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From: (Dani Eder)
Subject: Re: Microwave power transmission (was Re: ENVIRON-NUTS)
Date: Sep 19 1995

Rod Adams <> writes:
>Paul, one of the Florida utilities has a power generation capacity of
>14,000 MWe. This utility serves about 2/3 of one state.
>If they decided to use your solar satellites to serve the needs of their
>current customers, how many satellites would they need and how big
>would the ground receiving antennas be?
>(Rough calculation for the receiving antennas, figuring on 200 watts per
>square meter would result in 70 square kilometers of receivers.) That
>would be empty land with no buildings, no plants, no trees, and no

The microwave receiving antennae need not be solid metal, any more than
all backyard satellite dishes are.  The holes do need to be less than
1/8 wavelength so you don't get much transmission.  You can grow stuff
underneath the antennas as something like 50% of the sunlight will get
through.  Your calculation of the total land area is about right, though.
Note that a coal-fired plant of 300 MW covers about 200 acres, so the
50 or so conventional plants cover 15 square miles, or 37 square km,
not trivial either.

>If a satellite were as big as an aircraft carrier (300 meters by 30
>meters) it would be able to collect only 13.5 MW of sun energy.  If it
>was 30% efficient (a real stretch for solar cells) it would only produce
>only 4 MWe. It would take a huge number of such satellites to provide
>even the electricity needed for a portion of a single US state, much less
>the energy needs of the entire world.

The studies in the early 1980s done by the Dept of Energy assumed each
satellite was 10 km long and delivered 5,000 MW at the ground, so for
Florida you would need 3 satellites.  The large size of the satellite
was driven by placing them in synchronous orbit and the 2.45 GHz frequency.

We have made some progress in producing microwaves efficiently at
higher frquencies, and lower orbits could be used with some loss of
operating time, which would bring the minimum plant size down to 50MW.

Despite being as large as 10 km, the mass of the satellite is about
1 aircraft carrier (50,000 tons), being a thin sheet of material
backed by a structural frame.

>Since the Hubble's solar cells needed to be replace after just a couple
>of years, it would seem to me that your proposed system would take a
>huge amount of resources.

The solar arrays on the Hubble were replaced primarily because 
they jiggled too much, not because they wore out.  If a power satellite
is exposed to radiation, the cells gradually degrade.  For communications
satellites in synchronous orbit, this is not a major problem over 12
year service lives.  To maintain high efficiency, the power satellite
concepts included thermal annealing of the arrays periodically to
remove the cell defects generated by radiation.

A study which I worked on determined that 3% by weight (or 1500 tons)
of the satellite needed to be brought from Earth, the rest of the
material could be brought from the Moon, which takes 20x less energy
to launch stuff from.  At next-generation launch vehicle costs of
$1000/kg, this amounts to $300/kW of installed power, which is not
trivial but not too much either.

Dani Eder

From: (Dani Eder)
Subject: Re: Microwave power transmission (was Re: ENVIRON-NUTS)
Date: Sep 20 1995

Rod Adams <> writes:

>Paul Dietz <> wrote:

>The question, Paul is how much would 70 km^2 cost?  In many areas of 
>Florida, especially where the power demand is high, undeveloped land 
>costs about $10,000 - $50,000 per acre. That would lead to land costs 
>alone in the several billion dollars even before launching a single 
>vehicle.  Of course, you will also have to do rather intensive studies 
>on the effects of microwaves on the endagered species that may be living 
>under the "rectenna".

$50,000 per acre is indicative of suburban land costs.  An SPS receiver
can be several hundred miles away from the power users if you build
a suitable high voltage DC power line.  Where I live, 62 miles from
downtown Birmingham, AL, land costs $1000 per acre or less.  Also, 
offshore installations have been considered.

>I have seen the reference designs and find that my poor mind cannot 
>comprehend how we can get there from here. I chose the analogy of an 
>aircraft carrier because that is one of the largest man-made objects I 
>can think of that actually moves.  I assume that the satellites will 
>have to have control systems to keep them aligned to both the sun and 
>the receiving antennas.  

Think of it more like a building or a bridge, it's not going
anywhere once it's built.  The original studies assumed the powersat
was all brought from Earth.  1 km square sections are assembled
at a factory in low orbit, then they self-ferry themselves to GEO
by supplying power to ion thrusters.  The large aircraft assembly
building Boeing has in Everett, WA covers some 42 acres, or 1350 ft
square.  The powersat weighs about as much as an aircraft carrier,
but is more spread out.

Pointing is pretty straightforward.  The array always points at
the sun.  Using a piece of something several feet in front of some
cells can be used for sensing.  As long as you are pointed correct,
all the cells are in shadow.  If you get off-sun, whichever cell is
out of the shadow tells you which way you are off-sun.  The
microwave transmitter is a phased array.  It uses a reference
beam transmitted from the center of the receiving antenna on the
ground.  The reference beam, except at startup, is powered by
the received power.  Therefore if the beam wanders off the receiving
antenna, you lose power for the phase reference, and the beam
defocusses over the entire Earth.  This way you can't get a terrorist
trying to zap Disneyworld.  Being a phased array, the transmitter
can be a few degrees off from pointing right at the receiver
without affecting the beam quality.

>BTW, I also assume that the satellites will have to be parked in that 
>crowded orbit over the equator 22,300 miles up where geosynchronous 
>satellites that serve communication systems are concentrated.  Will they 
>fit without causing a traffic jam?

Since the SPS is a huge power supply, comm transponders will naturally
migrate to being mounted on the powersat.  Even with the most
ambitious scenarios, with 200 powersats in GEO producing 1000 GW,
the satellites will be 1300 km apart, and 10 km long, so there is
quite a lot of empty space left in between.

>If smart fellows designing an array producing a few hundred watts of 
>power have difficulty with support structures and thermal expansion, how 
>will future engineers avoid those problems with multi-megawatt sized 
>arrays?  I also seem to recall that there have been other satellites 
>with solar arrays that have been damaged by space debris, and which have 
>failed to fully deploy.  I also recall that the cells are susceptible to 
>damage from cosmic radiation.  How long does an average cell last in the 
>rather harsh environment of space anyway?

The only reason the arrays on Hubble were a problem is the extreme
sensitivity of Hubble to jiggling.  They were being bothered by
one part in ten million jiggles of the telescope caused by two
parts per million flex of the arrays.  The powersat as a whole can
flex by one part in a hundred with no measurable effect on performance.
Deployment will be done at an orbital factory, when you mount the
cell arrays on the support structure.  Radiation does affect the
cells, depending on where you are.  In the worst part of the 
radiation belt (1-2000 km) you will lose 50% of your power in a year.

GEO is in the outer fringes of the radiation belt, and you lose a
few percent efficiency a year due to radiation damage.  That's why
they planned to anneal the cell defects by heating them on a periodic
basis.  Heating can be done rather simply in GEO with a few mirrors
to focus several suns intensity on one spot.

I would assume as a side activity you would send out some ion-
powered tugs using smaller arrays to collect up dead satellites
in and near GEO so they don't become a hazard.  As for natural
meteoroids, they will poke holes in your array.  Live with it.
The meteoroid density at GEO is lower than that at the Earth,
since the Earth's gravity focusses them some.  Look up at
night and count meteors (a few an hour), and remember that they
are each a fraction of an inch across.  How long will it take
to have enough meteors to make a dent in 50 square km?  Think
of it as painting a square inch an hour.  That's real slow.

>Lest people accuse me of just taking more potshots against a non-nuclear 
>technology, please understand that I am just asking practical 
>engineering questions that must be addressed before obtaining any funds 
>for such an ambitious project.

The two valid criticisms from the original National Research Council
review of the Department of Energy studies (around 1981) were (1)
solar cells cost too much.  They were assuming (in 95 dollars) 
around $0.40 per watt, while cell costs then were more like $10-15
per watt.  Currently they are $4 per watt, still too high, but better.
(2) launching everything from the ground required launch costs of
$40 per lb, which was more than 2 orders of magnitude below then
(and now) launch costs.  I worked on a sucessor study that showed
that you could reduce the Earth-launch needs to 1% stuff too hard
to make in space, and 2% factory equipment, using lunar-derived
raw materials, so your Earth-launch needs are 3% of the satellite
weight, and a factor of 10 reduction in launch cost (about what
the SSTO is supposed to do) would be enough.

If you want to find out more about these things, search under
Dept of Energy studies with 'Solar power satellite' in the title,
and National Research Council reports around 1981.

Dani Eder

From: (Dani Eder)
Subject: Re: Microwave power transmission (was Re: ENVIRON-NUTS)
Date: Sep 27 1995
Newsgroups: (Mike Jamison (ADF)) writes:

>In article <>, (Dani Eder) writes...
>Seems like a lot of things would have to pan out in just the right way for
>space based solar power to be viable:

>1)  Launch costs reduced to $40/lb.

In fact, that's about the launch cost assumed for the Solar Power Satellite
studies ($25/lb in 1980 inflated to today's prices would be $40 or so).

>2)  Solar cell prices stay at current levels.

No, we assumed that cell prices would fall to around $1 per watt.  The
key ratio for SPS vs ground PV is the cost/watt ratio.  In orbit, a
square meter of PV material will produce about 7 times the average
power of a cell on the ground (factoring in night, clouds, and tmospheric
absorbtion).  Thus, if the overhead to install the PV in space is
less than 7 times the panel cost, it makes sense to do it in space.
If it costs more than 7 times the panel cost to put it in space, it
is uneconomic relative to ground PV.

>3)  Environmentally friendly launch system.

The launch system assumed was a 'big dumb booster' type with a 200 ton
payload (if all the satellite hardware comes from earth).  It uses
LOX/Kerosine for launch, and water lands in one piece.  To deliver one
satellite required 250 launches, at a cost of $17.5 million per launch.

4-5)  Fusion and nanotech dont happen: that's a given.

>Scrap the PV cells.  Use mirrors and a brayton cycle generator, thermal
>phase change battery - end to end efficiency goes up by a factor of 4, using
>today's technology.  And the thermal battery lasts longer than the Ni-H
>batteries used now.

The SPS concepts included a brayton cycle and a PV array as alternates.
Neither had energy storage, since you are in sunlight 99% of the time
(except for 1 hour periods near the equinoxes when the Earth eclipses
you).  The outages occur at local midnight at the receiving antenna, 
so it is assumed that you can cover for the outage by wheeling in
some power from neighboring sources.

>>A study which I worked on determined that 3% by weight (or 1500 tons)
>>of the satellite needed to be brought from Earth, the rest of the
>>material could be brought from the Moon, which takes 20x less energy
>>to launch stuff from.  At next-generation launch vehicle costs of
>>$1000/kg, this amounts to $300/kW of installed power, which is not
>>trivial but not too much either.

>This study must assume that a moonbase already exists?  Or does it include
>the cost of building moon factories?

The breakdown is 1% of the satellite is too complex to make in space,
the other 2% (1000 tons per satellite) represents the equipment delivered
to the Moon to mine and extract the balance of the materials.  For
example, the brayton cycle collector surface can be sheet metal with
a small overcoating of aluminum to increase reflectivity.  Iron can
be extracted from the lunar surface by magnetic separation to pick out
iron-nickel particles from meteorioid bombardment.  Then you melt
what you pick up to separate the iron bits from the glassy stuff they
are typically stuck to.  The iron sinks to the bottom, and you have
reasonably good metal.  So you need a machine to scoop up lunar surface
material, screen it by size, then dribble the small particles past
a magnet.  The iron-bearing particles get shoved sideways.  Melting
can be done with a mirror reflector.  All this can be done via remote
control of the mining machines.  Launching stuff off the moon can be done
by mechanical catapult.  One version uses a rotating arm with the
payload on one end and a counterbalance on the other mad of local 
rocks.  The arm is spun up over time by an electric motor, and 
both payload and counterweight are released at the same time to
minimize the reaction when you throw.  This operation can also be
automated (load payload, load counterweight, spin up, release,
de-spin, repeat).  It is assumed that you will have to bring 2%
of a satellite's mass after the first few as replacement equipment
and supplies to the moon's surface.

Dani Eder

From: (Dani Eder)
Subject: Re: LaGrange colonies: are they needed?
Date: Jun 21 1996
Newsgroups: (Martijn Faassen) writes:

>What about O'Neill's own argument, powersats? Eventually we could
>run out of energy down here (if we use uranium it'll take a while,
>if we manage to get nuclear fusion going, this point is not valid). 
>Solar energy is hard to obtain here on earth, but in space, you have
>plenty of it, 24 hours a day. If a way can be found to transfer it back
>to Earth (microwave radiation is what I read about), this could be
>very useful for the people back on Earth.

The relevant figures are that an array of solar cells in GEO produce
about 7 times as much power as the same array on the Earth's surface
in a typical North American location.  This is because in space you
don't have losses due to nighttime, clouds, and absorbtion by the 
Earth's atmosphere.  Therefore if it costs less than 7 times more
to set up an array in space than on the ground, you are better off trying
to do so.  The factor of 7 has to account for the extra stuff to
get the array in position and feed the power back to earth.

The next question is whether solar (ground or space based, whichever
is cheaper) beats out other power choices.

Solar power satellites were considered by O'Neill because energy
production is a big enough business to cover the then $200 billion
estimated cost of building a space colony.

There is another business big enough to be in that league - steel
production.  The world uses about 600 million tons of steel per 
year, worth about $200 billion-300 billion per year.  There's lots
of high grade steel floating around in the asteroid belt.  If you
can deliver it back to earth for only $100 billion per year, you
would stand to make a hell of a profit.  $100 billion per year 
pays for a lot of space program.  As far as I know nobody has
studied the details of how to do it like they have for powersats.

Another point - the $200 billion to build a space colony was using
1970's era technology, and is way too high.  

Dani Eder

From: (Henry Spencer)
Date: Mon, 9 Aug 1999 18:11:12 GMT

In article <>,
Christian Petersen  <> wrote:
>A physicist informed me that the efficiency of the present energy transfer beam
>(microwaves) devices are very low.

Not so.  DC-to-DC efficiencies circa 60% have been demonstrated, and the
microwave-to-DC efficiency at the receiver -- which is what matters most,
since that's where the waste heat goes into the biosphere -- is 80%+.
Further improvements should be possible with a bit of work.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: Re: Orbital microwave power stations
Date: Mon, 25 Oct 1999 14:03:51 GMT

In article <>,
Jan Panteltje <> wrote:
>> 3,456 full time window washers... and
>>maintenance staff to repair cells damaged by weather
>>such as hail, lightning, winds, sandstorms, etc. The rectenna farm is much
>>harder to damage or degrade.
>Yes, but in your case you also have some equipment to maintain up there.

Not very much maintenance.  Some powersat designs have no moving parts at

>There the efficiency is not that high either, converting solar energy
>to microwaves,even if you reached 90 %, then you will still have to
>radiate .1GW in heat away from your 'space powerplant'.

Quite correct.  But that inefficiency is out where it doesn't matter very
much -- it's not taking up surface area on the ground or putting heat into
the biosphere.  It needs engineering attention but isn't a big problem.

>Not to mention the incredible surface you need for your solar cells, and these
>break down to, so who is going to replace these? Costly job.

Uh, "break down"?  Solar cells generally don't just fail.  They do tend to
deteriorate with radiation exposure, so powersat designs usually have
built-in equipment for re-annealing the cells.  There's no need to have a
crew out there for that.

>How are you going to point that HUGE solar array :)?

Not a significant problem, since it only has to hold a fixed pointing
direction (well, more precisely, a direction which rotates continuously at
one revolution per year).  Light pressure (solar sailing) is an obvious
method, with all that surface area.  So are ion engines, since there will
be lots of power available.

>Also it is not clear how you could receive ANYTHING with such a low power level
>/ meter square.

It's been demonstrated.

>Finally the wavelength has imo not much to do with it, in the cm range, it
>makes no difference if your reception area is 100 sq meters or 10 sq kilometer.

The wavelength has *everything* to do with the reception-area size,
because it fundamentally determines how tight a beam you can get out of a
transmitting antenna of a given size.  Twice the wavelength means twice
the beam width.

>it is the construction of the antenna(s) that is the important factor...

For phased-array antennas controlled using a beacon transmitted from the
target, exact antenna shape is almost irrelevant, because the beam control
system just cancels it out.  The control system essentially echos back the
wave shape it receives, and so any irregularity in the antenna surface has
no effect on the transmitted beam.

>But technically the most problematic part is repairing things up there.
>If it breaks down today, how soon will your power plant be 'on line' again?

Breakdowns can be expected to be relatively rare, since there are few
moving parts and a very predictable environment.  Coping with occasional
problems would be done the same way that it's done for conventional power
plant:  have spare capacity to take over temporarily.  If there are a
bunch of powersats up there, it's quite likely that one or two will be
spares, rented out to whoever needs them.

>All that while all we need to get some heat is dig a hole in the ground deep

If only it were that simple, geothermal power would be running everything.
It's not.  The heat is there, but getting it up in quantity is not easy.

>Would you perhaps care to explain (I can only find a fuzzy picture) how
>that antenna works, whre the power lines are (that route the power from
>the centre say), what type of photocells you will use up there, what
>type of antenna, wha tare you using to convert electrity into
>microwave up there, what is the MTBF of those, and of
>cause a few more things I could not find on the links you provided?

You might consider reading books (remember them?) instead.  Not everything
is on-line.  The information you want has been published; there are a
number of different proposals.

If you want the design explained to you, in detail, I can send you my
consulting rates.  You won't like them. :-)  Reading books is cheaper.
The space program reminds me        |  Henry Spencer
of a government agency.  -Jim Baen  |      (aka

From: (Jordin Kare)
Subject: Re: SPS Reprise
Date: Wed, 15 Dec 1999 21:12:11 -0800

In article <838ntu$djq$>, wrote:

<Ongoing thread about SPS's using laser downlink to directly power
automobiles deleted...>

> A couple hundred horsepower on an antenna only half a meter
> wide is fairly stout... about 600 kilowatts per square meter, or more
> than 400 times solar insolation. Zap. "Ouch, I'm blind and my hair's on
> fire. And where'd my skin go?"

Lends a whole new meaning to a phrase one of my bosses used to use to
describe working flat out:  "Going 90 miles an hour with your hair on

There are some difficulties with parts of Mr. Mook's scheme as proposed
that have not been cited yet.  One is pointahead:  pure phase reversal via
4-wave mixing will cause the returned beam to focus where the beacon *was*
(in the satellite's frame) when the beacon emitted a signal, not where it
is when the amplified signal gets back.  For GEO, pointahead will offset
the returned beam by about 400 meters; if the target is moving at, say, 20
m/s (72 km/hr) there's an additional ~4 meters of pointing error.  The 400
m error can be compensated for in principle, since it's not time-varying
(although it varies with location on Earth); the error due to target
motion is pretty tough to correct.  Some other numbers seem highly
optimistic, such as the efficiencies of photovoltaics, even when
illuminated with bandgap-matched monochromatic light.  However, the most
obvious one is that cloud cover will scatter any radiation short enough to
allow practical transmission from GEO to meter-scale collectors (assuming
the transmitting aperture is not 100+ km across), which pretty much rules
out short wavelengths (infrared or visible light) for direct transmission
of power from space to distributed ground receivers.  (No, you can't burn
through clouds, and no, not even 4-wave mixing will let you propagate a
beam through them.  Millimeter waves will penetrate light clouds, but are
severely scattered by rain, and so are probably still unacceptable for
power distribution)

None of this should be taken to discourage thinking about innovative SPS
technology; I personally think there will eventually be an economic case
for SPS's that will support investment. But I don't think we're there

Jordin Kare

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