how does a heat pump work?

Question

I don’t get it. How is cold, even freezing air turned into heat? How is it less energy consuming than other heating systems?

In: 14

Answer 1

It works the same way as your fridge, which turns room temperature air “into cold”.

When gases expand, they cool down. When you compress them, they heat up. This is the principle that is used in both fridges and heat pumps.

In a heat pump, it works like this. You take some gas and you compress it. This heats the gas up. Now you run this hot gas through the space that you want to heat up. The gas transfers its heat energy to its surroundings and heats up the space a little (while the gas itself cools down).

Okay, but now what? Compress the gas even more so it heats up more? That won’t keep working as you’ll hit a limit to how much you can compress the gas. No, what you do instead is, you run the gas to an expansion vessel outside the building. In there, you let the gas expand in volume, which cools it down. For the heat pump to work, the gas has to cool to below the outside temperature. Once that happens, heat naturally starts to flow from the surroundings into the gas.

To complete the cycle, you run the gas through the compressor again and into the interior of the building. The gas heats up again and this heat can transfer to the space you’re trying to heat.

In this way, heat is “pumped” from outside to inside. When the gas is allowed to expand, it acts as a heat “sponge” that heat will naturally flow into. When it is compressed, it’s like you wring out the sponge, dumping all the heat you had absorbed.

The reason this is so efficient is because at no point are you using energy to *produce* heat (e.g. by burning fuel). All the heat is already there – you’re just moving it from one place to another.

There are limits to this. Yes, very cold air can be “turned into heat”, by making the gas even colder than the air so that heat will still flow from the air into the gas. But you can’t make the gas infinitely cold, so if it is too cold outside (colder than the expanded gas) then your heat pump won’t work (and even before reaching that limit you start to see a drop in efficiency). You can remedy this somewhat by running the outside part of the heat pump not through air but, deep into the ground where temperatures don’t get as low when it’s freezing outside.