Zubrin's panning of space solar power in Entering Space

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.

TomRC wrote:

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?
[...]
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.

The 'beam' from a mirror inevitably diverges, due to the nonzero
size of the Sun. The sun (at 1 AU) is about .01 radians across,
so the footprint of the beam from a mirror at altitude d is about
,01 d. This is too large for mirrors at GEO to be practical.

Mirrors in LEO suffer from shadowing by the Earth itself.

Paul

(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). 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.

(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.


Unfortunately, the sun is not a point source. It subtends
a half-degree arc. As a result, the beam dispersion from a
mirror in geosynchronous orbit is more than 8.7 milliradians.
This, in turn, gives a minimum spot size on the surface with
a width of over 300 km.

Mirors at low altitude may be useful for large solar ponds,
however. And lasers may work for solar furnaces in GEO.

Best regards,
Len (Cormier)
PanAero, Inc.
(change x to len)
http://www.tour2space.com

In article ,
says...
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.

What you're proposing is certainly interesting. However, I suspect that,
for a variety of reasons you'd run into trouble trying to implement it.
First of all, GEO space is expensive to get to and is prime real-estate.
Secondly there are lots of reasons why people would not be too keen on
losing the ability to have dark nights in a local region. Plants need a
dark cycle to grow for one thing. Also, one is likely to severely raise
temperatures in a region if there were extended periods of daylight.

On a more practical level, many of these space reflectors are quite
flimsy and will eventually burn up or need to be maintained on a regular
basis. Until we have more plain old solar electric collectors on earth
it won't make a lot of sense to put the mirrors up. But FWIW, I think
that space mirrors of some sort or other could have a lot of promise.

--
__________________________________________________ ___
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

TomRC wrote:

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?

Basic optics. Mirrors in the sky would reflect a trivial amount of solar
energy down to the ground.

A mirror can at best be as bright per visible area as the object it's
reflecting. so, if you want something in the sky that lights up the
ground as well as the sun does... it will need to look as big (or bigger
than) the sun. I leave it as an excercise for the student to determine
the diameter of a circle in geosynchronous orbit that would look as big
as the sun in the sky...


--
Scott Lowther, Engineer
Remove the obvious (capitalized) anti-spam
gibberish from the reply-to e-mail address

(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.

You forget that the atmosphere filters out some of the power, like UV rays,
for e.g. I don't know how much energy is lost this way however. It is
quite possible that a mirror system ends up being less expensive.

The closest thing to this I can remember is the old Russian plan to light
their polar circle cities with space based mirrors.

(Bill Bogen) wrote in message
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?

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). 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.

Thanks for your reply. Yeah, I did some more investigation of the
optics after posting.

However, I think there may be ways around this - though I haven't come
up with anything really economical so far. E.g. one might hang a
mirror on a tether from GEO to a much lower altitude, a la the space
elevator concept. That probably requires nanotube ribbon (not
available today, not likely to be cheap when available) and would
require more mass per sq-km - some in the ribbon, some in extra
supports to keep the mirrors flat under some fraction of 1G force.
Dangle it 6000km up, at about 1/4G, and I think the spot size falls to
about 55km, of which maybe a 25km diameter would be usable for power
generation.

Or, maybe use the mirrors at GEO to heat a high temp metal to a white
hot point source, and reflect THAT light to earth in a tighter area.
This would be massively inefficient, but would still probably be
better in terms of mass to orbit than the system Zubrin analyzed.

As to the heating - (a) put it in a desert to start with (b) shadow
the earth during the daytime to reduce net heat input (c) it probably
won't be terribly significant anyhow - a 30x30km net mirror size is
just too small to much affect the earth.

Scott A mirror can at best be as bright per visible area as the object
Scott it's reflecting.

A very nice explanation. It suggests a different view of what a solar
power satellite is required to do: convert solar photons into a tighter
beam of photons, improving the divergence by three or four orders of
magnitude.

I understand this can't be done purely optically. But it is interesting
to consider how little can be done in space, and have the SPS still do
useful power collection.

The photons have to be absorbed and then re-emitted to get away from the
optical limitation. Between absorbtion and re-emission, some of the
energy has to be concentrated (and most lost to heat), in order to get
a smaller spot size on the ground.

So can you build some sort of optically pumped laser? Maybe increase
the spectrum of light that pumps by converting out-of-band pump photons
with phosphors of some sort.

So you have a tube of this lasing stuff in a (very) long reflective
trough. One end points at the receiving station the ground. The other
end has a smallish cavity which has the reflectors that get the laser
going in the tube direction. Most of the tube is just a one-pass
amplifier.

Side benefits:
- Two axes of your station keeping are handled with tidal locking.
- Put an array of these troughs side-by-side, and you can rotate
the troughs to follow the sun by rotating against the array.
- Only one axis of sun tracking is necessary, and it doesn't interfere
with pointing at the ground.
- No high power moving parts, fluids, or electrical bits.

"TomRC" wrote in message
om...
I just finished reading Zubrin's "Entering Space", and was somewhat
disturbed by his economic analysis of space solar power.

To me, the biggest point that he missed (or pretended to miss) was that SPS
isn't better than ground-based solar because you can make the panels
smaller. It's better because the energy is there 24/7, which makes it much
better suited for baseload electric power than ground-based solar.

I write about this a bit he
http://members.aol.com/oscarcombs/spacsetl.htm#zubrin

Re space mirrors, in addition to the very good points others have made
regarding the optics, another disadvantage of space mirrors is that they
don't get you around the problem of overcast days in the same way that SPS
does.

--


Regards,
Mike Combs
----------------------------------------------------------------------
We should ask, critically and with appeal to the numbers, whether the
best site for a growing advancing industrial society is Earth, the
Moon, Mars, some other planet, or somewhere else entirely.
Surprisingly, the answer will be inescapable - the best site is
"somewhere else entirely."

Gerard O'Neill - "The High Frontier"