This past weekend took me on a bike ride across Portland, Maine's Tukey's Bridge. Thanks to the vagaries of the bicycle network in Portland (another long post), we looped under the bridge before crossing over it.
As we did, I noticed the tide going out. Portland's back cove is not a particularly large body of water, it's about one square mile. Nor is it deep—at low tide it mostly empties in to the Atlantic and becomes a large mud flat. But all the water has to go somewhere, and it certainly does: it flows under Tukey's bridge. Portland's tides—around 10 feet—are relatively large but nowhere near those up in the Bay of Fundy. But where the water rushes in and out of the Back Cove, there is a decent current which occurs several hours each day. Every day.
The flow is increased by the current incarnation of the bridge, which is built on fill to narrow the mouth of the cove considerably. So now the square mile of water has to flow through an opening only about 250 feet wide. There's power in that water, and while it's certainly not megawatts, the infrastructure seems to be such that you could pretty easily install a small-scale tidal power system to take advantage.
Tukey's bridge has two main passages. What if, in one of them, you installed a paddle wheel contraption between the supports with a generator on one end? There is no need to worry about building supports; the bridge abutments are in place. There is no worry about navigation; small boats could easily pass through the other half of the bridge. There would have to be some sort of floating barrier to keep boaters from running through such machinery, but that would be doable. The paddles would not reach the floor of the cove, so they wouldn't interrupt whatever ground fisheries there are there. The generator could be housed on the bridge abutment out of the water, which would keep it out of corrosive salt water.
So if we assume that all the agencies (and I'm figuring NOAA, Army Core, the DOT and others would be involved) could be satisfied, would this work as a demonstration project? Could it conceivably have a payback time that would make it economically feasible? It seems like a low-cost project. All you'd need is a generator, some power lines and some concrete to anchor it to the bridge.
Or do I have the physics of it all wrong anyway?