Is there any efficient way of using a large volume but low pressure (<2 bar) stored compressed air, either to drive an engine or some other application?
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$\begingroup$ Google "blowdown turbine" $\endgroup$– Phil SweetJan 29, 2020 at 22:44
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$\begingroup$ Thanks for the Prompt info sharing. $\endgroup$– Muralidhar MJan 30, 2020 at 12:44
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2 Answers
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To store a large volume of low pressure air requires a truly gigantic holding tank which would be completely uneconomical to build and operate. Consider the following: a wind turbine is the ultimate in extracting useful work from a high flow rate of low density working fluid (air) driven by an extremely small pressure difference. Assume a 100 foot diameter turbine feeding on air flowing at 22 feet per second, meaning that in one second, 173,000 cubic feet of air goes through it. In one minute, ten million cubic feet go through; in one hour, 620 million cubic feet and in 24 hours, 14.9 billion. That's a tenth of a cubic mile of air to run the turbine for one day and you want to store it all at a fraction of a PSI for the source pressure? From the standpoint of mechanical engineering, you'd never do it that way.
To make it economical dictates that you increase the energy density of the storage method which can easily be done by compressing the air to high pressure and releasing it at low flow rates instead of trying to do the same with low pressure and high flow rates.
answered Jun 28, 2020 at 6:13
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welcome on this SE. Your question might be a little vague for Engineering SE, but here is a possibility:
You can use this volume of air as a refrigerant: You put it at ambient temperature, wait for the air volume to come to the ambient temperature, and then you put it close to a volume to cool down: the difference of pressure will cool down the volume of air, and heat transfer will do the rest.
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1$\begingroup$ This feels like a brainstorming session, which SE isn't the best venue for. In terms of using it as an engine, sure, but the stored energy will be low per unit, so for an engine to do meaningful work it would need large pistons. $\endgroup$ Jan 29, 2020 at 18:48
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$\begingroup$ @ScottDunnington Yes this looks like brainstorming, that is why I warned the OP about the question being "vague". About using as engine, I think it could more efficient in the work I described (not taking in account it is a totally different output) $\endgroup$ Jan 29, 2020 at 18:54
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$\begingroup$ As you rightly said typically Heat engines require larger compression ratio for good efficiency and in this case energy per unit volume is also less. Thanks for indicating a possible use as refrigerant.. $\endgroup$ Jan 30, 2020 at 13:14
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$\begingroup$ I am envisaging rather theoretically of extracting heat from a low temperature but high Heat capacity medium using "Second law thermodynamics", where external work done on the closed system will transfer heat in the reverse direction. I theoretically arrived the heat thus extracted can be used to raise the pressure of air to 2/3 Bars whereas the COP of the system is of the order of 300%, i.e. one Joule work done would extract Four Joules of heat out of the medium. $\endgroup$ Jan 30, 2020 at 13:57
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$\begingroup$ But the limitation is that the heat extracted can only be used to rise the Air pressure to at most 2/3bar and the COP will reduce tremendously for larger pressure ratios. Hence I was wondering whether there is any use of storing the extracted energy as a low compressed air!! Thanks again for your reading and patience. $\endgroup$ Jan 30, 2020 at 14:00