Xref: utzoo talk.politics.misc:8649 sci.misc:1183
Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!bloom-beacon!athena.mit.edu!jfc
From: jfc@athena.mit.edu (John F Carr)
Newsgroups: talk.politics.misc,sci.misc
Subject: Re: greenhouse effect
Message-ID: <4091@bloom-beacon.MIT.EDU>
Date: 28 Mar 88 05:30:39 GMT
References: <34557@kestrel.ARPA> <2430@umd5.umd.edu> <2116@mit-amt.MEDIA.MIT.EDU> <22089@bbn.COM> <35092@kestrel.ARPA> <22285@bbn.COM> <2963@sfsup.UUCP> <22670@bbn.COM>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: jfc@athena.mit.edu (John F Carr)
Distribution: na
Organization: Massachusetts Institute of Technology
Lines: 59

In article <22670@bbn.COM> eli@BBN.COM (Steve Elias) writes:
>In article <2963@sfsup.UUCP> glg@/guest4/glgUUCP (xt1112-G.Gleason) writes:
:
:	(regarding solar power satellites)

:!In any case something seems bogus here, because in order to be significant
:!we would have to be considering SPS's with a total area that is *very*
:!large (significant relative to the earths total collection are).
:
:	indeed something is bogus:  the very idea of Solar Power Satellites
:	being a useful energy source.  they would be enormously expensive
:	and enormously dangerous (thermally), if enough were built to 
:	actually provide a decent amount of energy.

If we ever get a lot of energy out of them, we will need a cheaper launch
system.  We will also use very large thin films to minimize weight and
cost.  The SPS's will not be built if they are too expensive.  

(Assuming "thermally dangerous" refers to global, not local effects.)
Whatever power generating capacity we need to build to keep up with
demand, we will build.  The relevant question is, what are the relative
dangers of each type.  Assuming international cooperation in space (else 
the SPS is too vulnerable to attack) the SPS is more reliable than other
systems, and has a greater thermal efficiency on earth.  (the efficiency
of the space based part may not be high because of the difficulty of
radiating heat, and that will affect the cost and possibly the decision
whether to build).  If the only criterion for choosing a generator were,
"what is the danger to man, both short- and long-term", then solar power
satellites would be a good choice. 

:	energy put into building earthside collectors is not leaving the
:	earth/atmosphere system, so it doesn't negate the energy sent down
:	by the power satellites.  reflectivity changes due to the collectors
:	would have to be considered, as well.  but, regardless of reflectivity
:	changes, thermal equilibrium would be quite goofed up -- by waste heat
:	from the microwave receivers...

You have assumed <= 50% efficiency, others have claimed higher.  Either
way, this is better efficiency than for a heat engine.  So, it is better
(if trying to minimize waste heat) to use solar power.

:	the greenhouse effect can theoretically be kicked off by both CO2
:	buildup and by large thermal changes -- both factors are 'cause' and
:	both factors are 'effect'.  that's Frank Drake's point...  i'm with
:	him all the way on this one...  just call me "thermally correct".


The effect is far more sensitive to CO2 concentration; present power 
sources produce more CO2 as a fraction of the total present than they
do heat.  

By the time we are generating a few % of the solar flux on the earth as
waste heat, we will have to worry.  By then, I hope we will have moved
power intensive industry into space (if not, we can produce more power,
and use the excess to drive a heat pump [perhaps, a large laser] and 
refridgerate the earth.)

   John Carr           "No one wants to make a terrible choice
   jfc@athena.mit.edu   On the price of being free"           -- Neil Peart