Comment by ben_w

Comment by ben_w 17 hours ago

It sounds like you're mistaking PV for a thermal system.

In a PV cell, you have a semiconductor. Semiconductors have this thing called a "band gap", which is the energy needed to get an electron from the valence band to the conduction band: https://en.wikipedia.org/wiki/Band_gap

The limits to efficiency of a solar panel is that sunlight has photons of many energy levels; the photons with energy less than the band gap do nothing, those with more, waste the excess.

A laser can have energy tuned to this band gap, at which point the PV part becomes ~99.9% efficient. (The laser part is not close to that efficiency).

malfist 12 hours ago

I'm not talking about a thermal system, I'm talking about having to deal with the thermals of your inefficiencies. That energy that doesn't get converted to electricity is converted to heat. And you have to deal with it.

The type of laser based PV that you're taking about that's highly tuned is at maximum 27% efficient. Not 99%.

That's a 73% waste you have to manage

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ben_w 29 minutes ago

I don't know where you get these ideas from, but they're very very wrong.
You can already buy PV for sunlight with higher efficiency than 27%. Heck, even with a single band gap the limit for *sunlight* with all the problems I've just stated (because a broad spectrum will waste energy and the sun's spectrum is very broad) still gets you to the 33% Shockley–Queisser limit for one junction on the solar spectrum: https://en.wikipedia.org/wiki/Shockley–Queisser_limit
But for monochromatic light, tuned to the band gap, stuff people have already built is several times better than that, and the theoretical limit is basically how finely you can tune the laser bandwidth and how precisely you can control impurities that broaden the band gap.
Here's one designed for monochromatic laser use, that was actually built, with results published in early 2019, with 70% where they recon doing it better would get to 80%: https://krichlab.ca/wp-content/uploads/2019/02/Xia18-PVSC.pd...
As with all quantum systems, which is what both PV and lasers are, the ultimate efficiency of a tuned laser-and-band-gap pair is as good as your engineering, hence ~99.9% (on the cell side) if you control absolutely all parts of it properly.

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