I wonder what the net efficiency of operating LED lighting from solar power would be. The obvious advantage of traditional farms is that some inputs, most notably light, are free. But if you consider solar panel efficiency, the cost of mining raw materials and manufacturing panels, LEDs, and power components, I wonder how much worse indoor farming is. Is it 10% efficient? 50% efficient?
With an solar cell with an infinite number of layers the maximum efficiency for normal sunlight is 68.7% to go from light->electricity[1]. Real cells don't have infinite numbers of layers, so conversion efficiency is around 25% for single-layer monocrystalline silicon cells. Those have a maximum possible efficiency of 32%.
Current LEDs are around 40-50% efficient. So 45% efficient LED * 25% efficient solar = about 12% total efficiency.
[1] A. De Vos & H. Pauwels (1981). "On the Thermodynamic Limit of Photovoltaic Energy Conversion". Appl. Phys. 25 (2): 119–125. Bibcode:1981ApPhy..25..119D. doi:10.1007/BF00901283. S2CID 119693148.
Thanks for the numbers on LED lighting. As a comparison, photosynthesis efficiency is about 5% [1]. One could at first think that 12% is better than 5%, but the plant still need to grow even with LED lighting.
The first step in the photosynthesis is the light absorption. The loss there is due to the limited light absorption (range 400 - 700nm). So maybe this step could be improved by using LED lighting dialled in to the range of the plant. The rest of the loss chain would remain mostly the same.
Doing the math with the wikipedia numbers
Plant growing in open field
===========================
0.53*0.7*0.76*0.32*0.6 = 0.05413632
==> 5.4%
Plant growing with LED lighting
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Considering 100 % LED light absorption by the plant (which is an exaggerated hypothesis) and the 12% LED lighting efficiency
0.12*1.0*0.7*0.76*0.32*0.6 = 0.01225728
==> 1.2% efficiency
However, you also get the uplift in efficiency from the fact plants prefer certain wavelengths of light, and have nearly no use for others (eg. green). And LED's are good at producing certain wavelengths of light.
I think that's just because full spectrum LED's are cheaper because they're made in massive quantities for lighting homes and offices.
If you were designing a set of LED's purely for plant growth, and making them in sufficient quantities that you can design your own dopant concentrations etc, then you probably wouldn't end up with full spectrum LED's.
I wonder about using partial LEDs and also solar concentraters and fiber-optic light pipes. Design the building deliberately with a curved-window face like the building that melts cars [0], but capture and redirect the sunlight. Might even be better to use a mirror+pipe system than fiber optics often used in lightpipes. Done well, should cut the electricity bill massively
But, from the article, it seems like the biggest bill to be cut is the 7-figure executive salaries. They are obviously not worth it.
I thought about fiber optics too. The thing is, the square cube law hits vertical farms(it's even worse when they only collect sunlight at their roof). There's some wasted energy in standard farms (the spaces between plants) so I guess it's not as clear cut. But as soon as you start to stack layers it starts to become a problem.
