In order to be able to "collect" heat, the air conditioning gas in the circuit must be brought to an appropriate temperature. So there is a temperature difference between the air conditioning gas and the outside air. In order to absorb heat, you have to cool it down in a special way. You do this by reducing the pressure so that it can expand. Sounds counterproductive to what you see in nature and that's also the aim of the game.
In the graphic below, the circuit begins on the left with the fan and evaporator. The supercooled air conditioning gas is heated by the outside air on the left and then flows on towards the pump at the top. It is then compressed by it so that as much air conditioning gas as possible is in a small space, so that the temperature rises even more.
This increased temperature can then be extracted from the air conditioning gas and discharged with the fan on the right in the condenser. Since the pressures on the right and left are different and this system is based on that, you need an expansion valve at the bottom. Together with its sensor bulb, it only opens sporadically and closes the circuit so that the cooled air conditioning gas can be heated up again by the evaporator and the circuit is closed.
Air conditioning systems use the principle with the warm side outside the car and the cold side inside the car. Fridges too. In this way, the heat from the interior can be transported to the outside.
Heat pumps use the principle in reverse. Here the warm side is inside the car and the cold side is outside. They have the advantage that you can heat much more efficiently in this way than if you were to convert the electricity directly into heat. Because you extract the heat from the surrounding air and concentrate it on the desired point. So they are 2-3 times more efficient than classic electric heaters.
However, because they extract the energy from the outside air, they do not function arbitrarily in all cold weather. Commercial heat pumps reach their limits below -20 °C and you have to then switch to a conventional electric heater in order to reach the desired 22 °C. That's why even cars with a heat pump still have got a PTC heating element.
Heat pump and air conditioning combination
In electric cars it still costs extra if you want to have a heat pump. In some it is not even available at all. But all you have to do is let the pump run backwards?
Unfortunately, it's not quite that simple. For example, the pump (called air conditioning compressor) in the third generation smart electric drive only works in one direction. Rotating in the other direction, its air conditioning compressor cannot build up any pressure. To do this, the air conditioning compressor would have to be a little more complex. Also, expansion valves only work in one direction.
You pay a surcharge for the heat pump because additional directional valves are required. These then switch the circuit so that the condenser and evaporator swap roles, but the air conditioning compressor, for example, continues to rotate in the same direction. It doesn't take too much effort to do something like this, but it's not as simple as just reversing the air conditioning compressor.
Air to air, air to water and what great terms there are
If you also want to be particularly specific, then you can specify where, for example, the heat pump gets its heat from and to what it then gives it off. This then only describes the question of what the evaporator and condenser are flushed with. Whether air, water or another medium. This becomes especially interessting in houses, for example, if you want to use geothermal energy instead of outside air. Or even if you want to heat water with it.