre in Orkney, will look at tailoring its design and the economics of deployment at the different TIGER sites.

In general, installation of the turbines does not require major construction operations. The seabed-mounted type stand either on steel gravity structures or simple piled foundations, and the driveline and generating equipment are preassembled and contained in a nacelle. An export cable takes power to the shore, where some horizontal drilling under the beach for the cable is required.

Installation challenges arise because the turbines have to be installed where currents are strong, so most work has to be done at slack tide, which lasts only 30-40 minutes. Offshore work will be done by remotely operated subsea vehicles to avoid the use of divers, but they can only work at low tidal velocities. Further development of installation and support vessels that can operate in shallow waters with high flows is another area to be investigated with the supply chain.

Numerous academic partners in the consortium will be evaluating the data emerging from the project and measuring the performance of the installations. The results will be disseminated by ORE Catapult through a series of workshops, conferences and reports.

In addition to demonstrating the technology’s maturity for the UK market, the TIGER project is also hoping to open up export markets, particularly for off-grid island communities which currently depend on diesel generation.

Tidal power has had false dawns in the past, but Cheeseman is confident the project will succeed, not least because of its broad-based nature involving some of the leading developers, engaging with a knowledgeable supply chain and expert marine energy academics from universities in the two countries.

“We’ve got the cost reduction curve broadly accepted,” he said. “We’ve got the technology. It’s all been tested at independently so we know it works. This is about collecting hard evidence to support a cost strategy and putting it into practice.”