Offshore tidal power
The construction of a revolutionary new tidal power station, which promises to supply half the power needs for a city the size of Swansea, is due to begin early next year.
Tidal Electric’s £35m project, the first of its kind in the world, is to be built in Swansea Bay, South Wales, and is scheduled to start production in late 2005. There are currently two commercial-scale tidal power stations in operation around the world: a 240-megawatt plant at La Rance near St Malo in Brittany, and a 16-megawatt facility at Annapolis Royal, Nova Scotia, Canada. The development of further projects has been held back by a range of significant environmental and logistical problems, including:
• tidal-power barrages confuse and kill migratory fish, which must pass through the turbines to access estuaries or seas – the mortality rate is around 6 per cent;
• navigation channels become blocked – locks can be installed, as at St Malo in France, but access is slow and expensive;
• barrages change the size and location of the inter-tidal zone;
• barrages can alter the tidal cycle and change water levels – local wildlife is forced to adapt, move, or die;
• barrages obstruct the natural ‘flushing’ of tidal basins;
• and the St Malo barrage has been plagued by the silting up of the enclosure. Tidal Energy in numbers 200 miles – the longest tidal bore in the world runs up the Chientang River from the China Sea;
• it reaches a height of 30 feet 60 the height in feet of the largest tidal range in the world, in the Bay of Fundy, Nova Scotia, Canada 10 feet an hour – the rate at which the water level in the Bay of Fundy rises;
• as fast as the water in a bath tub with both taps opened full 0.25 pence per kilowatt-hour – the estimated cost of electricity from the Swansea Bay project once the development and construction costs have been repaid 12 knots (15 mph) – the speed of the fastest flowing tidal stream in the UK, in the Pentland Firth between Orkney and mainland Scotland 25 factor by which the volume of the Gulf Stream, the world’s best known tidal current, exceeds the combined flow of all the world’s rivers Tidal Energy France built the first and, with a 240-megawatt capacity, still the biggest tidal power station in 1966 at La Rance near St Malo in Brittany.
Tidal energy is actually one of the oldest forms of energy used by humans. Tidal mills were common in Europe as far back as the Middle Ages, when water captured in millponds at high tide was used to drive water wheels as tides fell. Power was typically used for grinding grains into flour. The daily tidal cycle lasts for 24 hours and 50 minutes. During a 24-hour period, the moon travels about 12 degrees off its 360-degree orbit. The tidal cycle’s extra 50 minutes accounts for the time it takes the earth to ‘catch up’ with the moon’s new position.
Tidal currents are vital to many forms of marine life. For example, sessile filter-feeders such as clams, sponges and seasquirts rely on plankton carried to them by tides. Offshore tidal power (OTP) involves minimal environmental impact; there are zero emissions, a limited affect on shoreline geomorphology, and migratory fish can simply swim around the structure. It could provide habitats for many species – from micro-organisms to birds.
A small number of large OTP plants in the Severn Estuary could be a direct replacement for Somerset’s ageing Hinckley Point nuclear power station, which is due for closure in 2011. OTP plants produce cheap electricity: the projected initial cost of electricity from the Swansea Bay project is two pence per kilowatt-hour – that’s similar to the cost of conventional gas-fired power. None of the technology needed is new. An OTP generator uses conventional hydroelectric generation equipment, which has been in existence for more than 120 years. The structure that holds the water itself can be made from local waste materials. Tidal power is inherently predictable.
This article first appeared in the Ecologist July 2003