[IncreasePosts]

AIUI plants are actually only responsive to a few wavelengths of light for most of their growth. I've wondered, if solar panels can collect energy over a broader spectrum, if it could actually be more efficient to drive LEDs tuned to just what plants need, driven by broad spectrum solar power. In this way you could, theoretically, power a 3d growing operation based solely on the solar panels on the roof.

[imtringued]

Every vertical farming company says this in their advertising. That didn't stop them from going bankrupt.

Conventional agriculture works much better. You can build acres of greenhouses and make a profit.

Vertical farming is such an abject failure that every single vertical farm is biomass constrained, meaning that they have to stretch their biomass with water. This is why vertical farms generally only sell "leafy greens", a marketing term that tries to sweep the inherent technical failure of vertical farming under the rug.

[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.

[zhivota]

I love this idea and it's one of those ideas I categorize into the bucket of "when all the other lower hanging fruit has been picked", just because it's more complicated.

When we've got actually braindead policy like ethanol fuel mandates, the ROI of switching a corn farm to solar is so incredibly high that solutions like this just aren't competitive.

I wish some of our billionaire class would turn their attention to these things rather than building yet another rocket company. Maybe that's why Gates is buying up farmland, who knows.

[tbrownaw]

Maybe present it as proving out technology that could be helpful in building a self-sustaining Mars colony?