Solar concentration mirrors in the outer solar system

Christopher James Huff wrote:

For the first generation mirrors, I'd just look at using something like
sheet mylar stretched over rigid frames, approximating the parabola with
flat sections.

Here is a pattern for such a mirror:
http://clowder.net/hop/railroad/mirror.html

You wouldn't want the diameter of each polygonal face to be equal or
less than the cross section of the object you're focusing the sun's
rays on.

--
Hop David
http://clowder.net/hop/index.html

(Johnny1A) wrote in message . com...
Bryan Derksen wrote in message . ..
On Sat, 19 Jun 2004 20:09:04 GMT, "Perplexed in Peoria"
wrote:
It takes a lot of energy to find and refine that aluminum and to form it
into mirrors. Such an installation will have to pay for its own cost
of construction in less than 10 years to be worth doing. So, if you
can estimate the energy cost of construction, you can construct a
graph showing how time to pay back construction energy increases with
distance from the sun. The point where that line rises past 10 years
or so represents the real economic "edge of sunlight". It may be farther
out than 3AU, but I'll bet it is well inside the Oort cloud.

Solar mirrors of this type would probably be extremely easy to move
around the solar system; they're basically giant solar sails. So how
about doing all the manufacturing deep in the inner solar system,
where the aluminium and energy are extremely abundant, and send the
finished mirrors sailing out to the Oort? They could be "paid for"
with Oort cloud resources such as long-baseline astronomical
observations, volatiles, living space, etc. - whatever it is that
habitats and/or ships are out there for in the first place. There's no
need for complete material self-sufficiency as long as they're
_economically_ self-sufficient.

There's also the safety element, of course. It strikes me that a
couple of compact, shieldable fission/fusion reactors might be a
_safer_ source of energy than a fragile film of aluminum. OTOH, the
fragile film of aluminum has fewer components and a very reliable
ultimate source of energy.

It's the sort of tradeoff that we'd need experience to make, since
it's a judgement call.

Shermanlee

If you have redundant cabling, it's difficult to see what could damage
your mirror. Meteorites would just pass through it, so after a few
centuries it might be degraded, but not destroyed.

"Perplexed in Peoria" wrote in message igy.com...


The edge of sunlight, commonly put at 3 A.U., could possibly be
extended to hundreds of times that by using extremely thin mirrors to
concentrate the attenuated solar light. The mirrors could be aluminum
a few tens of atoms thick, and an array of flat mirrors could be aimed
with a support structure to focus this for useful solar energy and/or
light.
[snip]
distance area required mass required
(A.U.) (m2) (kg)

orbit of Pluto 35 5.83 x 10^9 2.33 x 10^6

current outer
edge of the Kuiper belt 70 2.33 x 10^10 9.32 x 10^6

furthest orbit of Sedna,
inner edge of the Oort cloud 900 3.86 x 10^12 1.54 x 10^9

These could be made from chunks of aluminum only 10, 16, and 83 meters
across respectively.

It takes a lot of energy to find and refine that aluminum and to form it
into mirrors. Such an installation will have to pay for its own cost
of construction in less than 10 years to be worth doing. So, if you
can estimate the energy cost of construction, you can construct a
graph showing how time to pay back construction energy increases with
distance from the sun. The point where that line rises past 10 years
or so represents the real economic "edge of sunlight". It may be farther
out than 3AU, but I'll bet it is well inside the Oort cloud.

The economics of far out settlements are a completely different
subject. Suffice to say for the moment, that:

- The masses are small compared to the settlement mass, upto the inner
edge of the Oort cloud.

- Current nuclear reactors weigh about 25 tons per MW, or 25 E6 Kg per
GW. Sure - this will improve, but they could still cost more than
mirrors at 70 AU, especially as the radiators need to use fluids
operating across ~5K to 800K temperature range.

I suspect (using forseeable technology - which is not realistic) a
large space settlement at 100 AU would use big mirrors for light, and
nuclear power for electricity, with each able to back up the other.

On Fri, 9 Jul 2004 10:59:36 +0000 (UTC),
(Alex Terrell) wrote:
The economics of far out settlements are a completely different
subject. Suffice to say for the moment, that:

- The masses are small compared to the settlement mass, upto the inner
edge of the Oort cloud.

At 3 kw/per person, 1 Gw of power would serve the needs of 3.33 x 10^5
people. For a cylinder settlement of a million people at 100 square
meters per person density, I get a figure of 5.03 x 10^11 kg for the
total mass of the settlement. Using equations derived from the ones
in my original post, I get the following distances for each given
percentage of the mass of the mirror system compared to that of the
settlement:

Percentage Distance (A.U.)

1 1626
10 5144
25 8129
50 11502
100 16267

So for a mirror comparable in mass to that of the settlement, the
maximum distance is 16267 AU: a quarter of a light-year. The mirror
system's area would be 1.26 billion square kilometers.

I suspect (using forseeable technology - which is not realistic) a
large space settlement at 100 AU would use big mirrors for light, and
nuclear power for electricity, with each able to back up the other.

I suspect that by that time they won't use mirrors, but instead will
build gigantic photovoltaic sheets. When they're able to manufacture
the thousands or millions of square kilometers of mirror cheaply, they
should be able to manufacture photovoltaic sheets not too much harder.

Photovoltaics would have lots of advantages over mirrors. The
problems of aiming the mirrors and keeping them relatively flat and the
support structure this requires is not needed. Instead a much lighter
tensional structure slowly spinning would be used just to keep the sheet
more or less oriented to the sun.

Photovoltaics can probably be made out of the organic elements, more
common in the outer solar system than aluminum or other reflective
metals.

In the outer solar system where temperatures are near absolute zero the
photovoltaics might work much more efficiently.

At over 10000 AU the sun would be dim enough and the sheet would be
big enough that light from starlight would become a factor. Put
photovoltaics on both sides of the sheet and collect light from the
rest of the galaxy. Then don't even bother pointing it toward the
sun, point it toward the galactic center. Then there's no limit to
how far out from the sun the settlement can go.

-------
WLM

In sci.space.policy WLM wrote:
On Fri, 9 Jul 2004 10:59:36 +0000 (UTC),
(Alex Terrell) wrote:
The economics of far out settlements are a completely different
subject. Suffice to say for the moment, that:

- The masses are small compared to the settlement mass, upto the inner
edge of the Oort cloud.

At 3 kw/per person, 1 Gw of power would serve the needs of 3.33 x 10^5
people. For a cylinder settlement of a million people at 100 square

3Kw/person is probably a bit low.

Consider for example if you could grow all the food you need in 6-12 square
meters.

Some quick sums indicate that for potatoes, for example, you get around
10Kg/m^2, and you need 3Kg/day (2000 Kcal)

With 3 crops a year, that's 30Kg/m^2, and you need 1100Kg/year, so
some 40m^2.
On earth, the light energy falling on that 40m^2 will be on average
some 250-300W/m^2, or a total of about 10-12Kw.

And that's leaving no power for cooling, ...

(WLM) wrote in message om...
At 3 kw/per person, 1 Gw of power would serve the needs of 3.33 x 10^5
people. For a cylinder settlement of a million people at 100 square

I suspect (using forseeable technology - which is not realistic) a
large space settlement at 100 AU would use big mirrors for light, and
nuclear power for electricity, with each able to back up the other.

I suspect that by that time they won't use mirrors, but instead will
build gigantic photovoltaic sheets

Oops, make that "a cylinder settlement of 3.33 x 10^5 people". And for
just light (for growing crops, etc.), settlements would still use mirrors
on out to quite a ways, say 100 AU.

Ian Stirling wrote in message ...
In sci.space.policy WLM wrote:
At 3 kw/per person, 1 Gw of power would serve the needs of 3.33 x 10^5
people. For a cylinder settlement of a million people at 100 square

3Kw/person is probably a bit low.

Consider for example if you could grow all the food you need in 6-12 square
meters.

Some quick sums indicate that for potatoes, for example, you get around
10Kg/m^2, and you need 3Kg/day (2000 Kcal)

With 3 crops a year, that's 30Kg/m^2, and you need 1100Kg/year, so
some 40m^2.
On earth, the light energy falling on that 40m^2 will be on average
some 250-300W/m^2, or a total of about 10-12Kw.

And that's leaving no power for cooling, ...

Point well taken. I suppose they could live on spirulina grown in
tanks at 3 kw/person, but that wouldn't be much of a life, not having
any real food.

As far as the cooling is concerned, that would probably be a power
*source* instead of requiring power. The temperature of the space
outside the settlement is going to be near absolute zero in the
places discussed. So a heat engine could be used to take that excess
heat from the atmosphere and by cooling it, generate electrical power
with very high efficiency. And so alot of that 10-12 kw could be
recovered.

At 10 kw/person, the furthest distance for a mirror comparable in
mass to the space settlement is at 10366 A.U. Still a long ways out.

Point well taken. I suppose they could live on spirulina grown in
tanks at 3 kw/person, but that wouldn't be much of a life, not having
any real food.



http://www.spirulina.com/

It's the pic on the left that gets me - I think these people intend to
terraform the _Moon_!

In sci.space.policy WLM wrote:
Ian Stirling wrote in message ...
In sci.space.policy WLM wrote:
At 3 kw/per person, 1 Gw of power would serve the needs of 3.33 x 10^5
people. For a cylinder settlement of a million people at 100 square

3Kw/person is probably a bit low.

Consider for example if you could grow all the food you need in 6-12 square
meters.
snip
And that's leaving no power for cooling, ...

Point well taken. I suppose they could live on spirulina grown in
tanks at 3 kw/person, but that wouldn't be much of a life, not having
any real food.

As far as the cooling is concerned, that would probably be a power
*source* instead of requiring power. The temperature of the space

For this to be true, you've got to have much larger radiators.
Say the heat source is at 273K.
The heat loss per square meter of radiator is of the order of
500W/m^2 or so.
Let's say you want to run the heat engine between 273K and 137K (50%
efficiency with an ideal engine), your radiators need to be 16 times as
large, assuming the same heat output.

On the flip-side, your energy gathering area can be 50% smaller.

[ Sorry for resurrecting a thread from July. The background was discussion of
how economical it'd be to use solar energy in the outer solar system courtesy
of concentrating mirrors, and how big mirrors you'd need... ]

Ian Stirling wrote:
In sci.space.policy WLM wrote:
On Fri, 9 Jul 2004 10:59:36 +0000 (UTC),
(Alex Terrell) wrote:
The economics of far out settlements are a completely different
subject. Suffice to say for the moment, that:

- The masses are small compared to the settlement mass, upto the inner
edge of the Oort cloud.

At 3 kw/per person, 1 Gw of power would serve the needs of 3.33 x 10^5
people. For a cylinder settlement of a million people at 100 square

3Kw/person is probably a bit low.

My conservative estimate for a comfortable colony would be a full megawatt per
person. The Earth gets about 10^16 watts, and has not quite 10^10 people.
Divide. You could argue some of that energy is supporting "waste" ecosystems,
but they're part of what makes the world nice, and we're said to be
intercepting 40% of primary productivity as it is, while at the same time
supplementing that with fossil and nuclear power.

I suppose if we just consider food... photosynthesis is about 1% efficient,
right? So that 40% is actually .4*10^14 watts, at best. Going the other way,
human bodies are roughly 100 W heat lamps, so 10^10 humans need 10^12 W of
food energy. Which seems about right, consider losses as you climb the
food chain and our eating a lot of meat.

So yea, 3-10 kW is probably a minimum just for food, but there's also the
elemental recycling (though at least some of that would be handled by the food
production), and industrial uses, and some amount of transportation and
"parkland", depending on whether your habitat is more like a nuclear submarine
or Soviet apartment block on one end, or an O'Neill park cylinder or Culture
GSV on the other.

*I* want a megawatt.

-xx- Damien X-)