- #1
Cyphear
- 8
- 0
I have two questions/ideas about the efficiency of A/C's that I'd like to bounce off you guys. I couldn't find anything on Google about either of these items being discussed before.
1) Precooling
The first thought I was wondering was whether it was more efficient to pre-cool your house in the morning, since an A/C will blow colder air when it is cooler outside. It may be blowing 50 degrees at the registers first thing in the morning, but 60 degrees when it's hot in the middle of the day. Obviously it's more energy efficient to get colder air coming out of the registers considering the energy use should be constant. I think the value of this technique would depend on how well your house is insulated, but with good enough insulation, I believe it could make sense to use this technique and avoid running the A/C in the heat of the day.
2) Humidity
My second idea I was able to hash out a little more, and it was in regard to humidity. It appears that it takes a descent amount of energy to remove humidity from the air (a byproduct of running the A/C). My calculations show that it takes 8.9kJ to dehumidify one kg of air from 75% RH to 50% RH at 25 degrees C, while it takes 1.012KJ to cool one KG of air 1 degree. With these assumptions, this means that if you let 75% RH air in your house, it will only be cost effective if it keeps you from cooling that air by 9 degrees.
This was fairly surprising to me, as I would previously have opened my windows if it's a little bit cooler outside in the morning (regardless of humidity). I'll be considering the humidity in the future before opening my windows.
Here are my math notes in case anyone is curious or wants to check my math:
1) Precooling
The first thought I was wondering was whether it was more efficient to pre-cool your house in the morning, since an A/C will blow colder air when it is cooler outside. It may be blowing 50 degrees at the registers first thing in the morning, but 60 degrees when it's hot in the middle of the day. Obviously it's more energy efficient to get colder air coming out of the registers considering the energy use should be constant. I think the value of this technique would depend on how well your house is insulated, but with good enough insulation, I believe it could make sense to use this technique and avoid running the A/C in the heat of the day.
2) Humidity
My second idea I was able to hash out a little more, and it was in regard to humidity. It appears that it takes a descent amount of energy to remove humidity from the air (a byproduct of running the A/C). My calculations show that it takes 8.9kJ to dehumidify one kg of air from 75% RH to 50% RH at 25 degrees C, while it takes 1.012KJ to cool one KG of air 1 degree. With these assumptions, this means that if you let 75% RH air in your house, it will only be cost effective if it keeps you from cooling that air by 9 degrees.
This was fairly surprising to me, as I would previously have opened my windows if it's a little bit cooler outside in the morning (regardless of humidity). I'll be considering the humidity in the future before opening my windows.
Here are my math notes in case anyone is curious or wants to check my math:
25C = 298.15 Kelvin
Lwater(T) = - 0.0000614342T3 + 0.00158927T2 - 2.36418T + 2500.79
0.01831660673 + 0.4738408505 - 704.880267 + 2500.79
=1796.40189 kJ/kg (latent heat of vaporization @ 25C)
100% RH = 20g H2O per kg of air
75% RH = 15g H2O per kg of air
50% RH = 10g H2O per kg of air
To remove 5g of H2O:
1796.40189/1000*5 = 8.98200945 kJ to dehumidify one kg of air
1.012 J/g specific heat of air at 25C
To cool 1KG of air 1 degree: 1012J