This might be a silly question but I was wondering if a car AC unit be run off entirely on battery or alternator?
As this could help in
Another reason that I could think of is for those people who might want to take a short nap and want to use AC without the car running.
I can't give you numbers or calculations without some work, but I can tell you than energy is never free. Cars have an A/C compressor that is mechanically driven by the engine because this is the easiest way to get the job done in a typical consumer car.
An A/C compressor actually takes a huge amount of energy to operate. In fact a central A/C unit for a house uses more electricity than anything else (in a hot climate). If you want to electrically power the A/C that power needs to come from somewhere.
If you wanted to generate enough power to drive an electric compressor, the required alternator would take even more engine power than the mechanical compressor because of inefficiencies and losses. If you wanted to use battery power, you would need very large batteries and a more powerful charging system.
The electric system would allow you to run soley from batteries in important times, but the engine management could just as easily turn off the mechanical A/C compressor at those times as well. Normally people won't melt if they have to do without it for the 20 seconds it takes to pass someone on the highway.
Well, Teslas (and presumably other all-electric cars) have A/C so it's not impossible, but A/C takes a fair amount of power.
On the other hand, Teslas store a lot of energy. The A/C is something like 2.4kW which is about 8,000 BTU/h or about 3HP. So using an electric motor on a conventional car might give you a few more HP briefly, but the alternator has to eventually make up the deficit, and a bit more for its own inefficiencies.
An easier approach would be to drop the A/C clutch out when you tromp on the accelerator. That might allow you to use a smaller engine for the same performance (passing especially) with a simple software change. Probably cars already do that. The A/C would run a bit more after you do that, to keep the cabin air temperature down. No free lunch.
You could always do it with a window air conditioner and a huge inverter, but the efficiency would be pretty bad I bet. The aerodynamics in the below example are also a bit dubious. In a pickup it might actually make sense=- put it in the bed- since a cheap Chinese window A/C is almost free.
The short answer to your question is no. As @JPhi1618 noted, the compressor is mechanically driven. Without the compressor you don't have any cooling. The AC runs by changing the state and pressure of a liquid, and compressing the liquid is a big part of that cycle. If you bolted an electric compressor onto the car you would have to find a way to switch between the mechanical and the electric compressor, and that no one makes. You would have to design and machine it yourself.
Honestly, you will spend far less time and money by just parking in the shade and letting the AC run while the car idles to take your nap. (How do I know this, lol) Not a perfect solution, but much cheaper and easier than trying to compress the AC coolant electrically.
Hope that helps!
Yes, this is possible. Several companies (VAG, BMW) have switched to AC units that are driven by electric motors. For BMW, this is part of their Efficient Dynamics strategy, where they'll switch the compressor on/off depending on engine load to optimize fuel consumption.
When the weather's not too hot, you can run the compressor intermittently without the passengers noticing anything, so they run the compressor when you're slowing down and switch it off when you're accelerating. Other companies use similar strategies, BMW is just the first I could find that offered some explanation of why they switched to electric AC.
Electric AC has another advantage. On smaller engines, you can feel when an engine-driven compressor engages. That hiccup in the drivetrain can get annoying. An electric AC is unnoticable.
Running the compressor while the engine is off is another matter. The compressor draws several kW. At 5 kW you'd drain a (really big) 100 Ah battery in 12 minutes.
I don't know how feasible it is to retrofit an existing AC system to electric.
There is NO free lunch, you will need many batteries and a much better alternator.
You could in fact attach a window mounted air conditioner like @Spehro Pefhany suggests.
OR
TEC Thermo Electric Cooling
There are peltier elements that also generate a cooling effect compressor free. They are essentially a plate of ceramic postively charged, and one negatively charged. This creates a heat flow where one side is incredible hot and the conversely the other side is cold.
So you could mount these either inside your door, or perhaps behind the backseat.
Either way you would need to transfer the resulting heat outside of the cabin. If you don't mind an inferno in your trunk, behind the back seat. You could attach the cold side to the back seat and put giant set of heat sink fins in the trunk, with 12v fan to keep it cool.
If you were mechanically inclined you could also use water cooling. Install a second radiator under the hood and 12v pump and fan. Then the heat would get exhausted that way. If you don't have enough cooling you will break your peltier element.
http://www.heatsink-guide.com/peltier.htm
here is an example: http://www.amazon.com/TEC1-12706-Thermoelectric-Peltier-Cooler-Volt/dp/B002UQQ3Q2
Larger ones exist. However, if you purchased a bunch, say 20 you could have a setup where you control how cool it gets by turning some off. I have no idea how many you would need, but give these are 1.5 inches many would be needed.
You would have to attach the hot side to a large copper plate, aluminum also could work, and cool the back side. The down side to aluminum is it can't store any heat, and having used these to cool CPU on a computer I have come to the conclusion copper is better. On the cold side copper or aluminum would work, but make sure the 2 sides don't touch. The peltiers are less than 4mm in many cases so you will need something thin to keep them apart, depending on the total size of your device. Also a thermal paste will increase conductivity and heat/cold transference.
You will then need a large array of heat sink fins and several 12v fan to cool the back side. Either that or a water cooling loop.
The good news is all of this can be done with 12v-15v. The bad news is even 1 peltier element requires 7 amps at 12v. The fans and pump (if you go that way) will use very little compared to this. You can buy 12v 120mm computer fans cheap that use very little power. Even 10 of these is probably 90A with the pumps and fans. Your battery will die quickly.
In electricity, the smaller the gauge, the thicker the wire.
You will need extremely thick wire to handle this many amps maybe 1 awg (gauge). For simplicity you can connect up to 20A on 12 gauge wire so 3 of the listed peltiers. Then you will have to connect those wire bundles to even thicker wires and so on. Finally to a car battery, hopefully a 2nd car battery. In fact you will need many car batteries to keep this array powered for any length of time.
I have seen vehicles with multiple alternators, such as BASS competitions. I can hook up 2 alternators to my car but have seen 10 to 15 alternators hooked up to the subwoofer competition trucks. There would have to be enough power to run a small 5kbtu rv ac unit through your sunroof (with a custom painted fiberglass roof scoop to cover the whole unit ofcourse, allowing adequate air flow).
2 105 amp alternators, big high amp inverter, custom capacitor setup for the surges?
Would be nice for turbo cars that have to run a big front mount intercooler. I would sacrifice 50 to 75 pounds no problem for ice cold ac in Phoenix.
The key here is where does the power come from... If you use the vehicle battery then the charge come from the alternator and therefore the engine - btw the alternator can take approx 10bhp or more if producing full charge.
The possibility of a second battery with a split charge system still requires the energy to come from the alternator and engine.
A second alternator to charge the second battery is still taking power from the engine and, now the engine has more rotating weight to drive.
So, to remove the load of your AC from the engine, either direct or indirect, needs a separate power source - you could consider a bank of batteries that you charge at home and give sufficient power for your trip, but the weight will not improve the fuel consumption and you need to have the space available.
