The core idea
Nothing actually makes cold. Heat is energy, and it flows spontaneously from hot to cold — never the reverse, any more than water flows uphill. A refrigerator or air conditioner is a heat pump: it spends mechanical work to carry heat against its natural direction, out of a cold space and into a warmer one.
The trick is that a fluid's boiling point depends on pressure. Drop the pressure and a fluid boils at a low temperature; squeeze it and it condenses at a high one. The machine exploits this with four components in a loop:
- Evaporator (indoor/cold side): low-pressure refrigerant boils, absorbing latent heat from the room or food compartment — this is where the cooling happens.
- Compressor: squeezes the vapor to high pressure, raising its condensing temperature above the outdoors.
- Condenser (outdoor/hot side): the high-pressure vapor condenses, dumping that heat outside.
- Expansion valve: throttles the liquid back to low pressure; it flashes cold and re-enters the evaporator.
The energy books must balance: Q_hot = Q_cold + W — heat rejected outside equals heat removed inside plus compressor work W. Efficiency is the COP = Q_cold / W — kilowatts of heat moved per kilowatt of electricity. A COP of 3 means three units of heat moved per unit of work; no resistance heater can touch that, because resistance heat is capped at COP 1.
Real-world example
Your kitchen fridge and a home split A/C are the same machine with different geometry: the fridge's evaporator hides in the food compartment, its condenser is the warm grid on the back. A winter heat pump is also the same loop, run so the useful output is the hot side — and with a reversing valve it becomes an air conditioner in July. One cycle underlies most of the world's cooling and a growing share of its heating.
Common pitfall
Leaving the fridge door open to cool a hot kitchen. Every joule of heat removed from the room goes right back in through the condenser — plus the compressor's electrical work W. Net effect: the room gets warmer by W. You'd be running a 100-watt room heater with extra steps.