What I don't understand about the idea is this. This stretch of water has one of the highest tidal ranges in the world, but this presumably is a matter of resonance, i.e the Bristol Channel and Severn Estuary just happen to be shaped in such a way that their natural resonant frequency is a close match with the driving force's twelve and a half hour odd period. But surely this means that the extra energy in the water, over and above places with lesser tides, is not something that can be tapped again and again each tide at any significant scale, it's a one time only bonus in its potential and kinetic energy which is gone after you extract its power through dampening. The ultimate non renewable energy, gone in half a day!
Anyway, I'll be giving serious consideration to dragging my family out early to see a five star Severn Bore the weekend after next, it could be the last one ever.
That would be true if it were a freely oscillating system, but since the tide is 'powered' by the moon I'd think the tidal cycle should keep going until the moon stops orbiting. (though I'm not 100% sure I understand how tides work)
I don't see that. Surely if you have two swings hanging from a swaying tree and one of them is swinging wildly because it's the right length to resonate with the moving branch above, you can't say hey this x kg tyre is rising and falling y cm every s seconds, lets harness that. It only has the same renewable energy as the other swing, plus a one time build up of energy that its taken to get it up that extra height.
Flip the oscillators, the sea is the tree branch, and the swing is the moon.
The tree branch is moved slightly each swing, and if I am to extract energy from the branch by forcing it to do work, I will damp the motion of the swing and it will eventually stop.
You're right in that it's not renewable, the energy is being extracted from the kinetic energy of the moon, and it will slow the moon to some minute degree.
No one's pushing my swings! I've already accounted for the input, it's the great mass of the swaying tree (and in turn the wind etc.), and like the tides' moon it's the same input to both swings, only one's them is swinging like the clappers whilst the other barely moves. Even though the former clearly has more energy at a given point in time most of that is a one off build up, and there can't be any more repeatedly extractable energy in the big swinger than the small.
Could be missing something but I don't see resonance as the right model. In my mind the high tides are as a result of a large body of water being pushed into a confined space by a the geometry and currents of the area. That seems like it'd happen regardless of whether some energy had been tapped from a previous tide.
I agree that tidal power isn't "renewable" though, but for different reasons, as it (making a semi-educated guess) decreases our gravitational pull on the Moon, letting it drift away slightly faster than it might naturally, but the effect would be microscopic.
(My disclaimer is that I wrote a Newtonian model of the Solar System and its effects on tides as part of a Physics degree. Doesn't mean I know what I'm talking about, just that I thought a bit about it 15 years ago)
It has nothing to do with resonance. It's much the same principle as a pulley where you focus little force over a long distance into a lot of force over a smaller distance.
It's an oscillating system with a dramatically greater amplitude than its neighbours sharing the same driver. I'm having a hard time believing it has nothing to do with resonance.
Edit: Resonance may pay a tiny role, but it's a long way from the star of the show.
It also works with normal waves, storm surges, and Tsunamis which are non-oscillating systems. It's due to the large amount of kinetic energy in moving water which allows a much wider range of waves to benefit than you get with a resonance effect. Picture a large rock in a fast moving stream, the water bunches up in front of it and in some cases you can see a bump the water over some obstruction. http://ak.picdn.net/shutterstock/videos/6538901/preview/stoc...
PS: Because the land slopes up the water nearing the shore can have a slightly lower angel over a long distance which adds up. Thus, the total height at the end might be 20+ feet, but at the edge of the bay it's only vary slightly above sea level. However, the reverse at low tide does not work nearly as well because the slope dampens the effect vs reinforcing it. Resonance effects require both the height and trough of each wave to reinforce each other.
Anyway, I'll be giving serious consideration to dragging my family out early to see a five star Severn Bore the weekend after next, it could be the last one ever.