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?
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It’s because what’s being measured in this case is how much energy you spend in order to move a given amount of energy. Because heat pumps aren’t generating energy in he same way a heater does, they just move it. Hence the name “pump”
Like a big oil tanker truck might spend 100 gallons of gas to transport 10,000 gallons of gas (I have no idea if these numbers are accurate, just an example) you could say it then has a coefficient of performance of 100. It didn’t turn 100 gallons of gas into 10,000 gallons of gas. It just moved it where you want them.
It’s the same thing with heat pumps. You spend one 1 unit of energy to move more than 1 unit of energy then you have a COP of over 1.
>How is it that a machine (like a heat pump) can consume 1kw of power and produce an amount greater than that of heat?
Because it doesn’t “produce” the heat
Heat pumps pump heat, they don’t make it
They’re air conditioners and you can set the hot side to be inside the room. Just like an AC or a fridge, refrigerant runs around through the coils and when in heatmode it goes outside the house, gets expanded so it transitions from a liquid to a cold gas, absorbs some heat from the outside air, gets squished from a gas into a much hotter liquid, then gives off its heat to the air inside the house
The power consumed is running the compressor which is pushing the refrigerant around, but the heat coming into the house is being stolen from the air outside and moved inside and that’s not related to the power consumed by the compressor.
Coefficient of performance does not measure heat *produced*. It measures the heat which is added to the hot end.
In a regular heater this is just heat generated from throwing away energy. In a heat pump, there *is* that heat, but *also* heat *moved* by the pump.
Heat pumps work like they sound. They pump heat around. It’s not a cheat of the laws of thermodynamics. They use energy to transfer heat from one area to another. And the coefficient of performance is “how much heat is moved” over “how much energy was put in the system in order to do so”.
So let’s use a fridge as an example. As made-up round numbers, we’ll say it uses 200W, and has a coefficient of 2. So it uses 200W to remove 400W of heat from the inside of the refrigerator. What isn’t explicitly stated is that 600W of heat is ejected out of the back of the refrigerator (the 400 that came from the interior, and the 200 that came from running the heat pump).
Coefficient of performance must be greater than 1.
The formula is Th/(Th-Tc) where Th is the temperature of the hot side and Tc is the temperature of the cold side both measured in Kelvin. The only time it can be 1 is if the cold side is at absolute 0, which can’t happen in the real world.
Take a heat pump for example. If it is operating with a CoP of 5, then for every 5kWh it uses of electrical energy, 25kWh of heat are added to your home. That’s because it’s taking energy from the air outside and moving it inside, even if the outside air is cold, it still has heat. Any losses that the heatpump experiences will still be turned into heat, and will therefore still heat you home. That’s 20kWh from outside and 5kWh from the electricity.
If we compare that to and electrical space heater (100% efficient by definition) it will consume 5kWh of energy and heat your home with 5kWh of heat.
Heat pumps don’t make heat, they move it from one place to another.
A resistive heater is as close as we can get to perfect efficiency since it can convert 100% of electrical energy in to heat.
The heat pump simply gathers existing heat from one location and transports it to another. Therefore it’s COP will fluctuate depending on the amount of heat available to be moved whereas a resistive heater will crank out the same number of BTUs no matter what ambient conditions may be.
If you use a bicycle pump to inflate a tire, you’ll feel it get hot. That’s the result of the air being squeezed so much it heats up.
No additional energy has been put into the air, it’s just now all in a smaller space so feels hotter (because Physics!). If you can find a way to take that heat energy out and put it somewhere else, like into water, then that’s how an Air Source Heat Pump works.
The coldest anything can ever get is minus 273C, so even 0C air still has a LOT of heat energy in it. If it takes 1kW of electrical energy to squeeze air enough to get 4kW of that heat energy out, then that’s a COP of 4.
When you let the air expand again, it will get very cold because you’ve removed that heat, which is why there’s cold air being blown out of the ASHP fan.
So you’re not using electricity to heat anything. You’re using it so squeeze existing heat out some air.
Heat pumps use a refrigerant that is compressed and uncompressed in a certain order and pumped around in order to move heat from one place to another.
The pump that moves the refrigerant might use 1000w. But the refrigerant might hold equivalent 3000w of heat energy.
The heat already has to exist but the energy you Input is just to move it. So the coefficient is considered only the energy you put into it.
When you compress something it gets very hot. You can then use a radiator to remove that heat. Then you move the compressed thing somewhere else and expand it and it cools down and absorbs heat. Rinse and repeat.
OP – it does not produce, it MOVES. It is using electrical energy to carry heat from one side to the other.
Both heat and energy are measured in kwh, so you’re saying “I moved 3 kwh of thermal energy and this cost me 1 kwh of electrical energy”.
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