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Comment by BostonEnginerd | Hacker News Reader

Heat pumps are somewhat limited in the heat they can provide. For example, it would be difficult for a heat pump to produce several hundred degrees C temperatures for an industrial process.

For applications in their wheelhouse, though, they are absolutely amazing.

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There are a couple physical principles that work together to limit solar thermal collectors: heat flows from hot to cold, and mirrors and lenses work both ways. Together, they imply that a solar collector can't heat up anything hotter than the surface of the sun because if the target was hotter than the sun, it would heat the sun up and not the other way around. The surface of the sun is 5.7 thousand Kelvin. This is enough to melt, but not boil tungsten, which I must imagine is a major issue for the tungsten gas industry.

But if you compress the energy collected on a large surface onto a small surface, getting higher heat should be possible, not?

No. [0]

Counter-intuitive for most people, but it cannot be done. The explanation is a mixture of heat flowing both ways, and a law called "conservation of étendue" (you cant focus light beams down as much as you'd think). The link explains the details much better than I could.

[0] https://what-if.xkcd.com/145/

That only talks about lenses and optics.

I'm still convinced the electrical energy collected on a few acres of solar panels fields, could be used to heat a small piece of metal hotter than the surface of the sun. Or am I understanding this problem entirely wrong?

I mean, I do understand why a large array of mirrors and/or lenses won't heat a surface up more than the source. But if we convert and then "upgrade" it (probably with a lot of efficiency loss) i'd think it possible.

And if we collect over time and store that (e.g. as hydrogen, or in batteries) I'm pretty sure we can get it even hotter.

Oh, for sure. Using photovoltaic panels and electric heating, none of this applies.

But the comment you replied to initially talks about the limit of solar thermal collectors.

This is probably irrelevant, but due to the finite speed of light the target would be radiating to the place the sun was, rather than the place it will be.

The article brings this up towards the end,

"These [eddy current heaters/induction heaters] are comprised of a magnet mounted on a rotating shaft, and can reach temperatures of up to 600 degrees Celsius. Using eddy current heaters, windmills could provide direct heat at higher temperatures, making their potential use in industry even larger."

There's plenty of research going into very high-temperature heat pumps. It's likely that eventually that will absolutely be 'in their wheelhouse'. Sure, the COP might only ever be something like 2-3 instead of lower temperature heat pumps (under 80 or so °C) that can be > 4 COP but still should be more than 1...

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