Re: How will air travel work in a green solar economy?

Wed, 09 Jul 2014 11:19:53 -0700 



________________________________
 From: John Clark <[email protected]>
To: [email protected] 
Sent: Wednesday, July 9, 2014 10:51 AM
Subject: How will air travel work in a green solar economy?
 


>>Liquid Hydrogen would be a pretty good fuel for airplanes,


Liquid hydrogen is hard to make and store (it needs to be liquefied to near 
absolute zero -- an energy intensive process all by itself); the volumetric 
density of liquid hydrogen is not all that impressive either. Liquid hydrogen 
is also aggressive chemically and causes metal tank walls to become brittle 
over time. Biofuels seem superior -- much easier to store and handle -- as a 
future fuel source for aviation needs. The US Navy and Air force (and Virgin 
Airways among others)  are experimentally blending it into jet fuel. Jet 
engines, apparently burn the mixes, quite well with no modifications required. 
Thus the entire existing infrastructure and stock of jets (and jet engine/ fuel 
systems) can be modified in an evolutionary sense to add support for biofuels. 
Going with liquid hydrogen is a radical departure from the current installed 
base.
Chris



 so let’s see 
how many solar cells would be needed to make the fuel to keep one in the air. A 
747 jet uses on average 140 megawatts of power, incidentally 
even the old fashioned nuclear reactor on a Nimitz class aircraft 
carrier  can generate 190 megawatts, a LFTR could be much smaller 
because it's much more energy dense. The electrolysis process to make 
hydrogen from water is only about 60% efficient so that brings the power 
requirement up to 233 megawatts, but then you need another 30% to 
liquefy the hydrogen (it’s not easy to do) so the grand total is you 
need a  solar cell installation that on average produces 333 megawatts 
each and every hour to keep a hydrogen powered 747 in the air. 

Averaged
 over 24 hours a square meter of solar cells might produce 30 watts each
 hour, so you’d need 11,100,000 square meters of solar cells, that’s a 
square 2787 meters on a side. We conclude that to keep just one jet in 
the air we need a fuel factory that covers 3 square miles of the Earth’s
 surface. And that is why I don’t think solar is the answer to all our 
energy needs. 

There are only 2 other sources that have the potential to power our 
civilization for the next billion years:

1) Fusion reactors, but nobody is close to figuring out how to build even a 
working model much less a practical machine. 

2) Thorium fission reactors, and we’ve known how to build them for half a 
century.

  John K Clark

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