Comment by amluto | Hacker News Reader

Comment by amluto | Hacker News Reader

As far as I can tell, this is functionally just a coating with low emittance in the IR outside a narrow band. A matched PV device that is kept cool can, in principle, approach the Carnot efficiency for the temperature difference between the emitter and the PV junction. If the emitter is the sun, then the source temperature is very high and the Carnot efficiency isn’t a major limit. If the source is a solar panel, I’m having a hard time seeing how this is useful.

I can see this being somewhat useful as a no-moving-parts heat engine for something like a solar concentrator, but there’s another relevant thermodynamic limit: even if this magic material has emissivity 1, it won’t radiate at a greater power per unit area than the blackbody spectrum predicts. At non-crazy temperatures, this is not very high, which will limit output for small things like solar concentrator targets.

So I can see this being useful to convert waste industrial heat, or maybe as a bottoming engine for a combined cycle plant, but I am having trouble understanding how it could be useful for solar.

One sketch of such a system is to physically mate an effective high T solar absorber to an effective narrow spectrum photon emitter (as described in this work). Then that can be coupled with a typical solar cell with bandgap matched precisely to the emitter wavelength. So your solar cell is near ideally efficient for the photons it receives. As I understand the emissivity of these materials can exceed blackbody radiation in the near-field but not the far-field (or something like that? Search "Superplanckian emission").

The devices as I am familiar are often called thermophotovoltaic cells: https://en.wikipedia.org/wiki/Thermophotovoltaic

See eg http://xlab.me.berkeley.edu/pdf/259.pdf for a great overview, esp section 3.3.

I saw this video recently that I found accessible from an undergrad physics background and got me interested: https://www.youtube.com/watch?v=XnVVyTD7CzM

> Then that can be coupled with a typical solar cell with bandgap matched precisely to the emitter wavelength. So your solar cell is near ideally efficient for the photons it receives.

One way or another, once you've converted sunlight to heat, you are limited by the Carnot efficiency. For the 80% efficiency they claim, if all of it comes from thermophotovoltaics, they need a hot side temperature at least 5x ambient, which is over 1000 C. I wish them luck getting anything resembling a solar panel up to 1000 C. (I'm not, in any respect, saying it's impossible -- I'm saying it's very hard. You'd need excellect spectrally or directionally specific absorption to avoid re-radiating all that heat out the top of your panel, and you'd need conventional transparent insulation to stop conduction.)

On top of that, super-Plankian emission or no, if it's limited to the near field, then the PV cell is very, very close to the hot surface. That PV cell needs to be kept near room temperature to get that efficiency.

This whole thing seems extraordinary complex for something that wants to be cost-effective.

The absorber/emitter assembly doesn't really resemble a solar cell. Demonstrated tungsten emitters have exceeded 1500K - it's not really that wild, it's what's in incandescent light bulbs. You concentrate sunlight typically.

For solar, it somehow has to integrate with the solar panel, so that the same square meter is used both for direct photovoltaic generation of electricity, and for this IR capture, so energy is obtained from a wider band.

I understand your point that it can't just be driven by a secondary IR emission from a warmed-up solar panel; that's not hot enough to be that useful.

In a more detailed article, there is talk about the carbon nanotube device being useful because it can withstand high temperatures:

https://news.rice.edu/2019/07/12/rice-device-channels-heat-i...

They of course understand that they need a big temperature differential.

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