Compressed air engine

rdwarr

Generali wrote:

According to the University of Oregon ( http://zebu.uoregon.edu/2001/ph162/l10.html ), compressed air can store 2kWH of energy per cubic meter (1000 litres). The capacity of a Toyota Corolla's fuel tank is 60l and so could store 0.12kWH.

Surely that depends on the pressure to which the air is compressed though. Under high pressure you can fit more compressed air (i.e. molecules) into a cubic metre. Energy is generated as each molecule escapes and becomes subject to normal pressure again.

wdyw

rdwarr wrote:

Surely that depends on the pressure to which the air is compressed though. Under high pressure you can fit more compressed air (i.e. molecules) into a cubic metre. Energy is generated as each molecule escapes and becomes subject to normal pressure again.

m .cp. (Delta T) thats what energy you can get out of the air . Minus loses, effieciencies, etc etc.

redux

Generali wrote:

According to the University of Oregon ( http://zebu.uoregon.edu/2001/ph162/l10.html ), compressed air can store 2kWH of energy per cubic meter (1000 litres). The capacity of a Toyota Corolla's fuel tank is 60l and so could store 0.12kWH.

According to Wikipedia, a modern battery powered car uses 0.2-0.3 kWH per mile ( http://en.wikipedia.org/wiki/Battery_electric_vehicle ). That means that, assuming the same efficiency, 60l of compressed air would take you up to 0.6km.

Another way to look at it is a litre of petrol contains 8kWH of energy (UofO), over 65 times more than a litre of air!

Yet another way to look at it would be that the gentleman that went to all the trouble of writing all that tosh about compressed air driven cars clearly hopes that the readers of his website can't count!

If in the next 5 years anyone produces a car retailing at less than £20k that can travel for more than 100 miles at 30mph powered solely by compressed air plus any energy that can be regained by braking, I will give £100 to the Alzheimers Society.

There may be some questions to answer about the viability, but being this facetious isn't necessary.

Putting compressed air in a normal car fuel tank would burst it. Think more like the bottles of gas used for welding, building site size - but you'd still need half a dozen or so, which would weigh half a ton.

This is why I wondered why they don't develop the carbon-fibre tanks they mentioned right from from the start, as they reckon this will save 300 kg or so on the car weight. I wouldn't expect them to be at all cheap though ...

I'm wondering just how hot the tanks get while being filled. Might need quite a bit of insulation between them and the car interior

tomstickland

m .cp. (Delta T) thats what energy you can get out of the air . Minus loses, effieciencies, etc etc.

No, that's the thermal engergy stored in the air, with Cp being the thermal heat capacity of the air.

The stored energy will be the energy of compression.

Generali

I wasn't trying to be facetious, just trying to work out the viability of this using my very rusty A-level physics. From the back of envelope stuff above I reckon these cars aren't feasible and this is just another internet hoax.

If your physics is better than mine (perfectly possible) then maybe you can show how it is possible to store enough energy in a practical way in compressed air to drive a car. From the numbers I can find it is not although I too would much rather be able to fill my car for tens of pence not tens of pounds.

redux

Generali wrote:

I wasn't trying to be facetious, just trying to work out the viability of this using my very rusty A-level physics. From the back of envelope stuff above I reckon these cars aren't feasible and this is just another internet hoax.

If your physics is better than mine (perfectly possible) then maybe you can show how it is possible to store enough energy in a practical way in compressed air to drive a car. From the numbers I can find it is not although I too would much rather be able to fill my car for tens of pence not tens of pounds.

I more or less agree with you, and was being a bit ironic at the beginning .

Tom found their reference to 300 litres at 300 bar, ie 90,000. Then I looked up gas bottles on the BOC website and found that a large welding bottle for oxygen holds 11,000 litres at 230 bar and weighs 85 kg when full, which could mean using 6 or 8 of them.

Clearly that sort of storage facility isn't practical on a small city runabout. The taxi car featured on their website has a few bottles underneath that look something like that size.

Making the bottles out of fibre-reinforced plastics would make them lighter, as they suggest, but also vastly more expensive than steel or aluminium - I've found a 3200 litre kevlar one for sale at $1500, weighs 8 kg, 80 x 20 cm, working pressure 130 bar.

So it's possible, but not practical for trips over city runaround lengths.

I think that is the whole problem with the stored energy to power a car - the extra weight. If electric we need loads of batteries; if compressed air loads of tank storage. If burnt fuel, there is just so much more energy there due to the explosion from liquid to gas, and all the thermodynamics - I can travel 1500km on 75 litres of diesel that weighs about 60kg.

And, going back round in a circle, the compression of the stored air consumes energy from somewhere, most probably from burnt fuel in a power station. My car engine is more efficient than a power station, and I don't need to lug around an extra quarter to half a ton of dead weight.

So I'm sceptical too ...

wdyw

tomstickland wrote:

No, that's the thermal engergy stored in the air, with Cp being the thermal heat capacity of the air.

The stored energy will be the energy of compression.

ah no, I do beleive you are wrong. Internal energy stored in a gas is (m. cv. T).
With enthalpy being (U + PV), thus encompassing the energy required to fill the tank we get to (m . cp . T). ...and as others have said filling the tank won't be an isothermal process, temperature will rise .....

As the gas is released to the engine, its temperature will fall dramatically (the blurb indeed states an "exhaust" temperature of -40degs C), giving you a delta T of about 60degs
assuming temp of air in the bottles is close to the ambient temp rather than the temp the gas gets to during filling.

redux

Hans_de_Vries wrote:

The sticky problem is how to attract investment in order to put these vehicles on the roads and highways of the world without relying on hydrocarbons or hydrogen and providing virtually free transport for billions of people.

And of course, how to attract investment into all the new power stations that would be required, which don't use any fuel, and don't have any revenues

Enron tried that and they ended up in jail

tomstickland

wdyw wrote:

ah no, I do beleive you are wrong. Internal energy stored in a gas is (m. cv. T).
With enthalpy being (U + PV), thus encompassing the energy required to fill the tank we get to (m . cp . T). ...and as others have said filling the tank won't be an isothermal process, temperature will rise .....

As the gas is released to the engine, its temperature will fall dramatically (the blurb indeed states an "exhaust" temperature of -40degs C), giving you a delta T of about 60degs
assuming temp of air in the bottles is close to the ambient temp rather than the temp the gas gets to during filling.

Right you are.
Can you do the maths on the amount of stored energy in the tank versus compression work then?

I might do it when I can be bothered.

ie: Isentropic compression.
Let it cool.
Then expansion in engine.

If they want their concept to be taken seriously then they need to do the maths and present it clearly on the website.

redux

If the air gets that cold when expanding, what stops ice build-up in and around the engine? Carb icing with no carb!

Or does the compressor include dehumidification?