[CGMthrowaway]

Back of the envelope math:

  Solar panel captures energy from an 800nm wide range (300-1100nm)
  Plant captures energy from a 300nm wide range (400-700nm) 
  The solar panel could reproject and amplify the 300nm range at (800/300=) 2.7X more power than the sun

[traes]

The reason plants capture energy from this range is because that's where most of sunlight's energy is concentrated, which is going to drop this quite a bit further. Glancing at a solar radiation spectrum curve makes it look a lot closer to ~1.5x. Combine that with inefficiencies of both the panel and the LEDs and it really doesn't look that good.

[magicalhippo]

But solar panels are only about 30% efficient, so that kinda kills any gains.

[pinkmuffinere]

Thanks, this is exactly the comment I was looking for. In addition to the 70% loss due to the solar panel efficiency, I think we should also lose some efficiency in the conversion to light via leds (although I expect that’s much more efficient, perhaps at like 80%).

I’m curious what is physically possible, if we assume we can achieve the max possible efficiency. I’m guessing there’s behavior like a Carnot engine, and the energy transfer can only be up to ~86% efficient (but please correct me if I’m wrong!!). In that case, conversion from light to energy via solar panels -> conversation back to light via leds should be 0.86*0.86 = 73% efficient in best case. And the full effect should be (800/300)*0.73 = 1.94, about twice as good as growing plants with the sun’s direct light. I’m surprised that seems possible!

p.s. My efficiency guesses are probably wrong. Please correct me.

[magicalhippo]

By using multiple junctions and stacking them, top one converting the most energetic photons, then the second-most etc, one can approach the theoretical limit of about 95% or whatever it is. However in practice it's very expensive and difficult as I understand. AFAIK the current state of the art is about 6 stacked junctions at around 60% efficiency, at great cost.

And as you say the LEDs aren't 100% efficient either, though both deep red and bright blue are among the most efficient, about 85% there.

So that leaves you with about 50% overall just from those two.

[ben_w]

You're basically fine about PV, but nobody's corrected the real error which is wildly over-estimating the efficiency of photosynthesis in plants, which is 0.1-4.3%, with crops in particular being 1-2%: https://en.wikipedia.org/wiki/Photosynthetic_efficiency#Typi...

[benj111]

I thought that efficiency was governed by the wavelengths they absorb? Ie absorb all wavelengths == 100% efficiency.

So that would imply they are inherently more efficient just looking at the figures provided.