Once in a while, people suggest using the Severn Estuary to generate tidal power. Many other countries with coastlines also have estuaries with sufficient tidal range to make them attractive candidates too. Tidal range is the vertical difference between the water depth at high and low tide, and when the shape of the estuary is factored in, this obviously represents the potential energy available to be harvested. The placing of the barrage determines most of the cost.
A good US overview is at
Building a dam is established technology, as is hydroelectric generation. The environmental problem is that estuaries are also valuable ecosystems, and it would be nice if we could get power that way without needing the estuary. Putting the enclosures, or impoundments off shore solves that. One option is to build a tidal lagoon. A nice UK site describes the idea:
As a diversion, you could also just float a lagoon one and tether it, but that probably isn’t a great idea. This is why: A huge man-made enclosure with high walls in a high tidal range area off shore could open its gates to let water flood in via generators as the tide comes in and/or hold it to be released via generators as the tide goes out. But if we make it from plastic, it wouldn’t be able to withstand much pressure and wouldn’t last long. If we make it from steel, it would be stronger, but would take a lot of steel to make a worthwhile enclosure. Then once we’ve made it, how would it be anchored to the sea floor to stop it just rising with the tide or to stop it falling as the tide goes out. Remember, tidal generators only become useful when there is a significant pressure difference. It would need very strong anchors and very strong cables to prevent it from floating up as the tide rises. The base for the enclosure would have to be very strong with strong supports to hold up the enormous weight as the tide goes out, or it would have to sit on a huge base of concrete (assuming it can’t just sit on the sea floor because it is at sea, which is after all the whole point).
So it’s obvious once you think about it for a few minutes why people want to use estuaries or lagoons to hold the water. Only the wall is needed, not the base. The difficult half of the problem and most of the cost goes away.
We are already building off shore wind farms. They sit in regions where the sea is shallower, but since they already present an obstacle to shipping, that obstacle wouldn’t be much worse if the whole farm were to be surrounded by a sea wall. Then tidal generators could be fitted in those walls. Wind farms therefore ought to be perfect candidates for tidal lagoons. It would produce an impoundment without further damaging shipping channels or fish migration paths, while making a less hostile environment for the wind turbines and making their maintenance easier and safer.
A steel wall would be theoretically workable, but would be expensive and resource intensive. A concrete sea wall would be less expensive, but making concrete generates relatively large amounts of CO2. Stone could be used but leaves an ugly mine behind. So, the best solution for tidal lagoons is using a conventional rubble mound breakwater.
This isn’t a new idea. It was thought through ages ago by others. One proposal for the UK that gathered support:
http://www.publications.parliament.uk/pa/cm200506/cmselect/cmenvaud/584/584we78.htm describes almost exactly this solution, identifies promising UK sites, and even does all the appropriate surveys and calculations, showing costs compare reasonably with onshore wind turbines. It is still expensive, but not as bad as off shore wind or even a tidal barrage (because the depth of water on the path the wall follows is low, keeping construction costs down). Worth a read.
It is a sound idea already. I like it, though it is still far more expensive than developing shale gas. But instead of using just rubble, why not also use the opportunity to dispose of other waste such as plastic by using it as breakwater filler? Maybe even other kinds of landfill might work as filler. A lot of waste plastic is shipped to far-away lands for disposal. Mixed with rubble, the density would be OK to make it sink and stop it being washed away. It would get rid of waste, while providing some of the substance of the breakwater, hopefully even bringing the price down further. It is unlikely to make a huge dent in costs, but it would reduce the madness of sending plastic to China for disposal and take pressure of landfill.