UK tidal: Ebbing it on

Harnessing the energy of the UK’s River Severn, which has the world’s second largest tidal range, has long been a dream for ambitious engineers and governments alike. Now, a new impetus on low carbon generation has prompted another re-examination of the issues involved. David Appleyard summarizes the latest findings and recommendations for policy makers.

The UK’s outstanding tidal resources could provide at least 10% of the country’s electricity, according to government advisory body the Sustainable Development Commission (SDC), which has recently released a report on prospects for developing tidal energy in the country.

The report, Tidal Power in the UK, discloses that the best tidal stream sites are in the north of Scotland, with significant potential also around north Wales, Northern Ireland, and the Channel Islands. The tidal range resource, meanwhile, is concentrated in the estuaries off the west coast of Britain, including the Severn, the Mersey and the Humber. Around 5% of UK demand could come from each resource, says the SDC, which carried out a comprehensive study, including an evaluation of proposals for a Severn barrage, and which makes a series of recommendations to government.

The project, announced in the 2006 Energy Review, was funded by the Department for Business, Enterprise and Regulatory Reform (BERR), Welsh Assembly Government, South West of England Regional Development Agency, Scottish Government, and the Department of Enterprise, Trade and Investment (Northern Ireland).

Tidal energy overview

Exploiting tidal energy resources requires concerted action on a number of fronts and to add to the complexity the tidal power technologies that could be deployed are very different in both design and level of development.

With an excellent tidal stream resource, the UK is leading the world in the development of a wide range of tidal stream devices – several of which are at the pre-commercial stage – but must continue developing such technologies in order to capitalize on potentially very large export and climate change benefits. Tidal stream devices will require many years of targeted support to reach commercial maturity.

Despite the encouraging progress made so far, the government could do more to assist these emerging technologies, particularly through flexible financial support, says the SDC. Innovation, and the development of new low carbon technologies such as tidal stream generators, needs to be a fundamental part of the UK’s response to the challenge of climate change. The government must, therefore, increase R&D expenditure and become less risk-averse in supporting innovation. One mechanism that is currently under consideration is a government plan to introduce technology banding to the Renewables Obligation, which could better serve the needs of the tidal stream industry by providing access to funds aimed at encouraging pre-commercial demonstration.

This conceptual drawing shows a device, under development by UK-based Marine Current Turbines, that may be deployed in large arrays

The government should also provide additional resources to the European Marine Energy Centre (EMEC) in Orkney, which provides a testing site for wave and tidal devices, and which must be used to its full potential.

Furthermore, the government should explore the opportunity to develop a regional tidal stream cluster, or ‘hub,’ around the Orkney Islands and parts of the Caithness and Sutherland coastline. This could make good use of the less challenging conditions in these locations to develop a co-ordinated pre-commercial testing programme. The SDC is also concerned over the long-term ability for tidal stream generation to connect to the electricity transmission system due to a lack of capacity, a particular threat to the development of tidal stream in the north of Scotland. Energy regulators Ofgem and the government must urgently increase the capacity of the electricity transmission system to accommodate renewables over the long-term, the Commission says.

Tidal barrages, on the other hand, are a proven, but highly capital-intensive option that would require a strong lead by the government to be built. As yet there has been no attempt to exploit the UK’s large tidal range resource, despite numerous project proposals going back many decades. Virtually all of these have focused on the construction of tidal barrages, which use similar technologies to hydropower dams and are therefore relatively mature. However, the high capital cost and concerns over environmental impacts have prevented a barrage ever being built in the UK, despite examples in France and Canada operating successfully.

Likewise, the concept of a tidal lagoon is not a recent proposition. Not one has ever been built anywhere in the world, and although the technologies used would themselves be classed as mature, the concept itself is unproven due to a number of remaining uncertainties over design, construction methods and physical impacts. This means there is a lack of evidence with which to assess the long-term potential of tidal lagoons, despite a potentially significant resource in shallow water areas around the UK.

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To help fill this information gap, the SDC believes there is a strong public interest in developing one or more tidal lagoon demonstration projects in the UK, which would help provide real-life data on their economic and environmental viability and recommends that the government provide financial support. The government could also encourage private sector engagement through joint initiatives, increased support under the Renewables Obligation or by announcing a one-off competition.

However, all tidal technologies have a number of environmental, social and economic impacts that need to be considered. In particular, the impact of a Severn barrage on internationally protected habitats and species is of great concern and conditions under which such a scheme would be approved must be consistent with the principles of sustainable development, says the SDC.

The Severn barrage

The Severn Estuary is a unique and dynamic environment. It has the second largest tidal range in the world, combined with a high suspended sediment load, and has a number of special features, including extensive areas of salt marsh and mobile sandbanks. Developing a Severn barrage would result in significant climate change and energy security benefits with a single barrage capable of supplying 4.4% of UK electricity supply, generating some 17 TWh annually for over 120 years.

However, such a development would have a major impact on the local environment, with the loss of up to 75% of the existing intertidal habitat, which is internationally protected. It is an important site for migratory birds, and for fish movements in and out of the estuary’s tributaries, such as the Wye and the Usk. For these reasons the Severn Estuary has been designated a protected site under national and international legislation.

The most important pieces of conservation legislation for a prospective Severn barrage are the EU Directives on Birds and Habitats, which protect sites designated as Special Protection Areas (SPA) and Special Areas of Conservation (SAC). The aim of designation is to protect against biodiversity loss by conserving a series of important or at-risk habitats and species that make up the Europe-wide Natura 2000 network and the Severn Estuary is a SPA and a candidate SAC.

The La Rance tidal barrage generator in France, one of the few operational examples médiathequeEDF

There would also be a number of impacts on local communities and the regional economy, and a high risk that unsustainable ancillary development would take place alongside any barrage project. Therefore the SDC has laid down a series of tough sustainability conditions including public ownership, full compliance with European Directives on habitats and birds and further investigation of opportunities that might exist for combining climate change mitigation with adaptation through a habitat creation package that actively responds to the impacts of climate change over the long-term.

However, the SDC also warns that development of a Severn barrage must not divert government attention away from much wider action on climate change and energy policy aims.

Two proposals

A number of different barrage options have been proposed for the Severn Estuary. The Cardiff-Weston scheme is one of the larger options proposed, and would have a generating capacity of around 8.64 GW. Meanwhile, the Shoots scheme, which would run near to the two Severn road crossings is a smaller, 1.05 GW proposal, with an annual output of around 2.75 TWh.

The assumption is that both barrages would be operated on the ebb tide, with the addition of ‘flood pumping’ to increase the total energy output and generate electricity for around 7–8 hours on each tide. Output would vary within this period and as a result, the annual output of each barrage is less than that implied by the rated capacity. For example, if built, the Cardiff-Weston scheme would generate 17 TWh per year, slightly more than would be produced by two conventional or nuclear 1 GW power stations.

The timing of output from a Severn barrage, regardless of the scheme, is not optimal either. On average, both proposals would produce more power at the times of the day when demand is lowest although the variability in output from a barrage, which is highly predictable, would raise few technical challenges for the operation of the electricity grid.

The high capital cost of a barrage project leads to a very high sensitivity to the discount rate used. At a low discount rate of 2%, which could be justified for a climate change mitigation project, the cost of electricity output from both barrage proposals is highly competitive with other forms of generation.

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However, at commercial discount rates of 8% or more, these costs escalate significantly, making private sector investment unlikely without significant market intervention by the government. Nevertheless, electricity from a barrage would displace output from fossil-fuelled power stations, and would make a genuine and sizeable contribution to meeting the UK’s targets on renewable energy and on reducing carbon dioxide emissions.

The SDC also concludes that there would be substantial flood risk benefits from a barrage, but these are only marginal to the economic case for its construction.

Furthermore, a Severn barrage has a number of disadvantages that are similar to those of nuclear power, says SDC, adding that developing such a large amount of electricity generating capacity in a single location would not of itself move the UK any closer to a more decentralized energy system.

A positive decision

The UK is in a unique position, with a superior tidal resource combined with the largest collection of devices being developed or tested anywhere in the world.

Tidal stream technologies could make a substantial contribution to sustainable energy strategy and in many ways the tidal stream industry is at the same stage of market development as wind power was 20 years ago. This will require continued support and political commitment for at least the next two decades, and a willingness to invest widely in the knowledge that not all the devices under development will succeed. The challenges for this nascent industry are considerable: securing the necessary public and private investment, achieving cost reductions, accessing the grid, satisfying environmental and regulatory requirements – along with working in the difficult marine environment.

But the rewards are also potentially large: the generation of a sizeable percentage of UK electricity supply, the long-term contribution to highly skilled jobs and a knowledge economy, the export potential, and as a contribution to global efforts to reduce carbon dioxide emissions. It is this long-term perspective that must be adopted when taxpayers are asked to invest in the development of such technologies.

The UK’s potential for developing a number of different tidal barrage options other than the Severn is also extensive. The main reason why this potential has not been developed in the past is that the schemes studied have appeared not to be economically viable.

Moreover, tidal barrages, like large hydropower schemes, can be hugely disruptive to the local environment and estuary systems, and can have a number of regional economic and social impacts, all of which need to be considered. The SDC is therefore supportive of further investigation into UK tidal barrage options outside of the Severn Estuary, although each should be considered on a case-by-case basis.

If built, a Severn barrage would be designed to generate electricity for at least 120 years. It would be a major addition to the landscape, and would have fundamental environmental, social and economic impacts on the surrounding area. These timescales emphasize the need for any barrage project to be designed and delivered in a way that ensures the long-term public interest, rather than a short-term, profit-led approach.

The SDC has a number of concerns over the apportionment of risks and benefits for any barrage scheme, particularly one that is led and owned by the private sector. It is very unlikely that a proposal for a Severn barrage would ever come forward without significant government intervention, and a substantial funding package to pay for the initial research and evaluation. Once construction begins, the government will almost certainly effectively underwrite the project due to its size and political significance. This increases the risk that underinsured risks will be picked up by the taxpayer.

A project of this kind also raises concerns over short-termism, with private sector developers requiring a high rate of return. There is also the risk that a short-termist approach could lead to the use of sub-optimal construction methods and materials, possibly leading to higher levels of ongoing maintenance.

The SDC consequently believes that any barrage project should be publicly-led and publicly-owned as an asset to ensure long-term sustainability in its design and delivery, and a fair allocation of risks and rewards. A publicly-led approach would also enable the use of a low discount rate, justified by the long-term climate change benefits and potentially facilitated by the government’s access to low cost capital.

At a low discount rate, the cost of electricity output becomes highly competitive with other sources of generation, even if the cost of the compensatory habitat package is high.

A crucial first step in the development will be to obtain an early indication of the feasibility of compliance with the European environmental legislation, and the cost of achieving this.

Even so, the SDC believes that there is a strong case to be made for a Severn barrage, but a decision that should only be made as part of a major effort to deliver at least a 60% cut in greenhouse gas emissions by 2050, with action loaded towards the next 20 years. Indeed, the Committee warns that a narrow focus on just one major project such as a Severn barrage could be detrimental to the development of a whole class of emerging tidal stream technologies, some of which could be sizeable generators of renewable electricity in the UK, with great export potential over the long-term.

But, despite recent progress with the Climate Change Bill and the 2007 Energy White Paper, the SDC believes that the government does not yet have the policies in place to deliver the carbon savings that will be required to 2050 – and in particular, the delivery of emissions reductions over the next 15 years. Nevertheless, in the light of increasing public concern over climate change and a greater political willingness to tackle the issue head-on, the SDC believes that a Severn barrage could be pursued as part of a major drive to reduce emissions substantially over both the short and the long-term.

A robust climate change and sustainable energy policy is an essential pre-requisite for development of a barrage. If this exists, there is the potential for a Severn barrage to be used as a symbolic example of the scale of action that is required.

David Appleyard is Associate Editor at Renewable Energy World


  • Renewable Energy World's content team members help deliver the most comprehensive news coverage of the renewable energy industries. Based in the U.S., the UK, and South Africa, the team is comprised of editors from Clarion Energy's myriad of publications that cover the global energy industry.

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