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Zubrin's panning of space solar power in Entering Space



 
 
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Old February 25th 04, 12:26 PM
william mook
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Default Zubrin's panning of space solar power in Entering Space

(Bill Bogen) wrote in message om...
(TomRC) wrote in message . com...
I just finished reading Zubrin's "Entering Space", and was somewhat
disturbed by his economic analysis of space solar power. Not that I
found it off the mark for what he analyzed - but that it seemed to
ignore what seems like an obvious alternative. Too obvious - there
must be something I'm missing here.

Can someone explain why, given Zubrin's arguments about launch costs
for the commonly conceived solar to microwave power sat, it isn't far
more effective to simply put large mirrors up at GEO to light up solar
power farms on Earth at night? That should about double the power
output of a solar power farm, without greatly increasing operating
costs. The mirrors could also be used during early and late daylight
hours, to augment lighting of the solar farm - eliminating most of the
value of expensive sun-tracking hardware.

My back of the envelope estimate (including LEO launch costs coming
down to $2000/kg due to frequent launches of solar mirrors - a side
benefit) comes in well under 2 cents per KW-hr for the power added by
the space mirrors. I assumed that the mirror could either solar sail
up to GEO, or that there'll be enough LEO to GEO traffic to justify an
inexpensive solar powered tug.


Assume the mirror is perfectly reflective. To double the output of
the solar farm would require that the mirror 'look like' the complete
image of the Sun, that is, span about 0.5 degrees in the sky, just
like the Sun. To do this from the distance to GEO (35844 km) would
require that the mirror be 313 km (188 miles) in diameter. Also, most
of the energy from the mirror would be used to heat the atmosphere
(adding to global warming).


Absolutely right in all details.


Solar power satellites, made from lunar
materials, with cheap rectenna farms on the earth's surface, just seem
smaller, simpler, cheaper, and more enviro-friendly.


The logistics of setting up to build stuff on the moon destroy the
economics here. If you don't count those costs into the charges
against the solar power plants, you might have a shot.

But, TomRC has a valid point change the paradigm and you change the
economics. The paradigm he chose was wrong, but his desire to solve
the problem by changing the underlying physics of achieving the end
you want is an utterly valid approach.

Zubrin's analysis is also spot on accurate as all Zubrin's work is.
If you asked him about other feasible processes he would likely reply
that its up to you to come up with a valid approach, he cannot spend
time analyzing things that aren't proposed.

But, TomRC is right that Zubrin looks at a classic Glaser design.
That's because that's what's out there. Glaser proposed thin film PV
collectors feeding a large phased array microwave emitter beaming
energy down to a single site at baseload rates of pay at very low
power densities. TomRC fastened on the idea - quite rightly - that
the incredible masses of PV material involved killed the project. Get
that areal density down and voila! - something that's not feasible at
a certain price point becomes feasible. Having a thin film mirror on
orbit won't do it because of optics. But, a thin film mirror
concentrating light onto a solar pumped laser just might.

First, large thin film mirrors can be deployed in orbit with modest
masses. That's no problem. Echo II passive satellites were launched
in 1960 that massed less than 80 kilos and was around 50 meters in
diameter

http://www.hq.nasa.gov/office/pao/Hi...4209/ch2-5.htm

Here's a photo of one and a little of its role in history.

Lots of aerospace engineers have considered how to make use of this
incredibly low mass system to do useful optics.

http://www.de.afrl.af.mil/News/2003/03-38.html
http://www.nasatech.com/Briefs/Jan01/NPO20952.html

These can be used for telescopes to look into space or back at Earth.
These can be used to project beams of energy where you want them.
These can also be used to concentrate sunlight to a point nearby.

Having lots of concentrated sunlight and usefully processing that
sunlight into either electricity

http://www.mokindustries.com

with which you can make maser or laser energy

http://www.physik.uni-wuerzburg.de/T...opto/power.htm

fairly efficiently or laser energy directly from sunlight

http://hep.uchicago.edu/solar/laser.html


Now, if you make electricity in this way and use it to make microwaves
with klystron tubes then you've got the problem of beaming the
microwave energy to Earth. This really isn't a problem, but what
you're really up against is the Rayleigh limit - which depends on the
spot size you want to make on the ground, the size of the optics you
have in your emitter, the distance between the two points - and the
wavelength of the beam you're using.

Microwaves mean big wavelengths, and big wavelengths mean big emitters
and collectors. So, you still have a big ass antenna on orbit if you
use microwave beams that penetrate cloud cover well.

But, if you efficiently generate laser beams in space and beam them to
receivers on the ground, the Rayleigh calculation says you can get by
with really tiny optics, which is a good thing, especially if they're
made of thin film reflectors!

So, TomRC's objection to Zubrin's analysis stands. His solution is
not right, just incomplete. Mr. Bogen's analysis of the shortcominb
of Tom's proposed solution is also correct. Mr. Bogen's own proposed
solution of building things on the moon - similar to what was proposed
by Gerard O'Neil many years ago - is one way to reduce costs of
traditional PV/microwave approach. If you don't count the cost of
building the industrial infrastructure on the moon in the first place
against the solar panels.

I believe that Tom's observation that mirrors, especially thin film
mirrors have a lot to recommend them. I'm not alone in this belief.
That's why the air force, nasa, and others are spending considerable
sums on thin film space optics.

One can imagine a proposed solution as follows;

(1) Build a fully reusable, multi-element heavy lift launcher out of
near term technology - to reduce the cost of placing big things
into space;

(2) Use thin film optics along the lines of ECHO II, to create a
very
large concentrating mirror in space launched in a single rocket
launch aboard (1), and deployed in space. This powersat
consists
of the following;

(a) Inflatable concentrating primary
(b) Solar pumped laser array
(c) Laser beam steering apparatus
(d) Beam energy to large numbers of users simultaneously
(e) Receivers use bandgap matched PV to efficiently produce
electricity


This last bit, 2c - laser pumped beam steering is the optical
equivalent of phased array beam steering in a microwave emitter. The
wavelengths are different, and the physical mechanism to achieve the
phase changes in both systems are slightly different, but the wave
physics are the same.

Optical processing of the laser energy can be used to reliably and
safely deliver billions of high power beams anywhere on earth (or in
space) by the use of nonlinear optical materials;

http://www.photonics.cusat.edu/Resea...ptics_OPC.html

Basically, customers beam a low power laser to the satellite. A
window of special material reacts to these beams. A powerful laser
pulse generated in the satellite using sunlight as its primary power
source shines through this window. The window breaks up the powerful
beam pulse into a conjugate beams that trace each lowpower beam back
precisely to each customer's site. There optics take the powerful
beam and cause it to illuminate a high intensity PV cell of very small
area (and cost).

Since the low power beam passes through the atmosphere on the way up
to the satellite, it is able to cause the returning beam to be
predistorted so that when it arrives at the ground receiver, all the
effects of the atmosphere are removed.

This can be done with an adaptive mirror as well, as shown below;

http://www.ctio.noao.edu/~atokovin/t...art5/mcao.html

But the measurments indicated here are all taken by the low power
pilot beam and the nonlinear optical window does the rest.

There are some more things that we need to do before we can publish a
complete engineering analysis to rival Dr. Zubrin's analysis of
Glaser's initial conception. But, Tom's idea of reducing machine mass
and complexity through the use of lightweight optics is a good one
despite the error he made in his first suggested implementation.

William Mook
 




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