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Comment by MrsPeaches | Hacker News Reader

Seawater ain’t no joke.

I remember reading a few papers about essentially underwater turbines in tidal estuaries. Where you have natural flow of water that you can just tap. If I recall correctly impact on local wildlife (seals iirc) was lower than expected.

Main argument I could see was that water had 1000x the density of air so 1000x the amount of power for a rotor with the same swept area. Of course fluid speeds are lower so it’s not quite that easy. But much more predictable. They did some fancy optimisations in the blades and came up with decent efficiencies. It’s a great little series of papers by Batten and Bahaj et. al from 2006.

I assume the art has progressed since then.

But yeah, anything with moving parts in seawater is a nightmare as far as I can tell.

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>I assume the art has progressed since then.

Not really, no. Even if you completely solved the issue of corrosion (which we absolutely haven't), you'd still have to deal with biofouling - anything you leave underwater will rapidly become covered in algae, seaweed and invertebrates. We have antifouling coatings, but a) they're quite toxic, for obvious reasons and b) they need to be regularly renewed.

That would be enough of a problem, but you've also got the cost and difficulty of maintaining underwater infrastructure. It's inherently dangerous, very slow and often made impossible by tidal or weather conditions.

Offshore wind turbines are already much more expensive to build and maintain than onshore, but tidal generators are on another level entirely.

The Rance tidal power station [1] in France has been operating since 1966 so I don't think that there are technical issues, at least with this type of tidal power generation (ie. a dam across a tidal estuary and good old turbines to produce electricity). That specific power station doubles as a much needed road bridge, as well.

[1] https://en.wikipedia.org/wiki/Rance_Tidal_Power_Station

It has been operating - but it was a research project and it's not like tidal has proliferated since then. Not quite a dead end yet - people are still working on submerged tidal kinds of things. But close.

It wouldn't be legal to build Rance in the EU today, because of the enormously disruptive effects on the local ecosystem. It's not something that bothers me personally, but it's a total dealbreaker in a country like the UK where wildlife activists hold considerably more political sway that climate activists.

Yes, that's what I was thinking.

There was a project to build a tidal lagoon in Cardiff [1] that was eventually abandoned, and I don't know about the environmental impact. But it had the merit of being ambitious.

[1] https://www.bbc.co.uk/news/uk-wales-41213528

Copper does not foul. It is widely used in aquafarming applications. As I understand the environmental impact is nil.

Tidal energy is not renewable! We'd be tidally locked to the moon in 1000 years if we used Tidal for 1% of our energy consumption.

https://cs.stanford.edu/people/zjl/pdf/tide.pdf

That article is based on ridiculous assumptions of future energy consumption growth, and was probably created as a joke. Which backfired since it got seriously quoted in so many places.

They have a 2% increase in energy use each year for 1000 years, currently 10^12W

That would mean we’d be well into the type 1 civilisation about 10^19W a year and we’ll be on the way to a type 2 civilisation.

Sunlight hitting Earth is around 10^17 W, so that civilization would be rather toasty.

In theory if you used the power and radiated it to space (via laser, RF, etc) you’d be ok. You could even use also use that energy to put into rotational energy to increase the earths rotational speed

Lasers can't radiate heat, as a laser beam has no entropy. Ditto for coherent radio waves.

Fire a 1GW laser at the moon and you will transfer the best part of a billion joules of energy from the earth to the moon every second.

You have to be very careful of Maxwell's Demon when it's a question of entropy: how efficient is that laser? How does the efficiency change when it is in a heat bath of X kelvin?

There may be some neat tricks, this is an area I know I'm not good at, but you have to be very careful.

Sure. And you will transfer no entropy, so this cannot be used to cool the Earth.

SF authors (like David Brin) have screwed up on this very topic.

Lasers can be used for cooling, by shining a laser on a target carefully set up so that anti-Stokes scattering carries away entropy. The light scattered is quite incoherent, though.

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