# What temperature can you attain with a solar furnace?

A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at about 6000K. The target will then heat up to 6000K and start to radiate as a blackbody, reaching thermal equilibrium with the sun.

The question is: is there any way to surpass this temperature, perhaps by filtering the light to make it look like a BB spectrum at higher temp, then concentrating it back on the target?

• That would violate the second law of thermodynamics, assuming all the optics are passive. – Mark Eichenlaub Jan 10 '11 at 15:34
• If you assume equilibrium, then there is no way. Black body must always emit as much radiation as it receives. The upper limit is given by total Sun's output corresponding to 6000K. – Marek Jan 10 '11 at 17:03
• By the way, you might want to read up on Dyson sphere. – Marek Jan 10 '11 at 17:04
• If you surround the Earth-sun system with ideal mirrors, everything on the inside will eventually heat up well beyond 6000K. Just divide the total mass by the heat derived from fusing everything to iron. – Scott Carnahan Jan 11 '11 at 13:11

Theoretically the answer is yes. That is because the sun is not a blackbody emitter, there is an excess of UV radiation. So if you were able to achieve radiative equilibrium with only UV light (which is maybe 1% of BB radiation at those temps), you could do it. Practically, I'd think it would be just about impossible, as your filter would have to have it's innermost surface at nearly 6000K.

Note: The solar UV primarily comes from the chromosphere and corona, which is heated (in some not too well understood way) by mechanical/magnetic energy derived from convective processes. The X-ray excess is even greater than the UV excess. Even the earth gives off detectable gamma rays, and that would be impossible thermally.

According to wikipedia, it can reach 3500-4000 °C

You can increase the temperature by choosing material which is black at visible light range and white at IR range.

There is no limit to the degree of concentration. In theory, the entire output of the Sun could be concentrated into a small point.

• Basic thermodynamics... a passive process like reflection cannot build a greater temperature than the thermal source. – Muggs McGinnis Sep 17 '18 at 1:18

## protected by Qmechanic♦Nov 18 '17 at 1:22

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