[Nycturne]

Frost build up in the evaporator is still a problem and no refrigerant can beat Sadi Carnot.

Agreed that frost can hurt efficiency, but ultimately the goal is to outdo electric heaters and other refrigerants. From the research I linked:

Under −20℃ ambient temperature with external air circulation, the maximum heating capacity is 7502 W with COP of 2.16.

COP of 2 at -20C is quite acceptable. And still using half the energy of an electric heater for the same output.

Not sure what you are getting at with the reference to Carnot. Heat pumps are not heat engines in the Carnot sense, and the principle behind them isn’t violating thermodynamics. No more than cooling systems built the same way.

There is more energy in “freezing” air than one would think, and the triple point of CO2 gives a couple options for moving that energy without excessive energy use.

 

[jpm8766]

COP of 2 at -20C is quite acceptable.

COP of 1 at ultra cold temperatures is perfectly acceptable to me as well, but at those temperatures, a completely frozen evaporator would stop the heat transfer entirely, no? I guess you're saying that R744 wouldn't suffer those consequences because it is CO2 based and a properly balanced system should be able to provide a larger efficient operating range without potentially freezing.

What I think I am getting out of this is the following lesson: "You might be able to look at performance of vehicles without heat pump in cold weather to get an idea of what range loss at extremely low temperatures and a heat pump could look like."

I'm really looking forward to the conclusion of Bjorn's ID.Buzz test in the arctic to see some more numbers that are easier to digest than thermodynamic intuition.

 

[Atlant]

Not sure what you are getting at with the reference to Carnot. Heat pumps are not heat engines in the Carnot sense

Of course heat pumps are heat engines, they're just running in reverse. And I never claimed they violated thermodynamics, I was just (obliquely) making that point that pumping heat from a colder reservoir to a warmer reservoir takes more and more energy as the cold reservoir gets colder, just as Carnot predicts.

 

[Atlant]

COP of 1 at ultra cold temperatures is perfectly acceptable to me as well, but at those temperatures, a completely frozen evaporator would stop the heat transfer entirely, no?

Right! That's why heat pumps operating in the regime where their evaporators go below 32°F/0°C MUST have some way of defrosting their evaporator coils (whether its an external electric heater or briefly running in cooling mode or what-have-you). As you say, once the evaporator coil is full of frost, it's not going to acquire much heat from the air that can no longer pass through it.

 

[Nycturne]

Of course heat pumps are heat engines, they're just running in reverse.

Fair enough.

And I never claimed they violated thermodynamics, I was just (obliquely) making that point that pumping heat from a colder reservoir to a warmer reservoir takes more and more energy as the cold reservoir gets colder, just as Carnot predicts.

And I don’t think anyone has disputed the reality of the bolded bit. It’s precisely because of that reality that using different refrigerants provides benefits in the first place. Ones with better heat capacity to maintain a useful COP in a wider temperature range for example.

COP of 1 at ultra cold temperatures is perfectly acceptable to me as well, but at those temperatures, a completely frozen evaporator would stop the heat transfer entirely, no? I guess you're saying that R744 wouldn't suffer those consequences because it is CO2 based and a properly balanced system should be able to provide a larger efficient operating range without potentially freezing.

If we just want a COP of 1, electric heaters get us there. But it means dealing with the range drops in winter we see today when you use the heater. So being able to use a heat pump in these conditions means saving power, and improving winter range of EVs. It’s not so much that R744 doesn’t suffer the consequences, but rather it’s able to maintain a useful COP in worse conditions than the refrigerants we’ve been using in residential applications for a while. So if you want a heat pump that works in sub zero C temperatures, using the right refrigerant helps. The cost is that R744, being CO2, requires high pressures to operate, so it can be a liquid rather than shifting directly between solid and gas. So it’s not a drop-in replacement for other refrigerants, you need to use a heat pump built for those higher pressures.

Frost that is colder than the air around it will absorb energy from the air around it, and can have energy extracted from it, but does provide resistance to transfer of energy into the refrigerant loop. Consider a freezer, it doesn’t stop working when the evaporator gets frosted over, but it can result in more power draw to maintain temperatures in the freezer, and in some cases damage could result by running things too hard for too long. I do wish there was better data for the public when it comes to COP vs temp/frost. I’m honestly having a heck of a time finding a good source, and sometimes a simple graph makes a concept much easier to understand.

Defrosting for EV heat pumps is an active area of study on how we can improve the defrost cycles. It’s being suggested that how we trigger defrost for heat pumps in other applications isn’t necessarily the right call for EV heat pumps. I’m curious to see some of this get applied in the future:

In summary, the defrost initiation criteria of the conventional com-
mercial HPs are entirely unsuitable for EVHPs, and it is imminent to find
a new efficient and accurate defrost initiation criterion.

 

[ChargedVW]

Yes, the PTC cabin heater pulls 5 kW:

Here's part 2 that he posted about 8 hours ago

 

[VW TECHNICIAN]

There is clever way for heat pump how to deal with frost up...but I'm not in power to make statement....no need for outside electric heater on the evaporator ( condenser in heat cycle)
Depending on humidity and outside temperature....( PTC heater may have same efficiency in extreme situations there) but on average heat pump will be more efficient and it is integrated more efficiently with cooling loops.
Canadian fellas can provide more insight on this matter. Especially on 2023 models.

 

[cosev]

According to Byorn, the heating system was pulling 5kW. That's a lot

The Chevy bolt I used to own pulled 7kW. It was pretty exciting to watch the gom drop when you turned it on…

 

[d287]

What probably DOES help, at least a little, is to recycle the waste heat from the drive into the passenger compartment (assuming that there's any to spare after heating the battery). But the ID.4 doesn't do that, either.

Where did you get that idea? If you look at page 43 of the High Voltage SSP, there is a heat exchanger for the HVAC condenser that does exactly that… transfer heat from the coolant loop into the cabin.

 

[ChargedVW]

Here's part 2 that he posted about 8 hours ago

Part 3 is up now and I'm sure there will be many others after this.

 

[VW TECHNICIAN]

When you're down below freezing, I'm not sure a heat pump helps.

What probably DOES help, at least a little, is to recycle the waste heat from the drive into the passenger compartment (assuming that there's any to spare after heating the battery). But the ID.4 doesn't do that, either.

Before spreading wrong information, i would advise you to take look at pdf files posted on this forum...there are schematics for all different types of heating and heat pumps.

 

[Atlant]

Several folks have told me that I'm wrong about waste heat from the electrical system not being able to be discharged into the passenger cabin of the ID.4.

I do not for a moment claim that I am never wrong, but I guess I do need further explanation.

Here's the diagram of the non-heat-pumped ID.4's battery and electric drive cooling system from MC-10186407-0001.pdf, the “Self Study Program 811213 The High-Voltage System in the ID.4”:

The only question is what is component 6, the “Heat Exchanger for Heat Condenser”? To what does that exchange heat?

Meanwhile, please note that there doesn't seem to be any direct path for waste heat from the drive electronics to reach the cabin air; the waste could warm the battery and any remaining heat in the battery exhaust coolant that isn't routed right back to the drive electronics could reach the mysterious component 6, but that's a pretty indirect path.

 

[VW TECHNICIAN]

Several folks have told me that I'm wrong about waste heat from the electrical system not being able to be discharged into the passenger cabin of the ID.4.

I do not for a moment claim that I am never wrong, but I guess I do need further explanation.

Here's the diagram of the non-heat-pumped ID.4's battery and electric drive cooling system from MC-10186407-0001.pdf, the “Self Study Program 811213 The High-Voltage System in the ID.4”:

View attachment 18970

The only question is what is component 6, the “Heat Exchanger for Heat Condenser”? To what does that exchange heat?

Meanwhile, please note that there doesn't seem to be any direct path for waste heat from the drive electronics to reach the cabin air; the waste could warm the battery and any remaining heat in the battery exhaust coolant that isn't routed right back to the drive electronics could reach the mysterious component 6, but that's a pretty indirect path.

If equipped with heat pump it will use #6 heat exchanger block for heat harvest from battery pack to assist in cabin heating.

 

[Atlant]

If equipped with heat pump it will use #6 heat exchanger block for heat harvest from battery pack to assist in cabin heating.

Thank you for that clarification — That's EXACTLY the situation that I expected.

 

[VW TECHNICIAN]

Thank you for that clarification — That's EXACTLY the situation that I expected.

#6 is heat exchanger block .....refrigerant to coolant ( heating or cooling if necessary)

 

[Atlant]

#6 is heat exchanger block .....refrigerant to coolant ( heating or cooling if necessary)

Just to get this completely explicit:

So in a North American car without a heat pump, which portion of the Air Conditioning refrigerant loop is this heat exchanger dumping its heat into? (I'd assume a cool/cold part of the refrigerant loop?)

 

[SunWizard]

So in a North American car without a heat pump, which portion of the Air Conditioning refrigerant loop is this heat exchanger dumping its heat into?

It dumps any waste heat into the battery, nothing in AC loop. There is not much heat because the car is so efficient, so it rarely warms the battery above 50F if its <32F ambient. At a coolant temperature of 59°F (15°C) or higher the thermostat opens, allowing battery coolant to flow to the radiator (passive cooling). It starts using the AC loop to cool the battery at 86°F (during charging) or 95F when driving.

 

[VW TECHNICIAN]

Just to get this completely explicit:

So in a North American car without a heat pump, which portion of the Air Conditioning refrigerant loop is this heat exchanger dumping its heat into? (I'd assume a cool/cold part of the refrigerant loop?)

No heat pump...heat exchanger is used only when air conditioning is called for cooling HV battery pack....heat would be dumped to the refrigerant.

 

[Atlant]

It dumps any waste heat into the battery, nothing in AC loop. There is not much heat because the car is so efficient, so it rarely warms the battery above 50F if its <32F ambient. At a coolant temperature of 59°F (15°C) or higher the thermostat opens, allowing battery coolant to flow to the radiator (passive cooling). It starts using the AC loop to cool the battery at 86°F (during charging) or 95F when driving.

The picture doesn't show that, though, does it? It shows hot fluid entering this heat exchanger and cool fluid leaving it. (It also ignores the fact that the PTC Heater heats the fluid, but showing heating/cooling on the same diagram was probably viewed as too confusing).

BTW, my Volt has the exact same heat exchanger that @VW TECHNICIAN is describing and it's for exactly the same purpose: cooling the battery (and the drive electronics? I can't recall.) cooling loop in the summertime when its hard to reject heat to the ambient air via the radiator.

 

[SunWizard]

The picture doesn't show that, though, does it? It shows hot fluid entering this heat exchanger and cool fluid leaving it.

That is because that loop is only used for cooling the battery, as I described. Those temps and description are all from the NHTSA doc.