[whatshisface]

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.

[im3w1l]

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.

[berkes]

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

[pbmonster]

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/

[berkes]

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.

[pbmonster]

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.