# Cooling/refrigeration explained

The essence of a refrigerator and air conditioner is the same, relatively straightforward cycle:

1. A compressor constricts a cold substance (often called "refrigerant"), which raises its pressure and pushes it into the coils near the "hot area" as a gas (per [Boyle's Law](https://en.wikipedia.org/wiki/Boyle%27s_law)).
2. When the hot refrigerant in the coils meets the comparatively cooler air temperature in the "hot area", it becomes a liquid.
3. The refrigerant cools down and flows into the coils inside the "cool area".
4. The heat inside the "cool area" moves into the refrigerant.
5. Finally, the refrigerant evaporates to a gas and flows back to the compressor, where the cycle starts all over again.

When an engine needs cooling, the process is typically reversed: the hot area is inside the engine and the cool area is outside it. Most engine coolant is a mixture of water and an antifreeze, which catalyzes water to lower water's freezing point and raise its boiling point.

Heat pumps are effectively the same principle, but transfer heat from a wide variety of sources to something else that would use it (e.g., geothermal heat to heat water). Unfortunately, heat pumps are constrained by extra energy required proportionally to the difference in temperature between the hot and cold areas.