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Ending Winter Range Anxiety

Discussion in 'Model S: Battery & Charging' started by ThosEM, Mar 31, 2015.

  1. ThosEM

    ThosEM Space Weatherman

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    #1 ThosEM, Mar 31, 2015
    Last edited: Mar 31, 2015
    Recently, I convinced myself that the Tesla HVAC system does not have a reversible heat pump, capable of heating the cabin and batteries as well as cooling them. It took a while because I was convinced that Tesla’s design was too clever to omit such an obvious feature, requiring so little additional complexity. Someone posted the Tesla patent on the thermal management system, which clearly refers to a “refrigeration unit” rather than a heat pump, and the system diagram very clearly omits the reversing valve necessary to switch the working fluid flow.

    Subsequently, I also convinced myself that the Nissan LEAF does in fact have a reversible heat pump system that both heats and cools the car. I was amazed by this, since Nissan seems to have been a bit cavalier about thermal management for the batteries. Perhaps this has evolved since the LEAF was first released.

    My point here is that Tesla has seriously missed the boat by failing to offer a heat pump capable of heating. Using electrical energy to heat a resistor is just as inefficient (compared with a heat pump) as using an internal combustion engine to turn a shaft (compared with an electric motor). The only thing worse would be to use an internal combustion engine to turn a generator shaft and then use the generated electricity to heat a resistor, but that is no consolation for Tesla owners.

    I submit that cold weather heating is the single biggest cause of range anxiety for electric cars, as compared with ICE cars. It has played a prominent part in criticisms of EVs, reaching an apogee with the infamous Broder review of the Tesla for the NY Times. It is certainly the principal cause of range reduction in cold weather, and therefore the principal cause of range uncertainty, which is the true source of range anxiety. If Tesla wishes to decisively and effectively eliminate range anxiety, it is essential to implement a reversible heat pump. Ideally, the modifications of the current system could be readily (i.e.: economically) retrofitted to older (i.e.: my) vehicles. It should require only the addition of a reversing valve with servo and suitable software controls.

    Now it might be argued that a reversible heat pump would still not suffice during extremely cold conditions, and this is true. But I submit that it would relegate the current resistance strips to much reduced use as “auxiliary heat” and would make a big difference in all but the most severe weather, when impacts on range are predictable and hence less uncertain.

    For truly frigid conditions, as part of the “cold weather package”, Tesla should also implement a biogas or propane fired auxiliary heater that would only be used at such low temperatures that a heat pump cannot produce heat efficiently. The resistance strip heaters could be retained for the case where the heating fuel has been exhausted. Many Tesla or other EV owners find this concept inconsistent with their desire to rid themselves of all burning fuels, but if you have this reaction, please consider the argument above concerning the use of electricity for resistance heat. A combustible fuel is an extremely efficient heater, though a very inefficient motivator or shaft turner. Conversely, a battery is a very efficient motivator / shaft-turner, but a very inefficient heater. When trying to survive in the Arctic, one should seek to use combustible fuels as efficiently as possible, that is, in heaters and stoves, and not for motion. Correspondingly, electricity should be reserved for motion and not used for heat, on energy efficiency grounds alone. To address the emissions aspect of energy use, the right approach is to develop clean and carbon neutral fuels like biogas, which is being used to heat electric city buses in Scandinavia. If you still find these arguments about the cold weather package off-putting, please do not let it cloud your assessment concerning the reversible heat pump point above.

    In summary, the implementation of a reversible heat pump with biogas fired auxiliary heating may not increase the range of an electric vehicle, but it will stabilize the range near the values experienced in temperate weather, even when the temperature plummets. This will effectively extend the cold weather range of electric vehicles and, above all, reduce uncertainty about the range that will be achieved when driving in winter conditions. That, alone, will drastically reduce if not totally eliminate range anxiety for EV drivers in most north american locales.

    Discussion? Can we persuade Tesla of the necessity of addressing cold weather range as part of "ending range anxiety forever?"
     
  2. efusco

    efusco Moderator - Model S & X forums

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    That solution was used in the earliest RAV3 EVs back in the ideas. So it is certainly a workable idea. That said, I think the general distaste for such a system amongst EV buyers and potential buyers is so great that it would not be accepted at all. I think a better focus would be on improving the efficiency of the heating, maybe with a heat pump system, in future generations.
     
  3. Saghost

    Saghost Active Member

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    I'm not sure how much a full heat pump would help Tesla.

    The piece I didn't set you mention here is power train heat harvesting. One of the fancy features Tesla put into the model S is the ability to use drive motor/inverter waste heat to warm the cabin.

    This is useful in exactly the same moderate cold conditions that the heat pump helps with - neither is enough for the extreme cold, of course.

    I don't have charts or test data to know if there's a meaningful benefit to the heat pump in a car that harvests drivetrain heat (this might vary a lot with driving habits - folks that drive fast and accelerate hard might have no benefit, while folks that spend hours sitting in traffic might see a substantial one.)
    Walter
     
  4. RDoc

    RDoc S85D

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    I think you're correct about the distaste issue, but I'm afraid that's a case of aesthetics trumping reality. An on-board fossil fueled heater would likely produce far less CO2 than electrically based heating since the efficiency of such a heater would be very high compared to using electricity which is mostly produced by burning coal and natural gas, and yes I know that there are several people, perhaps a dozen who use solar for some of their charging.
     
  5. ThosEM

    ThosEM Space Weatherman

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    Which idea was used the RAV3 EV?
    1. The reversible heat pump should be the baseline for all Model S cars.
    2. The cold weather package should be augmented with a combustible aux. heat system for those who need one.
     
  6. lphe

    lphe Member

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    It takes a tremendous amount of energy to heat cold outside air. Hopefully, Tesla is at least recirculating some of the inside air so the car doesn't consume so much energy to warm the cabin. Unfortunately, if you don't draw in some outside air, the windows will eventually fog/ice up.
     
  7. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    My understanding has always been that heat pumps don't work particularly well in very cold weather, so I'm not sure they would solve the problem.

    If any kind of a combustion device is put in a car, it will to be subject to the same regulations as if there was a ICE. I'd suggest that more insulation would be a better way to go.
     
  8. Bangor Bob

    Bangor Bob Member

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    Modern DC-inverter scroll-compressor heat pumps work fine in cold weather, mine work fine down to -18F and are very efficient at more moderate temperatures!

    While it's nice that the heating system can use lost drivetrain heat, if it's in the 90% efficiency range and your highway cruise is using say, a 15kW average, that's only 1.5kW of heat, and the battery will be getting some of that (and rapidly leaking it out to a wet aluminium plate exposed to high wind speeds). Supplementing that with a 1kW heat pump could add another 2-4kW of heat output for the cabin to temperatures well below freezing. Insulating the bottom of the battery pack would help too...
     
  9. CHG-ON

    CHG-ON Still in love after all these miles

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    If they could dehumidify the air when in recirc when it's cold, would that save more energy than requiring us to leave recirc off in order to avoid fogging? From my POV, I would not like the idea of having to maintain a tank for a fossil fuel heater. But I totally get what you are saying. Of course, I live in CA, where it is always sunny and 70 degrees and all the girls are in bikinis all year 'round!
     

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