I don’t get it. How is cold, even freezing air turned into heat? How is it less energy consuming than other heating systems?
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The fun part is that it is actually not less energy consuming than other heating systems. You still need the same amount of energy to heat your house. The point is that it uses ambient heat as energy source, which is freely available. You just need some kind of auxiliary energy to power the process, electricity in most cases. And that power consumption is much less than just use the energy directly because you don’t use that additional energy source.
The heat pump uses the laws of thermodynamics. If you compress a gas, it gets hotter. If you expand it, it cools down. So you compress it, extract the heat, transport it outside and expand it to cool it down. Now it can absorb heat from the ambient, and you can restart the process. The absorbed heat plus the energy you used to compress the gas in the first place is the usable energy (minus some losses of course).
An AC or fridge uses the same principle by the way, just in the opposite direction. In fact, many AC units double as heat pump heaters because they can reverse the process.
Edit: [That comic](https://xkcd.com/2790/) demonstrates the process in a very simple way.
You ever heard about how it takes as much energy to turn 100 C water into 100 C steam as it takes to heat 0 C water to 80 C water? That’s important, and called the enthalpy (or latent heat) of ~~fusion~~ vaporization. Phase changes consume (or release) a massive amount of heat energy. Have you ever used a can of air duster or a CO2 cartridge in food prep and it got super cold? That’s why. This is also how thunderstorms feed themselves, by the way. Also remember that compression and decompression will heat and cool fluids thanks to the ideal gas laws even without phase changes.
You can take advantage of this in a heat pump. One one side, you have a compressor, a pump that crushes some gaseous refrigerant (they can operate in wider temperatures than water) and heats it up in the process. Since it’s now hotter than its surroundings, it is run through a condenser – essentially a long conductive pipe that lets it radiate that heat until it’s the same temperature as outside. In this process, because it’s already under pressure, it condenses into a liquid, further releasing more heat.
Now it gets run into an evaporator where the opposite happens. The liquid gets dumped into a low pressure chamber, where it evaporates immediately. Remember the latent heat of ~~fusion~~ vaporization above? It absorbs an enormous amount of heat from its surroundings when it does so, just like your can of air duster. The now ambient temperature gas gets sent back to the compressor, where it starts over again.
Why is it more energy efficient? In a resistive heating element like your toaster, you can only ever get 100% of the heat you put in as electricity out again as heat. In a heat pump, you’re only moving heat, not generating it (compressor motor excepted). A motor converting electricity into fluid motion will move much, much more fluid and therefore more heat than the same amount of power heating a thick wire.
Basically moving heat energy is always done in the same way, be it a fridge, freezer, air-conditioning or heat-pump (In fact, air conditioning and heat pumps are the exact same system but running in reverse)
When a fluid converts from liquid to gas, it absorbs a ton of heat energy from its surroundings. When it turns back, it releases it. The boiling point of liquids can be manipulated with pressure.
So you move a liquid from a high pressure area, to a low pressure area and back. During this, it changes from liquid to gas to liquid. Doing this you move heat.
The other important thing to realize is that everything has some heat energy unless its absolute zero (0 degrees Kelvin), meaning that even if it is -40 outside, there is still heat there (Admittedly, not much, hence heat pumps get less efficient the colder it is outside), which can be moved inside your house. As it turns out, its a lot more efficient to move heat than its to actually generate hate directly with electricity. Up to 5 times as much. Meaning a heat pump in ideal conditions can achieve a 500% efficiency rating compared to a plain electric heater
Heat pumps (at least refrigeration type heatpumps) work just like Air-conditioners in reverse.
They make use of 2 physical phenomena:
1. Latent Heat, which is the energy difference between a substance at the same temperature but two different states, so for example 100°C steam has a lot more energy than 100°C water, and this difference is the latent heat of vaporisation (you can actually see this difference in person when you boil water, it only takes a few minutes to bring a kettle or pot tona rolling boil, but takes a few dozen minutes to turn it all into steam). Another side effect of latent heat is that it makes it impossible for a gas to exist below its condensation point or a liquid above its boiling point in a stable state. Once a liquid reaches its boiling temperature, **all** of the energy you pour into it just makes it turn into a gas faster, it doesn’t make it hotter.
2. The change of a liquids boiling point depending on pressure. The higher the pressure, the hotter something has to get before it starts boiling (this is how pressure cookers work, by increasing the pressure you are able to heat water to more than 100°C without it boiling)
In a heat pump you take a refrigerant (a special gas) and you compress it to a really high pressure, in fact you compress it to such a high pressure that the boiling point of the substance becomes higher than the ambient temperature, so that when you then pump that gas into a radiator, it starts condensing into a liquid, and in that process it releases the latent heat that it had stored hy being a gas, making the radiator very hot. Once the gas has turned into a liquid you then pump it through a metering device that restricts it, and causes a much lower pressure on the other side. So low in fact, that the boiling temperature of the liquid shoots back down way below the ambient temperature. As you recall, a liquid can’t exist stably above its boiling point, so the refrigerant immediately starts boiling, and to boil it needs to absord the latent heat of vaporisation again, so it rapidly cools dowm to the boiling temperature because it’s turning its own internal heat into latent heat, and then starts absorbing more energy from the environment as you pump it through a second radiator. So long as the boiling temperature of the refrigerant is lower than that of the air, it will absorb all the latent heat from the surrounding air.
The refrigerant then enters back into the compressor to start the cycle anew.
Now all you have to do is put the hot radiator inside and the cold radiator outside, and you have a heat pump.
The Reason why the heatpump gives you more hest than it consumes in electrical energy is the aforementioned latent heat, which is abosrbed from the outside air and then transported inside.
So long as the temperature of the low pressure boiling refrigeratant is colder than the air, the refrigerant can absorb its energy, and work. Modern heatpumps can function down all the way to air temperatures of around less then -10°C
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.
Same way as ac system, just reversed
It takes heat from ground cooling it down and putting it in your house
My understanding is that it doesn’t work on freezing air, just cold air. At or below zero, there will be no energy left to transfer.
But above that temp, it works just like an air conditioner, only in reverse. The air conditioner works by giving the heat in your house a path of least resistance to the outside, and then releasing it.
A heat pump will create the opposite conditions, creating an area where heat will naturally be drawn along a path of least resistance, into the house.
To know physically how it does that you have to start getting into chemistry and material science. But that’s the gist – it gives the desired temperature a path of least resistance to follow, either towards or away from you depending on what you’re trying to accomplish.
It’s less energy consuming because it’s taking advantage of natural conditions – Instead of using electricity to heat up a bank of coils in a heating unit, it’s just moving some liquids around. You can probably tell just from that sentence how much less electricity we’re talking about.
The science behind what those liquids do is complicated, but the process itself isn’t particularly complex or energy consuming.
Every chemical has a relationship between heat and pressure. Some of them, when you compress them get hot in proportion to the pressure. Then when you expand them they get cold in relation to the drop in pressure.
So you find a chemical that does this very efficiently (refrigerant), compress it so it gets really hot. Run that through a radiator so that it comes down to ambient temperature. Then expand it, and it will come out colder than it started because you exhausted all that heat while it was compressed. Run air over that and you have air conditioning.
If you want to heat the room, you just do that in reverse. You run air over the compressed refrigerant to heat up the room. Then you expand it and run it through a radiator so it warms up to ambient. (The need to warm it up is why heat pumps lose efficiency at low outside temps.)
Even freezing cold air has a small amount of heat in it. If you use a lot of the air, you can remove some of its heat, accumulate it in an insulated container (like a thermos bottle), and use the stored heat to warm up something else. The freezing cold air becomes even colder as a result, but you’ve kept some of its former heat for your own use.
When the air gets REALLY cold though, it becomes too much work to try to squeeze out the little bit of heat available. Then the heat pump needs help, getting some additional heat from a gas or electric heater.
All the other answers explain the thermodynamics of heat pumps, but here’s something simple that really helped me understand it: **unless you’re at absolute zero, there is always something colder than another thing**.
If the outside air temperature is -40C, liquid nitrogen (-196C) is colder, right? If you put liquid nitrogen outside in the middle of winter, it would warm up and evaporate because the -40C air would warm it up.
So if you want to transfer heat out of the cold, winter air what you need to do is make something even colder than that, which will then cause that something to heat up.
Everything described in the other comments is explaining how we make that colder thing, and how we then get the heat out of it, but that’s a core piece that I think a lot of people forget.
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