How the heat pump works
Posted: Tue Jan 25, 2022 12:23 pm
I've been reading the official VW training information for the MEB platform heat pump (thanks zice for letting me know it was now publicly available). If you want to read it, search for "ssp 881213". Now I have a far better understanding on how the heat pump system works, and I'd like to clear up some misunderstandings (which I've been guilty of too), due to the lack of any real information from Skoda/VW/Audi/Cupra on their retail websites and brochures, and also give a summary of how it works. Any mistakes, please let me know.
TL;DR: if you do mainly do journeys where a petrol/diesel engine struggles to get the cabin up to the temperature you want, a heat pump probably isn't worth it on an Enyaq. It's also most effective at temperatures around freezing, so if you don't drive much in winter then it also might not be worth it for you. For everyone else, it can reduce energy consumption to heat the car (particularly at higher speeds).
Here are some summary points:
The heat pump uses R744 as a refrigerant - this is normal CO2 gas. It's non-toxic and a natural component of the atmosphere. It is however at a very high pressure in the system (over 1000psi). There are sensors in the cabin to detect leaks. The non-heat pump cars use R1234yf, which is 2,3,3,3-Tetrafluoropropene. Extremely flammable and not nice to breathe in, but not as damaging to the environment as older refrigerants such as R134 and R12.
There is only one compressor. I previously thought there were two, but no, there's just one. What there is however is an assortment of valves, expansion valves, heat exchangers, condensers and evaporators in both the cabin and engine bay. So it's capable of both heating and cooling at the same time, by using different combinations of components. For example to dry and heat the air, it first cools intake air (to dry it) and then heats it using the heat it extracted from it whilst cooling it, plus a little more from the compressor. It's all about the flow of heat, not the temperature of fluids
In normal operation, the heat pump uses waste heat from the motor and battery to heat the cabin. Exactly the same as a petrol/diesel engined vehicle does. The electric PTC heater is only used when a sudden increase in temperature is requested, or the motor/battery is very cold. On cars without a heat pump, this heat is simply lost to the environment through the radiator.
There are a number of other modes of operation for when the motor/battery coolant circuit is cold to help get it up to temperature (and hence increase the efficiency). In normal driving the 6kW battery PTC is not used. This is only used for pre-heating for departure schedules (and possibly for scheduled charging). If anyone's interested, I can post a summary of what it does in each mode. But I suspect this is going to be a long post as it is...
For cooling it's pretty much the same as a car without a heat pump, in that it uses the condenser behind the radiator to dump heat to the environment. But it can also cool the motor/battery independently of the cabin if it needs to (for example whilst charging) or at the same time (for example when driving immediately after charging).
Most of the time when the car is heating the cabin or the battery, the radiator and condenser are blanked off with an electric roller blind cover to improve aerodynamics. It will open this as much as it needs to in order to get the temperature difference across the radiator or condenser.
On cars without a heat pump, all heating comes from the 6kW PTC heater and all cooling/drying comes from the 5kW compressor. Demisting the windscreen could theoretically take around 10kW as result, though generally it'd be less than this.
On cars with a heat pump, the PTC heater is not used once there is sufficient heat in the motor/battery coolant system. All heating and cooling comes from the 5kW compressor, with the PTC only used for short periods until the temperature of the refrigerant fluid increases to meet the demand. Demisting a stone cold car will of course use the PTC, so be the same as the non-heat pump case. But once the car has driven for a few minutes the motor and battery will be hot enough not to need it.
It's this use of waste heat that gives the saving on electricity usage. For short trips, the motor and battery won't get hot enough to provide any heat - just the same as in a petrol or diesel car. But once it's up to temperature, that waste heat is able to heat the cabin air. It's most effective when the outside air temperature is below freezing, down to a point where the heat loss through the windows is more than the heat from the motor/battery can replace. At that point it'll use the PTC to supplement the heat pump.
Hopefully that should help people understand what the system does. It's much more sophisticated than the Stellantis (ie Peugeot, Citroen, Vauxhall, Fiat etc) system, and the Hyundai/Kia system - and they make you pay for it whether you want it or not. Personally, I think it would be better if Skoda/VW/Audi/Cupra made the heat pump standard like most other companies - but gave you the option to remove it to reduce the price, rather than it appearing as a very expensive option with no real description as to whether you would benefit from it or not.
TL;DR: if you do mainly do journeys where a petrol/diesel engine struggles to get the cabin up to the temperature you want, a heat pump probably isn't worth it on an Enyaq. It's also most effective at temperatures around freezing, so if you don't drive much in winter then it also might not be worth it for you. For everyone else, it can reduce energy consumption to heat the car (particularly at higher speeds).
Here are some summary points:
The heat pump uses R744 as a refrigerant - this is normal CO2 gas. It's non-toxic and a natural component of the atmosphere. It is however at a very high pressure in the system (over 1000psi). There are sensors in the cabin to detect leaks. The non-heat pump cars use R1234yf, which is 2,3,3,3-Tetrafluoropropene. Extremely flammable and not nice to breathe in, but not as damaging to the environment as older refrigerants such as R134 and R12.
There is only one compressor. I previously thought there were two, but no, there's just one. What there is however is an assortment of valves, expansion valves, heat exchangers, condensers and evaporators in both the cabin and engine bay. So it's capable of both heating and cooling at the same time, by using different combinations of components. For example to dry and heat the air, it first cools intake air (to dry it) and then heats it using the heat it extracted from it whilst cooling it, plus a little more from the compressor. It's all about the flow of heat, not the temperature of fluids
In normal operation, the heat pump uses waste heat from the motor and battery to heat the cabin. Exactly the same as a petrol/diesel engined vehicle does. The electric PTC heater is only used when a sudden increase in temperature is requested, or the motor/battery is very cold. On cars without a heat pump, this heat is simply lost to the environment through the radiator.
There are a number of other modes of operation for when the motor/battery coolant circuit is cold to help get it up to temperature (and hence increase the efficiency). In normal driving the 6kW battery PTC is not used. This is only used for pre-heating for departure schedules (and possibly for scheduled charging). If anyone's interested, I can post a summary of what it does in each mode. But I suspect this is going to be a long post as it is...
For cooling it's pretty much the same as a car without a heat pump, in that it uses the condenser behind the radiator to dump heat to the environment. But it can also cool the motor/battery independently of the cabin if it needs to (for example whilst charging) or at the same time (for example when driving immediately after charging).
Most of the time when the car is heating the cabin or the battery, the radiator and condenser are blanked off with an electric roller blind cover to improve aerodynamics. It will open this as much as it needs to in order to get the temperature difference across the radiator or condenser.
On cars without a heat pump, all heating comes from the 6kW PTC heater and all cooling/drying comes from the 5kW compressor. Demisting the windscreen could theoretically take around 10kW as result, though generally it'd be less than this.
On cars with a heat pump, the PTC heater is not used once there is sufficient heat in the motor/battery coolant system. All heating and cooling comes from the 5kW compressor, with the PTC only used for short periods until the temperature of the refrigerant fluid increases to meet the demand. Demisting a stone cold car will of course use the PTC, so be the same as the non-heat pump case. But once the car has driven for a few minutes the motor and battery will be hot enough not to need it.
It's this use of waste heat that gives the saving on electricity usage. For short trips, the motor and battery won't get hot enough to provide any heat - just the same as in a petrol or diesel car. But once it's up to temperature, that waste heat is able to heat the cabin air. It's most effective when the outside air temperature is below freezing, down to a point where the heat loss through the windows is more than the heat from the motor/battery can replace. At that point it'll use the PTC to supplement the heat pump.
Hopefully that should help people understand what the system does. It's much more sophisticated than the Stellantis (ie Peugeot, Citroen, Vauxhall, Fiat etc) system, and the Hyundai/Kia system - and they make you pay for it whether you want it or not. Personally, I think it would be better if Skoda/VW/Audi/Cupra made the heat pump standard like most other companies - but gave you the option to remove it to reduce the price, rather than it appearing as a very expensive option with no real description as to whether you would benefit from it or not.