Unfortunately it is unclear from the diagram at
whether the 88% energy efficiency of collection accounts for loss due to the
angle of the collector.
My guess is that it does. (The ground collector would assumably have the
same power loss problem so it would have been a significant error in the
diagram to not to have factored it in...) However without going to the
source data used or asking Mark Pardo, we don't know.
Tom M.
TomM@...
I hope Criswell is talking to Robert Forward, who's something of an expert on pushing around light, wire meshes with microwave beams.
>>>>>>>>>>
I have Bob F's email address, you can easily reach him via http://www.tethers.com
I found an email for David Criswell, but I got not resposne on a message I sent to it, not sure if it is valid.
>>>>>>>>>>>>
(Or Leik Myrabo, who's done actual demos on this.)
>>>>>>>>>>>>
I have not tried reaching Leik.
>
>That puts us at 50.8% efficiency... instead of 38.5% for the relay...
>
>Tom M.
>TomM@...
A reflector is going to act like a solar sail. Station keeping due
to momentum transfer will have to be calculated. The relay should
have the same problem. I don't know if the momentum transfer would
be signficant.
==========
Well yes, if 100%-50.8% = 49.2% of the microwave is converted to momentum that is significant. But it is a lot easier to deal with momentum than to dissipate all that as waste heat. For momentum we need a propulsion system, electric propulsion should work fine, needs very little propellant.
>Unfortunately it is unclear from the diagram at
>
>http://www.permanent.com/p-sps-tc.htm
>
>whether the 88% energy efficiency of collection accounts for loss
due to the
>angle of the collector.
>
>My guess is that it does. (The ground collector would assumably
have the
scenerio the rectenna would be placed perpendicular to the beam.
Mitchell James
mejames@...
http://www.InnerTransit.net (Email distribution for multilevel organizations)
http://www.InnerTransit.org (Homebase for collaborative engineering)
According to this link
_report
"Canada is expecting to export a good deal of its super-cheap hydro-power to
the USA. And Hydro-Quebec is considering exporting electric power to Japan
by microwave using orbiting "power relay satellites". This got the headlines
in the local newspaper - "Vendre de l'electricite au Japon... par
satellite" - and may possibly be the first application of space-based WPT
for terrestrial energy supply. "
http://www.hydro.qc.ca/
So, hydro quebec may have already done much of the research that we are
looking at. This was three years ago, so apparently they decided to not go
through with the plan, but perhaps we can find out why.
Tom M.
TomM@...
This is testimony before the Subcommittee on Space and Aeronautics Committee
on Science House of Representatives regarding SPS
by
Gregg E. Maryniak
maryniak@...
He also might be able to provide additional info to help our case.
Tom M.
TomM@...
Another interesting link mentions the most recent hearings on SPS were in
2000
also mentions NASAs development plans and estimates, etc.
Tom M.
TomM@...
>
> Is it possible that we could use the reflector to do more than just
> passively reflect the beam? After all, it's floating in orbit anyway.
> Why not have it collect solar energy of it's own to add as a "boost" to
> the energy it's reflecting? Sort of an energy from orbit/energy to orbit
rectenna target.
It is not possible to focus both at once.
How would the solar energy be collected ?
Solar PV cells are only 20% efficient and very heavy.
What you are talking about is essentially SPS, no ?
But what I have been saying is that simple microwave relays are a lot
cheaper than SPS, and could be a peofit center much sooner.
What you are proposing is the reverse of my proposal.
>
> orbit, but on looking closer, I see we're discussing actual power
> relays. Why aren't we just talking reflectors, since Criswell seems
> to think they can be made to work?
>
Do you have links or references ?
We are going in that direction.
The main problem is that the path length doubles so the rectenna
diameter doubles.
Do you have links or references ? http://www.hotbot.lycos.com/?search=1&MT=David+Criswell+lunar+solar+powerreturns some relevant hits. But I wan't able to find anything specifically on the subject of the Earth-orbiting microwave reflectors.
Mike Combs
See
has many of the relevant links
here is a list of Criswells papers
http://ilewg.jsc.nasa.gov/ILEWG/register/criswell_papers.html
And here is another relevant link
http://www.spacefuture.com/archive/power_from_space_for_the_next_century.sht
ml
it mentions that Angelini has proposed what we've proposed in has paper
Arnaldo M Angelini, 1988, "On the Possibility of Intercontinental Power
Transmission via Satellite", Space Power, Vol. 7, No. 2
Tom M.
TomM@...
>
> The angle to the Sun will be different than the angle to the Earth
> rectenna target.
> It is not possible to focus both at once.
point at the Sun while the transmitter/reflector is pointed at the Earth
rectenna target? I could make a sketch, but I'm no good at ASCII drawing.
>
> How would the solar energy be collected ?
>
> Solar PV cells are only 20% efficient and very heavy.
>
> What you are talking about is essentially SPS, no ?
>
Heavy is certainly a big problem. But I thought the whole point of
collecting solar energy from orbit was that you could achieve efficiency
ratings above 90%. Otherwise, why bother? I don't have the reference on
hand to quote from, unfortunately. Maybe I misread it.
> But what I have been saying is that simple microwave relays are a lot
> cheaper than SPS, and could be a peofit center much sooner.
>
> What you are proposing is the reverse of my proposal.
>
Not quite. I'm trying to figure out how we can maximize the power output
from these satellites we're putting up into orbit. I'm not saying we
shouldn't use simple microwave relays. All I'm suggesting is that if we
put the equipment we're talking about into orbit, how much more work would
it be to put a solar collector on top of them to add to the power they're
already redirecting back to the Earth rectenna. If it's a huge cost
increase with a small power boost, then never mind. But if it's a small
cost increase with a decent boost in transmitted power, then it's
something to consider.
--Justin
FYI, there's a SPS discussion taking place right now on the Usenet newsgroup sci.space.policy. Geoffrey Landis, who has done actual SPS work, just got through saying that there have been proposals for orbital power relays, but that they turn out to be not that much less complicated than full-up SPS.
Mike Combs
here is a list of Criswells papers
==============
Tom,
Thanks, but that link does not respond....the server does not exist.
CR
Here is the google cached version of it
gister/criswell_papers.html+Criswell+microwave+reflectors&hl=en
I've tried the link I first gave four different times just now, and it only
worked 1/4
Tom M.
TomM@...
>
> Why not? Couldn't the satellite be configured to have the solar collector
> point at the Sun while the transmitter/reflector is pointed at the Earth
> rectenna target? I could make a sketch, but I'm no good at ASCII drawing.
>
That would double the weight of the spacecraft.
>
> Heavy is certainly a big problem. But I thought the whole point of
> collecting solar energy from orbit was that you could achieve efficiency
> ratings above 90%. Otherwise, why bother? I don't have the reference on
> hand to quote from, unfortunately. Maybe I misread it.
>
I have been proposing microwave relay, not collecting solar energy.
The SPS is a separate discussion, which exceeds my present bandwidth.
>
> Not quite. I'm trying to figure out how we can maximize the power output
> from these satellites we're putting up into orbit. I'm not saying we
We need to maximize power output per unit weight.
> shouldn't use simple microwave relays. All I'm suggesting is that if we
> put the equipment we're talking about into orbit, how much more work would
> it be to put a solar collector on top of them to add to the power they're
> already redirecting back to the Earth rectenna. If it's a huge cost
> increase with a small power boost, then never mind. But if it's a small
It at least doubles the system weight and cost.
>
> FYI, there's a SPS discussion taking place right now on the Usenet
> newsgroup sci.space.policy. Geoffrey Landis, who has done actual SPS
> work, just got through saying that there have been proposals for
> orbital power relays, but that they turn out to be not that much less
> complicated than full-up SPS.
>
I know Geoff Landis quite well, we both worked at NASA Lewis for a
while, and we socialized quite a lot. He certainly knows space power
well, and from time to time we share iformation.
Geoff was commenting on microwave relay via the RF-DC-RF conversion.
Note that he did not quote any numbers, and simply saying "not much
less" is rather vague.
An SPS relay satellite will cost less than a full SPS. By how much is
not an easy question to answer. At the very least it removes the PV
solar array, which is the heaviest part of the system, over 50% of the
spacecraft weight, so will undoubtedly make a singificant dent in the
launch cost. Sorry, I do not have any numbers either.
But reflecting RF is a very different matter, and can be a lot cheaper
then either SPS or relay.
Note that I think the path length problem can be resolved by using a
curved reflector which re-concentrates the beam somewhat.
CR.
here is a list of Criswells papers I ended up reading one of Criswells papers somewhere on the net while following links that were provided earlier. I hit one statement that caused me to scratch my head. It said that his scheme was predicated on using the antenna near field. I have been doing airborne radar simulations for decades for flight training simulators. I have never had to consider the effects of the near field in any of those simulations some of which have been very intense. So I went back to a primer on radar and found a page discussing the near field in relationship to reflections from rain.
If I got the math correct it will require a square antenna 66.4 Kmeters per side (square is better than round for these equations for some reason). This has to be a phased array antenna which is totally mind boggling considering the cost and difficulties in creating an earth based 20 ft phased array antenna.
For those who like technical details I provide the following:
"The electromagnetic field in the Fresnel (near field) region is quite different from that in the Fraunhofer region although there is a smooth transition from one to the other. The most significant feature is that the radiation is largely concentrated within a cylinder whose cross section is the aperture area until the distance from the aperture is approximately that designated as the transition range. This behavior is also indicated by the on-axis power density which at large distance varies inversely as the square of the distance but in the Fresnel region oscillates about a constant value. It may also be noted that at the transition range k*r0 a cone with solid angle "theta" would intercept an area of (k**2)Ae ......"
So the transistion range is defined as k*r0. The rest of the task is trying to find what these letters stand for.
r0 = Ae / wavelength
Ae = (0.4 to 0.7 for different types of antennas) * Area of square antenna
Area of square antenna = Length of side **2
wavelength = speed of light / 2.4 Ghz
speed of light = 2.99*10**8
K = (1.0 to 2/3 depending on antenna shape, square = 1)
Dl = Distance from the earth to the moon = 384,401,000 Km
Solve for Length of side.
Also the radius of the moon is 1737Km which will also affect such a large antenna.
Mitchell James
www.InnerTransit.org (Homebase for collaborative engineering)
>
> Kmeters per side (square is better than round for these equations for
> some reason). This has to be a phased array antenna which is totally
> mind boggling considering the cost and difficulties in creating an
> earth based 20 ft phased array antenna.
>
Hmmm.
Reflectors are becoming more and more attractive.