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gavine

Petrol Head turned EV Enthusiast
Apr 1, 2014
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2,226
Philadelphia, PA
Let's assume it is 40F outside and cabin temp is set 70F, would it be more efficient to use recirc with AC-on or fresh air with AC-off. I realize recirc with AC off would be most efficient, but the windows get foggy so recirc basically requires AC, at least if using for any extended amount of time.

Any thoughts on what is more efficient? It seems counter-intuitive to run the AC compressor and the heat simultaneously, however, heating 32F outside air up to 70F also consumes a lot of energy.

I'm curious what the community thinks on this.
 
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The best is to not have anything on :)
But to be efficient, during fast warm up cycle (or nobody inside) keep recirculation on.
As soon as desired temperature has been reached, switch recirculation off but also
reduce fan speed to as low as possible. Slowest fan speed and all air to defrost should
keep window clean. If more warmth is needed, direct some air to feet as well.
If window starts to fog up with slowest fan speed, slightly raise fan speed.

Warm air gets out of the vehicle at the same rate as fresh air enters.
This warm air going out is the enemy of efficiency as there is no heat scavenging at exhaust ports.
Keep fan speed as low as possible.

Keep in mind AC with slowest fan speed is also pretty efficient as AC duty cycle will be extremely low.
 

David99

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Jan 31, 2014
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I have done a few tests measuring power consumption of the AC. It is really minimal when the car is heating the car. It's only drying the air to prevent the windows to be fogging up. That seems to take very little energy. The lowest power draw from the AC compressor seems to be around 500-600 Watt. But when the heater is on and the AC is just drying the air, it is turned on and off. So the energy consumption is really low. The vast majority of the energy used is the PTC heater warming up the air.

The power draw is directly related to the temperature difference. If you draw in air from outside, the heater has to use more power to heat up the colder outside air than just keeping the cabin air at temperature. I have done tests measuring the power draw switiching between recirculate and fresh air when in cold weather and the difference is clearly noticeable. If you leave the climate control on auto it will do an amazing job and be as efficient as possible.

On Average the Model S/X uses aprox 65 Watts per degree of temperature difference between inside and outside. You can simply do the math on this. If you drive in 25 defree weather and set your car to 75 to keep it warm, it uses aprox 3250 Watt just for heating.
 

SageBrush

REJECT Fascism
May 7, 2015
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New Mexico
I have done a few tests measuring power consumption of the AC. It is really minimal when the car is heating the car. It's only drying the air to prevent the windows to be fogging up. That seems to take very little energy
Latent heat.

I doubt one can generalize here. The local humidity and temperature delta determine best practice. One thing is certain: avoid hot drinks in the cabin.
 

David99

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Jan 31, 2014
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I have V9 and haven't noticed any change with the climate control. But Tesla changed a few things with the climate control over the production years so maybe it affects different production years differently. I just got another software update yesterday so maybe they fixed some issues.

I doubt one can generalize here. The local humidity and temperature delta determine best practice. One thing is certain: avoid hot drinks in the cabin.

Not generalizing, just what I measured in my car. It sure depends on moisture conditions. Hot drinks evaporate but I think 4 people breathing adds more moisture :) I'm guessing here but I doubt the AC needs more energy to dry very moist air from 4 people with hot coffees than using fresh air and heat it up. If it's rainy the AC is your only chance for clear windows anyways. I trust Tesla's climate control doing it efficiently.
 

SageBrush

REJECT Fascism
May 7, 2015
14,128
19,557
New Mexico
Not generalizing, just what I measured in my car. It sure depends on moisture conditions. Hot drinks evaporate but I think 4 people breathing adds more moisture :) I'm guessing here but I doubt the AC needs more energy to dry very moist air from 4 people with hot coffees than using fresh air and heat it up. If it's rainy the AC is your only chance for clear windows anyways. I trust Tesla's climate control doing it efficiently.
If my arithmetic is correct it takes about 0.7 Wh to remove 1 gram of water present as a gas. A cubic meter of air at 50% humidity holds ~ 10 grams of water at your cabin temperature.

I think your experience of low power consumption to defog you windshield is due to the high cabin temperature setting you choose. In my driving in the high desert of Colorado during the winter I do not heat up the cabin anywhere near 70F -- if at all. This makes it easier for the glass to fog up but I find that the 'blue' defrost keeps the glass pretty clear even if used intermittently. My winter driving works out to ~ 250 Wh/mile at 65 mph in 20 - 30F ambient and dry roads. My body is comfortable from the heated seats and my hands are comfortable from gloves. Alas, my feet are less than comfortable though tolerable.
 
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David99

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Jan 31, 2014
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250 Wh/mile at such low temperatures is darn good.
One thing I noticed with EV owners (including myself) is that we tend to be very conscious about energy consumption and efficiency. We always try to drive efficiently and conserve. The OP's question is a great example. I used to be that way as well and overall I'm still trying not to be wasteful, but after owning my Model S for 5 years now I prioritize comfort more now. I set the climate control to what makes me feel comfortable. Trying to optimize those few % by doing experiments with AC settings and such isn't really worth it. So I safe 0.5 kWh?

There are situations where I had to drive extra efficient to make it. In those moments it's good to know what makes a difference and what doesn't. But in daily driving, I prefer comfort.
 
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SageBrush

REJECT Fascism
May 7, 2015
14,128
19,557
New Mexico
A bit more on the underlying physical chemistry:

It takes one calorie of heat to raise the temperature of one gram of water 1C
It takes 540 calories of heat to vaporize a gram of water at 100C to a gas

Dehumidying air is an energy intensive process, although as I said above its use in our context of driver comfort and fog free glass is complicated and local conditions dependent. Your approach may well make sense in a relatively warm but humid environment but in my relatively low humidity but cold environment I'm more inclined to blow a small amount of outside air on the glass.
 

gavine

Petrol Head turned EV Enthusiast
Apr 1, 2014
2,639
2,226
Philadelphia, PA
If you leave the climate control on auto it will do an amazing job and be as efficient as possible.

It seems that auto-mode with heat defaults to outside-air and AC-on. That doesn't seem like the most efficient method. I'm surprised it doesn't default to recirc, especially when you first turn it on.

I agree wholeheartedly that AUTO is most efficient in summer with AC. Hands-down. I hate when people set the temp to LO and then turn the fan down. That's extremely inefficient.
 
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