> lot of land that isn't suitable for farming

> We're talking about powering a hydroponic greenhouse. The alternative to "Solar Powered LEDs" isn't "plant things in the ground". Its "make a glass window on your roof".

I think what they were getting at is that you can take land wholly unsuitable for farming or greenhouses (e.g. severe slopes), apply solar panels, and then funnel that energy to a vertical farm. Since the vertical farm does save space in the abstract (just not necessarily once accounting for space needed for electricity generation), the scheme overall is still a more efficient use of land.

> Since the vertical farm does save space in the abstract (just not necessarily once accounting for space needed for electricity generation)

I severely doubt that.

Solar panels are maybe 30% efficient. So 10-acres of glass-roofs need to be replaced by 30-acres of solar panels just to account for this inefficiency (let alone other inefficiencies: such as wiring, inverter, batteries, and LEDs). Maybe 50-acres of solar panels to be anywhere close to comparable against 10-acres of glass roofs once we include other inefficiencies.

> Solar panels are maybe 30% efficient. So 10-acres of glass-roofs need to be replaced by 30-acres of solar panels

Not remotely. Plants are also extremely inefficient, converting only about 1% of the solar energy that falls for their use. [1]

Most of this inefficiency is from solar energy being in the form of frequencies that the plants can't use, but solar panels can. So the panels can capture this energy, then funnel into the red and orange lights that are most efficient for plant growth.

I've read a bunch on this, and haven't been able to find an authoritative source for what the efficiency conversion is -- how many acres of solar panels power how many acres of vegetables, and is it greater or less than 1:1? -- but it's certainly not as simplistic as "solar would need 3x more land because they are 30% efficient."

1. https://phys.org/news/2012-01-energy-conversion-solar-cells....

The article's numbers are that 1m^2 of wheat needed 20m^2 of solar panels.

That implies a 5% efficiency in the "total solar captured area" of the solar panel + LED lights compared to just sticking the plats out in the sun, which is totally independent of the plants own efficiency in photosynthesis.

(I guess coming from a site about Low Tech, their slant on the numbers might be questioned, but they certainly hold up to initial scrutiny from here...)

> I've read a bunch on this, and haven't been able to find an authoritative source for what the efficiency conversion is -- how many acres of solar panels power how many acres of vegetables, and is it greater or less than 1:1? -- but it's certainly not as simplistic as "solar would need 3x more land because they are 30% efficient."

I own a townhome, so my only real ability to grow plants is through a grow-light connected to electricity.

As such, I've spent some time calculating the PAR values of a decent grow-light, as well as the amount of PAR that natural sunlight gives. Plants need a ludicrous amount of PAR (basically blue + red lights, green not needed cause green just bounces off of plants).

Sunlight is mostly broad spectrum: broader than plants need and therefore a source of inefficiency (green light is wasted) that LEDs can somewhat replace.

Grow-lights have a benefit that they can be placed very close to the plant (maybe just 1-foot away) to "focus" the energy a bit better. Nonetheless, the amount of PAR / PPFD from a typical day sun (or even a cloudy day) far exceeds what you'd get from 500W or even 2000W grow lights.

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Its just a hobby of mine, and I'm not growing anything especially hard (just Basil, which is really easy to grow... but Basil is a summer plant that really wants sunlight).

Still, once you start calculating PAR and actually mapping out how much electricity your "emulated sunlight" needs, you'll realize how grossly inefficient that "solar panel -> electricity -> LED" plan really is.

EDIT: Natural sun is like 2000 PPFD or something FAR in excess of what most plants need. Still, a good growlight solution might hit ~1000 PPFD constantly. Lets take this 650W LED and think about it: https://allgreenhydroponics.com/collections/american-made-le...

You'll get ~500 to ~1000 PPFD across a 4'x4' or 16-square foot area from that 650W LED (and most of that light is focused on the center: you'll want to overlap your lights a bit for more consistency).

Then think about how much solar panels you need to power a 650W LED for the 16-hours / day your typical plant would want (to account for the lesser PPFD indoor plants get, you run the lights for a bit longer than sunrise-sunset).

Just some napkin math. Nothing serious here: just guestimating the area in my head.

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EDIT: Now it should be noted: I've heard of good hydroponic greenhouses that have the "do both" approach: glass roofs to let the sun in most of the time, and LEDs to augment the natural sun (cloudy / rainy days, as well as winter-settings when you have fewer hours of sun). The sun isn't nearly as consistent as we'd like, but... that means that you need something aside from solar power powering those LEDs.

But the concept of building a all-LED underground (or "inside a building") without any natural light just... seems grossly inefficient to me. Such a setup only seems useful to those growing contraband IMO.

We can also increase yield and desirable traits by providing the right wave lengths at the right point of the plants lifecycle, we can make the basil more intense, the lettuce bushier and more red. And we can control the environment to easier prevent vermin without pesticides, bad weather, etc. We also do large scale installations in the desert under ground where we previously couldn’t farm at all.

I was an engineer at a company called Heliospectra, we specialized in this and I personally built systems that on an industrial scale enabled above light/environment control down to umol of individual wavelengths every minute based on variables such as sensor feedback, algorithms for specific types of plants and traits as well as learning to adapt based on the environmental daily patterns from sensor feedback over time. It was actually quite fun to work on, I do still keep in contact with the company.

I'm not sure if you noticed, but your argument is that I'm not doing a good enough job accounting for the space needed for solar, and the thing you're trying to counter is that if you _ignore_ the space needed for [solar] then vertical farming makes sense. Those are two completely logically independent ideas which can't be used to refute each other regardless of their respective veracities.

> "Plant things in the ground" is also cheaper, though it does suffer from potentially poor soil conditions.

"Poor soil conditions" to include "soil" that is alkali dust, sand, dry for eleven months out of the year, frozen solid and under multiple feet of snow for more than half the year, and so far from either a river or reliable groundwater that any and all water used must be trucked in. In tanks. On trucks.

Plus, you only get to dump more fertilizer in the water if you filter it back out again.

> "make a glass window on your roof".

Doesn't work as you can't stack plants with it nor can you grow them underground.

Where are you placing these solar panels? Why can't those locations be rooftop hydroponic farms instead?

Because rooftop hydroponic farms have to be tended to almost daily, or at least for planting/harvesting. Solar panels don't need nearly as much maintenance and allow you to concentrate the actual growing operations for the economies of scale.

Do you mind if I ask why you're so against the idea?

> Do you mind if I ask why you're so against the idea?

Because it is clearly inefficient to convert sunlight -> electricity -> simulated sunlight.

But you're ignoring the inefficiency of building separate hydroponic operations on individual rooftops and driving around to all of them, etc. Also as SamBam mention above, you don't have to make a simulated sun. Instead it can work as sun -> electricity -> narrow-spectrum-light-that-plants-convert-at-higher-efficiency. You might even genetically modify plants to be more productive with the smallest spectrum possible.

Or the obvious solution: put hydroponics on traditional close to ground greenhouses... Potentially best of both worlds, if they are sufficiently distributed on outskirts of cities.

Hydroponics is how many commercial growers operate their greenhouses

Or maybe something like what's being investigated for greenhouses on Mars (which gets less than half the sunlight that Earth receives) using mirrors and optical fiber cables to redirect light.

Yes! I wonder if there is a way to make a light battery? :)

Light has a tendency to zoom off at, well, the speed of light, and colliding with objects causes it to lose energy. There have been some experiments in slowing down or "stopping" light although they involve some elaborate setups that almost certainly use more energy to trap the light than you'd get from storing it.

You could also store it in or orbiting in a black hole, but good luck getting it back in its original form! https://en.wikipedia.org/wiki/Photon_sphere