Mike Combs wrote:
"Joe Strout" wrote in message
...

I know you're working toward a point here, and it's probably rude of me
to interject -- but to this particular question, the answer is not
difficult: the original light energy is in a different place, and
probably of a different spectral quality, than what and where you want
it. Compare to this question: why convert the rotational power of the
steam turbine at the power plant to electricity, pipe it to someone's
home, and then convert it into rotation in your blender, when rotation
is what you wanted in the first place? You could instead have long
rotating axles (connected by universal joints) transmitting the rotation
directly to your blender. But it's much more practical to use
electricity as the intermediary; it's easy to transmit and reshape into
whatever particular form of power you need. This is true despite the
efficiency losses.

I take your point, but I'll never consider getting sunlight inside of a
windowed habitat as being nearly as impractical as your axel to the blender.

That may be so, but this leaves some things out, like the Chevron
shields needed on top of any glass panels,

Bear in mind that the chevron shields are for one particular habitat design:
the Stanford Torus. The Bernal Sphere and Crystal Palace deal with this
problem via other (possibly better) methods. And once we get up to the
Island 3 scale, the thickness of the structure itself eliminates the need
for separate shielding (chevron or otherwise).

and the radiator mass needed
to reject all that extra heat,

Undeniably true. But I think one thing this issue hinges on is which is
more expensive per sq meter: simple panels of aluminum tubing, or panels of
PV arrays? The natural illumination option needs to reject more heat,
meaning somewhat greater area heat radiators, but the artificial
illumination option requires a lot more electricity than the habitat would
otherwise need just for routine electrical utilities, which means /much/
bigger solar arrays.

as well as severe constraints it imposes
on overall colony geometry, which can have serious impacts on your mass
budget -- for example, it would be hard-to-impossible to make much use
of natural light in a multi-deck design like Kalpana One.

I would agree, but have a personal dislike for multi-deck designs. They
pursue a different design goal, which is "provide as efficiently as possible
living area for X people". I still have a fondness for O'Neill's original
design goal, which was "recreate as closely as possible the most attractive
parts of the Earth's surface".

Yes, I found that section useful. I'm not convinced it's a good idea
either, though. I'm really not trying to be contrary, but it seems to
me that if your inhabitants really want windows, they probably expect a
decent image (you know, normal-sized sun during the day, ordinary stars
and maybe a glimpse of the Earth by night). If they're willing to forgo
having windows they can actually look out of, then I think it's probably
not worth bringing in natural light at all.

Yes, 70x sounds a bit extreme to me. I'm not sure what concentration factor
is assumed in the Bernal Sphere design, but it doesn't look to be beyond
10x. And that strikes me as about right. I'd agree that making the windows
substantially less than 50% of your overall hull is probably to be
recommended, but I think you can go to extremes here.

If we use some of the sunlight passing through the windows for power
generation, let's let the physics determine how 'extreme' we should go.
Months ago I read descriptions of cooled quartz windows passing
10,000X sunlight (sorry, can't seem to find it again). Let's assume a
more modest 1000X concentration of sunlight. Also assume, as described
elsewhere in the thread, a cylindrical colony, 8km long and 4km in
diameter, with the interior illuminated at 100W/m^2, and ½ the energy
of the sunlight (the 'useless' wavelengths) captured at the windows.
Let there be 1000 such windows scattered around the middle of one
endcap of the colony. A fluid, opaque to the appropriate wavelengths,
would circulate between quartz layers to capture the energy, cool the
window, and provide power to the colony (heat engines are still more
efficient than PV cells). At night, the fluid would change so as to
absorb _all_ the light; no reason to waste the power-generating
capacity of the windows.

Each such window would be 4.3m in diameter, much easier to maintain or
to repair if ruptured than huge expanses of floor-windows. The total
area of the windows would be about 1/9th of 1% of that endcap's area.

I think we need to forget about windows on O'Neill-style colonies that
allow views of the starry sky and of the Sun swinging by in an
Earthlike fashion. O'Neill may have proposed it to encourage the idea
that space development didn't mean being trapped in little, tin-can
space stations or having to terraform planets but I don't think it's
practical to force an actual design to mimic _every_ aspect of life on
Earth's surface. I would design the colony as a large version of many
houses built in desert climates, with a lush garden in the center and
walls all around. If you want to see the Sun and the stars, go down
into the colony 'basement' to a nice, dark chamber and watch through a
floor-window. Sunlight should enter the colony in the safest, most
efficient manner. Using ½ the floor area for relatively fragile acres
of windows doesn't seem like the way to go.