[TriLUG] OT: thermodynamics of A/C question

[Neil L. Little]

Older homes have whole house fans that do just that, remove those BTUs 
by creating a negative pressure to draw air from cooler locations.

The modern version of that would be to air from a buried culvert. Maybe 
take the fan out of the picture altogether by using a solar stack which 
is really just a cupola with vents at the top of the house. The phrase 
for all this is passive solar cooling.

73,

Neil Little, WA4AZL


Joseph Mack NA3T 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
>