Wave & Tidal Power in the Severn?

CONTENTS

• “best prospect is tidal current turbines”

• Severn Barrage Details

• Power from Marine Tidal Currents

• Map of Tidal Resource

• Tidal Pools, Lagoons or ‘Offshore Tidal Impoundments’

INTRODUCTION

A Tidal Barrage for the Severn estuary is again a hot issue, with Britain’s need to rapidly build up renewable energy generation. Barry lies close to the landfall of the proposed mega-barrage and the local FoE group (Barry & Vale) have looked into the various options. Our preference is for tidal current turbines, but we see the debate as continuing.

We recommend reading first this section 1. which outlines the issues.
The later sections provide further detail and links to documents.

Section 1 covers:

• Severn Estuary – the second highest Tidal Rangetidal range The difference in height between high and low tide in the world
• The Severn Mega-Barrage
• Comparison of Tidal Barrage and Tidal Currents
• Barry & Vale Friends of the Earth statement
• Tidal Impoundments or Lagoons

1: “best prospect is tidal current turbines”
Barry & Vale FoE contributes to the debate

Friends of the Earth (FOE) strongly supports tapping tidal energy around Britain, viewing this as an important part of our energy policy which looks for a big contribution from renewable sources. Members of Barry & Vale FoE who live by the Severn Estuary and use it for recreation are very aware of the power in the estuary’s tidal ebb and flow.

However the mega-Barrage across the Severn from Lavernock to Brean Down (map below) is neither the only option nor an obvious choice. Instead of endorsing this, Barry & Vale FOE believe that the National Assembly for Wales should be more seriously promoting the best prospect – tidal current turbines. Wales has abundant resources of marine energy, in wind and waves as well as tidal currents. Efforts to use these via moderate scale arrays of turbines and wave power devices can give quick dividends. Until progress is made on this moderate-scale technology, the mega-Barrage should be put on the back-burner.

FOE’s briefing on the Severn barrage and lagoons was issued in 2004 and generated much interest and debate. Early in 2006, a specific plan for a tidal lagoon 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 Select Committee reported (pdf file) in July 2006 after taking evidence on these and other energy resources in Wales. This briefing takes account of the new material, but recognises the debate is ongoing.

The Severn Mega-Barrage

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 wall would pass close to and just east of Steep Holm Island and two miles west of Flat Holm Island. The Barrage would incorporate lock gates to allow shipping and smaller craft to access the port at Bristol, other docks and the River Severn. The installed capacity, or maximum output, of the proposal would be 8.64 gigawatts (GW).

The scheme as drawn up by the Severn Tidal Power Group (STPG) could generate 17 terawatt hours per year (TWh/yr). A revised report was published in 2002 by the DTI
The proposed Barrage would generate about 5.4% of current England and Wales electricity demand of 350 TWh/yr and cut 18 million tonnes CO2 per yr (from the UK’s ~600 Mt CO2 per yr total). The scheme would generate only on the ebb-tide, operating about 5 hrs out of the 12.5 hr cycle. To help phase with electricity demand, discharge times would be adjusted and pumping used for ~ 1hr in the cycle (See Severn Barrage Details – Article 2).

Diurnal fluctuations can 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.

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

Barrages tap energy in the tidal rise, yet there is also energy in the tidal currents. Generating energy from the currents is considered much more feasible nowadays. Tidal currents are tapped in both ebb and flood directions, by submerged turbines much smaller than wind turbines, because the power density in water currents is much larger than in air currents.

Tidal currents deliver more continuous power than a barrage and use of several sites with a range of tidal timings would give reasonably steady overall power. Unlike the Severn Barrage and (to lesser extent) Severn tidal lagoons, they would not need supporting power stations to fill in the slack periods.

Barry & Vale Friends of the Earth statement of 3rd August 2006

• Marine current turbines are real prospects now (Marine Tidal Currents – See Article 2) with large-scale installation of the leading design from the company, Marine Current turbines (MCT), from 2009. Alternative designs (See Article 2) 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 (Tidal Current resource – See Article 4).
• Cost estimates appear similar at 6p/kWh, but the Barrage costs will have inflated since 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 only 40% of the tidal cycle and needs several major power stations operating to back it up. It also generates over 4 times as much at spring tides compared with neap tides, out of phase with the winter peak demand.
• The Barrage power being huge for limited parts of the tidal period requires the National Grid to build and upgrade power lines.
• The visual intrusion and footprint of the Grid lines will impact seriously on the eastern Vale of Glamorgan.
• Immense tonnages of aggregates for the Barrage embankment would have to come from limited supplies – potentially from south Wales quarries and the Mendips.
• The road building and construction site for the Barrage at Lavernock Point would over-ride the Unitary Development Plan and destroy the rural landscape in the Penarth-Sully coastal strip.
• The conservation interests of the Severn Estuary protected area, a European Natura 2000Natura 2000 A European network of protected sites developed to maintain or restore natural habitats and species of wild flora and fauna to favourable conservation status within the European Union. site, for wading/migration birds, special fish species and tidal mudbanks will be strongly impacted, though not destroyed.
• The conservation interests and a management plan for the inner estuary need defining (ending the delay by the Countryside Council for Wales) before potential impacts of the Barrage can be assessed.

Tidal Impoundments or Lagoons

The Tidal Lagoon proposal has come into contention since Tidal Electric Ltd. came to Wales to float the concept in 2000. A quasi-oval embankment in shallow waters would have entrance/exit channels containing low-head turbines driven by height difference on the incoming and outgoing tides.

An area in Liverpool Bay north of Rhyl (map above, courtesy of Cllr Dr. Stuart Anderson) is one proposal, but that for Swansea Bay (article 5) has won official credibility. For this, Tidal Electric Ltd. (TEL) are proposing a single-basin lagoon of 5 sq km area, with ~22 MW av. output (maximum 60MW = 0.06GW). They envisage generating power on both flood and ebb tides, as silting is said to be less of a problem than for a barrage. Like a tidal barrage, lagoons have much higher output during spring tides and lower during neap tides (factor ~2 either way) so add less to generation capacity over the peak winter demand. The company estimates that large schemes in the most optimum sites could impound up to 300 sq km with an installed capacity of about 4.5 GW.

The environmental issues have been given a little attention. The tonnage of aggregates for constructing embankments is significant and geo-textile bags filled with dredged silt or scrapings from within the impounded area could avoid some of it. Silting up of the lagoons is seen as manageable, but the great uncertainty in Swansea Bay (article 5) is potentially large effects on erosion and deposition, which the consultants have not addressed. Deposition of muddy silt from the two rivers discharging into the Bay would increase. It’s likely that sandbanks will build up between the lagoon and Swansea beach, from the large amounts of sand in suspension in the wider estuary. That the suspended sand might be replenished from beaches on the Gower is a sensitive concern that could scupper the scheme. TEL and their partners need to face up to this serious issue, which is difficult if not impossible to predict.

2: Severn Barrage Details

The new DTI government report judges that ebb-flow only generation makes a contribution to “firm” capacity of 1.1GW, compared with the installed capacity of 8.6GW. The new DTI report identified two ways of enhancing the firmness of supply:

1. varying the start time of generation
2. including a low head pump 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 of the Severn Barrage is £10.3-£14.0 bn at 2001 prices. Costly strengthening of the Grid is needed for such a large scheme and that figure includes the full cost of grid reinforcement, escalated to 2001 prices (approximately 13% of total capital cost, ~ 1.0p/kWh).

The DTI argued 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 negatives of destruction of a European conservation area (the government has failed to notify the EC of this Special Area of Conservation, presumably because of the Tidal Barrage possibility, but this failure on “socio-economic grounds” contravenes the Habitats Directive) and said little about impediment to shipping.

The mitigation of flooding risk in the Severnside Region is given substantial 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 £40-200M/yr is expected to rise due to climate change to at least £4,000M/yr by 2075. This risk and the associated capital expenditure costs to improve flood defences would be avoided if the Barrage were constructed. However, coastal retreat has been identified as the preferred strategy for much of the estuary, so the real costs must be much lower.

At current values, the DTI 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 attract favourable prices, yet the uncertainty of big construction projects with turbine technology well beyond the present experience in size and hostile operating conditions (abrasive sediment) goes 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 then required.

Impacts on the conservation areas: WWF-UK in their 2006 Response to the Energy Review explains that the barrage would have a direct impact on four sites protected under European conservation legislation: the Severn Estuary Special Protection Area, and proposed Special Area of Conservation, and the rivers Wye and Usk SACsSAC Special Areas of Conservation. The project would have to pass the stringent tests contained in the EU Birds and Habitats Directives: that there were no alternative solutions and there were 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. While 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-UK.

The statutory conservation Agencies have come out strongly critical, saying that the Severn barrage would cause “irreversible impacts to features of international importance”.

3: Power from Marine Tidal Currents

The Carbon Trust’s Future Marine Energy (January 2006) www.carbontrust.co.uk/Publications/CTC601.pdf says the costs are uncertain but concludes:
tidal streamtidal stream The flow of water through channels or around coastlines as a result of tidal water movement 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. The 2.8 GW total capacity was said at the REA (Renewable Energy Association) Bristol conference in July to be grossly underestimated, by Prof. Stephen Salter.

A particular design shown in Marinet’s report of the Bristol REA conference was tested in the Severn estuary off Lynmouth, Devon by the company Marine Current Turbines (MCT) in small version (‘Seaflow’) and now the commercial prototype, the 1MW power ‘SeaGen’ is to be erected in the Strangford Loch narrows. The major supply company EDF reported at the Bristol conference that they are confidently backing this design. EDF plan to follow the Strangford Loch commercial trials with a 20-strong array of these turbines in 2007-8 and predict they will supply power at ~ 6p/kWh. They show costs reducing to 4p/kWh or less, much better than the optimistic 6p/kWh predicted for Severn Barrage power. EDF plan from 2009 to be building commercial installations of over 50 MW wherever they can and look to a global market of thousands of MW. Off the north and west of Anglesey is the first target area in Wales. In comparison, the Severn Barrage would not come on stream until 2019-20.

There are alternative tidal current designs, but not as far advanced as MCT’s monopile carrying twin turbines.
Norway’s Statkraft promotes turbines suspended from a floating structure. The TidEl system uses floating turbines anchored via chains to the seabed. Both these 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’ pole carrying a single turbine is carried on a large base sitting on the seabed – being completely submerged, it’s feasible for the deeper water (>30m deep) when 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 they are aiming to site 30-50 units by 2010-2011 in the seas off Pembrokeshire or south of Barry in the Severn Estuary.

4: Map of Tidal Resource

The tidal resource includes both deep water (depth >30m) and shallow water (depth 20-30m) with a mean peak spring tidal current speed >2m/s. The Path to Power: Stage 2 report gives higher definition maps for deep and shallow water resource separately. The four filled coloured circles refer to the times by which the tidal currents could come on-stream. The poor prospects for northern Scotland is inexplicable, in view of the fact that Pentland Firth is one of the most promising sites.

5: Tidal Pools, Lagoons or ‘Offshore Tidal Impoundments’

An early favoured location was off the Welsh coast north of Rhyl, where there’s a ready supply of aggregates (slate waste) and it would shelter this vulnerable coast from storm surges in wave height. Friends of the Earth’s Briefing in 2004 accepted TEL’s (Tidal Electric Ltd.) attractive cost estimates and gave the idea very favourable comparison with the Severn Tidal Barrage (in both cost per unit and total resource around Wales).

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.

The Rhyl proposal is being taken forward by a still informal group including County Councillors, Cardiff University engineers and RWE/npower (of the North Hoyle offshore wind farm). They call it an OTI (Offshore Tidal Impoundment) and three of the team described it at the MAREC conference (see above). The OTI team believe (Dr Anderson Ev.248 in the Welsh Select Committee report) more turbines are needed and total cost is higher than TEL’s figure, probably higher than offshore windpower. As a pilot project, their OTI would be about 6 sq km in area and 100 MW peak capacity. There are ideas of providing for leisure moorings, a commercial harbour and constructing a pier part or all way to the embankment. A wind turbine could also be erected on the embankment. The aerial sketch below (provided by Dr Stuart Anderson) is a provisional layout only.

TEL went public with their specific proposal of a 5 sq km lagoon about a mile off-shore in Swansea Bay early in 2006. This 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 Friends of the Earth, WWF Cymru and some politicians (Welsh Lib-Dems) as an environmentally preferable alternative to the Severn Tidal Barrage. Bridgwater Bay across the channel is talked of as a site for a second larger lagoon. The Welsh Affairs Committee took evidence on TEL’s scheme in March and May 2006, while the DTI have encouraged the company to submit a formal application. As an outcome of the Energy Review, the government is asking the Sustainable Development commission to examine the Lagoon scheme as well as the Severn Tidal Barrage.

Strongly differing figures for cost of lagoon generated power have been given by Friends of the Earth, AEAT (for the DTI and WDA). 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 has factor 36% for a single basin and 2-way generation at 60 MW maximum whereas the OTI team calculates 31.5% for this configuration. AEAT in April 2006 use a factor 24% similar to the Severn Barrage’s 26% for ebb-generation only. AEAT consider the structure dimensions adopted by Tidal Electric’s consultants (Atkins) are too small and recalculate the costs for a more substantial structure including contingency etc., obtaining 3.6 times Tidal Electric’s £81.5M, which gives ~12 p/kWh for TEL’s 2-way generation and 8% discount rate (the Carbon Trust’s review of wave and tidal currents took 15% discount rate). Others consider the capital costs come between the two – eg. at the Welsh Affairs Committee on 8th May 2006, a figure higher than the 6p/kWh from the Severn Barrage was thought likely.

Structure of embankment for the TEL scheme.
The lower structure of mainly rock with little fill is for the inshore embankment.
The upper, more substantial structure is for the deeper outer embankment, subject to stronger seas. (from TEL www.tidalelectric.com)

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