Why does Tesla use a Resistance Heater instead of Heat Pump

[taraquin]

Don't get me wrong. I am extremely impressed with many of efficiencychoices Tesla have made. Light builds, aerodynamics, efficient motors (3 and S/X-Raven), choice of lightweight rims and LRR tires on Model 3, goid BMS-system etc, BUT compared to the competition the HVAC-system ought to be improved a lot. Kia, Hyundai, VW and most of the other lead the way there, partly due to heat-pumps. I would gladky pay 1-2k more for a Tesla with HP

 

[arnis]

Tesla is made for hot climates from ground up.
There is almost nothing besides seat heaters and mediocre wiper defrost that is specifically designed for cold climates
Besides cabin this vehicle also heats up battery to above freezing.. with efficiency up to COP 1.0.
And the reason is... battery chemistry is appropriate for hot climate, not cold.

headlights, washing nozzles, rear wiper, camera cleaning, thermal blinds (sunblinds), seals (charge port), door sills...
and hundreds of other things, all made for mostly hot/dry/clean weather.

 

[taraquin]

Tesla is made for hot climates from ground up.
There is almost nothing besides seat heaters and mediocre wiper defrost that is specifically designed for cold climates
Besides cabin this vehicle also heats up battery to above freezing.. with efficiency up to COP 1.0.
And the reason is... battery chemistry is appropriate for hot climate, not cold.

headlights, washing nozzles, rear wiper, camera cleaning, thermal blinds (sunblinds), seals (charge port), door sills...
and hundreds of other things, all made for mostly hot/dry/clean weather.

I agree. In cold weather climate several of the competitors have some advantages. The battery in the Model 3 is better in cold climates than S\X, and allows more regen at lower temps, but still not as good as several of the other EVs out there.

 

[tpoltron]

At least in the Model 3, Tesla uses a single cooling system for batteries, motors and cabin:
Tesla 'Superbottle' proves that the Model 3's disruption lies in its vertical integration

It’s possible that a heat pump doesn’t fit into this elegantly simple design. Eg heating battery while cooling cabin or some such.

 

[Galinette]

It’s possible that a heat pump doesn’t fit into this elegantly simple design. Eg heating battery while cooling cabin or some such.

Of course it works. A heat pump is basically a device which produces cold on one side and heat on the other side. Deciding to heat or cool something just relies on routing air/fluid to one side, and use outside air on the other side as a heat source/sink.

So if you can cool the cabin, and if you can heat the battery, it should be straightforward to do both at the same time. You don't even need the outside air then.

The only reason why the S hasn't such a system is because they relied on available OEM car HVAC systems for this, and since they had been developped for ICEs, where using the heat pump for heating the interior would be a bad idea. Developing a full custom system for the model S would have added costs.

With the development of EVs, OEM will for sure develop fully reversible HVAC systems for the interior and the battery.

 

[mongo]

So if you can cool the cabin, and if you can heat the battery, it should be straightforward to do both at the same time.

If you only want to do that, and the amounts of heat to transfer match, it is straight forward.
However, if you want to heat or cool the cabin while also heating or cooling the battery while also dumping or collecting the imbalance of heat from the environment (heating ot cooling the world), it is not straightforward.

 

[Galinette]

If you only want to do that, and the amounts of heat to transfer match, it is straight forward.
However, if you want to heat or cool the cabin while also heating or cooling the battery while also dumping or collecting the imbalance of heat from the environment (heating ot cooling the world), it is not straightforward.

AFAIK car heat pumps are simple systems that run at full power or nothing. Throttling is done by doing on/off cycles.

If you need to heat the battery with full power and need only a fraction of that power to cool the cabin, you just need to cycle between :

Outside (cold side) => Battery (hot side)
Cabin (cold side) => Battery (hot side)

You basically have three heat exchangers (outside air, cabin air, battery fluid) and two sides of the heat pump (cold and hot) that you can route to one of the three exchangers. By cycling between routing states with whatever duty cycle you can achieve any need.

 

[mongo]

AFAIK car heat pumps are simple systems that run at full power or nothing. Throttling is done by doing on/off cycles.

If you need to heat the battery with full power and need only a fraction of that power to cool the cabin, you just need to cycle between :

Outside (cold side) => Battery (hot side)
Cabin (cold side) => Battery (hot side)

You basically have three heat exchangers (outside air, cabin air, battery fluid) and two sides of the heat pump (cold and hot) that you can route to one of the three exchangers. By cycling between routing states with whatever duty cycle you can achieve any need.

Right, which is a lot of plumbing (and valve cycles). Cabin would need both and evaporator and condenser if you wanted to to do heat with de-humidification (defrost/ defog).

The Tesla compressor is variable speed for efficiency, so it does not just toggle between 0 and 100% (that would give bad user experience). ICE run the compressor 100% and regulate via the TXV.

 

[wdolson]

Tesla does more sophisticated battery cooling that Nissan and many of the other car makers out there. They also need to keep the batteries cool during supercharging, most EVs on the market don't need to charge as fast. Even in freezing weather they sometimes need to run the air conditioner during supercharging and they need the cold on demand as the batteries heat up. Heat pumps take time to come up to the desired temperature and if you're trying to cool when the predominant environment is already cool, you will get very little cooling.

The thermodynamics of how to make a heat pump that can cool the batteries as well as an AC makes my head hurt to think about it. It is probably impossible to make it react as fast as AC, which will slow down supercharging or damage the batteries. It will probably also require a lot more complex plumbing than AC which adds weight and complexity to the car as well as takes up space.

Everything in engineering is a trade off. I don't think Tesla is going to do away with the AC anytime soon.

 

[arnis]

Incorrect Incorrect Incorrect. A lot of wrong stuff happening here.
Firstly, compressor that can be either in cooling cycle or heating cycle (same compressor actually)
runs at adjustable power. Starting at around 100W up to 2500-3000W - around 1% increments.

Secondly, Tesla can replace existing AC compressor plumbing with heat pump plumbing
without changing superbottle design in its core. Glycol loop doesn't even need to change.
Though another refrigerant radiator is required inside HVAC box.

In short, adding heat pump capability, that supports cabin and battery heating will add 300-600€ of cost
for vehicle manufacturing. Almost no compromises will take place. Few extra kg of weight, more external
fan noise in winter.

Thirdly, heat pump equipped vehicles can skip heat pump when necessary. For example battery needs
cooling while supercharging and cabin needs heating. Just use existing heating elements - that's all.
Every EV can do it, incl Tesla. It's called dehumidify mode, sometimes necessary on a rainy day.

In case of Tesla, heat pump will have significantly more effect on range and power bill than Leaf for example.
Leaf doesn't heat the battery when cold (and doesn't actually need that due to different chemistry).
Tesla does heat the battery. Heating the battery at 300% efficiency will save significantly more electricity
in the long run.

 

[mongo]

An A/C system can run at an arbitrarily high condenser temperature. The higher the temp, the more heat transfer to the environment per unit time/ air flow/ fan energy. In heat pump mode, the system is limited by the 0C freezing point of water. Hitting the dew point is great since you get the heat of vaporization bump, but once the coil goes below freezing, you can start developing ice. In that range, you need to increase airflow, area, or reduce heat transfer.

Plumbing complexity increases if the system supports mixed modes, as does the potential future energy savings.

User experience can be negatively impacted due to a system that drops range due to drive time and humidity (dew/ freeze point) dropping range due to temperature only is more relatable. Also a non-mixed mode system system would have a dip in efficiency between the pure A/C mode and pure heating mode where the resistance elements are used.

If the system cost 600 Euro, manages to achieve a COP of 3 all season, and is in use 50% of the time (seasonal) at the max power input of 2kW with a 1.5 hour commute per day, 5 days a week (25,000 km/ yr, 15,600 miles per year at 40 MPH). For a car driven in these conditions:
(50%*52 weeks)*2 hours a day * 5 days a week * (3-1) * 2kW = 780 kWh savings per year
Output from a solar panel in Michigan:
Cost of solar: $1 per watt * 600 Euro ($650) = 650 Watts * 2.9 kWh/day/1kWpanel * 365 = 688 kWh/yr

If the fleet trends to less than that drive cycle, it is more advantageous to invest the cost of heat pump into solar.

 

[Reeler]

Put down the koolaid everyone and just accept that the lack of a heat pump is a Tesla fail. As others have stated, this car was designed for Elon's commute in LA to Space X or the Bay Area to the Fremont factory and no other opinions matter. For the other 99.99999999999% of the world, your "little" problems are irrelevant. Tesla did a poll with one voter and you lost.

I wish EM would get interested in skiing so that he would see the total fail here with the HVAC and with the trunk/doors dumping water/snow inside the car. That is the best we can hope for.

 

[taraquin]

Anyone knows how they solve this in the I-pace, i3 and Taycan which all have Liquid cooled batteries and heatpumps?

 

[Reeler]

If the regulators used cold temperatures as part of the range testing, Tesla would quickly add a heat pump. Although the tests are supposed to approximate real world, it is not reality for EVs if you don't have part of the cycle in cold temperatures. Until the regulators have a better temperature model for the real world, the uninspired will ignore a glaring issue for most of the world.

 

[Fobble]

Tesla is made for hot climates from ground up.
There is almost nothing besides seat heaters and mediocre wiper defrost that is specifically designed for cold climates

Probably you gives Tesla too much credit. I have scorching heat during summer in Arizona, and Tesla AC is very under powered, and literally worst AC on any car I have owned, including the cheap ones. The entire HVAC system needs to be improved for sure.

 

[arnis]

. The entire HVAC system needs to be improved for sure.

Yes, HVAC box itself is very simple. Excessively simple. It doesn't understand partial air mixing (recirculation), different temperatures from different outlets... etc

 

[arnis]

In heat pump mode, the system is limited by the 0C freezing point of water.

No. In cold weather there is very little humidity that air can hold.
Also HP equipped vehicles do defrost cycle.

 

[mongo]

No. In cold weather there is very little humidity that air can hold.
Also HP equipped vehicles do defrost cycle.

Yeah, total humidity is low, but what is there can still freeze, just like frost forms on your car. The longer the system is running, the worse the potential problem.
Defrost cycle cuts into your efficiency since you need to re-phase change at least part of the ice. Meanwhile, your cabin heater has to pick up the full heat load.

There are trade offs.

 

[arnis]

Well I know how it works and I've lived with a car that does that for 5 years. What are we debating about?
I also own air sourced HP for my house for 6 years. I've watched the process with my own eyes.

 

[mongo]

Well I know how it works and I've lived with a car that does that for 5 years. What are we debating about?
I also own air sourced HP for my house for 6 years. I've watched the process with my own eyes.

Well, I'm not debating that a HP works. I've been eyeing one for my barn in Michigan. My co-worker house runs off a geothermal setup.
My interest is in the discussion of whether it makes sense for a global vehicle fleet. Cost vs return vs environmental operating range. If someone says Tesla is dumb to not put them on all their cars without data to back up that statement, I'm going to raise the unaddressed issues.