David,
I agree, cool project.
Can you give some definition of the speed range that you want to use?
My approach to wind turbines has been to use a "synchronous generator", but basically it's been just a hobby for me, so my hacks aren't necessarily what you're looking for.
Still, using a 3-phase permanent magnet generator will allow you to operate the prime-mover at a wide speed range and simplify the power conversion by just rectifying to DC.
In a system like this, starting from a dead stop the machine speeds up to a "cut-in speed", below which current doesn't flow. Up to that point the generator has not produced enough voltage to equal the voltage blocked at the rectifiers. Once that threshold is met, then the peaks of the generator's AC is "clamped" to the DC system voltage as the machine speeds up. The current flows in proportion to the voltage drop and resistance/reactance in the generator windings. As my system stands, it cuts in at about 250 RPM and power generated increases steadily up to about 600 RPM. The speed would continue to increase if wind speed increased, but above a certain limit the wind speed could be damaging so the machine takes measures to prevent higher speed and protect itself (called furling).
I think I've posted enough of the guts of my system -here:
(Hope this doesn't look hokey - my WT is just a hobby project but I thought it might be relevant. There are plenty of more subtle and intelligent schemes (ie using SCR's) but I don't use them.)
Your tidal generator may operate at a roughly constant speed - I can only guess. I suggest the synchronous generator because it can allow operation at a speed range roughly 1:3 which may be flexible enough to deal with the variant speeds of your system. The more you can tell about that, the more helpful my answers can be.
My inverter can be grid-tied though I don't need it to. It's a 20th century beast with massive transformers but it may have the features you need; battery charging, inverting, generator support and grid-tie. It's flexible enough that if there is no grid, meaning it's permanently "islanded" - as long as there is NO chance it will EVER see a grid - I must emphasize - then I am told that it can be configured to BE the "grid" for its own particular island. Then other similar inverters can share the AC line on their grid input. I'm sure it gets complex very quickly, and out of my depth. Others have accomplished this - but not me. If you want to pursue this, I know exactly who to ask. Alternatively, it can accept the AC frequency input of another inverter/generator based power supply, as long as that supply is stable enough. The inverter will disconnect from any grid AC frequency or voltage input that falls outside certain parameters.
Mine is too small for your 5kW power range. Its big brother, the XW6048, may have the muscle to do what you need.
STF
