Re: How does air conditioner coolant fit the theory of thermodynamics?

You seem to be missing important distinctions in your statements about 
heat, energy, etc.  In particular, "hot" and "cold" are not descriptions 
of "energy", they are descriptions of *temperature*.  And temperature is 
not simply a matter of energy -- temperature specifically represents the 
kinetic energy of the molecules but a fluid can have other types of energy 
(such as the potential energy of interaction between its molecules).  As a 
simple illustration, when water boils, you put a lot of energy into it, 
but the temperature remains constant during the boiling process (the heat 
input is in some sense being used to separate the molecules as the liquid 
becomes vapor).  Your statement about heat going from high to low energy 
concentrations is similarly incorrect; heat flows from high to low 
*temperatures* which is not necessarily the same thing.

The standard refrigeration cycle (which is what most air conditioners use) 
takes advantage of the difference in energy between vapor and liquid.  
Imagine starting with the coolant as a hot high-pressure vapor (just after 
the compressor).  Then you run it through a heat exchanger (like the coils 
on the back of your refrigerator) where heat flows out of it into the 
cooler air.  This loss of energy causes the vapor to condense into a 
liquid.  Then (this is the key step) you run the warm high-pressure liquid 
through a throttling valve.  No energy is being put into the system or 
removed, but because the pressure is reduced the fluid vaporizes.  Since 
vapor is a higher-energy state in general (actually we should be 
using "enthalpy" rather than energy, but that's a nuance I'll ignore 
here), the temperature goes down as the total energy is conserved (this is 
called the Joule-Thomson effect).  So now you have a cold vapor.  You run 
this through another heat exchanger where it can pick up heat from 
something at higher temperature (for example, the inside of your 
refrigerator).  At that point, you have a less cold vapor.  Then you put 
it through the compressor, where the compression energy put in causes the 
temperature to go up, and we are back where we started.  The net effect 
was transfer of heat energy from the inside of the refrigerator, into the 
coolant, and out to the environment.  The thermodynamic price for this is 
that you have to put energy in to the compressor.

If you want to learn more about the thermodynamics of refrigeration 
cycles, I would suggest any mechanical engineering or chemical engineering 
thermodynamics textbook.  I also found a couple of explanations on the Web:
 fbox.vt
 nwu

Dr. Allan H. Harvey, aharvey@boulder.nist.gov
"Don't blame the government for what I say, or vice versa."