By Mark Kinver
Science and nature reporter, BBC News
In January, the UK government and Welsh Assembly launched a two-year feasibility study into the possibility of harnessing tidal energy in the Severn estuary in order to generate electricity.
It has the second largest tidal range in the world, which could be used to meet up to 5% of the UK's electricity needs.
Two forms of technology are being assessed by the study: tidal barrages and tidal lagoons.
The concepts are not new and there have been plans to tap into the river's resource since the middle of the 19th Century.
The big tidal range also makes the area an internationally important habitat for migrating birds.
Environmentalists say that a 16km (10 mile) barrage stretching from Cardiff to Weston-super-Mare would destroy vital feeding sites.
The feasibility study, which is expected to conclude early in 2010, is tasked with identifying a "single preferred tidal range project" from a number of options proposed.
TIDAL ENERGY TECHNOLOGIES
There are two different types of systems that use tidal energy to generate electricity: tidal stream and tidal range.
With a tidal range of about 15m, the Severn is an untapped energy source
Tidal stream systems utilise the energy in fast-flowing current to turn the blades of free-standing turbines, in a similar way that the wind turns turbines on land.
This technology is gaining favour because it has lower capital costs and is deemed to have a lower environmental impact than tidal range systems.
Tidal range technology, such as barrages and lagoons, exploits the potential energy in the height difference between high and low tides (known as the tidal range).
It works by impounding a large volume of water at high tide, which is then released at low tide through a bank of turbines.
Because the tidal range of up to 14m (46ft) in the Severn estuary is the second largest in the world, technologies that can exploit the energy of the twice-daily tide, such as barrages and lagoons, are seen as the preferred option.
Also, engineers say that the relatively shallow waters within the estuary make it unsuitable for tidal stream devices, which need to be placed within depths of about 30m (100ft) or more in order to operate most effectively.
SEVERN BARRAGE PROPOSALS
Although there are a number of proposals for tidal barrages stretching across the Bristol Channel, the feasibility study is likely to concentrate on two options.
The first is the widely reported scheme that plans to build a 16.1km (10 mile) barrage from just south of Cardiff across to the Somerset coast, near Weston-Super-Mare.
The other is the "Shoots" proposal, a much smaller project that would see a 4.1km (2.5 mile) barrage stretch across the Severn at roughly the same point at the current two road crossings.
The Sustainable Development Commission (SDC), the government's independent advisers, published a report in October 2007 that examined the potential of tidal power around the UK's coastline.
In it, The SDC compared the two proposals:
17 terawatt hours
2.8 terawatt hours
(Source: Sustainable Development Commission)
The Commission's report estimated that the large Cardiff-Weston plan would cost in the region of £15bn to construct, while the smaller Shoots scheme would require £1.5bn.
If given the green light, the barrage is expected to take about 12 years to build.
Although the construction of a barrage would be a complex undertaking, engineers say the basic concept is well understood and the technology is also commercially available.
The La Rance barrage in northern France, which has been generating electricity since the 1960s, is often used as an example of how tidal power can be successfully utilised.
SEVERN LAGOON PROPOSALS
Tidal lagoons operate in a similar way to barrages in that they exploit the difference in tidal height to generate electricity.
However, the structures do not fully obstruct the estuary; they are either free standing offshore or use the shoreline to form part of the lagoon's edge.
The schemes are generally considered to be technically feasible from an engineering point of view, but there are currently no examples of an operating lagoon in the world.
Although the feasibility study will consider possible locations for the lagoons, the most well-known proposal in the past was the "Russell lagoon" concept, which was a series of three enclosures built against the banks of the Severn.
The SDC estimated that the Russell scheme would have been capable of generating about 6.5 terawatt hours of electricity each year.
Because there are no operational lagoons, there is a degree of uncertainty over how much electricity they can produce.
For the same reason, experts say that it is also difficult to estimate how much it would cost to construct a tidal energy lagoon.
The Severn estuary is subject to a series of national and international designations that are aimed at protecting the area's unique habitat.
In its 2003 Energy White Paper, the UK government even decided that the construction of a tidal barrage was not a good idea because the project raised "strong environmental concerns".
Pintails (Anas acuta) could be among the species to suffer, the RSPB warns
However, it did add that officials would continue to explore opportunities.
The key concern relates to the loss of inter-tidal feeding areas for water birds.
Groups such as the RSPB and WWF say that if the Cardiff-Weston project went ahead, much of this vital feeding area that currently supports tens of thousands of migrating and wintering birds would be permanently submerged.
Recognising this concern, the SDC recommended that any project given the green light would have to be in "full compliance" with EU habitat and wildlife directives.
The Commission also stated that it was vital compensatory habitat was provided "on an unprecedented scale" in order to replace the lost feeding sites.
The SDC added that it was essential for the scheme to be publicly funded and publicly owned in order to ensure compliance to the environmental legislation.
A report commissioned by a number of organisations, including the RSPB and WWF, said that it would be a waste of public money because other renewable energy sources, such as wind, would be more efficient and effective.
Friends of the Earth Wales compiled a report backing the use of tidal lagoons in the area.
Although a scheme such as the Russell lagoons would cover about 60% of area affected by the Cardiff-Weston proposal, the environmental group argued that it would leave much of the inter-tidal mudflats and salt marshes unaffected.
However, the SDC report concluded that it did not "consider that a large-scale tidal lagoon development in the Severn estuary would offer any environmental or economic advantage over a tidal barrage".