Yar, especially since pack temperature is really pack coolant temperature or cell exterior temperature and the system needs to protect the parts of a cell at the most extreme temperature. So they need to assume worst case thermal propagation through the cells and limit charge current to what that section of the cell can support in a minimally-degrading-over-vehicle-lifetime sort of way.
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Heat pump: cabin vs. battery warming?
- Thread starter scaesare
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SageBrush
REJECT Fascism
I've read that a somewhat typical specific heat capacity for EV battery packs is in the range of 800 - 850 J/K*Kg
Your number is about in that range
I think it is ballpark correct to say that after Supercharging, the car wants to reduce pack temperature about 15C quickly (say, within 20 minutes). If the heat can be utilized or stored elsewhere -- great. If not, it gets expelled to ambient. A 15 C drop in pack temperature using 825 J/K*Kg in an 80 kWh pack (Model Y LR) calculates out to 1.8 kWh, or about 5.4 kW over 20 minutes. After that heat dump, the pack has another 10C or so to 'spend' before the next Supercharger stop. Unless ambient is **really** code, I have read that a COP of ~ 2.0 is realized, which implies another 2.4 kWh of heat into the cabin.
All told, the Heat pump/Octavalve can transfer about 4.2 kWh of pack heat into the cabin between Supercharger stops without causing other performance issues. If ~ 40 kWh are sent to the battery every charging stop, this heat scavenging approach improves range about 10% so about 1/3 - 1/2 of the EV range winter penalty has been mitigated.
Amazing engineering, but as I posted above, the driver can help by giving the car a place to dump heat.
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Editorial comment:
This is why the Model Y LR has a 330 mile EPA range, and why people conclude the EPA result "wrong." The much improved winter performance jacks up the overall rating.
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Going with COP of 2, you get 2 units of heat out for each unit expended, or 1 free (resistive is COP of 1, 0 free). So, if you want to pull 1.8kWh from the pack, you need to supply the heat pump with 1.8kWh at that 5.4 kW level.
Total energy would then be 3.6kWh *3412 BTU/kWh = 12k BTU in 20 minutes for a 36k BTU equivalent cabin output.
However, we are talking 50C or 122F, to cabin at 70 F, so the COP will be way more than 2. If the cabin had a coolant loop, the only supplied energy would be the pump. With pack at 30C, patent calls out COP of 5.
Yes and yes. And I believe that the heat pump could harvest heat from the cabin, if it's been heated by the sun on the glass roof during your drive, put it into the battery for storage, and then harvest it from the battery to heat the cabin after the sun goes down and you start driving again.
SageBrush
REJECT Fascism
My impression is that the first 1.8 kWh of pack heat removal does not use the heat pump. During that stage, the car just needs a sink for that amount. If a heat pump were to augment the heat delivery the sink would be overwhelmed.
Only an impression. I am not an engineer, let alone a thermo guy.
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I am not sure, but I think the octovalve could use the heat pump to transfer heat from the motor to the pack (or cabin). If this is the case, then the system would use both “the motor-phase-angle trick” and the heat pump at the same time. Sourcing heat from a higher temperature reservoir would enable a high COP for extra efficiency vs. heating the pack directly from the motor and/or resistive pack heater like the original Model S did.
GSP
brainhouston
Active Member
im curious if there's a way to see which mode the car is in, like with SMT or diag thermal screen like on legacy...
would be nice to know how diff modes correlate to weather conditions n such
my legacy x regen is about 20kWh at 50F, wonder whats it like for heat pump cars?
ie is limited really limited or just kinda matching older cars or some other algorithm
would be nice to know how diff modes correlate to weather conditions n such
my legacy x regen is about 20kWh at 50F, wonder whats it like for heat pump cars?
ie is limited really limited or just kinda matching older cars or some other algorithm
SageBrush
REJECT Fascism
Wild Musing (just to show I really do not know what I am talking about) ....
I imagine the holy grail of winter driving to be a case where the extra heat added to the pack during Supercharging covers the cabin heating and pre-conditioning before the next charging stop. Due to the necessity of dumping heat quickly from the pack soon after Supercharging, I keep imagining the inclusion of a high capacity heat sink, perhaps in the form of a phase change material.
This way the driver would not have to be involved in heat sink games.
I imagine the holy grail of winter driving to be a case where the extra heat added to the pack during Supercharging covers the cabin heating and pre-conditioning before the next charging stop. Due to the necessity of dumping heat quickly from the pack soon after Supercharging, I keep imagining the inclusion of a high capacity heat sink, perhaps in the form of a phase change material.
This way the driver would not have to be involved in heat sink games.
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SageBrush
REJECT Fascism
brainhouston
Active Member
slight tweaks sure but in general at least same generation packs should be in the ballpark
regen decreases with temps drop for all of them, im just curious how heat pump affects that.
SageBrush
REJECT Fascism
So far as I know, if you are heating the cabin, you are not heating the battery to improve regen. So it goes back to what I wrote you earlier.
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Seems strange that those would be mutually exclusive. I could see not exceeding heat pump capacity purely for regen for efficency... They can heat the pack and cabin simultaneously (winter supercharger preconditioning)
SageBrush
REJECT Fascism
Yeah, they call out in the patent that heating the pack just for regen can be a net loss versus leaving it to self heat (or not)...
I'm skeptical that intentionally adding mass to the car for a thermal-battery is going to be a net improvement. Especially over adding a larger battery pack, which holds electrical energy (I guess in chemical form) and thermal energy.
The thermal-battery materials would probably be heavy and/or bulky, and you're adding additional coolant loops to move the heat around. And all of this only helps you in cold weather! If it's hot out it's purely a loss (totally useless mass). I really can't see Tesla doing something that hurts efficiency for probably the majority of miles Tesla's drive and adds complexity and mass.
Consider: cold weather happens in mountains. Adding mass, even if beneficial as a thermal reservoir as you describe, can still hurt your range a lot on the uphill legs of a trip. I'm thinking about situations like driving from Denver to a couple hours up into the mountains.
What Tesla is doing today is great because heating up the battery is already necessary and they're turning a weakness into a small benefit.
The only type of mass it might make sense to add is material that stores thermal energy in the form of a phase change. But that introduces another issue: when the substance vaporizes, it expands. Where do you store the gas?
SageBrush
REJECT Fascism
How much volume/mass are you imagining ? Storage of ~ 1.5 kWh would be more than enough
I presume the PCM is not restricted in its shape, and perhaps it can also function as a mechanical barrier.
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Creates a bit of a problem actually getting the material to change phase given that solids do not flow. Heat transfer would happen eventually but would not be very efficient.
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