Glass windows

On Sep 14, 2006, at 16:09 UTC, Combs, Mike wrote:

> > The area of solar panels you'd need to grow food in high-Earth
orbit
> > is roughly the same as the area of the ground you're growing crops
in.
>
> So your argument is that the efficiency losses at the PV and at the
> artificial lights are made up for by the fact that we are illuminating
> crops with a lower wattage at tuned wavelengths?

No, they're made up for mainly by the greater amount of solar energy
available in HEO vs. on the ground. Here's the BOE calculation:

If we assume 20% efficiency for light->electricity, and 55% efficiency
for electricity->light (which is the current best rate, in sulfer
discharge lamps), the product is 11% efficiency. Multiply by 7, for
amount of sunlight available in GEO vs. the ground, and you have 77% as
much light available in this way as compared to using the light
directly (on Earth).

This calculation doesn't take into account the gains you'd get from
tuning your light for photosynthesis. I haven't done that calculation,
but my guess is that it would at least make up the other 23%.

Of course you may want to do better than terrestrial crop growing, for
example, by illuminating your crops 24/7. In that case you'd need
proportionally larger
PV arrays. But the point of this calculation was mainly to discredit
Zubrin's claim (and the common misconception) that growing crops via PV
and artificial light requires ridiculously large PV arrays.

> This might even be right, but I still can't get past the cost
> differences between square miles of glass panels and Mylar versus
square
> miles of PV panels and artificial lights.

Square miles of glass panels, Mylar, and radiators. That's an
important (and often-overlooked) addition, though I see you're not
overlooking it (below). Also, don't forget the Chevron shields --
Mylar won't do it for them. Now we're talking about square miles of
aluminum shaped into very specific structures and polished to a mirror
finish.

> And I wouldn't necessarily
> accept that the heat rejection issue turns this around. I don't think
> being able to get by with 2/3 or 1/2 of the square area of heat
radiator
> would pay for having to increase the square area of your PV panels by
3x
> or 4x.

Perhaps not, but the other big factor is how choosing natural light
constrains the overall design of the colony. If you are trying to save
money, you build a multi-deck colony. The outermost deck has to have
radiation shielding and pressure containment, and these make up by far
the majority of the mass (and cost) of the colony. All the other decks
need neither, and are essentially free. If you have five decks, you've
effectively reduced your cost per unit area by 4/5ths.

But a multi-deck design almost (though not quite) precludes windows.
There are some ways to get light in the sides, but it'd have to be
pretty concentrated, and it's coming in at a very oblique angle. Still
worth exploring, though.

Best,
- Joe

Joe Strout -- joe@...