If you've ever heard your air conditioner struggling to cope with a summer heat wave -- and been disappointed in the success of its efforts -- you've probably wondered what you could do to alleviate the machine's suffering -- and yours. You could plant trees and shrubs close to your central air conditioning unit and hope that the consequent lower temperature helped the unit cool your home more effectively. For that to work, the outside temperature would have to somehow be related to the effectiveness of your air conditioner. Luckily for you, it is.
Air conditioners are heat exchangers -- units that transfer heat energy from one place to another. They are limited by the laws of thermodynamics, which are the rules constraining the movement of heat energy. The most common way of seeing this is that two objects of different temperatures put in an environment where they interact will come to equilibrium at some temperature between the two starting temperatures. Heat transfer also can have other effects, as when you put an ice cube tray full of water in the freezer and the water solidifies. Heat energy flows from the hotter object to the colder object -- that's a statement of the second law of thermodynamics. It's why you need an air conditioner in the first place: Heat will naturally transfer from the hot outdoors to the inside of your home.
Air conditioners work by circulating a refrigerant. That's a fluid that transitions from a liquid to a gas at a relatively low temperature. When the fluid is a gas, its properties are governed by the ideal gas law, which says that the pressure, density and temperature of a gas are all connected. If you raise the pressure of an enclosed gas, you'll increase its temperature, and if you lower the pressure of a gas, you'll lower its temperature.
It takes energy to change a fluid from a liquid to a gas. A refrigerant transitions from a liquid to a gas at high temperature, while at low pressure the transition is at a lower temperature. The exact temperatures, pressures and the energy of the transition are all dependent on the specifics of the refrigerant.
The Air Conditioning Cycle
Your air conditioner uses the principle of the second law of thermodynamics and the refrigerant's properties to cool your house. Outside your house, the air conditioner compresses the gaseous refrigerant, which -- according to the ideal gas law -- increases its temperature. A fan blows outside air across coils containing the hot, high-pressure refrigerant. The outside air is cooler than the fluid, so -- according to the second law of thermodynamics -- energy will flow from the refrigerant to the outside air. When the high-temperature gaseous refrigerant gives up energy, it transitions to a liquid.
The high-temperature, high-pressure liquid goes through an expander, which converts the refrigerant to a low-temperature, low-pressure liquid as it enters the inside of your house. There another fan blows inside air across the coils, where the warmer air sends heat into the cool coils, converting the liquid to a gas. The gaseous refrigerant enters the compressor and the cycle starts again.
Imagine your refrigerator without the coils that interact with air. It would be a machine that converted a low-temperature, low-pressure fluid into a high-temperature, high-pressure fluid, and back again to the same low-temperature fluid. In real life, inefficiencies heat the fluid, gradually raising the temperature with each cycle. Now add inside cooling coils to your imaginary air conditioner. Those cooling coils pick up heat from the inside air, but they have nowhere to get rid of it, so the temperature of the whole system increases.
So you add an outside heat exchanger to get rid of the extra heat. The amount of heat transferred and the rate at which it's transferred depends upon the temperature difference between the outside air and the refrigerant. The lower the temperature of the outside air, the more cooling is done by the heat exchanger, instead of the compressor.
Air Conditioner Efficiency
There are a number of different methods for evaluating air conditioner efficiency. The one that is most closely tied to physical principles is the coefficient of performance, or COP. The COP is the ratio of heat removed from a system to the energy required to remove the heat. The theoretical maximum is equal to the coldest temperature of the refrigerant divided by the difference between its coldest and hottest temperatures, where the temperatures are expressed in Kelvin. Even the perfect system decreases in efficiency with increased outside temperature, dropping about 2 percent per degree Celsius (about 1 percent per degree Fahrenheit). In the real world, the drop in efficiency is even more dramatic; so it's not an illusion when your air conditioner seems to be working harder as the outside temperature increases.