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.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?
To defrost the inside of a windshield, you need both AC and heat. A heat pump can one or the other, so you'd need two heat pumps. The Leaf also has a resistive heater because of this.
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.
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.
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).
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.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.
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.