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Comment by dilippkumar | Hacker News Reader

> The heat pump has a coefficient of performance (COP) of three to four, which means that it produces 3 kW to 4 kW of heat for each kilowatt of power it consumes.

This is setting off some massive BS alarms in my head. Why is this not violating conservation of energy?

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This is a common headscratcher in the heat pump space. They're taking advantage of a thermal gradient to get more out than they put in. Very much like driving downhill, as I understand it. If you put an air conditioner into a closed system, you can't beat Carnot efficiency. But if you're allowed to dump/absorb heat to/from the environment, the coefficient of performance measures the heating/cooling on the control side versus energy input -- which is allowed to go over unity.

https://en.m.wikipedia.org/wiki/Coefficient_of_performance

(I'm no authority here -- I only bought a heat pump and had the same reaction as you)

Heat pumps do not generate heat (well, only as a side effect of moving parts). Heat pumps compress gas. By using a compressor, they cycle gas between a low pressure chamber and a high pressure chamber. The low pressure chamber is outside, absorbing heat from the surrounding air. It is pumped into the high pressure chamber using the compressor, turning a large volume of cool gas into a small volume of hot gas (conserving energy). This hot gas warms your water or air. The now cooler, but still high pressure gas is leaked into the low pressure chamber, where it expands turning a small volume of warm gas into a large volume of cold gas (conserving energy), and the cycle continues. This is how you spend 100 joules of electrical energy to move 300 joules of heat energy from outside to inside and 300% heating efficiency.

The thermo-acoustic ones are using sound waves instead of a compressor, but the same principle.

It’s cheating! The extra energy comes from the environment. It’s much easier to steal the heat energy out of the outside air (or out of the ground in colder climates, because it stays warmer) than to create the heat yourself.

Producing heat can only be COP=1 if it’s 100% efficient, but you can use the same amount of energy to move three, four, even five times as much heat to or from the outside (moving heat the opposite direction is the way air conditioners work).

This just means that you are moving heat energy from one resovoir to another. The total amount of energy the system releases is the energy moved AND the energy used to move that energy. COP is calculated by dividing the amount of heat moved by the energy used to move that heat. In short no violation.

The wording is bad. It’s moved the equivalent of that energy.

Technology connections did a good video on this.

https://m.youtube.com/watch?v=7J52mDjZzto

What a heat pump does is move heat from outside your house to inside your house (hence the name). You can do this for free if the outside of the house is warmer than the inside (just have a thermal conducter). If the outside is colder though, it costs energy to move the energy - you're pushing the heat "up" the temperature gradient.

[Of course, this is a heat pump used in the 'heating your house' use case - heat pumps are more general. Fridges and air conditioners are also heat pumps that use energy to move heat from a cold place (fridge/room) and release it in a warmer place (the outside)]

Heat pumps do not generate any heat at all instead as the name applies they move (pump) heat.

It just happens that physics allow you to move 3 or 4 kW (maybe even more in future) of heat energy from outside air inside with 1kW of energy and thus the COP. And if you run the loop in reverse you get what is traditionally called air conditioning.

How traditional heat pumps do this is with a refrigeration cycle just like your fridge or freezer does basically by compressing and decompressing a gas.

Heat pumps rearrange existing heat in the environment, they don’t release it chemically by burning which is inherently <100% efficient.

What you actually care about is entropy for these systems. Conservation of energy doesn't care whether the energy is inside your house, or outside your house.

0 = change in energy inside + change in energy outside + added energy

This is not really a downvote-worthy post, I upvoted it. You are right to sound your BS alarm, but as others have described, heat pumps are 100%+ efficient by cheating. So, not really, but also, yeah they are.

If there was a massive alarm going off in my head and I was already at my computer, I'd probably take 2 mins to Google "how does a heat pump work". But I appreciate not everyone thinks like that.

Heatpumps are an extremely widely deployed technology by now (I have one in my house, so have everyone I know); this is an article about inventions in heat pump space, and the post is sceptical about the concept of heat pumps.

Kind of like a post about a new kind of airplane getting a comment saying that the concept of flying makes their BS alarm go off. Yes, flying is a bit "magic" and seems to defy the laws of gravity....

It's a misunderstanding of energy conservation. It's not cheating if you count all the energy in the system.

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