I was listening to a programme on Radio 4 last night in which was discussed the proposal for tidal power generation by building a barrage across the River Severn (UK). One of the chief objections to plans of this sort is that they could muck up surrounding wetland and estuary ecosystems.
So it occurred to me… instead of building barrages across rivers, why not build huge circular ‘ponds’ a little way out to sea - let them fill up when the tide rises, then when it falls, drain the water out of them, through turbines, to generate electricity.
They would require more material to build - for a couple of reasons:
–unlike barrages across estuaries or natural harbours, they can’t take advantage of land masses to form part of their perimeter
–of course being offshore a bit, that means they’re in deeper water, so need to be taller in order to rise above the surface.
But we build artificial islands, harbours and barriers for other reasons, so why not tidal power ponds?
Seems easier to just run a pole out of the water and add pontoons that rise and lower, turning a gear or such.
Going from Wikipedia, it looks like there isn’t a lot of “damless” hydropower in the world at the moment. It doesn’t really say why that is so though. I’m moving on to Google.
Hm, no real answers there either. To offer some thoughts that were spurred though:
It’s cheaper to construct something from land than from water, probably.
It’s cheaper to build a dam than to fully separate out the ocean from 360 degrees. You’re doing essentially the same thing in either case, but in one you can use the topography of the earth to reign in a large section of your pool, which is essentially free walling.
Anything which allows a continuous flow of water in and out will need to be regularly cleaned of various things growing on it. Again, this will be cheaper if you are based off of land. Since you’re barracading off a large section of the ocean, you either need to create a hollow section in the walls where you can have human workers and room for moving parts around internally in a dry space–which more than doubles the cost of the wall–or you need to have some pretty fancy dive equipment that allows you to interswap gigantic underwater parts for cleaning. Either way, the less wall you have to build to section off the ocean, the better off you are.
There is a prototype tidal generator in the East River (actually a tidal estuary) in New York, though the aggressive tides there are proving a bit much for the generating equipment (N.Y. Times link). Putting generators in tidal choke points like this looks like an effective way to capture energy from natural differences in the rise and fall of the tides.
The problem I see with the artificial tide pool idea is that the height of the tide almost everywhere in the world would likely produce insufficient hydrostatic pressure (head) to run a turbine. It is easier to use the several knots of tidal current (both ebbing and flooding) that can be found to occur naturally at the mouths of bays and harbors.
Not a definitive answer but one thought. Tidal range (the difference between high and low tide) is strongly dependent on the shape of the coastline, bay , estuary etc. and in the deep oceans tides are negligable - less than a couple of feet. A tidal barrage across the Severn is potentially viable because the tidal range there is the second largest in the world - up to 14 metres. If you move off-shore you massively reduce the power available while increasing the cost.
Yup, it’s been tried. There are texts on power plant design where you can find designs for this. And, I believe, it is being used on the Bay of Fundy where the tides are the highest in the world. I don’t have a link, but a quick visit to a college library can readily provide texts on techniques for harnessing tidal power.