How does the temperature of the condenser impact an air conditioner's energy usage?

Question

Air conditioners have a condenser after the compressor, in the high pressure side of the system. I've found graphs that show the compressor takes less energy if the condenser is colder. That makes intuitive sense, an AC doesn't have to work as hard if it's cold outside.

First I want to confirm I understand why they are more efficient at lower condenser temperatures. Is it that the compressor doesn't need to compress to as high of a pressure to get the refrigerant to it's liquid form, as the vapour/liquid point of the refrigerant is lower on that side of the system? Or something else?

Secondly, are there limits to this efficiency boost? I see some charts from HVAC companies showing it down to 18c condenser temperatures, but not lower as they aren't practical. If I had a unlimited supply of 5c cooling water to cool the condenser, and can lower the high-pressure side vapour temp to just above the low pressure side's vapour temp, can the compressor just barely work, and the system becomes very energy efficient, with the compressor only keeping the high pressure side just a bit higher pressure that the low pressure side? We can ignore the energy of the fans, and just focus on the compressor.

Bonus points: do systems like the one described exist. Aka using a cold water/coolant supply to cool the condenser, increasing energy efficiency.

Answer 1

if you had 5C cooling water you could possibly run the rest of the system as a heat pipe (if it were built the right way)

Using rivers, the sea, ground water, the ground itself, and even swimming pools as the heat sink for cooling systems is a well established practice.

The air is commonly used not because it is the best heat sink, only because it is the easiest to access.

Answer 2

That makes intuitive sense, an AC doesn't have to work as hard if it's cold outside.

It also makes sense when one considers the Carnot refrigeration coefficient of performance (COP), the theoretical maximum for any refrigeration cycle, which is

$$COP=\frac{T_{L}}{T_{H}-T_{L}}$$

Where $T_H$ is the temperature at the condenser (outside temperature). The lower the value the greater the coefficient of performance.

First I want to confirm I understand why they are more efficient at lower condenser temperatures. Is it that the compressor doesn't need to compress to as high of a pressure to get the refrigerant to it's liquid form, as the vapour/liquid point of the refrigerant is lower on that side of the system? Or something else?

That's essentially correct. Intuitively, transferring the same amount of heat to a higher outdoor temperature is analogous to carrying the same bucket of water up a higher hill, where the hill elevation is analogous to the difference between the indoor and outdoor temperatures.

Secondly, are there limits to this efficiency boost?

If what you mean by "efficiency" is the COP, the theoretical limit, which is independent of any limitations on the particular coolant used, is the Carnot COP.

The rest of your description has more to do with engineering. For a discussion of water cooled AC's see: https://www.forbes.com/home-improvement/hvac/water-cooled-air-conditioners/#:~:text=They%20tend%20to%20be%20more,are%20the%20most%20effective%20choice.

Hope this helps.

Answer 3

Typical electric motors draw more current under load (the effective impedance is reduced).

If you reduce the head-pressure of the condenser, the current through the motor will reduce (the effective impedance is increased), the power (current * voltage) will be reduced: the system will be more efficient.

This assumes that the voltage remains constant, and that the compressor doesn't simply spin faster. Real life is always limited by complications like that, but to a first approximation: ac systems are more efficient if the condenser is kept cool.