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HVAC power usage

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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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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:
4. Ancillary losses are caused by all “other” electrical loads in the vehicle, particularly the 12V cooling blowers and pumps, the 12V radio, internal and external lighting, etc. For the modeling above, we assumed that there was not heating or air conditioning load, but if there were it would show up here. These losses are somewhat different than the others because they represent a roughly constant power draw on the vehicle regardless of speed, winds or elevation changes. Because of this, they cause the energy usage per mile to start becoming high again at very low speeds. This effect would be even more pronounced if the heater or A/C system were operating. Likewise, the impact of ancillary losses is extremely small at high speeds because the primary propulsion power is very high and these small power draws make a relatively tiny contribution.
http://www.teslamotors.com/blog4/?p=70
 
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.
 
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.
 
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.
 
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.
 
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.
 
Tesla Roadster test drive event Westport CT 5/23/2010

the heat ~2.8 kW (excluding the fan) and the AC ~1.7 kW.

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.
 
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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