> As for energy and light needs. Yes, it would need lots of energy but nowhere near what you think. Your back of the envelope math is talking about acres which isn't that useful of a metric for your energy needs.
It's true that acres were an awkward way to do the calculation. Let's redo the calculation in megawatt-hours.
- According to https://www.nature.com/articles/s41586-018-0706-x, grain crops yield about 0.24 grams of dry weight per mol of photons. (This assumes 100% of the photons are delivered to the plant, e.g. the LED is shining directly on the leaf with nothing lost.)
- One mol of photons embodies about 0.1kWh of energy. (That's for red photons; higher-frequency photons would require more energy. And note this is a physical limit, where we're assuming the LEDs are already 100% efficient.)
- So we'd need 1,125,000 TWh/yr of energy to grow the world's supply of grain through indoor farming, or 45x the world's current electricity production. And this calculation is just for grains; to grow all the non-grain crops too, you'd need even more energy. So I stand by my original estimate.
What about genetic engineering? Looking at https://www.nature.com/articles/s41586-018-0706-x, the "0.24 grams of dry weight per mol of photons" calculation is taking into account that e.g. grain plants are not as efficient as some other plants at absorbing light, and not all of the grain plant is edible. The most efficient plants (leafy greens) produce 1.33 grams of dry weight per mol of photons. So if we could genetically engineer a grain plant that absorbed as much light as the most efficient plants, and didn't have an inedible stalk, then we'd need 203,000 TWh/yr of electricity to grow the world's supply of grain, which is "only" 8x the world's current electricity production.
And at that point, we've squeezed out all the obvious sources of inefficiency; to do better, you'd need to fundamentally change plant biology, or just do photosynthesis directly in a vat, or something.
It's true that acres were an awkward way to do the calculation. Let's redo the calculation in megawatt-hours.
- According to https://www.nature.com/articles/s41586-018-0706-x, grain crops yield about 0.24 grams of dry weight per mol of photons. (This assumes 100% of the photons are delivered to the plant, e.g. the LED is shining directly on the leaf with nothing lost.)
- One mol of photons embodies about 0.1kWh of energy. (That's for red photons; higher-frequency photons would require more energy. And note this is a physical limit, where we're assuming the LEDs are already 100% efficient.)
- So that works out to 0.0024 metric tonnes dry weight of grain per MWh. Global grain production is about 2,700 million metric tonnes per year (per https://www.statista.com/statistics/263977/world-grain-produ...)
- So we'd need 1,125,000 TWh/yr of energy to grow the world's supply of grain through indoor farming, or 45x the world's current electricity production. And this calculation is just for grains; to grow all the non-grain crops too, you'd need even more energy. So I stand by my original estimate.
What about genetic engineering? Looking at https://www.nature.com/articles/s41586-018-0706-x, the "0.24 grams of dry weight per mol of photons" calculation is taking into account that e.g. grain plants are not as efficient as some other plants at absorbing light, and not all of the grain plant is edible. The most efficient plants (leafy greens) produce 1.33 grams of dry weight per mol of photons. So if we could genetically engineer a grain plant that absorbed as much light as the most efficient plants, and didn't have an inedible stalk, then we'd need 203,000 TWh/yr of electricity to grow the world's supply of grain, which is "only" 8x the world's current electricity production.
And at that point, we've squeezed out all the obvious sources of inefficiency; to do better, you'd need to fundamentally change plant biology, or just do photosynthesis directly in a vat, or something.