# Eli5 How do cold climate air source heat pumps work

245 views

I am hearing about this as a low carbon method to heat or cool buildings but I can’t find a good article or video to see how it works. The closest I can figure is like a fridge in reverse… the pump or refrigerant gets hot and you use that heat. Cold climate systems supposedly extract heat down to -25c. How?

In: Physics

It’s a heat transport cycle either using evaporation or a chemical reaction. The evaporation one is simpler so I’ll explain that:

1. You evaporate a medium in your heat source at low pressure. It absorbs energy to make that phase change (low pressure means low boiling point)

2. You use a pump (compressor) to move the medium to the other side and increase the pressure

3. At high pressure the medium condenses (despite being warmer) releasing the stored energy

4. You let it stream back through a small opening that forces the pressure to reduce

Cycle closed. You transport a lot more energy than the pump requires, at least if the temperature difference isn’t too big. (Therefore the most efficient ones don’t use outside air but ground water that is around 10°C all year around, and in norway there is a large scale heatpump supplying district heating from 4°C fjord water)

The chemical variant replaces the evaporation with dissolving one medium in another and later forcing it out with pressure.

That is exactly it. Like a fridge. The laws of thermodynamics say that you cannot have cold without heat. Even at -25c there is heat above absolute zero that can be extracted from the air. As long as molecules are moving there is ‘heat’ in any given item/system, and this happens at all times above absolute zero. Heat pumps will run in reverse to pull heat from the outside air, and bring it inside, this causes condensation on the outdoor unit since it is getting colder than the air. When the condensation reaches a point where the system won’t correctly draw heat from the outdoor air due to ice buildup, the system will automatically go into a defrost cycle where it acts as a normal a/c pulling heat from indoors and using the outdoor air to dissipate it. This causes the metal coil to heat up melting any ice buildup. Then it switches back to running as a heat pump. Once the outdoor air reaches a temperature that ices the coil up immediately, the system becomes redundant. That is usually at about -20 to -40c depending on the system design parameters.

Put water in a freezer and it will freeze. Freezing water requires removing quite a lot of heat from the water.

Where’d the heat go? It’s clearly not in the freezer, because it’s cold in there. Heat cannot ever disappear due to the laws of thermodynamics. This means it must have ended up in the room the freezer is in.

Now take these blocks of ice you just stole a lot of heat from and put them outdoors so that they melt. Now put that water in the freezer again and presto, you’re heatpump heating your house by taking heat from outside.

This example of course only works if the temperature outside is above the melting point of ice, so 0°c.

A heat pump is going to use something better suited for the job, maybe something that melts at -25°c? And instead of using the liquid-solid phase transition it’ll use the gaseous-liquid transition, mostly because transporting a liquid and a gas can both be easily done in tubes, while a solid complicates things quite a bit.