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From: Henry Spencer <>
Subject: Re: energy concept
Date: Mon, 20 Oct 1997 15:30:38 GMT

In article <>, Steve  <> wrote:
>The generator would have a solar collector to focus sun light and
>produce heat. The heat would be used to boil off liquid which would
>then...generate electricity...

It's called solar dynamic power, and it's been a perennial also-ran for
large space power systems for a long time.  It was almost the primary
power system for the space station, since it's more efficient than solar
arrays (which means less collecting area and hence less air drag) and it
can use thermal energy storage to ride out short periods in darkness
(which avoids the need for a massive and short-lived battery system for
operation in low Earth orbit).

Disadvantages?  Some.  It needs big waste-heat radiators for the condenser
(although they can be kept edge-on to air drag, at least).  Precision
pointing is required -- power output falls rapidly to zero if it's not
pointed fairly exactly at the Sun.  It's relatively complex, with a
spinning generator and other moving parts, which raises reliability
worries.  Vibration is a concern.  Micrometeorite punctures are largely
harmless to solar cells but are potentially disastrous to plumbing.  And
there is not much experience with it in space, so the technology is not as
well understood.

>Instead off using water this generator
>would use a liquid with a much lower boiling point, say liquid methane.
>Since methane evaporates at such a low temperature it would not need as
>much sunlight and or could use a smaller solar collector.

Actually, with a system like this, that isn't a problem:  if there's not
enough sunlight, just make the mirror bigger.  Mirrors are very light.
It's typically better to run at high temperatures, in fact, because that
makes the radiators smaller and lighter.  Liquid metals were the preferred
fluids for a long time, but of late there is a tend to prefer gas-based
cycles which have no liquid in the system, avoiding the complexities of
liquid behavior in free fall and worries about the liquid freezing if the
system is shut down for a while.

Also, the size of the collector is set almost entirely by the amount of
power needed, the efficiency of the system, and the brightness of the
sunlight.  To get a certain amount of power out, you have to collect a
certain amount of sunlight.  The working-fluid temperature isn't much
of a factor in mirror size.

>Could it:
>operate in the Jovian system?
>in Saturn's system?

Certainly, given large mirrors.  One advantage of solar-dynamic power is
that it adapts easily to low-light operation, since bigger mirrors don't
add much weight.

>power an electric rocket?

Certainly.  It would even have some small advantages for this, since most
electric rockets need high voltages, and there are some annoying technical
problems in getting high voltage out of solar arrays.

>weigh less than current methods of generating electricity?

Questionable.  Mirrors are lighter than solar arrays, but the generating
equipment itself weighs something, and radiator weight is a major issue.
In large sizes, maybe; in small sizes, it would probably be heavier
because of the greater complexity.

>cost less than current methods of generating electricity?

Right now, no, because of development costs.  Given development, maybe.
Current solar arrays are not cheap.
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

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