Any thermodynamic engineers here?

Thread Starter

MikeA

Joined Jan 20, 2013
227
I'm observing an interesting relationship between outdoor temperature and the energy used by my air conditioning compressor.

The graph below shows:

Y axis = how many watts the compressor is using
X axis = hour of the day

Red line is the compressor cycling. The grey line is outside temperature (F) multiplied by an arbitrary number (45 in this case) to scale it to the Y axis.

I wasn't surprised that the two independent inputs correlate, they should, but what surprised me is just how closely the outside temperature and energy usage tracks each other.

What would be a scientific way to explain what I'm observing? :p

thermo.png
 

Hymie

Joined Mar 30, 2018
861
The power used by the compressor is the power consumed by the compressor motor; as the outside temperature increases, so does the motor load.

I suspect that the increased outside temperature is proportionally increasing the overall pressure of the system, resulting in a proportionally increased motor load (power consumption).
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
I suspect that the increased outside temperature is proportionally increasing the overall pressure of the system, resulting in a proportionally increased motor load (power consumption).
Right, that's what I expected.

What I can't explain is why this proportion seems to be described by such a simple formula, 1F = W * 45.

That is for each 1 degree increase (or decrease) of outside temperature, power usage goes up (or down) exactly 45 watts. This linearity must be some formula taught in Thermodynamics 102 (or 103 maybe).
 

Reloadron

Joined Jan 15, 2015
5,646
I don't get it. Our AC is a basic 3 Ton, 36,000 BTU, 3 HP unit running on 240 VAC 60 Hz. Running load it draws 15 Amps and outside temperature matters not. What is called a 3 KW load (even though 240 * 15 = 3,600). I have not seen an AC compressor that draws current proportional to the outside air temperature. New to me? Then too always a first. The compressor is not working any harder if the outside air temperature is 80, 90 or 95 degrees F. Unless in this case it's a "smart compressor" of sorts. Additionally if it was some type of smart compressor wouldn't it care more about the inside air temperature than the outside?

What are the compressor specifications and manufacturer?

Ron
 

Hymie

Joined Mar 30, 2018
861
You are observing this linear relationship over a delta of less than 20⁰F in outside temperature. If this relationship held true at all temperatures, theoretically the compressor motor would be drawing zero watts with an outside temperature around 80⁰F below that in your experimental data.

I suspect the factor of 45 would change depending on the precise configuration of the pipery/cooling capacity of the particular air conditioning installation.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
If this relationship held true at all temperatures, theoretically the compressor motor would be drawing zero watts with an outside temperature around 80⁰F below that in your experimental data.
It seems to be linear within all outside temperatures when A/C has ever run (70-100F). I'm not claiming or expecting that this relationship holds true past that.

There must be some underlying physical principle behind the linearity.
 

Chris65536

Joined Nov 11, 2019
227
The compressor is not working any harder if the outside air temperature is 80, 90 or 95 degrees F.
I think it would be working harder. The compressor is pushing against the high pressure in the condenser, and the higher the condenser temperature, the higher the pressure. The condenser temp would be close to a fixed delta-V over ambient.
 

Tesla23

Joined May 10, 2009
406
I'm observing an interesting relationship between outdoor temperature and the energy used by my air conditioning compressor.

The graph below shows:

Y axis = how many watts the compressor is using
X axis = hour of the day

Red line is the compressor cycling. The grey line is outside temperature (F) multiplied by an arbitrary number (45 in this case) to scale it to the Y axis.

I wasn't surprised that the two independent inputs correlate, they should, but what surprised me is just how closely the outside temperature and energy usage tracks each other.

What would be a scientific way to explain what I'm observing? :p

View attachment 215597
It's probably an inverter unit and part of the algorithm, when cooling, is to scale the unit's power to the outside temperature. You will probably find that the COP drops as the power increases. It would help if you told us what the unit is. If it's an old unit with a simple induction motor and a bang-bang controller then this would indeed be curious.
 

Reloadron

Joined Jan 15, 2015
5,646
I would like to know how the measurements were taken as in setup and instrumentation. While I understand X and Y axis I don't get how they apply nor do I understand the scale used?

While I am not and never was a refrigerant type the system will have a low side and high side pressure which should not vary based on outside temperature. The pressure will vary based on the refrigerant used. At 95F a common refrigerant like R22 will have a high side pressure of about 250 PSI and a low side pressure around 70 PSI. While the vapor pressures at sea level vary based on outside temperature the high and low side condenser pressures shouldn't. My compressor, 240 VAC a typical residential unit has no change in current draw other that start-up and run and start up is a pretty quick happening.

If there is a new design, similar to a Panosonic Microwave Oven that actually scales the power level to the compressor somebody point me to it. About all I can do is look at my 10 year old home system. All I am saying is that if there is a new design and more efficient design I am not seeing it anywhere. :(

Ron
 

Tesla23

Joined May 10, 2009
406
If there is a new design, similar to a Panosonic Microwave Oven that actually scales the power level to the compressor somebody point me to it. About all I can do is look at my 10 year old home system. All I am saying is that if there is a new design and more efficient design I am not seeing it anywhere. :(
I am also a long way from being an AC expert, but some years ago when I was buying a new unit I did investigate the claims of inverter units. I couldn't see why running the compressor at lower power was any more efficient that just running it say 50% of the time. Typically it is, detailed information was hard to find, but from my notes I found this useful:
https://www.osti.gov/servlets/purl/1171749/

1598405192514.png

If you run the unit at full rated power the efficiency is typically less than running it at low power, so modern inverter units try to set the compressor power to maximise efficiency. You would see this as reduced power consumption based on some algorithm used by the unit to estimate capacity needed - and external temperature would clearly be a significant variable.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
I couldn't see why running the compressor at lower power was any more efficient that just running it say 50% of the time.
By that logic buying an A/C that's 4 times the needed size and running it at 25% of the time would be the same as buying a system that's 1 times the needed size.

But it's not since humidity removal is one of the primary goals of A/C. Humidity removal is most efficient when the system is running continuously, at full speed, or half speed, or quarter speed.

Even having a blower time delay after the A/C turns offs can be detrimental. As the moisture on the coil that would otherwise drain out, would be picked back up into the air stream.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
I would like to know how the measurements were taken as in setup and instrumentation.
Current + voltage monitoring, and an outdoor temperature sensor.

While I understand X and Y axis I don't get how they apply nor do I understand the scale used?
The scale is for power, in watts. Temperature is overlayed and scaled with a constant multiplier of 45. To get temperature at any time, just take the figure of watts, and divide by 45.

My compressor, 240 VAC a typical residential unit has no change in current draw other that start-up and run and start up is a pretty quick happening.
It should. That was the whole point behind the adoption of SEER ratings, vs EER.

The seasonal energy efficiency ratio (SEER) is also the COP (or EER) expressed in BTU/hr/W, but instead of being evaluated at a single operating condition, it represents the expected overall performance for a typical year's weather in a given location. The SEER is thus calculated with the same indoor temperature, but over a range of outside temperatures from 65 °F (18 °C) to 104 °F (40 °C), with a certain specified percentage of time in each of 8 bins spanning 5 °F (2.8 °C). There is no allowance for different climates in this rating, which is intended to give an indication of how the EER is affected by a range of outside temperatures over the course of a cooling season.
https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio
 

Tesla23

Joined May 10, 2009
406
By that logic buying an A/C that's 4 times the needed size and running it at 25% of the time would be the same as buying a system that's 1 times the needed size.
The issue is that the 'needed size' is for the hottest/coldest days of the year, most of the time it is an A/C that's several times the needed size, and so there may be a substantial performance improvement to be had running it at lower power most of the time.
 

Reloadron

Joined Jan 15, 2015
5,646
Time to consider digging out some sensors and monitor and record my system. Should have everything I need. Just something that has me real curious now.

Ron
 

sagor

Joined Mar 10, 2019
257
One has to remember that an A/C unit is basically a heat exchanger. When the gas is compressed in the outside unit, it creates heat, which has to be sent to the outside air (the fan blowing on outside coils). The hotter it is outside, the harder it is to cool the compressed gas. That means it has to run longer to get the same net "cooling effect". If the gas is not cooled enough, it enters the house and when it expands to provide cooling in the inside coil, it just does not cool as much as when the gas is better cooled outside. Thus compressor runs longer, even if the heat in the house is constant.
Finally, a house heats up during the day, and latent heat still resides in the walls and roof when the temps start to fall outside. However, the A/C unit still runs a fair bit to remove the heat from inside, hence it tends to run just as long as in mid-day (somewhat, it varies by house structure and weather). Even thought the heat exchange efficiency in the outside unit is a tad better as temps fall, there is still a lot of heat in the building structure that has to be removed.
 
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