This is a bit of a crazy idea for generating energy I just thought of. It would involve rediculous amounts of materials and construction anyway so this is just to ask about the theory behind it really. Our planet is an oblate spheroid and so the distance from the North to South poles and is about 42KM less than the diameter spanning the equator. (From research on the internet) A siphon will work when water is being moved to a point lower than where it is taken from. So my idea is: Theoretically if you had: A vertical tube submerged slightly in the sea at the equator say 1 Meter high, Another longer tube not submerged at some point near the north pole. Imagine you joined the top of the tube at the equator with the North Pole one following the curvature of the Earth 1 Meter above sea level, adding to it slightly as you went along to meet the height of the taller tube at the end. Because the Earth being an oblate spheroid, the end of the tube at the North Pole would actually be much lower relatively than where water would be drawn from near the equator. Also because it would be closer to the centre of the Earth the pull of gravity would get greater as you travelled towards the end at the North Pole. If you sucked water through the tube at the North Pole end until it came out, in theory would it then come out continuously? I realise that the North Pole is frozen over lol; it would have to be a location close by which isn?t.
My guess is that whatever forces are acting upon the earth to make it the shape that it is, would also be acting upon the water within the pipe; preventing the water at the high side from flowing to the low side. Were it not this way then what would prevent the water upon the open oceans from already flowing in this direction?
But, like you said, the price-tag would be high for such an experiment. We may never know for sure.
A siphon doesn't really work on change in altitude but due to change of pressure.
At sea level in the arctic the pressure is relatively close to the pressure at sea level near the equator.
If you experiment on a small scale, you will find that with greater difference in pressure change (altitude) you get more pressure with your flow.
In order to extract energy from the flow you will need a greater amount of pressure difference than you would find between areas of barometric pressure across the (weather) map.
Keep thinking.
The more ideas we throw out the closer we get to the one that works.