Battery Electric Vehicle HVAC

[dmielke]

Is anyone familiar with BEV vehicles running 100% recirculation at low ambient temperatures?

I understand that eHVAC have a big impact to BEV range when its cold and running 100% recirculation is less energy consumption compared to 100% fresh air, so it seems like there is a motivation to do this. I also understand that recirc air at low temperatures has issues with fogging and co2 build up, but i'm sure there are solutions to prevent those issues.

So i'm curious if anyone has seen BEV HVAC solutions to extend range by utilizing only recirc air and also providing solutions to fogging/co2? or is there any other reason fresh air should be used at low temperatures?

 

[HVAC-Novice]

The amount of fresh air needed and the need to dehumidify (foggy windows!) Is the same as in an ICE vehicle. Maybe they use some better insulated glass in BEV. But the humans in the car still need oxygen.

I hope no one uses 100% recirculation or skimps on heating/dehumidification.

They have to be a bit more efficient to get the heat needed. Research the Tesla super-manifold and octa-valve how they scavenge all available heat. Most other BEV manufacturers are much less efficient an rely on more resistive heat.

I'm sure a smart person has figured out if increasing recirculation (and dehumidifying) is more efficient than just reducing recirculation. Since they can scavenge condenser heat from de-humidification i suspect they get away with smaller amounts of fresh air (but not zero). You also could heat the glass. but then you need to add a heating film on all windows.

 

[BrianPetersen]

I have a Chevy Bolt. It has simplistic HVAC by today's standards, no heat pump, no heat recovery, but it does have automatic climate control (which I end up overriding when I can judge better than it does).

You can't use recirc when the temperature is low enough that the air-con cannot operate to dehumidify the air, because otherwise the windows will fog up or freeze up. The only way you could conceivably do this, is to run the air-con to dehumidify the recirculated air before re-warming it to interior temperature. But if you're going to do that, what's the point of recirculating? You might as well just draw in outside air and warm it up as needed (which inherently reduces its relative humidity), and vent the humidity outside. Dehumidifying takes quite a bit of power.

The automatic climate control takes a conservative (it guesses "more airflow" to make sure) estimate on how much fresh air it needs to blow through in order to keep the windows clear. My own normal operation is to press "auto" until the interior gets some warmth into it, then turn the fan speed down to the minimum that allows just a little window fog/frost around the edges, which I can see around and live with. Dropping the fan speed cuts down the HVAC loading quite a bit, and this car uses resistive heating, so it's worthwhile. I'm not sure there's any way to automate that.

Recirc is really useful in cooling mode in summer, because the recirculated interior air is cooler than the outside air, and in a high-outside-humidity situation, probably less humid also. In heating mode, you really need the defog/defrost function.

Automatically-regulated heated seats and heated steering wheel allow HVAC heating demand to be reduced, and those work well.

Maybe I'll do an experiment tomorrow by hitting "recirc" just to see what happens. Forecast is for just below freezing, so it will be in heating mode. I suspect the windows will fog up unless I also turn the air-con on. (Same as any other car would.)

 

[BrianPetersen]

Simple test done.
Vehicle is a 2023 Chevrolet Bolt EV.
Outside temp today was around -5 C, partly cloudy.
Climate control set to 20 C on auto. Heated seats and steering wheel on auto.
One person in the vehicle.
Pressing "recirc" eventually leads to the windows starting to fog up after about 10 or 15 minutes. (Unsurprising. It doesn't do this if you have it on outside air.)
If there's a difference in energy consumption or in the amount of time it takes to warm up the interior compared to just leaving the fresh/recirc control on "auto", it's too small to be readily discernible.

If you have to run the air-con together with recirc mode to act as a dehumidifier, what's the point compared to using low-absolute-humidity outside air to start with?

The most economical way of using heating-mode HVAC in this car that still keeps it decently comfortable inside, is to hit "auto" and then after the chill is off the interior, turn the fan speed down a notch at a time until you find the minimum that is right where the edges of the windows are just starting to fog. (Usually 2 or 3 steps out of 8)

 

[Compositepro]

Seems like a good application for a heat interchanger between the fresh air inlet flow and the exhaust air flow.

 

[BrianPetersen]

Question then is whether the cost and weight and space of such a thing is worth the trouble. Not much temperature difference means huge surface area is needed.

 

[3DDave]

You might think an exchanger would be good, but it needs to exhaust air at a temp above freezing or the exchanger just plugs with ice the same as the window freezes over.

 

[HVAC-Novice]

The fresh air inlet is in front of the car and air gets exhausted at the back. An ERV would require ducting, an additional fan and the fresh air fan to be larger. That needs space and adds weight.
I think the exhaust of the car also acts a de-pressurization of the cabin. S it must have little air resistance. Like in old cars were closing the doors created an over-pressure.

I also suspect at extreme cold or hot ambient the rear of the car isn't at exact desired temperature and a bit warmer (summer) or colder (winter). So the ERV effectiveness will be a bit diminished.

Building ERV can deal with freezing. But they are in buildings that constantly exhaust room temperature. All depends on dewpoints etc. When the car starts up, you start with cold air and the ERV likely will freeze. So you couldn't really use it before the car warms up, which is where I would like the most heat recovery.
for an ICE vehicle that makes it pointless since after longer driving, heat is free anyway. For an EV, it may or may not be worthwhile at all. Someone smart has to really design and investigate. There is a chance schlepping all the weight and added fan energy over the life of the car uses more energy than what will be saved. Plus more filters, cost, and maintenance.