On: Why vertical farms are a crock.

[dalke]

"In a controlled environment the remaining 99% of the energy could be recycled."

Thus violating the second law of thermodynamics. Do tell how we can recycle that energy, turning waste heat back into light, since that would also make solar cells more efficient, increase farming capabilities in cloudy/cold regions, and reduce our dependency on petroleum by making biofuels easier to grow.

For a starting point of the calculations, see http://en.wikipedia.org/wiki/Photosynthetic_efficiency .

[DougWebb → dalke]

Obviously you just put buckets under the plants to catch all of the light that falls through, and you have a machine that picks up the buckets and dumps them out over the top of the plants again. I think it'll help to paint the inside of the buckets white... or maybe black; not sure. Oh, and the bucket-dumping machine should be solar powered.

</sarcasm>

[dalke → DougWebb]

Oh, of course - Maxwell buckets will do the trick! Just need little robots with mirrors on each one to let light in and not let it out.

[tocomment → dalke]

Put mirrors on the walls and ceiling and put reflective particles over the soil. That should let more light eventually end up on the plants.

[dalke → tocomment]

You'll need mirrors which don't block the light. One-way mirrors are only half-silvered, which means they'll block more light than they'll recover. What you'll have to do is figure on something on the other side of the plants than the light source.

Take a look at the link to "Photosynthetic efficiency". Only about 13% of the light ("incomplete absorption") could be reused this way. Most of the rest of the energy is turned into heat.

Now, it is possible to be more efficient through layers. Take a jungle, where most of the energy is absorbed by the top level while lower-level plants make use of what energy comes through, eg, more efficient in dim light or better use of other parts of the spectrum.

You will be able to get some more efficiency by doing this, but it's going to be expensive, and nothing like "99%".

[tocomment → dalke]

I think you're imagining something else. Here's what I mean:

   _________   Mirrored ceiling
   L  L  L     lights
  |        |   Mirrored walls
  |        |   
  | P P P     Plants
  | M M M     Mirrored surface with holes for plants
  | S S S     Soil

So any light that misses a leaf, reflects off a leaf or goes through a leaf will simply bounce around the room until it hits a plant.

I'm sure mirrors still convert some light to heat, but it seems like it would be a big improvement.

[dalke → tocomment]

I see - you are talking about artificial lighting of the plants, while I was thinking of sunlight. With sunlight the ceilings will block the light going to the neighboring plants.

What you are saying seems to make sense for some circumstances, though I earlier pointed out there's isn't that much light which is going to waste which could be used for photosynthesis. I looked around for "mirror garden", on the thought that home gardens would be the most likely candidate for using mirrors, but the only mentions there were for decorative purposes.

I suspect there's a strong economic reason which makes it not practicable. Perhaps it's that a greenhouse with glass is more efficient because it makes better use of free natural light.

[tocomment → dalke]

Maybe when my kid is older we will do this as ascience experiment. See if a "mirror garden" plant can grow with less light and how much less. Expect a blog post in ten years..