LeftieBiker
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Re: Official Tesla Model Y Thread

Mon Mar 16, 2020 12:58 pm

The issue is that since there are now heat pumps that still deliver heat at -25F, are the ones that deliver usable heat down to about +20 to +25F still good enough? The mini-split in my bedroom will deliver usable heat down to about 8-10F. If my car could do that I'd be thrilled.
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Re: Official Tesla Model Y Thread

Mon Mar 16, 2020 1:11 pm

I think the answer to that depends on cost and where you live.
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Re: Official Tesla Model Y Thread

Tue Mar 17, 2020 3:15 pm

With regard to the heat pump on the Model Y, Tesla has a patent that describes how their system works:

Optimal source electric vehicle heat pump with extreme temperature heating capability and efficient thermal preconditioning
Abstract

A vehicle thermal management system includes a vehicle heat pump system, a battery system coolant loop, a drive train coolant loop, and control electronics. The vehicle heat pump system includes a compressor, a cabin condenser, a cabin evaporator, a cabin blower, and a chiller. The battery system coolant loop is in thermal communication with a battery system and with the chiller and selectively in thermal communication with the drive train coolant loop. The control electronics control the components of the vehicle thermal management system to heat the cabin, cool the cabin, heat the battery system, cool the battery system, and cool the drive train. The control electronics may control the compressor to operate in an efficient mode or a lossy mode in which the compressor generates heat. The control electronics may also control the components of the vehicle thermal management system to precondition the battery.
Whether this is the same system used in the Y is unclear (to me) but it seems likely.

This system is way more sophisticated than those used by other car manufacturers in that it does not include the usual sort of resistance backup heater. Rather, it uses the AC compressor and blower motors detuned to be less efficient to generate waste heat that is then used for heating purposes. It also harvests the waste heat from the battery (if any).

Running AC motors in a "lossy mode" is the same method used in the Model 3 to heat the battery in cold weather: the detuned traction motor generates the heat. The idea is that the cooling system for the motor is already in place, so it adds another use — to harvest heat for battery heating. A remarkably clever idea. (I presume that this has been discussed in the Model 3 forum.)

This is Figure 32 from the patent, which gives an idea of the efficiency versus ambient air temperature:

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GaleHawkins
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Re: Official Tesla Model Y Thread

Tue Mar 17, 2020 5:32 pm

Thanks for your detail post about their automotive heat pump. That is one example of why Tesla is without competition yet in the EV space.

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Re: Official Tesla Model Y Thread

Tue Mar 17, 2020 9:55 pm

I'm not entirely following this. Tesla's heat pump scavenges heat from the motors, which are adjusted to produce waste heat for this purpose. At what temp does this process begin? IOW, how low can the heat pump move enough heat from the atmosphere to heat the cabin, without the car having to start "running too lean" as it were, to provide cabin heat? How much less energy does this use at frigid temps than a PTC? Graphs were never my strong suit...
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Re: Official Tesla Model Y Thread

Wed Mar 18, 2020 9:42 am

This does sound like an interesting idea but I'm not completely sold on how wonderful it is. A resistance heater has an efficiency of about 100%, meaning virtually all the power consumed is turned into heat. It's also a pretty simple and cheap device that is small enough to be placed just about anywhere to make it as useful as possible. Creating waste heat in the motor that can be piped to the cabin sounds like an interesting idea but it's not as if any energy is being provided 'for free'. The energy of the heat being supplied could have been used to propel the car and there is no net energy gain vs a resistance heater element, as far as I can see.

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Re: Official Tesla Model Y Thread

Wed Mar 18, 2020 12:48 pm

Indeed.
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Re: Official Tesla Model Y Thread

Wed Mar 18, 2020 1:28 pm

goldbrick wrote:
Wed Mar 18, 2020 9:42 am
This does sound like an interesting idea but I'm not completely sold on how wonderful it is. A resistance heater has an efficiency of about 100%, meaning virtually all the power consumed is turned into heat. It's also a pretty simple and cheap device that is small enough to be placed just about anywhere to make it as useful as possible. Creating waste heat in the motor that can be piped to the cabin sounds like an interesting idea but it's not as if any energy is being provided 'for free'. The energy of the heat being supplied could have been used to propel the car and there is no net energy gain vs a resistance heater element, as far as I can see.
However, with a resistance heater you still need a blower running so it's less than 100% conversion of energy to heat from a system standpoint. Maybe getting the heat from retarding the blower motor ends up being slightly more efficient overall?
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dgpcolorado
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Re: Official Tesla Model Y Thread

Wed Mar 18, 2020 3:17 pm

goldbrick wrote:
Wed Mar 18, 2020 9:42 am
This does sound like an interesting idea but I'm not completely sold on how wonderful it is. A resistance heater has an efficiency of about 100%, meaning virtually all the power consumed is turned into heat. It's also a pretty simple and cheap device that is small enough to be placed just about anywhere to make it as useful as possible. Creating waste heat in the motor that can be piped to the cabin sounds like an interesting idea but it's not as if any energy is being provided 'for free'. The energy of the heat being supplied could have been used to propel the car and there is no net energy gain vs a resistance heater element, as far as I can see.
There is no suggestion that the energy being used is "free," although using waste heat from the traction motor or battery to heat the cabin does improve efficiency, much as an ICE car uses engine heat to heat the cabin, rather than radiate all of it to the outside air. The detuned motors could also be expected to provide heat at "100%" efficiency, or a COP of 1, much the same as the resistance heater that is usually used. Using a motor this way is just a different way of providing the equivalent of resistance heat, without the need for an extra resistance heater circuit. The amount of heat generated can be finely controlled by the rate of detuning the AC motor.

Tesla has been using this method for battery heating in the Model 3 for some time: the AC traction motor (if I understand it correctly, no guarantees on that) is run in lossy mode to provide extra heat, rather than using a separate resistance heater to heat the coolant that circulates through the battery (as is the case with my older Model S). Since the motor is part of the cooling system anyway, running it in lossy mode just increases the amount of heat for the cooling system to capture and use, in this case for battery heating when it is cold. Using the compressor motor in this way to provide extra heat for the cabin, when it is too cold for the heat pump to be efficient, is just an extension of something Tesla was already doing with the traction motor.

This sort of "outside the box" thinking is typical of Tesla IMO.
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dgpcolorado
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Re: Official Tesla Model Y Thread

Wed Mar 18, 2020 3:30 pm

LeftieBiker wrote:
Tue Mar 17, 2020 9:55 pm
I'm not entirely following this. Tesla's heat pump scavenges heat from the motors, which are adjusted to produce waste heat for this purpose. At what temp does this process begin? IOW, how low can the heat pump move enough heat from the atmosphere to heat the cabin, without the car having to start "running too lean" as it were, to provide cabin heat? How much less energy does this use at frigid temps than a PTC? Graphs were never my strong suit...
As I understand it, the energy savings of the heat pump (COP >1) depends on both the ambient temperature and the heating load (how much heating is needed). If Figure 32 is representative of the system in the Model Y, the extra heat generation begins to be used in blend mode between 10ºC to -10ºC, depending on load, and is the primary source of heat below -10ºC. At very cold temperatures the efficiency would be much the same as a conventional PTC heater.

It is a bit early to know what the actual numbers on a Model Y will be.
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