Air Conditioning Efficiency (Coefficient of Performance) Questions
June 21, 2014 7:37 PM   Subscribe

I have come to understand that a typical modern window or residential-central air conditioner (which I believe is referred to in the industry as a "Direct Expansion" or DX unit) has a coefficient of performance which typically ranges between 3.0 and 3.5. That is to say, for every unit of energy consumed, the air conditioner removes between 3 and 3.5 units of heat from the conditioned space. I have a few questions regarding this, inside.

1) For a standard window/central residential type unit, what (if any) factors determine how much power the compressor draws (assuming steady-state operation; I don't mean "if it's cooler out, less heat will enter the conditioned space, so the thermostat will call for cooling less frequently")? For example, does the difference between the indoor and outdoor (evaporator and condenser) temperature make a difference somehow? Does the compressor motor draw more power if its input pressure is higher / lower / etc? I would be very interested to hear about this at as much length as you are willing to entertain.

1a) Is it true that the type of air conditioning system that is similar to a window or home central unit (in which there is a compressor, condenser, and evaporator, and the condenser and evaporator exchange heat, via air, directly with the conditioned and heat-rejection spaces) is equivalent to the term "Direct Expansion System"? Or, does Direct Expansion/DX have some other connotations that I don't know about?

2) If the CoP of a typical home system is 3.0 - 3.5, what kind of CoP do you see in commercial/industrial units, the kind that you see next to or on the roof of commercial or academic buildings, which often appear to be emitting some form of steam or water vapor?

2a) Is the CoP of those units significantly higher than 3.0-3.5?

2b) If not, why are those units used instead of "regular" residential type units?

2c) How do those types of units work, anyway? What are they doing -- using the enthalpy of vaporization of (for instance) tap water to their advantage? Or is there something else to it?

In case it matters, the climate I am thinking of here is that of southeast Michigan. The relative humidity here is high enough that a residential "swamp cooler" would be largely useless, but not so constantly high that there is never any advantage to be had from evaporating a liquid.

For what it's worth, I have an educational background in engineering, but no formal education concerning HVAC systems. So, please feel free to go into as much technical/physical/mathematical detail as needed to make a good explanation.

Thanks in advance!
posted by Juffo-Wup to Science & Nature (3 answers total) 3 users marked this as a favorite
You'll find answers to most of this stuff in the ASHRAE handbook at your local library. It's mostly thermodynamics and practicality (the kind of compressors we can make, the cost of making pressures higher, etc.).

DX usually just means that the fluid that's moving heat to/from the conditioned space is the refrigerant, vs. chilled/hot water systems used in a lot of larger installations.

You can get a better COP by making your hot or cold reservoir hotter/colder, but you are restricted by what you can do with the refrigerant. For instance, you can't really run a mix of liquid and gas in a compressor, so that limits you with the usual CFCs. You can use other substances, like CO2, but then you need much higher pressures, so you don't see that in residential units very often.

On a big building, what's you'll usually have for AC is a chilled water system, with chillers creating cold water. They will often use cooling towers (essentially huge swamp coolers) to get a bit of "free" cooling, but that requires a lot of care because cooling towers can become an ideal environment for the Legionella bacteria (like in Quebec City two years ago)
posted by Monday, stony Monday at 8:55 AM on June 22, 2014

1) The efficiency of the unit is theoretically proportional to the inverse of the difference between the two sides. So, the cooler the outdoor temperature, the more efficient the A/C is. Exact proportionality will vary a bit with different A/C design, but the inverse relationship is true in general.

Insufficient refrigerant pressure can definitely cause performance and efficiency issues.

2) There is actually a pretty wide range in COP for residential A/C from cheap window units below COP 3 to the best mini-splits above COP 5. The best large units, commercial or central residential, are a little worse than the best mini splits, falling under COP 5. Obviously, this doesn't account for the use of cooling towers which would increase the system COP. Energy Star data is readily available to compare the best or each type.

b) Commercial units are bigger and may be built to different standards.
posted by ssg at 1:19 PM on June 22, 2014

Compressor power for a given unit is primarily a function of the high side pressure. High side pressure is in turn determined by the condenser temperature, since this is what determines the pressure required to make the refrigerant condense.

Condenser temperature will be whatever it needs to be to reject the required amount of heat. You need to have some temperature difference above the outside air temperature in order to reject any heat. So you could say that the minimum possible condenser temperature is the same as the outside air temperature when zero heat is rejected, and it goes up from there based on things like:
  • Condenser size/fin area (larger = lower temperature) which is why increasingly efficient units are getting larger
  • Condenser airflow (more = lower temperature)
  • Condenser contamination (dirty = higher temperature)
  • Refrigerant type (the pressure vs. temperature curve is different for each refrigerant)
  • Amount of heat absorbed in the evaporator and produced by the compressor itself (so blocking off airflow or a dirty evaporator would reduce compressor power)
The cooling towers you see for large air conditioners exist for a couple of reasons. In these units, the condenser is water-cooled rather than air-cooled, and the water from the condenser flows to the cooling tower where it rejects heat into the air and some of the water evaporates. This means you can avoid running refrigerant lines to a condenser outside and can keep all the refrigerant inside the mechanical room or even inside an entirely self-contained chiller.

But perhaps more importantly, the portion of the water evaporating in the cooling tower cools the rest of the water down below the ambient air temperature. This allows the condenser to see a lower temperature than the outside air to reject heat into, so the high side pressure is lower and less power is needed to run the compressor. Efficiency is improved.
posted by FishBike at 4:46 PM on June 22, 2014

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