An underwater turbine destined to form part of a major green energy project is already using tidal power to produce electricity for homes on a northern island.

The 100ft-long device, which can provide enough energy to meet the electricity needs of 500 homes for a year, has completed its initial testing period.

Scottish Power Renewables said the results gave them "great confidence" in the turbine which will be installed in the world's first tidal array in the waters off Islay.

The one-megawatt turbine was installed in the sea off Orkney in December in some of the worst weather experienced for more than decade. It is already providing power for homes and businesses on Eday, one of Orkney's northern isles.

The firm plans to use the device in a 10-megawatt tidal array in the Sound of Islay. Planning permission was granted by the Scottish Government in 2011 and it is hoped work can be completed by 2015.

Keith Anderson, chief executive of Scottish Power Renewables, said: "The concept of generating electricity from the natural movement of the tide is still relatively new - and test projects like this are vital to help us understand how we can fully realise the potential of this substantial energy source."

The performance of the turbine "has given us great confidence so far", he said.

"Engineers were able to install the device during atrocious weather conditions, and it has been operating to a very high standard ever since.

"We have already greatly developed our understanding of tidal power generation, and this gives us confidence ahead of implementing larger-scale projects in Islay and the Pentland Firth."

Mr Anderson added: "Scotland has the best tidal power resources in Europe and that's why we are seeing world leading technologies tested here."