Why can’t you siphon water from one part of the sea to one part 10km away to generate electricity




Okay, a little bit of backstory. My uncle was arguing that you could siphon water from one part of the sea to another part to generate electricity if they fell on turbines on the other end. He used a siphoning example , as in siphoning gas from a car on anything else really, on how it would work. He also mentioned that because of earth curvature that it would have about a 10m fall.

I immediately saw some faulty things but don’t really have the background to be so sure about it. What I mostly opposed was the fact that it wouldn’t be a fall if the pipe followed the earths curvature . The sea to pipe height would be the same at both ends and therefore not work because there is no fall in height. Even though one end would be 10m lower but that’s only relative to each other not an actual different height.

So who is actually right here?

In: Physics

The amount of power needed to move that much water would take more then the power generated from it.

The sea actually *does* bulge out at the equator due to the spin of the planet. However, this wouldn’t get you power because the water wouldn’t “fall”. If you go at it from another perspective, you could say that the effective force of gravity is lower at the equator (add the two force vectors together, they are parallel but opposite), so the two water masses are essentially at the same potential energy level. Which means you won’t be able to extract potential energy out of it by letting it flow through your pipe.

Also said pipe would have to be stupid long.

If he bases it on just the curvature of the planet, make him consider how the vector of the gravitational pull also follows the curve of the planet. Basically, if you try to use a straight pipe, the end spout of it is pointing upwards and the gravity there is pointing backwards along the length of the pipe, pulling the water back *in*.

Earth’s curvature has a radial bulge, yes, but so does the surface of the ocean. The difference in altitude he’s talking about is a difference measured from the center of the earth, not a difference measured from above or below sea level, and that’s the relative measurement you want to use if you want to talk about hydrostatic equilibrium.. Sea level is… the level… of the sea. So wherever you are on earth, whatever altitude the sea is at, that is already the maximum height that water would flow to if siphoned from some other part of the sea.


You may wish to read with him about the shape of a [geoid](https://en.wikipedia.org/wiki/Geoid). The VSauce video [Which way is down?](https://www.youtube.com/watch?v=Xc4xYacTu-E) might also help shed some light on how altitudes work on an irregular spheroid.

This would only work if you ran the pipe past a thermal vent like a caldera. With a one way valve, water would enter, leave as steam, travel uphill a short distance and then condense back to water able to be used in a gravity turbine. Not exactly siphoning. The main problem I see is if it is scalable enough to be useful. Scaling of another type would also be an issue.

The average area level is zero – in fact it is so average that we use it as the reference point to work out altitude.

To create a siphon, you need a difference in height – when measured above sea level, so to siphon the sea into the sea, you would be effectively trying to siphon from one pool of water into the same pool of water.
It is also worth remembering that the sea is one continuous body of water, so any water will naturally move to the lowest point on its own – the water won’t flow down your siphon, because it could just flow around it anyway…

The place I can see a confusion appearing to make his idea sounds plausible is the fact that the earth isn’t perfectly spherical. So the distance between the centre of the earth and sea level will vary between the poles and the equator. This measurement isn’t relevant in this case though, as the spin of the earth causes this, and centripetal force will hold the water at the equator rather than letting it flow ‘down’ to the poles.

A broadly related idea that we are considering is capturing energy from tidal forces – the sea level does vary over the planet in a continual cycle based on the moon causing the tides. Because we know it regularly flows back and forth it is possible to do things like placing turbines or other devices to generate energy from where there will be notable tidal flows – this hasn’t been developed to three level of mass usage yet, but is something being considered as we push towards green energy sources.