--- In spacesettlers@yahoogroups.com, Al Globus wrote:
> --- In spacesettlers@yahoogroups.com, hitssquad wrote:
>> The paper also said: "just 600 solar power satellites, 1 km radius each, could supply a terawatt (one trillion watts) [Assuming 10% end to end efficiency] of energy continuously and extremely predictably."
Assuming a minimum (because the produced electric-power is supposed to be continuous) solar flux density of 1321 watts/sqm, the amount of solar power intercepted by such an SPS fleet would be 600 * .7854 * 1e6 * 1321 watts/sqm = 622.5 gigawatts. Multiplying that by your 10% efficiency figure gives us only 62.25 gigawatts (to the busbar?). Even increasing the assumed solar flux density to 1400 or 1500 watts/sqm wouldn't bring the total to anywhere near a terawatt.
> Given one million square meters and 1,000 watts/m2 thats a gigawatt.

Actually, I mistakenly used your radius figures as diameters. Therefore my numbers should all be converted by a factor of four. The number of square meters of a round collector area would be pi (3.14) million, and of a square collector area would be 4 million. Assuming 600 round SPS we get 2.49 terawatts (at 1321 w/sqm) at 100% efficiency, and a quarter terawatt at 10% efficiency. That's closer.

Assuming 600 square SPS of 1 km "radius" each (4 million square meters of collector area on each), and 1667 w/sqm, we get 4 terawatts at 100% efficiency, and 400 gigawatts at 10% efficiency. At 25% efficiency, we get a terawatt of electricity.

>> 10375 GW. To produce 15 TW would require 15000 GW / .10375 GW = 144,579 SPS units.

Since that's 1 km radius, and not diameter as I mistook it for, that should say "36,145 SPS units."

>> Next: "At a 10 km radius, fewer than 400 satellites are necessary."
the real answer is that 1,446 satellites would be necessary.

That should be 362 10 km radius satellites.

>> They wouldn't fit in GEO

That's still true.

>> Even without any comsats in the way, they would need GEO to be 20 km * 109 * 1,446 = 3,152,280 km in circumference.

That equation should read: 20 km * 109 * 362 = 789,160 km (still almost 3x the 264,869 km circumference of GEO).

I've corrected http://alglobus.net/NASAwork/papers/PathsToSpaceSettlement2011.pdf
(Paths to Space Settlement) wrt the number and size of PowerSats
needed to supply Earth's energy. Thanks to hitssquad for finding the
error. Although the error was much smaller than we thought at first,
it was still about a factor of 4 off. Please check the new numbers
but I think I got it right this time (NOTE: radius and 20% end-to-end
efficiency).

I do not think the spacing between sats needs to be nearly as large as
proposed. I don't know where the numbers came from, perhaps comsats.
SSP doesn't need to space like comsats because each is transmitting to
a different antenna so there is no interference. Thus, so long as the
sats don't run into each other (or anything else), its fine.

Also, particularly in the high numbers case, these sats will need some
propulsion for station keeping. Probably solar electric, there's
plenty of power! With a large customer base one can easily imagine
refueling sats that go from PowerSat to PowerSat refueling with
whatever the reaction mass is. Obviously, just another engineering
problem to solve. That's the fun part :-)

I also fixed the fraction of the Sun's energy that falls on Earth to
2.2 billion, thanks again to hitssquad for finding the error.

On Feb 1, 2012, at 2:26 AM, hitssquad wrote:

On March/02.2012 23:19, Al Globus wrote:

> I do not think the spacing between sats needs to be nearly as large as
> proposed. I don't know where the numbers came from, perhaps comsats.
> SSP doesn't need to space like comsats because each is transmitting to
> a different antenna so there is no interference. Thus, so long as the
> sats don't run into each other (or anything else), its fine.

>
> Also, particularly in the high numbers case, these sats will need some
> propulsion for station keeping. Probably solar electric, there's
> plenty of power!

Hi Al,
I would propose a combination of solar sailing and solar electric.
Solar sailing could provide the fuelless propulsion part for station
keeping, steering and pointing, while solar electric would be used to
place the power sat from LEO into GEO or allow other orbits than GEO or
even change orbits as needed.

Using sail panels covered with or made of solar cells, connected with
conductive threads, would allow to launch or construct in space large
thin film solar cell areas along with the sail-power sat.

As such a sailcraft has ample cell area for power generation, the
generated power could be used to operate very fuel efficient electric
thrusters, like the new ESA/ESTEC DS4G ion thruster, which is about
eight times as efficient as regular ion thrusters (about 250 km/sec.
ejection speed).
To use such thrusters in regular satellites would be not possible, since
the energy needed to power them is not available.

"Roller Reefing" would be a way to combine power generation, fuelless
station keeping, steering and pointing and would allow also to launch or
construct large solar cell areas in a simple and relative inexpensive way.
See the paper Solar Sailing and solar power generation by
'Roller-Reefing' http://solar-thruster-sailor.info/PaperISSS2010V7.doc

With a large customer base one can easily imagine
> refueling sats that go from PowerSat to PowerSat refueling with
> whatever the reaction mass is.

For operating the craft without refueling for most of the mission time,
fuelless seems to be the best option to go. For simple refueling through
docking a mobile thruster unit I would propose this:
http://solar-thruster-sailor.info/figs/fig16.html

For more information on Solar Sail Power Stations see also
http://www.solar-thruster-sailor.info/PresentationISSS2010V13.pdf

Best wishes

Frank