Aircon keep vs. letting car heat up and cool down

[ZeeDoktor]

I finally got around to running a somewhat representative test of energy consumption for the aircon in my 2020 Model S (Raven). Note it does not have the heatpump like all newer models do so is significantly less efficient.

Inside set temperature is 21C for all three tests, outside temp 30C for all tests.

Starting drive in the warm car with 30C ambient temp in the garage (no solar heated surfaces, just uniform 30C on all surfaces from the garage temperature) consumes an average of 14 Wh/minute for the aircon compressor on a 58 minute long drive.

Starting drive in the solar heated car (2h in full sun with overtemp protection active at 40C), rolling windows down for half a minute to get rid of the accumulated hot air inside the car, then letting the AC do its thing consumes 41 Wh/minute on a 50 minute drive.

Leaving the aircon running in keep mode while parked in the direct sun with car pre-cooled to 21C inside consumes 11 Wh/minute of energy. Interesting to note that 75% of that energy was used by the heater, not by the aircon (it was exceptionally humid that day, 85% on 30C day).

What does that mean?

You can save net energy in 30C weather and 21C set temperature by keeping the climate running if you are:
- parking the car in the shade if you're leaving the car for 14 minutes or less
- parking the car in the sun, if you're leaving the car for 131 minutes or less (~2 hours).

 

[paulp]

I finally got around to running a somewhat representative test of energy consumption for the aircon in my 2020 Model S (Raven). Note it does not have the heatpump like all newer models do so is significantly less efficient.

Inside set temperature is 21C for all three tests, outside temp 30C for all tests.

Starting drive in the warm car with 30C ambient temp in the garage (no solar heated surfaces, just uniform 30C on all surfaces from the garage temperature) consumes an average of 14 Wh/minute for the aircon compressor on a 58 minute long drive.

Starting drive in the solar heated car (2h in full sun with overtemp protection active at 40C), rolling windows down for half a minute to get rid of the accumulated hot air inside the car, then letting the AC do its thing consumes 41 Wh/minute on a 50 minute drive.

Leaving the aircon running in keep mode while parked in the direct sun with car pre-cooled to 21C inside consumes 11 Wh/minute of energy. Interesting to note that 75% of that energy was used by the heater, not by the aircon (it was exceptionally humid that day, 85% on 30C day).

What does that mean?

You can save net energy in 30C weather and 21C set temperature by keeping the climate running if you are:
- parking the car in the shade if you're leaving the car for 14 minutes or less
- parking the car in the sun, if you're leaving the car for 131 minutes or less (~2 hours).

I always leave the aircon on when parked out in the sun. having a cold car to return to is one of the reasons ai can never go back to ICE cars. I dont really care if it costs me 5c or even 50c in lost FIT.

 

[Quickst]

When my car is in the sun in summer and not plugged in to charger , I have no hesitation in using camp mode and keeping the car at 21C. I do use a windscreen pop silver shade but no roof shade - I don't like the pop ups nor the installed ones.

EV use is all about convenience and comfort.

 

[bcarp]

consumes an average of 14 Wh/minute for the aircon compressor on a 58 minute long drive.

How did you measure the energy used by the aircon compressor alone, independent of the other systems? Was this from the energy graph in the car or an independent energy meter that you rigged up yourself?

Leaving the aircon running in keep mode while parked in the direct sun with car pre-cooled to 21C inside consumes 11 Wh/minute of energy

I presume this number was while the car was parked? What was the energy use during the drive? One might assume that it remained the same, but it may not necessarily have been the case.

 

[bay74]

Interesting that the heater used 75%. How would that change if you had the temperature set to LO (and manually reduced the fan speed), I wonder?

 

[ZeeDoktor]

@bcarp I'm using a dual bus CAN server as the data source and a customised version of the teslax app to measure integrated energy consumption for the various subsystems. The end user GUI stuff Tesla provides doesn't give you that kind of information.

The aircon energy consumption is the same whether driving or not. The aircon compressor and the heater are measured separately.

It was a tremendously humid day when I tested that - something like 85% at 30C. I suspect the heater is used to remove more humidity from the cabin air - it does that while driving too occasionally, but uses less energy. I suspect the presence of a built-in 80W heater (per person sitting in the car) serves to warm the return flow.

 

[bcarp]

Thank you. Very comprehensive answers. I’m assuming the “doktor” part is some sort of sciency background. One more question. What was the ventilation system energy use during the overheat protection in the sun while parked time, before starting to drive.

 

[ZeeDoktor]

I didn't measure the overheat ventilation system alone, however, in an earlier test some years ago I measured the fan at max level to consume about 80 watts and about 15 Watts at the lowest. So, negligible...

 

[bcarp]

I didn't measure the overheat ventilation system alone, however, in an earlier test some years ago I measured the fan at max level to consume about 80 watts and about 15 Watts at the lowest

But the overheat protection isn't just the fan. It's also the AC, so it may not be negligible. It might skew some of the numbers for the full sun parking. This is the sort of test I'd run if I had the time. It's the sort of thing I think about at home too... run the AC all day or just when I get home?

 

[ZeeDoktor]

Fair point - I'll run that test on the next sunny day. Looks like that'll be on Wednesday at the earliest in the Sydney area:

 

[Rosier]

Given that heat pumps are more efficient, wonder how it would affect these results.

On one hand; it will use less energy if left on. On the other; it could possibly cool down the car quicker and more efficiently.

Whilst it sure is nice and convenient to have a cooled down car to get in to, I’m also very curious to find out how it’s going to affect the results.

 

[meloccom]

All Air Conditioners are a type of heat pump, so when in cooling mode, heat pump and resistance heater cars will show little difference. However in winter when a heat pump car uses the AC in reverse versus the resistance heater on cars without heat pump the better efficiency of the heat pump cars would show.

 

[ZeeDoktor]

Well the overheat protection measurement experiment failed earlier this week, the iPad that was recording the data overheated and shut down... I didn't consider to protect it from direct sun (which overheated it - not the inside cabin temp). Will try that again another day when there's sun.

Indeed as @meloccom states, it's mostly the heater that is more efficient for the heat pump equipped models.

 

[bcarp]

the iPad that was recording the data overheated

Thanks for giving it a go. Would be interested in the results if/when you manage to get it.

 

[Rolond]

so when in cooling mode, heat pump and resistance heater cars will show little difference

Not quite, if you say set the temperature of a resistance heater car to 22C the air conditioner will cool the air down to to about 4C and then the resistance heaters will heat the air up to give you 22C at the vents, the heat pump uses the octovalve to achieve the same result with out requiring extra energy to do so

The most efficient way to use cooling with a resistance heater car is to set the temp to Lo, and use the fan speed to control cooling, it then never uses the heaters

You would think to conserve energy you would raise the temperature setting, but it actually uses more energy that way

 

[JonDarian]

Not quite, if you say set the temperature of a resistance heater car to 22C the air conditioner will cool the air down to to about 4C and then the resistance heaters will heat the air up to give you 22C at the vents, the heat pump uses the octovalve to achieve the same result with out requiring extra energy to do so

The most efficient way to use cooling with a resistance heater car is to set the temp to Lo, and use the fan speed to control cooling, it then never uses the heaters

You would think to conserve energy you would raise the temperature setting, but it actually uses more energy that way

Are you sure about this? It is certainly not how airconditioning normally works, and I can't see any advantage in Tesla doing it this way.

 

[Rolond]

Some one worked it out, from memory there was a thread about it somewhere, If you set the temperature to a certain temperature that is the temperature that the air is at the vents, the only way to stop the heaters from coming on is to set the temperature to Lo

 

[Rosier]

Are you sure about this? It is certainly not how airconditioning normally works, and I can't see any advantage in Tesla doing it this way.

It is in most cars. It is controlled by the fan speed at lower temperatures 16~19 but as you dial it up it also uses the heater core to warm up the air because slowing down the fan to the lowest doesn’t cut it.

 

[doggy1]

It is in most cars. It is controlled by the fan speed at lower temperatures 16~19 but as you dial it up it also uses the heater core to warm up the air because slowing down the fan to the lowest doesn’t cut it.

I've worked on the aircon in my last 8 ICE cars (all European). Although I acknowledge that one COULD use the heater in this way and maybe early Teslas did this, it sounds rather inefficient. None of mine has used the heater in this way. To maintain temperatures less than ambient, they have always cycled the compressor on and off as required. Thermal mass in the system (incl Evaporator) ensured that you did not experience wild fluctuations in temperature

 

[Rosier]

I've worked on the aircon in my last 8 ICE cars (all European). Although I acknowledge that one COULD use the heater in this way and maybe early Teslas did this, it sounds rather inefficient. None of mine has used the heater in this way. To maintain temperatures less than ambient, they have always cycled the compressor on and off as required. Thermal mass in the system (incl Evaporator) ensured that you did not experience wild fluctuations in temperature

What do you mean by worked on?