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Comment by londons_explore | Hacker News Reader

This raises a question in my mind...

Current hydroelectric systems rely on building a dam, say 50m high, and generating electricity from a large mass of water falling that 50m.

Might another approach be to build a tower as high as possible, say 500m, and catch rain up there. You might catch less rain, but you'd have a lot more height.

Obviously the challenge with this is getting a large catchment area very high up. I guess you'd need some kind of aerodynamic design like a kite to allow a massive square meter-age to stay aloft during storms.

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Rivers collect water over thousands of square miles. Thus turning inches of rainfall into serious amounts of water. Going 10x as high, but collecting 1/1,000,000th the water is not going to provide much power as energy is linear with height. https://en.wikipedia.org/wiki/Drainage_basin

The Hover Dam for example collects water across 167,800 square miles.

One can argue that these dams are already catching rainfall indirectly, from a very large area with very high efficiency.

I suspect theoretical energy generation efficiency is in disproportion to the difference in potentials (IIRC the hotter something is, the more efficiently you can extract energy from it - not more energy, but more in proportion - but IANAexpert).

So while dams may catch rain efficiently, if they were 10x higher perhaps the extra potential energy usable is more than 10x

Dams are very efficient to construct, because you take advantage of natural valleys - erosion has done most of the heavy construction work for you. The catchment area isn't just the surface area of the reservoir, but huge areas of surrounding land. Major hydroelectric reservoirs are unimaginably vast.

https://en.wikipedia.org/wiki/List_of_reservoirs_by_volume

Interesting idea. As jdietrich says, I presume the reason we don't see this is simply the question of scale and cost. If you want to catch kilotons of water, a reservoir is far cheaper than an enormous raised platform.

On the flipside, there's the question of how far down you can get the water to drop, before feeding it to your generator. This is something already taken advantage of - https://en.wikipedia.org/wiki/K%C3%B6lnbrein_Dam#Reisseck-Kr...

Some dams use this with their generators being situation in the valley below. I see little point to do this unless you are restricted by some other factor, for example if you are a flat country with no access to natural reservoirs or vertical drops.

Catching much rain very high up sounds stupid but at the same time that's something that needs considering. Stupid suggestions can be made un-stupid in my occasional experience. Could you elaborate please? Genuinely interested.

[1] probably explains the downsides best (which I had not considered).

I guess considering that, the only way to make it viable is if there isn't much rainfall, or if the terrain is very flat/porus, or if you can make the collection devices very cheap (for example a kite that flies into a cloud and can collect a few tens of liters per second 1km up could generate 100 kilowatts, which is a lot less than a big wind turbine, so needs to be a lot cheaper to compete)

[1]: https://news.ycombinator.com/item?id=22994225

Yeah, the effectiveness of wide area water collection is why your suggestion is 'stupid'. That's ok. Where it's possibly not stupid is in the comment you linked to "...as energy is linear with height"

Sure potential energy is linear with height, but energy conversion efficiency is I think non-linear. You can I think collect more than 10x useful energy from an object that is 10x as hot. Equally I suspect you can get more than 10x useful energy from a column of water 10 miles high than from a column of water 1 mile high.

I'm pretty sure your idea won't work but that doesn't mean it can't possibly work. It's more likely harvesting wind energy via kites (as has been proposed) is more realistic, but still, I don't like dismissing dumb ideas. Sometimes they can be made useful.

> Equally I suspect you can get more than 10x useful energy from a column of water 10 miles high than from a column of water 1 mile high.

For a given flow rate, no.

High pressure water can be fairly efficiently converted to electrical energy, which itself can fairly efficiently be converted between low voltage+high current to high voltage+low current.

All the conversions are linear and well understood - there is no magic squared here to make this idea magic.

OK, you sound like an expert (not sarcasm) so if I may (and please note I never suggested a squaring relationship),

> High pressure water can be fairly efficiently converted...

And lower pressure less efficiently converted? Therefore the higher the column, the higher the pressure and the better the efficiency of the conversion?

I think that even if it is a good concept, this reservoir would never fill.

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