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Comment by kragen | Hacker News Reader

Conventional “mechanical” drilling isn’t very heat tolerant because heat softens metal. For oil and even gas drilling it doesn’t need to be; they’re located in cooler rock. For supercritical steam EGS heat tolerance is a sine qua non. Conventional approaches might still work best, but it’s also plausible that what’s best for gas isn’t best for EGS.

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But if we get to temperatures that destory metals, isn't it enough?

Why don't we drill to whatever is the termal limit, make a big cavity there, install radiators there and just heat the watter to that temperature and take it out to surface where we can heat homes.

I know this is done in Bucharest for a water park (to much lesser depths because apparently we have hot springs near Bucharest).

Do we need to reach magma?

Metals start to get weaker at relatively low temperatures. https://www.engineeringtoolbox.com/metal-temperature-strengt... We want to extract 100’s of MW of heat which presents several issues.

You need a thermal gradient to move heat from the surrounding rock to the water your pumping in. The higher the initial temperature the larger the difference and therefore faster you can extract heat. Aka even if the working fluid is at a constant temperature you can simply pump it faster.

Also, Carnot efficiency limits means you want the hottest steam you can handle. Further, you have various inefficiencies such as losing heat as water moves back up a borehole.

This is about running your air conditioner, charging your Tesla, and molding your kitchen sink, not heating homes. Think Carnot efficiency, power density, supercritical steam, and shutting down gas pipelines.

And the temperature to destroy steel under the immense mechanical stress of a drill string is surprisingly modest. Jet fuel can’t melt steel beams, but softening them is enough.

In a fire, A wooden ladder remains viable king after an aluminum one! Soft happens WAY before melt

Thermal mass and thermal conductivity also play a role in this example. Two more reasons a wooden ladder is better in this scenario. For ladders at the same temperature, the aluminum one would conduct more thermal energy, quicker, into a user's hand then a Wooden one. Making a wooden one more usable for longer in this scenario even though the aluminum one would technically last longer.

This is why wooden sauna seats work well, aluminum seats in the sauna would not go over so well.

Lots to consider when solving thermal problems, it's not just about turning up the dial to be able to withstand more heat. Sometimes it's about the nuance of moving the heat efficiently and putting it where it's wanted, and not where it isnt.

Wood has about the same specific heat as aluminum, but a wooden ladder weighs much more, so a wooden ladder at the same temperature contains much more sensible heat to eventually conduct into the user's hand.

I looked it up. Actually wood had about twice the specific heat as aluminum. So I must revise my original comment to remove specific heat as a driving factor. This leaves only thermal conductivity. A wood ladder would have more heat to give than an aluminum one, but it still is able to give it much slower, making it ok to touch for longer.

Agreed. I'm surprised the difference in specific heat is so large! Maybe that's fairly humid wood? Because glucose is about 220 J/mol/K https://webbook.nist.gov/cgi/cbook.cgi?ID=C492626&Mask=2#The... and 180.2 g/mol https://en.wikipedia.org/wiki/Glucose, working out to 1.2 J/g/K, and aluminum is 24.2 J/mol/K and 27.0 g/mol, giving 0.90 J/g/K.

My general rule is that everything is 1 J/g/K except water, which is 4.2. It's obviously not true, but almost always within a factor of 2, and 1 is an especially easy number to multiply and divide by.

Reading your first paragraph, I was thinking "water. the answer is always water", when it comes to estimating the properties of life-stuff vs everything-else-stuff within a factor of 2. Not obviously, of course, but it's just such a powerful heuristic.

Reading your second paragraph, I see you're familiar with the principle, and added a good heuristic to my stockpile, thank you.

Happy to share :]

> Jet fuel can’t melt steel beams

ISWYDT.

> Do we need to reach magma?

Nooo --- don't dig the Well to Hell!

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

Steel in particular softens at a much lower temperature than it melts. And it doesn't need to soften much to make a really long thin drill bit bend.

> Do we need to reach magma?

Nope. Over the last ~20 years the drilling industry has gotten good at: angle drilling (not slant drilling but turning the drill angle at the bottom) and fracking, or pumping fluid through rock to break it up. This means conventional methods can dig far down, turn the drill and make a reserve with fracking. The heat is much less but it's conventional technology.

https://www.ted.com/talks/jamie_c_beard_the_untapped_energy_...

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