P's post on energy reminded me of an idea I had for a tidal generator. I doubt it would ever be built in the U.S. due to 'environmental' considerations like those facing the costal wind generators.

Here goes:

On a beach somewhere build a large concrete vault and make the top even with the high water mark. The length should be- long, preferably what the budget and terrain allow. Width- get to that later [1]. Along the entire length of the container on the seaward side would be rows of inlet holes say, around 4 inches in diameter. [2] Space the inlet holes at 5-foot intervals and about a foot of vertical separation between rows. Tides vary so how about 12 active rows of holes?

Now drop a shaft from the top of the seaward wall down through the vertical center of each set of 12 holes. At each hole, attach a turbine to the shaft. (A preformed turbine shell in the concrete and ducting the holes so water will hit one side of the blades might help. Other types of arrangements like a vertical turbine-perpendicular to the flow with gearing or maybe offsetting the shaft from center might be better. I don’t know.)

Now, you guessed it. Every 5 feet place a generator on top of the seaward wall and attach it to the top of the turbine shaft.

How it supposed to work:

Starting at low tide with the tank empty, the tide rises covering the first row of holes. When they are completely submerged, the gate valves [3] inside the tank open allowing the water to flood in and start turning the generators. Once the water level inside the tank reaches the first row of holes, the gate valves in that row are closed and at this point the tide level outside the tank should be approaching (covering if lucky) the second row. Once the second row is submerged the process repeats onward up the remaining rows.
Notice the water’s GPE is being trapped inside the tank. After the tank is full and the tide starts to go out, the process begins in reverse. The tank empties out the same holes it was filled from, opening the gates from the top down.

[1] The size of the holes should be large enough to efficiently turn the turbine but should be kept small so the container won’t fill too fast cutting run time short.

[2] The width of the tank should provide enough volume so that water will continue to come through the inlet holes until the tide level reaches the row of holes above. If there is not enough volume, the level inside the tank will rise and block off the flow from the active row.
Conversely, if the volume is too large, the tank may empty too slowly. Ideally, the system would be almost empty by the time the rising tide returns to the base of the structure. The variables in tidal action and resulting tank volume could be compensated for with a variable volume tank – floodgates.

[3] On the inside of the ‘tank’ there needs to be computer controlled gate valves to prevent premature filling of the tank. The energy needs to build up outside the tank and then be released in force. Not just dribbling into the tank wasting precious volume.