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
Efficiency is (what you get)/(what you put in). In this case, what you get is hotter air, and what you put in is work. There is also heat input from the cold air, but we aren’t “paying” for that and so it isn’t included in the efficiency metric.
When the heat pump is operating steadily, the energy input to the cycle matches the energy output. This means the heat out to the hot room will be equal to the heat input from the cold outdoors plus the work input from electricity. This means the efficiency fraction as defined above must be bigger than 1.
Heat is extracted from the cold side. Because this is against the natural flow of hot to cold, energy is required to get the heat pump’s cold side cold enough to be warmed by the surroundings.
The power required to run the heat pump is used to work against the natural flow of heat from the hot side to cold side.
It doesn’t take much energy to pump gasoline, and yet it powers your car to travel vast distances. That’s similar to what a heat pump does. It isn’t creating heat, just moving it around. Since it’s relatively easy to move heat, you can move a huge amount of heat around without using that much energy.
You’re thinking of _efficiency_. How much energy goes in, versus how much work (energy) gets done. Usually you lose energy to heat, noise, light, etc., hence why efficiency is always less than one.
I promise you that a heat pump isn’t defying this rule, and it’s not magic. The work that a heat pump does is to move around a fluid in a closed loop. That fluid is losing energy to friction in the piping. If you tried to measure the energy going in versus the mass flow flow and velocity of the fluid in that system, you would definitely measure an efficiency of less than 100%.
But that’s not what interests us about heat pumps, so it’s thermal efficiency is sort of meaningless. A heat pump allows that fluid to evaporate from liquid to gas and condense from gas to liquid exactly where we want. In doing so, we are able to remove heat from one place (where the evaporation /boiling occurs) and transport it to another (where the condensing occurs).
If you consider the amount of heat that you’re able to transport, that can be quite a bit more than the energy you put in to the system to operate the pump, compressor, fans and other equipment. We call that ratio a “performance factor”.
Because you are ignoring the initial heat added.
Your thermodynamics professor would tell you to draw an energy boundary.
You have some amount of heat energy: heat source, and a heat sink (destination). If you add a heat pump between them, you would add electric energy to the heat pump to move the heat from the source to the sink. If you draw your energy boundary around the heat pump, you have: electric energy going in to run the pump, heat energy going in from the heat source, and heat energy going out to the heat sink (heat energy in + electric energy in > heat energy out).
Saying the COP is greater than 1 ignores the heat energy going in. They are simply saying (electric energy in < heat energy out). It is intentionally misleading.
To use another metaphor: this is a dude standing at the top of a waterfall, peeing into it, and pretending he’s responsible for the entire waterfall.
The electricity consumed by a heat pump does not *create* heat.
It moves it.
It removes heat from atmospheric air and transfers it to the air inside your house. This is why heat pumps begin to lose there efficiency once outside temps get below a certain point, it must work much harder to remove heat from air that is already cold.
But u/bitter_mongoose, how does a heat pump move heat from *cold* air?
It’s pretty simple, actually. The “cold” outside air is nowhere near as cold as the refrigerant inside your hvac system.
In the summer, your heat pump “runs backwards” to provide the same effect to achieve air conditioning.
ELI5: You can grow the food for your lunch, or you can reach over to your friend’s desk and steal their lunch. Both lunches have a similar energy input, but stealing your friend’s lunch costs a lot less of *your* energy.
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Efficiency is useful energy output (usable heat energy) divided by energy input. When using a heat pump, the energy inputs include:
(a) the energy from the natural environment that previously created the heat, and
(b) the energy we apply (electricity) to move the heat where we want it.
A heat pump has a high efficiency (say 400%) because we’re only counting the energy inputs *we* applied. The energy from the natural environment is not a cost to us.
By excluding some of the inputs that don’t matter to us from the equation, it’s no longer really efficiency. That’s why we have other terms like coefficient of performance (COP). COP of 4 can be thought of as an efficiency of 400%.
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Some further commentary:
Heat pumps don’t create heat, unlike many of heating systems we use. They take heat from somewhere else and move it to where we want it. Heat wants to naturally flow from places of higher heat to lower heat. Using the refrigeration cycle, heat pumps allow us to move heat more effectively and in the direction we want (such as from places of lower heat to higher heat). Pressure and temperature of a substance are interconnected, by controlling these pressures we can adjust temperatures and create a system to pump heat in ways it wouldn’t flow naturally. Moving heat takes much less energy than creating heat. We get to take heat from somewhere that has already had energy sunk into it to make it.
For comparison:
1. **Electric Resistance Heating:** Examples include electric baseboards. Creates heat via the electrical resistance that occurs when running electricity through a material. This process is essentially 100% efficient as there are no losses.
2. **Combustion Furnaces:** Such as a natural gas furnace in many homes. Creates heat by combusting a fuel. These range in efficiency from 80% to 95% (questionable if you’ll reliably meet that higher end of that efficiency range though).
It can’t. It does not produce that heat. The heat pump creates a bit of waste heat, but that isn’t the point.
A heat pump can be like a fridge. Mount your fridge such thet the cold part is outside your house and the warm coils are inside. Now turn the fridge on and set it to refrigerate outside.
To be clear, this is a terrible heat pump in real life. It works well in your kitchen to pump heat out of your food and i to your kitchen, but don’t do this to heat your house.
“Explain how a heat pump works like I’m a 5-year-old:
A gnome pulls a little heat from cold outside air.
He dumps hot air inside house to heat it.
The gnome snacks on electricity to keep going.”
https://twitter.com/NicoleKelner/status/1562457925443936257/photo/1
The “gnome” is a refrigerant gas running in a loop. When it’s outside, it’s colder than the outside air. As the heat from the outside air flows into it, it begins to boil, going from a liquid to a gas. Much like when water boils, a lot of energy is going in, but the temperature isn’t changing at all, just the phase. The refrigerant is then compressed (using a pump powered by electricity) as it moves inside the house. Moving through the compressor heats up the refrigerant, and now it’s hotter than the inside air, so the heat flows from the refrigerant into the home. The refrigerant is then pumped back outside, and goes through an expansion process, cooling it back down again.
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
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