I think your description of the Sandia achievement is essentially
correct, but it does seem a little grudging of the step forward it
represents (and please correct me if, as you hint, this step has been
taken before). I see it like this-
I respect the work the Sandia team has done, though I do have some
concerns about their claims to have violated Plank's Law. The negative
tone of my notes is based on the press release and the folks who wrote
it. The Sandia press release just one of far too many press releases
that claim far more than has actually been achieved.
1. A blackbody radiator becomes a more efficacious, visible emitter
the hotter it gets, i.e as a greater proportion of energy is emitted
at visible wavelengths.
Right
2. Tungsten incandescent lamps can demonstrate an impressive efficacy
when the filament is run at a temperature that will evaporate it
quickly.
Right in principle, but not that impressive without "help".
3. If you want to create a tungsten blackbody radiator that has both
high efficacy and long life then it would make sense (tungsten
chemistry tricks aside) to run the lamp at a modest temperature, but
somehow try and mitigate the level of power lost in the generation of
unwanted longer wavelengths.
Yes!
4. Lamp makers of course, already use this trick with heat reflecting
filters on several different kinds of lamp, including tungsten
incandescent. The result is a blackbody radiator that is seemingly
more efficient than normal by virtue of its (selective)thermal
insulation. Looked at close up, the radiator is behaving like it
should given an in bound flux of additionally warming, long-wavelength
photons.
Not only "behaving like" - it actually is. The IR reflector allows the
electrical input power to be reduced while the filament size and
temperature remains the same.
5. A photonic lattice (of the right dimensions) is simply a more
elegant way to achieve the same effect. In a manner of speaking it is
the selective reflector and blackbody emitter combined in a single
entity. The thwarted long wave emission is part of the energy input
into the radiator, that would otherwise be lost were it not for the
photonic structure.
Well, I would say that because the lattice prevents radiation in the
unwanted IR region, it is not an energy input, just less energy loss.
6. A blackbody radiator cannot be more efficient radiometrically than
physics allows, but a blackbody radiator as an element in a well
thought out system can be more efficacious photometrically than a
blackbody on its own.....
This is no longer a blackbody radiator. It is now a selective
radiator, and if it slight selective, like nature tungsten, it is
called a gray body radiator. But at any wavelength, the selective
radiator cannot radiate more energy than a blackbody of the same
temperature at that same wavelength.
7. Surely, Sandias efforts are all about creating long lived systems
with sufficiently small geometry to do the job intended. IR is where
they have got to now but other press releases strongly suggest a
determination to get to visible scales.
P.S. I would love to hear about earlier attempts in this direction if
you have the time.....
It is not time, but protection of proprietary technology. Here is what
is public. People have created "patterns" on tungsten filaments and
have shown that these patterns will selectively suppress radiation in
the IR while also increasing the emissivity of the tungsten in the
visible. (Remember that the emissivity of tungsten is not as high as a
blackbody so there is room for improvement.) These patterns then work
like the Sandia lattice. The problems with these prior efforts is that
the patterns disappear in less than 1000 hours when the tungsten is
operated to the temperature required to produce a lamp. The
disappearance is caused by temperature-induced tungsten migration. The
smaller the features, the faster they are lost.
The Sandia lattice may be more resistant to this problem than the
prior patterning efforts. However, to prove the concept and move
beyond what has been demonstrated before we need a lattice optimized
to enhance the emissivity in the visible, not the near IR, and we need
life test at operating temperature so show the lattice is stable for
at least 1000 hours. These are two well known issues. The short life
of small tungsten patterns is perhaps the only reason we do not have a
lamp with a selective emission tungsten filament right now.
