I suspect tidal power is one of those things that pays off only the course of many decades and is immensely expensive at first, hence is hard to persuade the public to build at first.
Even if it saves billions of dollars over the course of 70 years.
Here is tidal power in action - built in France way back in 1966. It does work.
China now has a larger one but mostly the idea has been ignored. Sad.
the tide can be harnessed where the large water flow is channeled through a small area.
there are a few natural locations where this occurs.
you otherwise would need to build large containment for this. it would be a large expense. it would impede shipping and boating. it would alter the coastal environments changing a fast uniform water flow to a slow graduated flow.
I agree with your breakdown of the types, but wouldn’t the one proposed in Scotland go under method 2?
Ah - I see what you mean. The MeyGen Pentland Firth project does indeed use water currents but it isn’t operating yet.
I was instead thinking of the Pelamis Wave Power system which started in 1998 and is the best known wave power technology. It was producing commercial electricity by 2004 and the concept has been watched globally.
Unfortunately wave power cannot generate a lot of electricity and is limited to a range of wave heights. Too calm is no good and violent storms can wreck the array.
Incidentally there was another project using strong currents which was absolutely novel but can’t find a reference and it may have failed.
Anyway the concept was to use a venturi pipe to drive a wind turbine. One end of the pipe was placed in the strong current under the Golden Gate Bridge. Water rushed across the mouth of the pipe and sucked air from inside the pipe - the sort of setup you can see in a school science lab.
The other end of the pipe was on land and had a fan in the mouth. The fan turned as air was sucked in. Brilliant, simple, controllable, predictable, very few moving parts, and all that was in the water was a pipe.
It probably worked, but was only capable of generating small amounts of power.
The East River project in NYC is ridiculous when you look at what they put into this effort and what they will get out of it.
http://energy.gov/articles/turbines-nyc-east-river-will-provide-power-9500-residents
If they are gaining significant knowledge, it might be justified, but I don’t think they are learning anything they don’t already know.