Half a croissant, on a plate, with a sign in front of it saying '50c'
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Cooling towers

Super-tall conductive cooling
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So, a heat engine. not a specific type of engine. Just a nominal system to extract mechanical work from a temperature difference between two “heat reservoirs” (not bodies of water, but an infinite source or sink of heat at a fixed temperature).

A heat engine’s maximum efficiency is dictated by the absolute temperature difference between the hot and cold reservoir. (Hence intercoolers on car intake, for instance) Given limitations of engineering materials, Earth-bound engines are usually limited by the approx 20 degree cold reservoir. This idea is to use conductive cooling to transport “coldness” at -50 degrees from the stratosphere (approx 10km) to a heat engine at ground level.

Perhaps a Stirling engine. Which could use 1. Ambient heat, 2. Geothermal, 3. Solar, 4. Waste industrial heat… as it’s hot-side reservoir.

10km towers, insulated (perhaps with aerogel, for reasons), with large surface-area heat exchangers at the top (fins, branches, fractal) Or giant kites, with big thermally-conductive cables.

Sparkly colours of course.

Ooh, could also supply air conditioning and refrigeration this way.

Frankx, Sep 05 2021

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       Solar updraft towers are an engine working on this principle. Only instead of thinking about transporting cold down, they're transporting heat up and extracting (the relatively small) energy from that.   

       In general, humanity doesn't give much of a stuff about efficiency. Accessing the -50C 10km up is tricky, because it's 10km away, in the least convenient direction. On top of that, there's not much cold up there. That is, the air is thin, so you're heat exchanger has to be larger, tricky, because you have to suspend it 10km up. Thermally conductive cables... how are these working? The closest we've got is heat pipes. You'll have them in a laptop for example. But you get efficiency losses with distance which overwhelm your gains from the enhanced delta t.   

       It makes more sense to go down. The ground is reasonably cool, and thermally speaking, there's a lot of it.   

       Some adjustments might be worth it. Mounting an AC condenser on a roof vs the ground might get you a couple of C, if you can shade it. But you get inefficiencies from any extra pipe length, and it becomes tricky to service.   

       I'm all for 10km towers however. Although they might be vulnerable to the very large legacy heat engine we like to call "weather".
bs0u0155, Sep 07 2021
  

       Just hang them from satellites in geosynchronous orbit. Ok, 35,786km is more than you need. But you'd get to add carbon nanotubes to your materials list.
Worldgineer, Sep 07 2021
  

       ^String bridges to the islands in the stars.   

       Enough towers and a weather ask system is born.
wjt, Sep 09 2021
  

       If we were to plant on Mount Everest the thinnest spine of titanium capable of bearing its own weight, which could simply conduct heat out into space with no moving parts, then we would have the added benefit of watching climbers stumbling on to the first step of a spiral staircase with the realisation that they weren't really at the top after all.
pertinax, Sep 09 2021
  
      
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