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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. |
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Zubrin's panning of space solar power in Entering Space
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 |
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Zubrin's panning of space solar power in Entering Space
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Zubrin's panning of space solar power in Entering Space
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Zubrin's panning of space solar power in Entering Space
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 |
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Zubrin's panning of space solar power in Entering Space
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Zubrin's panning of space solar power in Entering Space
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Zubrin's panning of space solar power in Entering Space
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. |
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Zubrin's panning of space solar power in Entering Space
"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" |
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