Air Again

[ppi]

Well if this guy can get 50 miles at 65mph with the curb weight of this car, what can we do with a 900lb car??? in my build we have always thought that air should be used in city mode, add a
on Board compressor and look out.
http://www.autobloggreen.com/2008/09/25/santa-monica-altcar-expo-2008-preview-rocks-a-parking-lot/

[westonlgray]

Gone tomorrow

I wonder what he uses to make it move?

I have my doubts though, compressed air is not that cheap to get, especially at very high pressures. I would like to see a km/kWh for this car to see how it compares to electric or gasoline.

[ppi]

there is an early video of it on utube i guess before he went to composite tanks, because you need very high psi for range and speed, all the items can be had ez but the composite tanks are a lil harder to come by and are the main cost, the air motors can be had for cheap on ebay and there is no need for complex controlers etc, your pump can be ran off home solar or bio generator for little cost.

[westonlgray]

I know the components are relatively cheap. My concern is whether you are getting a good energy efficiency or not. From an energy perspective, I believe compressed air is a lot more expensive than electricity. That is why I want to know if anyone has looked at a miles/KWh number for this type of propulsion. I would be interested if it is somewhat close to electric or even gas.

If the effiicency of air is a lot less than electric, then you have to start looking at the life-cycle costs to see whether the more expensive electric drive systems would actually save energy in the long run or not compared to the air.

My estimate is:
It looks like he has 5 scuba tanks. Assuming that they are all 100 cu-ft tanks, then we have 500 cf of air. I am also assuming 3000 psi which is a typical scuba tank pressure. A typical 5hp 3200 psi air compressor puts out 3.5 cfm of compressed air. This is 3.5 cfm of 1 atm air compressed down to the size at 3000 psi.
Therefore, that would be about 500/3.5 = 143 minutes. The power would then be 143 minutes/(60 min/hr) * 5 hp * (746 W/hp) = 8.9 kWh. So by my back of the napkin calculation is 50 miles/8.9kWh = 5.6 miles/kWh.

For comparison, my 99 Saturn SL2 has gotten a 36 MPG lifetime average. That means 36 miles/gallon (1gallon/33.56 kWh) = 1.07 miles/kWh

Ok, well, I guess my calculation says that air may be a good idea. I haven't taken into account any drying of the air, etc, so my number might be a bit high, but it is certainly interesting to look at. I wonder if air is really noisy? Does anyone see any glaring errors in my calculation?

[RAN]

weston, at this time of the morning, I'll use your calculations :-{)]
I'm glad you decided to look at compressed air from this angle, and it would seem that it's at least a viable alternative.
I've always had the picture in my mind of these air motors sounding like jackhammers, which, along with like-sounding mothers-in-law, is not something I would subject myself to willingly.
Anyone have more info to share?

PS: I just found the youtube vid of the Miin-Aer Boxter... it sounds every bit as bad as I thought. Miin-Aer Boxster (http://www.youtube.com/watch?v=wQ8S6D2ZGeI)

[ppi]

you can get the sound down with a custom muffler again power to run a compressor is almost free from the sun at least out here in the west, next air power is about the same as ev power 1hp in air is like 10hp in ice, suba tanks are a good stand in at the moment but the custom tanks from france and a few other place are night and day and are safe
the cost to maintain an air car is night and day, not many parts wear out at all over years try that with any battery bank
plus there are better designs coming down the pipe rotary designs and weight 60lb motor moving 1200lb car

[westonlgray]

Ran, to tell you the truth, I was expecting a much different answer. With all of the energy used to compress air, I wouldn't have thought that it would be that efficient. I also didn't think that my car would be that comparatively inefficient.

Maybe the air system could be used to store power and then runn a small generator when the power goes out. The air tanks and a small high pressure air compressor are probably comparable to a bank of batteries and an a power inverter. I wonder how high pressure air systems do when they are just left sitting...

[danbucks]

westonlgray, I will assume your numbers are accurate for some kind of compressor ... but that's in fact the most inefficient way to compress air.
High efficiency comes from a "staging" station - e.g. N tanks (usually 3) : one gigantic one at low psi, one medium at medium psi, and one at high psi - a series "step-up" transformer.
An air car really should have one of these at home, and perhaps an "emergency" compressor on-board (low-efficiency)
With that (ignoring air-engine inefficiencies), loss should be minimal to traditional electric storage designs. I don't have specifics, however.

[westonlgray]

I just picked a small (inefficient) 3 stage air compressor that someone might be able to buy on a budget. If they set up an air distribution station (gas station), then they would probably be able to afford a much more efficient design.