[sandspar]

This might be a stupid question, but could a vertical farm use systems of mirrors to direct sunlight to the crops? That way the vertical farms can collect sunlight from a wide area. Maybe the mirrors could be positioned in such a way as to take up less ground space overall than an equivalent flat farm.

[lmm]

That doesn't seem very plausible: fundamentally all accessible sunlight hits the ground somewhere or other (until we start talking about space mirrors and things like that), so while you can use mirrors to concentrate sunlight (as solar thermal plants do) you can't really beat collecting x amount of solar radiation from y square meters of ground. You can build a tall structure that collect the sunlight hitting the side of it, but obviously that's going to block said sunlight from a correspondingly large area of ground in its shadow; at extreme latitudes where the sunlight mostly comes in sideways you get more concentrated sunlight that way (at the cost of an even bigger shadow), but I doubt it'd be worth the overhead.

[akiselev]

Mirrors can neither increase photosynthetic efficiency nor reflect more light than they receive, so to get an equivalent amount of light you'd need a massive structure of mirrors the size of the fields you replace. There might be some clever optimizations possible, but between cooling to keep the plants from burning and occlusion from nutrient delivery systems, it'd be a very large net loss once capital costs are figured in

[gorgoiler]

If the sun shines sideways then you don’t need mirrors. (In many places in the North, at this time of year, the sun occasionally shines sideways.)