Tidal power
How can the technology be used?
What is tidal power?
The tides are reliable and predictable: we know they come twice a day, filling and emptying natural basins along the shoreline, and are caused by the interaction of the gravitational fields of the earth, moon and sun.
There are three ways of utilising tidal energy: tidal barrages, tidal lagoons and marine current devices.
How does the technology work?
Tidal Barrages
These schemes are work much like hydroelectric dams, except on a larger scale.
A 'barrage' is constructed across a tidal bay or river estuary. When there is a suitable difference in the height of water on either side of the barrage, gates are opened and the water flows through tunnels in the dam, which are fitted with turbines and generators.
Locks are included, much like in canals, so that ships can pass through. As there are two high and two low tides in each day, the barrage will only generate every six hours. In order to produce practical amounts of electricity, a difference of at least five metres between high and low tide is requried.
Tidal lagoons
Tidal lagoons are a new approach to generating electricity using tidal power. They work by constructing a rubble mound structure to temporarily trap tidal flows.
At low tide, visually this would look like a typical breakwater seen around marina and harbour approaches. The lagoon enclosure fills at high tide, and as the tide goes down this creates a 'head', which powers turbines and generators in the walls.
An animated explanation is available on the Tidal Electric website. It is possible to subdivide the lagoon using less substantial walls. This optimises the power output to supply variations in demand and act as storage, similar to hydro pumped storage.
Marine current turbines
Marine current turbines make use of tidal streams through devices that are similar to submerged wind turbines to harness the energy of tidal currents.
The turbines are installed at places with high tidal current velocities or continuous ocean currents and drive generators in the same way as hydro or wind turbines. Current test turbines are rated from 750 to 1500kW per unit.
Suitable for the UK?
Tidal Barrages
There a relatively few sites around the world where it would be suitable to use this technology, but several of these are on the west coast of the UK.
A 10-mile tidal barrage has been proposed for the Severn estuary from the west of Cardiff to Somerset. The barrage would have an average output of between 1.95 - 2.17GW, producing 17-19TWh of electricity per year.
This is equivalent to 4.7% of the current UK demand for electricity, or the total demand for electricity in Wales.
Before this plan could go ahead, a range of environmental effects would need to studied, such as salinity, oxygen levels, sewage dispersion, fish migration, sediment transport, erosion and deposition.
The Severn Estuary is a sensitive, environmentally protected area that is being proposed for Special Area for Conservation designation due to the importance of its ecology.
Tidal Lagoons
The world's first tidal lagoon is being proposed for Swansea Bay. It would be sited near Port Talbot and enclose two square miles of sea and have an installed capacity of 30MW.
This could be a more environmentally sensitive option for harnessing the tidal energy of the Severn Estuary. Due to its high tidal range the Severn Estuary is a prime site for tidal power.
It is estimated that using sites in the Severn Estuary for tidal lagoons could enclose up to 115 square miles and generate 24TWh per year, or 7% of the electricity demand of England and Wales.
As these schemes are near-zero carbon emittors, they could save 23 million tonnes of carbon dioxide per year.
Liverpool Bay has also been identified as a potential site for tidal energy generation.
Marine current turbines
Many site have been idenified by Marine Current Turbines as potential locations. This technology is still in prototype stage, but there is a great potential in the UK.
Links to companies involved with tidal power
The benefits
Marine energy in the UK has the potential to create long-term employment, and will provide opportunities to export technologies and skills around the world.
There are negative impacts on ecosystems caused by damming an estuary or bay, but lagoons may offer a more ecologically sound alternative.
Marine current technology is still in its infancy but, according to the manufacturers the rotor speeds will be low enough to enable marine wild life to avoid being harmed.
Other green energy sources
Artists impression of a Marine Current Turbine
Map of possible sites for marine current turbines. Source: Marine Current Turbines
