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Comment by msandford | Hacker News Reader

That's not even close to an explanation.

If 1kg of water descends 1m the most power you can hope to extract from that is U = m x g x h or 1 * 1 * 9.8 ~ 10 J (https://en.wikipedia.org/wiki/Potential_energy)

If you can extract 90% of that, it's 9J. If you can double that efficiency, it's 18J. Which would make the "over unity" generator folks on YouTube pretty happy, but the universe generally says you can't really do.

So which is it? Is hydro 90% efficient, or can you get double the power output if you want mechanical energy? You can't have both.

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Hmmm. I'm not claiming to defy physics although I see how it came across that way. That was not my intention.

If you read the literature you can see why I was phrasing it like that so perhaps I too was misled.

http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/eng...

The potential energy harvest from a hydro setup must be 90% of available energy.

In practice half is lost due to the reasons outlined in the low tech magazine article I linked to. So the electricity production is lower than if we had the turbine hooked up to a motor. The other possibility is that small scale hydro is of a lower efficiency and that this explains the paradox. That is: the literature is talking of a large scale operation that can achieve 90% efficiency but smaller schemes incur losses that explain why converting a small hydro setup back to providing mechanical energy can double the output.

Assembling and paraphrasing your information:

Turning the shaft horsepower from a hydro turbine into electricity, transmitting it through wires, transformers, etc, and turning it back into shaft horsepower through motors, etc is inefficient due to conversion losses. Just using the shaft horsepower directly incurs none of those conversion losses. They can be substantial enough in some situations to approach 50%. This doesn't mean you can harvest more energy than the water can provide, it means that by eliminating losses you can USE more of the energy that the water provides, instead of losing it to heat or noise or whatever other conversion losses.

That's an explanation. What you have linked to and hinted at is only the start of one.

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