compressed air energy storage

[muffinman]

i know im not the first to think of this, but why isnt it way more common? compressed air seems to be a far better method of storing energy than batteries, when you factor in lifespan and maintenance cost... at least i think so. electric motors/generators and CA turbines/motors are serviceable, and air tanks would have exceptionally long life spans. batteries, not so much. plus, battery care and protection seems fairly complicated.
what am i missing?

 

[Joseph C.]

Three words. Poor energy density. Another word - inefficient.

 

[muffinman]

but more sustainable... right?

 

[spinningmagnets]

Believe it or not...the Amish do this a lot. They use air pressure from windmill-driven compressors to run air-tools in their workshops. http://www.popularmechanics.com/how-to/blog/how-the-amish-use-power-tools-15171374

 

[Joseph C.]

but more sustainable... right?

If you can live with inefficiencies and the poor energy density. Perhaps but to be sustainable you would have to have a clean source of energy to power the compressor. Batteries will get remarkably better, compressed air energy storage won't.

 

[muffinman]

cost seems to be the other major factor... batteries cost a ton, and have relatively short lifespans...

 

[Chalo]

cost seems to be the other major factor... batteries cost a ton, and have relatively short lifespans...

Reliable pressure vessels aren't cheap either, in sizes and pressure ratings that would do any real good.

When we say "inefficient", we're talking like 25% efficient under ideal conditions. Really inefficient, like photovoltaics but with lots of wear parts and maintenance attached. The basic problem is that when you compress air you get adiabatic heating, which is energy that escapes from your grasp. Then when you discharge the air you get adiabatic cooling which does the same again. There is no way around it if you are going to store the energy for more than a brief moment.

In the course of all this, you take hits from flow turbulence and mechanical friction both ways too. It's a totally goofy, Rube-Golbergian way to store energy.

 

[Ykick]

IIRC from air powered car articles it required about 7HP to compress enough air to produce 1HP from air motors.

 

[el_walto]

I think the energy density is the largest problem with an air powered bike. Too much weight for not enough range.

 

[dogman dan]

I saw compressed air storage as having some advantages that make it worth the inefficiency in certain conditions.

An air powered vehicle would be completely smog free, much like an EV, if you don't consider where the "fuel" came from. So it could be a good choice in a congested city with bad smog problems, like Mexico City for example, that's in a bowl.

But unlike an EV, the compressed air vehicle, though short range, could be refilled very quick. The uses I saw proposed were along the lines of food delivery.

Other than that, I can't see why it would be good for storing a lot of power. Too inefficient. Too costly.

 

[Ykick]

I saw compressed air storage as having some advantages that make it worth the inefficiency in certain conditions.

An air powered vehicle would be completely smog free, much like an EV, if you don't consider where the "fuel" came from. So it could be a good choice in a congested city with bad smog problems, like Mexico City for example, that's in a bowl.

But unlike an EV, the compressed air vehicle, though short range, could be refilled very quick. The uses I saw proposed were along the lines of food delivery.

Other than that, I can't see why it would be good for storing a lot of power. Too inefficient. Too costly.

The application I saw was for large covered food markets in Asia, airports, etc. Short, frequent trips with no pollution. Hard to beat electric for those situations but I do like air motors.

There might still be a practical application for hot climates since you could cool the interior with the exhaust from a CA motor? However, cold climates would just as likely freeze up....

 

[muffinman]

i have an extreme lack of knowlege when it comes to physics and energy. my thought was more along the lines that compressors could be repaired and tanks made more easily than batteries and chargers in post apocalyptic type conditions... damn, im outing myself as a potential prepper :?
essentially, im just curious about other energy storage solutions, besides batteries.

 

[Ykick]

Compressed air energy is definitely "low tech" and more easily accomplished sans battery manufacturing but as mentioned, 'requires roughly 7x the energy to produce what it can deliver via current air motors.

 

[muffinman]

in theory, that shouldnt matter too much if using something like wind power, right? lol im fogetting costs again, damn.

 

[amberwolf]

Except that wind power is itself typically unreliable, with smaller lower-to-the-ground equipment. Takes very tall towers and huge blades to get into the less-disturbed windflow patterns in most places.

Then you have the issue that the more air you have in the tank the harder it is to put more in, making the transfer less efficient the higher it's "state of charge", if you have a simple constant wind and fixed gearing in your wind-powered-compressor. Not sure if it would make a difference to have some sort of automatic gear shifting setup or not.


Then you also have air leaks to contend with. Gaskets and such wear out/dry out, fittings wear, etc., and you lose air (energy) within the compressor, delivery hoses/etc., and storage tanks. Given how much more energy it takes to put that back in again, it'd be extremely important to constantly check for such leaks and fix them.


You should do a search on ES for this topic, because it's been covered in some detail on the bike/motor end of things over the years, and that will give you more info to base your choices on.

No reason it can't work, but as stated by others, it's way more inefficient, with a host of issues to deal with.

 

[muffinman]

and THAT is why i hate logic. and physics. and voices of reason. and cats.

 

[spinningmagnets]

The downsides are that the electrical-grid driven compressors are somewhat dirty (mostly coal), inefficient, and air-driven cars are short-range. Air-drive seems to be only appropriate for small light cars with one passenger.

That being said...dogman has it right. They can be "refilled" quite rapidly from stationary air-tanks, and the "battery" (though short range), will last decades (10,000 cycles?).

 

[footloose]

I think the energy density is the largest problem with an air powered bike. Too much weight for not enough range.

+1 on that. Compressed air is reasonably light. Containment vessel, on the other hand, pretty heavy.

I believe a fully charged 80 cubic foot scuba tank stores ~400 w/hrs of energy, in a package weighing ~44 lbs.
A Ping 48v/20Ah battery for comparison stores 960 w/hrs of energy, in a package weighing ~22 lbs.

One scenario that might be interesting for compressed air propulsion: rapid discharge, continuous impulse, skip the inefficiencies of driving a motor.
Newton's 3rd in action!
How to validate utility of the approach? We need a simple experiment and a volunteer. Naturally I'd volunteer, but I'm kind of busy this afternoon, so I'll just design the experiment:
Take bike and tank to the dragstrip.
Strap tank onto bike, with valve at rear.
Mount bike, crouch down, hold on tight to handlebars.
Clear all bystanders from behind bike (we don't know what kind of acceleration we'll get from bike/tank/rider combo, but should assume... that low mass valve is going to accelerate wayyyy fast!)
Have buddy stand to the side, and whack the valve *hard* with small sledgehammer.
Off you go, and with luck maybe even in the direction you were hoping for!

To increase both speed and range... substitute a set of double tanks for the single

Unfortunately, due to destructive nature of triggering the system, no recharge possible... so charge-cycles of this battery limited to 1 ...

 

[gogo]

Yes, compressed air has similarities in energy transfer to capacitors. Its got its best punch right away and diminishes thereafter. Tyler Durden suggested it as a means of capturing braking energy to be released from a standstill, which is where ebike controllers need the most help. That amount of energy might be easily stored inside a frame tube? Two-wheel braking dynamics would seem to make implementation difficult, but tricycles might be easier.

 

[footloose]

... a means of capturing braking energy to be released from a standstill, which is where ebike controllers need the most help. That amount of energy might be easily stored inside a frame tube?...

Despite my skeptical comments, this is interesting. I met a guy at SF Makers Faire who had patented
a small hub-driven compressor that would pump up tires while you rode, or if you hand-spun wheel after fixing flat.
Precision engineering, expensive... and didn't really do anything that you couldn't do with $5 worth of hand pump or CO2 inflator. But a beautiful bit of engineering, so I just drooled and moved on.

Could such a gizmo, re-purposed to provide compression braking, actually provide worthwhile braking and in process store enough energy to provide worthwhile takeoff boost?

My intuition is "no", but I haven't done the math.

 

[muffinman]

i wasnt thinking of using this for a bike, but for home energy storage, where tank weight is a non issue.

 

[footloose]

My error, in assuming bike application.
For home use, compressed air does seem useful for energy storage, at least as long as the energy is used directly (not converted to electricity first). Pnuematic tools and paint sprayers both use compressed air "batteries", right?

 

[muffinman]

that was essentially my thought, though to have it double as a generator in a pinch, for lighting and other lowish power uses. not appliance running.

 

[dnmun]

i think compressed air is used to start large diesel engines. instead of an electric starter they use an air motor for turning over the diesel motor to start it. air tanks are common on locomotives for the air brakes. so they use that to spin up the motor when they need to start it.

 

[liveforphysics]

i wasnt thinking of using this for a bike, but for home energy storage, where tank weight is a non issue.

If you want highly inefficient yet costly and spacious energy storage at home that requires mechanical conversion/maintenance on both storage or usage, compressed air looks pretty good.