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Heat Pumps

Discussion in 'Roadster' started by bolosky, Oct 20, 2009.

  1. bolosky

    bolosky Member

    May 5, 2009
    It's been getting colder here in Seattle, and so I've started using the cabin heater more. It's really noticable how much battery it uses.

    I did a little experiment upon arriving home tonight. After pulling in the garage (no worries about carbon monoxide here!), I put the car in park and looked at the current draw. It was 1-2 amps. I turned on the heater full blast, and after a few seconds it went up to 9-10 amps. AC showed 4-5A. So, the heat uses about 8A, while the AC is only 3A, nearly three times cheaper.

    I got to wondering why the car couldn't just run the AC backwards and have a heat pump like my house does. This results in significantly more efficient heating than just running a big resistor, which I assume is how the Roadster works. I guess car AC units don't have the de-icing cycle that heat pumps need, but I don't see why they couldn't if they were properly designed.

  2. edo

    edo Member

    Jul 23, 2008
    Is your home heat pump underground (at least in the basement)? I would suspect that Seattle would suffer from the "efficiency" going down to <100% in the winter due to the "evaporator" getting too cold if it's outside.

    The Roadster would also suffer from this in the colder climes, and it would also need to carry around the weight of an extra condenser for inside the car and evaporator for outside.

    That all being said, it might work... if nothing else, one could mess with the pressure of the air before going through the heater to gain a bit of heat.
  3. GSP

    GSP Member

    Dec 28, 2007
    The EV-1 used a heat pump. This would be a nice upgrade for the Roadster someday.

    I wish Tesla had released more info on the HVAC updates for the 2010 "Roadster 2.0"

  4. lmore

    lmore Member

    Nov 6, 2011
    Trondheim, Norway
    Interesting, I would want a heat pump upgrade for my Tesla Roadster. But it must be effective in cold climates.

    If the current resistive heater uses 3 kW, an effective heat pump should be able to create the same heat using less than 1 kW.

    A heat pump will be even more important in cold climate weather for the Model S, since it has a larger cabin to heat.
  5. dmckinstry

    dmckinstry Model S - U.S. P - #1649

    Jul 9, 2011
    Medical Lake, WA (near Spokane)
    Remember though that effective heat pump efficiency drops as the weather gets colder. When you get below freezing, it probably won't do any better than resistive heating. Heat pumps have to defrost their coils after the temperature gets to a certain point, probably based on humidity as well as temperature.
  6. DrComputer

    DrComputer Active Member

    Jan 29, 2009
    Sherman Oaks, CA
    The numbers stated above are also wrong. The A/C compressor in the Roadster has a two speed design. On "low" it does only use about 3-4 amps of electricity. But when it ramps up to high it will draw about 10 amps of electricity. Turning the A/C on when it is cold outside will only get it to go on in low mode. Heat up the battery pack when it is hot outside and then turn on the A/C and you will instantly see (and unfortunately hear) the compressor ramp up to high mode and suck down electrons.
  7. jkirkebo

    jkirkebo Model S P85+ VIN 14420 EU

    Jun 13, 2010
    Fredrikstad, Norway
    Modern heat pumps give good performance down to at least -15C (5 F).
  8. ElSupreme

    ElSupreme Model S 03182

    Jan 13, 2012
    Atlanta, GA
    You might be able to pull the very little bit of waste heat out of the motor, inverter, and battery pack liquid cooling system. Probably would still need to run defrost cycles though.
  9. Eberhard

    Eberhard #421 Model S #S32

    Oct 17, 2010
    you can calculate the maximum efficiency of a heat pump according to carnot using this formula = 1 - (T low/T high) where T = absolute temperature in Kelvin.

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