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Comment by tkanarsky | Hacker News Reader

Hmm, off the top of my head I think that it takes as much energy to pump water down a 1-foot tube as it would for a 1000-foot tube. It's like rotating a long loop of chain on a pulley, but in three dimensions. One liter of water goes down the tube, one liter of water comes up around the tube. Both sides weigh equal amounts so there's no net force. Of course, overcoming inertia to get the water moving and keeping it moving despite friction might be significant. Ironically, a consequence of the oil industry is that we have powerful pumps that are proven to be economical in moving large amounts of liquids long distances through pipes :)

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> Both sides weigh equal amounts so there's no net force.

Water at 40m depth has a different pressure than at the surface level. Pumping at the surface and releasing at depth will have significant water pressure difference.

(A quick Google later). At 40m, it's 5atm or roughly 75psi pressure.

(Another quick Google). Oil pipelines run at a higher order of magnitude as that, so it's doable but would return energy, increasing as you go deeper.

Gravity takes care of the pressures. If you put a long pipe vertically in the water, with its bottom open and its top sticking up a couple meters on top, and then put some water into the top of it, what happens to the water column inside?

You do not need to get the water at the surface to 10, or 100, or 1000 atm. You just need a long pipe that goes all the way to the bottom.

Likewise for transporting water upward.

There is significant friction between the water and the tube that needs to be accounted for. The longer the tube, the more friction there is.

But the bigger its cross section, and the slower the water inside moves, the less friction there is. A fabric tube 10 or 20 feet across would work as well as a pipe. It could stick up a few feet above the surface, and the pump just lift water over the edge into it.

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