[TriLUG] OT: thermodynamics of A/C question

[Michael Hrivnak]

I am very interested in reducing energy consumption, and I recently
purchased a wifi-enabled thermostat that might be of interest.  A lot
of searching eventually led me to a $100 thermostat available locally
at Home Depot.  You can manage it through their website and through a
smart phone app.

http://www.radiothermostat.com/filtrete/products/3M-50/

I also wrote a python library to control it:
https://github.com/mhrivnak/radiotherm

If nothing else, using the website to program it makes it much more
friendly than many other programmable thermostats.  Then you have the
advantage of making adjustments from your phone.  When on the same
LAN, the phone app communicates directly with the thermostat, so
changes are instantaneous.  Otherwise updates go to the cloud-based
service, and the thermostat polls every 5 minutes for changes.

It was important to me to get a thermostat that was easy to interface
with so I wasn't locked in to a vendor that might not exist in 5
years.  This one has extensive API docs published by the manufacturer,
which is very nice.  On the whole, the product isn't particularly
polished and has a few rough edges.  However, it's inexpensive and
lets you program and control it with a computer or your phone, which
is great.  I like it a lot, and it has helped me save a lot of energy.

Michael

On Fri, Jun 22, 2012 at 9:46 PM, Joseph Mack NA3T <jmack at wm7d.net> wrote:
> On Fri, 22 Jun 2012, matt at noway2.thruhere.net wrote:
>
>> 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).
>
>
> ah. the condenser, not the compressor.
>
> I keep replacing compressors in my cars. Each generation of cars have made
> is more difficult to replace the compressor. I have compressors burned into
> my brain.
>
>
>> 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.
>
>
> I've read about SEER, but it only lasted till I clicked on the next link.
>
>
>> 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.
>
>
> don't understand how humidity plays much of a role here. Even at 100%
> humidity at 30degC, the air is only 4% water
>
> https://en.wikipedia.org/wiki/Dew_point
>
> moist air is lighter than dry air (MW water=18, MW air=29). However the heat
> capacity of water vapor is about double that of air
>
> https://en.wiktionary.org/wiki/humid_heat
>
> so moist air would absorb slightly more heat for a fixed temperature rise
> than would dry air.
>
>
>> 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.
>
>
> Yes. I plot the wetbulb along with the temp each day with rrdtool. The wet
> bulb temp stays pretty much fixed during the day, unless a new body of air
> moves in. The RH on the other hand drops as the temperature goes up during
> the day.
>
>
>> 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.
>
>
> The thing I keep track of wrt humidity and A/C, is that when you're cooling
> air, first you have to remove the water by changing it to the liquid phase.
> This requires a lot of energy, without changing the air temperature (this
> energy is called latent heat). Water has just about the highest latent heat
> of vaporisation of any material.
>
> https://en.wikipedia.org/wiki/Latent_heat#Table_of_latent_heats
>
> During this phase of cooling, the A/C unit is producing liquid water out the
> drain hole. It's only when the wet bulb temp drops to that of the
> evaporator, that you start to pull out the heat to drop the temperature of
> the house air (called sensible heat). I believe this is why people advocate
> leaving the house closed with the A/C at a fixed temperature. You aren't
> letting in humid air from the outside, from which you have to remove the
> water.
>
>
>> By comparison, water cooled units using cooling towers, make use of
>> evaporative cooling
>
>
> ah yes, the hyperbolic cooling towers at power stations (eg Shearon Harris)
>
> https://en.wikipedia.org/wiki/File:Shearon_Harris_Unit_1.jpg
>
>
> Joe
>
> --
> Joseph Mack NA3T EME(B,D), FM05lw North Carolina
> jmack (at) wm7d (dot) net - azimuthal equidistant map
> generator at http://www.wm7d.net/azproj.shtml
> Homepage http://www.austintek.com/ It's GNU/Linux!
> --
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