Reply #64: Carnot efficiency is actually pretty easy to understand.

[Tesha]

Basically, all of these "heat engines" work by
mining the potential energy that exists between
the source of heat and the temperature into which
the engine dumps its waste.

It's not unlike a water wheel or hydroelectric dam:
The higher the reservoir of water compared to the
height at which you dump the "used" water, the more
power you can obtain. Raise the dam or lower the
river and things get better.

And for temperature and Carnot efficiency, it all
comes down to absolute temperature: the temperature
of things above Absolute Zero (about -273° Celsius
or -460° Fahrenheit). To make things simple, scientists
and engineers use the Kelvin scale which uses Celsius-
sized degrees but places its zero point at right at
Absolute Zero.

So now we come down to it: the absolute maximum
efficiency you can get out of any "heat engine" is
the distance in Kelvins between the hot and the cold
divided by the total distance from the hot to
absolute zero.

For example, let's take a steam engine (or a steam
turbine; doesn't matter!). Maybe its boiler makes
steam at 273° Celsius / 523° Fahrenheit. That's
546 Kelvins. Let's also assume that our steam engine
rejects heat into the atmosphere or a lake or some
such and that the temperature of the air or lake is
just freezing: 0° C / 32° F / 273 Kelvins.

0----:----1----:----2----:----3----:----4----:----5----: Heat source: 546 K

0----:----1----:----2----:-- Heat sink: 273 K


The *ABSOLUTE BEST* efficiency that our steam engine can
ever reach will be about 50% because that's the proportion
of the distance from the heat source to the heat sink
compared to the total distance from the heat source to
absolute zero. That is to say that our steam engine will
*ALWAYS* waste at least 50% of the heat you provide to
it; gotta shovel more wood into the boiler's firebox!

And our steam engine will usually do worse than this
because of things like friction and the fact that we're
often rejecting the heat into air that's warmer than
freezing. (That's why nuclear power plants are so
often located next to water; that water provides a
nice, convenient, relatively-cool cheap heat sink.)

Gas turbines can be more efficient because the "heat
source" can be a lot hotter than our measly 546 K;
maybe a thousand Kelvins or more.

0----:----1----:----2----:----3----:----4----:----5----:----6----:----7----:----8----:----9----:----10 Heat source: 1000 K

0----:----1----:----2----:-- Heat sink: 273 K

Now we're talking almost 75% potential efficiency!

Perhaps this new engine does this (compared to an
internal combustion engine)?

Tesha