How do heat pumps work and why are the energy efficient?
Edit: the question is sufficiently answered. Thanks guys!
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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
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