How It Works: Fridges

The onset of summer makes us appreciate our trusty fridges all the more. Without them we’d be throwing away fuzzy cheese every day and lollies would be liquid. But how does it all work? (See the diagram).

The type of refrigeration that occurs in our domestic fridges and freezers is known as absorption refrigeration, and is all to do with changing the pressure of coolants in those coils at the back of the unit. Ah-ha, we knew they were there for something.

Coolants are chemicals with low boiling points which alternate between being a liquid and a gas at different stages of the refrigeration cycle. In the past, coolants in domestic fridges and freezers were the now notorious CFCs, but these have been replaced with a range of more eco-friendly substances, such as hydrofluorocarbons (HFCs).

^{The process of absorption refrigeration, however, cannot be used to cool items down much further than -90°C, whereas some technologies, such as superconductors, require far lower temperatures to be able to function properly - temperatures nearing absolute zero (-273.15°C). To chill things this far requires a different cooling technique called adiabatic demagnetisation - woah there!

What's adiabatic demagnetisation?

This process works using magnetism; holding weakly magnetic molecules in a strong magnetic field prevents them from moving around. By slowly lowering the strength of the magnetic field the molecules are free to start moving again, but movement requires energy. The molecules take this energy, in the form of heat, from the surrounding area, which cools everything down around them.

When the magnetic field is switched back on again and the molecules are prevented from moving they start to release the energy they’ve gained, which can be siphoned off and away. Repeating the process can cool items down to a few billionths of a degree above absolute zero.}