Feasibility of a hohlraum as a solar absorber

[DrMrLordX]

Would a hohlraum function well as a solar absorber when used in conjunction with a solar concentrator? From what I've read, it would seem that a properly-constructed hohlraum will do a very good job of absorbing electromagnetic radiation that enters the cavity, and if you could insulate the walls of the hohlraum (perhaps by integrating the hohlraum into a vacuum flask) and keep the orifice leading into the cavity as small as possible without necessarily limiting the size/output of the concentrator, it would be possible to build heat inside the interior of the hohlraum faster than it could escape, at least up to a certain threshold temperature beyond which the hohlraum would emit too much radiation from its orifice for the concentrator to continue heating its interior.

Vacuum-sealing the cavity of the hohlraum, as well as the space inbetween the concentrator and the orifice leading into the hohlraum's cavity, might also help.

I have constructed a crude proof-of-concept for this idea that, sadly, I have never had the time to test, and that I may never have the time to test, for a variety of reasons. It's just some cardboard treated with Rustoleum paint, of all things, along with a toilet paper core similarly-treated with Rustoleum. The idea was to use two flat pieces of treated cardboard with the treated toilet paper core inbetween, and a massive washer sitting on top of the top piece of cardboard to weigh down the assembly and help define a space where the orifice to the "hohlraum" would be carved. I bought a ~$10 magnifying sheet from an office store which was a fairly cheap, if tiny, Fresnel lens.

All I'd have to do is test the lens to determine the size of its focal point and then carve a hole in the top piece of cardboard to (nearly) match the size of the lens' focal point to serve as the "hohlraum"'s orifice. I was also planning on putting a penny at the bottom of the "hohlraum" to serve as a guinea pig, of sorts. Obviously this would not serve to test the concept of a hohlraum integrated into a vacuum flask, nor would it allow for vacuum-sealing the interior of the hohlraum, but I was curious to see how hot it would get in there. The goal was material failure of some sort.

The reason for the Rustoleum was fairly simple: other than the fact that it is a bright white variety of the paint (good for reducing emissivity, I suppose), I was hoping that the paint's base might melt/run off at around 200F (which is the advertised temperature limit for the paint), leaving behind deposits of titanium oxide on the cardboard's surface. That should, hopefully, prevent too much oxygen from getting at the cardboard, preventing combustion. I really have no idea how much metal oxide is inside Rustoleum, and, in retrospect, it might have been better to use calamine powder or something along those lines.

 

[CycloWizard]

I suppose it could work. I think you're getting a bit ahead of yourself regarding the final system, as you would need to transfer the heat to affect power generation (assuming that's the goal of all of this). I think the optics will be fairly tricky due to the broad spectrum of EM you'll be dealing with, whereas it looks like these things are usually used with lasers.

 

[Modelworks]

I really have no idea how much metal oxide is inside Rustoleum, and, in retrospect, it might have been better to use calamine powder or something along those lines.

You can get titanium white paint at art supplies that will contain much more oxide than any other paints. Best would be lead carbonate but with the health concerns it is hard to find. You can make your own with lead metal. Combine the metal with a strong vinegar in a clay pot and set it inside a bucket of ammonia and allow to sit in a warm enclosed area for about a month. It will form white lead flakes on the outside .