Long before we knew about global warming, we developed technologies that keep us comfortable indoors regardless of the weather outside. On a hot day, we can stay cool inside by running an air conditioner. It's a common misconception that air conditioners create cold air, since that would violate the laws of thermodynamics!
There are a few ground rules we need to follow when making an air conditioner.
- The first is that energy can't be created or destroyed. If we suck thermal energy out of the air in one place, we need to dump it somewhere else.
- The second rule is that heat naturally travels from hot to cold. Moving heat from a colder place to a warmer place (like an air conditioner on a hot day) takes energy.
In physics, these rules are known as the first and second laws of thermodynamics.
An air conditioner follows these laws to remove heat from the air inside a building and release it outside. In particular, an air conditioner has four stages involving the flow of a liquid refrigerant called a working fluid:
- It compresses the fluid with a compressor, raising its temperature.
- The hot fluid runs through a radiator outside the building, heating the outside air.
- The fluid expands through an expansion valve, lowering its temperature.
- The cold fluid runs through a radiator inside the building, cooling the inside air.
While running, the compressor and expansion valves in an air conditioner continuously control the temperature of the working fluid. This process consumes energy, but allows the fluid to continuously absorb heat inside and release it outside.