French Air-powered Car To Trials

[Rajiv]

Re: French Air-powered Car To Trials

Look up enthalpy again and read it and understand the concept of enthalpy -- Work is done in compressing the air (air gets hot, compressed, and then loses its heat to the surrounding atmosphere - and gets back to room temperature) -- thus the primary energy has to come from somewhere else -- in this case, electricity generated either from oil, natural gas, coal, nuclear, wind, solar, tidal or hydroelectric -- I may have missed a few sources. In order to expand from its compressed state, air needs to absorb energy (the energy it lost to the surrounding air) from somewhere -- in this case from the surrounding atmosphere -- when the air expands, that energy can be used to do useful work.

The main problem why compressed air cars have not been used is that traditionally, the storage tanks that are required to store the compressed air are heavy and bulky. But that state has now changed. These cars do not use steel or metal containers to store the compressed gas. They use carbon fibre tubes. These are light weight and can store compressed air at higher pressures than steel tanks can. and hence the 200 km ranges that can be aceived in these cars.

The speed and power of these cars is determined by the quality of the heat exchangers -- in other words, how rapidly can the compressed air heat back up!

[Guest]

Re: French Air-powered Car To Trials

I'm really enjoying reading you two bat it back and forth here. Takes some poking and prodding to get Rajiv sounding snappish.

Actually (hope Metalman doesn't bite my head off for this outlandish comment) that design looks pretty damn good - the mechanical one that is ( not the aesthetic one! ). 200 Kilometers after a 2 minute compressed air recharge? Hot diggety. I am guessing if this thing actually moves on compressed air, that two minutes worth of highly compressed air requires notably less conventional fuel to produce than do 200 Kilometer's worth of travel in a Toyota Corolla. Or no? As for the aesthetic of the above smartcar or whatever the heck that blob of plastic is called, it looks like a cross between a fat suppository and a pencil sharpener. Not hugely inspiring. But hey, if it runs all that distance on 2 minutes refueling with compressed air, it's still pretty nifty if you are skosh gasoline.

[Munger]

Re: French Air-powered Car To Trials

You have to realize that in a compressed air car, the energy is not stored in the compressed air -- the compressed air acts as a medium to extract energy from the surrounding air. During compression, energy is used to compress the air and heat the surrounding air -- most of the work is done in heating the air -- and the surrounding air is the reservoir of energy that is tapped when the air expands again.

Those are the fundamentals of "enthalpy" -- This is the main reason that a compressed air car can and does work! It just needs a good heat exchanger!

See also Engineair’s Ultra-Efficient Rotary Compressed-Air Motor

The issue is being confused. Here are the steps:

(1) store the energy (in this case the form of compressed air)
(2) extract the energy

The details don't really matter. It must work this way. You are not going to get more energy out of the process than you put in.

One detail that matters in this case is the maximum amount of energy that can be stored as compressed air.

With compression, you are really just exploiting the pressure differences between the compressed gas and the surroundings. For example, a container of compressed air placed within a bigger container at the same pressure will not be able to release any energy. This is like putting a lid on a glass jar and taking it off again. No pressure difference = no energy available.

The amount of energy available in a pressure differential is approximately given by the formula 2.5 natural_log(pressure_after/pressure_before) kJ/mole. Crunch the numbers for air and you will find that it has a maximum energy storage of around .46 kJ / gram.

air liquifies at ~ 200 atmospheres
air ~ 29 grams/mole

This gives the upper limit. There are some details like phase change that will throw it off a little, but that's in the ballpark. Doesn't matter how you extract the stored energy - heat engine, pressure engine, whatever. You are not going to extract more than the maximum amount of energy that is stored. When you release the pressure the container will cool, sure. Add up all the changes in states and you are going to find that they equal the amount of energy stored in the first place. Principle of conservation of energy. It's a real phenomenon.

Gasoline has about 46 kj/gram.

[Rajiv]

Re: French Air-powered Car To Trials

Yes you are absolutely correct on what you have said --

However, MDI stores its air at 300 bar (4500 psi) so -- it is likely liquid at that pressure

Also from Compressed-air energy storage

A standard 200 bar 5 liter steel bottle has a mass of 7.5 kg, a superior one, 5 kg. Bottles reinforced with, or built from, high-tensile fibers such as carbon-fiber or kevlar can be below 2 kg in this size, consistent with the legal safety codes. 1 m3 of air contained inside such a full bottle has a mass of 1.225 kg (at 0°C). Thus, theoretical energy densities are from roughly 70 kJ/kg at the motor shaft for a plain steel bottle to 180 kJ/kg at the motor shaft for an advanced fiber-wound one, whereas practical achievable energy densities for the same containers would be from 40 kJ/kg to 100 kJ/kg. Comparing to the data given for rechargeable batteries, this makes the advanced fiber-reinforced bottle example comparable to the lead-acid battery in terms of energy density and advanced battery systems are several times better. Batteries also provide nearly constant voltage over their entire charge level, whereas the pressure of compressed air storage varies greatly with charge level. It is technically challenging to design air engines to maintain high efficiency and sufficient power over such a wide range of pressures. Compressed air can transfer power at very high rates, which is a principal objective of transportation prime-movers, for acceleration and deceleration; particularly for hybrid vehicles.

The above figures are for compressed air at 200 bar -- at 300 bar, as you yourself note, the air would in fact be liquid, and have a higher energy density

Also, here are the links to the MDI air car site

More Air Car - Compressed Air Technology

[open4]

A few red herrings have entered this debate,

Air has a critical temperature of -140c and therefore cannot exist as a liquid above this temperature whatever the pressure,

Enthalpy other than some adiabatic expansion within the motor is a zero sum game and insignificant in car with an energy source of compressed air [effectively a spring composed of air] such as the car of this debate providing that it is isothermally expanded that is allowed to gain ambient [free] heat through a heat exchanger to its pre-compression temperature, prior to feeding the air motor,

I worry about confusing things but, a car, not the object of this debate, using ambient heat as an energy source extracted by way of an on board store of refrigerant such as cryogenic liquid air [intrinsically having less potential energy than liquid water] could theoretically have some practicality, in this system when the air is liquefied by compression and cooling, heat energy is lost to atmosphere [unlike the car in question having a store of compress air, you won’t find any useful energy in the cars liquid air tank as atmospheric heat is the cars fuel store the liquid air is just a way of extracting ambient energy] when motive force is required from the car, liquid cryogenic air is released into a heat exchanger re absorbing the ambient heat lost at the time of its liquefaction boiling the liquid air to gas air under pressure and thereafter powering the air motor, think of it as a steam engine running in a world where the ambient temperature is above the boiling point of water, you wouldn’t therefore need to carry any fuel, only water, you would non the less be providing energy for the system by condensing steam [atmospheric water gas] to water liquid.

The theoretical energy density of compressed air says nothing about overall energy conversion efficiency a significant problem exists in air ,being a compressible fluid, only partly transferring its available energy as motive force on expansion within a positive displacement type motor and significant energy loss occurs on uncoupled expansion lost to atmosphere on air discharge,

Multiple cylindrical high pressure gas storage tanks are for their weight, a more efficient method of storage than a single large diameter tank [the hoop stress is directly proportional to the circumference of the tank] the major problem with spiral wound composite carbon fiber etc tanks is that they can be dangerously explosive on impact or fire,

[petertribo]

Re: French Air-powered Car To Trials

I am not surprised at how much controversy a new technology generates, particularly when it is one which threatens a 130 year old technology and the OIL/AUTO/ICE Cartels which now have brought so much misery, pollution and war to the world. Not to mention a Car Culture promoted by the US Government, Madison Ave and Hollywood for at least 90 years. Old thinking dies very hard. (The US Interstate Highway System was a direct subsidy to the OIL/AUTO/ICE Cartels and has contributed to the suburban sprawl we are paying for today.)

I certainly don't pretend to know if it is a viable car but, if some folks are willing to throw some money, research, development and testing at it, that at least indicates some viablility. And, yes, some of that may be Government $$$ and cooperation but is that not true of much or all alternative tech? Maybe if OIL/AUTO/ICE paid their taxes and did not Lobby and Corrupt Governments (OIL wars made to order), that would be the best alternative of all. And, as with many new inventions, this may lead to new discoveries and new concepts.

As far as slurs against the French, any Americans who make them might want to get back to their history books and realize that without the help of France, the Revolutionary War would have been lost. The Siege of Yorktown:

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

The French, being a gracious people do accept digital apologies. Vive La France, french fries and, of course, l'amour.

[Jam]

Re: French Air-powered Car To Trials

The amount of energy available in a pressure differential is approximately given by the formula 2.5 natural_log(pressure_after/pressure_before) kJ/mole. Crunch the numbers for air and you will find that it has a maximum energy storage of around .46 kJ / gram.

air liquifies at ~ 200 atmospheres
air ~ 29 grams/mole
[...]
Gasoline has about 46 kj/gram.

OK. I actually ran the numbers. Got some intermediate results different (the formula should be 0.086 natural_log(pressure_after/pressure_before) kJ/mole. Incidentally, changing the pressure ratio from 200 to 300 only marginally changes the result as natural_lot(200)=5.3, and natural_log(300)=5.7) but got the same final result: the energy density of compressed air is ~0.01 times the energy density of gasoline. (see e.g. http://hypertextbook.com/facts/2003/ArthurGolnik.shtml). This also seems to be confirmed by Wikipedia

http://en.wikipedia.org/wiki/Air-powered_car
Compressed air is a heavy way of storing energy. 300 litres (11 cu ft) of air at 30 MPa (4,500 psi) contains about 16 kWh of energy (the equivalent of 1.7 liters [0.44 US gal, 0.37 imp gal] of gasoline, assuming a 100% efficiency of the engine).

So, it does not seem like a particularly practical way to propel a vehicle (as compared to gasoline).

[metalman]

Re: French Air-powered Car To Trials

OK. I actually ran the numbers. Got some intermediate results different (the formula should be 0.086 natural_log(pressure_after/pressure_before) kJ/mole. Incidentally, changing the pressure ratio from 200 to 300 only marginally changes the result as natural_lot(200)=5.3, and natural_log(300)=5.7) but got the same final result: the energy density of compressed air is ~0.01 times the energy density of gasoline. (see e.g. http://hypertextbook.com/facts/2003/ArthurGolnik.shtml). This also seems to be confirmed by Wikipedia
So, it does not seem like a particularly practical way to propel a vehicle (as compared to gasoline).

[leegs]

Re: French Air-powered Car To Trials

. . . In order to expand from its compressed state, air needs to absorb energy (the energy it lost to the surrounding air) from somewhere -- in this case from the surrounding atmosphere -- when the air expands, that energy can be used to do useful work...

I don't think the above is correct. Imagine a vessel of compressed air at say 80F. The vessel is contained within a perfectly insulated vacuum chamber. You open a valve on the vessel and the compressed air leaks out into the vacuum. The temperature of the air will decrease, as described by the equation PV = nRT (I think that is Boyle's law), but due to the insulation of the chamber, no energy will be absorbed.

If the expansion of the gas takes place in 'the world', the decrease in temperature will cause heat to flow, but 'absorbing energy' is not a prerequisite for the air to expand.

[Rajiv]

Re: French Air-powered Car To Trials

The stored energy of compressed air is very low -- but that is not where the energy to power the car is coming from

Read this discussion at the Oil Drum -- maybe then you will understand

Compressed Air Energy Storage - How viable is it?

So what about compressed air? Surely a cylinder of compressed air contains energy that could be used to drive something?


This is where it all becomes a little strange. The energy content of compressed gas isn't very different from that of uncompressed gas (this is the number that you are bandying about!) at the same temperature. For an ideal gas, the energy contents are identical. How come we can get work from the compressed gas?


The answer is that compressed gas has a lower entropy than the uncompressed gas, and that the amount of useful work you can get out of something when it changes depends both on the change in energy content and the change in entropy. We usually focus so much on the energy side of things that we ignore the entropy side.
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Metalman, I know you have a short attention span ;) -- But please do read this and understand it if you want to take a meaningful part in this discussion -- if you do not understand the facts, keep your nose out of it! ;)

[goadam1]

Re: French Air-powered Car To Trials

What do you people do for a living?

[Rajiv]

Re: French Air-powered Car To Trials

I think using a formula mechanically without understanding what it means! See my response to Metalman above

[metalman]

Re: French Air-powered Car To Trials

The stored energy of compressed air is very low -- but that is not where the energy to power the car is coming from

Read this discussion at the Oil Drum -- maybe then you will understand

Compressed Air Energy Storage - How viable is it?

'Notice that in a conventional car you get free heating, but in a compressed air car you get free cooling. '

and how do you heat a compressed air car? you can't unless you bring a hydrocarbon fueled heater with you.

i'm raising my stupid grade on the compressed air car to a 4.

[Rajiv]

Re: French Air-powered Car To Trials

'
and how do you heat a compressed air car? you can't unless you bring a hydrocarbon fueled heater with you..

The same way you cool a car which is producing excess heat -- use a heat pump! In other words an air conditioner working in reverse!

Anyway with "global warming" Heating requirements should reduce

[metalman]

Re: French Air-powered Car To Trials

The same way you cool a car which is producing excess heat -- use a heat pump! In other words an air conditioner working in reverse!

Anyway with "global warming" Heating requirements should reduce

right.

'honey, pull over. i'm gonna turn on the heat pump!'

'don't we have enough air to get there if we don't pull over?'

'nah, honeypie. we have just enough air to make it to the next air station if we don't turn on the heat pump'.

'aw, shucks'.

silly. no one's gonna do this.

hey! here's a practical application! for non dreamers only...

requirements...

- flat surface
- indoors
- short range
- no toxic exhaust allowed