Energy Storage and Efficiency: Compressed Air vs. Batteries

In my undersanding these two storade systems are of two extremes.

*Compressor Air storage can handle higher power in terms of H.P etc. The power storage potential is surely at the higher side. But the release out rate is the draw back. the sharp deceleration on recovery of potential energy is a major set back. It is more of a high power but quick release system. The weight of air to be carried also add to load in vehicles use.

*Battery storage has lesser power storage capability compared to compressors, but is best suited for low load vehicles. The potential energy release mode is steadier and lasting long compared to compressor storages.

Comparisons can be made only on these criterias, since both systems have deficiencies on comparison to fuelled vehicles.

Without being an expert in this field and with no specefic parameters outlined I WOULD SAVE THE STORAGE OF ENERGY AS COMPREESED AIR IS GOING TO BE LESS EFFICENT FOR SEVERAL REASONS. tHE FRICTION OF THE COMPRESSOR AND THE HEAT GENERATED BY COMPRESSING THE GAS MUST RESULT IN A SUBSTANTIAL LOSS OF ENERGY, ATTEMPTING TO USE THE ENERGY STORED IN THE FORM OF COMRESSED GAS RESULTS IN SOME OF THE SAME PROBLEMS ONE IS FACED WITH WHILE STORING IT . THE FRICTION AND TEMPERATURE GAIN CAUSED BY COMPRESSING THE GAS WILL RESULT IN MORE ENERGY LOST AND THE COMPRESSED AIR HAS TO BE CONVERTED INTO SOME FORM OF MECHANICAL ENERGY USING TURBINES AND OR PISTONS AND THESE ARE INEFFCIENT WAYS TO CONVERT THE ENERGY BACK TO A USABLE FORM, ALL THE ENERGY CONVERTED TO HEAT IS LOST IN STORING AND DISCAHRGING.. iF THE ELECTRICAL ENERGY IS STORED IN BATTERIES AND IF THE ENERGY IS USED FOR LOCOMOTION THE BRAKING ENERGY CAN BE RECAPTURED AND CONVERTED BACK TO ELECTRICAL ENERGY AND CONSERVED. OBVIOUSLY THIS IS A MOST BASIC HYPOTHESIS AND DOSENT HAVE THE MATH TO DO A VALID COMPARISON

This has been thoroughly beaten to death on this forum. Just search the topic. Compressed air for energy storage is a bad idea in so many ways and there's not much point in re-listing them here just read what has already been written.

An educated guess is that the electricity solution will win on efficiency and probably on other grounds like energy density (vol, and weight(?)) too.

Current practice is always a good guide, unless an idea is truly innovative, and it's no accident that electrical solutions abound while the alternative of using compressed air does not.

But to the specifics.

In simple terms the electrical cycle will be, at best, the product of:

mechanical generating electrical, 90% (that's high)

store electrical, 95% (guess)

discharge electrical 95% (guess)

convert electrical to mechanical, 90% (that's high)

which is about 70% overall.

Whereas the electrical cycle could in theory approach very high efficiencies if one tried, for example by using super conductors. Using compressed air starts to involve a thermodynamic cycle and the losses associated with this on top of other flow and mechanical losses.

I'd expect the mechanical and flow losses to run at about 80% each way, at best, so the system is already looking like 64% and a loser to the electricity solution.

The thermodynamic part arises because compressing the air will heat it up and heat energy will leak out during both the compression phase and during storage. When it comes time to expand the air to get the energy back, it will then cool down and any balancing heat to complete the expansion will not have time to get back in. That is how the thermodynamic loss occurs. There will also be a loss because, in practice, physical considerations will mean that the air will probably not be entirely expanded before it is exhausted to atmosphere.

A pure guesstimate here is that all this will waste another 25% of the energy originally invested....so our 64% is down to something like 40%.

The above is pretty rough, but is probably good enough reality indicator to show that there is no real chance that air will beat electricity as a storage solution (on efficiency grounds).

Efficiency is a bug bear term. On the surface it is fairly straightforward...power in versus power delivered. Where the problem arises is when we speak of "useful" power.

If you had an automobile which had a small engine which would kick in once in a while to keep an air tank filled versus an automobile which had a small engine which would kick in once in a while to charge a battery, you certainly do get into some real debates. The air would be more efficient in the physics sense, but the air motor can only use the first 25% of the power stored in the air tank so mechanically it suffers from a lack of usefullness for three quarters of its stored energy. (I can prove that.....lets take it as given hmmm?) The losses occur in the heat produced by compressing the air, and the friction of the pumps and motors, don't really amount to all that much, but any leakage would result in more losses.

Compare to the losses incured by changing the state of electricity to a chemical reaction and back again to drive an electric motor. There we have real losses in efficiency, but on the other hand, the motor can use almost all the energy stored in the battery, and so therefore the useful energy stored in the battery trumps the useful energy stored in the air tanks.

Of course, for real efficiency, you have the engine drive the wheels directly, with no change of state energy systems to cause losses. But that results in its own problems. The most "efficient" engine won't provide much acceleration! Gears are expensive, heavy, and often break. Electric motors are smaller and lighter than the gears and transmission systems, plus they are easier to repair, and more reliable, which is of course, why locomotives use diesel-electric systems. Not so much for the "efficiency", but for the reliability and usefulness of the system.

My personal feeling is that in a more holistic sense, compressed air would be more environmentally friendly, it would not have batteries which need replacing every two years for instance, but it would ice up something awful. And not just in the winter....my air powered sanders here in my shop can only run for about a half hour before you can't hold them any more, and the ice stops them dead in an hour if you soldier on with frozen hands! Compressed air suffers from leakage which requires a fair amount of time and effort to keep under control. (loss of time, loss of efficiency, a maintenance nightmare.) Essentially a different set of problems. Its not economical to recycle a lithium ion battery when its usefulness has expired, yet easy and economical to recycle a lead acid battery. In an accident, the spill from a bank of Lead Acid batteries can destroy a whole section of concrete road, and possibly injure rescue workers or occupants, but L.I. batteries are not dangerous in that way. Yet it takes more energy to make a lithium ion battery than you would ever get out of using it...a consideration which is often overlooked in "efficiency" figures.

The modern "hybrid" automobiles use an engine which would be too small to be really useful for direct drive city driving to keep a battery charged. The general concensus here on this forum has been that replacing the generator and batteries with an air pump and tank of compressed air would not provide any substantial benefits.