Pure Air or Hot Air?

I am not getting into this again.

See numerous previous threads regarding this scam.

I missed all of the earlier threads about the "air car". But I would guess that the main issue concerns the energy required to compress the air, and the resulting overall energy efficiency? And maybe also that compressed air might not give much of a driving range? Someone probably mentioned the danger of exploding compressed gas cylinders in a collision. Someone please bring me up to speed on the major objections of compressed gases as an energy storage medium.

The key parameters for an energy storage medium for any off-grid vehicle are:

• energy stored per unit of vehicle weight (determines the range between recharges and the recharging time), and
• power available per unit of vehicle weight (determines vehicle performance and influences range between recharges).

Compressed air needs a vessel to contain it and there is a theoretical limit to its size that determines the practical limit of compressed air use. At the limit, the compressed air pressure cannot exceed a predetermined fraction of the ultimate tensile stress of the material containing it. So it's going to be hefty. As stored air pressure rises, the contribution to the above two parameters expressed as a fraction of the vehicle's overall weight increases until there comes a point where the properties of other energy storage media and their conversion equipment (gasoline, battery, hydrogen) overtake it as more viable options for a practical off-grid vehicle.

In compressing air, energy is used and heat is liberated, which has to be dissipated. As the air expands in some form of motor it cools, taking in heat from the environment. The surrounding air is loaded with varying degrees of humidity, which condenses out on the equipment in the form of ice, adding to the vehicle's weight. As air flows, heat is dissipated as it succumbs to friction imposed at the pipe wall. So the overall efficiency of the system is rather low, and certainly lower than many other options. So while it can be done and has been done, it is seen as impractical in comparison to the other options in most cases.

Thanks WSlack you for your detailed answer. Just as I suspected, both the low energy efficiency and the pressure containment problems make this approach impractical. I had not thought about the additional problems caused by humidity. Maybe compressed air might have niche applications (like driving a fork lift in a warehouse), but it certainly doesn't sound feasible for long-distance travel (and hence the reason people have cried "scam").

This reminds me of an idea I once proposed for a related form of energy storage: vacuum-power! No, I don't mean the zero-point energy. I mean using an evacuated vessel as a source of energy. Simply use a bottled vacuum to suck air through a turbine (which in turn powers a vehicle or generates electricity). Note that since a vacuum has no mass, it has *infinite* energy density! And since the universe consists mostly of empty space, we have free access to unlimited amounts -- we need only travel outside our atmosphere to "refill our tanks with nothing". Maybe we could even build a pipeline to outer space so that we could harvest vacuum energy without the need for space travel. As a bonus, nothing is non-polluting!

Isambard Kingdom Brunel had a similar idea for propelling rail vehicles, and experimented with it between Exeter and Newton Abbot on the South Devon Railway in the 19th Century. In this system, a number of fixed pumping stations evacuated the air from a slotted pipe fixed between the rails. A piston was attached to a rail vehicle. The piston fitted inside the pipe so that the pressure difference between the atmosphere behind it and the partial vacuum ahead of it propelled the vehicles along. The downfall of the system was the leather flap used to seal the pipe either side of the piston and its tendency to perish when subjected to sunlight and sea-spray. It was never commercially successful. Indeed, a lot of venture capital was written off when the line was converted to locomotive traction.

There is a section of original pipe fitted-up as an exhibit at the Didcot Railway Centre.

The very enthusiastic presenter Adam Hart-Davies did an interesting demonstration/experiment with a replica system on television as part of the Open University programmes a few years ago. Instead of a rail vehicle he used a modified go-kart, and sat on it. Instead of a cast iron tube he used a slotted 4in Ø PVC wastewater duct. Instead of a coal-fired pumping station he used two electric vacuum cleaners and a load of duct tape. It all worked!

Of course, the system doesn't qualify as 'off-grid' as the source of energy is not contained on or within the vehicle.

<Maybe we could even build a pipeline to outer space so that we could harvest vacuum energy without the need for space travel. >

It wouldn't work for long. Air entering the pipe at the bottom would assume the same density/height relationship as that of the surrounding atmosphere, over time, and create a gas column that is similar to what is outside it, although formed of the same proportion of components as sea-level air. Then it would stop.

Oh, yes. Just remembered this: http://en.wikipedia.org/wiki/Atmospheric_railway

You wrote an interesting post, thanks.

Part of it was:-

It wouldn't work for long. Air entering the pipe at the bottom would assume the same density/height relationship as that of the surrounding atmosphere, over time, and create a gas column that is similar to what is outside it, although formed of the same proportion of components as sea-level air. Then it would stop.

I am not so sure about that. If we forget the practicalities of building a pipe (which I will now call a chimney for the rest of this article) to beyond the earth's atmosphere for the moment, huge though they are! It may work.

A Chimney or the "Chimney effect" does some strange and wonderful things, it is used to bring cold and very deep seawater to the surface without a pump, it helps a fire to draw even before the fire starts to warm the air above it up.....if you have ever been in the fireplace of an old (and cold!) 19th Century furnace chimney, you would be astounded at the storm of air (on a perfectly calm day!) that goes up that pipe, without any help at all!!

Somewhere in Spain I believe, the effect allows people with a special costume on to "Fly around" in a wide chimney!!!

The reason I am told is because you have joined a low pressure area of air at the top of the chimney to a high pressure at the bottom!!! Plus the warming effects of the sun, thinning the air and making it lighter....

If that is true, then going to outer space will just increase the efficiency of that as there the air pressure is ZERO!!!!

What are your thoughts on that???

Its a good question with which to open up the thread further for the benefit of the younger reader's studying:

• Does it make any difference if this hypothetical vacuum pipe is 1000mm, Ø or 1000km Ø?
• Does it make any difference if this hypothetical vacuum pipe is 1mm thick or 1km thick?
• Does it make any difference if this hypothetical vacuum pipe is made of a high thermal conductivity material, like copper, or a low one, like aerogel?

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

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

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

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

Thanks!

Hi,

I like you picking up on this great, only problem, although I know about this chimney effect from a practical point of view, from the theoretical side I am more than a little hazy......(downright useless is probably better said!!)

I was hoping that someone else could sort of pick up the torch and run on with it!!!

Hows about it Mr Slack!!

A Chimney or the "Chimney effect" does some strange and wonderful things, it is used to bring cold and very deep seawater to the surface without a pump, it helps a fire to draw even before the fire starts to warm the air above it up.....

This "Chimney effect" really does work, but it relies on a temperature difference (which causes a density difference), not pressure stratification due to gravity. I've heard about real and planned applications (e.g., a really large-scale solar chimney in Australia) that use this approach to heat air using sunlight, then let the heated (buoyant) air rise through a turbine. Sounds like a good approach for harvesting solar energy.

My thougts are that, if that were so, the Earth's atmosphere would long ago have escaped to the vacuum of space.

<<The reason I am told is because you have joined a low pressure area of air at the top of the chimney to a high pressure at the bottom!!! Plus the warming effects of the sun, thinning the air and making it lighter....

If that is true, then going to outer space will just increase the efficiency of that as there the air pressure is ZERO!!!!

What are your thoughts on that???>>

Are we not missing these "theoretical" chimneys everywhere?

Also, as air has a weight, would it not come out of the chimney and fall back to earth?

Isambard Kingdom Brunel had a similar idea for propelling rail vehicles...

Wow, I only suggested the idea as a joke -- didn't realize that anyone had attempted a practical application. It did not succeed, but I would not expect it to. Since this "vacuum-as-energy-source" approach would generally employ a pressure difference of only one atmosphere, it should have much less utility than a system using pressurized gas (which could have hundreds of atmospheres pressure difference).

<Maybe we could even build a pipeline to outer space so that we could harvest vacuum energy without the need for space travel. >

It wouldn't work for long. Air entering the pipe at the bottom would assume the same density/height relationship as that of the surrounding atmosphere...

Agreed. The initially empty pipe would fill with air until the weight of the air inside it balanced the atmospheric pressure at the bottom opening of the pipe. If such pipe (which I suggested in jest) could maintain a constant flow of surface air into space, why wouldn't the atmosphere just bleed away into space even without the pipe? Fortunately for us, this won't happen (at least not at any rate faster than that at which in-falling comet fragments replace the escaping gases).

<<<In compressing air, energy is used and heat is liberated, which has to be dissipated. As the air expands in some form of motor it cools,... So the overall efficiency of the system is rather low, and certainly lower than many other options. So while it can be done and has been done, it is seen as impractical in comparison to the other options in most cases>>

Who says the heat has to be dissipated? If the compressor is cooled by boiling water (which may be entrained in the air) and the steam is then used to reheat the air as it expands, very little heat is dissipated and the efficiency is very high. US Patent number 5.832,728 explains this, but it turns out that experiments in 1930 proved the principle, showing that a compressor and air motor were a more efficient energy transmission means than an electric generator and electric motor. A diesel-pneumatic locomotive out-performed a diesel electric locomotive, used less fuel. However, since 99 per cent of good ideas go nowhere, diesel electric locomotives are the norm today.

Ha ha ha ha ha ha ha ha ha... oh boy...

They must have seen it in the USA and want to try the scam here......NO CHANCE!!!

The USA does seem to have the greater proportion of historically interesting compressed air propulsion advocates, though European trams have been powered this way on occasions in the past! See the link in post #6 for some nutty pictures (to today's reader, that is).

The wise money is betting on 'hot air' at 100/8 'on' favourite.

From my previous posts on wind farms.

http://cr4.globalspec.com/comment/23703

Sometimes the government can be useful .

I recently caught the debate on CR4 and have since seen the speil on, I believe, The History Channel.

The center seating, the mission statement, and the gs Hybrid sounded and looked promising. The idea of riding around on a Haus Campbell and all the chatter of the machine are distracting from the appeal.

Also no HP claims have been published that I am aware of.