Air Powered Airplanes?

I think this is the answer to our fuel consumption needs for light autos. $2-5 to fill up with air and go 500+ miles? Are you kidding me? NO! They are being built today and will be available next year.

http://www.popularmechanics.com/automotive/new_cars/4217016.html

http://www.popularmechanics.com/automotive/new_cars/4251491.html?series=19

I wonder how long it will take for someone to put an air powered car engine in an airplane.

No pun intended but... DON'T HOLD YOUR BREATH!

Red Voodoo said:

No pun intended but... DON'T HOLD YOUR BREATH!

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Now that was funny.

I saw this baloney on TV a while back. They even said that the wheels would turn an onboard pump that would recharge the tanks so that you would never have to pay for fuel. I'm no engineer, but my feeble brain says that whatever energy I get from that compressed air would exactly equal the amount expended in cramming the air into the tank in the first place. I'm calling B.S. on this one.

szicree said:

I'm no engineer, but my feeble brain says that whatever energy I get from that compressed air would exactly equal the amount expended in cramming the air into the tank in the first place.

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Sorry, wrong.

It will be less, no mechanical system is 100% efficient.

In theory, this would be very similar in operation to the re-generative braking used by electric/hybred powered vehicles.

Just a way to get back some of the potential energy in the vehicles inertia.

Guys, this vehicle is being produced already. This is not a pipe dream, it is reality. You plug it in to run the onbard compressor or stop at an HP air filling station, or have a HP compressor at home filling a standby tank.

Did you read the article?

Geico266 said:

Guys, this vehicle is being produced already. This is not a pipe dream, it is reality. You plug it in to run the onbard compressor or stop at an HP air filling station, or have a HP compressor at home filling a standby tank.

Did you read the article?

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Mike S.,

You got me, it's a net loss.

Larry,

I'm not saying it's not real, just that compressed air costs a lot more than plain ol' air. If it takes a 5hp motor an hour to fill that thing up, then the most I can hope to get from it is a 5hp car for one hour (or less for longer). I can also tell you for sure that my garage compressor would never be able to fill an 88 gallon tank up to 4000 psi. To make this happen you'd need a little tiny pump, a strong motor and some time to kill. I'm also not exactly thrilled about driving around with an 88 gallon bomb in my car. Finally, the bottom line is that burning coal in some powerplant to generate and send A/C to my house so that it can run an electric motor (loss) to drive a pump (loss) to fill a tank (bomb) to drive an air motor (loss) is just not realistic. "Green" has been snapped up as just the latest marketing ploy.

There used to be a CO2 free flight class

In the 40s to early 50s there was a free flight model airplane class. I never competed in it so I don't know how they worked but the fuel was supplied by a small CO2 cartridge I believe connected through a line to the top of a cylinder driving a piston, crank and prop series. They were very low powered I believe but the basic technology is not new.

Bob Axsom

http://blacksheepsquadron.com/generic_info.html

Neat stuff.

That's not the point

szicree said:

Mike S.,

You got me, it's a net loss.

Larry,

I'm not saying it's not real, just that compressed air costs a lot more than plain ol' air. If it takes a 5hp motor an hour to fill that thing up, then the most I can hope to get from it is a 5hp car for one hour (or less for longer). I can also tell you for sure that my garage compressor would never be able to fill an 88 gallon tank up to 4000 psi. To make this happen you'd need a little tiny pump, a strong motor and some time to kill. I'm also not exactly thrilled about driving around with an 88 gallon bomb in my car. Finally, the bottom line is that burning coal in some powerplant to generate and send A/C to my house so that it can run an electric motor (loss) to drive a pump (loss) to fill a tank (bomb) to drive an air motor (loss) is just not realistic. "Green" has been snapped up as just the latest marketing ploy.

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Obviously this compressed air is neither free, nor can it be produced without energy. Right now you are driving around with enough gas in your car to level your entire neighborhood. Danger is relative. It can be pollutant free in a nuclear energy economy.

This is a super simple alternative to electric power. Instead of storing energy in a battery, it is stored in the form of compressed air.

From what the small article said - 125 miles & up to 68 mph. Probably produces about 15hp max, and that 125 miles is probably at a slower speed.
The engine, would work the same as a steam engine. That means no transmission or clutch, with full torque at zero RPM.

Really, there is no doubt this will work. Regenerate braking can also work. The only question on the whole deal is on how well it will work. The challenge is compressed air capacity.

Filling a tank to over 4000 psi is also a non-trivial issue, but seemingly easer than safely filling a tank with hydrogen. Also faster than charging a battery.

A small compressor could be put on board so it can be refilled slowly just by plugging it in.

Some smart ME should crunch the numbers to see how viable this is (for auto, not airplane). So, with the numbers quoted in the article would this thing be able to go 100+ miles, or just to the end of your street????

Clearly compressed air cars can work. As always, in today's era of junk science on a mass scale, one needs to do the analysis to determine the BS factor.

When I read the first article, my eye caught the statement:

"It will take only a few minutes for the CityCAT to refuel at gas stations equipped with custom air compressor units"

I wondered how they would handle the heat involved with compressing that much air. Without heat transfer, the temperature of that glob of air would be enough to melt anything near it.

It would be interesting to do a little further analysis, determining the amount of gasoline that would be equivalent to the compressed air tank in the subject car. Heat transfer would seem to be a major player, both cramming air in and getting it out.

From what I've read they use the heat build up to expand the air more into the cylinders in a proprietary design. It will be interesting to watch this technology unfold, but clearly something has to be done to cut gasoline usage so my AVGAS is cheaper!

I also wish we as a nation that would take on the task of alternative fuels & engines with the same youthful enthusiasm as we had when we decided to go to the moon.

As far as the filling process goes, of course a home compressor won't work to get the air to 4,300 PSI. They come with a compressor that plugs into the wall and pumps the pressure up in a multi stage compressor, or you can buy a multi stage compressor that will do the job. Either way, the cost of $2-5 per fill up is pretty attractive. We have the distribution system inplace for electricity so any vehicle that uses electricity as a means for power is a good start.

In Utah, a real movement is going forward to convert autos to compressed natural gas (CNG), a resource that is domestic, plentiful, and extremely clean burning. The reason, the gas system is owned and operated by Questar, a utlility that owns its own gas wells, and they maintain the price at local pumps throughout the state at $0.63/ GGE (gasoline gallon equivalent). Utah cannot get converted vehicles fast enough to fill the demand.

I fill my dual-fuel F150's tank for around $7/tank and drive ~150 miles with no noticeable loss in power. The tank fills to 3600psi under ideal conditions. FWIW, the octane rating on cng is around 130; it might be an alternative for aircraft should lightweight tanks be made available.

rtry9a said:

In Utah, a real movement is going forward to convert autos to compressed natural gas (CNG), a resource that is domestic, plentiful, and extremely clean burning. The reason, the gas system is owned and operated by Questar, a utlility that owns its own gas wells, and they maintain the price at local pumps throughout the state at $0.63/ GGE (gasoline gallon equivalent). Utah cannot get converted vehicles fast enough to fill the demand.

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That is outstanding. How & where do you fill it up? Liquid or gas? What is the distribution system?

Remember 1973?

Well if not I can give you a few personal experiences. A fuel crises was recognized and the president Richard Nixon said we had to get away from dependence on foreign oil. I was working for McDonnell Douglas Astronautics - East at that time covering the development of the large nicad batteries being developed by Eagle Picher for the Skylab Project in Joplin Missouri while living in St. Louis. I drove home on weekends and one night a week to attand class at Washington University (a very long night but I was young). Gas stations were running out of gas and Lines were around the block at gas stations where they had some. Ones that had gas were limiting sales to 5 or 10 gallons per customer. I had to find stations with gas at several locations to continue my schedule. A nationwide 55 mph speed limit was imposed to conserve fuel and goverment rules on vehicle fuel consumption were initiated. Government contracts were awarded to develop a wide variety of alternate energy sources. When I transfered to MDAC- West in Huntington Beach in 1978 the contracts were still very active. MDAC-W was working on the large solar array station at Barstow and I felt pretty good about the efforts that were being made to push the application of science to solve the energy problems. I later learned that my final employer JPL was working on solar cell technology at this same time. The whole effort seemed to peter out and we developed SUVs instead. Hopefully, this time we will press the science a little harder.

Bob Axsom

AlexPeterson said:

Clearly compressed air cars can work. As always, in today's era of junk science on a mass scale, one needs to do the analysis to determine the BS factor.

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Exactly. I'm not exactly filled with confidence when I read that the thing is being produced by the largest car maker ... in India?!

As others have said, this is very old technology (I actually had a model plane that ran off compressed gas). So why hasn't it been used for cars before? We've had compressors and pumps, right? I also remember that my little plane would run slower and slower as the pressure fell off. Is this what my air car is going to do? Drive to work at 60 and come home at 15?

Regarding the claim that the gasoline in my car's tank could level the neighborhood, I'm not worried. It's in liquid form and well protected. To release it's stored chemical energy would require vaporization and ignition. On the other hand, the only thing needed for that air tank to release it's massive energy is a ruptured tank.

Geico266 said:

That is outstanding. How & where do you fill it up? Liquid or gas? What is the distribution system?

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CNG is a 3000-3600psi compressed gas; LNG is liquid (stored abt 100psi, like propane, located mostly near the coasts but not yet common where natural gas pipelines exist). There are something like 25 fueling stations spread around Utah (10 near SLC), located mostly at gasoline stations and mass transportation terminals. A compressor feeds storage tanks that supply a two nozzle pump that looks a lot like a normal gasoline pump, except that the nozzle has two hoses and a valve attached. The actual nozzle looks much like a slightly larger fitting at the end of the air compressor in your garage. You can also buy a small compressor that hooks to your natural gas heating supply line and fill at home if you want.

Several states have large public cng distribution networks in existence- notably California, Utah, and Oklahoma, though California allowed Clean Energy, a monopoly, to take over the system and boost prices way higher than costs justify. http://www.cngprices.com/ I can drive to LA or almost to Denver (abt 750mi) no problem on cng for around $25.

szicree said:

Exactly. I'm not exactly filled with confidence when I read that the thing is being produced by the largest car maker ... in India?!

As others have said, this is very old technology (I actually had a model plane that ran off compressed gas). So why hasn't it been used for cars before? We've had compressors and pumps, right? I also remember that my little plane would run slower and slower as the pressure fell off. Is this what my air car is going to do? Drive to work at 60 and come home at 15?

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That is what is embarrassing India! Americans have lost their ability to think outside the box. We are too consumed with who got voted off American Idle, or the lastest Britney Spears "train wreck".

Why hasn't this been done before in automobiles? When was the last time gas prices were headed to $5 / gallon for regular?

I'm pretty sure the engineers will put a tad more thought & engineering into the car than your toy airplane. The fact is the car is here and working as advertized.

Why do you doubt something exists when it is already here?

Further Thoughts...

szicree said:

I'm not exactly filled with confidence when I read that the thing is being produced by the largest car maker ... in India?!

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You'd be surprised how much engineering is already performed offshore (mainly India & Mexico) these days on "American" cars. Having seen the labor ranks eviscerated (not an editorial, an observation) in my lifetime, don't be surprised if the engineering ranks are next. Automobiles are mostly rehashed and re-rehashed old technology (hence vigorous effort put into style and such). Most of what passes for automotive engineering is routine and repetitive, and can be sent to lower cost areas just as easily as labor.

AlexPeterson said:

I wondered how they would handle the heat involved with compressing that much air.

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As opposed to filling up directly from a massive high-power compressor, I expect the car would be filled from a larger tank which is then filled by a compressor or gang of compressors working more evenly. The heat would be dissipated or used for some other purpose accordingly.

------------
Some thoughts.

1> Gasoline engines waste something like the first 70 cents out of every dollar as heat. When stated in those terms I (my wallet too) yearn for something better (Of course, in winter as my teeth chatter I tell myself that its ok). I'd be willing to bet that a scaled compressor system would be more efficient than scattered reciprocating engines. How do you compare total system efficiencies?

2> Along those lines, the compressed air long having dissipated its heat, could the process of expanding it in the vehicle be a source of air conditioning?

3> 1 + 2 together mean that the compressed air car is more suitable for warm climates like Indias.

4> Didn't UPS test a compressed air regenerative braking system a couple of years ago? The compressed air was used to get the vehicle moving for the next stop.

5> It kinda is silly to use gasoline for mundane tasks to shuttle kids, retreive groceries and reasonable commutes. We do it because it has always been cheap and convenient (it still, gulp, is). Gasoline is more well suited for special purposes where energy density is important like: long automotive trips, aircraft and racing.

6> One big knock against electric vehicles is their range. It seems that postal vehicles are an ideal EV application. Frequent start-stops, never needs to go on a long trip, can reliably be plugged in every night. When I lived in suburban Boston the letter carrier would shut off the vehicle, get out, walk up to the house, deposit mail, go back, start the vehicle, drive 100 feet to the next house. Its hard to imagine a worse use for a gasoline engine. They'd get pretty beat up, I called them Spewmobiles.

airCon

ergie63 said:

.......

2> Along those lines, the compressed air long having dissipated its heat, could the process of expanding it in the vehicle be a source of air conditioning?
........

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The Tank should cool as it empties. Enough to use for cooling? IDK

LPG very common in Europe!

After having used it in 4 more cars before, I drive my Lexus RX300 on LPG (Liquefied Petroleum Gas) or a mixture of Butane and Propane. It used to be a waste product of the oil industry, that they burned-off on the oil rigs out on the North sea. But, nowadays, we pay about EUR 0,45/liter for it (in Belgium, more expensive in Holland, Germany and France). But, that is still less than 1/3 of the gasoline price out here today. So after you forked out a couple of "grand" (EUR) for the installation of the tank and system, you are saving more money as long as you are driving more km's. In Holland and Belgium you can fill up the tank at every third petrol station (some countries are better than others, but there is plenty of petrol stations around Europe that supply LPG, to keep you going). Also the car will still run on normal gas, in fact it starts on gas and after it has reached a certain temperature it will automatically switch to LPG. Since the energy in LPG is slightly less (per ltr.) it will use about 15% more than when driving on ordinary gas. But because of the price-offset that is not a problem. Also the little loss in power is hardly noticeable in todays cars with 200 or so HP.

For airplane use it will never work, since the system uses the hot coolant of the car engine to prohibit icing of the "expander" (turning liquid in to gas). Also the pressure-tank will be heavier than a normal tank and filling up will not be easy (you can not use a simple cannister if LPG is not available at your local airport).

For car use it is fine though, and it is also good for the environment. Can you imagine, the more I drive, the better it is for the environment? Yes, the more LPG I use in my car (which is very efficient and pollutes very little) the less the oil rigs have to burn off (which is very polluting with no filters, catalyzers, etc!).

Regards, Tonny

I agree with the previous poster. We need something akin to the Manhattan Project for alternative energy. Unfortunately, as with most things, peoples' political sensitivities are affected or corruption would be involved someway or another when anyone talks about energy.

I think this is an area where America could really excel and prove ourselves again in technological innovation. We can't continue to call ourselves the "greatest country in the world" forever for things we've done in the past. What are we doing now? Where are our so-called "leaders" (of both parties) in Washington DC?

Steve Brown said:

The Tank should cool as it empties. Enough to use for cooling? IDK

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The tank will cool as it depletes, yes, but I think what he was referring to was the exhausted air exiting the engine. This air would be significantly cooler after having done work in the engine through expansion.

airguy said:

The tank will cool as it depletes, yes, but I think what he was referring to was the exhausted air exiting the engine. This air would be significantly cooler after having done work in the engine through expansion.

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The act of expansion is where the cooling takes place, actually a release of the heat/energy acquired during the compression of the air.

Dont forget that heat=energy.

The engine appears to have multiple "expansion chambers" (for lack of a better word) with each having a output shaft, all coupled together with a belt----or maybe the belt is driving rotary valves??? Who knows.

Just like the compressor out in your garage, I would assume that the different chambers have different volumes, to take advantage of the latent energy still in the air being exhausted from the previous chamber.

Old steam locomotives used the same principal, sometimes 3 separate pistons being driven by the same "puff" of steam.

Bottom line here is that the air exiting the final stage is going to have pretty much given up a lot of energy------probably enough to cool the car as suggested.

Ever noticed how cool an air drill gets when you use it a lot???

http://media.popularmechanics.com/images/air-car-engine-0607.jpg

Quote:
Originally Posted by AlexPeterson View Post
I wondered how they would handle the heat involved with compressing that much air.

Originally Posted by ergie63
As opposed to filling up directly from a massive high-power compressor, I expect the car would be filled from a larger tank which is then filled by a compressor or gang of compressors working more evenly. The heat would be dissipated or used for some other purpose accordingly.

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It won't matter how the compressed air is produced or from where it comes - if the tank in the car starts at low pressure and ends at high pressure, it will get hot. Really hot in this case if it is done in only a few minutes.

It's worth taking a closer look at the numbers quoted.

Stored energy density is the normal killer for non-chemical energy storage (compressed air, flywheels, springs etc).

340 litres at 4350 psi by my reckoning will release roughly 500 MJoules of energy, or in more readily used terms, about 20 BHP hours, which is about the equivalent of 1-1.5 USG of 100LL. And thats allowing for very high mechanical efficiencies.

So figure how many of these very expensive and big carbon fibre tanks you're going to need to go places.

AlexPeterson said:

It won't matter how the compressed air is produced or from where it comes - if the tank in the car starts at low pressure and ends at high pressure, it will get hot. Really hot in this case if it is done in only a few minutes.

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Not so much as you might imagine, though. I actually work in this area, I own a company handling high pressure air systems and I have 15 years experience with air at pressures up to 10,000 psi. Heating and cooling of the air (or any gas above its inversion temperature for that matter) is a function of expansion (or compression) ratios. A 10:1 ratio will produce the same amount of heat increase per mass quantity of air regardless of initial or final pressure. For example, if you take one pound of air at 1 psi and compress it to 10 psi, it will release exactly the same amount of heat as it would if you compressed that same quantity of air from 1,000 psi to 10,000 psi.

In the case of a high pressure tank for an air-powered vehicle, say you run it down to 360 psi and fill to a max of 3600 psi - that is a 10:1 compression ratio, but only the remaining air inside the tank (the original quantity of air at 360 psi) is compressed and produces heat. The air you are filling with is not compressed - in fact it is expanding from a higher pressure tank through a regulator and so enters the tank cooler, the net effect is a slight temperature increase, perhaps 30 or 40 degrees F by the time the mass of the tank walls absorb the heat differential. Where you get into trouble with tank heating on fills is when filling a nearly empty tank. Take that same tank down to atmospheric pressure and run it up to 3600 psi (245 atmospheres) and you have a 245:1 compression ratio - that's enough to create SERIOUS heat - but the total quantity of air is much lower also, and it's diluted by the incoming pressurized air. The net result is that the lower the initial pressure, the more tank heating you'll get - but it's entirely manageable. I flash-fill bottles to 4500 and 6000 psi every day, filling several hundred per week, and a completely empty bottle at room temperature will settle out to about 120 F at pressure.

Mark Albery said:

So figure how many of these very expensive and big carbon fibre tanks you're going to need to go places.

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Ahhh, and there's the rub. The weight and cost of these bottles will be quite ugly - and even a small compressor (3.5 CFM at 4500 psi) that can be run off household current (220 volt single phase, 18 amps) starts at about $4,000.

Greg, thanks. I vaguely recall the energy is related to the log of the ratios - which is exactly what you are saying. Your second paragraph is very nicely explaining things - thanks.

airguy said:

Ahhh, and there's the rub. The weight and cost of these bottles will be quite ugly - and even a small compressor (3.5 CFM at 4500 psi) that can be run off household current (220 volt single phase, 18 amps) starts at about $4,000.

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And, that is before the lawsuits begin....

AlexPeterson said:

And, that is before the lawsuits begin....

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Right. If you're using it for paintball, SCUBA diving, or breathing air for a fire department, I'll sell you that compressor for $4k. If you're using it for aviation, it's gonna have to be the aviation model at $12,500....

Geico266 said:

Why do you doubt something exists when it is already here?

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I guess I'm just a show-me kinda guy. Do I think an air powered car is possible? Absolutely. Do I think it'll be cheap, safe and practical? No. Every story I've seen on this thing makes it sound as if a tiny little bit of energy is used to fill a big tank with air, and then the air magically makes the car go. One article even suggested that the electricity we already use to pump gasoline into our tanks could be used to fill the air car's tank with compressed air!

Consider this: Think of how much work it takes to push a real car at 68 miles per hour for two hours. This is the amount of work that the pump is going to have to do to fill up your air car, and according to the press it'll have to do it in just a few minutes. The mech. engineers can chime in here on just how much work this is, but it seems like an awful lot.

I do not doubt that this vehicle exists, but I'm fairly confident that the performance claims are WILDLY overblown.

There are some interesting entries on Wikipedia for this technology. As with anything on Wikipedia you'll proably want to read then verify.

http://en.wikipedia.org/wiki/Compressed_air_energy_storage

It might be a worthwhile mental exercise to trace the energy path through the whole system. If you can factor in approximate transfer efficiencies at each step you can get an idea of how factual some of the air car claims are.

szicree said:

I guess I'm just a show-me kinda guy.

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Once upon a time rational people though that the human body could not go faster than 60 miles per hours in a car least the body would melt, the best minds in the world also thought the world was flat, man would never fly, going to the moon was impossible, breaking the sound barrier meant certain destruction, exploding an atom bomb would ignite the atmosphere, and WWI was the war to end all wars.

We MUST start thinking outside the box if we are to survive as a nation.

Lionclaw said:

It might be a worthwhile mental exercise to trace the energy path through the whole system. If you can factor in approximate transfer efficiencies at each step you can get an idea of how factual some of the air car claims are.

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You are totally forgetting about the flux capacitor. It runs on a banana peal and a few beer cans.

Its the refill problem

szicree said:

I guess I'm just a show-me kinda guy. Do I think an air powered car is possible? Absolutely. Do I think it'll be cheap, safe and practical? No. Every story I've seen on this thing makes it sound as if a tiny little bit of energy is used to fill a big tank with air, and then the air magically makes the car go. One article even suggested that the electricity we already use to pump gasoline into our tanks could be used to fill the air car's tank with compressed air!

Consider this: Think of how much work it takes to push a real car at 68 miles per hour for two hours. This is the amount of work that the pump is going to have to do to fill up your air car, and according to the press it'll have to do it in just a few minutes. The mech. engineers can chime in here on just how much work this is, but it seems like an awful lot.

I do not doubt that this vehicle exists, but I'm fairly confident that the performance claims are WILDLY overblown.

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Not an ME, but will chime in anyway. Your skepticism about the claims is well founded. This car can work, and I personally think its a great idea if the range numbers add up. I'm not sure of that at all, and not sure I can even figure out how to analyze it (need smart ME).

The refill issue, however, is exactly as you stated. It is also analogous to the battery refill problem for electric cars (I am an EE).

With that mode of energy storage there is also periodic wild claims about fast recharge that just doesn't math out. This has been at play for years in the development of battery operated equipment, such as laptop PCs. Yes, you can charge quickly. The power loss of doing so, however, increases at the square of the charge rate increase.

So, double the charge current, quadruple the power loss during charge. Assuming the battery will take charge that way (it won't), you have been dissipating 4x the power for 0.5x the time, and therefore wasted twice as much energy to do the same job.

From an overall standpoint, a maximum charge rate of 1C (the normal discharge rate) is ideal, since the overall system is designed to deal with that already. That means it takes as long to charge as it did to discharge. From an energy conservation standpoint, the slower you charge, the less power you waste.

OK - not an ME, but I'll go out on a limb with this relationship:
Air pressure ---> volts
Air flow ---> amps
For the compressed air storage of power, air is going to have to flow into the tank (like current into a battery). The faster it flows, the more heat is generated. I'm guessing its a flow^2 relationship, so loss will increase with the rate of refill.

Like in the electrical analogy, you can over design the charging paths, but that leads to an impractical system. I suspect, for example, that designing a valve that will handle 4000+ psi becomes more difficult at the square of the diameter.

Geico266 said:

Once upon a time rational people though that the human body could not go faster than 60 miles per hours in a car least the body would melt, the best minds in the world also thought the world was flat, man would never fly, going to the moon was impossible, breaking the sound barrier meant certain destruction, exploding an atom bomb would ignite the atmosphere, and WWI was the war to end all wars.

We MUST start thinking outside the box if we are to survive as a nation.

Click to expand...

I suspect that the belief about people needing to stay under 60 was not held by trained physicists or engineers. The flat Earth theory was held by superstitious simpletons and religious zealots, not the best minds, and obviously the scientists knew we could go to the moon.

I agree wholeheartedly that we need solutions to our current energy supply troubles, but this thing just ain't it. As was already stated, the energy contained in that tank is equivalent to 20 Horsepower-hours. In other words 20 horsepower for an hour or, if you prefer, 10 horsepower for 2. So how do you suppose you get 10 horsepower to get a car to 68 mph? It isn't magic. You make the car weigh a couple hundred pounds, fair the heck out of it, and pray that you never run into anything larger than a small, slow moving dog. I wish I could be more optimistic about this one, but energy is energy. Do the math and you'll see.

Steve Brown said:

OK - not an ME, but I'll go out on a limb with this relationship:
Air pressure ---> volts
Air flow ---> amps
For the compressed air storage of power, air is going to have to flow into the tank (like current into a battery). The faster it flows, the more heat is generated. I'm guessing its a flow^2 relationship, so loss will increase with the rate of refill.

Like in the electrical analogy, you can over design the charging paths, but that leads to an impractical system. I suspect, for example, that designing a valve that will handle 4000+ psi becomes more difficult at the square of the diameter.

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Refilling the bottle from a compressor directly would take time - there's no two ways around it. To be able to do a reasonably quick fill (rough time scale equivalent to a gas pump) you would have to have large storage tanks at a fill station with a large compressor keeping them full, and when a vehicle pulls up you flow air from those large storage tanks through a pressure regulator into the vehicle tank. This would allow even large quantities of air to be moved in only a few minutes, I regularly fill 500-cubic foot bottles from empty to 6000 psi in about 3 minutes through a 1/4" OD stainless steel line with .035" wall thickness, so my minimum diameter is .180". The valves end up being the maximum restriction in the flow path, and even then it's not a major issue. Dollar-wise, it's going to be MUCH more cost-effective to use a large compressor and storage system to flash-fill a vehicle in this manner than any other method. It's simply not practical to have small onboard compressors to plug in and slowly fill the tank - they would literally take hours (read "overnight") to do so.

The recharge problem for air bottles is not nearly as much an issue as it is with batteries. Batteries MUST undergo a chemical reaction to charge or discharge, and are thus limited as to the total rates of energy transfer. For air flow, the mass and size of the filling system is miniscule and does not realistically scale with flow rate. I can fill a 500-cubic foot bottle in 4 minutes with a fill system weighing about 4 pounds (which is about minimum weight actually), or I can fill it in 15 seconds with a fill system weighing about 6 pounds. The cost of the fill system will roughly triple, but the mass and size don't increase much. The air bottles are simply storage vessels - filling them fast is very much like filling your fuel tank fast. If you had a 4" gas filling port and ran a 4" fuel hose (think jet transport aircraft here) you could fill a 30-gallon tank in about 4 seconds. The trick comes up in supplying that much air (large storage vessels at a higher pressure) and then finding a happy medium with regard to your filling system size and complexity. I don't think 3-4 minutes is unreasonable, that's very close to what you spend at a gas pump today and is very easily done with cheap and small components.

As for the valve size at pressure, yes they become more complex and expensive, but this engineering and design work was long ago taken care of by the oil industry, they regularly use large valves (up to 14") at pressures well above the range we are talking about. Realistically however, for this type of application you don't need more than 1/4" ports on a hand valve, I sell those all day long (rated to 6000 psi) for $48.

As Szicree just said - it just ain't practical. We can do it, yes, it's fairly easy - but there's no point to the exercise. This type of energy storage simply isn't practical.

Practical depends...

Whether something is practical is a function of the context in which it exists. The phenomena we metaphorically call the invisible hand automatically sorts though all the variables and thus determines whether something finds a niche, becomes widespread, or ends up being a curious moment in history.

Sure, if the compressed air thing were more practical than it is, then one can imagine it having been adopted sometime in the past. Yet, a gasoline engine without gasoline is not practical. Since gasoline is likely far more precious relative to income in India than here, there is likely a greater calling for alternatives however kludgey.

True statement.

I supposed if fuel became outrageously expensive (even compared to today's prices) and we focused on a nuclear-generated electric economy, we would find other ways and some of the "non-practical" items would suddenly become viable - but even then I'm banking on superbatteries, supercapacitors and/or hydrogen fuel cells being more likely to come to maturity for GA aircraft power supply than compressed air. The energy density of compressed air and the mechanics of releasing it are not conducive to installation in a vehicle that requires both low weight and high power output.

I too am betting on some combo of supercapacitors, batteries and fuel cells. Besides, who wants to ride in something that sounds like a fart.

Am I missing something?

airguy said:

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As for the valve size at pressure, yes they become more complex and expensive, but this engineering and design work was long ago taken care of by the oil industry, they regularly use large valves (up to 14") at pressures well above the range we are talking about. Realistically however, for this type of application you don't need more than 1/4" ports on a hand valve, I sell those all day long (rated to 6000 psi) for $48.

As Szicree just said - it just ain't practical. We can do it, yes, it's fairly easy - but there's no point to the exercise. This type of energy storage simply isn't practical.

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It sounds like the fast fill problems have been solved with other applications. Therefore, why isn't it practical?

I still haven't seen anyone math out the range. That is the key.

I have a 14 mile round trip to work. That's 70 miles a week. If that could be done on one "air up", then it is practical.

Truthfully, I won't do it until the road is cleared of SUVs, but the technology at least is feasible if the range is adaquate.

Energy Flow in the USA 2006

I think in any discussion of this type it is nice to have a look at the big picture:


Also here is some detail on what goes to what:



I think that if we increased nuclear by a factor of eight, and renewables by a factor of three, then we could keep more of the nice energy-dense petroleum/coal liquefaction/natural gas products for the transportation sector. The clean (zero air emissions) nuclear-generated electricity could take over a lot of stationary energy needs now met by coal/NG/petroleum, leaving more fuel for boring holes in the sky!

nucleus said:

The clean (zero air emissions) nuclear-generated electricity could take over a lot of stationary energy needs now met by coal/NG/petroleum, leaving more fuel for boring holes in the sky!

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That's what I want, a coal fired airplane!!! Sign me up.

Now for a dose of reality. My mom, big airplane fanatic, sat next to a guy on an airline who was noticeably nervous. Over the course of the flight in getting acquainted she discovered that this guy was in hydrogen energy research and the reason that he was so nervous was that he and his family had already received death threats. No doubt from supporters of the petroleum cartel.