Energy consumption is definitely an important disadvantage—I indeed assume that energy is not a concern for the post, either through nuclear power or other non-land renewable sources.
Even with this key disadvantage, this direction for agriculture might be advantageous because:
Yield will scale linearly or even super-linearly with number of stacks in a multi-storied building because the constrained environment will allow for better optimizations (robotics, precision agriculture, genetic engineering) than current distributed horizontal farming.
Energy consumption for climate control, artificial lighting will scale sub-linearly for the same reasons, in addition to economics of scale.
Energy costs in general have been decreasing. Plus, widespread adoption of nuclear power due for AI capabilities might have a fortunate side-effect of reducing energy costs.
These are intuitive beliefs though and I currently don’t have numbers to back it...
Thanks for the numbers—I am starting to see that this might not be a good idea in terms of energy consumption.
I am curious about the 500-800 W/m^2 estimate—it seems too large—could you describe the calculations in a bit more detail? I am saying it is too large because on average earth only seem to receive about 250-300 W/m^2 based on solar irradiance data[1]. Additionally, this is over a wide range of wavelengths—plants don’t really use all the wavelengths so the effective power required in terms of energy of photons might be lower[2].
Just to double check, I tried to estimate the same thing in a different way. Sun irradiance provides ~800-2800 kWh/m^2 every year[3]. Assuming an average of ~1500 kWh/m^2, the total energy provided by the sun on the land devoted to crops (1.6 billion hectares) is about 24 EWh. Given that the total electricity generated is around 30 PWh[4], the energy provided by sun is 3 orders more.
What I am currently uncertain about is the the proportion of the total sunlight energy is meaningful required for a growing plant—that is:
how much effective area is used by the plant during its lifecycle (growth rate, leaf distribution, farming strategy, etc)
how much of the sunlight is actually absorbed by the plant (does not account for photosynthetic losses)
This is important considering that the land use for crop is not equal to direct photosynthesis/crop growth. If we assume only 1⁄2 of the energy is in usable wavelengths and 1⁄10 is used for effective crop growth, then we need 50x the current electricity production, very similar to your estimate.
However, what is not clear to me is the error margins in this estimate. Is it factors or orders of reduction of incident solar energy? That is, is there is an upper limit to the number of photons a photosynthetic cell can effectively use. If that amount is a couple of orders less than sun irradiance, then the electricity production requirement might be an overestimate.
Regardless, even if I am very aggressive in my estimate by assuming crops use only 1% of the total sun irradiance per year, it still is 5x times the current electricity production and does not seem feasible.
What I am currently wondering (hope to look up and a comment in a day or two) is:
Is there an expected doubling rate for electricity production and how long is it?
What are the cascading benefits of fractional vertical farming on earth? That is, if 1⁄10 of the crop area was replaced by vertical farming, is the energy cost worth the carbon capture and habitat restoration benefit?
It is clear to me that energy is a far bigger constraint than I thought—I definitely underestimated the amount of energy provided by the sun, especially considering the amount of electricity generated by humans.
From a quick estimate of this and this figure.
This figure. I did a quick estimate using STM G173 data—the power in 300-700 nm is about ~50%.
Based on this figure.
https://ourworldindata.org/grapher/electricity-prod-source-stacked