"Ignoramus26567" wrote in message
...
On 2009-03-11, Leo Lichtman wrote:

"Ignoramus26567" wrote: (clip) This is because compressing air to higher
pressure heats air more than
compressing air to lower pressure, and that heat is completely wasted
(clip)
^^^^^^^^^^^^^^^^^^^^^^
Dropping the pressure to a lower value through a throttling valve or
regulator wastes the energy that could have been obtained by running it
through an air motor to lower the pressure. So you lose on the way up
and
you lose on the way down.

Great point.

Uh, actually, not. Throttling doesn't affect the pressure-volume product. No
energy is lost by throttling. However, expansion of the air to a
lower-pressure downstream value does give up heat upon expansion. Whether
that "loses" energy depends on what the temperature of the air was before it
was throttled, and what it is just before you actually use it to perform
some work. It can regain that energy from the ambient heat in the room, if
the expansion dropped the temperature of the air below room temperature when
it expanded, and then the air re-heated by running through the copper pipe
in a relatively warmer room.

Running that same air through a motor results in a greater PV loss -- lower
pressure or less volume -- than it would through a throttle that produced
either the same downstream pressure, OR the same downstream volume. The
difference is the work performed by the motor.

Most of the energy loss is from what you suggested in the first place:
losing the heat of compression. That's why those compressed-air cars they've
been experimenting with in France are so inherently inefficient. You can, in
theory, recover all of that heat. But it's like recovering lost heat in a
steam engine or turbine: exceedingly complicated, with multiple heat
exchangers.

--
Ed Huntress