Don, The heat issue is with the compression and storage of the air itself, not the operation of the motor. When a gas is compressed, it heats up, and either this heat is retained in something like metal or molten salt, or it has to be rejected to the atmosphere through heat exchanges, where its usefulness is lost. If the heat is ejected (which is the only technically feasible process we have now), then when the compressed gas is released, it has to be heated up because gas that is decompressing gets much colder, and motors or turbines can't take being frozen (you've probably seen frost at the nozzle when you've used those small cans of compressed air for cleaning electronics). So in the compressed air energy storage projects that exist today (Iowa, Germany, Alabama), natural gas is used to heat the compressed air up before it reaches the turbines. Even with the heating, the overall process is more efficient than burning natural gas directly. High pressure storage vessels are also a problem, since they have catastrophic consequences when they fail and thus the engineering requirements are severe. It's the storage vessels that make the cost of such systems more expensive than batteries. Underground storage in airtight caverns is best, and geologically, old salt dome formations (such as the ones we use to store natural gas in) are very suitable. They are very stable geologically. It seems unless small scale adiabatic (that is, no heat loss) storage could be figured out, the efficiency of a small system for power generation would likely be very low. First the PV or wind would have to drive a compressor to compress the air, the engineering of the pressure vessel would require some dealing with heat ejection, then energy would be needed to heat the air up again before driving a small motor to generate DC power, then pass through an inverter to AC. Manufacturing of the compressor, pressure vessel and motor entail environment impacts beyond the boundary of the compressed air system, so to weigh the tradeoffs with batteries, this would have to be considered as well. Storage of intermittent energy sources is indeed going to be a major challenge to bringing true renewables into play for a 24/7 economy. David -----Original Message----- From: Don Eichelberger [mailto:done7777@sbcglobal.net] Sent: Friday, July 18, 2008 1:51 PM To: Jeanne; david-fridley.net Cc: sustainability@sfgreens.org Subject: Re: [Fwd: RE: [Fwd: Compressed air car]] Thanks, Jeanne and David- I found this very interesting, thank you. I do have some questions. I am more interested in the compressed air motor as a decentralized stationary energy source that can act as an emergency back-up, like a battery but without the toxins. It seems that, as a stationary source, some type of heat exchanger technology could be devised that could provide hot water as a byproduct. Either submerge the motor in water or through use of cooling coils, it seems the heat could be made useable. Heat sources would be from motor friction and compression of air molecules, right? Couldn't such a motor be made to operate more coolly by using the air to cushion moving parts? I don't understand the part about heat ejection having to heat the air up again on decompression, can that be explained in lay terms? It seems if there is already a less efficient (50%) design that can deliver energy density equivalent to lead batteries, but without the toxins, is making huge steps. I would be interested in checking out the midwest studies you referred to. As for geologic storage, could such geological formations be made seismically unstable storing air at pressures of 2500 psi or more? I am thinking more smaller, less centralized storage unit, say at a local market, that can be fed with a small windmill or solar panel powered pump for emergency power back-up for lights and refrigeration. I look forward to hearing your thoughts. Thanks again, Don At 10:15 PM 7/16/2008, you wrote:
Don,
Here's David Fridley's response to my query on compressed air energy storage. Thought you might be interested.
Jeanne
-------- Original Message -------- Subject: RE: [Fwd: Compressed air car] Date: Mon, 14 Jul 2008 19:39:41 -0700 From: David F <mailto:david@fridley.net><david@fridley.net> To: 'Jeanne' <mailto:greengal@netvista.net><greengal@netvista.net> References: <mailto:487BA2C6.9060200@netvista.net><487BA2C6.9060200@netvista.net>
Jeanne,
Yes, but there are technical challenges of course. The main problem is how to deal with the heat. The best way would be to store the heat with the compressed air, but that hasn't been worked out on large scale. What has been worked out is storage with heat ejection (meaning having to heat the air up again on decompression), which lowers the efficiency to about 50%. This method has about the same energy density as a lead-acid battery, so it's good, but not as good as advanced batteries. But you don't need the same toxic materials as you do for batteries. There's some trials of wind/compressed air storage in the Midwest I've read about, so it's definitely progressed to the demonstration levels. As long as there are appropriate storage conditions (underground caverns being best for large scale storage), then it's a good option.
As for the cars, the compressed air storage mechanism works best with slow release, which is why these cars run on the air only up to 35 mph, then rely on the alternative energy source. Since decompressing air cools it, you have built in air conditioning.
David
From: Jeanne [<mailto:greengal@netvista.net>mailto:greengal@netvista.net] Sent: Monday, July 14, 2008 12:03 PM To: Fridley, David Subject: [Fwd: Compressed air car]
David,
Don has an interesting question. Suppose we forget about this as a car, but consider it as a way to store intermittent sources of energy. Does it make any sense?
Jeanne
-------- Original Message -------- Subject: Compressed air car Date: Tue, 06 May 2008 22:47:13 -0700 From: Don Eichelberger <mailto:done7777@sbcglobal.net><done7777@sbcglobal.net> To: <mailto:ber8832@aol.com>ber8832@aol.com, <mailto:richard@katznip.com>richard@katznip.com, <mailto:greengal@netvista.net>greengal@netvista.net
Here are links I said I would send on the compressed air car being developed in France. I am most interested in the motor as an energy storage device that's cleaner than batteries.
I'd be interested to hear what you think,
Don
<http://zeropollutionmotors.us/?page_id=43>http://zeropollutionmotors.us/?p age_id=43
http://www.popularmechanics.com/automotive/new_cars/4251491.html?series=19
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The Hegelian/Marxist goal is emancipation. Marx said it best in 1843:
"Human emancipation wll only be complete when the real, individual man (sic) is absorbed into himself the abstract citizen; when as an individual man, in his every day life, in his work and in his relationships, he has become a species-being (politically accountabe to the whole); and when he recognizes and realizes his own power as social powers, so that he no longer separates this social power from himself as political power."
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Don Eichelberger DJ Fix Independent Productions My Blog www.greenuprising.blogspot.com My You Tube <http://www.youtube.com/DonnyFix>http://www.youtube.com/DonnyFix My Space http://www.myspace.com/donnyfix Donny Fix del.icio.us book marks http://del.icio.us/donnyfix The Hegelian/Marxist goal is emancipation. Marx said it best in 1843: "Human emancipation wll only be complete when the real, individual man (sic) is absorbed into himself the abstract citizen; when as an individual man, in his every day life, in his work and in his relationships, he has become a species-being (politically accountabe to the whole); and when he recognizes and realizes his own power as social powers, so that he no longer separates this social power from himself as political power."