Could grow lights on a massive scale replace 100% of sunlight for 100% of the growing season?
In a world where pollution and population have drastically reduced the effectiveness of sunlight to grow food, would light-panels be capable of completely replacing the sun?
The problem I'm trying to solve is rationalizing indoor farming for 100% of the Earth's agriculture. Piping in water is a big deal, but believable. Fertilizing is believable. Temperature control might be believable.
But is it believable that light panels could replace 100% of the sun for 100% of the growing season?
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For the purpose of the question, the building in question is 14,500 square kilometers (a bit bigger than the U.S. State of Connecticut) It has ten stories (allowing for agriculture roughly the size of the U.S. State of New York). Don't worry about how the building can be that large and carry that much weight. This question is only focusing on the lights.
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The lights are timed to fade-to-starlight-black and fade-to-full-sun with appropriate changes in timing to emulate natural growing seasons. For the purpose of this question, let's assume we're dealing with wheat, which allows for a winter wheat during the cold months and a spring wheat during the summer months.
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You can hand-wave the right types of lights that produce the right mix of wavelengths, but you can't hand-wave away the power requirements.
Sunlight reaching the earth's surface is about 1.2 kW/m2 on a clear day with the sun high in the sky. You probably don't need all that if you focus on particular wavelengths that plants use. Let's say you still need 500 W/m2 just to pick something. Even if your lights and power delivery system are 100% efficient, you still need 500 W for every square meter of growing area when you want to simulate sunshine.
You say you have 10 floors of 14,500 square km each, so 145,000 square km to illuminate. That's 145 Gm2. Times 500 W, the power is 73 TW. You are simulating day/night cycles, so let's say you only need ⅓ of that on average. Since this is all indoors removed from the actual day/night cycles, you can have different parts of your farm on different schedules so that you need the average power all the time instead of the full power part of the time. That gets you to 24 TW. To put this in perspective, the total electric generating capacity of the United States is a bit over 1 TW.
Sunlight is a huge source of energy to earth, with the energy generated by humans a tiny fraction of that. Any system that tries to replace sunlight with something artificial will require huge amounts of power.
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There are multilevel automated food plant farms in existence that are artificially illuminated. Sure they may use skylights but they have many times the growing area so the plants are not getting full sunlight that way.
There are lots of underground growing systems that are in old mines that are used for the purpose of hybridising suicide gene seed stock for sale of GMO variants. These variants are usually made to have sterile seeds so farmers have to buy the seeds over and over again (kind of nice when we move away from Round-up and they go away) While these sites do not have massive areas like OP proposes they will be totally dependent on external power.
In the cultivation of hemp plants for illicit and sanctioned production of hashish and marijuana it is common to grow the plants in closed indoor spaces for secrecy, security and species management reasons. They routinely make use of artificial lights, I expect almost exclusively LED lighting these days and I have seen advertised units with adjustable spectrum light that gives the optimal light profile for each time of day and each grow season to trigger and support leaf and bud growth at the correct times.
So the process is possible but as Olin mentions the power supply and heat dissipation may become an issue. If there are so many people that there is no room to grow food then it also means there is not enough space for animals and plants and the ecosystem has collapsed. This will cause runaway erosion and loss of transpiration and run off capture resulting altered water cycles and eventual loss of aquifers.
Long term it will result is total loss of human life and resetting of land based evolution from rats or whatever managed to stay alive.
It always brings to mind the massive error found in numerous Sci-Fi stories and films. Planets with (near) complete ice or desert cover still have breathable oxygen atmospheres. Sure almost anything is possible but plant life is the only KNOWN way to make the oxygen and absent this there would be no breathable atmosphere (sort of like Mars). On Earth the oceans may be able to carry the burden if not badly damaged but replacing all terrestrial plants with a small amount of food crops will make a substantial difference in the O2 supply.
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