9/11/08 - Air Cars vs. Electric Cars vs. Hybrids - Which are Greener? - gas2.org

[jstdadd]

Source: http://tinyurl.com/5qr5v3

AirCar - 0.176 lbs CO2/mile
Prius - 0.34 lbs CO2/mile
Aptera - 0.044 lbs CO2/mile, based on PG&E carbon generation footprint; .114 lbs CO2/mile based on US Average CO2 Electrical Powerplant.
^^^^^^
Edit Here: I originally mis-stated the PG&E number as being the US Average number.

The Tesla CO2 footprint was approximately twice the Aptera.

[KarenRei]

MDI (the Air Car company) has been producing vaporware and specious claims for over a decade. A car with their claimed specs has always been just six months away. Don't hold your breath. Compressed air has very poor volumetric energy density and air compressors are extremely inefficient; the only real energy and CO2 savings would be if they made the car light and streamlined enough that the gasoline power, which would be providing the overwhelming amount of the energy for the drive, would take you further on a gallon.

[KarenRei]

I should also add that contrary to MDI's claims, Tata has ruled out producing the car in the immediate future:
http://www.cartradeindia.com/news/tata-compressed-air-car-to-take-time-110388.html

And I'd recommend anyone listening to the buzz about the Air Car read this reality check:
http://edition.cnn.com/2008/TECH/08/08/air.car/?imw=Y&iref=mpstoryemail

It is possible to power a car with compressed air, but the mileage claim is "at the edge of possibility," said John Callister, director of the Harvey Kinzelberg Entrepreneurship in Engineering program at Cornell University's College of Engineering.

He noted that such dramatic fuel efficiency is associated with tiny experimental cars, not bigger mainstream ones.

Another expert expressed concern about the amount of energy it would take to generate the required air pressure: 4,500 pounds per square inch, or more than 120 times the pressure inside the tires of a typical four-door sedan.

"That is above what you normally find even in an industrial setting," said William Bulpitt, senior research engineer at the Georgia Institute of Technology's Strategic Energy Institute.

"That takes quite a compressor to do. ... It takes horsepower to compress the air up to that pressure."

If you count that energy, it's hard to believe the car would be that much more efficient than an electric vehicle, Callister said.

Again, don't hold your breath.

[n_dawg]

Oh, in response to Karen's claims about compressor inefficiency: chaining compressors together in multiple stages with intercoolers allows you to approach adiabatic performance. The MDI compressors have 3 stages, and are supposed to be 70% efficient.

[KarenRei]

70% polytropic efficiency perhaps. Certainly not system efficiency. Even massive regenerative industrial compressors going for pressures like this don't get that. Polytropic efficiency can approach 100% because it's the ratio of the minimum amount of work theoretically needed to the actual work needed. System efficiency is very low because there's a big difference between the minimum amount of work needed and the amount of energy stored.

There is no single efficiency measure for compressors; in various literature, you'll hear about volumetric efficiency, adiabatic efficiency, isothermal efficiency, mechanical efficiency, overall efficiency, and system efficiency. System efficiency is the number that really matters here. System efficiency is the overall adiabatic efficiency times the motor efficiency times the controller efficiency times the efficiency of any other components.

Just from a fundamental perspective, it is essentially impossible for an air car to have a higher system efficiency than an equivalent li-ion car because they both have electric motor losses and li-ion batteries are nearly lossless. So even if the compression cycle itself was nearly lossless -- which it's not even close to -- it still wouldn't win. Yet just from a friction perspective alone, let alone the far more critical waste heat issue, there's far more friction in a multistage compressor and then a subsequent compressed air engine than just a direct electric drive.

[Rhodomel]

Just from a fundamental perspective, it is essentially impossible for an air car to have a higher system efficiency than an equivalent li-ion car because they both have electric motor losses and li-ion batteries are nearly lossless...

I STRONGLY DISAGREE with this one about Lithium Ion, perhaps KarenRei got the FACTS WRONG! Proof is that without even doing any work by electricity, Lithium Ion batteries on standby mode lose their energy... Do you want me to spell out how much they lose by not doing anything? I believe it is certainly very significant to be far from nearly lossless!

[KarenRei]

Some references for li-ion efficiency:

99.9%:
http://www.batteryuniversity.com/partone-12.htm

Greater than 95% efficiency:
http://www.electricitystorage.org/pubs/2000/summer2000/ConventionalBatteries.pdf

94%:
http://www.smartsparkenergy.com/pdf/VPPPaper.pdf

95.1% to 98.8%, depending on charge/discharge rate
http://www.greencarcongress.com/2008/02/nissan-tests-ne.html

Now, you seem to be referring to self discharge rate instead of charge-discharge efficiency. I've seen numbers ranging from 1% per month to 5% per month; I can get you refs on that too if you want. That's likewise a trivial amount compared to normal usage.

And lastly.... who are you, and why did you create a new account just to write a single angry post?

[AndyH]

Source: http://tinyurl.com/5qr5v3

AirCar - 0.176 lbs CO2/mile
Prius - 0.34 lbs CO2/mile
Aptera - 0.044 lbs CO2/mile, based on PG&E carbon generation footprint; .114 lbs CO2/mile based on US Average CO2 Electrical Powerplant.
^^^^^^
Edit Here: I originally mis-stated the PG&E number as being the US Average number.

The Tesla CO2 footprint was approximately twice the Aptera.

Hmmm...we're either really close to, or have been beaten by the Honda Civic GX natural gas car.

I haven't found actual CO2 rates for the car, but did find that it's Tier 2 Bin 2. Tier 2 Bin 2 (http://www.dieselnet.com/standards/us/ld_t2.php) has a max CO2 per mile of 2.1 grams - .00462 lb/mile. A 1993 LNG Accord came in under .8g/mile in CARB testing (http://www.arb.ca.gov/regact/cng-lpg/appb.doc) - .00176 lb/mi. I expect the later Civic, with their advanced computers and emissions devices to beat the old Accord numbers.

If those numbers and assumptions are good, we'll need to use wind/solar/hydro to refuel our electric cars to beat the GX.

(Jay Leno's Garage) (http://www.jaylenosgarage.com/video/video_player.shtml?vid=193319) (As an aside - notice how 'charging overnight' is seen as a negative for electric or PHEV cars, but is a plus for an NGV?)

Andy

[KarenRei]

You're confusing CO with CO2. A car like the Aptera will emit essentially zero CO, since even fossil fuel power plants emit dramatically less CO per joule than internal combustion engines. In terms of pollutants, turning an equivalent gasoline car into electric, on our current national average grid, will cut your CO2 by a third, keep your NOx the same, keep your SOx the same, nearly eliminate CO, nearly eliminate VOCs, and increase PM. California power is cleaner than the national average, and overall, we're moving in the direction of grid cleanup.

Of course, an Aptera is no conventional car's equivalent ;)

[AndyH]

Right, Karen - I dropped a 2. :( Sry. Ceiling Cat's gonna bat me in the head for that.

The best I've found for real numbers is a somewhat generic 1999 study (http://www.ipd.anl.gov/anlpubs/2000/01/34988.pdf) by Argonne Nat. Lab that shows a nearly 20% reduction in CO2 in a natural gas car when compared to running gasoline. This is a full fuel cycle analysis, not just tailpipe emissions. (caution..quick scan thru bleary eyes...)

That would bring a CNG/LPG Prius to about .27 lb/mile. This car has been the undisputed best of the ICE world for emissions for the past 5 years and is rated better than the Prius by both the EPA and the ACEEE (http://www.greenercars.org/highlights_greenest.htm). AT-PZEV/SULEV/ILEV I'd really like to see some actual tailpipe emissions numbers. I found one Gov't report referenced but the website's down.

Yes - electric is where we need to be. Absolutely. Better than any ICE.

Grid cleanup - a good thing. I'm looking forward to an EV living in my off-grid wind/solar yard. Not this week, though...or next...

Andy

[jstdadd]

Hmmm...we're either really close to, or have been beaten by the Honda Civic GX natural gas car.

I haven't found actual CO2 rates for the car, but did find that it's Tier 2 Bin 2. Tier 2 Bin 2 (http://www.dieselnet.com/standards/us/ld_t2.php) has a max CO2 per mile of 2.1 grams - .00462 lb/mile. A 1993 LNG Accord came in under .8g/mile in CARB testing (http://www.arb.ca.gov/regact/cng-lpg/appb.doc) - .00176 lb/mi. I expect the later Civic, with their advanced computers and emissions devices to beat the old Accord numbers.

If those numbers and assumptions are good, we'll need to use wind/solar/hydro to refuel our electric cars to beat the GX.

(Jay Leno's Garage) (http://www.jaylenosgarage.com/video/video_player.shtml?vid=193319) (As an aside - notice how 'charging overnight' is seen as a negative for electric or PHEV cars, but is a plus for an NGV?)

Andy

Andy:

The production, delivery and fueling costs need to be considered. You have to get the well drilled, get the NG out of the well, down a high-pressure pipeline to your local distribution center, and then run an in-home or fueling station compressor to put the gas into your car. Electric moves the NG to your car. The NG in the US is often stored in underground 'vaults' until needed, and there is an energy consumption budget there, also.

The production and delivery costs of fuels cannot be ignored for the carbon budget of using those fuels.

All that said, NG is WAY, WAY better, IMHO, to moving crude to the US on big ships, refining it here and delivering it to your car. NG keeps the money spent in the US.

Good catch on the car, though. If not for the Aptera, I would look into buying one!

[jstdadd]

I should also add that contrary to MDI's claims, Tata has ruled out producing the car in the immediate future:
http://www.cartradeindia.com/news/tata-compressed-air-car-to-take-time-110388.html

And I'd recommend anyone listening to the buzz about the Air Car read this reality check:
http://edition.cnn.com/2008/TECH/08/08/air.car/?imw=Y&iref=mpstoryemail





Again, don't hold your breath.

One of Tata's biggest problems is actually building the factory (for a wide range of vehicles.) They have had huge demonstrations against them in India by people working in agriculture, they don't want agricultural lands converted to industrial use. Source: http://tinyurl.com/3wxq9b
They are also being demonstrated against at every manufacturing location in India and at their headquarters in Mumbai.