Compressed Air for Electricity Supply

Intercontinental ballistic missiles used to be powered by compressed air. They would sit in their silos for years, so batteries were not an acceptable option. Upon being launched, a pyrotechnic charge would pierce a can of compressed air. The resultant air stream would drive a turbine and generate the electricity needed to power the missiles electronic.

That's all I know about it.

Do you have more details on this. Maybe there is something of interest in that.

If you have a link please can you post it .

Sorry, that's just something I heard from a professor in college.

This is a difficult question to answer. It depends on your machine design. First of all, lets see pressed air in a container. It has engergy: E=p x v; where p=pressure(N/m^2), v=volume (m^3)[MkgS system][ideal gas equation], suppose your machine's system efficiency is 80%, you can give a max of energy Ex80%. If the energy takes out in a minite, you will give a power of Ex89%/60, of cause if your machine has a huge function to take off in a second, you will give a large power of Ex80% Watt.
Now you can calculate your electric generator easily.
Not very long ago I saw a story on tv, a middle cshool teacher located in brazil who creats a bycicyle drived by pressure air. and has a speed of 90mils/h, approach as fast as motorcar, very interesting.

Generated power will depend on turbine characteristics (rotational speed, blade design, produced voltage and current, etc) and air flow pressure among other things.

1mW is a very small quantity of power. Are you trying to power a sensor in a remote location where battery, solar cells, vibration power generators, etc are not suitable but where an existing compressed air supply exists?

For your mentioned application (assuming that the compressed air supply is existing)You would need to tie into the air line using a second pipe connected to a tiny turbine. A limiting valve would be necessary to limit the air flow thru the turbine out into the atmosphere to a very small amount. Oil in the compressed air stream may cause problems thou. Efficiency will be poor at best. Placing the turbine directly in series with the main turbine would not work (won't generate power unless the air supply is being bled off down stream). This application is a mechanical system and will have reliability issues.

How about a tiny 3.6V Lithium Thionyl Chloride battery with a life of 10-15 years? Would be a much better long-term solution than trying to fit a small turbine (MUCH better efficiency and reliability, minimal maintenance issues (MUCH less than with a small turbine), etc).

What is the application?

Compressed air is an inefficient in just about any application. That's not to say it isn't the best solution for some. Since your power level is very low it will be fairly easy to do but if you have a power source creating the compressed air it would be far better to hook it up to your generating device directly. Good luck.

I can't speak for the generation side of this, but on the compression side, a typical air compressor system is about 50% efficient to begin with. So in other words, when compressing air, the potential energy stored in the air is about 50% of what went into it to compress it in the first place. This can be increased to maybe 70% with the most modern and sophisticated systems (translate: expensive) under ideal conditions (translate: difficult to attain). Conversion of mechanical energy to electrical energy is rarely better than 70%, so at BEST, the efficiency of doing this is probably around 50% (70% of 70%) and at worst may be as bad as 25% (50% of 50%). Not a great way to store energy, but it has been done. I believe there was a car powered by compressed air back in the earliest years of automobiles.

Most people don't understand how inefficient compressed air truly is. I once looked into using a "vortex cooler" on an electrical enclosure as an alternative to an air conditioner. What I could have done with a 1000 BTU AC unit (roughly 1/2HP) would take a vortex cooler that required 20CFM at 100PSIG, which requires about 5HP to compress that air!.

But if you are still interested, here is a decent paper on the energy in compressed air.

http://www.k-k.pi.titech.ac.jp/pdf/Powermeter.pdf

One other issue of which to be aware. If you store large volume of compressed air at a very high pressure there is a risk of the tank experiencing catastrophic failure. In other words it could blow up and kill you. Truck tires used to have split rims that they eventually did away with (I think) because too many people were getting killed by them.

I guess you meant 1MW. not one milliwatt... where do you get your compressed air from? If from a conventional compressor, than just forget it. compressing air in the conventional way, be it piston or screw, is very inefficient. Compressing any gas produces a lot of heat all of which will be lost, unless you are thinking of using that heat? I don't think so.

Hope it helps

Wangito.

Damn poor effieciency, too many parts wearing out to do the task and a high expense. one of the poorest ways to do the task.

Use an engine directly coupled to the generator.

Just use a battery if all you want is one milliwatt (mW). If you want one megawatt (MW), you would require a huge high-pressure storage tank. By and large there are better ways than compressed air.

Hi Mr Guy Negre of MDI (www.mdi.lu) build a 10Kw Generator driven by Compressed air. Also a Sth Korean co. (Energin.com) build a similar generator. Mr. Negre is only interested by his Air compressed car. Regards

Not so sure about compressed air, but if you want power in the order of mWatts, a miniature gas turbine powering a generator seems good. Have a look at http://www.technologyreview.com/InfoTech/11635/

Strong steel or Titanium spheres filled with highly compressed Helium can store more energy than batteries of the same weight, I do not know to what use they have been put possibly in missiles or torpedos.

Yes, and from post "Air as a Source of Energy", the theoretical latent heat embedded in 1 kg of vapor is 2.31 MW. Both sound all well and nice but how much extra infrastructure do you need to convert it and what is the actual conversion efficiency to get a single watt of power out for use compared to other technologies!

I have seen generators that were used in the exhaust stream of a turbine (helicopter), so there is a body of math to support it; for powering electronics on an older small helicopter, I would imagine they were only generating somewhere in the 1 to 10 KW range - mw, easy; MW, I would probably look for something else.

I've never heard of the compressed-gas generators on large missiles - I thought we just put batteries on them. With all of the other support required for such a device, changing out batteries every few years would likely just be another maintenance task.

Well, this thread has polarised people into three separate groups.

1) 1mW generation

2) 1MW generation <sigh>

3) Military use in missiles

Could the original Guest poster PLEASE provide more information on what they actually want to do. A lack of information coupled with the possibility of a mistake between mW and MW is just too confusing. I may very well start a blog to try and address these and other similar points that make trying to figure out what people ACTUALLY want so infurating.

If 2) 1MW generation using compressed air, then my answer is simply "Won't work - hopeless efficiency (laws of the universe and all that jazz)".

Hay unidentified guest,

You made a question, a pretty blured one. Participants are asking you questions, requesting more clarifications/information, and you don't even bother to answer. I think that you show a definite lack of respect and appreciation for people who are donating you their time and knowledge. You should be ignored.

Wangito.

Keep in mind that as a guest, he doesn't recieve notifications of these posts. Not everyone checks in here every hour.

Didn't know about the notifications, Maybe I was too quick, but this is a result of many similar cases. Ya know, The straw that broke the camel's back?

Wangito.

Well the Guest did only post yesterday.

I will put a blog togeather soon pointing out many of these straws so that new guests will hopefully read and heed and not continue to make the same mistakes over and over again. The last thing I want to happen is to see engineers eventually get so frustrated that instead of detailed answers, all guests get are answers such as "It won't work", "No", or silence. This is, afterall the only really good discussion site I have been able to find.

Sorry I was out of office yesterday. The idea here is to use energy efficient techniques to generate wireless power for driving electropneumatic controllers (electropneumatic positioner). I was thinking compressed air as an option. Do you think options like magnetic coupling or fuel cells are a better option for this.

Well, if you already have compressed air at the site (I'm assuming you have since an electro-pneumatic positioner needs compressed air to function), then making a compressed-air-driven turbine to spin a generator should not be difficult but it will not be efficient. If you're just going to provide power for the positioners, the size of the turbine and generator need not be too large so the quantity of compressed air use might not be an issue. If it's a large facility or you want to power a lot of devices then this idea might not work out very well.

If the site is out in a desert or plain, you could use solar or wind power instead. Use these to charge batteries for stretches of low sun or wind (or no sun or wind) and you could have a pretty good set up.

yes it is for powering the positioner. But some estimate of the requirements of how to do it will be useful for me. Like how much power can I get out of the compressed air system and how does this actually work would be useful for me.

Also do you suggest alternatives. Like I have seen some work at ABB with magnetic coupling. And what about fuel cells as a source of electricity supply. I also have problems understanding the 4-20mA interfacing in an electro-pneumatic positioner. Some help would be handy to get into more insight.

Thanks

I cannot give you equations or calculations, if that's what you're asking.

Like I said, it depends on how many devices you want to provide power to and what their power requirements are. That will determine the size of your generator or dynamo. When you have the generator size, you can determine the size of the turbine that will spin it.

I'm a little curious. You don't sound very knowledgeable in instrumentation and yet, you're getting into something that's seems out of your league. I hope you haven't made any promises to anyone about this. If I were you, I'd get the services of a good contractor who knows what he's doing.

The reason I'm saying this is that positioners usually don't need an external power supply. They're driven by the controller or plc analog output. If you're going to be providing power for instruments, I don't think it's going to be the positioners.

See here, this is what everybody is asking you for. There's not much information in you posts and we can't help you if you don't provide them. There's a lot of knowledgeable people in CR4 and they come from a wide range of expertise. If there's one thing they don't have, it's the ability to know exactly what you need based on just a few words or sentences.

The 4-20mA is the signal that tells the positioner to move the valve to whatever position it needs to be in. 4mA usually means 0% position or fully closed and 20mA usually means 100% or fully open. I say usually because some valve operate in the opposite direction, that is, 4mA is full open while 20mA is fully closed. It all depends on what the process needs.

Actually i am not very knowledgeable in the instrumentation field. I can try to describe my idea. We have a electropneumatic positioner driven by a 24 V supply and the 4-20 mA current to control and indicate the valve position. Now we are planning to supply the power from a compressed air system driven by a wind turbine or the other alternative is to use the magnetic coupling like the one described with the ABB.

Brilliant! How come I didn't think about it!?The windmill will drive a compressor, The compressor will drive a deep well submersible pump. and Another pump will lift the water to an elevated Tank from which the waterfall will gravity turn a hydro-electric driven generator, which will supply all your system requirements from 1mW to 1MW.. What the catch? none. We have already pointed out that hydro-electric generators are extremely efficient. You made it buddy!

Wangito.

There are holes in your plans that beg answering. Using wind turbines to drive the compressed air system sounds terribly unreliable and will result in inconsistent pressure in the system (wind speed being inconsistent and all).

So it's just one positioner and not several? And how do you plan to control it? Do you have a controller (which also needs power of course)? Do you intend to use a potentiometer (which doesn't require power) to vary the current?

If it's just the one positioner and one controller, I'd go with solar cells with battery charging than try something novel but unsure.

I am just curious to know about this calculation. Isnt it that the amount of compressed air required to generate about 1 W with a pressure of 1 bar is given by

Compressed air(cu.mm/s) = (1000 N-mm/s) / (0.1 N/sq.mm)

Well if you look at the parallel discussion "Air as a Source of Energy" the theoretical latent heat embedded in 1 kg of vapor is 2.31 MW (apparently). Of course it will be a little bit less after actual conversion and efficiencies are taken into account..

LOL

Hey, a built-in spell checker function has been added! Lets give it a try........ Seems to work.

Maybe I made a mistake with my formula by not taking into account the enthalpy as air being a compressible fluid.

But just to know if there is any general set of value like A cu.m/min of compressed air at 100 psi gives B watts of power.

The only question here is how you will get compressed air, as there is no natural source for it.

Any thing that will make the compressed air for your plant can directly run plant. So why to add more steps, further loss of energy, and this subject had been discussed earlier too. It doesn't look viable at all. Any way do you have some block diagram?

ok .. i will try to explain this .... For an electropneumatic controller , I want to know if it would be possible to use the compressed air available to use as the source of electric supply. I am also trying to find out how much of compressed air is required to produce ´A´ Watts of electric power just to get a feel for the numbers.

Can someone tell me if any univ or industry is working on some similar product

To generate Electrical energy you _have different methods.

1: Taking advantages from magnetic field. In this method you need together with magnetic field conductors, and prime mover. The advantage you can take from the compressed air in this method is to use a turbine or a steam engine to change the compressed energy of the air in to kinetic energy to set the shaft of the rotor of the alternator of the generator in to motion. Thus you can provide electrical energy. For the detail calculation some has to study thermodynamics and the turbines or steam engines which is beyond the capability of this short paragraph to be covered here.

but as a general relationship of the pressure present in the gas chamber and the work done by the gas camming out from the say a turbine some one can use this approximate relation ship.

Considering the Gas tank and the outlet of the gas are all at the same level and the pressure of the gas in the tank to be P₀ and the initial velocity to be zero. The out camming pressure of the gas to be P₂ and its velocity leaving the turbine after doing work w in moving the turbine be v and density of the gas under the existing pressure and temperature to be D and D₂ and the gravity be g then:

p₀/D=p₂/D₂+v²/2g+w

2: But if you want to take advantage from the piezo crystals which under pressure generates electrical energy, some one can do it by different methods. In this case you should know about the properties of the Crystal and depending on the characteristics of the crystal you can take electrical energy.