Compressed Air "Batteries"

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ChrisB
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Postby ChrisB » Mon May 28, 2007 9:26 am

EVguru wrote: It's been discussed on the EV list many times and people who can do the maths have shown that the energy density of compressed air is less than that of good old 'wet' lead acid. The energy efficiency is about half that of lead acid!


I have to admit that I personally cant see how they can be as efficient as something like a battery, shearly down to amount of energy used to compress the air in the first place, BUT one has to think that the engine could be very useful in areas where it wouldnt be practical to use a normal engine or battery vehicle.

Theres also the discussion that if you where able to use free energy to "charge" the air vehicle in the first place then does it matter that its not that efficient, its not like ICE engines are wonderfully efficient :lol:

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Postby ChrisB » Mon May 28, 2007 9:31 am

aminorjourney wrote:Okay, but I'm talking about lifespans here.

A regular EV will use many sets of batteries in a lifetime. Would an Aircar over a ten year lifespan use less resources? After all, air is free. Yes, you'll need to compress it and that will use energy - but will the total resource footprint be less than half a tonne of lead acids? Just curious :)


Now that is a interesting point of chat , as in my other post if your using free electrical energy to "charge" it with air then I would "think" the overall resource footprint would be considerabley smaller than that of a ICE or even an EV.

I supppose it depends on productions costs and damage at that end of its life ??

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Usable Energy Storage

Postby k81ng » Mon May 28, 2007 10:18 am

EVguru wrote:It's been discussed on the EV list many times and people who can do the maths have shown that the energy density of compressed air is less than that of good old 'wet' lead acid. The energy efficiency is about half that of lead acid!


Paul, could you expand on that? According to the calculations I've run, a 230 bar, 12 liter tank would have about 1242000 joules of energy, though I am not sure of the point at which the system would not be able to do any more useful work. A typical lead-acid battery is going to drop about 3-4 volts during use, releasing about 386160 joules. Considering that a tank is going to become lighter as you travel but a lead acid would not, it seems that there is still a decent potential here.
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Postby EVguru » Mon May 28, 2007 10:46 am

You need to convert to Whr/Kg which is the standard way of expressing energy density for batteries.

What battery capacity were you basing your calculations on?

You've also touched on another problem with compressed air and that's pressure regulation. The pressure starts off very high and ends up low. Regulating it to a constant pressure so the engine gives a consistent performance is difficult without creating another loss. The common type of pressure regulator represents a resistor.

In steam engines they try and control speed mostly by varying the valve timing, but still only have to deal with a reasonably constant pressure source.
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Postby qdos » Mon May 28, 2007 2:22 pm

EVguru wrote:In steam engines they try and control speed mostly by varying the valve timing, but still only have to deal with a reasonably constant pressure source.


Interesting you mentioned steam engines at this point Paul as I was just thinking this is really not a lot different. I hear what you say about the conmen and them snaffling up all the grants and hijacking the ecological flags etc. (At this point a flyer from Tescos which hit my door mat the other morning comes to mind!) The thing to learn however is how they get all this publicity and obtain the money. Why is it we don't seem to be able to do the same? Hopefully we can learn a thing or two and pull ourselves together and make steps in the right direction I'm quite sure we can do it if we all pull together. :)

The worst that can happen is the likes of Mr Ford buy us up and stuff us on the shelf like the other organisations that actually seem to be close to wrenching the oil addiction away from Joe Public. Yep I'm a sceptic too :wink:

I particularly think the range claims are are bit far fetched and I really can't see compressor stations popping up all over the place but then again they were selling air to inflate tyres at the local Shell Station so who knows! LOL God how expensive is that going to be if it costs 20p to inflate 4 tyres a few extra PSI ?????

Picking up on Nikki's comment about areas that ICE and Electric vehicles not being allowed. Yes I've worked in a lot of zones where this is the case. Underground and Petrochemical to name but two such industries.

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A few figures

Postby k81ng » Mon May 28, 2007 4:44 pm

Okay, I've run a few figures:

The basic formula I've been working with has been this

E = PV * 450

which approximates the energy stored in a tank of air at 24 C. See http://biobug.org/scuba/scubatank/ for details.

So, using a carbon fiber tank I found online as an example (http://www.airhog.com/tanksets.htm),

E = (310 bar) * (10.1 L) * 450 = 1,218,060 Joules

This tank, when empty, is 11.3 lbs and holds approximately 6.5 lbs of air, totally 17.8 lbs when full, or 8.09 kg.

So, the WHr/kg for this tank would seem to be

Full
(1,218,060 J) / (8.09 kg) * (2.78 * 10(-4) W / 1 J) = 41.86 WHr/kg

Empty
(1,218,060 J) / (5.14 kg) * (2.78 * 10(-4) W / 1 J) = 65.93 WHr/kg

Average = 53.90 WHr/kg

According to Westbrook's _The Electric Car_, this performance puts carbon fiber tanks on par with valve regulated lead-acids, nickel-iron, and nickel cadmium batteries. For reference, Lithium ion batteries have a WHr/kg of 120-150 according to the same book.

It should be noted, too, that the same book lists valve regulated lead acids as having 700+ 80% discharge cycles before replacement. I haven't seen any figures on replacing carbon fiber tanks, but the general consensus in metal tanks has been to replace them every 5 years with heavy use and 10 with best care.

Further, the figures I've run up are based on non-ideal conditions. I've happen to find a standard tank make of carbon fiber, but much larger tanks might have much better or worse WHr/kg.
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Postby EVguru » Mon May 28, 2007 5:19 pm

I did the conversion more simply.

1 W/hr = 3600 Joules.

But one HUGE error, when the tank is empty it contains no energy!

So you have rougly 42 Wh/Kg of total energy and rather less usable energy.

By the way. Raising the pressure of a gas by 100 bar would raise it's temperature by some.

EIGHT HUNDRED DEGREES!

A really sneaky bit of Negre's hype is that he's not only dumping the heat created during compression, but actually chilling the air in the tank to minus 100C (yet more energy input). That gives him enough temperature difference to pull some energy back out of the ambient air.

Have a look at some of the responses at http://www.futurepundit.com/archives/000129.html
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Postby ChrisB » Mon May 28, 2007 7:02 pm

EVguru wrote:Have a look at some of the responses at http://www.futurepundit.com/archives/000129.html


Interesting link that, the war on air engines has been raging for many years as I've always thought, with the believers and the calculators fighting their corners for years and I suspect the debate will rage on for years.
But one thing "I" cant overlook, is the fact theres still no sign of these engines as such, yet apparently the ideas been around for years :?

So with this thinking ( rightly or wrongly ) I have to think its not a workable machine or if it is its not profitable to use, which actually could well be the crucks of it.


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Postby qdos » Mon May 28, 2007 9:52 pm

ChrisB wrote:But one thing "I" cant overlook, is the fact theres still no sign of these engines as such, yet apparently the ideas been around for years :?

So with this thinking ( rightly or wrongly ) I have to think its not a workable machine or if it is its not profitable to use, which actually could well be the crucks of it.


ChrisB


Bit like hydrogen power really :wink:

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Update from MDI

Postby k81ng » Mon Jun 11, 2007 11:31 am

AutoblogGreen just interviewed Miguel Celades from MDI. He stated that he's expecting people to be able to purchase compressed air hybrid cars "hopefully by next year". You can read the interview at http://www.autobloggreen.com/2007/06/09/autoblog-qanda-miguel-celades-sales-manager-of-mdi-they-make-th/.
They are expecting produced cars to offer about 158 mpg using both petrol and compressed air but the cars will also offer a compressed air only mode with reduced range.
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