Extreme Heat's Effect on Range

[BS8899]

The car a/c is likely around 20,000 btu/hr for pull down. If 6800 btu/hr is required once the car is cool and at high ambient COP is only 2.0 (guess on my part). The car will need 1kw-hr for a/c. At 60 this will decrease efficiency by 0.02 (mile/kw-hr). The ac impact will be lower at high speeds and greater at low speeds. At 20 mph it will decrease efficiency by 0.05.

Heat being a larger delta T, will require more energy. If 6800 is needed for a 30F temperature then if it's 20F out side you'll need 11,000 btus

Forgive my ignorance, clarification please:

> why would the AC impact be lower at higher speed?

 

[segbrk]

Forgive my ignorance, clarification please:

> why would the AC impact be lower at higher speed?

You're increasing the mi part of mi/kWh without increasing the kWh used by A/C. He's not saying it uses less energy, just that you cover more ground while using the same amount of energy (for A/C specifically).
Another way of looking at it is just that you use more A/C on the same trip sitting in traffic vs cruising, simply because you spent more time in the car.

 

[BS8899]

You're increasing the mi part of mi/kWh without increasing the kWh used by A/C. He's not saying it uses less energy, just that you cover more ground while using the same amount of energy (for A/C specifically).

Thanks, that makes sense.


However, if you look at just the AC portion, if the outside temp is (say) 115F (e.g., Phoenix to though Palm Springs to OC on I-10) and the desired cabin temperature is 72F, a faster car speed will likely require more cooling (BTUs) as the ambient hot air will be more aggressive in heating up the cabin.

 

[segbrk]

Thanks, that makes sense.


However, if you look at just the AC portion, if the outside temp is (say) 115F (e.g., Phoenix to though Palm Springs to OC on I-10) and the desired cabin temperature is 72F, a faster car speed will likely require more cooling (BTUs) as the ambient hot air will be more aggressive in heating up the cabin.

I'm no physicist, I could be mistaken, but air isn't a good thermal conductor. I think the majority of your heat comes from solar radiation hitting and penetrating your roof/windows, not from the air.

 

[BS8899]

I'm no physicist, I could be mistaken, but air isn't a good thermal conductor. I think the majority of your heat comes from solar radiation hitting and penetrating your roof/windows, not from the air.

Yes, radiation and the glass dome effect of the Lucid cabin is a big factor. You are correct that static air is nor a good thermal conductor. But moving hot (115F) air at 75mph can convey a lot of heat.

 

[Pragmatic]

Lots of assumptions in my quick calculations. At 115F a lot more energy will be needed than at 90F. Delta T is more than doubled and COP will go down so I'd multiple my numbers by 2.5 for 115F. Also high humidity would increase my numbers.

 

[BS8899]

Lots of assumptions in my quick calculations. At 115F a lot more energy will be needed than at 90F. Delta T is more than doubled and COP will go down so I'd multiple my numbers by 2.5 for 115F. Also high humidity would increase my numbers.

Agreed!

Also, as we are cooling the (glass) cabin and 75mph (or higher) 115F air is moving over a (relatively) cooler cabin, more heat will be absorbed by the cabin in addition to the "static" sunlight/radiation/glass-dome effects.

 

[Pragmatic]

I estimate at 90F a decrease in miles/kw-hr of .02, at 115F it should be 0.05 (again my estimate with an average speed of 60mph). Higher speeds will lower this number (more energy per hour but less energy per mile) still 40% less than the impact of heating the car when it's 20F.

 

[BS8899]

I estimate at 90F a decrease in miles/kw-hr of .02, at 115F it should be 0.05 (again my estimate with an average speed of 60mph). Higher speeds will lower this number (more energy per hour but less energy per mile) still 40% less than the impact of heating the car when it's 20F.

OK, I understand your analyses. I think your point vis-a-vis, the efficiency impact and how it varies with speed is a valid point. That said, I think we need to scope the bigger problem first. Specifically,

We want to establish a pseudo-quantitative understanding of the degradation of mileage driving in hot weather by building a reasonable model. To that end, we need to identify the heat sources and the required AC unit.

Without going into the absurd details of doing a finite-element model, perhaps we can do a simplified lumped model as follows:

> Heat Sources:
[1] conduction and radiation from the sun's rays that heat the cabin (i.e., car at rest, no cabin occupant). Perhaps we can treat the cabin as a glass greenhouse?
[2] heat from the airflow at XX-mph over the passenger compartment that brings additional (in addition to the sun's radiation) heat flux into the passenger compartment, and
[3] heat emanating from the inside of the car (electronics, etc.) and by the occupants.

> Required AC unit:
> once we have the above, we can calculate the "steady-state" heat load and the BTU required to maintain a given steady-state temperature taking into account the efficiency of the AC unit.
> and from the estimated BTU above, we can estimate the resultant degradation of the mileage efficiency.

Perhaps there are aspiring physicists who want to tackle this problem. I know I am not qualified!

 

[koflo]

I’m averaging about 2.5 mi/kw. I was in the high 3s (3.6ish) before summer. Sure the AC matters but the external fan to cool the battery if constantly running which has lead me to believe that the real difference is the battery cooling.

 

[BS8899]

I’m averaging about 2.5 mi/kw. I was in the high 3s (3.6ish) before summer. Sure the AC matters but the external fan to cool the battery if constantly running which has lead me to believe that the real difference is the battery cooling.

Fair input. I'd think fans would not be a big factor compared to AC units. Others can chime in as you all know better than I.

 

[skndrni]

Yes, radiation and the glass dome effect of the Lucid cabin is a big factor. You are correct that static air is nor a good thermal conductor. But moving hot (115F) air at 75mph can convey a lot of heat.

+1 on that. Isn’t that how a convection oven works?

Same concern I had given all the Tessies with occupants broiled to medium rare. Hence, metal roof here, living in icy bliss.

 

[BS8899]

+1 on that. Isn’t that how a convection oven works?

Same concern I had given all the Tessies with occupants broiled to medium rare. Hence, metal roof here, living in icy bliss.

Don't forget "Air Fryers"....that's simply moving "Hot Air"

 

[koflo]

Fair input. I'd think fans would not be a big factor compared to AC units. Others can chime in as you all know better than I.

It’s possible, honestly I haven’t changed my AC habits much. I’m a “auto” only user and park in my insulated garage. Many times I precool my car as well while the cars still plugged in. Usually am at 72ish. My range went down so much that I actually called lucid and they said my consumption is expected and the explanation was due to the battery needing to be cooled down as the pavement is 150+ degrees when it’s 120 out in phx.

 

[joecrna]

My 12 hour run, usually 6 or less, was primarily do to charging. Waiting in line at the chargers and then long times charging do to what I feel is excessively conservative charging curves. I had to charge to 90% to reasonably expect to reach the next charger and have enough juice to run the AC for a while if all or most chargers were down on arrival. The run back from Phoenix "only" took 9 hours as the lines at chargers were much shorter. I was going through 70-80% of battery capacity between chargers and they are even less reliable in the desert then usual EA chargers.

The car is an Air Pure, when my GT became available, the people at the "sales" sites couldn't explain the difference to me except for the electric specific features. I wanted to know why as a car there was a $50k difference in price, so I opted to wait on the Pure. I wanted a hard top, better rear seat leg room and the soft close doors didn't matter to me. As it turns out an AWD Pure, fully loaded with post delivery PPF and a ceramic coat didn't cost much less then the GT-P. At least I got the color I wanted. With the Hugh drop in resale I'll definitely find out what can be done by the aftermarket once my 3 year experiment is over. It will be an exercise in performance only.

 

[Luigi S]

When I have driven in 118 degree heat I notice about a 10% loss in efficiency compared to the same drive at 95 degrees.

I live in the Palm Springs, California desert. I’ve had my Lucid for about two months. Have noticed a BIG drop in efficiency while driving in the local desert. Stop and go perhaps 45 to 50 miles an hour max. I get somewhere between 1.6 to 2.0 miles/kWh depending on the heat which could be anywhere from 110° to 120°. I did put shades, but they are not completely occlusive. I noticed in the same temperature drive on the highway. I get 3.5 miles per kilowatt hour or more. The cooling motor for the battery management system is extremely loud and drains a lot of power. I cannot for the life of me understand why driving the highway is more efficient than driving in the city at the same temperature. I set my regenerative breaking maximum and still and very disappointed. Currently in service to check out battery . I am an owner of three previous Teslas and none of them performed this way. I expect about 30% drop in range with very high heat in the desert and I’m getting with the Lucid somewhere in the neighborhood of 50 to 60% range drop. I would like to know if anybody else is getting this experience

 

[emoylv]

Yes, my range drops to about 50% during the height of summer (July/August here in Vegas). The cooling fans sound like jet engines, and they are loud! That's for driving between 3-5pm, usually the hottest time(s) of the day. I also avoid charging during these hours in the summer. Fans go wild and the charge rate plummets. Early morning, when the summer temp is around 95 yields really good charging rates.

 

[Luigi S]

Yes, my range drops to about 50% during the height of summer (July/August here in Vegas). The cooling fans sound like jet engines, and they are loud! That's for driving between 3-5pm, usually the hottest time(s) of the day. I also avoid charging during these hours in the summer. Fans go wild and the charge rate plummets. Early morning, when the summer temp is around 95 yields really good charging rates.

What is really weird? Is that in this same temperature, the highway arranged doesn’t suffer nearly as much as driving around in the city. I found I could still get about 3.1 to 3.5 miles/kWh or more driving on the highway at about 70 to 75 miles an hour even when it’s 110°. Not a chance I’m close to this when driving in the city.

 

[emoylv]

That's been my experience as well. Maybe it's the difference in temperature between the highway (light gray) vs. city streets (black)?

 

[Luigi S]

That's been my experience as well. Maybe it's the difference in temperature between the highway (light gray) vs. city streets (black)?

I think that moving air by driving means that some of that allows cooling a little easier without the jet fans coming on as much.