Ah, summertime, at least for those of us in the northern hemisphere. The time when thoughts turn to... refrigeration. We use it every day in refrigerators and air conditioning. But how does it work? There's really no magic, all you need is principles from your high-school physics class. Mainly that matter heats up when compressed and cools off when decompressed.

A refrigeration loop has two parts: the condenser and the evaporator. The condenser is a set of coils and usually has the compressor and blows hot air. The evaporator at a minimum is more coils but often has fans to blow air through the coils and distribute the cold. The two halves are connected by air-tight piping and is filled with some sort of refrigerant gas. It used to be Freon but these days more eco-friendly alternatives have been found.

So it all starts at the compressor. The refrigerant is in a gaseous form and is compressed to high pressure. The rise in pressure heats up the refrigerant quite a lot, so when it passes through the condenser, it's eager to shed that heat. Usually a fan moves air over these coils to help pull the heat out of the refrigerant and cools it enough that it condenses into a liquid in the pipe. So now you have a warm high-pressure refrigerant liquid.

Between the condenser and the evaporator is a pressure restrictor that keeps the refrigerant at high-pressure on the condenser side, but allows it to uncompress once it passes on and into the evaporator. When the refrigerant uncompresses, it evaporates back into a gas and absorbs a lot of heat. Or in the way we usually view it, it gets very cold. This makes the evaporator coils cold and air blown over it will be cold. The refrigerant gas then comes back to the compressor at the cycle starts all over again.

Now if this is an air conditioner, the out door unit is the condenser and has the compressor, while the indoor unit is the evaporator. In refrigerators, the condenser is at the bottom or along the back, the compressor is at the bottom and the evaporator is inside in the freezer compartment. Commercial refrigerators are the same idea, only the condenser might be in a pack on top of the refrigerated room, or even outside (making it more like an air conditioner).

See? Just physics. No magic.

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A heat pump is a reversible air conditioner. There is a switchable set of valves that can send the hot refrigerant from the compressor to either the coils inside the house or outside, and the uncompressed refrigerant to the other set of coils. Essentially, when heating your home with a heat pump, you're air conditioning the outdoors!

Now you may think, "Wait... how can I pump heat from outside to inside? The whole point is that it's so cold, I need to warm things up!" Fair point, but it's just 30 degrees outside, not 0 degrees Kelvin. It's cold to you and I but there's still heat energy out there. Once you can pull it out of the air and concentrate it indoors, it will be warmer inside.

That being said, there are still many units (often in commercial installations) that have a set of electrical heating coils to provide "emergency heat" should the need arise.