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Why does Tesla use a Resistance Heater instead of Heat Pump

Discussion in 'Model S' started by GJ79, Feb 1, 2017.

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  1. GJ79

    GJ79 Member

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    I was wondering why Tesla doesn't use a more efficient way such as a heat pump for the cabin heater as the resistance heater eats up the Range significantly. I am sure there is a reason other than cost, does anyone know ?
     
  2. tyson

    tyson Member

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    It could be because heat pumps don't work well in colder temps, especially extreme cold. This is why all homes in the northern states have gas furnaces. In order for a heat pump to produce heat on one coil pack it has to make the other coil pack cold and then use ambient air to re-heat/boil the refrigerant so that it can pass through the compressor again. This does not work in freezing temperatures at all and works poorly below ~40 deg F.
     
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  3. wdolson

    wdolson Supporting Member

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    Heat pumps don't work very well in extreme cold. Heat pumps also take up more space than resistive heating do though not working well in extreme cold is probably the primary reason.
     
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  4. Haxster

    Haxster Member

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    I wondered this too.

    Maybe it was easier and cheaper to just use a readily available electric automotive A/C (like is used in the Prius) and then add some cheap resistance wire to whatever needed heating. And this approach is smaller, cheaper, and more flexible than ducting, valves, dampers, tubing, and additional heat exchangers.

    I don't know what's used for heating the battery, but a heat pump would certainly seem more efficient there than resistor heating too.

    Anyone know for sure?
     
  5. croman

    croman Active Member

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    My leaf worked great in the frigid north called Chicago. I think heat pumps are probably something Tesla should consider but they seem to be stuck on a Cali mindset. Its a definite point of weakness when compared to my lowly Leaf.
     
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  6. JohnnyG

    JohnnyG Weee!

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    #6 JohnnyG, Feb 1, 2017
    Last edited: Feb 1, 2017
    It's exactly what @wdolson and @tyson said... It's thermal dynamics. You have two coils used to transfer heat from one area (inside car) to another area (outside car). Not only does the efficiency dramatically reduce in colder outside temps, but it almost completely does not work in extremely colder outside temps. As @tyson said, this is why heat pumps are not common for homes in the northern states, and for homes in northern states that do have them, they still have gas/oil/electric furnaces for the winter, while the heat pump mainly only functions in the Fall/Spring when it's more efficient.

    The Tesla MS & MX are huge vehicles, with a massive area to heat, especially being a hatch. In colder climates, with a heat pump, you would never get that whole area warm by the time (the average person) got to work/home.
     
  7. brkaus

    brkaus Active Member

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    I believe this has been discussed, my understanding -
    1. Heat pumps do not work as well in a very cold environment. Great for CA through. So much for the argument that they design everything for California.
    2. Most cars use AC in the winter for dehumidifying, defrosting, and generally keeping the windows fog free. If you had a single unit, it would have to alternate modes. Blowing cold and then warm air. It would be cost, space, and weight prohibitive to have two HVAC units.
    I don't believe the cost of a HVAC unit that supports heat & cool modes is that much more expensive, so I doubt cost is the issue.
     
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  8. McRat

    McRat Active Member

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    #8 McRat, Feb 1, 2017
    Last edited: Feb 1, 2017
    To defrost the inside of a windshield, you need both AC and heat. A heat pump can do one or the other, so you'd need two heat pumps. The Leaf also has a resistive heater because of this.
     
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  9. croman

    croman Active Member

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    Yes, so why is everyone treating this as an OR situation when it clearly could be a AND situation? Why doesn't the S have a heat pump? I'm not sure about everyone else but I use the heat all the time when its cold and not only is the S far more inefficient, it also doesn't heat as well as a lowly leaf. That's not good to be beaten by a car 1/3 the cost (before incentives, 1/5 after) in anything, much less heating.

    I do believe this a california mindset still because those of us in the frozen north lose a bunch of our range to an inefficient heating system that draws 6kw of energy to provide less heat than my leaf which used 1.5kw.
     
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  10. BerTX

    BerTX Active Member

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    I thought the main heat source for the car was waste heat from the battery. Since there is none in cold climates, the resistance heat is used until the battery warms. If you are only doing short hops and the battery doesn't heat, then an inordinate amount of power is spent on resistance heating.

    If I remember correctly, the Leaf does not have liquid cooling, so that source is not available. (well, unless you want heating equivalent to a 1965 VW Bug). I suspect the Leaf uses resistance heating in defrosting or extreme cold.
     
  11. McRat

    McRat Active Member

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    Heat pump heat is not free. It is better, but not free. It also sucks as a heater when the air temperature over the heat exchanger is below freezing. Resistive heaters do not rely on an airflow heat exchanger.
     
  12. croman

    croman Active Member

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    The leaf does use resistive heating in extreme cold but it switches to a heat pump after a while. Also the peak draw of the leaf heat is 3kw and is VERY hot. The peak draw I've observed in my new S is around 6.6kw and its not terribly warm even with the fan down to 3 or 4. The S does get to temperature but it does so in a lot longer timeframe. I'm just saying that Tesla can't just rest on its laurels and perhaps it can steal some ideas from competitors (or maybe not, I'm not an engineer).
     
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  13. apacheguy

    apacheguy S Sig #255

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    The LEAF uses a heat pump. How do they make this work in cold climates?

    Ah, so it has both?

     
  14. jaguar36

    jaguar36 Active Member

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    They already have
    Modern heat pumps will put out their full BTU rating even when its -10F out. At that temperature, they are still more efficient than resistance heating. At around 30 degrees they only use 1/4 of the energy of resistance heating.

    The car would still need resistance heating for really cold days, however for the vast majority of the time the heat pump would be a huge benefit. In addition the added cost to make an air conditioner into a heat pump is very very small.

    It clearly just wasn't a priority for Tesla. It doesn't effect the top line advertising range number, and so they devoted their very limited resources to other priorities.
     
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  15. dhrivnak

    dhrivnak Active Member

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    Heat pumps can work in colder areas. Even at 5 F my heat pump keeps the house comfortable without the strip heaters. And a heat pump in reverse is an A/C unit. So it does not need more space than a typical A/C unit in a car. As many have pointed out Nissan has done this with good results.
     
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  16. JohnnyG

    JohnnyG Weee!

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    Not including the vastly larger passenger seating areas, the cargo area alone of the Model S is double that of a Leaf. Taking that into consideration, there is far more air that needs to be heated. This may have also been taken into consideration.
    Also, the heating ability (amount of time, temp, etc) is drastically (and I do mean drastically) affected by whether you have [Range Mode] turned on or not.
     
  17. croman

    croman Active Member

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    I don't believe that a Model S' interior area is double that of a LEAF. I own both. Model S is maybe (being generous) 20% larger (not counting Frunk). The hatch storage area is about 40% larger in the S than the LEAF but the interior cabin is not that much larger. I put a car seat in both cars (The same car seat) and it fits about the same. Honestly I was disappointed that the S wasn't significantly larger (its wider and a bit longer on the inside). Vastly larger is a very incorrect characterization.
     
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  18. MP3Mike

    MP3Mike Active Member

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    #18 MP3Mike, Feb 1, 2017
    Last edited: Feb 1, 2017
    Nope, the battery/motor heating/cooling loop is separate from the interior cooling loop.
     
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  19. BerTX

    BerTX Active Member

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    So why is there a "loop" for interior heating? Just heat the air.
     
  20. Bangor Bob

    Bangor Bob Member

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    This. If your experience with heat pumps is with the big 3+ Ton Carrier units, you just don't know what heat pumps can do.

    The 3/4 ton and 1 ton minisplits have taken over up here in northern Maine. Seems like every house I drive past up here has two or three of them stuck on the wall now. Mine work down to around -20F.
     
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