Farting Along With Air Cars
Posted by Max Dunn Sat, 08 Mar 2008 00:32:00 GMT
I was trying to explain to my wife how an air car worked, and with a smile on her face she said “So it farts as it moves forward?” Well maybe that sums it up nicely.
But do air cars really make sense? The information from the manufacturers seem like they are the solution to our energy crisis, and to everything else too [1]. But there are several fundamental problems with air cars.
The first is energy density. This air car has a 300 liter tank with a pressure of 4500 pressure per square inch (psi). This is a pretty big tank, about 80 gallons, and it needs to be really strong to withstand the tremendous pressure. Even so, the compressed air in this tank only contains about 14 kilo-watt hours (kWh) of energy [2] [3] [4]. Assuming the car is pretty streamlined, it should get about 3 or 4 miles per kWh, giving it a range of about 50 miles. To get more range would require another tank (unlikely because it would take too much room) or to compress the air more (unlikely because the tank would need to be even stronger and it is hard to find air compressors above 5000 psi).
The second problem is that it is inefficient to compress air to fill up the tank. The best we can do get is about 46% efficiency for the larger compressors. To put this in perspective, for 100MJ of electric energy input, an air car can travel 46km versus over 120km for an electric car [5].
One advantage air cars have over electric cars is that it is possible to refill them quickly if there were stations that had large compressed air tanks to refill from. In this case, an air car’s tank could be refilled in only about 4 to 5 minutes.
However, unless a lot of air car filling stations become available, users will have to fill them up at home.
To figure out what it would take to fill up your air car at home, we can look at filling scuba tanks which are normally 80 cubic feet (cf) and are filled to 3000 psi. The 300 liter tank on the air car will hold about 3200 cf of air, or about 40 scuba tanks worth. The Max-Air 90 air compressor puts out 10.8 SCFM and is quoted to fill a 80 cf scuba tank from 500 to 3000 psi in 7.5 min [6]. So to fill the equivalent of 40 scuba tanks at a pressure 1.5 times higher would take about 7.5 hours. That is okay, since we can let it run overnight.
How much energy would this take? The Max-Air 90 has a 9 horse power (hp) gas engine, which is equivalent to a 6.75 kilo-watt (kw) electric motor. So running this overnight would take about 50 kWh of electricity. This sounds about right because it would mean the compressor has about a 30% efficiency.
But now we run into another problem: these compressors are not cheap. They are not like the 150 psi shop compressors that you can buy at the auto store. In fact, they run about $8,000 and up! [7]
As Larry Rinek, senior research analyst for automotive technologies at consultancy Frost & Sullivan said “Compressed air does not contain much energy—that’s the killer.” [8] I would add that it is inefficient to compress air, and the compressor itself adds significantly to the cost of the car.
So I don’t see a big future with air cars. The physics of compressed air means that it is unlikely that they will be able to expand their range. It is better to keep working on improving battery technology – increase their energy, improve their life and decrease their cost. That will prove to be the future of transportation, not a car that farts as it moves.
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Hahahaha. I would like to get to talk to your wife. She sounds so witty! Hahaha. I can’t stop laughing. Anyway, I’d like to see how these cars will work. I get none of your technical terms but all sounds so interesting.
Thanks for the thorough analysis as usual Max. You list three problems with air cars, but I disagree that they are really disadvantages. First, the 50 mile range is more than enough for the average commute in America which is l6 miles (ABC News). Second, the inefficiency of storing energy as compressed air is more than offset by the savings over batteries. Unlike batteries, gas cylinders do not wear out, and since the battery replacement represents the vast majority of the per mile cost of driving an electric car, compressed air can be half as efficient (46% you mention vs. 80% for batteries) and still more be much more economical. Finally, perhaps we can be optimistic about the benefits of real R&D on compressed air cars – more efficient engines may be possible (hinted at in Tata PR – heating the gas), as well as better/cheaper compressors.
Thanks for your comments J.R. It is always healthy to get critical feedback. :-)
In regards to battery cost, in a previous post, I calculated the battery cost of an electric vehicle to be about $0.08 per mile. If we figure that the air compressor will cost $8,000 and that it will last for 10,000 hours of use, then the compressor cost would be $0.12 per mile! (Assuming that every hour the compressor is running adds 6.67 miles worth of compressed air – 50 miles / 7.5 hours.)
Of course, the big unknown is how long the compressor will last. I tried to find this out by looking at used compressors and how many hours they had been run. The highest I could find for a 5000 psi compressor was only a couple of thousand hours. This doesn’t mean that they won’t last substantially longer, but it is hard imagining that they could go much past 10,000 hours.
Another disadvantage of a compressor is that they are loud. I don’t think I would want a compressor running all night in my garage!
Yes, okay. I tried to find flaws in your logic or math, but alas I must accept that the compressor cost and time may be the Achilles heel of the air car concept. It will need to be replaced eventually, and that represents a cost similar to replacing batteries in an electric car. I still think it warrants further R&D investigation to find better ways of compressing air, but I don’t suppose it will be a viable choice for transportation in the near future. Darn! :-(
How about giving a run down on the engine that runs on salt water?
Regular servicing of compressors should be feesable to ensure a long life, just like your car now…..you don’t go more than 6 months without aat least an oil/filter change?? or coolant check what about your servicing of your vehicle? Or even servicing the air con compressor???
why should an compressor in your air car be any different…..
Here in Australia compressor manufacturers are looking at better, quieter and cheaper compressors that using solar energy can re-compress the used air and refill the cylinders. failing that a large tank of compressed air at your nearest fuel station just like the lpg gas should suffice
In their own test the air car traveled 7.22 KM., that is less that 5 miles. everything else that they claim is just words, not proven at all. They prey upon the public’s ignorance of science and their desire for alternative energy. Even their plan for many factories is inconsistent with economics, they plan to have factories producing 10,000 cars per year, that is 40 per work day. What ever happened to economies of scale, maximizing the use of capital resources etc? The more factories the more downpayments on factories they can accept, it all comes down to money. A thorough analysis of the entirety of their claims and press release reveals a company that will say and claim anything, proof is not something they provide.