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Old 06-25-2021, 04:09 PM   #1 (permalink)
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Electric AC compressor?

Hello everyone, I am wondering if anyone here has ever somehow used an electric AC compressor in a vehicle or has any ideas on how to do so. I like being cool but running the AC really kills my MPG, so I am wondering how practical an electric AC compressor would be so I could stay cool without killing my MPG. The other really nice benefit of an electric AC compressor is that I could run the AC while parked without running the engine so I could be comfortable while sitting in my car eating and such.

Obviously for this idea to be beneficial, the electricity to run the compressor would need to come from a source other than the alternator. I'm thinking a gen 4 Prius battery would be a good power source due to its 8.8 Kwh capacity, relatively small size and weight, and relatively low cost at a junkyard. If an electric compressor draws 2Kw I should be able to run my AC for 4 ish hours off of a charge, which is way more than enough for me.

Initially I was thinking of using the Prius compressor since those are available used for pretty cheap, but after a bit more research it looks that may not be possible since it appears to be a 3 phase inverter driven AC motor and the inverter seems to need a signal from the climate control module to function.

After seeing how expensive DC powered AC compressors are, I'm starting to think a better option may be to utilize the OEM compressor by using a belt drive and an electric motor to spin it. This method would also have the advantage of not requiring custom AC lines as long as I could find a motor small enough to drive the compressor in its current location, perhaps something that could fit where the power steering pump used to be. I would just need to figure out a way to power the compressor from the battery pack.

Although this setup would no doubt be complicated and pretty expensive, it also has some very real benefits and I think it should be doable.

If anyone has input, ideas, or suggestions, please feel free to share them. I appreciate any input.

Thank you very much in advance, looking forward to hearing your thoughts on this concept!


Last edited by EcoCivic; 06-25-2021 at 04:15 PM..
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Old 06-25-2021, 04:45 PM   #2 (permalink)
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I'll do the math when I get home, but you can likely expect an electric compressor to hurt your fuel economy even more - because ultimately you're still drawing that power from the engine, but you're converting it from mechanical to electrical and then back to mechanical energy, with large losses with each conversion.

I would expect a compressor to drain a 12v battery flat in maybe 10-15 minutes, at a guess.
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Old 06-25-2021, 05:03 PM   #3 (permalink)
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Quote:
Obviously for this idea to be beneficial, the electricity to run the compressor would need to come from a source other than the alternator. I'm thinking a gen 4 Prius battery...
Upsize the battery and you could do an alternator delete.
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Old 06-25-2021, 05:44 PM   #4 (permalink)
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How sunny is it where you live? What kind of driving do you do?

Since I drive around town slow a lot I've thought of building a big solar panel over the top of the Prius. One that goes from bumper to bumper and side to side. It would double as a portable shade. It's very sunny here and aerodynamics wouldn't be hurt that much since the town speed limit is 25mph. If I add a boat tail I might be able to make it as big as 2kW if I could angle it towards the sun. That's not enough to satisfy the Prius A/C at full blast, but would be enough once it's cooled the interior down some.
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Old 06-25-2021, 06:05 PM   #5 (permalink)
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I did a quick search for "Car A/C horsepower", and Wikipedia suggests the typical energy consumption is ~4hp or 3kw. That seems a little high to me for a compact car, but let's roll with it.

3kw @ 12v is 250 amps.

A typical 90's and early 2000's Civic alternator is 75 amps, so you'd need around 4 alternators maxed out to run the A/C compressor and the car. Alternators are optimistically 50% efficient, so the load on the engine would be ~8hp, effectively doubling the fuel economy loss.

A group 51 battery has around 30 amp hours when new and fully charged. Running the A/C compressor would run the battery flat in 7 minutes, assuming zero losses. You likely wouldn't be able to start the car with it after a few as 4 minutes.

Also:
Quote:
Lead-acid batteries can only be discharged up to 50% before irreversible damage occurs.
So, let's make that 3.5 minutes and 2 minutes.

The last part is the maximum safe draw from the battery. I expect you'd need about 3 typical Civic car batteries to not damage it from the compressor current draw. The bright side? You'd get a few more minutes of run time.
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Old 06-25-2021, 06:19 PM   #6 (permalink)
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How about a parallel electric/battery system with ample lithium battery capacity to drive an electric compressor. Plug in for battery recharge and a moderately sized solar panel for a little return of power on the road. When the battery hits a specified drain level, no more AC. In the winter the system would just power the heater fan. Pricey AC system, probably.
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Old 06-25-2021, 06:30 PM   #7 (permalink)
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If aiming to run a 3kw compressor for an hour, it would be around $1700 in unmanaged lithium batteries, and the battery would weigh around 65lbs. That would also be roughly the minimum safe battery size such that the compressor wouldn't cook/destroy the battery.
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Old 06-25-2021, 06:57 PM   #8 (permalink)
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The question is how much power really is needed. It's one thing to get into your car that's been baking in the sun, and another 10, 15, 20, 30 minutes and beyond after you've started to cool the car. Is the 3kW average over the entire period? Wouldn't you need less once the car had cooled a while? Is 3kW what a Civic needs or average between a Civic and a Suburban?

Somewhere on this forum there was a guy who made his own cooling seat and just filled a chest with ice water before heading out. It's much more efficient to just cool your body instead of the entire cabin.
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Old 06-25-2021, 07:06 PM   #9 (permalink)
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Thanks to everyone who responded. To be clear, I realize that running an AC compressor off of the alternator would be pointless and likely detrimental. My idea was to add a large lithium ion battery to the trunk to run the AC compressor off of and recharging the battery by plugging in when I get home.

That's a good idea to get a battery large enough to run the whole electrical system off of and doing an alternator delete, I didn't think of that. I have an Optima Yellowtop D34 battery and an alternator shutoff switch, so I have adequate capacity to do alternatorless driving for my normal routes. But if I got a large enough battery and an electric AC compressor I could remove the Optima battery as well as the alternator and the belt. Then my only accessory losses would be the oil pump and water pump.
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Old 06-25-2021, 08:13 PM   #10 (permalink)
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If you really want efficient A/C and have good fabrication skills, you can try fitting a modern variable displacement compressor. Another strategy is to modify the existing system to reduce its output so it's more efficient when you're driving faster.

If I understand how they work correctly, slightly decreasing the amount of refrigerant in the system would make the compressor (which has a fixed pressure ratio) consume slightly less power at any given speed. It may need to be running at a higher speed before the condenser side has enough pressure to work well.

Another method is an underdrive pulley.

AFAIK, electric A/C is probably best left for retrofitting cars without A/C, since its main advantage is simplification of packaging, not efficiency or cooling effectiveness. You're going to need a pretty crazy amount of battery to run A/C for more than a few minutes.

What sort of driving are you doing where the AC is killing your mpg? At idle, I find A/C uses a pretty modest amount of fuel (like 0.1gph ish). If it's travelling at high speed with the engine turning faster, I would first look to underdriving the compressor somehow.


Last edited by serialk11r; 06-25-2021 at 08:22 PM..
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