# Range extender using stored compressed air



## ga2500ev (Apr 20, 2008)

courtney2018 said:


> I did a few searchs on here to see if this had been talked about before but couldn't find anything.
> 
> I was wondering if anyone had thought of using compressed air tanks to drive an alternator once you got to the halfway point in your route. For me, that would be at work. I was thinking you might be able to drive an alternator, maybe coupling it up to a used turbo or something similar, to recharge the batteries slightly. I know it wouldn't give you a full charge, but it might give you an extra 15% in distance.
> 
> ...


There's a compressed air discussion going on the blog section. I just posted to it today.

Compressed air seems like a dead end. Much of the energy in the compression process is converted to heat, then lost as the decompressed air chills as it's released.

The other problem is that you need a lot of air. That means you're going to need high compression and really sturdy tanks.

But one potential advantage is that you can gain a form of regenerative braking even when using a series traction motor. If you run a compressor when braking, you can pump the tanks as you slow. That would get you back a measure of lost energy that you can reuse later.

But at the end of the day it's going to cost a lot of setup to get going for a very limited energy return.

ga2500ev


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## courtney2018 (May 22, 2008)

I think you're talking about using the vehicle in some way to compress the air in the tanks. I'm talking about doing that at home. Have a compressor in the garage or whatever compress air in the tank to be used for the next day. 

I'm not concerned about the temperature drop of the air as it comes out. I want to utilize the force in the pressurized air to put mechanical work on something - an alternator - and transform that into something I can use - electrical stored energy in batteries.


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## mattW (Sep 14, 2007)

the range of EVs and Compressed Air vehicles are pretty similar anyway so without a recharging infrastructure of high pressure charging stations I don't think you would really get much of a range advantage compared to using just one system of storing energy (batteries).


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## ga2500ev (Apr 20, 2008)

courtney2018 said:


> I think you're talking about using the vehicle in some way to compress the air in the tanks. I'm talking about doing that at home. Have a compressor in the garage or whatever compress air in the tank to be used for the next day.


I'm aware of your suggestion. Started down that path about a month ago. Looked at paintball compressors, scuba compressors, oxygen tanks, and the like.



> I'm not concerned about the temperature drop of the air as it comes out. I want to utilize the force in the pressurized air to put mechanical work on something - an alternator - and transform that into something I can use - electrical stored energy in batteries.


The problem is that if the air is coming out cold, then you're losing energy because it isn't an isothermic reaction.

There are several articles on the Wikipedia about compressed air economies. Take a read when you get a chance.

The bottom line is there is absolutely no way you're going to get more energy out of compressed air than you're going to get out of batteries. So unless compressed air is going to give you some other advantage (like regen braking for example), there is no reason to carry compressed air over carrying batteries.

A better economy would be liquid nitrogen or liquid air. Each has better than 10 times the energy content of compressed air (@ 300 bar) at equal weight. In addition if you can heat the liquid nitrogen you can get fuel economies approaching 100 MPG. The U. of Washington group that studied using LN2 for cars wrote this comparison paper of different configurations:

http://www.aa.washington.edu/AERP/CRYOCAR/Papers/sae99.pdf

But right now there's nothing better than batteries. Low cost lightweight, high energy battery technologies are a better direction to search.

ga2500ev


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## weelliott (May 12, 2008)

courtney2018 said:


> I think you're talking about using the vehicle in some way to compress the air in the tanks. I'm talking about doing that at home. Have a compressor in the garage or whatever compress air in the tank to be used for the next day.
> 
> I'm not concerned about the temperature drop of the air as it comes out. I want to utilize the force in the pressurized air to put mechanical work on something - an alternator - and transform that into something I can use - electrical stored energy in batteries.


Even though you are not concerned with the air being cold, fact is, thermodynamics has stolen energy from you. When you compress the air it gets hot. That is energy that you put into the system being converted to heat. It then cools. That is your precious energy going away. When you release the pressure by 'using the air' it cools to lower than the temperature it was when it went in.(This is how a refrigerator, AC unit, or heat pump works.) If you were to in theory keep the airtanks superinsulated, then you would reduce the energy loss.(the compressor still needs to be cooled.) You could also heat the tanks to attempt to keep the pressure high as it dropped because air is used. That seems dangerous to me though. It also requires energy for that heat.

Air tanks are also really heavy for the amount of energy you can store in them. 

If you decide to go air, a turbo is not an efficient way of getting air to power something. Using a positive displacement pump would cut your losses. Like a power steering pump or compressor run backwards.

There is a guy from France I believe that has developed an "air powered" car. I put that in parentheses since air is not a fuel. Energy is only stored there, and inneficiencies on each end really make it non-optimal. He is another guy who claims to have been the best thing that ever happened to automobile racing and that now his genius is available to everyone in the form of his latest invention.


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## dfwheelman (May 15, 2008)

mattW said:


> the range of EVs and Compressed Air vehicles are pretty similar anyway so without a recharging infrastructure of high pressure charging stations I don't think you would really get much of a range advantage compared to using just one system of storing energy (batteries).


Using compressed air and EV together kinda makes sense but kinda doesnt.

Consider the air cars get better range, normal speed off one tank full that takes 5 hours to charge at home. Compare that with an EV that gets half the range, more cost, more weight and takes all night to charge fully.

However, compressed air only conversions, I don't know if thats feasible because of the vehicle weight. Guess that depends on the car.

You could get a lot of amps out of a heavy duty alternator (300+ at 24v). Since you cant charge while driving, you would simply be adding current to your pack. However, you *could* charge while parked, shopping, dining, etc and that would add range. Every time your contactor goes "off" your charging circuit could go "on"

I'm by no means an expert but like Spock said, "It's only logical"

Mark


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## dfwheelman (May 15, 2008)

http://www.mdi.lu/eng/affiche_eng.php?page=moteurs

Maybe someone disagrees with your negative assessment.

Ironically, the engine looks a lot like a Stelzer motor (two free pistons on common axis)

Been studying the designs for stelzer for a while and 2 cycle engines/compressors. Looks promising.


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## mattW (Sep 14, 2007)

dfwheelman said:


> * Since you cant charge while driving*, you would simply be adding current to your pack. However, you *could* charge while parked, shopping, dining, etc and that would add range. Every time your contactor goes "off" your charging circuit could go "on"
> 
> I'm by no means an expert but like Spock said, "It's only logical"
> 
> Mark


Whoever said you can't recharge while driving? Have you ever used your phone while its plugged in? You car battery is almost always being charged while powering the lights, ignition etc. There is no reason why you can't recharge while moving, that's the whole concept behind the Volt.


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## Manntis (May 22, 2008)

ga2500ev said:


> Compressed air seems like a dead end.


[/thread]

Compressed air as an 'energy source' is about as practical as hyrodgen. You are transferring energy from one form to another (potentially explosive) one, then back again, losing energy in the inefficiencies in between.


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## dfwheelman (May 15, 2008)

Manntis said:


> [/thread]
> 
> Compressed air as an 'energy source' is about as practical as hyrodgen. You are transferring energy from one form to another (potentially explosive) one, then back again, losing energy in the inefficiencies in between.


I'd have to say the same thing about EV though.

You're transferring energy from your house, through a not-so-efficient battery charger, into your batteries, back through a not-so efficient controller, into a not-so-efficient electric motor, into a inefficient transmission, onto high friction tires to propel your car down the road. 

So, everything has its inefficiencies .


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## mattW (Sep 14, 2007)

If you haven't read my blog about them, this is the key to how you store your energy:

"According to the efcf, the cars can have a theoretical compression efficiency of 46% and a tank to wheel efficiency of up to 84%, the report concludes that given 100MJ of energy a hydrogen FCEV could travel 42km, an air car 46 and a Li-ion BEV an impressive 133km."

EV's drastically trump the efficiency stakes (by a lot) and therefore are the best option. The only advantage H2 or O2 have are the quick recharge options, which may or may not be a deciding factor. I'd take O2 over H2 but I still rather have an EV.


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## dfwheelman (May 15, 2008)

mattW said:


> Whoever said you can't recharge while driving? Have you ever used your phone while its plugged in? You car battery is almost always being charged while powering the lights, ignition etc. There is no reason why you can't recharge while moving, that's the whole concept behind the Volt.


your Alternator is powering things while you're driving, not your battery.

I'm not a physicist but I'm pretty sure that current battery [lead acid] technology cannot be simultaneously charged and discharged. The current flow would be going two directions.

Now, with that said, there is a way to do it but that involves advanced stuff that we do not have in our batteries.

Read this really long article on the subject

http://keelynet.com/bedmot/bedbear.htm


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## dfwheelman (May 15, 2008)

Here's my "What If" on this subject.

Your electric motor is a perfect heat source to serve in expanding the compressed air.

Wrap copper tubing around the motor, taking care to avoid the terminals. This copper tubing would serve as the inlet to the air-motor. Basically, route the air though the now heated tubing into the air-motor to drive the alternator. You could have a home charging system filling a tank while your away. You come home, connect your line to the filled home tank, in 3 minutes your charged. Off you go.

The tubing also serves as a heat pump to cool your motor.

This of course is just theory


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## mattW (Sep 14, 2007)

dfwheelman said:


> your Alternator is powering things while you're driving, not your battery.
> 
> I'm not a physicist but I'm pretty sure that current battery [lead acid] technology cannot be simultaneously charged and discharged. The current flow would be going two directions.
> 
> ...


Current wouldn't flow in two directions, you just get a net current, depending on the voltage which is the sum of the two (positive and negative, in and out). You would not be simultaneously charging and discharging just one or the other depending on the generator output/motor draw.

An EV with a generator is exactly the same as the car battery. There will be a slight voltage difference between the generator and the battery pack, i.e. charging voltage minus pack voltage, which will draw current based on the internal resistance of the batteries. The motor will also draw current from the generator and so you are officially charging while moving as requested.


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## Manntis (May 22, 2008)

dfwheelman said:


> I'd have to say the same thing about EV though.
> 
> You're transferring energy from your house, through a not-so-efficient battery charger, into your batteries, back through a not-so efficient controller, into a not-so-efficient electric motor, into a inefficient transmission, onto high friction tires to propel your car down the road.
> 
> So, everything has its inefficiencies .


"transferrring" does not mean changing location. Re-read what I wrote: "_transferring energy *from one form to another*_".


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## Manntis (May 22, 2008)

dfwheelman said:


> I'm not a physicist but I'm pretty sure that current battery [lead acid] technology cannot be simultaneously charged and discharged. The current flow would be going two directions.


Current 'flow' does not block the charging of a battery that is discharging. Electrons creep through a wire at a rate of a few centimeters per minute in DC, and oscillate in a relatively fixed position in AC. It is the electromagnetic field _around _a wire that does the work.

Yes, a battery can be charged and discharged at the same time.


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