Copyright 2014 Robert Clark
In the blog post, "Short travel times to Mars now possible through plasma propulsion", I suggested current solar concentrator methods and lightweight space solar sails make possible fast flights to Mars with solar powered plasma propulsion.
Interestingly this technology also now makes possible economical space solar power (SSP). For SSP a key detriment has been the huge weight thought needed to be sent to space. For instance solar cells typically have a 100 watt per kg weight, though more recently they are in the 200 watts per kg range. So if you wanted to get a 1 gigawatt system, about that required for a large city, you would need to send 10,000,000 kg to orbit just in solar cells alone, hugely expensive
However solar concentrators using mirrors or lenses can now concentrate light thousands of times, requiring orders of magnitude lower weight in solar cells. Say, you had a 1,000-times solar concentrator. Then you would only need 10,000 kg in solar cells, which could be launched by a single mid-size launcher.
BUT you would also need to send the mirrors to orbit. And that is a second key advance we have also now reached, lightweight space mirrors. The Sunjammer space mirror to test solar sail technology is scheduled to be launched January, 2015. It has a 1,200 sq. m area at only a 50 kg weight.This can collect about 1 megawatts of power. So at 1,000 times larger, it could collect 1 gigawatts of power at only 50,000 kg mass, which could be launched by a single Falcon Heavy. Another consideration though is solar cells are not 100% efficient. They are actually about 30% efficient. So you might need 3 times larger collecting area. Still only 3 launches of the Falcon Heavy.
Actually though some recently work on solar concentrators have also been able to use the heat created, thereby increasing the energy efficiency to 80%. So you may get close to the area size for a 100% efficient system.
Interestingly some recent work on carbon nanotubes may be able to make the mirrors even lighter:
Researchers produce strong, transparent carbon nanotube sheets.
Aug 18, 2005
"Strength normalized to weight is important for many applications,
especially in space and aerospace, and this property of the nanotube
sheets already exceeds that of the strongest steel sheets and the Mylar
and Kapton sheets used for ultralight air vehicles and proposed for
solar sails for space applications, according to the researchers. The
nanotube sheets can be made so thin that a square kilometer of solar
sail would weigh only 30 kilograms. While sheets normally have much
lower strength than fibers or yarns, the strength of the nanotube
sheets in the nanotube alignment direction already approaches the
highest reported values for polymer-free nanotube yarns."
http://www.physorg.com/news5890.html
This is more than 1,000 times better than the Sunjammer sail. The transparent nanotubes sheets would have to be given a thin reflective layer. But this is commonly done with telescope mirrors and add little weight to the mirror. Actually it's been found that nanotube properties are highly tunable so it may be possible to create these thin, strong nanotube sheets that are themselves reflective rather than transparent.
Notably, this would provide a market for getting large amounts of mass to orbit for the space solar power to be applied globally for electricity generation. Then this may finally be the "killer app" for generating a large enough market for space access to bring the costs down and thereby make space access routine.
Bob Clark
