We recently had our heater changed to a heat pump and whilst producing the same amount of heat, it’s also very energy efficient, using a good chunk less energy than our previous heater.
How do they work and how are they so damn efficient?
It’s basically because they don’t directly convert electricity into heat, they just pull it in from outside. It still takes energy to ‘push the heat uphill’ from a cold place to a hot place, but less than directly heating.
To take my previous comment to the next level, have you ever put your finger over the end of an old-school bicycle pump and tried to push it in? If you have, you’ll know that the pump gets hot. The reason is that you are not only compressing air, you are compressing the heat that the air has. This raises the temperature and we can use this phenomenon to move heat around.
Imagine you were outside where it was cool and you extended the bicycle pump and blocked the end permanently.
Then you went inside hour house and compressed the pump. The air (and heat) in the pump would be compressed into a smaller space, so it the air temperature of the air in the pump would increase. If you compressed it a bit, the air in the pump might go up to be the same temperature as the air inside your house. If you compressed it even more, it would get hotter than the air inside the house. If you then held it there, over time, the heat inside the pump would transfer through the wall of the pump to the air in your house. This would cause the air inside your house to warm up and the air inside the pump to cool down until they are the same temperature. In doing this you have taken the heat in the air outside, and released it inside.
If you then went back outside and allowed the pump to extend again, then the air would decompress and, because the heat previously left the air in the pump (when it was inside), it would get quite cold. Colder than the air outside. If you then waited again, the air in the pump would gradually warm up, drawing heat from the air from outside of your house. This happens because even though the air outside is cool, it’s still warmer than the air in the pump.
Rinse and repeat. An air conditioner on heating mode, or a heat pump basically work in the same way. However, rather than using a bicycle pump, they have fluid running in a loop from inside to outside and back inside. The evaporator (outside the house) collects heat by allowing the fluid to ‘expand’ and cool below the ambient temperature outside. The condensor (inside the house) releases the heat by allowing the fluid to ‘contract’ and heat above the ambient temperature inside.
In this way, no heat is directly created from electricity. It is just moved from outside to inside. Believe it or not, this takes less energy than converting electricity into heat directly.
I’ve deliberately not talked about phase change here to keep it simple, that doesn’t change the basic idea behind it.
Wow, that’s amazing. Thanks for such a great explanation!!
I’d never thought of the cooling pipe used in CPUs before, but it’s really no different
heat pipes run on evaporation/condensation, i’ve explained this in some more detail
This would be a great post for an “explain it like I run Reddit” instance.
When you compress a fluid, it gets hot. When you decompress a fluid it gets cold. The energy required to do this is pulled from or given to the surrounding environment.
If you make it so the compression and decompression takes place in isolated environments, such as outside and and insulated house, then you can move heat outside or inside as needed. Hence why it’s called a heat pump.
Basically you are storing heat in a compressed fluid and moving it outside the box you’re trying to cool, or moving heat from outside to inside the box you’re trying to heat.
Technically all air conditioners are heatpumps, but they just work in one direction, where as a proper heat pump can work in either direction.
It’s like mopping up water with a sponge, and wringing it out into a bucket.
Gas molecules bounce around in the space they’re in - and the more heat energy they have, the faster they go.
The more you compress the gas, the more often they hit the walls - like shrinking the room down on a bunch of angry bees: you’re going to be hearing a bunch more thudding noises per second.
And that rate-of-collisions is what we call temperature.
When you bring two things into contact, it’s the temperature difference, not the heat difference, that determines which way the energy difference evens out.
So if you squeeze a gas, its temperature rises, and the heat energy leaks out through the sides of the container it’s in, until the temperature drops down to match the surroundings. The molecules get slower and slower, until the rate of collision on the inside matches the rate on the outside.
Lots of slow bees in a small room, smaller number of fast bees outside, the rate of thudding noises on either side of the wall will be the same.
If you now decompress that gas, let it out into the original-sized space it was in before - the now slow-moving molecules collide with the walls a whole lot less often than they did before; their temperature is a lot lower.
That low-temperature gas is now really eager to soak up any heat energy that’s going.
Pump the gas over here, squeeze it down, heat gets dumped out of it.
Pump the gas over there, un-squeeze it, suck heat back up into it.
Rinse and repeat. Sponge and bucket.
It’s more efficient than a heater, for the same reason a bicycle courier can bring you a gigantic feast in 23 minutes: they didn’t have to make it, they just carried it. The heat energy isn’t coming from the power lines, it’s coming from the atmosphere. All the electricity is doing is moving it.
imagine a container with no gases inside. let’s introduce some liquid here - it will evaporate until some kind of equilibrium is reached. pressure of vapor achieved is called vapor pressure and is strongly dependent on temperature. if pressure of gas above liquid is equal to vapor pressure, liquid is boiling.
you can make any liquid boil by decreasing pressure or increasing temperature, and conversely you can any gas condense by decreasing temperature or increasing pressure. if you look at a phase diagram there are regions where liquid or gas is stable. if you have liquid in “gas stable” area, it will evaporate, and if it’s the other way around, it will condense.
let’s cool down top surface of that container. vapor will condense on it, lowering pressure, making liquid on bottom boil, which decreases its temperature until it can’t boil at a given pressure. more vapor condenses and eventually bottom part reaches temperature of our cold source. this device is called thermosiphon. this way we transferred heat from bottom to top of container. if you make it so that liquid covers entire surface of container, for example by use of capillary forces, this device can transfer heat from any orientation to any other. this is called a heat pipe.
if you want to transfer heat from colder to hotter area, you need to make liquid boil at lower pressure and condense at higher pressure. you need to put some energy into that and this device is called a heat pump. notice that the lower the temperature difference is, the lower pressure ratio becomes and the more efficient entire thing is. the trick is, at low temperature differences amount of energy needed to run the pump is small compared to amount of heat moved around. there’s hard physical limit on this - it’s inverse of maximum (carnot) heat engine efficiency at these conditions, it can easily reach 3x-4x in practical implementation.
you can even reverse this and extract useful energy from temperature difference. this is called heat engine and about all (non-hydro, non-wind and non-photovoltaic) electricity generation runs on it. you can even treat these renewable energy sources as a heat engines, in some cosmic sense - taking heat source and heat sink as temperature of sun and temperature of photovoltaic panel, for example (with additional limitations)
A heat pump is basically a reverse air conditioner and works on the same principles. Same principles as a refrigerator too. Really such devices should be built to do both things more often.
This video goes into how they work.
Came here and was hoping to see this - this is THE video to watch on the topic.
That channel is the channel to watch on a lot of things in the realm of “how does this tech work” but not software or computers.
Great for learning about stuff you didn’t realize you’d want to know about or just background noise. The man made a video about the color brown and I enjoyed it.
Aside from the more technical details about the refrigeration cycle, a heat pump is effectively an air conditioner run backwards.
When you run an AC unit, the coil in your air conditioner/furnace absorbs heat from the air stream and transfers it to the refrigerant (through phase change)…the refrigerant then goes to the condensing unit, where it condenses out of gas, expelling the heat absorbed, which is then radiated to the outside air.
With a heat pump, you’re taking heat from outside (yes, even when it’s cool or cold, there is still heat in the air outside), transferring it to the refrigerant, and then the heat is radiated inside the coil in the fan airstream…so you take heat from outside and bring it inside.
This is also why heat pumps lose efficiency when things get REALLY cold. It becomes much harder to extract heat from outside when it is extremely cold outside, plus the need for heat inside is much higher, and so the heat pump can struggle to keep up. This is also why a lot of heat pump systems in very cold climates use supplemental electric heat (or gas heat) to compensate for the drop in heating ability.
Came here to point to this
I’ve been hooked on his channel for 2 years now, and I watch every video he puts out. Always informational, and entertaining to boot.
This video is a great explanation. https://youtu.be/7J52mDjZzto
Moving heat is cheaper than creating heat.
An air conditioner runs a motor to run a compressor which pushes a fluid through a set of pipes moving heat from a cold spot to a hot spot. A heat pump in heating mode does the same thing, except the cold spot is outdoors.
The amount of heat moved to the hot spot is equal to the amount of heat moved to the cold spot plus the work done (heat created).
Imagine a terrible, broken-down heat pump that just barely works. The amount of heat moved from the cold environment to the hot environment is near zero. The amount of heat moved from the hot environment to the cold one is near zero. The main source of heat in the whole system is the pump which just gets hot because it’s doing work pumping a fluid. An electrical heating system is basically just that. It’s a heat pump moving zero heat, instead of using a motor, it just uses a resistive electric wire. A motor doing zero useful work that just gets hot is essentially an electric heater. If you can get that motor to do any useful heat-pump work at all, of course it’s going to increase the efficiency of the system.
The only drawback of heat pumps is that you need all the fiddly bits to move heat from A to B. An electric heater is easy because it just heats up the area where it is located.
