Air Powered Car

[momo44]

Momo, I pointed out that there are large (Stytooner they were compound screw type) industrial compressors that can fill the target tanks in a few minutes but they would be huge and expensive (on a scale about 10,000 times the kind of compressors most of you see). This is understandable as we are basically transferring enough energy to propel a car 125 miles, in a matter of a two-3 minutes. So for a commercial "refueling" station for air cars, you'd want to charge the tank in 2-3 minutes, you're going to need a compressor costing several hundred thousand dollars. Won't run off normal electrical outlet either - probably diesel (so we're losing part of the green appeal). I suggested a charge of $20-30 for a charge, I still think that would be reasonable to pay for one, the energy used to charge the tank, and two, the amortized cost of the expensive facility.

Loring,

I was just trying to calculate the physics of a reasonably small compressor for a station to maintain (15HP). You are correct that the compressor (if you can find one) would be large - 450HP by my calcs. But to do that would be unreasonable by any stretch, and I am not sure $30 a pop for filling up is enough unless you have a pretty steady stream of customers.

[Black wallnut]

Interseting topic with plenty of thoughtful answers so far. In a previous job I worked with Anhydrous Ammonia, a liquid under pressure with a very low boiling/ gass off point. Our delivery trucks usually had a capacity of 2000 gallons and the pressures involved on a very hot day might have reached 200 psi. We used special 4" hoses to transfer the product into the trucks. These hoses were stiff and heavy. They required wrench tight coupling between tank and storage facility. Keep in mind we were working with a much lower pressure than what would be needed to power these cars. The hoses, couplers, and valves that would be needed to handle extreme high pressure of 4,000+ psi plus the required training of workers and certification of them, facilities, and transfer gear boggles my mind. Just thinking of the check valve that would have to be installed to prevent catastropic loss in the case of a simple hose rupture would require a very strong spring that would add to the captured energy and would present another lethal part in this eqation.

I'm all for realistic, renewable clean energy sources. All ideas should be considered. Given the risks involved in compressed air and the inefficency of these cars compared to gas/ diesel. I think we need to keep looking.

[Russianwolf]

http://www.max-air.com/maxair11.html

thinking along the lines of Scorpio.

The Cubic feet of the take stays static, correct? So how big is a is a 55 gallon tank in cubit feet? I have a 55 gallon aquarium at home that is approx. 48x13x40 (guessing the height is 40 inches) so that's 14.5 cubic feet, right?

This compressor says it can fill a 80 cubic foot tank from 500 PSI to 3000 PSI in less than 4 minutes. and has the ability to go up to 6000 PSI.

It also seems reasonable that in a filling station type application that the station would have a storage tank at maybe 10000 PSI that could fill maybe 10 cars (before the tank pressure drops below 5000PSI) without the compressor even needing to come on.

[momo44]

RussianWolf,

Converting from cf to gallons is Cf x 7.48 = Gal, so 55 gallons is 7.35 cf.




It takes 80 cf of air to fill an 80 cf tank to 1 atmosphere of pressure (14.7 psi). It take 200 times that amount to fill it to 200 atmospheres (2940 psi).

However, the scuba tanks that are 80 cf @ 3000 psi are only 80*14.7/3000 = .3920 cf or 2.93 gallons at 1 atmosphere. Scuba tanks are rated for how much air they have in them, not the static volume.

The example given in the website you link is filling an 80cf cylinder from 500psi to 3000 psi. This is done in 3.17 minutes. To figure out what is being done, take 3000 psi - 500 psi = 2500 psi pressure change. Now the 80 cf tank is 0.3920cf static, so 0.3920 cf/14.7 psia(atmosheric pressure) = .0267 cf/psi change . This means if you put 0.0267 cf of air in the cylinder, the pressure will go up by 1 psi.
Now 0.0267 cf/psi X 2500 psi change = 66.667 cf added to the cylinder ( not much). 66.67 cf/ 3.17 min = 21.03 cf/min which is the rating of that compressor.

In the air car however, the tank is 52 gallons (6.95 cf ) static. So 6.95cf / 14.7psia = 0.4729 cf/psi. Filling to 4350 psi would require 0.4729 cf/psi x 4350 psi = 2057.12 cf . That would take the scuba compressor 2057.12 cf / 21.03 cf/min = 97.82 minutes to fill.

A 10000 psi tank holding air would have to be 40 gallons to fill 2057 cf and remain above 4350 psi.

[Russianwolf]

RussianWolf,

Converting from cf to gallons is Cf x 7.48 = Gal


It takes 80 cf of air to fill an 80 cf tank to 1 atmosphere of pressure (14.7 psi). It take 300 times that amount to fill it to 300 atmospheres (4410 psi).

However, the scuba tanks that are 80 cf @ 3500 psi are only 80*14.7/3500 = .3360 cf or 2.51 gallons at 1 atmosphere. Scuba tanks are rated for how much air they have in them, not the static volume.

then they shouldn't be able to say is an 80 cubic foot tank if it can't hold 80 cubic foot of water, lead shot or other non-gaseous mater.

80 cubic foot is a measure of volumn and is static.

[momo44]

then they shouldn't be able to say is an 80 cubic foot tank if it can't hold 80 cubic foot of water, lead shot or other non-gaseous mater.

80 cubic foot is a measure of volumn and is static.

They will respond that you are paying for the amount of air in the tank, not the volume of the tank itself.

[Russianwolf]

They will respond that you are paying for the amount of air in the tank, not the volume of the tank itself.

and that makes their terminology more right? Don't think so.

no, I would be paying for the service of having the air placed inside the tank. Not for the air itself unless it were a special mix (ie. deep sea as opposed to shallow water)




You didn't address the second part of my post. The holding tank?

[momo44]

and that makes their terminology more right? Don't think so.

no, I would be paying for the service of having the air placed inside the tank. Not for the air itself unless it were a special mix (ie. deep sea as opposed to shallow water)

The proper way would be as Loring suggested - pay by the pound. Mass flow does not have the volume issue to deal with.

You didn't address the second part of my post. The holding tank?

I edited my post above to show a 10000 psi tank sized for one car fillup. If you want to fill 10 cars, then 10*2057cf/(10000psi - 4350psi) = 4.73 cf/psi . 4.73 cf/psi x 7.48 gal/cf x 14.7 psia = 520 gallon tank rated for at least 10000 psi. Yikes!

[Russianwolf]

520 gallon tank rated for at least 10000 psi. Yikes!

That doesn't sound too bad for something that could be built to withstand a Sherman tank considering it would be stationary.

[scorrpio]

You are too hung up on the standard refueling logistics. Do gas station receive and store crude oil, and refine it into gas on the spot as cars pull in to refuel? Nope - they receive the ready product from a central distribution point. And transferring the gas from storage to a car's tank is a simple process, hence it is done this way.

But consider propane. When your BBQ tank is out, do you have to wait for it toget refilled? Nope, you drop off the empty and pick up a pre-filled one. Consider SCUBA - on a two-dive trip, people don't line up at the compressor after the 1st dive - thay simply move their rig over to another tank.
Same here. Make air tank removable. A car pulls up to a station, attendant, unhooks the tank, a mechanical arm removes it and places it into a rack, then loads in a full tank, and attendant hooks it up. That's 2-3 minutes total.
Every day, a truck pulls in, unloads a rack of full tanks, and picks up a rack of empties. They are taken to a location someplace away from residential areas, where a massive compressor charges them quickly. Stations need not run, pay for, or maintain a compressor. Just like they don't need to run their own refineries.

[TheRic]

..... Make air tank removable. A car pulls up to a station, attendant, unhooks the tank, a mechanical arm removes it and places it into a rack, then loads in a full tank, and attendant hooks it up. That's 2-3 minutes total. .....

That tank would be heavy, sure would not be a self-serve. I have no clue how much the tank would weigh full or empty, sure someone here might know. I would guess close to 400lb full. You need more of a support structure to be able to hold 400lb that can easily slide in and out, but be safe in a collision. Having something permanent would be easier to design around than something that is mobile.

[Black wallnut]

So if the tanks were replaceable employment in the mini-mart service station business would double overnight. Service stations would have to purchase or lease a lift truck to unload the trucks that also would have to be specially designed to haul these compressed gas tanks. I'm thinking that the added labor could quickly force this enterprise to no longer be worthwhile in terms of cost to the consumer.

As far as means to secure a removeable tank, there are ways. Freight containers lock to special chassis' at the four corners. Everything you can imagine gets shipped on our freeways each day in a container. Perhaps mostly within a days drive from a port or rail container yard. These loads in some states reach a gross (that's containr plus cargo) weight near 60,000 lbs. These containers are loacked and unlocked as fast as walking around the trailer with a momentary pause at each corner. BTDT many times.

I think the bigest hurdle to this is the releative inefficiency compared to alternitives.

[LCHIEN]

That tank would be heavy, sure would not be a self-serve. I have no clue how much the tank would weigh full or empty, sure someone here might know. I would guess close to 400lb full. You need more of a support structure to be able to hold 400lb that can easily slide in and out, but be safe in a collision. Having something permanent would be easier to design around than something that is mobile.

52 gallons of compressed air at 300 Atm would weigh 170 lbs.
52 Gallons empty (e.g. at 1 Atm) would weigh basically nothing.
The tank itself, of carbon fiber, would have to be substantial.
Your number of 400 lbs, full, might be nearly correct.
That's a big chunk of a 1700 lb car.

[TheRic]

Would moisture be a problem in the tank, and therefore the lines, valves, etc.??

I know moisture forms in the compressor because the air IS being compressed. Moisture can also get into your air tools from said compressor. Would filling, draining, filling, draining of the tank cause water to build up?? Yea, a water/moisture trap would help, but even at that high pressure?

[Black wallnut]

Would moisture be a problem in the tank, and therefore the lines, valves, etc.??

I know moisture forms in the compressor because the air IS being compressed. Moisture can also get into your air tools from said compressor. Would filling, draining, filling, draining of the tank cause water to build up?? Yea, a water/moisture trap would help, but even at that high pressure?

Yet another excellent question. I can only take a stab at it knowing what I do of air brakes on trucks. Moisture in truck braking systems is a big problem. Especially in cold weather. Trucks have systems to reduce and sometimes eliminate this by product of compressing but these systems are not 100% reliable. However I think this issue would and could be addressed prior to the end user's tank.