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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It’s like an air conditioner running in reverse. It generates cold air outside with an evaporator and pumps the vapor indoors where it gets compressed to generate hot air inside with a condenser, before pumping the condensate back outside to continue the cycle.
The conceptual problem with understanding heat pumps is that what we think of as cold isn’t, physically speaking, all that cold. On the “true” temperature scale – where 0 is “the coldest anything can be” – water freezes at 273 Kelvin and human body temperature is 320 K, i.e. only 17% hotter (one degree Kelvin is the same temperature difference as one degree Celcius, they just have zero in a different place). Thus, there is still a lot of heat in air/water/earth/rock at freezing temperatures, some of which can be extracted (reducing the temperature to, say, 5 degrees below freezing) and dumped into the space you want to heat up. This heat is “cheap” – you haven’t had to make it all from burning something or running electricity through a resistor, you’ve just had to scoop it up and put it somewhere else. It does take some energy to move heat from a colder place to a hotter place, but not very much if the colder place isn’t all that much colder than the hotter place. And since, in the absolute scale, a frosty morning isn’t actually much colder than ideal room temperature (something like 10%), you can move something like 3 units of heat energy for every one unit of electrical energy you use up in moving it.
Let’s establish a few principles that we’ll use later:
* When you compress a gas, it heats up. If you compress it enough, it will turn to liquid.
* If you remove the pressure, the liquid will turn back into a gas and cool off again.
* Heat always moves from hot to cold. So if you set a glass of boiling water outside, it will cool down, even if it’s really hot out side by human comfort standards.
To generate heat, the heat pump compresses a gas to its liquid state so that it gets nice and hot. We blow air across the tubing that contains the hot liquid, and that heats our homes. The liquid exits the coils cooler than when it entered.
After we’ve used it to warm the air, the tubing containing the liquid runs outside. There, we let the liquid expand so that it returns to a gas state. When this happens, it gets cold. *Really* cold. Much colder than the outside air. Because heat always moves from hot to cold, the outside air actually heats the cold coils up, putting energy back into the liquid. The now-warmer liquid goes back into the pump where it is compressed again, and the cycle repeats.
It does require energy to pump the liquid around, constantly compressing it, but the actual heat energy that warms our homes comes from the transfer of warmth from the outside air to the cold coils. As crazy as that sounds.
For cooling in the summer, we simply reverse the direction of the pump. Now the gas is compressed outside where it gets *very* hot. The air outside may be hot by human comfort standards, but it’s cool enough to cool a coil that is near boiling.
It’s easier to understand if you realize that when we refer to “cold” air, that is a relative term. It’s “cold” compared to what humans fund comfortable. But in ABSOLUTE terms, that is still pretty warm. Absolute Zero is the absence of any heat energy. Thats 0 Kelvins (or -273 * C, -460 * F). Compared to THAT, a freezing temp (0 * C, 32 * F) is still pretty hot. That means there is still a LOT of thermal energy floating around in “cold” air. What heat pump does is use a small amount of electrical energy to move a large amount of that thermal energy from the cold air into the house. It gets into tricky thermodynamics to explain how/why (explained in other comments in detail), but the short version is it uses pumps and expansion valves to make a refrigerant a lot colder than the outside air, so that it absorbs heat from it, then does some more thermodynamics to make the refrigerant ‘reject’ that heat back into the house.
But the main ELI5 part is that “cold” air still has a lot of heat left in it, and heat pumps are a way we figured out of concentrating and moving that heat energy inside the house.
Compressing a gas takes energy which is transferred to the gas from the pump. This is the simplest stage. The gas heats up and is used to heat whatever you want.
The second stage is more complex. Say you add 1000J of energy. Then 600J is transferred to the air you want to heat. But the loss in energy condenses the gas to liquid. Now you have to release the pressure to create a gas again. This loss of pressure reduces the energy, let’s say by 700J. Now the gas is colder than it started. As long as the ambient air is warmer than the gas it can start absorbing heat. In this example it can potentially absorb up to 300J virtually for free. Then the cycle continues.
Air at -40 degrees is still +273 degrees Kelvin! That’s a lot of heat that can be pulled out!
Heat pumps work just like air conditioners, (or your fridge.) The only difference is that they have a valve that allows them to push the heat in either direction. So it would be like a fridge that can make the inside either hot or cold.
Because they’re not converting energy into heat, rather using the energy to gather and move existing heat, the effect can be many times more efficient.
Just want to add a clarification to the energy efficiency discussion: “more energy efficient” does not necessarily equate to “reduced carbon emissions” unless you have a relatively clean grid mix. If you switch from natural gas to a heat pump but your grid electricity comes from mostly coal, then you’ve take a step backwards. If carbon reduction is your aim, look into your grid mix using a tool like eGrid to see if an air source heat pump makes sense.
I know the OP didn’t ask about carbon, this is more for people who might be looking into from a carbon standpoint.
> How is cold, even freezing air turned into heat?
Ever stand in front of your fridge while it was actively cooling? If it’s modern, you probably felt a slight warm breeze on your feeties? That warmth is the excess heat from your fridge and freezer getting pumped out.
The real ELI5 is when you squeeze a gas it gets hot. (See adiabatic heating)
If you expose the hot pipes to outside air or water they will cool to about that temperature.
If you later release the stored pressure, the gas gets cold.
If you allow those pipes to touch air or perhaps water they will cool what they’re touching.
A heat pump does this continuously through a restricter valve to keep one side compressed and the other in a vacuum as the pump runs.
Moving heat to where you want it is about 5-6 times more efficient than making it through electrical resistance heaters
Trying my best to give an ELI5 answer:
If you push your palm down on your leg, you feel heat where your palm is pressing. It’s similar with gasses—if you put a gas in a cylinder and you have a device that can compress the gas, you will increase the pressure and the temperature of that gas. Now if you have a little tube connected to the cylinder, that hot pressurized gas will shoot out of the cylinder through the tube. If you make the tube really long, bunch it up, attach some metal that conducts heat really well to the tube, put that bunched up coil of hot tubes in your house, and blow air over it with a fan, the air will get hot! But all you did there was “move” the heat from the hot pressurized gas to the air that you blew over it through the metal that made up that tube because the tube was hotter than the air that blew over it and hot always moves to cold. The only “energy” that you’ve actually added so far was the electricity to compress that gas and to run the fan.
But what happened to the hot pressurized gas inside the tubes when you blew the cooler air over it and made it colder? Well it lost a lot of heat energy (moved it into the air) and it actually condensed into a liquid because gas needs a lot of energy to stay a gas. Just like when moisture in the air condenses on a cold soda can. So now it’s a lower temperature liquid in the tube but it’s still a high pressure. And since you don’t want that colder liquid inside anymore, you keep it moving outside. Because you want a net gain of heat inside the house and you want to be able to keep this cycle going and going.
Now outside in the tube we have a high pressure lower temperature liquid and we need to get it back to a gas. And how do you get a liquid to become a gas? You boil it! If you take that high pressure lower temperature liquid and smash it through a little hole/choke point in the tube, you’ll have a lot of pressure on one side of the choke point and a really really low pressure on the other side of the choke point. Once the liquid gets smashed through the hole it will get broken apart just like when it comes out of a spray bottle. This is the part that seems like magic—what was high pressure and lower temperature (but still pretty hot) on one side is now a really low temperature and really low pressure on the other side. So low that if you blow outside air over the same type of tubes surrounded by conductive metal as you had inside the house and with a similar fan, heat will actually move from the outside air into the tube and boil that low temperature/low pressure liquid immediately! And the outside air actually has enough heat energy in it to eventually boil it all so that it all turns back into a gas! And the cycle can continue!
I know it’s weird to think about cold outside air “boiling” a liquid, but that’s the magic/science of refrigerants and gasses. They can boil at crazy low temperatures and they can get to crazy high temperatures when they’re compressed.
The energy that you’re adding to that system is the electricity to compress the gas, to run the inside fan, to run the outside fan, and any other little components. So you’re only “consuming” enough electricity to help that heat move around. It’s kind of like how a bicycle helps a human move really far while not expending nearly as much energy as running.
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