How does a heat pump work and why does it need so little range?

Why a heat pump?


Normal PTC elements are usually used in electric cars to heat the interior. These work like the hair dryer and convert 100% of the electricity used into heat. Although these heating elements are in fact already completely loss-free, heat pumps can bring even more heat into the interior with the same amount of energy used.

What at first sounds like a perpetual motion machine is of course not one. A heat pump, like an air conditioner, is a heat exchanger. It collects heat at one point and moves it to another point in a concentrated manner. In the case of a heat pump, it collects the heat from the outside air and moves it into the interior. In terms of content, this is practically a backwards running air conditioner. This is why a heat pump actually has as little of an impact on range as an air conditioner. This is exactly why I always recommend the heat pump if it is available with an electric car.



How does a heat pump work?


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 climate 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.

Wärmetauscher EN


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.


Nevertheless, every vehicle with a heat pump still has at least one PTC heating element:

In order to be able to get heat from the surrounding air, heat must of course still be available. This works much longer than you think, heat pumps work at temperatures as low as - 20 °C. But at some point, of course, it's over. For such cases, a PTC heating element is installed, which can heat up. In Central Europe and large parts of the world it is almost never needed. But on the few occasions that it does, it's a very good thing to have.

In addition, another PTC heating element in the cooling circuit is often used to heat up the HV battery. It is very compact and absolutely sufficient for the purpose of battery heating.



Why does the heat pump cost when only the air conditioner runs backwards?


Because from a technical point of view it is still more complex than simply letting the air conditioning run backwards. Because the pump (called air conditioning compressor) in the smart electric drive of the third generation 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.


Offener Klimakompressor


You pay a surcharge for the heat pump because additional directional control valves are required. These then switch the cycle so that the condenser and evaporator swap roles, but the air conditioning compressor, for example, can continue to turn in the same direction. Therefore, it usually costs a small surcharge.