Solar concentration mirrors in the outer solar system

Whenever settlement of the outer solar system is discussed in this
ng, it's always assumed that it will be done using fusion power to
provide energy, with an "edge of sunlight", beyond which solar
photovoltaics which cannot be used, around 3 A.U.

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.

To supply 1 gw. of electrical power at earth orbit using the best
photovoltaic cells available (~ 30% conversion efficiency), an
area of 2.38 x 10^6 square meters is required. For getting the
same electrical power at distances further from the sun, an area
of mirrors is required amounting to

Area = A_e x Distance^2 / E

where

A_e = area at earth orbit = 2.38 x 10^6 m2
Distance = distance in A.U.
E = efficiency factor based on the reflectivity of the mirror,
and how well the light reflected hits the photovoltaics
due to possible aiming problems or mirror deformation.

and

Mass = Area x T x M_sp x Factor_s

where

Mass = entire mass of mirror system (kg)
T = thickness of mirror (m)
M_sp = mass density of mirror material (kg/m3)
Factor_s = factor for extra mass required for the support structure
for the mirrors.

Using an efficiency factor of 50%, a thickness of 40 nm, a mass
density of 5 gm/cm3 (= 5000 kg/m3) (I don't know the density of
aluminum off-hand so I used this), and a support structure factor of
2, one gets

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.

Problems

erosion of the mirrors by dust and meteorites
damage by the solar wind and/or cosmic rays
reflectivity being obscured by dust particles or
organic matter collecting on the surface
solar sail effect which makes the mirror system
(or what it is attached to) move
manufacturing the huge mirrors in the first place

Solutions?

Give the mirrors a static charge so they might have net mass
collecting capability instead of mass loss.

Attach the mirror system to its space settlement and orient it so
that the settlement slowly spirals down toward a more desirable
location closer to the sun.

Other possibilities

Instead of using mirrors, use photovoltaic cells themselves.
Assumes an extremely cheap way of manufacturing them of course.

Have mirrors with a reflector made from organic materials
(if there are any), which would make it lighter than an aluminum
one and from more common organic elements. Or make a concentrating
lense instead of a mirror.

Have there been any studies done on this subject? I think it's very
worthwhile looking at, given how much time and trouble we've had
getting controlled fusion power to work. Also a space settlement out
there in the Kuiper belt might want to save their deuterium for more
useful things like propulsion, self-defense, etc.

-------
WLM

(in order to get the correct address, make "EugeneFreenet" into "efn")

"wlm" wrote in message
...
Whenever settlement of the outer solar system is discussed in this
ng, it's always assumed that it will be done using fusion power to
provide energy, with an "edge of sunlight", beyond which solar
photovoltaics which cannot be used, around 3 A.U.

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.

Incidentally, once the mirror costs begin to dominate the costs of the
photovoltaics, there is no particular economy of scale AFAICS. So,
whatever works at Pluto, just use four of them at 70 AU.

If you are interested in sf technologies for providing energy in
the belt, here is one I have never seen explored. I haven't "done
the numbers", so it may not be practical. The idea is to tap the
accessible orbital energies represented by the eccentricities and
inclinations of the comets' orbits. For example, if your home comet
is headed south, and mine is headed north, we throw projectiles
at each other. Throwing takes energy, but catching provides
even more energy, since we were moving toward each other. However,
opportunities for such profitable exchange may be few and far
between. Furthermore, you have to invest energy to gain energy,
and the payback period may be prohibitive. In any case, this
pool of extractible energy is finite. When it runs out, all of
the belt comets will be in circular orbits in the same plane.
However, unlike with most energy extraction schemes, this long-term
changing of the environment may actually be a good thing - at
least it increases the density of settlement to a level that can
support some kind of civilization.

In article ,
says...
Whenever settlement of the outer solar system is discussed in this
ng, it's always assumed that it will be done using fusion power

Yeah, we can always hope that fusion will be ready by the time we're
exploring the outer reaches of the solar system. Of course, in all
probability the first fusion reactors are going to be monolithic and
it's probably going to take some time to miniaturize them. Then there's
the fact that the radiation given off by fusion reactors tends to weaken
the lattice structure of many materials, meaning that reactor life may
be short. But hey, if we have fusion technology we might also have the
ability to make and store enough antimatter to meet energy requirements
for our mission.

to
provide energy, with an "edge of sunlight", beyond which solar
photovoltaics which cannot be used, around 3 A.U.

Right, but many PV systems for space applications are envisioned to use
fresnels or other simple concentrators. Furthermore, if you use direct
solar thermal systems, as opposed to PV you will get superior
efficiency. Thus you will not need as much total energy input.


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.

Sure. Of course you could also just design systems that beamed high
energy radio waves over the distance required.

--
Quibbler (quibbler247atyahoo.com)
"It is fashionable to wax apocalyptic about the
threat to humanity posed by the AIDS virus, 'mad cow'
disease, and many others, but I think a case can be
made that faith is one of the world's great evils,
comparable to the smallpox virus but harder to
eradicate." -- Richard Dawkins

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.

In message , Bryan Derksen
writes
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?

That resolves an issue I had with an idea. If you melt a blob of glass
in zero G you could inject some gas into it and inflate it into a
bubble. If you then coated one side of the bubble with aluminium you
have a solar furnace that can hold an atmosphere, for instance hydrogen.
(The spherical shape isn't ideal, I think spinning the bubble could
create something closer to a parabola.

My problem was that these would be useful in the inner solar system, but
would eventually drift outwards unless some energy was used to
counteract the drift.. I wasn't sure what to do with bubbles that
drifted away. Of course they would continue to be useful in the outer
solar system, and might even be habitable.




--
Bernard Peek
London, UK. DBA, Manager, Trainer & Author. Will work for money.

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

On Sun, 20 Jun 2004 15:24:47 +0100, Bernard Peek
wrote:
My problem was that these would be useful in the inner solar system, but
would eventually drift outwards unless some energy was used to
counteract the drift.. I wasn't sure what to do with bubbles that
drifted away. Of course they would continue to be useful in the outer
solar system, and might even be habitable.

A solar sail can move inward toward a star by "tacking"; angle it so
that the reflected light pushes it against the direction of its orbit,
slowing it down. Gravity would then pull it inward.

If you're building your sail out of a glass bubble thick enough to
hold a usable atmosphere, though, it's probably going to be extremely
heavy compared to the solar sails that are usually considered, which
are aluminium films (possibly with plastic backing) only a few microns
thick. It will take a long time for the impulse from reflected light
to move those anywhere, I suspect.

In article , Bernard Peek
wrote:

(The spherical shape isn't ideal, I think spinning the bubble could
create something closer to a parabola.

Spinning the bubble and using uneven heating could produce something
very close. Perhaps you could start out with a hollow cylinder, "doped"
with varying amounts of pigment along its length to influence absorption
of solar heat. Spin the cylinder up and put it in an existing solar
furnace.

More likely, use gas jets and more controlled heating to shape it the
way you want. A relatively small reflection concentrator moving along
the spinning mirror, adjusting the heating to make sure it expands
evenly into a parabolic shape.

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.

--
Christopher James Huff
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG:
http://tag.povray.org/

In message ,
Christopher James Huff writes
In article , Bernard Peek
wrote:

(The spherical shape isn't ideal, I think spinning the bubble could
create something closer to a parabola.

Spinning the bubble and using uneven heating could produce something
very close. Perhaps you could start out with a hollow cylinder, "doped"
with varying amounts of pigment along its length to influence absorption
of solar heat. Spin the cylinder up and put it in an existing solar
furnace.

More likely, use gas jets and more controlled heating to shape it the
way you want. A relatively small reflection concentrator moving along
the spinning mirror, adjusting the heating to make sure it expands
evenly into a parabolic shape.

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.

For the first generation a hemispheric reflector might be good enough. I
was thinking of a bootstrap process, where each furnace would be used to
build progressively larger furnaces.

Another possibility would be to build a tubular structure, creating a
linear furnace. Billets of material could be pushed in one end of the
tube and be heated as they travelled the length of the tube.
Alternatively a ring-shaped furnace could be used, melting a zone of
material that could be moved along the length of a bar of rock, in the
same way that zone refining is used to purify silicon for
semiconductors.

This sort of furnace would require quite pure silica. If the furnace had
walls thick enough to hold a significant atmosphere the walls would have
to be thick. To get thick but transparent walls the walls would have to
be made of reasonably pure silica without impurities like iron.


--
Bernard Peek
London, UK. DBA, Manager, Trainer & Author. Will work for money.

Bernard Peek wrote:
In message ,
Christopher James Huff writes
In article , Bernard Peek
wrote:

(The spherical shape isn't ideal, I think spinning the bubble could
create something closer to a parabola.

Spinning the bubble and using uneven heating could produce something
very close. Perhaps you could start out with a hollow cylinder, "doped"
with varying amounts of pigment along its length to influence absorption
of solar heat. Spin the cylinder up and put it in an existing solar
furnace.

More likely, use gas jets and more controlled heating to shape it the
way you want. A relatively small reflection concentrator moving along
the spinning mirror, adjusting the heating to make sure it expands
evenly into a parabolic shape.

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.

For the first generation a hemispheric reflector might be good enough. I
was thinking of a bootstrap process, where each furnace would be used to
build progressively larger furnaces.


I brought up the "blow a bubble, make a mirror" idea a few months back
(sst, IIRC, but maybe I'll take the time to look it up).

The responses indicated that for the size mirror needed for any useful
purpose, blowing a bubble was impractical, would still require a
framework for stability, and making flat sections was a much more
practical idea. In addition, at the spherical radius of the mirror,
it would be very hard to distinguish between the curved and the flat
sections; it would be big enough that, like the parking lot outside
mission control, the sphere would locally measure as flat.

Perhaps you could make flat section in situ by spinning the melt like
pizza dough, so don't put away that axle and driveshaft yet.

/dps