Tidal Power for the UK — the Severn Estuary debate


Max K. Wallis
MARINET, Marine Network of FOE Local Groups and Affiliates

Abstract – Both tidal stream and wave power devices have suffered from stringent economic assessments which have cut off R&D. The total tidal stream resource is assessed as low (by the Carbon Trust) thus banishing it to the fringe, and tidal lagoons get contrasting pro and anti-assessments while the Severn mega-barrage gets optimistic treatment.

We find good reasons to distrust narrow economics, driven by interested parties. However active engagement requires us to define and cover the range of non-monetary issues – including learning and technology enhancement, decentralised power, and wider environmental and socio-economic benefits.

The Sustainable Development Commission (SDC) currently has the task of addressing the tidal power area, but has run restricted events dominated by an in-group of official bodies and project promoters. With WWF and FOE arguing different options, we on the fringe need to understand and discuss the issues in a wider context, and to make an input into the SDC process and the 2007 Energy Review.

Introduction – What options?

Tapping tidal energy around Britain is an important part of an energy policy that seeks a transition to reliance on renewable sources. The Severn tidal barrage has burst back into prominence, after years on the shelf, as a way to quickly boost our renewables commitment.

The mega-barrage across the Severn from Lavernock to Brean Down (Fig. 1) is neither the only tidal option nor an obvious choice. A smaller barrage further up the Severn (Shoots barrage) has various advantages, but still takes over 10 years in planning and construction. The best current prospect is tidal current turbines, with wave power devices being another possibility, both capable of quick dividends within a few years. Offshore tidal impoundments come between the two. How do we compare multi-project moderate-scale technology with mega-barrages?

FOE’s briefing (FOE 2004) on the Severn barrage and lagoons generated much interest and debate. Early in 2006, a specific plan for a lagoon which impounds the tidal rise was proposed for Swansea Bay. In spring 2006 the Welsh Assembly Government, in responding to the Energy Review, recommended the mega-barrage scheme for further study. The Welsh Affairs Committee (2006) reported in July after taking evidence on these and other energy resources in Wales. The 2006 Energy Review endorsed the mega-barrage for further study, and gave to the Sustainable Development Commission the task of evaluating options for tidal power, primarily in the Severn. www.sd-commission.org.uk/pages/tidal.html

The Sustainable Development Commission say their review
‘will look at the costs, the environmental impacts, the social side of it, whether it can actually be done in the UK’s energy market, and how it fits into the UK power grid. The intention of the DTI is to give it a clear idea of where they should prioritise in future. If we came out heavily in favour of going in a particular area then that might encourage a review of a particular option.’
However, both the 2007 Energy Review and politicians are already firmly opting for the Severn mega-barrage.

Map of Severn Estuary

Figure 1 Site of the Severn mega-barrage, south of Cardiff and Weston, shown dotted between the islands Steep Holm and Flat Holm. The Shoots barrage proposal is close to the southernmost of the two road crossings north-west of Bristol.

Comparison of Tidal Barrage and Tidal Stream – different forms of Tidal Energy

Tidal range is the difference in height between high and low tide. Tidal stream is the flow of water through channels or around coastlines as a result of tidal water movement. Some people talk of marine currents, as energy in the Gulf Stream and other circulating currents could also be tapped.

Barrages to tap the tidal range are sited in upper estuaries, while tidal stream turbines tap the currents in lower estuaries and straits. As a result the two may not conflict. Tidal streams are tapped, in both ebb and flood directions, by submerged turbines. These are much smaller than wind turbines because the power density in water currents is much larger than in air currents. Though tidal barrages have a long history, tidal stream generators are considered much more feasible nowadays.

A major drawback with the Severn Barrage, and to a lesser extent Severn tidal lagoons, is the need for supporting power stations to fill in the slack periods.

Tidal currents, on the other hand, deliver less discontinuous power than a barrage and if tidal current technology made use of several sites with a range of tidal timings it would give reasonably steady overall power.

A second drawback of barrages and lagoons is that they change the sedimentation and erosion pattern. This is especially important in the Severn because the strong currents make for very high loading in silt and sand. Note: Spring tides contain about 4 times the energy of Neap tides, and about 10 times the silt loading.

Major structures reduce currents, so the silt tends to deposit inside the barrage or lagoon and in backwaters outside the structure. Scour may also be enhanced outside the structure. For the former reasons, proposals for a major barrage in the Bay of Fundy (Annapolis, Canada) have been dropped and, because of shoreline erosion, a causeway is to be removed.

Each proposed barrage or lagoon would need specific studies – as far as a scale hydrodynamic model, as done for the old Severn Barrage study – and even then predictions would be uncertain.

Ebb Flow Generation Only

Figure 2 left, Ebb-flow generation only, in the Severn Tidal Power Group (STPG) study.The problem of siltation has led designers to consider ebb generation only (Fig. 2). Note that an appreciable height difference is needed to make good use of the volume discharged. As indicated, the upstream basin refills during the first 3 hours of the 12-hour cycle when the height difference is low. Thus this design generates for only about 40% of the time, with overall efficiency ~25% of the theoretical energy available. Clever designs might increase this.

Graph showing power output on Ecostar concept

Figure 3 right, Optimising power output on the ECOSTAR concept. The right sketch shows pumping (negative) and generation (positive) phases. The left sketch shows water levels in the inner basin lagging the tidal cycle by ~3 hours; the continuous line shows maximal generation, the broken lines show a practical modification, cutting off the peaks in pumping and generating power.

The barrage at La Rance, France, generates up to 240 MW and has run successfully for 40 years. It was planned for 2-way generation, but the turbines are less efficient in reverse. Instead the operators found that pumping extra water in during high tide (when the height difference is small) gives extra volume to drive the turbines when the height difference is large.















Stuart Anderson’s ECOSTAR (Eco-Storage for Tidally Amplified Release) concept outlined below is available in summary as is also his full articlewritten for the British Hydropower Association. Please note that both of these are large pdf documents.

In principle, one could design for two-way generation with pumping, generating only when the height differences are large, as in the ECOSTAR system (Fig 3, courtesy of Stuart Anderson). This author calculates that the total efficiency could amount to four times that of the design above. However, the ECOSTAR concept (Energy Capture Obtainable by Storage for Tidally Amplified Release) exaggerates still further the discontinuities in output. The DTI-sponsored studies have been directed instead to extending the generating cycle and phasing with electricity demand, by adjusting discharge times and using pumping for ~1hr in the cycle.

The Severn Mega-Barrage

The ebb-tide generating scheme as drawn up by the Severn Tidal Power Group (STPG) could deliver 17 terawatt hours per year (TWh/yr). A revised report was published in 2002 for the DTI (STPG 2002). The scheme would deliver about 5% of current England and Wales electricity consumption of 350 TWh/yr and cut 18 million tonnes CO2 per yr (from the UK’s ~600 Mt CO2 per yr total).

Diurnal fluctuations could be smoothed out by a two-basin design, whereby off-demand power is used to pump water into a higher second basin.

The DTI recommended this for further study. Operating the pair of basins to meet the demand cycle gives electricity of much greater value. The timing issue has been largely ignored for renewables, but will have to be taken into account for any large tidal scheme. If back-up power stations or storage systems are needed, economic assessment will need to compare total system costs with a two-basin design.

The Barrage proposed would stretch 10 miles from Lavernock Point, east of Barry, to near Brean Down in Somerset, impounding an area of 185 square miles. The scheme’s embankment would pass between the two islands (Steep Holm and Flat Holm) between which there is a deep channel. The Barrage would incorporate locks to allow shipping and smaller craft to access the port at Bristol, other docks and the river Severn. The proposed generating capacity (maximum output) is 8.6 gigawatts (GW). To balance slack periods, it needs operating together with our largest power station (Drax) cycling up and down (at the maximum output from spring tides, it would require two such power stations to cover the slack times).

The Shoots barrage scheme sited upstream of Bristol is sized at 1 GW, to generate 2.75 TWh/yr (one sixth the mega-barrage). The capital cost is an eighth and construction time a half of the mega-barrage. Other advantages claimed for it by promoters PB Power (of Parsons Brinkerhoff Ltd.) are – no impacts on the major ports (only on Sharpness), sufficient grid capacity (the mega-barrage requires ~ £1.5 bn to reinforce the electricity grid), and suitable to carry a high-speed rail link. The sedimentation problem is severe, but considered manageable via a design with high turbines and limiting intake at storm times and spring tides.

The mega-barrage as a development project

Protection against sea-level rise – the DTI assessment argues for multi-£bn benefits in terms of coastal protection. The Shoots Barrage would secure part of these benefits. However, present policy is for coastal retreat (‘realignment’) and spending of ~£600M on defences, so the Environment Agency has challenged the DTI claims.

The mega-barrage is inevitably a large development project. As well as the construction sites at each end, electrical substations with new National Grid lines and a new road link are envisaged. Marinas and executive housing islands linked to the barrage structure are also proposed. Prof. Brian Morgan (ex-Wales Development Agency) promotes the concept of a Severnside urban metropolis. Some, if not all, of the CO2 saving would be absorbed in energy-hungry development and on-going activity.

Conservation and Biodiversity

Studies have shown that fish-stocks and ecology in the La Rance estuary are flourishing in the post-barrage regime, despite being wiped out when the estuary was blocked for construction when the salinity was strongly reduced. Robert Kirby’s study (2007) details how the estuary ecology, in a similar sediment and salinity regime, has been re-established. He shows that the biology is richer because of greater protection from tidal surges, and because the reduced sediment load allows sunlight to penetrate deeper and sustain phytoplankton which form the basis of food chains.

In comparison, Robert Kirby argues that the Severn estuary ecology is highly impoverished due to its aggressive environment with high sediment loading, evidenced by a 3-4 times lower number of winter wading birds per sq km compared with other UK estuaries.

His argument has some force, though La Rance is tiny (20 times smaller in area and 30 times less powerful than the Severn) and the Bay of Fundy is a better comparator. There the small barrage has led to reduction in fish as well as shoreline erosion downstream (mentioned above).

A second response to Robert Kirby is that biological richness does not equate to biological diversity. The severe regime in the Severn does support an especially hardy ecology. The designation of the Special Area of Conservation (under the Habitats Directive, the European Natura 2000 designation was submitted in August 2007) applies to its current natural state. If it were to be barraged, the compensatory allocation that is required is virtually impossible. At the 17th May 2007 Cardiff Conference, the head of Environment Agency Wales (Chris Mills) called this a “show stopper”.

Indeed, the three Conservation Agencies’ report of April 2006 said the Severn barrage would cause “irreversible impacts to features of international importance” (reported by WWF Cymru, 2006).

Their arguments, listed below, supported by WWF-UK (2006) were made in their response to the Energy Review. This explains that the barrage would have a direct impact on four sites protected under European conservation legislation. Such a project would have to pass stringent tests. Namely that there are no alternative solutions, and there are imperative reasons of over-riding public interest.

If these tests were passed, then compensatory habitat would have to be provided in order to maintain the coherence of the Natura 2000 network. As the ecosystem as a whole is arguably irreplaceable, this last requirement is likely to prove especially difficult to meet in the context of the Severn, say WWF. Further discussion is given in a Conference Presentation by WWF Cymru (Morgan Parry 2006).

The view of the statutory agencies on the Severn Tidal Barrage

  • The Severn Estuary is an internationally important habitat with unique ecology
  • The nature and scale of barrage proposals would cause irreversible impacts to features, species and habitats of international importance
  • There would be a direct impact on Severn Estuary Special Protection Area (SPA), proposed Special Area of Conservation (pSAC), and the rivers Wye and Usk SACs
  • A Severn Barrage development would not be possible within the current legal framework provided by the EU Habitats and Birds Directives
  • It cannot be envisaged how required compensatory habitat could be provided to replace those that would be lost
  • The case for a barrage is being promoted without proper consideration of other potentially less environmentally damaging options
  • Water quality would be affected as a direct result of the barrage through the modification of the sediment regime and nutrient flux
  • The barrage would fundamentally alter the basis for determining appropriate limits for discharges to and abstractions from the estuary and the rivers that feed into it
  • Existing defences, and new strategies in preparation, will manage the flooding impacts of climate change without a barrage
  • There may be a downstream increase in vulnerability to tidal and storm surge flooding, and consequent erosion caused by reflected tidal waves at the barrage face
  • There will be major environmental impacts due to the vast amounts of construction materials required
  • There will be substantial ancillary and infrastructure development, including the need to modify or build new port facilities
  • It is difficult to envisage how the proposal would fit with the requirements of the Water Framework Directive, e.g. requirement to aim to meet good status by 2015
  • A major programme of predictive environmental studies will be needed, taking several years to complete

English Nature, Countryside Council for Wales, Environment Agency 2006

Summary Arguments

  • Marine current turbines are real prospects now ( see below), with large-scale installation of the leading design (MCT) from 2009. Alternative designs are also coming forward. The mega-barrage would not come on stream till 2019-20.
  • A large area of the Severn Estuary below Cardiff is assessed to have suitable currents  but only totalling ~200MW (5% of the mega-barrage average).
  • Cost estimates appear similar at 6p/kWh, but the Barrage costs date from 2001, while the marine current turbine costs are based on 2006 construction contracts and promise to reduce to 4p or 3p/kWh.
  • The real value of Barrage power is lower because it generates for only 40% of the tidal cycle and needs major power stations operating to cover its slack phases. Also, the 4 times higher generation at spring tides, compared with neap tides, is out of phase with the higher winter demand.
  • The Barrage power requires high National Grid investment in power lines, which will have high visual intrusion and footprint in rural south Wales and Somerset.
  • Immense tonnages of aggregates for the Barrage embankment would have to come from limited supplies – potentially from south Wales quarries and the Mendips.
  • The conservation interests of the Severn Estuary protected area (a European ‘Natura 2000’ site) for wading/migration birds, special fish species and tidal mudbanks will be strongly impacted.
  • Compensatory allocation of a similarly wild habitat, required if the Barrage develop-ment were to proceed for over-riding socio-economic reasons, is virtually impossible.

Severn Barrage quasi-economics

The project has always presented a problem of severely intermittent generation.

The 1989 study (EP57) said that ebb-flow generation would equate to “firm” capacity of 1.1GW, compared with the installed capacity of 8.6GW, but that’s an accounting fiction. The STPG (2002) review identified two ways of enhancing the firmness of supply:

  1. varying the start time of generation
  2. including a low head pumped storage reservoir capable of generating at any stage of the tide, constructed integral to the Barrage

The scheme would take a long time to come on-stream – a lead-time of at least 14 years before full generation (five years for environmental monitoring plus nine years for construction). The estimated cost was £10.3-14.0 billion at 2001 prices. Costly strengthening of the Grid would be needed for such a large scheme (~ £1.5 bn) is included.

The STPG argued that the Project would have positive environmental impacts in addition to CO2 avoidance, in particular the mitigation of coastal erosion and flooding risk, and the avoidance of costs for flood damage in the whole region. The report was optimistic that the project can potentially demonstrate a surplus of benefits over costs, once its positive externalities are taken into account. Yet it ignored the destruction of a European conservation area and minimised the impediment to shipping.

The mitigation of flooding risk in the Severnside Region was given high value. A study for government (National Appraisal of Assets at Risk of Flooding and Coastal Erosion in England and Wales DEFRA, September 2001) detailed the assets at risk from flooding and coastal erosion, including the potential impact of climate change. The area of urban development and high grade agricultural land at risk in the Severnside region was estimated as some 40,000ha.

The DEFRA report estimated the current annual average damage cost associated with this risk as £1,000-£5,000/ha and expected this to increase by at least 100-fold by 2075 as a result of climate change. This annual average flooding damage cost risk, which currently totals £40-200 million/yr was taken to rise due to climate change to at least £4,000 million/yr by 2075. This risk and the associated capital expenditure costs to improve flood defences would be avoided if the Barrage were constructed. Such numbers are clearly unreliable. Moreover, coastal retreat has been identified as the preferred strategy for much of the estuary, so the real costs must be much lower (£600 million given by the Environment Agency).

At current values, the STPG report said, electricity generated from the Barrage could be worth at least 7.7p/kWh (2.7p for the electricity itself, 3p for the value of its carbon credits and 2p flooding cost avoided (£3000/ha above)), compared with capital plus operating cost of 6p/kWh. The 2.7p/kWh includes 0.7p for “secure” supply, despite the high seasonal and daily fluctuations. The capital cost was based on rapid completion with generation by 2014, whereas 2019-20 is the current estimate; each year’s delay means significant extra cost in capital charges with no income.

The high level of predictability from barrage schemes over a period of several years could, the STPG say, attract favourable prices, yet the uncertainty of big construction projects with turbine technology is well beyond the present experience in size, and the hostile operating conditions (abrasive sediment) go unmentioned.

The 1.7p/kWh surplus of ‘worth’ over cost on the above figures depends significantly on intangibles and thus could run foul of EU subsidy law. Huge subsidies could be justified on the future flooding costs implied by the above figures – effectively the Barrage becomes a coastal protection scheme, with power generation a marginal extra. A much wider study with validated economics is required in this case.

Power from Tidal Streams

The Carbon Trust’s Future Marine Energy (2006) says the costs of tidal stream devices are uncertain but concludes:

tidal stream energy could become competitive with the current base costs of electricity within the economic installed capacity estimated for the UK, 2.8 GW.”

Indeed, the ultimate price given of 3p/kWh is very competitive. This makes it a very good or even the best renewable option. Yet the figures given in the Executive Summary are off-putting – central estimates of 12-15p/kWh, within an uncertainty band of 9-18p/kWh.

At the Renewable Energy Association conference (Bristol, July 2006) Prof. Stephen Salter said 2.8 GW total capacity was a gross underestimate, for he had estimated 12GW from the Pentland Firth stream alone. Details of Prof. Salter’s claims on Pentland Firth Tidal Turbines

The Carbon Trust took 18 TWh/yr as the UK resource, from a Black & Veatch study for them. Compare this with the DTI’s previous estimate of 70 TWh/yr in the UK Tidal Stream Review (1992), totalled over the eight largest sites. Total resource is normally related to a price level.

As a very good price is found for the best 18 TWh/yr, a lot more is likely to be accessible for the target cost of 6p/kWh. One question is – why did the Carbon Trust give such a negative presentation?

The costs in their summary apply to prototypes and are quite misleading if applied to commercial developments. The consultants ENTEC wrote this in the text. They also assumed a very high (15%) rate of return.

EDF Energy have bought into the MCT technology (see below) and plan from 2009 to be building commercial installations of over 50 MW. They look to a global market of thousands of MW. EDF’s Simon Merriweather (2006) predicted that they will achieve 6p /kWh with their first 20MW array, reducing to 4p or even 3p later. Yet ENTEC predict 6p /kWh would not be reached until after much learning and economies of scale, at the point when between 500 and 1500 MW had been installed.

This big disagreement surely shows that “consultants” reports cannot be taken on trust. Let’s remember that consultant economists some 15 years ago killed off the UK’s wave power programme with false costings (Ross, 1995, reassessed by Schofield, 2007). Commentators have adopted the 17 TWh/yr total, 5% of England & Wales’ demand, and dismissed tidal currents as marginal. Their importance changes completely if, as Prof. Salter suggests, the UK’s tidal resource is ten times higher.

Marine Tidal Stream Devices

The leading design, produced by the company Marine Current Turbines (MCT), has horizontal-axis twin-blade turbines, mounted on a steel monopole. The 300kW experimental prototype underwent 3 years of testing in the Severn estuary off Lynmouth, Devon, assessing installation, operation and performance (SEAFLOW 2006). The commercial prototype consists of twin turbines mounted on the monopole – this 1.3 MW ‘SeaGen’ generator is being erected in the Strangford Lough narrows (MCT 2006). The first commercial array of about 10 generators is to follow off the north-west of Anglesey (the Skerries).

The alternative tidal current designs are less advanced, examples being shown on the Marinet website. Norway’s Statkraftt promotes turbines suspended from a floating structure. The TidEl system uses floating turbines anchored via chains to the seabed. Both are small compared with MCT’s twin half-MW rotors, but Lunar Energy’s design has a single 1 MW turbine in a large pipe duct. Swanturbines’ monopole carrying a single turbine is set in a large base sitting on the seabed – being completely submerged. It is feasible for the deeper water (>30m deep) where most power is available. Following trials in the Tawe estuary, a prototype at one third MW size is planned for installation in 2008. The company told the Welsh Affairs Committee (Swanturbines 2006) that they are aiming to site 30-50 units by 2010-2011 in the Severn off Pembrokeshire or south of Barry. Other tidal stream devices are listed by MCT (2006).

Tidal Pools, Lagoons or ‘Offshore Tidal Impoundments’

An early favoured location was off the Welsh coast north of Rhyl, where there is a ready supply of aggregates (slate waste) and it would shelter this vulnerable coast from storm surges in wave height. The Rhyl team call it an OTI (Offshore Tidal Impoundment) and three of them described it at the MAREC conference (see box).

The North Wales offshore tidal impoundment scheme: a preliminary study of requirements, constraints and opportunities
S E Evans, J E P Poole and K P Williams
Cardiff University School of Engineering, UK

The concept of an offshore tidal impoundment (OTI) for generating electricity has been described, with water passing back and fore through turbines in the impoundment wall. Tidal cycles are predictable allowing accurate estimation of long-term power output. Such a scheme has been proposed for a site in North Wales, close to the existing North Hoyle offshore wind farm. The present paper scopes out this proposal as a prelude to any formal assessment. It considers the theoretical maximum power generation capacity and compares this with likely operational performance. The size, shape and siting of the structure are examined in relation to cost, and the likely impact on tidal currents and coastal flood protection. Three options are proposed for particular attention. Appropriate power generation devices are identified. Construction methods and materials are discussed, including the use of geomembrane bags, filled with material from the adjacent seabed. Costings for a single option are prepared and the overall sustainability of the scheme is assessed from economic, environmental and social perspectives. The likely power output is compared with that of an offshore wind farm covering the same area.
Proceedings of MAREC 2004 3rd International conference on marine renewable energy

As a pilot project, their OTI, would be about 3 sq km in area and 52 MW peak capacity (NWCRT 2007). There are ideas of providing for leisure moorings, a commercial harbour and constructing a pier part or all way to the embankment. Wind turbines could also be erected on the embankment. The layout in Fig. 4 (courtesy of Stuart Anderson) is provisional.

map of Liverpool Bay


Figure 4 pilot OTI – Offshore Tidal Impoundment – north of Rhyl, of area 3 sq km, installed capacity 52 MW, mean net output 15 MW, cost £150 M

Tidal Electric Ltd. (TEL) launched an alternative proposal of a 5 sq km lagoon about a mile off-shore in Swansea Bay early in 2006, on the basis of an Atkins Consultants’ study (TEL 2004).

Swansea Bay is advantageous for its shallowness and relatively high tidal range (4m neap, 8.5m spring tides) and having some shelter from Atlantic westerly gales. The proposal attracted support from FOE and politicians as a preferable alternative to the Severn mega-barrage. Bridgwater Bay across the channel is talked of as a site for a second larger lagoon. The Welsh Affairs Committee took evidence (TEL 2006) in March and May 2006, and were critical of the DTI for a hostile economic assessment. As an outcome of the Energy Review, the government is asking the Sustainable Development Commission (SDC) to include the Lagoon scheme.

Strongly differing figures for cost of lagoon generated power have been given by Friends of the Earth (2004), TEL and AEAT (2006, for the DTI).

FOE’s 2-2.5 p/kWh was the company’s figure in 2004 with a high capacity factor 61% (for multi-basin 30MW scheme).

Tidal Electric’s 3.5p/kWh in 2005 had factor 36% for a single basin and 2-way generation at 60 MW maximum whereas the OTI team calculated 31.5% for this configuration.

AEAT in April 2006 used a factor 24% similar to the Severn Barrage’s 26% for ebb-generation only. AEAT considered that the structure dimensions adopted by Tidal Electric’s consultants (Atkins) are too small and re-calculated the costs for a more substantial structure including contingency etc. at 3.6 times TEL’s £81.5M, which gave them ~12 p/kWh for TEL’s 2-way generation and 8% discount rate (cf. 15% discount rate taken by Carbon Trust’s review of wave and tidal currents).

Others have considered the capital costs between the two – eg. at the Welsh Affairs Committee on 8th May 2006, and a figure higher than the 6p/kWh from the Severn Barrage was thought likely. After the Welsh Affairs Committee challenged the DTI’s hostility, the Energy Minister Malcolm Wicks conceded to the Committee on 28 June 2006 that the DTI accepts tidal lagoons are technically feasible and would be eligible for support through the Renewables Obligation.

On the environmental impacts, the Atkins report (TEL 2004) accepts it was a pre-feasibility study and that risks remain, particularly:

“the insertion of a large artificial breakwater within Swansea Bay will create major changes to the water circulation patterns within the bay. This may have significant impact upon patterns of erosion and deposition within the bay.”

Atkins’ wording should have read …will have significant impact… What is curious is the lack of appreciation of the importance of this issue and the absence of any proposals to address it. As in studying the Severn Barrage sediment regime, the feasibility study would probably need a scale hydrodynamic model, with special regard to the sensitive sandy beaches of the Gower and Swansea Bay.


Politicians and pundits have jumped to decisions on tidal power on the basis of limited and biased information.

The information comes from project promoters and consultants for clients.

The DTI operates as one such client, rather than impartial assessor, while the Carbon Trust and the Sustainable Development Commission also seem influenced by business interests.

This has resulted in a gross under-estimate of tidal stream resource and turbine economics, when this could supply a significant percentage (20-50%) of the UK electricity needs.

Tidal streams, lagoons and the mega-barrage lie on a continuum of increasing environmental disruption, time-scale to operation, and decreasing flexibility. Intermittency and demands on the National Grid also present large problems for the mega-barrage, in the absence of storage. The Natura-2000 status and value of the Severn is a show-stopper.

Barrages and lagoons are respectively major and minor development projects, which can also give greater or lesser coastal protection from rising sea level and storms.

The three tidal technologies are largely compatible, using different marine sites. So it makes sense to deploy tidal stream turbines (several designs), construct a pilot OTI-lagoon and progress studies of the Severn ‘Shoots’ barrage.

As a means to help meet CO2 and renewable energy targets up to 2020, and for the transition to a low carbon and renewable power future, the mega-barrage can be discounted (cannot be replicated) while tidal lagoons and ‘farms’ or fences of marine turbines appear of high significance. They could be at the large technology end of a decentralised power system.

The 2007 Energy Review is very short on commitment to renewables (due to its nuclear fixation), so it’s time to press for a positive reassessment of tidal power.


  • Welsh Affairs Committee, Energy in Wales, 2006; Third Report of Session 2005-06 (House of Commons) www.publications.parliament.uk/pa/cm200506/cmselect/cmwelaf/876/876-i.pdf
  • WWF Cymru, Engineering our future nature: The dilemma of the Severn, by Morgan Parry, 2006
  • WWF-UK Response to the Energy Review 2006 www.wwf.org.uk/filelibrary/pdf/cc_rspnsenrgyrvw.pdf
  • Robert Kirby, Comparisons between Environmental and Water Quality Issues at the Tidal Power Scheme at La Rance, France, and those expected in the Severn Estuary, UK, 2007
  • EP57, The Severn Barrage Project: General Report: Energy Paper 57; HMSO 1989
  • SPTG, The Severn Barrage: definition study for a new appraisal of the project., report for DTI/ ETSU ref.T/09/00212/00/REP, 2002 www.dti.gov.uk/files/file15363.pdf
  • Morgan Parry, Environmental Impacts of Tidal Technologies, WWF Cymru presentation, Nov. 2006
  • Carbon Trust, Future Marine Energy, Jan. 2006 www.carbontrust.co.uk/Publications/publicationdetail.htm?productid=CTC601
  • Simon Merriweather, EDF Energy, The Utility Perspective, Renewable Energy Association, Wave & Tide Symposium, July 2006
  • David Ross, Power From the Waves, Oxford, 2nd ed. 1995, ISBN 0 19 856511 9
  • Steven Schofield, BASIC Report Oceans of Work: Arms Conversion Revisited, January 2007 www.basicint.org/nuclear/beyondtrident/oceans.pdf
  • SEAFLOW marine current turbine, DTI project summary PS244, Jan. 2006
  • MCT, Preliminary Works Associated with 1MW Tidal Turbine Final Report to DTI, Aug. 2006 Ref: T/06/002/00233 33 33/00/ /00 www.dti.gov.uk/files/file35033.pdf Includes listing of competitive tidal stream technologies and full description of Stangford Lough project.
  • Swanturbines, evidence to Welsh Affairs Committee, 2006 Link
  • NWCRT (North Wales Coastal Renewability Trust, Steering Group), ECOSTAR and Pilot OTI Project, 2007
  • TEL, evidence to Welsh Affairs Committee, 2006 Link
  • TEL, Feasibility Study for a Tidal Lagoon in Swansea Bay, report by Atkins consultants, 2004
  • Friends of the Earth, 2004, A Severn barrage or tidal lagoons? www.foe.co.uk/resource/briefings/severn_barrage_lagoons.pdf
  • AEAT, Tidal Lagoon Power Generation Scheme in Swansea Bay, report for DTI and WDA, April 2006

Understanding Electrical Units

  • 1 megawatt = 1 MW. This is the power from a large wind-turbine, or standard marine turbine.
  • 1 gigawatt = 1 GW. This is the power from a large power station.
  • 1 terawatt-hour = 1 TWh = 1 GW for 1000 hour = 110 MW for 1 year.
  • Capital costs are judged against £1000/kW = £1 Million per MW. Construction of large fossil fuel power stations costs less, but renewables cost more (a few times) than this figure.

Questions for Student/group discussion

  • Politicians (Peter Hain) say that the Severn Barrage could contribute about 5% to our electricity, no other renewable energy scheme compares with this so it cannot be turned down. Discuss.
  • Politicians (Eluned Morgan MEP; Andrew Davies AM, Minister) say it’s a choice between sustainable development and conservation (ie. bird habitat). On the opposing side, WWF Cymru’s head (Morgan Parry) says “We cannot take the attitude that Climate Change is so important that we can trash habitats”. Analyse these positions.
  • How much importance do we give to project timescale, topped by 14 years for the mega-barrage?
  • Why is intermittency important for the mega-barrage, but unimportant for tidal stream turbines? Can storage be a solution?
  • What is “embodied Carbon” and can it be assessed for tidal projects?
  • How do we decide the potential for power from Marine Currents? Do we see the assessment of the consultants ENTEC and Black & Veatch as ‘conservative’ or hostile?
  • David Ross tells of the conspiracy against wave power in the late 1980s. Discuss if something similar may be happening today.
  • Electricity supply companies in the past have been oriented to large-scale power stations. Are they now open to decentralised provision? And what are the benefits of decentralisation in the marine area?
  • How do consultants produce quite difference economic assessments of new technologies? Are assessments for government bodies not ‘impartial’?
  • New marine technologies struggle to get established because of £ multi-million R&D costs. Does the UK government sufficiently help the learning and development needed for ‘take-off’?
Presented at the Sustainable Science Symposium, Centre for Alternative Technology (CAT) with Univ. of East London, July 2007. Available, including presentation slides at Sust. Science Symposium 2007

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