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Author Topic: Can energy be stored as a compressed gas?  (Read 3490 times)

Offline thedoc

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Can energy be stored as a compressed gas?
« on: 26/07/2013 11:30:01 »
Tad Davison  asked the Naked Scientists:
   
A couple of years ago, I made a very poor attempt at asking a question concerning energy and its containment - essentially a pressure vessel of sorts - and I wondered if this might be a feasible way of storing energy until it was needed, to then generate electrical power. I see from the news this week, that scientists are looking at ways to do precisely that with compressed air. It is compressed to such high pressures, it becomes a liquid, then, when turned back into a gas, it is used to drive the blades of a turbine.

Regards

Tad

What do you think?
« Last Edit: 26/07/2013 11:30:01 by _system »


 

Offline alancalverd

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Re: Can energy be stored as a compressed gas?
« Reply #1 on: 16/07/2013 11:43:29 »
Air is not a good choice as its critical temperature (above which it cannot be liquefied) is around -190 deg C,so you need to build a storage pressure vessel that is also a good insulator, and also work with very cold gases. Carbon dioxide, ammonia and propane are much more engineer-friendly in this respect.

Anyway the answer is fairly easy to calculate: a liquefied gas will turn into about 4000 times its volume at 1 atmosphere pressure, from which you can estimate the stored energy using PV = RT (the gas law).

CO2 is used as a propellant but once you have gone to the trouble of liquefying propane, its specific energy as a fuel far exceeds its potential energy as an evaporating gas.

Far better, in my view, to generate electricity from wind or wave power and use it to electrolyse water, storing either hydrogen or hydrogen/oxygen mixture in our existing gas distribution systems - most of which were designed for hydrogen many years ago.   
 

Offline peppercorn

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Re: Can energy be stored as a compressed gas?
« Reply #2 on: 16/07/2013 17:47:18 »
Hydrogen from electrolysis of H20 is inefficient, plus transporting and storing it is challenging as it leaks so easily. I think it will react with commonly used metals as well.

An alternative approach to storing intermittent electricity generated via compressed air by renewables may be this:
http://www.treehugger.com/renewable-energy/new-jersey-utility-invests-20-million-into-compressed-air-energy-storage.html
 

Offline syhprum

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Re: Can energy be stored as a compressed gas?
« Reply #3 on: 16/07/2013 22:08:35 »
I understand that the military (where cost is no object) use highly compressed helium for power storage in some applications.
 

Offline alancalverd

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Re: Can energy be stored as a compressed gas?
« Reply #4 on: 17/07/2013 10:31:46 »
Compressed gas is a good short-term buffer store: a storage vessel known as an accumulator is fitted to practically every pneumatic system from dentist's drills to aircraft engine starters - but you need a truly vast pressure vessel to store an industrially useful quantity of energy. The practical limit of energy density is about 100 kJ/m3 so a tank the size of a garage, pressurised to around 50 atmospheres, will run the house for about 2 hours unless you want to cook dinner, in which case you will be lucky to roast a chicken in a microwave cooker before the power runs out.

I think the idea of generating electricity, converting it to compressed air, and using that to generate electricity, is bizarre. Industrially, compressed air is considered the most expensive form of practical motive power! Why not use your windmill to compress air in the first instance, and cut out two stages of inefficiency?

Helium is not much better as an energy store, and is frankly unfeasible on an industrial scale: there just isn't enough accessible helium on the planet to be useful, and any loss is irrecoverable.

Regarding hydrogen from electrolysis, it could be argued that efficiency isn't really important if the electricity is free* (though the electrical energy that doesn't form hydrogen does at least heat the water, and we have plenty of uses for hot water - possibly half of all the electricity we use is for heating water anyway!). "Town gas" was/is (is anyone still making it?)  up to 50% hydrogen: the losses were  tolerable and the infrastructure already exists:  we are still using most of it to distribute natural gas. Hydrogen does not react with metals.

*the main argument against windmills is that far from being free, they are horrendously expensive to build, maintain and scrap, so over a lifetime they produce ridiculously expensive electricity, even without storage.
« Last Edit: 17/07/2013 13:30:16 by alancalverd »
 

Offline syhprum

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Re: Can energy be stored as a compressed gas?
« Reply #5 on: 19/07/2013 09:38:23 »
Windmills were very rapidly abandoned at the start of the twentieth century when gas and diesel engines became readily available, why this craze to re-introduce them is it just a money making scam ?
 

Offline peppercorn

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Re: Can energy be stored as a compressed gas?
« Reply #6 on: 19/07/2013 11:38:46 »
....you need a truly vast pressure vessel to store an industrially useful quantity of energy. The practical limit of energy density is about 100 kJ/m3 so a tank the size of a garage, pressurised to around 50 atmospheres, will run the house for about 2 hours
- So, a 'truly vast pressure vessel' such as an "underground storage chambers (depleted gas wells, salt caverns, or some other formation)", as suggested in the article, that would be pretty vast right?

Industrially, compressed air is considered the most expensive form of practical motive power! Why not use your windmill to compress air in the first instance, and cut out two stages of inefficiency?

Is it considered more expensive than the -economically impractical- process of electrolysing water into hydrogen then back again (to do what? run expensive fuel cells, or a less efficient GT)?
And, in a perfect world it would probably make sense to have windmills compressing air by direct mechanical drive but even with "two stages of inefficiency" this loss is probably compatible than a full cycle losses of, say, pumped storage (also, one can do a lot more clever power matching and control with electricity than mech. drives).

Regarding hydrogen from electrolysis, it could be argued that efficiency isn't really important if the electricity is free
But you've been arguing the economics (where efficiency matters) against other energy storage solutions, so 'free' is kind of irrelevant.


[electrolysing] hydrogen does at least heat the water, and we have plenty of uses for hot water - possibly half of all the electricity we use is for heating water anyway.

Not in Europe we don't.  Like the mechanically coupled wind turbine/compressor argument, it's fine and dandy if you have your water heating requirement right next to your electrolysis station, but how often does that happen?

Hydrogen does not react with metals.

It causes embrittlement.  Without advanced carbon nanotube matrix to adsorb it, or energy intensive liquefaction, hydrogen will  leak out of most tanks relatively quickly.

the main argument against windmills is that far from being free, they are horrendously expensive to build, maintain and scrap, so over a lifetime they produce ridiculously expensive electricity, even without storage.
Can you provide some figures on that (compared say to other sustainable sources) ?
The new London Array seems to have been a worthwhile civil project.
 

Offline Bored chemist

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Re: Can energy be stored as a compressed gas?
« Reply #7 on: 20/07/2013 14:18:25 »
"The practical limit of energy density is about 100 kJ/m3 "
That's the energy storage density at just 1 bar pressure.
A few hundred bar is perfectly reasonable.
 

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Re: Can energy be stored as a compressed gas?
« Reply #7 on: 20/07/2013 14:18:25 »

 

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