"Concerned citizens" only hope for SPS......by Dr. Gerard K. O'Neill

Henry Spencer wrote:

Not quite. For one thing, with cheap solar arrays you're lucky to get 15%
of the light out as electricity, whereas the microwave conversion is 90%+
efficient (and the hardware should be a good deal cheaper, per square
meter, than even cheap solar cells).

For another, what happens when the Sun goes down, or it's cloudy? The
microwave beam is there *all the time*. (That advantage is even bigger
than it looks, because large-scale energy *storage* is difficult and very
expensive -- the payoff for needing *no* storage is much bigger than for
merely reducing the size of the storage.)

Better yet, rectennas aren't opaque like solar panels. They can be
deployed in a large open mesh that allows enough sunlight through to
allow agriculture underneath the rectenna. So one gets multiple land
use, too.

--
Dave Michelson

Henry Spencer wrote:

For another, what happens when the Sun goes down, or it's cloudy? The
microwave beam is there *all the time*. (That advantage is even bigger
than it looks, because large-scale energy *storage* is difficult and very
expensive -- the payoff for needing *no* storage is much bigger than for
merely reducing the size of the storage.)



Question of the week: I assume we put these in GEO; is there ever a
point during the year that they get eclipsed by Earth's shadow?
Also, does the sun getting behind them cause any problems, the way that
it causes interference with satellite transmissions from GEO here twice
a year?
For that matter, does the whole SPS solar array rotate once a day so as
to keep it facing dead on at the sun?
If we get a breakdown on one of the SPSs, how do we compensate for
losing its energy from the power grid if we use them as our main power
supply?
I picture only a few dozen of these of huge size in GEO supplying power
to each country, and losing 2% of your nation's power supply could
create problems.
Or do we use them in LEO or HEO, and deal with the tracking problems of
having them move across the sky in relation to their receiver antennas
and going into Earth's shadow on a regular basis, as well as air drag in
LEO?
If nothing else, the lower orbits would make aiming the microwave beam
at a point on Earth easier in regards to the size of the transmitting
antennae needed on the SPS.
Degradation of the solar cells due to radiation as well as crewed
construction and maintenance would be problems if they end up in the Van
Allen belts.
Further, the lower the orbit, the easier these would be to destroy - as
they are going to be pretty good-sized targets that would be vulnerable
to direct ascent attacks via missiles, given their necessarily
lightweight construction.
You could also hit them when they are on the far side of the Earth from
the nation they are supplying power to, and annihilate a nation's power
supply inside of a few hours if they are in orbits that traverse the sky.
This is the perfect target for the "Bucket of Sand" attack method.

Pat

Dave Michelson wrote:


Better yet, rectennas aren't opaque like solar panels. They can be
deployed in a large open mesh that allows enough sunlight through to
allow agriculture underneath the rectenna. So one gets multiple land
use, too.


There's some artist's conceptions of them he
http://ssi.org/assets/images/SPS_wi_rectenna.jpg
http://members.aol.com/sandycombs/rectenna.jpg
Gigantic, but probably quite cheap to make on a per-acre basis, and easy
to maintain.
HAARP's antennas are big, but quite low tech, and this would be fairly
somewhat similar, greatly scaled up:
http://www.sydweedon.com/images/haarp.jpg
http://www.ecplanet.com/pic/2003/06/...struttura2.jpg
I'm still not going to be the first person to walk under one when the
beam's coming down, mind you. :-)

Pat

On 24 Oct 2006 21:35:11 -0700, in a place far, far away,
made the phosphor on my monitor glow in such a
way as to indicate that:

If the power beam from space is safe because quote: at that spot size,
the
power density is about half that of sunlight.unquote.


so you want to spend billions having a space based system that delivers
about half the amount of power that a terrestrial solar panel would
provide. ?

doesnt seem worthwhile somehow.

It is. It delivers it continuously, regardless of weather or night,
and it has much higher conversion efficiency than solar panels, so
despite the lower power density, it's much more effective than solar.

On Wed, 25 Oct 2006 03:11:02 -0500, in a place far, far away, Pat
Flannery made the phosphor on my monitor glow in
such a way as to indicate that:



Henry Spencer wrote:

For another, what happens when the Sun goes down, or it's cloudy? The
microwave beam is there *all the time*. (That advantage is even bigger
than it looks, because large-scale energy *storage* is difficult and very
expensive -- the payoff for needing *no* storage is much bigger than for
merely reducing the size of the storage.)



Question of the week: I assume we put these in GEO;

Not necessarily. Other architectures have been considered.

is there ever a
point during the year that they get eclipsed by Earth's shadow?

A few minutes per day during solstices. You'd have to have redundant
beams from adjacent satellites to avoid the outages.

Also, does the sun getting behind them cause any problems, the way that
it causes interference with satellite transmissions from GEO here twice
a year?

I don't know.

For that matter, does the whole SPS solar array rotate once a day so as
to keep it facing dead on at the sun?

Not necessarily. One could use a spherical collector, which would be
simpler, but also more massive and costly.

On Wed, 25 Oct 2006 12:09:01 GMT, h (Rand
Simberg) wrote:

is there ever a
point during the year that they get eclipsed by Earth's shadow?
A few minutes per day during solstices. You'd have to have redundant
beams from adjacent satellites to avoid the outages.

Or a generating system that can sustain itself for the few minutes
of eclipse. A solar-thermal system, for example, rather than
photovoltaics, where the hot-spot can continue heating the working
fluid for a short time.

Also, does the sun getting behind them cause any problems, the way that
it causes interference with satellite transmissions from GEO here twice
a year?
I don't know.

I wouldn't think so. We're not talking about a beam that is
modulated to carry data, after all... but a beam of steady intensity.
Solar interference might cause a slight drop-off, but this could be
compensated for in the design.

Joe Strout wrote:

:In article ,
: Fred J. McCall wrote:
:
: Joe Strout wrote:
:
: :Yes, if you want your enemies to receive free, abundant power with
: :hardware that's hardly any more complex than a metal grating, this would
: :certainly be the thing to use. (For the sarcasm-impaired, that was a
: :sarcastic remark meant to highlight the irony of proposing the use of a
: :harmless -- actually, beneficial -- power beam as a weapon.)
:
: And what's to stop you from simply refocusing the beam?
:
:To do what? Deliver the energy to a different customer? Basically
:nothing, though at high inclinations they'd have to put a little more
:effort into the rectenna.
:
:But if you mean, to focus it to a tighter spot so as to make it even
:remotely useful as a weapon, then the answer is: the laws of physics.
:The minimum spot size is a function of the distance and the size of the
:transmitter. It'll already be as small as it can be (why would you
:build your transmitter bigger than needed?), and at that spot size, the
ower density is about half that of sunlight.

And you only ever have one, right?

Yeah, sure.

--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn

In article ,
Fred J. McCall wrote:


: And what's to stop you from simply refocusing the beam?
:
:To do what? Deliver the energy to a different customer? Basically
:nothing, though at high inclinations they'd have to put a little more
:effort into the rectenna.
:
:But if you mean, to focus it to a tighter spot so as to make it even
:remotely useful as a weapon, then the answer is: the laws of physics.
:The minimum spot size is a function of the distance and the size of the
:transmitter. It'll already be as small as it can be (why would you
:build your transmitter bigger than needed?), and at that spot size, the
ower density is about half that of sunlight.

And you only ever have one, right?

Yeah, sure.

Nobody is going spend $100 billion and 20 years of development and
deployment time to blast their enemies from space with microwaves.
There are plenty of other much cheaper weapons available now. Or do you
think that wily terrorists will simultaneously commandeer 10 or 20 SPSes?

In article . com,
wrote:

OK , I am missing something here.

If the power beam from space is safe because quote: at that spot size,
the
power density is about half that of sunlight.unquote.

so you want to spend billions having a space based system that delivers
about half the amount of power that a terrestrial solar panel would
provide. ?

doesnt seem worthwhile somehow.

Actually, you're missing two things:

1. Microwaves (of a suitable frequency) can be converted into
electricity with enormously greater efficiency (around 90%) than
sunlight (10-20%), using extremely cheap hardware (little more than a
wire mesh).

2. SPS power is available 24 hours a day and regardless of weather (and
is so suitable for baseline power).

Cheers,
- Joe

In article ,
Pat Flannery wrote:

I picture only a few dozen of these of huge size in GEO supplying power
to each country, and losing 2% of your nation's power supply could
create problems.

If it becomes our primary power source, you'll need more than a few
dozen per country -- we'd need to have them by the hundreds. This can
help deal with the eclipse issue. (If it is an issue; as another poster
pointed out, this may depend somewhat on the solar energy conversion
technology used.)

Or do we use them in LEO or HEO, and deal with the tracking problems of
having them move across the sky in relation to their receiver antennas
and going into Earth's shadow on a regular basis, as well as air drag in
LEO?

Long-term, I'm not a fan of this approach, because it seems wasteful;
your hardware is useless while it's in Earth's shadow, and while it's
not within sight of any rectenna. Then of course you have the tracking
and drag problems you mention.

However, in the short term, these may be quite attractive, because the
transmitter can be a LOT smaller (as you pointed out). This lets you
start with a small demonstrator, whereas you simply can't do a small
demonstrator in GEO; the transmitter size places a bound on how small
you can build.

In addition to a demonstrator unit, you might be able to find customers
along a certain orbit (probably, near the equator) who would want to
time-share a LEO powersat, or a constellation thereof. You still can't
sell them power at night, though, which takes away one of the big
advantages of SPS over terrestrial solar.

Further, the lower the orbit, the easier these would be to destroy - as
they are going to be pretty good-sized targets that would be vulnerable
to direct ascent attacks via missiles, given their necessarily
lightweight construction.

True. But I would expect this type of powersat to be used only in the
early days, and to supply only a small fraction of each customer's
power, so they wouldn't make very useful targets.

Then of course you have some people seriously proposing that the solar
power stations should be on the Moon. That doesn't feel like a win to
me, given the even-larger transmitter needed and the Moon's annoying
habit of moving around in the sky. GEO seems like the sensible place
for large-scale space solar power.

- Joe