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

On Tue, 8 Apr 2008 21:11:19 -0700 (PDT), Luke Campbell wrote:

> 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

All true. The big problem with external combustion engines, both
Stirling and "steam", is that the exhaust heat is too cool to make
regenerative techniques practical. I didn't know until fairly recently
that the railroads never used compound steam engines extensively, for
instance -- the cost to build, run, and maintain them overrode the fuel
efficiency considerations in an era of relatively cheap fuel. The
secondary pistons had to be /big/, and therefore heavy.

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.

Regards,
Ric
** Posted from http://www.teranews.com
**