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There have been many proposals and government white papers written on the construction of a tidal barrage across the Severn estuary.
The most promising and feasible being carried out in 1989 by a committee of businessmen from various construction industries known as Severn Tidal Power Group (STPG), backed up by a government Sustainable Development Committee in 2009.
The STPG proposed to build the barrier across a ten mile stretch at the mouth of the Bristol Channel, which would produce electricity on the ebb tide only, but with an output capacity of over 8000MW.
In the following sections we shall have a look at two of the current tidal barrages in operation in France and Canada (other barrages are in Korea, Russia, and China) and examine the latest proposal to build a tidal barrage across the Severn Estuary, with the attendant effects on the estuary and river environment, shipping, and the local populace.
The first sections give an overview of the current operating tidal barrages in France and Canada.
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La Rance Tida Barrage
This was the world's first Tidal Barrage Power Plant, being commissioned in 1966 after 5 years construction at a cost of 620million French Francs.
The barrage that is half a mile long, supports a highway and contains the turbines, numerous sluice gates, and a set of lock gates; allowing the passage of ships up to the port of Rance from the English Channel.
It has a peak rated output of 240MW produced by 24 x 10 MW bulb Turbines, with 40% capacity factor giving an output of 96MW; supplying 600GWh/annum to the French Grid.
The bulb turbines are some twenty feet diameter and can be converted to run in reverse, so at low power periods they can pump seawater at high tide into the basin. This extra storage can then be used at peak power periods when electricity rates are higher.
Despite the high construction cost the La Rance has now paid for itself producing electricity at 1.5c/kWh to the French Grid.
Environmentally, some of the fish have returned such as cuttlefish and sea bass, but some sand eels and plaice have disappeared.
An Image of La Rance is shown below.
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Annapolis Royal Generating Station
Annapolis Royal Generating Station is a tidal barrage across the Annopolis river estuary, located in the Bay of Funday, North America.
Construction commenced in 1980, the project being completed in 1984, having an annual output of 30GWh; the power turbines being ebb-tide operating. The barrage was constructed to fulfill a few purposes other than produce power; replacing an aging iron bridge and ferry across the river, and now carries a trunk road.
Environmentally, the Annapolis Royal Generation Station has a few problems; the banks of the Annapolis River being constantly eroded. Also, a trapping zone was created where several whales have been stranded behind the barrage; presumably after following fish through the sluice gates.
An image of the Annapolis Royal Generating Station is shown below;.
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Overview of The Severn Tidal Barrage Proposal
In 2007, the UK government set up a Sustainable Development Committee, which has reported back in favor of construction of a tidal barrage across the Severn Estuary.
It has been proposed that the Severn Tidal Barrage will consist of a ten-mile span across the mouth of the Bristol Channel, effectively joining England and Wales.
The barrage will be constructed from prefabricated steel reinforced concrete caissons which will be built in various locations in the UK and towed to the final location on the estuary.
These caissons will contain the numerous sluice gates, shipping locks, and specially designed water turbines. Once in correct location, the caissons will be ballasted, sinking slowly and accurately onto the foundations on the bottom of the estuary. They will then piled, grouted, and joined up forming the ten-mile wide tidal barrage.
It is proposed that road and rail links will be laid on top of the barrage with special facilities provided where they pass over the ships locks section in the form of a bridge or a long sloping road.
The water turbines will operate on the ebb tide only this being more efficient than flood generation. This will enable the turbines to have two generating periods as the Severn estuary has a semi-diurnal cycle – two high and two low tides every 24 hours and 50 minutes.
Proposed locations for the Severn Tidal Barrage are shown below;
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Operation of the Severn Tidal Barrage
The rise and fall of the tides are caused by gravitational forces exerted on the earth by the moon and sun, either acting separately resulting in normal tides or in combination where we get the higher spring tides.
Tides can be diurnal; one rise and fall/day, or semi-diurnal having two rises and falls a day.
A large tidal range (difference in vertical height between the ebb and full tide) is essential for the turbines to operate efficiently as they depend on potential energy to drive them. The Severn Estuary has the world’s second largest tidal range of 14 meters. It also has a semi-diurnal tidal cycle which means that the turbines have two operating periods per day.
The proposed method of operation is ebb tide generation with pumping. This entails opening the sluice gates to the incoming tide which fills the area behind the barrage known as the basin, and then the sluice gates are shut.
As the tide starts to ebb the turbines can be used as pumos, pumping more seawater into the basin thus increasing the amount of stored potential energy.
Once the tide has ebbed sufficiently, the turbine inlet gates are opened and the turbines start to generate power; generation continuing until the water level in the basin is insufficient to drive the turbines.
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There are several water turbine designs for tidal barrages such as the proposed Severn Tidal Barrage. They are of the propeller type, running submerged in a draft tube incorporated in the barrage caisson wall. The report by the sustainable development committee came out strongly in favour of the two types of water turbine designs used in the Rance and Annapolis Tidal Barrages. These are the bulb and rim turbines and although both have proved successful another water turbine design has been developed known as the tubular turbine which has some innovative features particularly suitable to tidal barrage power production. Rim and bulb turbines require to be removed from the draft tube for maintenance, whereas the tubular turbine can be maintained in situ, but the bulb turbine is the only one capable of acting as a pump, pumping water into the basin to add to the stored potential energy contained behind the barrage.
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Types of Turbines Suitable for Tidal Barrage Generation
There are several current designs of water turbines suitable for tidal barrage power generation and these are predominately propeller types as examined below:
- Bulb Turbine
This is the type of water turbine used at La Rance Tidal Barrage. The turbine generator lies horizontally in a draft tube within the concrete caisson, operating whilst totally enclosed in water. The power generator is contained within a waterproof bulb at the front of the unit, whose contour is designed to enhance the flow of water to the turbine propeller drive situated behind the bulb.
The turbine and generator are reversible to enable it to generate power on the flood tide or act as a motor to pump seawater into the basin, as discussed earlier.
The main disadvantage with the bulb turbine generator is that it has to be lifted clear of the water for generator or turbine maintenance.
The bulb turbine has proved very reliable, operating nearly constantly without major problems for over 30 years in La Rance Tidal Barrage. A sketch of a bulb turbine is shown below;
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- Tubular Turbine
This turbine is the most recent tidal barrage water turbine design, having a three-bladed propeller turbine which is mounted at an angle in the draft tube. A drive shaft protrudes upwards at the same angle and drives the generator which is mounted above the waterline on the concrete structure of the caisson.
Maintenance can take place in location when water supply has been isolated.
Cannot be reversed to operate on flood tide or used to pump storage.
A sketch of a tubular turbine is shown below;
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- Rim turbine – the Straflo
Early designs were more suitable for run of river applications due to water seals leaking under pressure; however recent improvements have made them more reliable, there being a 20MW Straflo turbine currently installed in the Annapolis River Tidal Barrage in Canada.
In this type of turbine the generator rotor is fixed around the rim of the turbine with the stator set into and supported by the caisson concrete structure. The rotor is protected from ingress of sea water by especially designed water seals.
It can only operate on the ebb tide and cannot be used to pump storage to the basin.
It has to be removed for maintenance of the turbine; although the generator can be accessed when water inlet gate been closed and water drained off.
Water seals still give occasional problems
A sketch of a rim turbine is shown below;
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Advantages and Disadvantages of the Severn Tidal Barrage
- Installed capacity of 8.5 GW equating to 5% of UK power requirement.
- Renewable energy thus increasing our percentage of power from renewable sources.
- Savings of over seven million tons of CO2 every year
- Long-term construction project which will give employment to 35000 construction workers
- Protection from rising sea-levels and storms
- Access between England and Wales via road and rail links across barrage
- Protected area for water sports such as yachting, canoeing and fishing
- Boost to local businesses and tourism enterprises
- Power generating sequence.
The power will come in two large bursts which the national grid will have trouble distributing. It can coincide with a period where little extra power is needed such as during the night.
There are several endangered, rare species of fish which currently migrate into the Severn estuary and river notably shad, lamprey, Atlantic salmon, and seatrout.
There are thousands of migrating birds which use the estuary for overwintering and breeding grounds, among these shell duck, redshanks, and many species of geese and waders.
Water quality and feeding grounds
There will be changes to currents which will affect water turbidity and sedimentation leading to detrimental effects to plankton, and erosion of salt marshes, mudflats, and sandflats.
From the above environmental concerns, the tidal barrage would seem to herald an environmental disaster. However there may solutions to these concerns such as creating a new habitat close by to accommodate the wildlife, and methods of protecting fish against damage by the turbine blades and help for the migrating fish such as fish ladders used in hydropower dams.
In the end Environmentalists will have to decide between damage to the estuary wildlife through a tidal barrage or through global warming and climate change, escalating due to the use of fossil fuels.
Climate change will probably cause more damage to the Severn estuary through rising sea levels and species diversion, but can be somewhat mitigated by the use of a large scale renewable energy project saving millions of tons of CO2 emissions.
Projected costs have been put at 14 billion pounds, most of which will have to come from investment by businesses with the rest coming from the government.
However, given today’s economic climate, businesses, especially banks, are being very cautious and this does not bode well for the Severn Tidal Barrage investment, as any money invested will be for the long-term.
Update December 2011
In 2010, the British Government decided to shelve the Severn Tidal Barrage Project citing costs, lack of long-term investment from private companies, along with environmental problems. They have chosen Nuclear Power as the alternative to the CO2 emission from fossil fuelled power plants.