More About Compressed Air

It is very simple, air motors are VERY inefficient!

The best air motor in the world is maybe 15% efficient.

Ad in a compressor at about 60% efficient in compressing the air needed to power the motor and you are wasting energy at a tremendous rate.

Hydraulic motors can approach 90% overall efficiently but are more commonly in the 80% range.

A hydraulic power unit can approach 85% efficiency. Add in losses through control valves and piping and you are lucky to get a 70 - 75% efficient system.

Electric motors are commonly over 90% efficient.

Think about it!

There is a reason that almost all industrial machinery is powered by electric motors.

$$$$$$$$$$$$$

I could not have explained better myself. Excellent job Prbarry.

this is my first visit here. If you can store high pressure air (6000 PSI) you can create enoough energy to move people long distances. say 200 miles for $5. natural tune up will do. ssgx6=

OK, It is time for you to show the mathematics behind your claim.

When you make claims, you better have the real facts to back them up

The math needs to show:

→Vehicle mass.

→Frontal area of the vehicle.

→Coefficient of drag for the vehicle.

→Coefficient of friction of the drive train.

→Horsepower required to accelerate from a stop to a cruising speed, say 100 kph.

→HP required to maintain 100 kph.

You state that you would have a 6000 PSI air supply.

→What pressure will you use to power the motor.?

→Whose air motor will you use that is capable of producing this amount of power?

→How much air will the motor use: IE: ??CFM @ ????PSI.

→How large will your tank be to hold the required amount of pressurized air for the vehicle to travel the 200 miles? (I gather you know this since you state you can fill it for $5.00.) (In case of an accident that causes the tank to rupture, what is the amount of kinetic energy that will be released?)

→How much will it cost to produce this Air Car?

You can say you are God, but until I see you walk across my swimming pool, I am reserving judgment as to the truth of that statement.

Prbarry:

There is a good article that addresses compressed air:
Thermodynamic Analysis of Compressed Air Vehicle Propulsion

by Ulf Bossel. The link was down but here it is: www.efcf.com/reports/E14.pdf I saved it to my hard drive.

In the article he shows that how you compress and expand air is very important.

Efficiencies can be as low at 9% and up to 40% with 4 stages. The key is intercooling (interheating on expansion) and staging.

There is a place for compressed air, but not in a system like MDI advocates. As you know, MDI is the promotor of compressed air.

In any case how you expand the 6000 psi is massively important. Using it in an IC engine would yield very good efficiencies (70%+), but doing it is not easy. I don't think MDI has the correct solution, but that is my opinion.

Drag * V / 550 = HP = {0.008 + 3.24 * 0.0018 *( v / 100) 2. 5* W + ½ * rho * V * V * Cd * A} * V / 550

If you use this equation in a spreadsheet with a car at .2 Cd and frontal area of 20 sq ft, you can see that it does NOT take much to cruise at 25-35 mph (1-2.13 hp) when the car is 1100 lbs or less.

If you combine the expansion with IC fuel burn, you could net out zero heat transfer from combustion giving an efficiency around 83% (1-TL/TH) for combusion at 2000 k. In other words, by going compressed air in the RIGHT cycle you can make the fuel you do have get well over 75% efficiency at which point you are well above 250 mpg.

Just a friendly hint...

WOW! Thanks for the great explanation. It is mostly beyond my expertise but it give me a great start toward understanding the thermodynamics of efficiently.

I'm not sure that storing compressed air at 6000psi is any less hazardous than some of the proposals for using hydrogen. Even 2000psi, typical of an oxygen tank for welding is a very dangerous thing and is very heavy. Rupture one of these and you have quite a significant explosion. Knock the valve of the end of one and you have a missile that can penetrate a concrete block wall. Also, I would like to see the math on it but I don't think that the amount of energy stored in a tank that would be practical in a mobile application would take you as far as you might think, especially when you factor in the (low) efficiency of the system.

Thank you all for your comments.

The reason why electrical motors are so prevalent may be that electricity is our energy carrier of choice and indeed it is a very practical one. On the other hand, it is not a form of energy that is easy to store. I am sure that we can improve the efficiency of air or hydraulic motors but if you are using free energy like wind efficiency is less of an issue. So my question is what would be the best way to store wind energy:
compressed air, electrical battery, hydrogen or elevated water (What I mean by that is pump water up in a tank that would then be used to run a turbine)? In terms of full energy efficiency I am not sure the answer is obvious especially if you take into account for example the tremendous ammount of energy necessary to make a hydrogen fuel cell (i.e. minig platinium etc..).

Regarding using compressed air in transportation I suggest all interested to have a look at :www.theaircar.com In a nutshell: 90 cubic metres of air compressed to 300 bars in a carbon fiber tank that won't explode but crack. Estimated cost 0.75 euros per 100 km (think of the price of gas, think of what it will be tomorrow...), max speed is 110km/h. Autonomy is 300km. it includes a compressor so you can recharge it in a few hours using electricity if you have not yet built your wind or water powered air compressor...

Cheerio,

JF

I must admit that it looks interesting. I know it would take a bit of adjustment to go from a vehicle that will do 140mph to one with a top speed of ~22mph.

I've been working with applications for cpmpressed air and hydraulics for years now. I Agree Compressed air is very inefficient. it is the most expensive 'fuel' available.

however for some applications it is ideal. the motors are small and light, Air tools make service work much faster as they are small they can get into tight spots, as you don't have to move them you can access through much smaller openings than hand tools and they are faster!

Hydraulic power (oil) is also on the rise. Its advantages are high torque, external cooling (unlike an electric motor) small motor size and the ultimate power source can use 'waste' energy, be remotely located and is easily stored. the biggest advantage is teh lack of surge on startup and you can use seals, whaich are easily replaced, as overload protection that would burn an electric motor out. (of course it still makes the plant unusable but that stops the operator overloading it without costing the company heaps).

of course there is also the ancient hydraulics used during the inductrial revolution and previously, dams, waterwheels etc. these areralso on the rise with microhydro power systems and direct applications. Hydraulic lifts are again becoming popular too

Everything has its place!

I finally got the time to go to aircar.com. Very interesting, BUT! Like other Micro Cars, the lack of acceleration and speed would make it very dangerous to actually drive it in the real world. Well at least here in the US.

The motor design is quite clever but I had the feeling I had seen something similar many decades ago.

I finally remembered, steam locomotive drive systems, follow this link to see how a steam locomotive drive works and then look at the aircar motor again. http://home.new.rr.com/trumpetb/loco/

Seems that there is not much new in the world.

Oh c'mon...pneumatics aren't at all inefficient when you look at the total system. "Efficient" is, after all, the degree to which something does its job well. And if you've ever seen an Air Hogs air-powered airplane fly, then you see that these little guys might not be efficient in the sense that you put more power in than you get out...but they are exceedingly light, fly well, and cost practically nothing to fly.

Same with pneumatic cars, etc. If you have a cheap source of compressed air (maybe from a windmill-turned-pump?), then just "plug in" your air car and let the big, low-pressure and lightweight reservoirs charge overnight, then you have your free-to-drive commuter car, n'est ce pas?

The problem is only when you try to make a pedal-powered air car (like I did). Only then does the "inefficient" quality become something to deal with (oy vey!).

Sorry Andy, Efficiency is Energy in VS Energy out.

Do a little math on that "Air Hogs air-powered airplane" toy. Scale it up to a practical size and then show me some figures.

In the 1970's I converted an Ingersoll Rand crawler rock drill, (used to drill holes for explosives for quarrying, etc.) from pneumatic to hydraulic operation.

As built it had to have a 60hp air compressor to supply it with enough air to run. Most users ended up using 80hp to prevent stall-outs. After being converted to hydraulics, it was powered by a 19hp hydraulic power supply and had enough power to move the machine if the drill became stuck.

Air power is for windmills and sailboats.

That's not true. Efficiency in any system must include time, cost and other considerations as well. You've artificially limited yourself to a single parameter, and you can't do anything interesting that way.

True and other considerations must include environmental impacts as well.

But even when talking about efficiency, you must take into account all the energy going into the system, including the energy necessary to build it for example. This is why hydrogen and nuclear are actually rather inefficient, same with photo-voltaic. All these technology are so energy intensive to build that if you do full cost energetical accounting, well you loose.

While I cannot say that I can drive using hydraulics, the suspension of my car (Citroen) is hydro-pneumatic, and is reliable at keeping the car level, even during cornering.

The system is fed by a rotary pump, kept at 62Bar, and stops the car very sharply - pressurised fluid fed directly to front brakes.

There is a company in the US using a hydraulic drive for heavy refuse trucks. This way they can use a smaller engine providing the pressure at a constant rate, even while the machine is stationary, and releasing the power as required.

http://www.memagazine.org/july06/departments/tech_focus/techfocus1.html

Long live DS21 Pallas's.

That said, I have built and used a hydraulic motorcycle, with an accumulator and regenerative braking.

It worked well. I also built and drove a hydraulic Pinto. Same thing as the motorcycle.

I have built several drive systems for boats that use one motor driving several pumps so as to have two propellers, bow thruster, generator and anchor winch all powered by the one prime mover, (motor).

Virtually all Off-Ooad construction equipment is driven by hydraulics.

I think there is a confusion here.

Sure hydraulics is a great way to project forces but the energy in this case is not hydraulic, the source of energy is whatever you use to power your hydraulic engine (gasoline or electricity more likely). The beauty of compressed air is that it is a way to store mechanical energy. If you are using a compressor powered with gasoline or electricity to produce compressed air that you will then use to do work, that will not be very interesting. But if your air is already compressed using wind power for example then it makes sense because you are able to store free energy (cost of the wind mill excluded). Even if the compressed air engine is less efficient, it is better than having to use gasoline which you obtain by extracting petroleum from somewhere far away, transport it, transform it etc... And I am not even taking into account military expenditures to secure your supply of petroleum or the environmental costs associated with its use... For electricity, it all depends how it is produced but there is as you all know a lot of energy lost in transporting electrical power from the plant the user.

So again when we talk about efficiency let's be thourough and include the whole Joule content of any process or product, from the source...

If air can be compressed by the wind, then why not hydraulic fluid? Use the output pressure to drive an electric generator when the power is needed, and at the most efficient speed for the application.

Have you tried to compress fluids? Try with water.... The point of hydraulics is that compressing fluids is good to transfert power (push at one end it pushes at the other)
but not for storing energy. Fluid are not compressible

http://www.accumulators.com/

Problem overcome!!

with a massive accumulator(s) and some clever valving you could have regenerative braking and hybrid hydro/pneumo power for an "efficient" and green car.

This discussion started with the premise of using a windmill to compress air in order to use pressurized air as a portable source of energy. My contention is this is a non-viable way to make and store energy.

You can store hydraulic energy in an accumulator and have a much more efficient source than compressed air. You can store electric energy in batteries and have an even more efficient source of stored energy.

It comes down to how much energy can you store over a given amount of time, IE: 6 hours of winds above 20mph equal how many joules stored for each media, air, hydraulics or electrical energy.

Why anyone would want to waste time and money building the most inefficient form of gathering and storing energy this side of solar panels is beyond my comprehension.

How come there is never enough time to do it right but there is always time to do it over? P. G. Bahre

I don't mean to quibble; I do understand your point and I don't disagree with it. But an air tank/air tool (for example) is a very lightweight thing compared to equivalent batteries or, heaven forbid, a hydraulic system with some sort of accumulator/energy source.

I love hydraulics. I work with them all the time. I'm a hydraulics cheerleader. But pneumatics definitely have their place.

Lightweight borosilicate-lined air cylinders operate with practically zero friction, noise, or contamination risk. Plastic pumps and motors can be made extremely light in low-pressure systems. You've got no problems with electromagnetic fields, leaking oil or heat. Such things mean that you can do some things with pneumatics that you just can't do by other means.

I still think my pneumatic pedal car could be the coolest toy ever if I could get the engine in tune...

I have designed both Hydraulic and Pneumatic Systems. I spent 23 years in Silicon Valley and definitely know the pros and cons of all types of power systems, mechanical (gears, chains, belts, drive shafts, etcetera.) electrical and fluid power.

In every design application there are multiple solutions that are capable of accomplishing the desired outcome. As engineers, we are suppose to provide the design that offers the best possible answer to all the parameters put forth by the client. We need to be able to back up our designs with facts and answers to all the questions presented to us about the design.

One problem I faced involved pneumatic actuators in a class 100 clean room. They needed to be precisely controlled during retraction. You know the problems associated with slow, smooth actuation of air cylinders, seal friction and the compressibility/expansion ability of air.

The client had been wrestling with the problem for over a year before calling me in.

The short side is I solved the problem with an Air over Oil System using ISP as the fluid media.

I reiterate, there is a place for all types of power transmission and storage types, but air is the most inefficient of them all.

Bring your Pneumatic drill and compressed air tank and work along side me and my DeWalt 18vdc drill. Compare weight, complexity, cost and ease of use between the two.

I am not against Pneumatics, I have designed a thousand or more pneumatic systems including ones that used hundreds of Self-feed pneumatic drills and tappers.

I am against inefficiency.

Unfortunately, that's the only one Ive not had the pleasure to own (yet!) - GS, BX, CX, and XM to date.

Andy, Quite the collection!

I have had a 2CV, Mehari, DS19 and DS21 sedan and wagon.

Vive la diffenrence.

Build a tube from Seattle to Portland or for that matter from Seattle to Los Angeles. Use pods to carry people or cargo that travel on an air bearing. Send the pods from A to B limited to the speed of sound much like a dart in a blow gun. Windmills can compress air at intervals for auxiliary boost, bearing and braking. Aunt Milly and her Red Hat friends could zip down to LA in a couple hours for less fossil fuel than it would take to fry a hamburger. One way tubes eliminate collision. No energy is wasted getting a jumbo jet to 30,000 feet only to start decent into the destination.

Most (over 50%) manufacturing plants and processes from Mom and Pop to General Mommy use compressed air since it replaced steam almost 100 years ago. Several million people gladly or grudgingly use compressed air every day. They think therefore they would convert to electric or hydraulic remedies if that would serve them better.

"I think, therefore I am" convinced that some have the equivalent of penis envy about using hydraulics vs pneumatics.

Tom,

Penises are hydraulically actuated!

Touche.

I think you're over-enamored with your notion of "efficiency" (I've got thirty-year-old air tools that still work great. Where'll your DeWalt cordless drill be in thirty years?), but you get top marks for humor. And that is probably more important.

PR, Many won't acutate with out viagra which is transported by air.

Would PRBarry be a GBG (great big guy) I worked with in the Bay (Pneumatic) area?

Tom, Greetings from the Bear Wrestler, AKA Paul R Barry, Sales Engineer, Bay Pneumatics, (Retired)

The best to you my friend. Should you wish to send off the air it's tom@applied-pneumatic.com

sounds just like the London Tube before they electrified it!

I've always thought a neat way to use compressed air would be to generate it by using a tromp, then store the compressed air in an old sealed up, hard rock mine. Pressures would not be too high but the volume could be massive. When you could get paid peak rates for the power, generate electricity to supply the grid. Windmills may not generate electricity when you need it or can get paid most for it.

Here is a link about tromps:

http://members.tripod.com/~nxtwave/gaiatech/airliftreferences/trompinfo.html

Tromp would get you your compressed air w/a wind mill

the amounts needed to blow an over sized bank tube, 100's of miles would be enormous.

Spinning turbines w/ the water would make more sense.

Innovation may save us all. "Don't ask what if it doesn't work. Ask, What if it does."

I think the initial point of this exchange was that wind mills could be used to compress air by direct drive more efficiently.

One of the factors that makes air expensive is that electricity is generated by several different means including wind mills and then the electricity drives electric motors that compress air. If the wind, water, coal or nukes cranked out compressed air directly the cost would be much less.

Currently the distribution of electricity is very accomplished. Compressed air could be distributed in a similar if more limited grid as a utility in an industrial area or industrial park.

The air would branch from the grid main through a pipe that had flow meters to each facility and user would pay for the Clean Dry Air they consumed.

The economies of scale with central source air possibly including but not limited to wind mills and direct drive compressors could be significant.

We kicked this around fairly well on my blog, "Air Ways" in the General section.

Bank tubes are often motivated by vacuum. Our intercity transport tube might use both vacuum and compressed air. A six foot diameter pod area would provide 4,000 square inches of area. An air bearing might reduce the coefficient of friction to .01 or less. Then a 200,000 lb. (100 ton) pay load would require slightly more than 2,000 lbsf (1 ton) to accelerate the load. Actually then 1 PSIG would provide twice that force or 4,000 lbsf and much more might break things. With partitions that could be closed behind the passing pod to contain ingested compressed air and a PID loop to control the pressure the pod could be accelerated at intervals. Very small compressors could fill large reservoirs between traffic intervals.

Practical? Not at this time. Possible? Uhuh. Innovative?

I like air & use many air tools, but

It is not very efficient

Air bearings use great gobs of air, [see link]

http://bearings.globalspec.com/SpecSearch/ProductSpecs?Comp=800&VID=97914

their best uses are for when you need isolation, or just don't want to pay the gravity tax.

Garthh Thanks for the link you provided. Did you happen to notice that the air flow listed was in Standard Cubic Feet per Hour (SCFH)? The greatest consumer at 32.8 SCFH is approximately .55 Standard Cubic Feet per Minute? Is a little over 1/2 SCFM considered to be gobs of flow?

Tom

Tom

I read it as 412.3 scfh/ton [great gobs]

it is a fun idea!

& this is a place to "WHAT iF"

So what sort of compressor would the windmills use?

how bout a link?

here in central california, many of the orchards have wind machines, it always struck me as wasteful that they weren't both generators & blowers

Hi Garthh

I expect that the wind generator shaft is directly coupled to the shaft of electrical generator/alternators and wires carry the electrical juice down to hook into the common buss or battery pack on the ground.

It seems that the replacing the electrical generator and connecting directly to the drive shaft of a compressor would not be very difficult. Tubes could carry the compressed air discharge down to common pipe or receivers. Just a mechanical analogy of the electrical system. In a limited area such as an industrial park a grid of air mains could be installed similar to water and sewer service.

One Mill I know kept their old steam donkey and compress air with a turbine driven with steam. They make the steam by burning waste wood, chips, bark etc.

Compressed air is effective, safe and user friendly but "Don't get no respect."

• The majority of compressors are shut down and the systems are exhausted over night. The air stored in receivers is either dumped to exhaust or allow to bleed down through leaks in the piping. Very few would drain the battery of their car over night.
• The amount of leaks in an average compressed air system in many factories and mills waste 25 or 30% of the compressed air. Again very few would open their wallet and throw away one dollar for every four counted.
• In a discussion in my blog "Air Ways" subject, Compressed air as a utility" one commenter said his compressors, 2 x 200 HP, were the greatest users of the electricity consumed in his plant. In this case he was very conscious of the need to stop leaks and provide good maintenance.

Another factor that slips by most is that the the heat of compression is wasted also.

It takes 7.8 cubic feet of free air to make 1 cubic foot of air when compressed to 100 psig. very simply, when the compressed air temperature returns to 70 degrees F the heat or BTU's from 6.8 cubic feet of that compressed air has been removed with after-coolers, fans, convection etc. I think of it as having a heat pump but throwing the heat away.

With a compressor, vacuum pump, water pump, hydraulic pump or any other device that is driven with an electric motor connected directly to the same type of power that generated the electricity efficiency would increase dramatically.

For today we do what is easiest with what is already in place. For tomorrow we may need to re-think what can be done with wind, water, waves and my favorite bladders or spring returned rackets that traffic would drive across to operate.

We don't need to think outside the box we need to get ourselves out of the box to think and do new.

Soap box just tipped over and dumped me out of it, gotta go.

All very interesting. I seem to have hit the right thread for figuring out if my idea has any legs to it (I may be a genious but being untrained just makes me dangerous - evil was just sorta picked up along the way >:D )

Anyway. It seems like this is all about what is the better Battery. It's the common theme throughout every type of power generation scheme. Chemical battery, Gravity battery, Compressed air battery...etc, etc, etc. That and what is the overall cost of refilling the battery?

I'm thinking compressed air has some legs to it yet, just no one is thinking about it properly. And please try and understand that I'm no guru in pneumatics or any other type of 'matics or 'dynamics, so mistakes or incorrect assumptions may play a part in this discertation. Feel free to prove me wrong.

Couldn't relatively small air cylinders be used to store air on an individual or single home basis (and could be scaled depending on application)? Then use combinations of 'alternative energy' generators to renew the supply. Wind would be an easy one to convert to mechanically run a compressor to sporatically refill the tank, but pv could be used to run a small compressor as well when it there is no primary load during it's operating times. Micro-Geothermal (think Stirling - but that's still in development stages as far as I can figure), micro-hydrodynamic (rainwater run-off), true geothermal (where applicable), passive solar (stirling again) and any other way you can think of.

Then, when loads require it, a rotory pnematic engine could be used to run a generator to supply power for the load. There would be losses all across the system, but since the actual power provided is as free as the operating costs of the machinery, and produces no hazzards, and uses free, abundant and inert air, any energy gain would be a plus.

With the same pneumatic rotory engine powering individual wheels in your vehicle, the vehicle could be many hundreds of pounds (even thousands) lighter, thus reducing the energy consumption used in making it move...and wouldn't you love to be able to fill up at home? Longer trips could be completed using the same type of infrastructure we have now...except now it could be cylinder exchange (just a thought as I don't know what the fill times would be) or emergency fills from a small pv array, though the tradeoff there would be time.

Existing technologies are still there to supplement the changeover, but this could start as a nice little cottage industry.

I'm missing so many facts right now that this is all still a pipe dream, but I've found some of the devices, like the new pneumatic rotary engine, so the tools are starting to come into place (http://www.engineair.com.au/airmotor.htm). It's all about not finding just one solution, but using them all. It's all in the battery. While fossil fuels have been a good battery during our learning curve, it's time to move on. Besides, I hate paying anyone for my needs.

Like I said, I'm sort of a novice at the mechanics and the theoretical calculations, so any help or advice you can give would be greatly appreciated.

You can reach me here or at tom@kaighin.us

(and forgive any spelling or grammatical errors, I'm too tired to bother checking)