[Geezer (OP)]

We can concentrate solar energy by optical means (mirrors, lenses, etc.) to heat substances to extremely high temperatures and convert the captured heat into other forms of energy.

This is the only "heat intensifier" I can think of. Are there any others?

I have a nasty feeling they would have to violate more than a few fundamental laws, but I thought there was no harm in asking.

[Soul Surfer]

Yes and it is in daily use  see  http://en.wikipedia.org/wiki/Solar_furnace

[Geezer (OP)]

Yes and it is in daily use  see  http://en.wikipedia.org/wiki/Solar_furnace

Thanks, but are there any others?

[Bored chemist]

The solar furnace isn't really a heat intensifier. The thing you put in the furnace would get a lot hotter if you simply moved it to the Sun.

[syhprum]

I am uncertain whether any system of mirrors, lenses etc can produce a higher temperature than the surface of the sun but there is one sure fire way to produce higher temperatures that is to use the radiation to produce electricity and then produce heat from that.

[peppercorn]

A heat pump.

[JP]

Are we talking about energy concentrators or heat concentrators, and what's the difference?  The term heat concentrator seems to me to mean something like Maxwell's demon, that takes a volume of matter and acts on it to directly move its heat energy into a smaller volume.  Solar concentrators and the like kind of do this, but very indirectly.  You have to change heat into light, then concentrate the light, then turn the light back into heat. 

[Geezer (OP)]

A heat pump.

Yes, but only if something does work on it  [].

[Geezer (OP)]

Are we talking about energy concentrators or heat concentrators, and what's the difference?  The term heat concentrator seems to me to mean something like Maxwell's demon, that takes a volume of matter and acts on it to directly move its heat energy into a smaller volume.  Solar concentrators and the like kind of do this, but very indirectly.  You have to change heat into light, then concentrate the light, then turn the light back into heat. 

Beats me! If we could raise the temperature of a substance by only transferring heat from lower temperature substances (and not doing any work), we'd have it made.

Sadly, I think that's impossible, but I was hoping somebody might prove me wrong.

[JP]

Ah.  I believe light concentrator should be able to raise the temperature on the output above what the input is, assuming you're going from black body radiation to black body absorption. 

Here's a thought experiment.  Assume you have a radiating plate heated up.  (A flat plate makes it easier, since you can put optics on both sides of it.  A sphere like the sun radiates energy in all directions--including those where we can't put our lenses!)  If you put some collection optics on one side of it, so that you capture and concentrate it's radiation down onto a spot which you focus onto another, cold blackbody, you'll heat the second black body up hotter than the first at that one tiny point.  You haven't violated any laws of thermodynamics here because a higher temperature in that tiny volume hasn't actually increased the total energy at all.  In fact, you've actually lost lots of energy that didn't get captured by the lenses.  It might be more usable in that form, however.

[syhprum]

I am no mathematician but to look at the question from a more theoretical basis.
would not the black body be emitting its energy as photons with Gaussian distribution of energies.
what we need to do is collect the higher energy ones to heat another one and build a cascade of such devices.

[Bored chemist]

Ah.  I believe light concentrator should be able to raise the temperature on the output above what the input is, assuming you're going from black body radiation to black body absorption. 

Here's a thought experiment.  Assume you have a radiating plate heated up.  (A flat plate makes it easier, since you can put optics on both sides of it.  A sphere like the sun radiates energy in all directions--including those where we can't put our lenses!)  If you put some collection optics on one side of it, so that you capture and concentrate it's radiation down onto a spot which you focus onto another, cold blackbody, you'll heat the second black body up hotter than the first at that one tiny point.  You haven't violated any laws of thermodynamics here because a higher temperature in that tiny volume hasn't actually increased the total energy at all.  In fact, you've actually lost lots of energy that didn't get captured by the lenses.  It might be more usable in that form, however.

Two problems
Light goes both ways. As the small thing gets hotter it starts to emit radiation. Soon it is loosing heat back to the sun as fast as it's gaining it.
Secondly, I can rig up a heat engine to draw power from the temperature difference between the now very hot small object and the cooler big object. I can take that power and use it to heat the big object. The system will get hotter and hotter.
You cannot (at least at equilibrium) get a set of optics to make something hotter than a hot body ( lasers don't count as they are not black body radiators).

If someone looks hard enough they will find that this is already a proved theory in optics.

[syhprum]

In my experience the temperature of a hot spot produced by a lens seems to be related to the F number of the lens in an inverse exponential manner ie as the F number gets smaller the temperature more closely approaches that of the radiant source but can never actually reach it.
does this tie in with theory ?

[JP]

It can apparently be done, though the engineering is not easy:
S. Cook, et. al., "Sunlight brighter than the Sun", Nature, vol. 346, p. 802 (1990).

If you can't get behind the Nature paywall, the basic concept is to use non-imaging optics (something like a parabolic mirror) to focus the sunlight down to a point.  They use total internal reflection to keep the reflection efficiency near 100% and focus into sapphire (which has a high index of refraction to further concentrate the light).  They report an irradiance (energy flux per unit area) of 72 W/mm², where the surface of the sun is at 68 W/mm².

[syhprum]

I agree that using reflective optics you can get a very small F number maybe 0.25 but other than from a laser source you cannot focus to a point.
The secret seems to lie in non imaging concentrators

http://www.ee.ucla.edu/~photon/pubs/ey1990sem2123.pdf

The authors of this article are of the opinion that the source temperature cannot be exceeded

"the irradiance inside the concentrator cannot produce a temperature
higher than the sun's even if the irradiance is higher. In other words, since a
black body will radiate more when immersed in a refractive medium, solar absorber
temperatures can never exceed that of the sun (5777 K)."

[yor_on]

Isn't that what they're using in France? Canalizing sunlight into a beam that they use to melt stuff?

[syhprum]

They are using a simple parobolic system of about F 2 that is adequate to produce about 2000°C

[Bored chemist]

JP, a parabolic mirror is an imaging optical element. (Ask any astronomer who wants to image a distant point).
I'm, not sure if the optics law still applies to non-imaging optics, but I think it depends on the law of the conservation of energy so I guess it doesn't care what sort of optics you use.

[JP]

BC, I was careful to say it was "like a parabolic mirror."  I'd have to reread it, but they might be using multiple sections of parabolic mirrors to maximize the concentration without caring about imaging.  The point is that if you're projecting an extended object onto an image of itself, I'm pretty sure that you can't raise the temperature, but if you don't care about an image and you just want to take all the source light you've captured and project it onto a point, you can increase the temperature of that one point above the source point.  That's the point of that Nature paper.

[Geezer (OP)]

Of course, we could always cheat and convert the photons into electrons to produce any old temperature we like.