Re: [Phys-L] efficiency versus Carnot efficiency

[Bruce Sherwood <Bruce_Sherwood@ncsu.edu>]

On Mon, Mar 18, 2013 at 6:10 PM, John Denker <jsd@av8n.com> wrote:

> On 03/18/2013 02:46 PM, Bruce Sherwood wrote:
>
> > They compared their result with the
> > efficiencies of actual power plants and found agreement with their simple
> > formula, which is consistent with fuel having been historically cheap and
> > engines expensive.
>
> I don't believe that for a minute.  Nobody I know operates car engines
> or airplane engines anywhere near max rated power for more than a tiny
> percentage of the time.
>
> Besides, how do you even /define/ T_H for an internal-combustion engine?
>
> Uh, I have the distinct impression that power plants, whether coal-fired
or nuclear, don't use internal-combustion engines, and Curzon and Ahlborn
said nothing about internal-combustion engines. And of course "maximum
power" in this context has nothing to do with the full-power capability of
a power plant. If you feed in less fuel, you get less work out, but Curzon
and Ahlborn are talking about the operating parameters which, for a
particular power input, gives the maximum power output for a particular
input power (instead of, for example, getting zero power output).

For that matter, I have the impression that power plants when they run at
all run at one fixed power level, and load leveling on the grid tends to
mean turning on or turning off some generators.

Details: Their analysis consisted of running a reversible engine with the
"high-T" reservoir at a lower temperature than the real high-T reservoir,
and with a "low-T" reservoir at a higher temperature than the real low-T
reservoir, thereby having nonzero Q into and out of the reversible engine.
They then optimize on the intermediate "high-T" and "low-T" temperatures to
achieve the most power output they can get, given a fixed input Q.

Bruce