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HVAC power usage

Discussion in 'Roadster: Technical' started by Albern, May 25, 2010.

  1. Albern

    Albern Member

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    #1 Albern, May 25, 2010
    Last edited by a moderator: May 28, 2010
    The most interesting point that I took away from this video was the fact that in the Roadster, the heater requires more energy than the air conditioning. Still relying on ICE cars I always figured A/C always took up a lot of energy regardless of the application.


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  2. TEG

    TEG TMC Moderator

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    I wonder if he is right about the A/C. I assumed it was a heavy power drain similar to the main heater...
     
  3. Albern

    Albern Member

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    Yeah that was my initial assumption too. We know that in ICE cars the heater draws very little energy because of all the latent heat available from the engine bay. I would think that the PTC heater used in the Roadster would be quite efficient. Unless, Mr. Sexton's point about the A/C being more efficient was based on the fact that it's shared / already running to cool the battery pack, such that the additional draw for cabin cooling required significantly less energy than running a heater for the same space.

    Went through this but didn't find a definitive answer: http://www.teslamotors.com/blog4/?p=43

    From this:
    http://www.teslamotors.com/blog4/?p=70
     
  4. donauker

    donauker Member

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    Resistive heat is one of the least efficient heat sources available. A heat pump is easily 2 to 3 times as efficient as resistive heat. The AC unit is just a heat pump used to pump heat out of the car.
     
  5. AndrewBissell

    AndrewBissell Member

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    Tesla Roadster test drive event Westport CT 5/23/2010

    Electric heaters (as used in the Roadster) are electrical resistance based. These can only ever deliver at most one unit of heat for each unit of electricity they draw (if 100% efficient - actually they will be close!).

    Whereas Air Conditioning uses a heat pump. These don't generate heat, they move it from place to place. Typically these will (in practice) use one unit of electricity to move from 2 to 4 units of heat. This means they have a Co-efficient of Performance for cooling (COP_cooling) from 2 to 4.

    Assume that the inside of the car wants to be at 20 Celsius.

    First assume outside is 5 Celsius - then outside air drawn into the cabin needs to be heated by 15 Celsius. Now assume that outside air is at 35 Celsius - outside air needs to be cooled by 15 Celsius.

    The energy added/drawn from the cabin is the same. But in one case (heater) it takes one unit of electricity to heat up a given quantity of outside air. In the other (AC) it takes 1/2 to 1/4 units of electricity to cool down the exact same quantity of outside air.

    So Mike Sexton is correct.

    Side note: A reversible heat pump can also take heat from cold outside air and add heat to the cabin. A heat pump's COP_heating equals it's COP_cooling+1. So if the Roadster used a heat pump for heating it would be more efficient to heat than cool the cabin.

    In future I hope Tesla will use reversible heat pumps to improve range in the winter by reducing heating energy needs.
     
  6. TEG

    TEG TMC Moderator

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    #6 TEG, May 26, 2010
    Last edited: May 26, 2010
    Some examples of EV conversion A/C and heat do seem to suggest that a typical A/C compressor uses < 2kW, whereas a heater could be 2kW+

    A/C
    Heat
     
  7. bolosky

    bolosky Member

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    Why speculate when you can measure?

    I decided to do an experiment to see how much power the heat and AC use in the Roadster.

    I turned my 2010 non-Sport on in the garage and waited a little while. The display said 1 amp. I turned the fan on full blast with no heat and no A/C and waited again. It wavered between 1A and 2A, call it 1.5A.

    Then, I turned the heat on full blast and waited a minute. It bounced around a lot at the beginning of the minute, but by the end it had settled at 9A.

    I did the same thing with the A/C and it was 6A.

    I believe that the current reading is DC current at the battery. If it's at 375V, then the fan draws ~200W, the heat ~2.8 kW (excluding the fan) and the AC ~1.7 kW.

    Put another way, the heat (now including the fan) uses 5.6% of a full 53 kWh range mode charge/hour running. The AC uses 3.4%.

    This makes me think that a heat pump isn't such a bad idea, especially since they already have a compressor/refrigerant system with the AC.
     
  8. TEG

    TEG TMC Moderator

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    A resistive heater is simple, light weight, relatively inexpensive, and small, all of which are good things. Possibly enough to make it a better choice than a more efficient heat pump.
     
  9. AndrewBissell

    AndrewBissell Member

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    Tesla Roadster test drive event Westport CT 5/23/2010

    Except that they already have a heat pump - the AC unit! The only reason to have two separate systems today is that the automotive supply chain for AC has not been focussed on reversible heat pumps (cooling and heating from one unit) because ICE cars have more than enough waste heat to use for cabin heating.

    As electric cars grow in market share this will change. I believe some cars already have or are planned with reversible heat pumps - perhaps RAV4 EV, certainly Aptera.
     
  10. AndrewBissell

    AndrewBissell Member

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    Tesla Roadster test drive event Westport CT 5/23/2010

    Thanks for the experiment and the report. Very useful!

    Can I add that a heat pump, when heating, will do better than the AC did when cooling. Why? Because with any heat pump the COP_heating is one higher than the COP_cooling.

    Put another way, when cooling, the electrical energy used to run the compressor makes heat that must also be rejected; when heating all that heat value is added to the heat energy the heat pump moves in from outside.

    So at a guess heat pump heating might use sub 1kW for a like case.

    Furthermore, the COP_heating improves the higher the temperature source you start with. Even though an EV hasn't got the level of waste heat an ICE has, it certainly has some - battery, motor and PEM all produce waste heat at temperatures often above outside ambient. If the heat pump sources it's heat from these the COP_heating could get quite high.

    After all in the Roadster that is the primary duty of the AC unit today - to reject heat from these locations to the environment. It would be smart when there is user demand for heat to reject that heat to the cabin instead!

    So the Roadster is ever so close to having a smarter heating system that might run well under 1kW full blast, and thus possibly add back 10%+ to mid-winter range.
     
  11. Albern

    Albern Member

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    #11 Albern, May 27, 2010
    Last edited by a moderator: May 27, 2010
    Wow, this is great information. Thank you Andrew and bolosky. I wonder then if Model S will have a reversible heat pump.


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  12. SByer

    SByer '08 #383

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    Is there not a heat button in the 2010s? In my 2008, if neither the AC nor heat button is activated, the HVAC temp knob should be meaningless. And, only one of heat or AC can be active at any one time.
     
  13. vfx

    vfx Well-Known Member

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    I run them both at the same time for quick window defrosting.
     
  14. doug

    doug Administrator / Head Moderator

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    Is that true? You might want both if you were trying to defog the windows.
     
  15. vfx

    vfx Well-Known Member

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    #15 vfx, May 29, 2010
    Last edited: May 29, 2010
    Yes, typo.
    not defrost, defog
    :biggrin:
     
  16. ChargeIt!

    ChargeIt! Member

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    #16 ChargeIt!, May 29, 2010
    Last edited: May 29, 2010
    2008 and 2010 difference ?
    (see here for 2010 at about 3:00ff)
     
  17. TEG

    TEG TMC Moderator

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    RangerEV is a somewhat similar situation.

    <1 AMP with no heat or A/C on. (Coolant pump running, GPS running, etc.)
    attachment.php?attachmentid=696&stc=1&d=1275714114.jpg
    ~4 AMP with A/C on (1.2kW)
    attachment.php?attachmentid=697&stc=1&d=1275714122.jpg
    ~9 AMP with heater on (~2.8kW)
    attachment.php?attachmentid=698&stc=1&d=1275714137.jpg
     

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