[TriLUG] OT: thermodynamics of A/C question

[jonc at nc.rr.com]

Follow up...

Did you look at this any further?  Did you in fact do a test run where you cooled the house down at night and then let it heat up during the day?  I would be curious to see the data.

In the past I've turned off my AC and run hourly measurements of inside/outside temp to see how rapidly it heats up... I've been pleasantly surprised by how well my house conserved the cooler temp.

One other caveat to your plan - the humidity in your home. If you don't control the humidity well, you may get a visit from molds and fungi... and that could be very costly.

Jon Carnes 

---- Joseph Mack NA3T <jmack at wm7d.net> wrote: 
> Summer is no fun here with the heat. The cost of A/C cooling 
> is more than for heating in the winter. This lead me to 
> wonder if there is any way around the cost of A/C?
> 
> For the last month or so, with cool nights, I've had windows 
> open with fans blowing the cool outside night air in through 
> the open windows. Until a couple of days ago, the house was 
> in the 60's in the morning and stayed about 10degF below the 
> outside temp during the day, which was quite accepable.
> 
> Then yesterday, with the night not being cool anymore, I had 
> to turn on the A/C. It occured to me that I should run the 
> A/C at night, when the air outside was cooler, instead of 
> runing it during the day. If I'm trying to remove a certain 
> fixed number of BTUs, I expect the A/C works better, from a 
> Carnot cycle point of view, if the heat sink is cooler.
> 
> Let's say I want the house temp to not go above 82degF 
> during the day (and don't care how cold it is at night).
> 
> I can either
> 
> o set the A/C to 82degF and have it run during the day 
> dumping to air at 90degF (say). Then at night the A/C 
> doesn't run much (at least in the current weather). In the 
> morning, the house is still 82degF.
> 
> o set the A/C to 76degF when I go to bed. I wake up to a 
> cold house in the morning and set the A/C back to 82. The 
> A/C doesn't run during the day (at least for today, with 
> 90degF outside).
> 
> from
> 
> https://en.wikipedia.org/wiki/Carnot_cycle#Efficiency_of_real_heat_engines
> 
> Carnot efficiency = 1-Tc/Th
> 
> My head hurts trying to think about Carnot cycles at the 
> interface between the high pressure exchanger (compressor) 
> and the outside air on one hand, and the low pressure 
> exchanger (evaporator) and the inside air on the other hand. 
> I assume for my scenario, the only change to be considered 
> is at the compressor exchanger, which is dumping to outside 
> air, ie only the outside air temperature changes much. The 
> compressor exchanger is hot (too hot to touch) which puts it 
> at 160degF or so (the temperature of hot domestic hot water) 
> =60degC=330degK.
> 
> Lets assume that the outside day temp is 310degK and the 
> outside night temp is 300degK.
> 
> The efficiency changes from
> 
> 1-310/330=6% to 1-300/330=9%, ie an increase in efficiency 
> of 50%.
> 
> Now I know that the heat exchangers in an A/C aren't Carnot 
> cycle, but I don't know how else to model them.
> 
> This article models an A/C as a Carnot cycle machine
> 
> https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio
> 
> but the equation presented
> 
> https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio#Theoretical_maximum
> 
> has a singularity at Tc=Th. Clearly an A/C still works if 
> the inside and outside temperature is the same.
> 
> Got any idea, if I want to remove a fixed number of BTUs, 
> whether running the A/C at night is any better than during 
> the day, or am I having myself on?
> 
> Thanks
> 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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