Real World Heat Pump Effect

[Saghost]

The proof of how good the HP is is when Tesla installs it in EVERY new Tesla. Time will tell along with real-world winter driving in a few months.

I doubt it'll financial sense to redesign the S/X HVAC to use the new heat pump and octovalve unless they actually do the endlessly six months away forever rumored interior refresh with the landscape screen and slot. The unit volumes are small enough that it's hard to justify the engineering time in cost savings.

I wouldn't be surprised if the 3 got the heat pump soon though.

 

[Booga]

Split-type reversible inverter HVAC system has MASSIVE heat exchanger (outside). I estimate around 4-5 more surface area.
Cars do compensate with either more fan power or travelling at high speed but unfortunately stationary systems, due to
no size constraints, are more efficient that automotive systems. Though they both get excellent results, between 300-450%.

Their efficiency, measured by the COP, can be high. In reality, that efficiency is seen at temperatures when it is warmer and it's not as helpful. If you travel in mild climates, say 50-60F, it will help, but only marginally, because your heating needs wouldn't have been much to begin with.

The COP falls as it gets colder, but provides greater benefit. I'm being a little picky, because people will cite these figures of 300-400%, but it's not nearly as large when you're making real use of the heater. This is why another EV saw a 13% range improvement at -10C versus only 5.5% at +20C.

See this chart: Imgur
For an explanation, see page 80 in this document: Unitary Thermal Energy Management for Propulsion Range Augmentation (UTEMPRA) (Technical Report) | OSTI.GOV

It is a large benefit either way, but I don't think it's nearly that large when it's actually cold outside. I'm curious to see if it provides benefit in a place like Norway during their winter, which has many Tesla owners and very low temperatures.

 

[arnis]

Whatever the climate, it is never going from summer to -20*C without passing -5*C.
Even in Norway, weather in winter is mostly below zero (celsius) single digits.
Number of days with mild cold weather is large and number of days where heat pump
is not sufficient is low.
There are absolutely no reasons to NOT have heat pump.
Also when temperature goes really low (below what heat pumps are sufficient)
actual heating demands fall as air humidity drops. Low humidity means less heat required
and usually more range (less drag/rolling resistance).

when you're making real use of the heater.

Real use of heater is about 5 months here. Those rare days hardly matter. PTC is always available.

 

[MP3Mike]

Real use of heater is about 5 months here. Those rare days hardly matter. PTC is always available.

Nope, the Model Y doesn't have a PTC heater. It does have two small 12v heaters mainly for the zone differences. But they have other ways of generating heat.

 

[frankvb]

Nope, the Model Y doesn't have a PTC heater. It does have two small 12v heaters mainly for the zone differences. But they have other ways of generating heat.

I believe there's a way to run the heat pump (compressor?) inefficiently to generate heat.

 

[Feathermerchan]

They can run the compressor motor inefficiently as wall as the drive motors, and cabin air fan motor. A big problem for heat pumps is that when the outside air is below water's freezing temperature, the coolant must still get colder to transfer heat. The radiator then may collect ice which blocks its passage ways and prevents it from absorbing any more heat.
In residential heat pumps, the system detects the ice and temporarily runs as an AC unit to defrost the coil. Tesla may just divert the coolant from the radiator and use the drive motors and inverters to provide heat.

 

[MP3Mike]

They mentioned on the investor call this afternoon that the heat pump system was designed to be able to move heat around down to -20*/30*C ambient temperatures. So it will be interesting to see how it works out in a really cold winter.

 

[Feathermerchan]

It can always move heat from the battery and drive units but below freezing, how does it clear ice (frozen water) from the radiator fins? Does it have a defrost cycle? It could.

 

[Big Earl]

It can always move heat from the battery and drive units but below freezing, how does it clear ice (frozen water) from the radiator fins? Does it have a defrost cycle? It could.

In cold weather like that, the Tesla heat pump system does not draw heat from ambient. It only draws from ambient when it is advantageous to do so.

 

[acarney]

I believe it does have a defrost mode. You absolutely could build up ice at “moderately cold” temps of like 30F. It uses heat from the compressor to heat coolant and then moves that through the system to whatever sub-system needs it (cabin, motors, inverter, radiator, etc).

 

[arnis]

All EVs with heat pump have defrost cycle. It works only when vehicle is stationary for few minutes OR
vehicle has radiator shutters that are more-or-less airproof.
Tesla will also do defrost cycle. Refrigerant is rerouted to external heat exchanger heating it up making it hot.

 

[mongo]

All EVs with heat pump have defrost cycle. It works only when vehicle is stationary for few minutes OR
vehicle has radiator shutters that are more-or-less airproof.
Tesla will also do defrost cycle. Refrigerant is rerouted to external heat exchanger heating it up making it hot.

The Tesla system uses se compressor as a resistive heater in cold temps. They could ice the radiator over to get the additional heat energy, but there is little to no reason to ever defrost it (there is no non-cabin air to refrigerant hear exchanger).

 

[arnis]

Resistive heater is 100% efficient. Useful for cabin heating.

External AIR heat exchanger requires airflow to exchange heat.
So there is a reason to defrost a heat exchanger. There is nothing to get from ice.

 

[mongo]

Resistive heater is 100% efficient. Useful for cabin heating.

External AIR heat exchanger requires airflow to exchange heat.
So there is a reason to defrost a heat exchanger. There is nothing to get from ice.

If the ambient conditions are such that the radiator freezes over there are three choices:

1. Run the heat pump mode at a low enough rate to prevent frost (including no HP effect)
2. Run the HP, frost over, then steal heat/ energy from somewhere to defrost it.
3. Frost over once to gain the heat of fusion from the moisture.

There is no external air to refrigerant heat exchanger. The system has seperate refrigerant to coolant evaporator and condenser, then the coolant exchanges with ambient via the radiator. With this setup, a portion of the piping is also a heat exchanger and it can use the drive units as heat sources (both normal loss and resistive)/ exchangers also.

 

[arnis]

Ice is a good insulator. As soon as area between fins is full of ice and no air passes through, it's done.

External glycol heat exchanger will freeze and unfreeze in a similar manner to refrigerant heat exchanger.
For defrost to happen rapidly likely HP will generate a lot of heat during short period of time.
Drivetrain heat generation rate is too slow for it to be useful compared to heat exchanger capability*.
Also battery needs to warm up not cool down. Therefore drivetrain will be cut off by octovalve or
even be supplementarily heated (battery only) with energy harvested from heat exchanger.
And shutters are definitely required for defrost, whatever media is used for heat transfer.


*Except during supercharging session. ideal time to defrost.

My data suggest that on average, in bad (moist) weather, heat exchanger will be ok for at least half an hour.

 

[mongo]

Ice is a good insulator. As soon as area between fins is full of ice and no air passes through, it's done.

External glycol heat exchanger will freeze and unfreeze in a similar manner to refrigerant heat exchanger.
For defrost to happen rapidly likely HP will generate a lot of heat during short period of time.
Drivetrain heat generation rate is too slow for it to be useful compared to heat exchanger capability*.
Also battery needs to warm up not cool down. Therefore drivetrain will be cut off by octovalve or
even be supplementarily heated (battery only) with energy harvested from heat exchanger.
And shutters are definitely required for defrost, whatever media is used for heat transfer.


*Except during supercharging session. ideal time to defrost.

My data suggest that on average, in bad (moist) weather, heat exchanger will be ok for at least half an hour.

Mostly agree with all that, but what is the advantage of frosting/ freezing over the radiator just have to defrost it again? Why not instead run the system on the edge of frosting?
Unless your defrost drops off chunks of ice, the energy transfer to/ from the moisture is net neutral with net loss to the environment.

 

[arnis]

Because the edge of frosting is around 0.1*C (celsius). And it's usually well below that. For example -2*C.
Medium inside the exchanger gets closer to external temp. Therefore negative* temps for glycol fluid. Absolutely works that way too.
So heat exchanger inlet is for example -15*C and outlet is -5*C while external temperature is -2*C.
Whatever touches the fins will either get stuck (snow) or freeze (water droplet, air moisture, salty water droplet).

Love the idea of dropping off chunks of ice

*negative - I mean like freezing temps, or whatever word is used in imperial.

The amount of frozen material is not 1:1 compared to energy extracted.
For example, within 30 minutes of HP operation heat exchanger can extract 3kWh of energy (average rate 6kW).
After that it is "fully frosted". Defrost cycle required thawing 500ml of water (half a liter) from -2*C up to +0*C (incl phase change).
That requires much less energy than was extracted "in between freezing cycles".

0.5l of water temp to be raised for 2 degrees requires 0.5*2calories=1 calorie. 0.0005kWh+0,047kWh phase change.
Therefore we could round it to 0.05kWh per half liter of water (almost half kg).
So the reason to defrost is because defrosting required 0,05kWh of energy and after that heat exhanger can supply 3kWh
until it is fully frosted again.

For calculation to be true, frozen material may not be chilled by airflow while defrost cycle is happening.

PS: I forgot to mention, that "defrost" cycle is actually used for snow removal as well. When it is well
below 0*C usually there is much much less air moisture that freezes on the fins. But if there is
any precipitation, that will cover the heat exchanger. Making it not pass air. So that must be melted away.
Same applies to home HP systems.

 

[mongo]

Because the edge of frosting is around 0.1*C (celsius). And it's usually well below that. For example -2*C.
Medium inside the exchanger gets closer to external temp. Therefore negative* temps for glycol fluid. Absolutely works that way too.
So heat exchanger inlet is for example -15*C and outlet is -5*C while external temperature is -2*C.
Whatever touches the fins will either get stuck (snow) or freeze (water droplet, air moisture, salty water droplet).

Love the idea of dropping off chunks of ice

*negative - I mean like freezing temps, or whatever word is used in imperial.

The amount of frozen material is not 1:1 compared to energy extracted.
For example, within 30 minutes of HP operation heat exchanger can extract 3kW of energy (average rate 6kW).
Then if "fully frosted". Defrost cycle required thawing 500ml of water (half a liter) from -2*C up to +0*C (incl phase change).
That requires much less energy than was extracted "in between freezing cycles".

0.5l of water temp to be raised for 2 degrees requires 0.5*2calories=1 calorie. 0.0005kWh+0,047kWh phase change.
Therefore we could round it to 0.05kWh per half liter of water (almost half kg).
So the reason to defrost is because defrosting required 0,05kWh of energy and after that heat exhanger can supply 3kWh
until it is fully frosted again.

Sure, but could you not instead reduce the rate of energy transfer such that the ambient airflow prevents frost build up to begin with (sublimation)? Then you avoid needing to melt that which you froze. In a long time interval situation (versus a 30 minute commute where one could potentially avoid defrost entirely), is that more efficient? Yes, when operating in the freezing domain, you are pulling more energy from the air, but when defrosting, you are not pulling anything from the ambient and losing some (less due to baffles) energy to ambient.

Interesting thermo question.
(Agree, dropping ice cubes is bad from a share the road POV, but it is more energy efficient. Only included that for completeness.)

 

[arnis]

Can't slow down HP because energy is required ASAP. The most amount is required within first 10 minutes.
And 90+% commutes are up to 30 minutes. So it won't even get to first defrost cycle.
Also in demo scenario, if HP is off, heat exchanger temp is -2*C. If it is "slightly active" it is -3*C. Full power -8*C.
It hardly matters. For air moisture, for snow. It will just jam the fins if vehicle is in motion in all 3 modes.

And like I demonstrated, energy required to melt is insignificant. Time for defrost is more important. It can easily be done
within 4 minutes. And I believe Tesla can push it to 3 minutes. 10% downtime in worst case scenario is ok.
Slowing down HP to try to avoid it... Well, the only way for that is to waste lots of battery power (same as PTC).

PS: all heat pumps (AFAIK) are all inverter based. So... they pretty much never work at 100%.
They hardly can work at 100%. There are very specific requirements for HP to be able
to "be cranked up to 100%". My car does that maybe 10-30 minutes per year.

 

[MY-Y]

Sure, but could you not instead reduce the rate of energy transfer such that the ambient airflow prevents frost build up to begin with (sublimation)? Then you avoid needing to melt that which you froze. In a long time interval situation (versus a 30 minute commute where one could potentially avoid defrost entirely), is that more efficient? Yes, when operating in the freezing domain, you are pulling more energy from the air, but when defrosting, you are not pulling anything from the ambient and losing some (less due to baffles) energy to ambient.

Interesting thermo question.
(Agree, dropping ice cubes is bad from a share the road POV, but it is more energy efficient. Only included that for completeness.)

In order for a heat pump to extract a reasonable amount heat from the air, it must be several degrees colder than the air. If it is near or below freezing cold out, the moisture in the air will freeze. All heat pumps deal with this; it's a solved problem. I would expect the heat pump to run at least an hour before needing to defrost. My home units usually make it several hours between defrost cycles.