How do heat pumps work and why are the energy efficient?
Edit: the question is sufficiently answered. Thanks guys!
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Heat pumps work the same way as any air conditioners. They use atmosphere to boil a liquid into a gas, putting heat in the gas, then they compress the gas to recondense the gas into a liquid, discharging the heat in a different place.
I don’t know that there’s an eli5 way to describe how a heat pump is more efficient than traditional electric heaters that use electricity to heat coils, but the reason they’re in the news is that they’re a viable electric alternative to standard natural gas furnaces, which means that they can be run on power from solar, wind, and other renewables, where natural gas furnaces will always be run on fossil fuels, so they’re the best carbon neutral way to heat a home (at least potentially, depending on how electricity is generated).
A heat pump moves the heat that exists in a large area outside into a small area inside. It’s efficient because it’s not using energy to generate heat, just to move that existing heat.
Heat pumps are not energy effecient, just like any pump they require an input of energy.
There are several types of heat pumps, but an air conditioner is an example of a heat pump most people are familiar with. The purpose of a heat pump is to move heat against the concentration gradient. This means taking heat from your house and pumping it outside, even though the outside is hotter than your house. This is accomplished by using a condenser, which takes a gas and compresses it into a liquid (in this case its a special chemical called a coolant). After the coolant is liquified, it is allowed to expand. The expansion of a gas results in its temperature dropping significantly, so now we have cold coolant. The coolant is pushed through a long thin tube, and air from your house is blown across the tube. The warmer house air gives heat to the coolant, and the house air is cooled off. Repeat the cycle, and you have a heat pump!
Heatpumps do not work the same as other heating units. They cool the same way but they heat by concentrating existing heat (energy) in the air and pumping it into your home. This is why you need a secondary heat furnace or strip when the temp gets below 40f or so bc it cannot concentrate the heat enough to warm the house to above 65f or so at that point.
You know those canned air bottles used for dusting? If you’ve ever used one you’ll know that at first the air comes out really fast, but then it drops off. Eventually the can gets really cold. When you set it aside you can hear a fizzing coming from it. What’s happening is that the liquid inside is under pressure and would really like to expand and be a gas. Letting it spray out allows more liquid to convert to a gas state, but in doing so heat leaves the liquid with the gas. The fizzing when you set it aside is the cold liquid inside boiling as it is heated by the air in the room. A heat pump basically works like that but in a closed loop with a compressor to re-pressurize the gas back into a liquid. Heat is carried from one side of this loop to the other. Refrigerators and AC units work in exactly the same way as a heat pump, the term “heat pump” generally refers to a unit that can pump heat in both directions, not just one.
A thermodynamic heat pump is something that moves thermal energy from one place to another. Moving energy does not necessarily have any relationship to how much energy you are moving. For example, if I carry a gallon of oil from one place to another, I moved a tremendous amount of energy, but I didn’t have to expend a particularly large amount of energy to do so.
So, thermodynamic heat pumps, which include both cooling and heating systems, can move much more energy around than the electrical energy you are using to power the system. The only real limit are the laws of thermodynamics – you aren’t allowed to create such a large difference in temperatures that you could use it to eventually extract more work than you put in, or where you would decrease the entropy of the overall pump+room+environment system.
[The wiki](https://en.wikipedia.org/wiki/Coefficient_of_performance#Theoretical_performance_limits) has some numbers – given temperature differences found in the real world, you can expect to be able to move around 2-10x as much thermal energy as the electrical energy you’re expending on doing so, with the average of practically build able systems really being around 2-5.
The colloquial term “heat pump” refers to the product that is used to heat your home. This is a special instance of a thermodynamic heat pump, which also includes AC systems, fridges – anything used to make one place hotter and another colder. It is more efficient than a normal electric heater because a normal electric heater that is using 1000 watts heating your room is only ever putting 1000 watts into your room. A heat pump can use those 1000 watts to make a flow of several thousands of watts from the outside into your room. It almost looks like cheating, since you get over 100% efficiency – but of course the place we’re getting that energy is completely different, so conservation of energy is always respected.
Traditional electric/ gas heaters only convert one type of energy into another. For example traditional electric heaters convert electrical energy into heat energy. This process is limited because if you put in X amount of electrical energy then you can only get out X heat energy. Heat pumps on the other hand uses electrical energy to move energy around instead of simply converting it from one form to another. So if you want it warmer inside then it just moves energy from the outside inside (or vice versa for cooling). Moving energy is not restricted in the same way as converting energy types so you can reasonably put in X energy and actually move 3-4X energy, thus meaning that you are getting 3-4 times as much energy out as you are putting in. Because of this we tend to call heat pumps mor efficient, but remember we are only moving the energy, not creating/ destroying it.
A heat pump is a device that pumps heat. It takes heat from one place, and transports it to another.
Heat pumps can be used both to warm up a place (sourcing the heat from somewhere else, e.g. outside, and pumping it **into** the space you wish to warm), or to cool it down (pumping heat **out of** the space you wish to cool and dumping it somewhere else, e.g. outside).
The reason why we call them energy efficient is that they can move more heat than they consume, i.e. a heat pump that consumes 100W of power could move up to 400-500W of heat from one place to another.
This means if you use a heat pump to warm up an area, it is up to 4-5 times as efficient as electric heating (since a 100W electric heater will only generate 100W of heat, but as I said a 100W HP could pump up to 400-500W of heat into the room).
The way the most common type of heat pump works is based of the refrigeration cycle. This is used in almost any type of device that cools something from AC units to Fridges and Freezers. It is based of the physical principle of latent heat. Latent heat is a principle that means a material can not physically pass through its phase transition temperature before having fully transitioned. To phrase it less sciency, it means that a liquid can never be colder than its freezing point, trying to cool it more/faster will just make it freeze faster, not get colder. In the same vain it can never be heared above its boiling point, no matter how much you heat it, it will just boil faster instead of getting hotter.
On top of latent heat the refrigeration cycle makes use of the pressure dependancy of boiling points. If you increase the pressure of something, it increases that substances boiling temperature.
Using these two properties we can now create a heat pump. The first step is to get some room temperature gas and run it through a compressor, and get it to a really really high pressure. When we do that, the gas will get quite hot (because that’s what gasses do when they’re compressed). Now because we increased the pressure of the gas so much, the boiling temperature also increased. In fact the boiling temperature is now higher than ambient temperature. So we take our really hot gas and pump it through a condenser. This is a heat exchanger that uses ambient air to cool our gas down. When it cools it will drop reach its boiling temperature and start turning into a liquid. Doing so releases a lot of energy (called the latent heat) and makes the condenser quite hot.
We know take this liquid and pump it through some type of limiter, e.g. a short very narrow tube. The limiter causes pressure on the entry side to stay very high whilst pressure on the exit side drops quite low. Because of this decrease in pressure the boiling temperature of our liquid drops as well. In fact it drops back down to well below ambient temperature. As you will recall, a liquid cannot physically maintain a temperature higher than it’s boiling point because of latent heat. This means the liquid extremely rapidly cools down to its boiling and starts boiling away. We pump this boiling liquid through another heat exchanger, the evaporator. Just like condensing releases a lot of energy, boiling consumes a lot of energy. That energy is taken from the ambient air. This makes the air around the evaporator go very cold.
Now we just feed the end of the evaporator into the input of our compressor to close the loop, and et voilá we have a heat pump.
Now we simply add some fans on top of the two heat exchangers to allow them to work nore efficiently and stick the hot condensor inside for heating or outside for cooling, and the cold evaporator vice versa
Heat pumps work just like an air conditioner or refrigerator. They move heat from one place to another. It’s typically much more efficient to move heat around, rather than create it.
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