Joe Strout wrote in message ...
I'm getting tired of the widely-repeated claim that it is impractical to
grow crops under artificial light (particularly if your power source is
solar). It just doesn't make sense, for two reasons. First, if your
solar power plant is in orbit where it receives sunlight 24/7, you've
already got about seven times as much sunlight to start with as a field
on Earth.

Most pundits assume that if you're in orbit, you're going to use
reflected light, simply because its cheaper and more efficient.
However, if your solar power units are cheap enough, then there's no
reason why you can't use electrical power. But why would you bother?

Second, though there are losses in converting the sunlight to
electricity and back to light, you can make the light you convert it to
be 100% pure clorophyll-absorbed prime wavelength, whereas the light
that falls on Earth is mostly wavelengths that plants can't use anyway.

Put those factors together, and I suspect that a km^2 of solar cells (or
similar solar power collector area) could grow MORE than one km^2 of
crops.

The problem arises for growing food on the moon, and here it's not a
question of energy production, but of storage during the night. A
couple of solutions a

- use plants that can grow for 7 / 14 days. This appears to be
practicel for many plants if chilled at night to just above 0.
- Use a lunar orbiting power plant. The issue here is that there is no
Lunar Stationary orbit near to the moon, so it could be a long way to
beam the power.
- Use a lunar grid to transport electricity from one side to the
other. This could work, but is capital intensive.


But suspicions aren't worth much; I really need some numbers. And here
my ignorance is getting in the way, and I'd like to correct that.
Can anyone point me to sources of data on absorption spectra for
important crop species, etc.?

I've started googling but have turned up surprisingly little so far.
(E.g., I know the difference between chlorophyll A and B, but I have no
info on what the relative balance between them is for any relevant
plant.)

Can't help here, but bear in mind you want to optimise output, so it
won't be worth saving electricity if it reduces your yield.

For that matter, there seems to be no conclusions as to what yield you
could get with the optimum lighting, CO2 concentration, timings,
temperature, water etc, with good GM crops. I would suspect you could
get 100 tons per hectare per year for carbo hydrate crops.

Lets say with the right wavelengths, you need 500W/m2, mx = 300W / m2
average, = 3MW per Hectare. At 100W / m2 solar cell output, and 75%
efficient lighting, That needs 4 Hectares of solar panels weighting 40
tons. A 1 hectare "green house" will weigh a lot more than 40 tons, so
it would appear that using electricity is reasonable.

100 tons would feed 200 people. If they want more than rice and
potatoes, make it 100 people, and if they want chicken, you're
probably down to 50 people.

Thanks,
- Joe

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