[ExI] Space Based Solar Power vs. Nuclear Fission

hkhenson hkhenson at rogers.com
Mon May 19 17:36:25 UTC 2008

At 03:11 AM 5/19/2008, you wrote:
>On Sun, May 18, 2008 at 10:53 PM, hkhenson <hkhenson at rogers.com> wrote:
> > It may not be.  If we could build a space elevator, the energy
> > payback time is incredibly short--at least for the energy needed to
> > lift a power sat to GEO.
>Speaken of unproven, or rather non-existing, technologies... :-)

The major problem is the cable.  It's obvious that nanotech would be 
up to building the cable, and there are other ways that might do it 
such as enzymatic dehydrating of tri-hydroxy-benzine or using iron as 
a solvent that offer a chance we could make 100,000 tons of strong 
enough nanotube cable even before the singularity.

(The singularity is nanotechnology and AI.  It is almost impossible 
to imagine getting either one without the other element coming on 
line in a time measured in weeks.)

But the main reason to look at a space elevator is to see how other 
methods compare to the minimum lift cost.

>Having said that, needless to say I am absolutely in favour of
>investigating and implementing both satellite- or moon-based solar
>energy and researching the fundamentals that might lead to a space
>Speaking of lowest hanging fruits, I do not see such things as very
>rapid-deployment solutions, however.

If you want rapid deployment, then rockets are the way to 
go.  800,000 tons per year is a gigantic project perhaps on a par 
with the Iraq war, but we can put numbers on it and they are within 
reason.  For example, the production for rocket hardware is a bit 
less than the high point Boeing reached for one aircraft, the 747.

>Please note however that I am
>much better informed of the state-of-the art of nuclear fusion.

Unless you are talking about exotic reactions, fusion has the same 
problem as fission; it generates neutrons.  I have come to look at 
neutron sources and DU as a nasty combination, especially after 
figuring out ways to make nukes that are so simple a well funded 
street gang could make 
them.  http://www.kuro5hin.org/story/2007/10/30/18253/301

The other problem is the incredible number of power plants of any 
kind it takes to replace the energy we get from oil and other fossil 
fuels.  It's about a GW a day of new capacity.  I can make a case, 
even with rockets, for putting in a GW a day of solar power 
satellites.  As I recall, fusion plants are estimated in the 10 GW 
range.  Even if we knew how to make them, can you see starting up one 
every ten days?


>Stefano Vaj
> > Specific orbital energy is u/2r, (398,600/42,000)/2 or -4.75Mj/kg
> >
> > Potential is -9.5Mk/kg and kinetic is 4.75Mj/kg
> >
> > Potential at the earth's surface is -62.6 MJ/kg; the difference 
> is 53.1Mj/kg.
> >
> > Using a space elevator, the rotation of the earth provides the
> > kinetic energy.  Since a joule is a watt-second; 53,100
> > kW-s/kg/3600kW-s/kWh is 14.75 kWh/kg
> >
> > A kW/kg power sat repays its lift energy 14 hour and 45 minutes after
> > being turned on.  A 2kg/kW power sat would take 29.5 hours.
> >
> > There is a heck of a lot of sunlight out there, and you don't need
> > much structure to capture it in zero g.  Whatever rate you get for
> > paying off solar cells on the ground, they will repay it at least
> > three times faster in orbit.
> >
> > With rockets, it's still not too bad.
> >
> > http://www.ilr.tu-berlin.de/koelle/Neptun/NEP2015.pdf
> >
> > Neptune is about 3 times the capacity of a Saturn 5, and this design
> > was done by some of the same people so it's solid engineering.
> >
> > This vehicle delivers 350 mt to LEO, and 100 mt to lunar orbit or
> > GEO. To lift 100 mt to GEO Neptune uses 3762-mt of propellant for the
> > first stage, 1072 mt second stage and 249 mt for the third 
> totaling 5077 mt.
> >
> > SSME O2 to H2 ratio is 6 to
> > 1.
> > 
> http://www.pw.utc.com/vgn-ext-templating/v/index.jsp?vgnextrefresh=1&vgnextoid=75a0184c712de010VgnVCM100000c45a529fRCRD
> >
> > I.e., 1 part in 7 of the propellant is LH. or about 725 mt of
> > LH.  The launch site would make electrolytic hydrogen out of water
> > (the only long term source). That costs about 50 kWh/kg plus another
> > 15 kWh to liquefy the H2.  Add 5 kWh for liquefying oxygen at 6 x 
> .8 kWh/kg.
> >
> > That would be 70 MWh per mt, or 70 GW hours for 1000 tons, or 50.8
> > GWh for 725 mt. At a kg/kW, 100 tons of satellite produces 100,000
> > kW, or 0.1 GW.  Thus it would take 508 hours to pay back the lift
> > energy or 21.2 days, 42.4 days for 2kg/kW.
> >
> > Rocket efficiency here would be 14.75/508 or 2.9%.
> >
> > A nuclear tug stage shuttling between LEO and GEO might double this
> > efficiency raising the payload from 100 mt to 200 mt.  That is the
> > consequences of a 15 km/sec exhaust velocity.
> >
> > It's the high cost of rocket hardware, not lift energy payback, that
> > makes power sats expensive.  If we could get that down . . .
> >
> > As for conventional nuclear power, I can't see it being built fast
> > enough.  The need for replacement energy as oil fades out of the
> > picture is around a GW/day.  That's in addition to the problem of
> > neutrons being diverted to make plutonium 239 out of DU.
> >
> > Keith
> >
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