compressed air generators

91winsun
91winsun Registered Users Posts: 14 ✭
in General Solar Power Topics #1
If you buried a steel tank in the ground about 20 ft down(safety) and it was 1000 gallons.... how much compressed air could you pump into it
with a windmill and electric compressor and if full how long would it generate 24 volts thru an air powered alternator for battery charging...???
what do you think???
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  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    #2
    Re: compressed air generators

    I did think about using a 500 gal propane tank for that use as I had one after I got my gaswell. I never went thru with the idea as some here didn,t think it was very efficient. There is a local Amish family that uses a 500 gal propane tank as a giant air tank. The Amish in my area aren,t allowed to use electric. They are allowed to use air tools though. They use diesel power units with line shafts for belt power. This one uses a vacume pump for running his milking machines. Also he has an air compressor to pump air into the big propane tank while milking. They run an airline into the house to run a big dayton fan with an airmotor on it. Don,t know if they have any household appliances that run off the air beside the fan. There is a cabinet shop there too that has all the modern tools. Plaiiner, table saw, wood lathe, jointer and dust collector. All the tools had hydralic pumps with a big lister powering the hydralic pump that run everything. If you were going to do this I think a propane tank above ground would be plenty safe. I do have a couple air motors I bought but never tryed it. :Dsolarvic:D
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
    Re: compressed air generators

    Generating 24 volts is no big deal,,, how much current and for how long is a much more involved question. I have no idea of the efficiency of typical air compressors, but intuitively if it takes so much energy to compress air, a large air tank should contain a fair bit of potential energy.

    There must be seem mechanical engineer oour there who can give a calc. As to how much energy would be available from 1000 gallons at say 150 psi.

    Interesting idea,, one I have thought of in the abstract over the years,

    Tony
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #4
    Re: compressed air generators

    There can also be some fairly large losses with compressed air due to the thermal expansion/contraction of the air. Initially compressing the air raises it's temperature, thus increasing the pressure the compressor must work against. This extra pressure returns to "normal" when the now compressed air cools to ambient. Then on the other end of the cycle, as soon as you start releasing air to do work, the temperature inside the tank drops, lowering the available pressure, thus reducing the work available. So you loose on both ends and depending on how fast the air is compressed and /or released, it can have a huge impact on efficiency. Sort of like using an air conditioner to run tools. :D
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
    Re: compressed air generators

    Not to mention the accumulation of water, since the vapor in the air not compress.

    I wonder which is more fixing, high volume/lower pressure, or lower volume/higher pressure?



    T.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #6
    Re: compressed air generators
    waynefromnscanada wrote: »
    There can also be some fairly large losses with compressed air due to the thermal expansion/contraction of the air. Initially compressing the air raises it's temperature, thus increasing the pressure the compressor must work against. This extra pressure returns to "normal" when the now compressed air cools to ambient. Then on the other end of the cycle, as soon as you start releasing air to do work, the temperature inside the tank drops, lowering the available pressure, thus reducing the work available. So you loose on both ends and depending on how fast the air is compressed and /or released, it can have a huge impact on efficiency. Sort of like using an air conditioner to run tools. :D
    Additionally, since there is no liquid phase involved, the pressure drops with every cubic foot of air you release from the tank.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: compressed air generators

    Compressed air has been used in a few places (mining, even locomotive engines).

    Some other interesting links (don't make any claims about companies listed):

    http://www.engineair.com.au/
    http://auto.howstuffworks.com/fuel-efficiency/vehicles/air-car1.htm

    Here is another Wiki page that discusses efficiency--It appears, at best, a "typical" compressed air installation (that does not use insulated tanks to store the heated air) would be on the order of 50%.

    There have been a lot of marketing about compressed air vehicles bank in the mid 2000's and earlier--But nothing really has come from that so far...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • SCharles
    SCharles Solar Expert Posts: 123 ✭✭
    #8
    Re: compressed air generators

    If you get serious, plan for an inline drier [prior to the tank] [or a stand-alone drier tank maybe if you live in a humid area, with that much air being compressed] that you will plumb in above ground in a handy-to-reach place. You'll need to check it frequently, at least until you determine how often it needs checking.
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  • GreatBallofFire
    GreatBallofFire Registered Users Posts: 16 ✭
    #9
    Re: compressed air generators
    91winsun wrote: »
    If you buried a steel tank in the ground about 20 ft down(safety) and it was 1000 gallons.... how much compressed air could you pump into it
    with a windmill and electric compressor and if full how long would it generate 24 volts thru an air powered alternator for battery charging...???
    what do you think???

    Well, I just joined this forum and was presently surprised to see a topic about compressed air, which is kinda around my wheelhouse. First, everything that has already been stated here is completely accurate. Compressed air as a power medium can be horrible inefficient when not used properly. There are many different options available that can help address this, some are very common and some are a little less known.

    First, you need to narrow down the specific goals and then follow with different options for those goals. Compressed air may or may not be right for this application based on your goals. The previous link about compressed air for energy storage averaging around 50% is actually quite good for a total energy in/energy out calculation when using utility size outputs. Factor in the lower cost relative to batteries and its actually pretty attractive for utilities that need additional peak output.

    This sounds like a fairly ambitious project, if you end up continuing this let me know. I might be able to point you in the right direction while offering a few pointers as well. As was already mentioned, there really is no need to bury the tank other than aesthetics and that would require a lot of extra work.
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  • solorone
    solorone Solar Expert Posts: 257 ✭✭✭
    #10
    Re: compressed air generators

    I don't understand how he will recover the energy spent filling the tank sounds like the OP is trying to create energy. I can see the point of storing unused excess power in such a fashion. All this is new to me, I will be curious and tag along.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: compressed air generators
    solorone wrote: »
    I don't understand how he will recover the energy spent filling the tank sounds like the OP is trying to create energy. I can see the point of storing unused excess power in such a fashion. All this is new to me, I will be curious and tag along.

    He won't recover all of it; that is impossible. Always power is lost to heat when you change it from one form (mechanical in this case) to another (potential in the form of air under pressure). An inevitable consequence of compressing air, as mentioned, is heat. Ironically you can heat air to pressurize it, or you can pressurize it to heat it. Then you have hot, compressed air. When on of those energy factors starts to dissipate (i.e. the heat radiates off), energy potential is lost. So you might as well get rid of the heat to begin with by pre-cooling the compressed air before storage. Then again there is the issue that when the air is allowed to expand to do work, it cools losing more energy potential.

    And somewhere in all this is an ideal combination of pressure/volume/temperature that will minimize the energy lost during storage.

    So let's use solar thermal collectors to heat & compress the air in the first place, like propane-powered refrigerators, eh? :D
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #12
    Re: compressed air generators

    Just a thought that popped into my mind the very first time I saw the possibility of a 1000 gal tank buried 20 feet underground and filled with air compressed to a high pressure. Perhaps it partly comes from my years in EMO and the fire service, as well as every so often hearing of some mechanic being killed when a tire explodes off the truck rim he's trying to assemble and inflate.
    The damage done a few years down the road when corrosion weakens the tank and the whole thing erupts up out of the earth like a huge bomb, could be very interesting. :D
    Even at only 100 PSI, the pressure on the walls of that tank would be over 7 tons per sq ft.
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  • GreatBallofFire
    GreatBallofFire Registered Users Posts: 16 ✭
    #13
    Re: compressed air generators
    Cariboocoot wrote: »

    So let's use solar thermal collectors to heat & compress the air in the first place, like propane-powered refrigerators, eh? :D

    I like the way you think Cariboocoot.:cool:
    waynefromnscanada wrote: »
    Just a thought that popped into my mind the very first time I saw the possibility of a 1000 gal tank buried 20 feet underground and filled with air compressed to a high pressure. Perhaps it partly comes from my years in EMO and the fire service, as well as every so often hearing of some mechanic being killed when a tire explodes off the truck rim he's trying to assemble and inflate.
    The damage done a few years down the road when corrosion weakens the tank and the whole thing erupts up out of the earth like a huge bomb, could be very interesting. :D
    Even at only 100 PSI, the pressure on the walls of that tank would be over 7 tons per sq ft.

    Wayne brings up a very important safety issue. Any compressed medium has some risk potential and when it comes to air tanks ,moisture is the enemy. The obvious solution is to dry the air before storage, but this creates issues of its own.

    -Mark
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: compressed air generators

    If anyone is interested in more info along these lines check out Mark GreatBallofFire's homepage mac1 enterprises. You can get a direct link by going to his profile (click the user name) and then "homepage". There is a discussion forum there for this subject as well.

    That doesn't mean we're going to close this thread though; merely pointing out that the subject is a bit out of the realm of our solar-electric theme here. :D
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  • Danny T
    Danny T Registered Users Posts: 4
    #15
    Re: compressed air generators

    Hello All,
    Getting compressed air for free is the easy part. Last year I bought a windmill that drives an air compressor using a 17.4 foot rotor turbine The unit puts out 175 PSI and up to 30 CFM. I'm using several used propane tanks to store the air above ground. The tower is 60' up and has a condenser tank attached to the two stage air compressor that automatically drains for 4 seconds when the unit is shut off. So keeping the moisture out is not an issue. The company that I bought it from www.windcompressor.com recommends using at least a 70 foot tower. I'm using the air to run pneumatic tools and pump water. The hard part is trying to figure out how to use the extra air to make 110 VAC. The windcompressor has no problems keeping the tanks full 3,800 gallons total. Does anybody know if an air powered generator is available? If not, any idea's on the simplest way to build one?

    Thanks,
    Danny T
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #16
    Re: compressed air generators

    There are lots of air motors:

    http://www.ingersollrandproducts.com/am-en/products/air-motors
    http://www.grainger.com/Grainger/air-motors/pneumatic-motors/pneumatics/ecatalog/N-brn

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #17
    Re: compressed air generators

    Welcome to the forum Danny.

    We have to take issue with your compressed air being "free" unless someone gave you all the equipment. Eventually that will need repair/replacement too, so it does have a per unit cost if you amortize the capital expense over lifespan.

    As for the generator, basically you have to use the stored compressed air to run a rotor motor (piston type would be inefficient) to power a generator head. I've never seen any commercially available, but that doesn't mean there aren't any. I think you'll find it takes a large volume of air to produce any amount of electrical power. Think of the size of an engine that runs a given generator. Then calculate how many CFM's are flowing through it and the pressures involved.

    I bet Inetdog can calculate this out for you. He's clever like that. :D Just from my own real-world experiences I'd say the number will be "a lot". :p
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  • GreatBallofFire
    GreatBallofFire Registered Users Posts: 16 ✭
    #18
    Re: compressed air generators
    Danny T wrote: »
    Hello All,
    Getting compressed air for free is the easy part. Last year I bought a windmill that drives an air compressor using a 17.4 foot rotor turbine The unit puts out 175 PSI and up to 30 CFM. I'm using several used propane tanks to store the air above ground. The tower is 60' up and has a condenser tank attached to the two stage air compressor that automatically drains for 4 seconds when the unit is shut off. So keeping the moisture out is not an issue. The company that I bought it from www.windcompressor.com recommends using at least a 70 foot tower. I'm using the air to run pneumatic tools and pump water. The hard part is trying to figure out how to use the extra air to make 110 VAC. The windcompressor has no problems keeping the tanks full 3,800 gallons total. Does anybody know if an air powered generator is available? If not, any idea's on the simplest way to build one?

    Thanks,
    Danny T


    Hello Danny,

    Like it was already mentioned, you probably won't find an off the shelf air-powered generator. But, with the large volume of air storage that you have available, you should have no trouble using an air motor to turn a genarator depending on the amount of juice you're looking for. If you want to send me some info, I would be more than happy to help.

    -Mark
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #19
    Re: compressed air generators
    Cariboocoot wrote: »

    I bet Inetdog can calculate this out for you. He's clever like that. :D Just from my own real-world experiences I'd say the number will be "a lot". :p

    Thanks for the vote of confidence 'Coot, but I don't do hydro or plumbing right now. :-) I would look up the formulas if I wanted to find the answer for myself. But for a quick SWAG, I would take the power that an air compressor would draw to produce the same pressure and CFM and then divide it by at least 10, maybe more. An efficient air powered motor would either be fairly small or would take some pretty high pressures. A quick test would be to take an air powered drill motor and couple it to the shaft of an alternator. :-)
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #20
    Re: compressed air generators

    My experience with air tools tells me the ones with motors (rotor) are vicious consumers of cfm's, but they don't require much pressure.

    If this were for a small amount of Watts on an occasional basis it could be viable. But it's probably not practical for a high Watt and/or long term (hours on end) use.

    I like the air drill + alternator experiment idea. :D
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  • GreatBallofFire
    GreatBallofFire Registered Users Posts: 16 ✭
    #21
    Re: compressed air generators
    inetdog wrote: »
    Thanks for the vote of confidence 'Coot, but I don't do hydro or plumbing right now. :-) I would look up the formulas if I wanted to find the answer for myself. But for a quick SWAG, I would take the power that an air compressor would draw to produce the same pressure and CFM and then divide it by at least 10, maybe more. An efficient air powered motor would either be fairly small or would take some pretty high pressures. A quick test would be to take an air powered drill motor and couple it to the shaft of an alternator. :-)

    Whether the air motor is small or big doesn't really matter, mechanical output = pressure X change in volume. Lower pressure should have less pressure blow-by in between the vanes, assuming the air motor is a rotory vane, which most are. Here's some back of the envelope numbers. Air motor 50% efficient and the generator 60% efficent.

    -Mark
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #22
    Re: compressed air generators

    Maybe it would just be better to tap some rotational power right off the turbine. I could envision a clutch engaging once the storage reached max air pressure.
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  • Danny T
    Danny T Registered Users Posts: 4
    #23
    Re: compressed air generators

    Hi ,
    Let me start by saying thanks for the quick reply and the information everybody has posted. I have been looking for an air powered generator but could not find one. I'm new to this and looking forward to learning. Is there a type of generator that anyone would recommend to match up with the air motor? I'm guessing I have to match the air motor to the generator? I am trying to use the extra air in case I loose power at the shop as a back up. Something in the 3000 watt range is my target. From the information posted and looking on the web I know CFM is going to be high. Again thanks for your time and info :D

    Regards,
    Danny
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  • GreatBallofFire
    GreatBallofFire Registered Users Posts: 16 ✭
    #24
    Re: compressed air generators
    Danny T wrote: »
    Hi ,
    Let me start by saying thanks for the quick reply and the information everybody has posted. I have been looking for an air powered generator but could not find one. I'm new to this and looking forward to learning. Is there a type of generator that anyone would recommend to match up with the air motor? I'm guessing I have to match the air motor to the generator? I am trying to use the extra air in case I loose power at the shop as a back up. Something in the 3000 watt range is my target. From the information posted and looking on the web I know CFM is going to be high. Again thanks for your time and info :D

    Regards,
    Danny

    Hey Danny,

    I'm assuming this is for off-grid only on the electricity, correct? Without boring everyone to death with numbers, you'll need roughly 35-40 CFM for each HP of mechanical output with the air motor. So, 3000 watts from a generator that is 60% efficient needs 4800 watts of input. 4800 watts divided by 746 watts is just under 6.5 HP. So, you'll need 225-250 CFM at around 75 psi. It's doubtful that you would need the full 3000 watt at a constant load, so it really depends on how you're going to be using the juice. If you want to send me some more specifics, I can help you out further.

    -Mark
    solar_dave wrote: »
    Maybe it would just be better to tap some rotational power right off the turbine. I could envision a clutch engaging once the storage reached max air pressure.

    Dave brings up the simplest idea of all, but it also brings up the inherent issue of using wind to generate electricity. Wind is always intermitten and electricity needs to be dispatchable. You can't really have consistent electricity generation from wind without some form of storage.

    -Mark
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #25
    Re: compressed air generators

    There's a formula halfway down this page which gives the potential energy storage: http://mb-soft.com/public2/storing.html Using that:

    Assuming a 500 gallon tank at 100psi (1.9m3 at 689655 N/m2), the total energy is then 1 310 344 Newton meters, spread out over 1 hour = 363Wh of energy. Assuming 100% efficiency everywhere.
    Unless I went wrong with the numbers somewhere, that seems like a waste of time and money.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #26
    Re: compressed air generators
    GreatBallofFire wrote: »
    Hey Danny,

    I'm assuming this is for off-grid only on the electricity, correct? Without boring everyone to death with numbers, you'll need roughly 35-40 CFM for each HP of mechanical output with the air motor. So, 3000 watts from a generator that is 60% efficient needs 4800 watts of input. 4800 watts divided by 746 watts is just under 6.5 HP. So, you'll need 225-250 CFM at around 75 psi. It's doubtful that you would need the full 3000 watt at a constant load, so it really depends on how you're going to be using the juice. If you want to send me some more specifics, I can help you out further.

    -Mark



    Dave brings up the simplest idea of all, but it also brings up the inherent issue of using wind to generate electricity. Wind is always intermitten and electricity needs to be dispatchable. You can't really have consistent electricity generation from wind without some form of storage.

    -Mark

    How about a DC generator hooked to the turbine and a battery bank for storage.
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  • Lee Dodge
    Lee Dodge Solar Expert Posts: 112 ✭✭
    #27
    Re: compressed air generators
    stephendv wrote: »
    There's a formula halfway down this page which gives the potential energy storage: http://mb-soft.com/public2/storing.html Using that:

    Assuming a 500 gallon tank at 100psi (1.9m3 at 689655 N/m2), the total energy is then 1 310 344 Newton meters, spread out over 1 hour = 363Wh of energy. Assuming 100% efficiency everywhere.
    Unless I went wrong with the numbers somewhere, that seems like a waste of time and money.

    The use of compressed air energy storage (CAES) for energy from intermittent sources is of significant practical interest, but as stephendv points out, the 500 gallon tank is too small to be of interest. What is of interest is using underground salt domes that are present in many parts of the world as huge reservoirs for compressed air storage.

    As pointed out earlier in this thread, if you cannot retain the heat from the compression process, the efficiency is pretty low. For that reason, there is a big project going on in Germany to try to store the compression heat separately from the compressed gas, and to use the compression heat to reheat the compressed air when the electricity is generated. The ADELE project is a large commercial endeavor to demonstrate adiabatic CAES where the heat storage device assumes storage temperatures up to 600 C with storage in stone or refractory brick material. The overall efficiency goal is 70%, which I think drops to under 50% if the heat energy is not saved.

    It would be possible to play with a 500 gal. tank, but that size is probably only suitable for educational purposes.
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  • GreatBallofFire
    GreatBallofFire Registered Users Posts: 16 ✭
    #28
    Re: compressed air generators
    stephendv wrote: »
    There's a formula halfway down this page which gives the potential energy storage: http://mb-soft.com/public2/storing.html Using that:

    Assuming a 500 gallon tank at 100psi (1.9m3 at 689655 N/m2), the total energy is then 1 310 344 Newton meters, spread out over 1 hour = 363Wh of energy. Assuming 100% efficiency everywhere.
    Unless I went wrong with the numbers somewhere, that seems like a waste of time and money.

    The energy stored in compressed air is rather low compared to other technologies when at low pressure. However, you always have to put each specific application in context. The post from DannyT stated that he already had the turbine, air compressor, and a large amount of storage. Plus, his tanks were still full after using the air tools so he was wanting to generate some electricity. When you consider that he already owns the most expensive components and that the $/HP for air motors is very reasonable, it may not be a waste of money for his specific application.

    solar_dave wrote: »
    How about a DC generator hooked to the turbine and a battery bank for storage.

    Ahh, the good ole chicken and the egg scenario. Regardless of the method, he still needs some form of storage and the large air tanks that he already owns provides that storage, but I agree that a few SLA batts wouldn't be a bad idea.

    -Mark
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  • Danny T
    Danny T Registered Users Posts: 4
    #29
    Re: compressed air generators
    GreatBallofFire wrote: »
    Hey Danny,

    I'm assuming this is for off-grid only on the electricity, correct? Without boring everyone to death with numbers, you'll need roughly 35-40 CFM for each HP of mechanical output with the air motor. So, 3000 watts from a generator that is 60% efficient needs 4800 watts of input. 4800 watts divided by 746 watts is just under 6.5 HP. So, you'll need 225-250 CFM at around 75 psi. It's doubtful that you would need the full 3000 watt at a constant load, so it really depends on how you're going to be using the juice. If you want to send me some more specifics, I can help you out further.

    -Mark



    Dave brings up the simplest idea of all, but it also brings up the inherent issue of using wind to generate electricity. Wind is always intermitten and electricity needs to be dispatchable. You can't really have consistent electricity generation from wind without some form of storage.

    -Mark
    Hi Mark,
    Yes, you are correct that I am going to use the electricity for off grid only. After looking over the numbers that you posted I will have to scale back in the watt range that I was hoping for. I am simply trying to have a back up system in place to power a few lights and maybe a refrigerator as well as learn something new at the same time. My thinking is since I have the compressed air why not use it. If needed I can find another used propane tank and increase my storage. Thank you for your time,

    Regards,
    Danny
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  • GreatBallofFire
    GreatBallofFire Registered Users Posts: 16 ✭
    #30
    Re: compressed air generators
    Danny T wrote: »
    Hi Mark,
    Yes, you are correct that I am going to use the electricity for off grid only. After looking over the numbers that you posted I will have to scale back in the watt range that I was hoping for. I am simply trying to have a back up system in place to power a few lights and maybe a refrigerator as well as learn something new at the same time. My thinking is since I have the compressed air why not use it. If needed I can find another used propane tank and increase my storage. Thank you for your time,

    Regards,
    Danny

    No problem Danny. If you do end up setting up some form of air motor generator I would be more than happy to help out if I can. Just use the email address listed in my profile if you want to reach me. I'm not familiar with your turbine/compressor set-up, but I may be able to possibly help you increase the air compression efficiency depending on how your system is designed. Have fun!

    -Mark
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  • Danny T
    Danny T Registered Users Posts: 4
    #31
    Re: compressed air generators

    Hello All,
    I want to thank everybody for their help and input and let anybody that may have some information, is interested in how this project turns out or just wants to follow along that I am switching this topic over to Marks (Great Ball Of Fire) forum. http://mac1enterprises.com/forum/viewtopic.php?f=5&t=25 My friend Gary that is working on the project with me will be doing the posting.

    Regards,
    Danny T
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