can a car run on compressed air?

Recently someone posted a thread in GD about this vehicle. And folks there just gushed over how it would solve so many energy problems.

See http://www.theaircar.com

And I was skeptical that the amount of energy stored in a tank of compressed air could be a practical means of locomotion. It's been a while since I have solved physics problems like this.

Forget for the moment that the energy would have to be derived from some external source. My impression from the site is that a little energy sleight of hand is necessary to envision this actually working.

Maybe those who frequent this forum could give me a better "take" on whether this is a goof. Thanks.

--IMM

Is it a good storage medium? Yes. Locomotives have been run on compressed air for years in places where explosive vapors might have been present.

Does it solve the energy problem? Nope.

Takes a lot of energy to compress that air. A lot of the energy becomes heat (compressed air heats up due to Boyle's Law) and is radiated away uselessly. When the air is re-expanded for use, it cools down, and you get less expansion out of it than compression that went into it. Typically, you run the expanded air through a radiator to take in some atmospheric heat. On a really cold day, there might not be much or any temperature difference to work from, and your motor is much less efficient in the cold.

that run for quite a while on the compressed air in an ordinary two-liter plastic cola bottle.

http://www.pbworld.com/pbenergy/caes.htm
constructed the U.S.'s first CAES facility for peak-shaving in Alabama's McIntosh salt dome, where a 20-million cubic foot cavern supports a 26-hour, 110 MW power generation plant.
KBB constructed the world's first CAES salt dome storage cavern project at Huntorf, Germany, where two 10-million cubic foot caverns support a 2-hour, 290 MW power plant.

I'm a mechanical engineer and I specialized in energy and vehicle power in college. You are correct that the amount of power you can store with compressed air does not make it a practical energy storage medium for a vehicle. Further, it doesn't reduce pollution anyway - it only displaces it. Instead of burning gasoline to provide power, you would burn coal at a power plant (still the bulk of our electricity industry) to make the electricity to run the air compressor. Coal burns dirtier than gas and you'd end up with even more pollution. Not a good plan.

Rachael

Coming up with alternative means of storing energy is only intended to be part of the solution. The other part is to generate our power with non polluting sources.

For example, I use an electric lawn mower. At the moment, that electricity is mostly generated with fossil fuels. But now, if electricity were generated with renewables, I would have a pollution-free mower.

If I still had a gasoline mower, I'm guaranteed to be burning fossil fuels, no matter how my electricity is generated.

Also, I would maintain that an efficient storage battery and an electric motor/generator with regenerative braking would be more efficient if you really want to displace the pollution to a central power plant.

Now, if the grid were to be run by wind, hydro, tidal, and solar inputs, none of which work particularly well with a moving vehicle :) you could make a good case for a storage car. But even so, compressed air is not a great medium for the reasons I cited above (radiative loss of thermal energy in the compressed state.)

Good help here. The air car site does talk about capturing the heat of compression. Is this a little obfuscation here?

In my post I acknowledged that the energy has to be produced by an outside source.

The system they propose is not the size of an underground cavern. More like the propane tank on a barbecue.

As I explained it on the post in General Discussion a few weeks ago, compressing air is like winding a spring, and I can't see storing the amount of energy to make a practical vehicle in a small air tank.

And yet, as a friend pointed out to me, this web site is elaborate and possibly beyond your average bullshit artist. What is going on there. Does anyone see something that is clearly contradictory?

Thanks, folks.

--IMM

His car has less horsepower and less range than a gasoline powered car. I don't think he's making any implausible claims.

I crunched the numbers for an exam in a thermodynamics course. You can power a car entirely adequately with compressed air. The car was fully patented, but not really feasible for production since that much compressed air was prone to explosions in a crash. Which is one of the major problems in hydrogen cars.

I would think (that is I hope) that a hydrogen car could carry a smaller volume of gas.

Love your tag.

--IMM

I'm not a lab scientist but I recall an eighth-grade experiment in which heated wood gave off hydrogen gas and water (wood was heated in a stoppered tube, hydrogen collected in an inverted tube, water condensing in another tube). I seem to recall the waste product was carbonized wood (assuming that an electric heating coil was used).

Is this feasible on a larger scale?

I know that the explosive possibilities of compressed air would be difficult to overcome and hydrogen would be even worse. However, I also know that some German vehicles in WWII actually ran on wood, though I'm not sure if the above was the method used.

Just a thought from an interested lay person...

Mac in Ga

To do that, it'd have to be reinforced...making it heavier...reducing it's range, at very least.

--IMM

If you go to their web page you can see they have gotten 212 km on one charge.

http://www.theaircar.com/tests.html

Here are some links, and a picture. Notice how HUGE and HEAVY the air reservoir typically is!

http://www.dself.dsl.pipex.com/MUSEUM/LOCOLOCO/pneumat/pneumat.htm



http://www.nrhs.com/web_exclusives/fireless_cooker/

http://www.cdmrr.com/porter.html

http://www.cdmrr.com/other_porters.html

http://www.tramways.freeserve.co.uk/Tramframe.htm?http://mysite.wanadoo-members.co.uk/tramways/Articles/Compair4.htm

Air is made up of something like 70% nitrogen.

MUCH more energy goes into making liquid nitrogen that you get back out of a liquid nitrogen engine, especially since you need to heat it up to make it expand. You can do this with air, of course, but your heat exchanger will ice right up in moist air and be very inefficient.

I have read that they work without frost up



http://www.aa.washington.edu/AERP/CRYOCAR/CryoCar.htm

The heat exchanger, CryoCar's crowning achievement, before installation. It does not frost up, because it has a concentric triple-pass interior design.

I was not familiar with this form of exchanger.

Still, the energy equation isn't favorable. But if you needed a really zero emissions vehicle, and did not want to have the hazard of a high pressure tank, this would be it. I would not want to run a LN2 vehicle in a really confined space, though, due to the oxygen deficiency hazard it might create. A compressed air vehicle (as one of the above links points out) actually adds "fresh" air to confined spaces like mines.

in the construction of a railway tunnel in Europe. It brought mechanical energy into the tunnel and fresh air.

I would not breath air out of a normal mechanics shop air compressor nor would a scuba diver. Something about the oil used.

Liquid Nitrogen Manufacture

The cost of the LN2 "fuel" is expected to be reasonable. The primary expense for producing LN2 is the energy cost for compression of air.9 Cryogenic separation of nitrogen from other condensables in air typically requires only a very small fraction of the total energy,10 so the ideal work to manufacture LN2 from air is very nearly that for using nitrogen as a feedstock. This work is exactly the reversible work obtainable from an ideal cryo-engine, 769 kJ/kg. The actual work required in a modern LN2 plant is 2.0-2.5 times the minimum, or 1540-1920 kJ/kg.11 Assuming an industrial electric rate for interruptible power of 5¢/kW-h, the energy cost would amount to 2.6¢/kg-LN2, in accord with delivery prices of LN2 in large quantities. Marketing the other commercially important components of air will help offset the LN2 production costs. Since the equipment needed for air liquefaction can be powered solely by electricity, it is conceivable to decentralize the "fuel" manufacturing process and to place small scale production facilities at the LN2 dispensing sites. A cost-benefit analysis is needed to determine the smallest air liquefaction machinery that can be used to produce LN2 in an economical manner.

The basic idea of the LN2 propulsion system is to utilize the atmosphere as a heat source and a cryogen as a heat sink in a thermal power cycle. This is in contrast to typical thermal engines which utilize an energy source at temperature significantly above ambient and use the atmosphere as a heat sink. In both cases the efficiency of conversion of thermal energy of the source to work (W) is limited by the Carnot efficiency, h = W/Qh = 1 - Tl/Th, where Qh is heat input, Tl is the sink temperature, and Th is the temperature of the heat source. By using liquid nitrogen as the cryomobile energy sink (Tl = 77 K) this ideal thermal efficiency is impressively high (74%) with an atmospheric heat source at Th = 300 K. The key issues are the ability to design a practical energy conversion system that can take advantage of this high efficiency and the available energy of the cryogen while still being cost competitive with alternative EVs.

http://www.aa.washington.edu/AERP/CRYOCAR/CryoCar.htm

Please list for us all the forms of energy that provide electricity at this cost. I'd love to see them.

I'll start with one: Coal. (Assuming of course that one completely ignores, as I'm sure you will, external cost.)

Next?

http://future.pis.hitachi.co.jp/eng/h_sys/h-5.html


A new load leveling system that stores energy in the form of liquid air is being developed. High energy storage efficiency is achieved by the regenerator, which recovers cold heat from liquid air during the daytime. Then the regenerator is utilized to produce the liquid air during the nighttime.

SCOTSMAN
Sat 5 Jun 2004
JAMES REYNOLDS

A LITTLE-known invention by a Church of Scotland minister almost 200 years ago could help to reduce the world’s insatiable and ever-growing appetite for oil.

As prices on the oil markets continue to approach their highest for 21 years - threatening a repeat of the fuel protests of four years ago - a leading expert on the Stirling engine has claimed it could reduce petrol and diesel consumption in motor vehicles by more than half.

Dr Peter Waddell, a retired reader in mechanical engineering at Strathclyde University, believes the internal combustion engine - workhorse of the western world for more than a century - could be replaced by a modern interpretation of Robert Stirling’s 1812 engine.

He claims that, using new advances in technology, the Stirling engine could easily match a modern petrol or diesel engine of a similar capacity, but with an improvement in efficiency of about 30 per cent.

... more ...

link: http://news.scotsman.com/scitech.cfm?id=637532004&20040607050754

www.sunpower.com


They are linear...they have no rotating parts. Mostly the company makes cryo generators...if you run a sterling engine backward you have a cooler.