Vertical farming

Vertical farming is the idea that food could be grown in buildings with multiple floors/levels. This is supposed to save space and use less land (thus saving more of the environment) but on close examination we see that it doesn't.

Not a solution to feed the world

Powering the grow lights

Natural sunlight is proportional to land(...)( Sure, technically you could argue that a tall building captures more sun than the land it sits on, but that's just because it obscures the sun on the land beside it. ). In vertical farms, the amount of real sun is quite limited. Grow-lights are needed:

Thought experiment

If all crops were grown in vertical farms, how much energy would be needed to power all the grow lights?

sunlight.visible_fraction

43%

How much of the sun's energy is in the visible part of the frequency spectrum

ClimateBits: Solar Radiation - Science On a Sphere - NOAA

sunlight.photosynthesis_fraction

sunlight.visible_fraction

How much of the sun's energy is in the "photosynthesis-friendly" part of the frequency spectrum

We factor this into the equation because we assume that grow-lights are targeted at the frequencies that plants can use most efficiently for photosynthesis.

Due to lack of data, we have to assume this is roughly equal to the 'visible light' fraction of sunlight.
Plants may somewhat make use of infrared as well, but this is offset by their lower utilization of green light (which isn't that much lower (according to this source) - as long as the plant has enough leaves, it will still ultimately absorb most of the green light it receives).

crop_land

15000000 km^2

Agricultural land used for growing crops - global total

https://ourworldindata.org/land-use

sunlight_average

200 watts per m^2

Solar irradiance, averaged over a whole year INCLUDING nights, cloudy days, etc.

Note: This varies by region.
https://www.newport.com/t/introduction-to-solar-radiation
http://www.ftexploring.com/solar-energy/insolation.htm

solar_panel.efficiency

17%

Efficiency of an average solar panel

Some solar panels are more efficient than this, but they tend to be expensive and contain more rare metals.

led.efficiency

58%

Energy-efficiency of a typical LED light

Kusuma, P., Pattison, P.M. & Bugbee, B. From physics to fixtures to food: current and potential LED efficacy. Hortic Res 7, 56 (2020). https://doi.org/10.1038/s41438-020-0283-7
https://www.nature.com/articles/s41438-020-0283-7

"Blue LEDs are now 93% efficient, phosphor-converted “whites” are 76% efficient, and reds are 81% efficient when run at the near-optimal conditions of 100 mA mm−2 and a junction temperature of 25 °C."
...but then...
"Typical fixture efficacy
Using more typical parameters:
1. Drive current achieves 90% of the reported LED efficacy.
2. Temperature management achieves 90% of the reported LED efficacy (at 25 °C).
3. The LED driver is 90% efficient.
4. Protected LEDs in the fixture achieve 92% optical efficiency.
The resulting fixture efficacy would be 0.90 × 0.90 × 0.90 × 0.90 = 67% of the reported efficacy of the LEDs."

I decided to average 'ideal' red & blue efficiencies, and then multiply by the 'real world' stuff. Result: 58%

energy.tfc

9937.70 Mtoe/year

Global energy usage - total final consumption (TFC)

Includes: fuel (80.7%) + electricity (19.3%) AFTER it is generated.

Does not include the fuel used in generating electricity. See [energy.tes] for that.

Citation: "Key World Energy Statistics 2020" IEA
- Page 47 - Simplified energy balance table - World energy balance, 2018

energy.tfc (calculation loading)

In other words, we would need 168 times more energy than the world currently consumes!

Since most energy currently comes from fossil fuels, this would be disastrous for the environment.

But what if the energy came from renewables?

We already have a hard enough time scaling up renewables - but let's imagine it was easy. Still, vertical farms would defeat their entire purpose: They wouldn't save land.

(calculation loading) In other words, to grow an acre worth of plants(...)( and I'm not talking about the footprint of the building. By "grow an acre worth of plants", I mean for example having a 1/8 acre building with 8 storeys ), you would need over 4 acres of solar panels!

To convert...
sunlight → electricity → back to light again
can never be as efficient as...
just using the sunlight directly.

The land requirements for wind power would be even higher. Keep in mind that wind power is still indirectly a kind of solar power: winds are caused by the sun heating the air.

Uranium is far too scarce to power the grow lights with nuclear fission. Maybe fusion power could do it someday. Until then, vertical farming is just a bad idea.

But doesn't it take energy to ship food from farm to city?

Yes. Just nowhere near as much as what vertical farming would take.

But what about shade crops?

Shade crops could already be grown in the shade of full-sun crops. Most of the crops that feed the world require full sunlight. This is because calories & protein require energy in the form of light shining on the plants.

There might be a very small niche for growing low-calorie greens in vertical farms in the city, just so people can enjoy herbs at their absolute freshest.

Cases where vertical farming might actually be a good idea

Growing mushrooms

Mushrooms don't get their energy from the sun - they get their energy from fiber. Since food crops have inedible fibrous parts, there is plenty of material to cultivate mushrooms on. Mushrooms could be grown in tall buildings or deep basements with no light at all.

Disambiguation

Some people use the term "vertical farming" to refer to something different: a rich layered polyculture - no multi-storey buildings involved. This type of "vertical farming" actually has good potential to feed the world.