A Scottish energy firm's tidal array system has become the first in the world to deliver electricity to the national grid from two linked underwater turbines.

Nova Innovation said its Shetland Isles project represents major progress in using tidal energy as a long-term source of predictable renewable power.

The company installed its first turbine in the Bluemull Sound in March, with the device generating to full power across all tidal conditions.

A second turbine was installed in August to work alongside the first.

As the firm's goal is to have a large number of turbines connected in an "array", the use of a second one marks a major milestone.

Most existing tidal schemes use just a single turbine to produce electricity and Nova Innovation described its array system - linking two tidal machines - as a world first.

Other tidal turbines, such as one installed in 2008 at Strangford Lough, Northern Ireland, have provided power to local grids from one turbine.

Simon Forrest, managing director of Nova Innovation said: "We are absolutely delighted to be the first company in the world to deploy a fully operational tidal array."

He added: "Deploying the second turbine truly sets us apart and showcases our technology. I would like to thank all our staff, partners and suppliers for helping to make our vision a reality."

Jenny Hogan, director of policy at Scottish Renewables, said: "Scotland is already at the forefront of capturing power from the tides and waves, and Nova's latest news demonstrates that lead is well-deserved.

"The country is already home to some of the most advanced marine energy technologies anywhere, as well as the European Marine Energy Centre - arguably the most advanced marine energy proving site in the world.

"With companies like Nova and others all working on developing this cutting-edge technology, the sector holds huge promise for the future."

WWF Scotland director Lang Banks said: "News that power has been exported to grid for the first time by a pair of tidal devices marks yet another major milestone on Scotland's journey to becoming a fully renewable nation.

"With some of the most powerful tides in Europe, Scotland is well placed to lead in developing this promising technology, which will help to cut climate emissions and create green jobs right across the country.

"How big a role tidal power will play in our future energy depends on the ambitions of our politicians today.

"The Scottish Government's forthcoming energy strategy provides the perfect opportunity to set out a bold vision for how we could become Europe's fully renewable electricity nation by 2030, ensuring that we secure the maximum economic and social benefits that will arise from a shift toward a zero-carbon economy."

Scottish Energy Minister Paul Wheelhouse said: "This is yet another important step forward for Scotland's renewable energy industry and tidal power in particular.

"The Scottish Government is determined to support companies like Nova Innovation who are delivering the kind of innovative technology needed to fully capitalise on Scotland's vast potential to generate clean, green energy.

"I met with management at the company recently at their premises in Leith, to hear about the testing they have been undertaking, and I know they have received public sector backing to reach the point where they are able to deploy two devices at the Bluemull Sound site.

"I hope they will be able to go on delivering even more success in deploying tidal power and pushing the boundaries of marine energy generation."