PowerSwitch

I came across this article which lists some of the biggest marine power projects in the world, in terms of current installed electrical capacity. Due to the high investment in setting up the project, a typical tidal power project is expected to break even between 8 and 12 years after commissioning. However I believe despite the long gestation period to make it commercially viable, tidal power has unparalleled environmental advantages .

http://www.power-technology.com/feature ... -projects/

That second graphic implies that the tide only ever goes one way.

I think that in some places, such as the sea between mainland Scotland and Orkney, the tidal stream flows in one direction all the time. There's a very long wave that makes the tide come in and out when viewed from the sea-side but out to sea it's different.

edit - On second thoughts, I think the tidal stream in the Pentland Firth is mostly east-going but has a bit of west going as well. It's complicated

I think these turbines only generates electricity for the tidal current moving one way, but there are proposals for a two-way rotors with a 1 MW rated capacity off the coast of Northern Ireland.