Jim R
Member
Wouldn't that be friction? Drag would be air resistance when in motion. Aircraft drag comes to mind.
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Real World Heat Pump Effect
- Thread starter badboy1980
- Start date
Sure, the thing I'm juggling in my head is whether it is more efficient to have a lower delta T and no defrost cycles, or a higher difference with occasional defrosts. Partly depends on current environment/ relative humidity. Ice sublimes, so it could operate below freezing without icing up.
At the same time, energy spent defrosting is wasted, so it would be trading off defrost cycles for more steady state compressor based inefficiency (lossy motor waveforms) heating.
Regarding a defrost system, the Y does not seems to have the same louver system as the S./X (at least it is not named that in the parts catalog). The radiator is sort of tucked up out of the air stream, so it may only get flow via the fan.
From the Tesla patent, they mention using the pack as a thermal energy sink due to high thermal mass. They also discuss prewarming the pack via the heat pump, then using that to heat the cabin. Along with the thermal mass of the drive unit(s), the initial heat up may not rely greatly on ambient air.
Great discussion!
Sublimation between water and ice? That happens at 0*C at 1 atmosphere.
Can't change that. Defrosting energy is lost, that is inevitable. Much more is lost if
one opens a door of the cabin in cold weather
Maybe there is something to play with if ambient is just a degree over 0*C. But in that case
it's super easy to defrost heat exchanger. Just stop the energy extraction and heat exchanger
will melt with ambient airflow.
If ambient is -0*C or less, whatever the rate, humidity will freeze on the surfaces.
Actually, maybe amount of frost will be equal to energy extracted. Not sure, might be that way.
Using mass is a good idea. Especially for extremely rapid cabin heatup. But can't do that for
any longer. Tesla already has "cold sensitive" chemistry mix compared to some others.
It can't receive ANY charge (regen) below zero (metric). And battery is close to that in cold
weather when icing happens on heat exchanger.
Can't change that. Defrosting energy is lost, that is inevitable. Much more is lost if
one opens a door of the cabin in cold weather
Maybe there is something to play with if ambient is just a degree over 0*C. But in that case
it's super easy to defrost heat exchanger. Just stop the energy extraction and heat exchanger
will melt with ambient airflow.
If ambient is -0*C or less, whatever the rate, humidity will freeze on the surfaces.
Actually, maybe amount of frost will be equal to energy extracted. Not sure, might be that way.
Using mass is a good idea. Especially for extremely rapid cabin heatup. But can't do that for
any longer. Tesla already has "cold sensitive" chemistry mix compared to some others.
It can't receive ANY charge (regen) below zero (metric). And battery is close to that in cold
weather when icing happens on heat exchanger.
I love the disagree. Don't read the patent, then. If you do, you'll see that Tesla sources heat from the battery and drivetrain in cold conditions rather than from ambient air. If the battery and drivetrain do not have sufficient heat to scavenge AND the ambient air is too cold to source heat from without frosting the radiator (not a refrigerant heat exchanger), Tesla will run various components (compressor, cabin blower and/or drive units) in an inefficient, heat-generating mode to provide heat for the heat pump to transfer to the cabin (similar to how Model 3 uses the drive units to provide heat for the battery pack). I appreciate that you are clearly very well-versed in the design and operation of traditional automotive heat pump systems, but as the patent clearly states, this is not a typical automotive heat pump system.
Per the patent, for short drives, there is no point in trying to heat the pack. So it can be used as source of heat energy both in terms of thermal mass and also for the inefficiencies therein. Thermal mass combined with the previous night's charge cycle/ mild preconditioning is a large reservoir (use HP to warm pack from cabin/ ambient, then use HP to warm cabin from pack when driver present)..
The new integrated pack design also looks like the bottom cooling channels could become a massive radiator/ ambient heat exchanger.
I don't think you know exactly how a heat pump works. Ice just naturally builds up on one side. My house heat pump goes through periods of time where it's running the "A/C" to heat up the radiator to melt any ice. So during that time it's blowing cold air in to the house and the heat will be on the coils to melt the ice buildup.
I believe the Tesla system works like that but ALSO can shut off that air source loop and instead harvest heat from the battery, motor, inverter, and compressor. In low temps (it sounds like very cold, -20C or something) it completely ignores the loop that goes to the radiator (air source heat pump) and instead just mainly uses the compressor and then any extra heat from the battery/other electronics. Now that still doesn't address the issue where you might have temps high enough to run the heat pump (say right around 0C) but low enough that the localized temperature on the radiator will be below freezing and build up ice (just like it does in your house). I assume then that the flow can be adjusted so heat from the compressor can run by there and quickly melt off any ice.
I suggest watching this if you don't understand the diff between the house air to air and tesla system;
Yeah, which seems like a waste, but I realized that I was looking at the whole freeze thaw cycle incorrectly. When an evaporator frosts up, it is taking ambient moisture (vapor) through the liquid state to the sold/ frozen state. So you gain the heat of condensation and the heat of solidifcation. To defrost, if you can get away with only melting the ice (heat of fusion) and blowing the water off, then the system operates at a net positive since the heat of vaporization is not replaced (winch should be larger than the temperature shifts needed).
For reasonable sizes of heat exchanger, air flow, and BTU output: frost/ defrost cycles makes sense.
I don't believe air moisture has condensation heat to give out to the fins. Though solidification heat is available.
I believe the weight of air moisture that is in gas state (humidity) is much smaller than the part that is liquid (fog).
Mostly it is that "wetness" in air that solidifies on the heat exchanger.
But yes, overall amount of energy is ridiculously tiny to consider in the equation. That I did state
I believe the weight of air moisture that is in gas state (humidity) is much smaller than the part that is liquid (fog).
Mostly it is that "wetness" in air that solidifies on the heat exchanger.
But yes, overall amount of energy is ridiculously tiny to consider in the equation. That I did state
Disagree x 4
mongo
MP3Mike
Eugene Ash
Big Earl
Well.. as we see, resistive heaters are still there. Though they are still inside HVAC unit.
Though making them 14V ones simplifies the design. I bet they are around 40A each with just on/off relay/transistor.
If HP stops for more than few minutes car will be undriveable in winter. And there is absolutely no redundancy.
Resistive heaters reduce the risks and add comfort.
Coolant pumps are redundant to each other, therefore no worries there.
BTW, last winter that exactly happened with my car. HP stopped
(pressure sensor failure, possibly low refrigerant as well). All went well.
Fixed the problem a month later without problems with daily commuting.
How can you say it needs 4kW of heat, but then claim the LV PTC can fulfill that with only 14*40*2 = 1.12kW (assuming it is 40A)? Side note: Those are for split temperature control and are mostly likely medium speed PWM, so not relay operated.
Per the patent, the compressor can be set up such that the compressor and drive electronics are cooled via the refrigerant (they can even boil it to start the cycle), so high power levels can be achieved. Why do you think it cannot handle 12kBTU (4kW)?
FWIW, current HV PTC cabin heated cars do not have redundancy.
I don't believe air moisture has condensation heat to give out to the fins. Though solidification heat is available.
I believe the weight of air moisture that is in gas state (humidity) is much smaller than the part that is liquid (fog).
Mostly it is that "wetness" in air that solidifies on the heat exchanger.
But yes, overall amount of energy is ridiculously tiny to consider in the equation. That I did state
Indeed you are correct, in typical outdoor applications, humidity is low. I realize I was being highly influenced due to a previous job where we had a cold chamber that was dealing with nontypical conditions and it froze up a lot (and quickly).
Vapor vs droplets/ road spray mix is an interesting factor also.
Thanks for the discussion!
For vehicle to be driveable windscreen must be clear. Tesla does not offer heated windscreens.
But 1kW of heat can defrost windscreen for vehicle to be driveable at -20*C.
Also cabin will be at acceptable temperature (a little above zero).
Heated surfaces (steering wheel, seat) will add comfort.
For "normal" vehicle behavior in cold weather, 1, 2 and even 3kW is not enough.
Likely compressor maximum power intake is around 3kW. Maybe 3500W. Assume COP 1.0.
That is not enough in extreme cold temps (below -25*C). Adding one extra kilowatt will be somewhat
helpful and maybe, MAYBE, tiny amount (200-500W) could be extracted from drivetrain and electronics.
In total, that will be sufficient for Model 3 and Y (still not enough for X). Drivetrain and plumbing is cooled
with very cold air flow during driving. For HP to extract energy from there it must cool it noticeably
below ambient. If ambient is -25*C then HP can't actually go easily to -35*C. Also that starts to
mess with oil viscosity and frost on the drivetrain.
Also if driver is heading to supercharger, system will struggle with no spare capacity. Car might be
stuck at SuC for half an hour before charging starts. That actually is still a problem with Model 3 without HP.
All this is not important when COP is 2.0 or more. But often it is not. At some moment before defrost
cycle starts COP drops to near 1.0.
Energy extracted from vehicle itself is not multiplied with COP factor. 300W out of
vehicle drivetrain is 300W of heat to cabin.
I wouldn't bet that LV heaters are PTC.
But 1kW of heat can defrost windscreen for vehicle to be driveable at -20*C.
Also cabin will be at acceptable temperature (a little above zero).
Heated surfaces (steering wheel, seat) will add comfort.
For "normal" vehicle behavior in cold weather, 1, 2 and even 3kW is not enough.
Likely compressor maximum power intake is around 3kW. Maybe 3500W. Assume COP 1.0.
That is not enough in extreme cold temps (below -25*C). Adding one extra kilowatt will be somewhat
helpful and maybe, MAYBE, tiny amount (200-500W) could be extracted from drivetrain and electronics.
In total, that will be sufficient for Model 3 and Y (still not enough for X). Drivetrain and plumbing is cooled
with very cold air flow during driving. For HP to extract energy from there it must cool it noticeably
below ambient. If ambient is -25*C then HP can't actually go easily to -35*C. Also that starts to
mess with oil viscosity and frost on the drivetrain.
Also if driver is heading to supercharger, system will struggle with no spare capacity. Car might be
stuck at SuC for half an hour before charging starts. That actually is still a problem with Model 3 without HP.
All this is not important when COP is 2.0 or more. But often it is not. At some moment before defrost
cycle starts COP drops to near 1.0.
Energy extracted from vehicle itself is not multiplied with COP factor. 300W out of
vehicle drivetrain is 300W of heat to cabin.
I wouldn't bet that LV heaters are PTC.
The drive units are still capable of multiple kW of resistive (lossy motor) heating (the 3 uses that as the pack heater). So oil thickening impacts can be avoided.
While we're off in the technical weeds, I think I realized another benefit to the Tesla system. It should create less frost.
Normally, the evap is right after the thermal expansion value. The valve causes a pressure drop and the refrigerant condenses after it and is boiled by heat from the ambient environment. Normally, all/ most the refrigerant is a gas by the end of the evap. The critical feature though is the the entry point of the evap ends up cooler than the exit point. So frost forms there first and, as the evap fins get blocked, the cold temperature moves down the evap path (and heat transfer is reduced) until the entire exchanger is coated.
Contrast this with the Tesla system. The evap is linked to the glycol loop which does not freeze (at least not at most temperatures). So that effect is removed. Then, what of the radiator? Well, instead of having a cooled liquid to boil, the media is a cooled liquid that only pulls heat based on temperare rise (no phase change). The glycol temp temperature drop is proportional to the heat transfer, specific heat and coolant flow rate. The inlet of the radiator is still cooler than the outlet, but the delta temperature is less along with the amount of heat each section can pull, so the rate of frosting should also be reduced.
Basically, while the evap pulls heat energy until the refrigerant boils and then reaches anbient temp, the radiator only pulls energy until the glycol reaches ambient, so less loading on the inlet side.
On the micro scale, less energy pulled from the environment means less frost build up.
Total energy transfer must be the same, however it is spread more evenly.
Yea. I'm not sure will lossy motor mode will be used if compressor is not doing that. Maybe for SuC preparation. That would help with speed.
Frost that appears at the expansion valve will reduce effectiveness of that "corner" pretty fast. After a while most of the heat exchanger surface is equalized accordingly.
Yes. Glycol as a media for energy transfer is interesting. Ground source heat pump systems actually use ethanol blend not glycol (at least modern systems). It is less viscous and has better properties for heat transfer. Also better for environment. Totally drinkable
Frost that appears at the expansion valve will reduce effectiveness of that "corner" pretty fast. After a while most of the heat exchanger surface is equalized accordingly.
Yes. Glycol as a media for energy transfer is interesting. Ground source heat pump systems actually use ethanol blend not glycol (at least modern systems). It is less viscous and has better properties for heat transfer. Also better for environment. Totally drinkable
Yea. I'm not sure will lossy motor mode will be used if compressor is not doing that. Maybe for SuC preparation. That would help with speed.
Frost that appears at the expansion valve will reduce effectiveness of that "corner" pretty fast. After a while most of the heat exchanger surface is equalized accordingly.
Yes. Glycol as a media for energy transfer is interesting. Ground source heat pump systems actually use ethanol blend not glycol (at least modern systems). It is less viscous and has better properties for heat transfer. Also better for environment. Totally drinkable
Yeah, I had a leak in the fridge and part of the diagnosis was watching the frost form. If the system can move the energy without creating a frost generating cold spot, that would be ideal. Also had a home AC unit freeze up due to low refrigerant charge.
Gycol was just my shorthand for coolant (heatant?) (which really isn't that much shorter to type)
If the car gets into an extreme situation where it needs the raw heating power, we’ve seen that the drive motors can each do at least 3.5 kW of stator heating, and the octovalve can pull that heat into the cabin as a base for the heat pump to work on, potentially outputting over 10 kW to the windshield vs the 6 kW of the older PTC systems.
It has not been confirmed that HP equipped Teslas will do lossy motor trick.
Stators already changed some time ago.
Also all that doesn't work if HP is out of refrigerant or any other error with the loop.
10kW is ok, nothing special. Many EV-s with HP can do that. Even cheapass 2013 Leaf.
Not all Teslas are equipped with 2 motors. That also matters.
Stators already changed some time ago.
Also all that doesn't work if HP is out of refrigerant or any other error with the loop.
10kW is ok, nothing special. Many EV-s with HP can do that. Even cheapass 2013 Leaf.
Not all Teslas are equipped with 2 motors. That also matters.