[TriLUG] OT: thermodynamics of A/C question

matt at noway2.thruhere.net matt at noway2.thruhere.net
Fri Jun 22 17:07:54 EDT 2012


Excellent technical question.

As you said, the cost driver in air conditioning is moving the refrigerant
from the low pressure side (evaporator) located in the air handler to the
high pressure side (the condenser).  The amount of energy KW required to
perform this function is directly related to the temperature difference
between the two, with the higher the difference the more energy required.

The energy usage will be largely impacted not only by temperature, but by
the ability for the condenser to reject the heat into the atmosphere. 
This will be reflected in the SEER (seasonal energy efficiency ratio) of
the unit.  The more efficient this process, through things like increased
surface area of the heat sink, better fan flow, etc the lower the
condenser temperature and the less energy used.

Now, where things get tricky is that the ability to reject the heat is
also a function of humidity.  When the humidity is high, air cooled
systems, like your typical residential unit become extremely inefficient
because they can't get the heat into the air and hence more energy is
required to move the refrigerant across the temperature differential.

Consequently, by running the AC when the condenser can best reject the
heat into the atmosphere you will lower your costs.  This will occur when
the the temperature and humidity are lowest.

There is one other wrinkle here and that is what people think of when they
refer to humidity.  The weather report always refers to "relative
humidity" which is a worthless concept.  Cooling load, as well as human
comfort, is largely a function of what is called the wet bulb temperature.
 This is the temperature that takes into account evaporative cooling.  Wet
bulb is very closely related to dew point.  When the wet bulb temperature
is low, even if the relative humidity is high, you can still more easily
reject the heat into the atmosphere.

By comparison, water cooled units using cooling towers, make use of
evaporative cooling and have a design property called their approach,
which is how many degrees from the wet bulb temperature they can cool the
water flowing through them to.  In a cooling tower, the water is turned
into droplets at the top shower heads and air is forced up through it in a
manner to achieve the maximum air-water surface contact.  With high
humidity and high temperatures, cooling towers become inefficient, but are
still able to reject heat much better than a air cooled unit.

So in summary, by running your A/C when the wet bulb temperature is
lowest, you will achieve the lowest cost in terms of KW per BTU of
cooling.

There is also what is called the coefficient of efficiency, which also
factors into your SEER rating.  In cooling, a lot of the energy
contribution is from the atmospheric thermodynamics, rather than the pure
electromechanical energy, which you can think of as a multiplier in that
you get X times the (BTU) watts of cooling for every watt of electrical
energy put into the system, where X is a positive number.

<snipped original post for size considerations>





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