Re: How to make money fast (in science fiction)

On Apr 8, 9:27 pm, Ric Locke <warlo...@[EMAIL PROTECTED]
> wrote:

> Thank you very much, by the way, for the (to me) new insight of treating
> a solar cell as using the Carnot cycle! It's absolutely true, of course,
> but I'd never thought of it that way, and it's a useful way of looking
> at it.

I didn't realize this until I was teaching an undergraduate course on
thermodynamics and statistical mechanics.  When I got to the part of
what was and was not a heat engine, I tried to come up with various
well known examples of generators/motors that fell into both classes
(wind turbines, hydroelectric turbines, automotive piston engines,
methane fired gas turbines, nuclear power plants, etc) for the class
to consider.  When I considered a solar cell, I realized that sunlight
(and thermal radiation in general) has entropy, and thus has the same
limits on turning radiant heat to work as any Carnot engine.  It is
somewhat curious, though, that the same device can be a heat engine
when run off of radiant heat, but not a heat engine when run off of,
say, a laser beam (which has vani****ng entropy).

I don't think the class really appreciated this point, though, as much
as I tried to put into their heads that what really defines a heat
engine is the flow of entropy through the device, rather than any
particular set of isotherms/isobars/adiabats/whatever.

Luke