[TriLUG] OT: thermodynamics of A/C question

Matt Flyer matt at noway2.thruhere.net
Sat Jun 23 08:33:34 EDT 2012


One of these days I intend to get around to improving the graph and yes,
the scales are one of my biggest target items.  It is a $0 project and a
matter of time getting to it.  While it may be difficult to determine
from the graphs, usage and drops in energy efficiency really do
correlate heavily to wet bulb from our experience, to where it along
with dew point have become accurate predictors of performance.

One other thing that I was thinking about in terms of residential
issues, with respect to running AC at off times to super cool versus
running it throughout the day is that a home, doesn't have any mechanism
of sufficient thermal storage as the heat capacity of (dry) air is so poor.

On a side note, I am anticipating owning one more house in my lifetime
and I am seriously considering using a two water loop system, one hot,
one cold, with variable flow air handlers that will allow me to flow air
in only the rooms that it is needed.  I expect that this will gain four
benefits:
1 - the ability to control for temperature and humidity, at the cost of
energy efficiency, but this would be nice in the spring and fall when it
is humid but not warm enough for the AC to run
2 - efficiency improvement by not conditioning, or only marginally
conditioning the unoccupied spaces
3 - having "storage" capacity will allow running the equipment less
often, but for longer stretches minimizing wastes
4 - have the ability to rapidly heat or cool a (previously) unoccupied
area on demand by utilizing the stored capacity in a short time frame.


On 06/23/2012 08:20 AM, Joseph Mack NA3T wrote:
> On Sat, 23 Jun 2012, Matt Flyer wrote:
>
>> For example here is a graph showing total campus usage for the last
>> 24 hours relative to dry bulb and wet bulb temperature.
>> http://chwreports.unc.edu/pic2.php?time=1340450882&bldg[]=2654&bldg[]=2653&bldg[]=2533&timedate=2012/06/23%2007:26:26&interval=900&period=86400&dateForm=YYYY/MM/DD&source=tomkins
>>
>> You can see how even as the dry bulb temperature went down, the cooling
>> requirements went up as the wet bulb temperature changed.
>
> thanks. I didn't know people were making this sort of data available.
>
> This is a difficult graph to read. The temperature scales for the wet
> and dry bulb are offset and the spacing for the two scales is
> different; it's 1deg/div for wet and 2deg/div for dry. My immediate
> reaction is that the graph has been plotted specifically to obscur
> something. There's no way to correlate any of the data without
> replotting.
>
> Here's similar data, where the wet and dry bulb are plotted on the
> same scale. Here you can see that the wet bulb doesn't change
> significantly through the day compared to the dry bulb.
>
> http://www.austintek.com/belhaven_weather-index.gif
>
> With little change in the wet bulb temperature and large changes in
> the dry, it would be difficult to find a correlation between power
> usage and wetbulb. I would expect that most of the power usage
> correlates with dry bulb. If the case is otherwise, the unc graph
> obscurs it.
>
> Joe
>




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