How is it that a machine (like a heat pump) can consume 1kw of power and produce an amount greater than that of heat? What am I misunderstanding?
In: 320
In electronics you can have amplifiers that boost total wattage by watts multiplied amps. This may not increase the total power output but may increase the amount of power being delivered for electrical output. This may be done by combining power sources or using a buck boost power converter.
it isn’t taking in energy to *produce* heat, it’s taking in energy to *move* heat.
the extra energy is coming from somewhere else. for a house, it’s the air outside. a heatpump sucks energy from the air, making it even colder than it already is, then dumps that heat energy inside where it’s warm.
air conditioning is also a heat pump. it takes heat from inside your cool house and dumps it outside in the hot weather. fighting entropy like that takes energy, but we can do it.
The coefficient of performance is for heat pumps. As the name suggests, heat pumps “pump” heat from low temperature to high temperature (against the natural tendency of the heat to flow from high to low). The COP is calculated as the heat moved divided by energy added. The energy that is used to this is work and is a high grade energy. Further, it’s only being used to move heat, not to create heat. If all the high grade energy was “converted” to heat, then yes, it could not be greater than 1. Like electricity being used to turn a filament heater. But since we are only “moving” heat from one source to another, a heat pump can have COP greater than 1.
Its not generating the heat by itself, its just yanking it from somewhere, usually from outside or in your house, and putting it somewhere else, usually outside or in your house.
It turns out that, with the right materials (refrigerant) the amount of energy it takes to pump that heat from where you don’t want to to where you do is a lot less than the heat you’re moving.
It’s just like your fridge.. the heatpump in there, takes the heat energy from your fridge, and takes it out. So the fridge stays cold, and the surrounding air in the kitchen heats up. The fridge does consume energy but generally less than the heat energy that is being moved. Heat pumps for home heating work on the same principle. It takes heat energy from an external source (outside air, or geothermal, or surface water) and transports that to inside. If it uses 1kwh electrical to transport 5kwh of heat energy the COP is 5.
A heat pump doesn’t create heat. It moves it.
It consumes energy to move heat from one area to another. It just so happens that if you get a system with lots of heat in one area it doesnt actually take much energy to move that heat somewhere else, and can look like an efficiency greater than 1.
Even when it’s cold outside, there is still lots of heat energy in the air. Although heat naturally wants to move from hot to cold, the job of the heat pump is to move the heat from cold to hot. You can either use your energy to create heat, or use it to move the heat. Moving heat is often much easier–for example, it takes very little energy to move around a pot of soup, but a lot of energy to heat it up in the first place.
refrigerant based systems are drawing energy to power a compressor. An electric heat strip is like the baseline thing they compare heat pumps to. It’s the 1 in the equation. Compressor just pushes refrigerant around in a circuit, raising and lowering the pressure in specific places to absorb heat and reject it somewhere else and the energy required to do this is sometimes significantly less than resistive heating elements.
Latent heat is an important subject to try and grasp. Any time a substance has to change state from solid to a liquid or liquid to a gas it takes a lot of energy to do so. It’s why when you have ice in liquid water the water measures 32 degrees. The mixture can’t rise above 32 because any extra heat energy is being used to melt the ice, it’s only after the ice melts that the water is free to rise in temperature again. This measurable temperature is called sensible heat and now we can understand why the other is called latent heat, it’s not measurable on a thermometer. It takes something like 140 times the energy to heat 32 degree ice into 33 degree water than it would to raise 30 degree ice to 31 degree ice. It takes over 900 times the energy to raise 212 degree water to 213 degree steam (all fahrenheit here, sorry world) than it would to take 210 degree water and raise it to 211 degree water.
Refrigerant based systems exploit this by controlling the boiling and condensing temperatures of the refrigerant by controlling pressure. We know that water boils at 212F at sea level but in higher altitudes, less pressure, the boiling point of water comes down. it’s the same with refrigerant. A hot, high pressure liquid experiencing a large pressure drop will suddenly be way above its boiling temperature and because of this it has to pick up an enormous amount of heat to change phase into a gas, that’s pretty much what’s happening in an indoor coil on an AC unit. The outdoor coil in an AC unit is getting hot, high pressure gas pushed through it and the fan helps it reject enough heat to the outside air that it condenses back down to a liquid so the process can start again.
Probably more info than anyone needs, I’m not an engineer but this is my crude understanding of it, I work with this equipment and I have to think about this stuff a lot. All this to say that moving heat with refrigerant and a compressor consumes less energy than generating heat from electricity.
It’s about what you’re measuring as input and output. It has a specific definition in the case of a heat pump, and that is the change in temperature it causes.
A resistive heater is basically 100% efficient, because the energy in the electricity is basically ALL converted into heat energy. Input = 10, output =10, therefore efficiency of 1.
A heat pump is just moving around heat that is already there. It uses a little bit of energy to do that, let’s say input of 5. And it still outputs 10, so an efficiency of *2*.
Now imagine heating a room with natural gas. Basically the only energy you’d measure as input would be the ignition, and you could just keep burning more and more gas to get more output and up your efficiency as high as you want.
You aren’t creating energy, it’s just that the inputs of all these methods don’t take into account where all the energy is coming from, just how much energy is consumed in operation. The efficiency of natural gas doesn’t care about the process of natural gas forming, but just what it takes us to move it around and ignite it.
Imagine you have a pick up truck, you drive somewhere an pick up a bunch of hot rocks. It costs you $1in gas but provides $5 of heat
Latest Answers