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

On Apr 8, 7:58 pm, Ric Locke <warlo...@[EMAIL PROTECTED]
> wrote:
> On Tue, 8 Apr 2008 19:05:00 -0700 (PDT), Luke Campbell wrote:
> > On Apr 8, 5:35 pm, Ric Locke <warlo...@[EMAIL PROTECTED]
> wrote:
> >> On 8 Apr 2008 9:34:39 -0500, Ash Wyllie wrote:
>
> >>> The Stirling engine is by far and away the most efficient device
that converts
> >>> heat into mechanical work. Up to 80% of Carnot potential can be
obtained.
>
> >> Quite true.
>
> > I suppose this is a bad place to mention that some gas turbines can
> > get within 90% of the Carnot efficiency?
>
> > Luke
>
> Not at all.
>
> The downside of that is that the Carnot efficiency is low because the
> temperature differential isn't large enough, so the throughput
> efficiency (output energy divided by fuel heat energy) is low. Given a
> material that is immune to heat and suitable for making turbine blades,
> a gas turbine can be almost arbitrarily efficient. Alas, unobtainium is
> rare in Nature.

Although, sadly, the same limits apply to Stirling engines.  Still,
some gas turbines extract up to 60% (!!!) of the thermal energy of
combusting gases as useful mechanical work, about a factor of three
better than typical automotive piston engines.  Of course, they have
all sorts of complicated machinery for recapturing heat and
regenerating energy.

For high temperature differences, you can use MHD generators, although
they currently do not exhibit high efficiencies.  You can, however,
use the still-hot exhaust to drive another motor or generator.
Combined MHD/steam turbine setups can also reach nearly 60%
efficiency.

Photovoltaic cells also form a kind of heat engine, one which turns
radiant heat into electrical work, and these operate with very large
temperature differences (6000 K as the high temperature reservoir for
typical applications, about 300 K as the low temperature reservoir).
Some photocells made of multiple layers of decreasing bandgaps have
achieved up to 80% efficiency at turning solar heat into electrical
work, at least in the laboratory.

Luke