Would anyone be willing to clarify some points for me on SPS? Here's
my thinking so far.

Power can be generated on an SPS from a few technologies.
Traditional PV made from lunar silicon crystal growth. Can be made
completely with in situ resources, but this technology is not very
effective in power conversion. Also, radiation damage over a 30 year
period would neccesitate PV replacement.

A variation of the PV silicon process is Rare Earh PV technology
which yields higher efficiencies, but requires importation of
critical raw materials from earth.

We could use Thin Film technology which has lowest efficiency, but
quickest manufactuirng time.

Or we could use Solar Thermal which has the highest efficiency, but
introduces mechanical complexity and liquid transportation into our
scenario.

After considering these options, I tend to favor the Solar Thermal
route.

So my questions on how to construct a SPS follow.

Considering that whether the SPS is in GEO or at lunar L1 (for
servicing the lunar facility power requirements) the SPS will
require an occasional orbit maintaining burn, this leads me to
conclude that the structure of the SPS will have to be designed to
take these forces into account. It can't be a pure zero-g
construction.

So, what should the infrastructure of the SPS be constructed of?
Steel Pipe? Steel girder? Aluminum? What dimensions for the beams?
Straight beams or triangular trusses? I'd love to hear peoples
thoughts on these issues.

My operating assumption is that the SPS has an infrastructure on
which to lay the Solar collecting apparatus. For the moment I'm
working on the assumption that the SPS would be comprised of row
after row of parabolic troughs, where each trough concentrates the
solar energy on a liquid coursing through a pipe centered on the
focal point of the parabolic trough. The liguid would be taken to a
series of Stirling engines located behind the solar facing surface
(on the dark side) and the electricity generated from these engines
would be sent to a central microwave transmitter.

OK, open fire and let's fix this scenario.

One other assumption I'm working with is that once the microwave
transmitter and at least one engine are in place, power can be
generated and transmitted while construction on the infrastructure
continues and additional power is brought on line. Is this feasible?

> Power can be generated on an SPS from a few
technologies.
> Traditional PV made from lunar silicon crystal
growth. Can be made
> completely with in situ resources, but this
technology is not very
> effective in power conversion. Also, radiation
damage over a 30 year
> period would neccesitate PV replacement.
>

IMHO an even bigger problem is the cost of setting up
a lunar plant to
make Si PV cells. Pure silicon is extremely
difficult to manufacture,
and production require a massive investment in
facilities. On Earth
enromous quantities of pure water are required which
simply will not be
available on the Moon. So completely new processes
will have to be
invented which will be even more expensive than
terrestrial processes.
I actually believe that manufacture of pure
crystalline Si semiconductor
quality materials on the Moon is a pipe dream.

One alternative might be amorphous silicon, that is
about the only Si
technology with a chance. But there are still a lot
of practical
problems, and it would not be my first choice for a
solar energy
generator.

> A variation of the PV silicon process is Rare Earh
PV technology
> which yields higher efficiencies, but requires
importation of
> critical raw materials from earth.
>

There might be even other technologies. It is
possible to make cells
from Titanium Oxides, and that material is readily
available on the
Moon. That looks like a more promising technology to
me.

>
> We could use Thin Film technology which has lowest
efficiency, but
> quickest manufactuirng time.
>

This would be fine for first generation SPS
demonstrators which are made
entirely from terrestrial materials.

This would be a vital precursor before we can expect
to see SPS built
from extraterrestrial materials.

I actually doubt that we will ever see SPS made 100%
from lunar
materials. There will always be some technologies
which are high
value, small and lightweight and very expensive to
manufacture on the
Moon. It makes economic sense to import such items
from Earth.

For example, ion drives and hall thrusters require
high precision
engineering, and are difficult to make. But their
weight percentage of
the total system is quite small, so it would make
sense to bring them up
from Earth, and not attempt to make them in space.
Same thing for
computer chips, electronics and attitude sensors.

>
> Or we could use Solar Thermal which has the highest
efficiency, but
> introduces mechanical complexity and liquid
transportation into our
> scenario.

Mechanical complexity is actually not such a big deal,
compared to
manufacturing pure Si.

>
> After considering these options, I tend to favor the
Solar Thermal
> route.
>

Bingo, I completely agree. Solar thermal is by far
the cheapest and
lowest risk type of solar power producer which can
most easily be
manuifactured primarly from lunar resources. The
investment required
to do this will be a lot less than trying to build a
semiconductor
manufacturing plant on the Moon (or in space).

>
> So my questions on how to construct a SPS follow.
>
> Considering that whether the SPS is in GEO or at
lunar L1 (for
> servicing the lunar facility power requirements) the
SPS will
> require an occasional orbit maintaining burn, this
leads me to
> conclude that the structure of the SPS will have to
be designed to
> take these forces into account. It can't be a pure
zero-g
> construction.
>

The forces can (and must) be quite small, low thrust
high SI thrusters
would be used, e.g. ion drive or hall thrusters.

>
> So, what should the infrastructure of the SPS be
constructed of?
> Steel Pipe? Steel girder? Aluminum? What dimensions
for the beams?
> Straight beams or triangular trusses? I'd love to
hear peoples
> thoughts on these issues.

Good questions.

For the size and dimonesions, a lot more research is
required before
these questiosn can be answered. A variety of
mathematical models must
be built to simulate various designs.

>
> My operating assumption is that the SPS has an
infrastructure on
> which to lay the Solar collecting apparatus. For the
moment I'm
> working on the assumption that the SPS would be
comprised of row
> after row of parabolic troughs, where each trough
concentrates the
> solar energy on a liquid coursing through a pipe
centered on the
> focal point of the parabolic trough. The liguid
would be taken to a
> series of Stirling engines located behind the solar
facing surface
> (on the dark side) and the electricity generated
from these engines
> would be sent to a central microwave transmitter.
>

This sounds like a reasonable "reference design" to
stimulate dicussion.

One point to discuss are the pros and cons of this
versus and array of
parabolic reflectors. One disadvantage of long
tubes of fluid is
that it gives a lot of surface area through wqhich the
heat can be lost
via radiation. We probably want to keep the tube
runs a short as
possible

>
> OK, open fire and let's fix this scenario.

> One other assumption I'm working with is that once
the microwave
> transmitter and at least one engine are in place,
power can be
> generated and transmitted while construction on the
infrastructure
> continues and additional power is brought on line.
Is this feasible?
>

I think this is inevitable. Maintenance and
expansion of the SPS
facility will be continuous and ongoing.