Artificial vs. natural illumination for space habitats

I've lost count of how many times on these newsgroups I've encountered
someone saying that providing space habitats with natural sunlight via
mirrors and windows would just be too complicated, and we'll instead use
artificial illumination. Sometimes the advantages of using red LEDs to
raise crops are touted (less wattage, less need for heat rejection).

I've always argued against this, mostly from instinct, but also from knowing
the actual solution the original space settlement studies did settle on. I
was concerned about conversion efficiencies (why convert light to
electricity and then back to light again when light is what you wanted in
the first place?), and it always struck me that aluminized Mylar and glass
panels by the square mile would be pricey, but PV arrays and artificial
lights by the same magnitude would be more so.

Here's a paper that seems to agree:

"Effect of Environmental Parameters on Habitat Structural Weight and Cost"
http://www.nas.nasa.gov/About/Educat...eres/II-1.html

In the midst of looking into a variety of parameters for habitats for 1,000
(early construction shack), 100,000 (intermediate range earthlike habitat),
and 10,000,000 (long range habitat), and for toroidal, spherical,
cylindrical, and Crystal Palace geometries, the paper compares the costs for
artificial vs. natural illumination.

Table 6 indicates that natural illumination with mirrors should have only
about 20% the cost of the artificial illumination option.

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended. They say you could get up to a solar
concentration level of 70x before getting into problems with the glass
softening.

--


Regards,
Mike Combs
----------------------------------------------------------------------
By all that you hold dear on this good Earth
I bid you stand, Men of the West!
Aragorn

Mike Combs wrote:
...it always struck me that aluminized Mylar and glass
panels by the square mile would be pricey, but PV arrays and artificial
lights by the same magnitude would be more so.

Mylar does not survive long enough in the outer space. To the best of
my knowledge all plastics warp in the outer space. Inorganic mirrors,
for example aluminum mirrors are more durable.

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended.

This idea was first mentioned in Island One web site:
http://www.islandone.org/LEOBiblio/SPBI1GH.HTM
The Island One site also mentions the need to cool the windows to
minimize their size and cost.

In article ,
"Mike Combs" wrote:

I've lost count of how many times on these newsgroups I've encountered
someone saying that providing space habitats with natural sunlight via
mirrors and windows would just be too complicated, and we'll instead use
artificial illumination. Sometimes the advantages of using red LEDs to
raise crops are touted (less wattage, less need for heat rejection).

Right -- though not necessarily LEDs, and not exactly red (but rather
matched to the absorption spectrum of chlorophyll as efficiently as
possible).

I've always argued against this, mostly from instinct, but also from knowing
the actual solution the original space settlement studies did settle on. I
was concerned about conversion efficiencies (why convert light to
electricity and then back to light again when light is what you wanted in
the first place?)

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.

and it always struck me that aluminized Mylar and glass
panels by the square mile would be pricey, but PV arrays and artificial
lights by the same magnitude would be more so.

That may be so, but this leaves some things out, like the Chevron
shields needed on top of any glass panels, and the radiator mass needed
to reject all that extra heat, 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.

Also, aluminized Mylar is unselective (i.e. it reflects a wide range of
wavelengths, including those outside the visible range), which creates
an even bigger problem -- the paper you cite below advocates using
selective mirrors, though they don't explain exactly what those would be
and how much that would cost.

Here's a paper that seems to agree:

"Effect of Environmental Parameters on Habitat Structural Weight and Cost"
http://www.nas.nasa.gov/About/Educat...eres/II-1.html

Yes, I've read this, but I'm not convinced by it. They didn't consider
the heat budget, except to recommend use of selective mirrors. Also,
their argument hinges on "distribution costs" which are not explained,
and which can't be derived from the other figures shown as far as I can
see. I recently asked about this on the SSI list (and got no reply):

The only constant mentioned for artificial lighting costs, other than
the PV array, is: "The cost for light bulbs and fixtures was estimated
using a specific weight of 0.03 kg/W of rated fixture output." But doing
the math, starting with the 44900 kg PV array shown in the table, I get:

PVmass = 44900 kg: 44900 kg
PVpower = PVmass * 124 W/kg: 5567600 W
fixtureMass = PVpower * 0.03 kg/W: 167028 kg

The PV power calculated matches the table (0.56 kW per person in this
10,000-person scenario), but multiplying this by the specific weight of
the fixtures only gives is 167028 kg -- not the 281,700 shown. Since
this distribution mass (ergo cost) is the dominant factor in the cost of
artificial lighting, this discrepancy is important. Can anyone see what
I'm missing?

Note that the numbers don't work out even if we grant their cost of
"light bulbs and fixtures" -- but of course there wouldn't be light
bulbs and fixtures; technology marches on, and we'd almost certainly be
using some form of solid-state lighting. (A good though now somewhat
outdated review is http://eetd.lbl.gov/btp/papers/47589.pdf.) I'm
pretty sure the mass per W output is substantially lower for these,
though that needs more research to be sure.

So, in short, I think we need to redo the analysis in that paper you
cite in light of modern technologies. But it's going to be a little
hard to make it comparable because of the mysterious and unexplained
"distribution costs" which dominate their results.

(If I were the cynical type, I'd be tempted to think that "distribution
costs" means "hand-waving fudge factor needed to support the conclusion
the authors wanted to make.")

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended. They say you could get up to a solar
concentration level of 70x before getting into problems with the glass
softening.

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.

Best,
- Joe

In sci.space.tech, on Mon, 3 Jul 2006 12:07:28 -0500, Mike Combs
sez:

` I've lost count of how many times on these newsgroups I've encountered
` someone saying that providing space habitats with natural sunlight via
` mirrors and windows would just be too complicated, and we'll instead use
` artificial illumination. Sometimes the advantages of using red LEDs to
` raise crops are touted (less wattage, less need for heat rejection).

I would be concerned about using limited bandwidth for crops, because
other things are going on besides photosynthesis. The UV in sunlight
provides sterilization on the plant and ground surfaces, keeping down
mould and bacterial colonies. Without this effect, you would need
more chemicals (how much, and which?), with accompanying indeterminant
biohazards...

` I've always argued against this, mostly from instinct, but also from knowing
` the actual solution the original space settlement studies did settle on. I
` was concerned about conversion efficiencies (why convert light to
` electricity and then back to light again when light is what you wanted in
` the first place?), and it always struck me that aluminized Mylar and glass
` panels by the square mile would be pricey, but PV arrays and artificial
` lights by the same magnitude would be more so.

` Here's a paper that seems to agree:

` "Effect of Environmental Parameters on Habitat Structural Weight and Cost"
` http://www.nas.nasa.gov/About/Educat...eres/II-1.html

` In the midst of looking into a variety of parameters for habitats for 1,000
` (early construction shack), 100,000 (intermediate range earthlike habitat),
` and 10,000,000 (long range habitat), and for toroidal, spherical,
` cylindrical, and Crystal Palace geometries, the paper compares the costs for
` artificial vs. natural illumination.

` Table 6 indicates that natural illumination with mirrors should have only
` about 20% the cost of the artificial illumination option.

` The paper does agree that concentrating solar energy so as to minimize
` window area is to be recommended. They say you could get up to a solar
` concentration level of 70x before getting into problems with the glass
` softening.

There are lots of different ways of getting sunlight in, as well. Besides
big mirrors, you could play with things like light pipes running "up"
from the outside to appear as towers with a reflective cone on top.
These might make sense for street lights in a habitat with a night
cycle on its main mirror system, if the design was sufficiently simple
and led to significant reduction in power generation and distribution
costs. The article seems very concerned with the cost of thick glass,
but I speculate that for such auxiliary lighting systems, the glass
quality doesn't need to be as high performance as for the main lighting
system. The pipes could be mostly open space with reflective surfaces,
following a "maze" path of some sort to baffle radiation.


--
================================================== ========================
Pete Vincent
Disclaimer: all I know I learned from reading Usenet.

Mike Combs wrote:

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended. They say you could get up to a solar
concentration level of 70x before getting into problems with the glass
softening.

Island One web site describes windows made of fused silica or Pyrex to
reduce sunlight absorption. The windows are cooled with water so
that they can be very small and very cheap.

http://www.islandone.org/LEOBiblio/SPBI1GH.HTM

Mike Combs wrote:
I've lost count of how many times on these newsgroups I've encountered
someone saying that providing space habitats with natural sunlight via
mirrors and windows would just be too complicated, and we'll instead use
artificial illumination. Sometimes the advantages of using red LEDs to
raise crops are touted (less wattage, less need for heat rejection).

I've always argued against this, mostly from instinct, but also from knowing
the actual solution the original space settlement studies did settle on. I
was concerned about conversion efficiencies (why convert light to
electricity and then back to light again when light is what you wanted in
the first place?), and it always struck me that aluminized Mylar and glass
panels by the square mile would be pricey, but PV arrays and artificial
lights by the same magnitude would be more so.

There is also simplicity. I mean a mirror is a dam simple device. Sure
solar cells have no moving parts, but look at the output over time. I
think mirrors work better in a radation envourment.

But i'm not sure about concerntration it 70x --thats not trival optics
and you may have just throw'n away all the simplicity.

delt0r

"Mike Combs" wrote in
message ...
I've lost count of how many times on these newsgroups I've encountered
someone saying that providing space habitats with natural sunlight via
mirrors and windows would just be too complicated, and we'll instead use
artificial illumination. Sometimes the advantages of using red LEDs to
raise crops are touted (less wattage, less need for heat rejection).

I've always argued against this, mostly from instinct, but also from
knowing
the actual solution the original space settlement studies did settle on.
I
was concerned about conversion efficiencies (why convert light to
electricity and then back to light again when light is what you wanted in
the first place?), and it always struck me that aluminized Mylar and glass
panels by the square mile would be pricey, but PV arrays and artificial
lights by the same magnitude would be more so.

Here's a paper that seems to agree:

"Effect of Environmental Parameters on Habitat Structural Weight and Cost"
http://www.nas.nasa.gov/About/Educat...eres/II-1.html

In the midst of looking into a variety of parameters for habitats for
1,000
(early construction shack), 100,000 (intermediate range earthlike
habitat),
and 10,000,000 (long range habitat), and for toroidal, spherical,
cylindrical, and Crystal Palace geometries, the paper compares the costs
for
artificial vs. natural illumination.

Table 6 indicates that natural illumination with mirrors should have only
about 20% the cost of the artificial illumination option.

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended. They say you could get up to a solar
concentration level of 70x before getting into problems with the glass
softening.

--


Regards,
Mike Combs

Solar power and primary and secondary illumination works closer in toward
the Sun but space colonization once on a roll will spread speedily further
and further out into the farthest reaches of the solar system. In closer to
the Sun, it is my belief that solar power and illumination overwhelms all
other sources of possible power and illumination so much so that we can't
even discover the existence of them, much less use them, as potential
alternatives. But those human explorers and colonizers reaching those
distances from the Sun I believe will do both.

Remember something, Mike, and think hard upon it. Cosmologists have
already discovered from observed peripheral effects upon the observable that
there appears to be very little void or vacuum to any otherwise clearly
apparent void or vacuum whether situated locally or remotely in the
Universe. We've already peripherally detected [in the shadows so to speak] a
Universe of matter and energy far different from anything our forebears ever
suspected anywhere in anything but science fiction. Buried so deeply as we
are in our own Sun's overpowering overwhelmingness, we can only speculate
what may be in those shadows. Those explorers, those colonizers, those
openers, those would be free, reaching further and ever further out where
the Sun is not at all overwhelmingly overpowering are the sole ones who
could and would meet that now speculative but unknown frontier horizon (way
out there in the -- relatively speaking -- shadows and the dark beyond
that).

We cannot possibly know it from here and now, but there may possibly be a
frontier Universe out there between the stars, and between the galaxies, far
greater and far richer than even the Universe we observe to be there. We
happen to already be fighting for the rich possibilities, the rich
dimensions, of the one between the planets of our own nearer interplanetary
space. For all that we are just beginning to realize would acquire to Space
Colonization, those dimensions, those probabilities, those possibilities,
may only be the smallest nearest, stepping stones of beginning toward an
presently unimaginable enormity of frontier we cannot and will not observe
until we meet it head on in moving out into it (in going in motion into it).

GLB

Mike Combs wrote:
I've lost count of how many times on these newsgroups I've encountered
someone saying that providing space habitats with natural sunlight via
mirrors and windows would just be too complicated, and we'll instead use
artificial illumination. Sometimes the advantages of using red LEDs to
raise crops are touted (less wattage, less need for heat rejection).

I've always argued against this, mostly from instinct, but also from knowing
the actual solution the original space settlement studies did settle on. I
was concerned about conversion efficiencies (why convert light to
electricity and then back to light again when light is what you wanted in
the first place?), and it always struck me that aluminized Mylar and glass
panels by the square mile would be pricey, but PV arrays and artificial
lights by the same magnitude would be more so.

Here's a paper that seems to agree:

"Effect of Environmental Parameters on Habitat Structural Weight and Cost"
http://www.nas.nasa.gov/About/Educat...eres/II-1.html

In the midst of looking into a variety of parameters for habitats for 1,000
(early construction shack), 100,000 (intermediate range earthlike habitat),
and 10,000,000 (long range habitat), and for toroidal, spherical,
cylindrical, and Crystal Palace geometries, the paper compares the costs for
artificial vs. natural illumination.

I think the generation cost is rather academic. Looking at the
radiation requirements, these imply large mass. My proto-design for a
cylinder is 8km long, 4 km indiameter, and all in masses about 1
billion tons.

The inside surface area is close to 100km2, so it needs, at about
100W/m2 (quite generous) 10GW of light. I assume this could be provided
at 50% efficiency, so max power needs are 20GW.

This may seem a lot, but its only about 200,000 tons of standard SSP.
Compared to the habitat this is trivial.

More important is ability to control the light to minismise heat
rejection. Both electric lighting and filtered sunlight will do.

By and large I prefer beamed light because, once IR and UV are filtered
out, it's close to 100% efficient, thereby reducing cooling needs.

Table 6 indicates that natural illumination with mirrors should have only
about 20% the cost of the artificial illumination option.

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended. They say you could get up to a solar
concentration level of 70x before getting into problems with the glass
softening.

Coming back to my "standard" cylinder - not all the 100km" needs
direct illumination. I was originally thinking of a 1km diameter axial
window. This is in the realm of "problems". It would also need to be
built in sections reinforced with steel, so that individual panes can
be replaced. The steel might get too hot.

I eventually came to the conclusion that a Bernel sphere approach might
work. Light would be beamed in along two rings at either end of the
flat plane.

wrote in message
ps.com...

Mylar does not survive long enough in the outer space. To the best of
my knowledge all plastics warp in the outer space.

The designs mentioned in the original studies were aluminized Mylar sheets
stretched over and fastened to metal frameworks. So it would be the
frameworks providing straightness, not the Mylar itself.

Inorganic mirrors,
for example aluminum mirrors are more durable.

Yeah, and given that silicon and oxygen are so plentiful, I think even
ordinary glass mirrors are another possibility. If the habitat is designed
such that the mirrors do not need to rotate with the habitat to perform
their function, they could be much thinner than the kind of mirrors we
typically make here on Earth.

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended.

This idea was first mentioned in Island One web site:
http://www.islandone.org/LEOBiblio/SPBI1GH.HTM
The Island One site also mentions the need to cool the windows to
minimize their size and cost.

This design uses water to keep very small windows below the boiling
temperature. Which means illumination is provided by 6 sun-hot
continuously-erupting geysers. I was always a bit put off by how unearthly
it would appear.

--


Regards,
Mike Combs
----------------------------------------------------------------------
By all that you hold dear on this good Earth
I bid you stand, Men of the West!
Aragorn

"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.

--


Regards,
Mike Combs
----------------------------------------------------------------------
By all that you hold dear on this good Earth
I bid you stand, Men of the West!
Aragorn