* Plant could meet 9 pct of Welsh electricity demand
* Developer hopes for govt approval by early next year
By Nina Chestney
LONDON, Feb 7 (Reuters) - A British company has applied to build the world's first plant that generates power from the ebb and flow of tides in a lagoon, potentially helping accelerate the country's ambitions to harness marine energy by 2020.
The proposed lagoon in Swansea Bay, south Wales, would be able to generate enough renewable energy to power more than 120,000 homes, meeting around 9 percent of Wales' electricity demand, according to the firm.
The application by Tidal Lagoon Power will have to be reviewed by Britain's Planning Inspectorate before going to government for final approval, which the company hopes will happen early next year.
The lagoon could reach an installed capacity of 320 megawatts (MW) when spring tides create a larger head of water. With an average head of 4.5 metres (15 feet), power output would be 240 MW.
"The UK has the second-highest tidal range in the world and today we are submitting an application for a development that will prove that this resource can be harnessed in a way that makes economic, environmental and social sense," said Mark Shorrock, chief executive of Tidal Lagoon Power.
The company has spent three years developing the project, along with a consortium of engineering firms including Atkins, Costain, General Electric, Alstom, Andritz and Voith.
The project, costing up to 850 million pounds ($1.4 billion), would involve constructing a 9.5-km (6-mile)breakwater wall near the port of Swansea which would enclose an 11.5-square-km tidal area.
When the tide drops, there is a difference between the water levels in and outside a lagoon. The water would pass through turbines to produce electricity. Similarly, when the tide rises, power would be generated as water fills the lagoon.
If approved, construction would start in the first half of 2015, with power delivered to the grid by 2018.
In addition to Swansea, Tidal Lagoon Power hopes to build five more commercial-scale tidal lagoons in UK waters within the next decade.
Britain wants to boost renewables production to help it meet legally binding targets to cut carbon emissions and to replace ageing nuclear reactors and polluting coal-fired power plants, up to a fifth of which face retirement this decade.
The government believes marine energy can meet up to 20 percent of current electricity demand and hopes to have around 200-300 MW of wave and tidal power installed by 2020.
Some green groups estimate that tidal lagoons could be less damaging to wildlife habitats and the environment than other forms of tidal power, but say further research is needed.
Globally, the industry is in its infancy compared to other sources of renewable energy such as wind or solar power, with no large-scale commercial wave or tidal facility in operation.
Tidal projects, including lagoons, can negotiate with the UK government for support through a subsidy scheme, and Tidal Lagoon Power hopes to tap that resource, though construction will be privately financed.
Australian bank Macquarie has committed to lead the capital financing of the project, the firm said.
British lawmakers last year recommended the government reject plans to build one of the world's biggest tidal energy projects in the Severn Estuary between England and Wales, saying the proposal lacked commercial viability.
But Scotland gave the go-ahead to a 96-MW tidal array project in Pentland Firth, which separates the Orkney Islands from mainland Scotland.