Efficient compressed air turbines/motors

[swbluto]

I'm thinking about using a compressed air system to store excess solar energy (I'll have to figure the most practical way to store compressed air, later), but for now, I'm trying to figure out how to efficiently convert compressed air into electricity as that's the most important part of the system. (I can only store so much compressed air for a certain amount of $$$, so how much energy I can extract from that (That is, the 'capacity' of the 'compressed air battery') is going to be dictated by the efficiency of the turbine/air-motor.)

Any readily available sources for converting compressed air into electricity, efficiently?

I'm thinking I could just get one of those compressed air motors and hook it upto an electric motor to harvest electricity. I see no data, however, on how efficient those air motors are - I suspect turbines would be much more efficient, but those seem to be hard to find/purchase commercially for the average person.

I wonder what it would take to make a turbine?

I'm guessing I'd ideally use metal, but I wonder if I could get away with plastic for lower-pressure turbines. I'm not experienced with the 'blade fatigue' issue to know if it'd be an issue in a lower pressure system. Plastic would be nice to use because I can readily make parts for that to spec; replacing a couple of blades every month would be no problem. Making metal blades would probably mean CNC machining, but I wonder if I could get 'good enough' with casting? Seems like fine details can be achieved with spin-casting that's typically used with rings, seems like the same could apply to blades.

(Btw, being a former navy guy, I saw plenty of turbines to know what they generally look like. The blades can be quite small, the ones in the navy were like 6"-8" long, mounted on a 3-4 ft wide solid disk, and that was the kind of turbine that'd power the entire aircraft carrier.)

 

[liveforphysics]

Each scuba tank at 4500psi holds ~40Wh if energy.

Through a good turbine and generator, ~25Wh of that stored 40Wh are available for useful work.

That's about 2 modern 18650s worth of energy storage per 4500psi 40ft^3 (uncompressed volume) tank.

The rule of thumb is that with practical equipment, about 50% of the starting energy goes into thermal losses and compressor inefficiencies. On the conversion back, the losses are similar.

Storing energy in compressing gasses makes batteries look so good.

 

[swbluto]

Each scuba tank at 4500psi holds ~40Wh if energy.

Through a good turbine and generator, ~25Wh of that stored 40Wh are available for useful work.

That's about 2 modern 18650s worth of energy storage per 4500psi 40ft^3 (uncompressed volume) tank.

The rule of thumb is that with practical equipment, about 50% of the starting energy goes into thermal losses and compressor inefficiencies. On the conversion back, the losses are similar.

Storing energy in compressing gasses makes batteries look so good.

For mobile applications, that is so true.

But, air compression system that can last over 10,000+ cycles and many /decades/ has some longterm cost advantages over modern batteries. They can usually be used to 100% DOD with no affect on the longevity of the storage system.

However, I need some ballpark numbers to know what I'll need. I remember calculating something like $1500-$2000 for 1000Wh for someone's residential use, which seemed viable. For my industrial uses, I'll probably need something like 10,000 wh ideally, which would be somewhere around $15,000-$20,000. I might chalk up the $2000 for lead acid batteries, at least initially, lol.

Still, I would like to think this one through. Compressed air is not limited to scuba tanks, though that /appears/ to be the most cost effective form among most conventional forms. I have an inkling, however, there are more cost effective forms if one takes advantage of being underground. (And, cost effectiveness matters with 10+ kwh, lol.)

 

[spinningmagnets]

The Amish earn a lot of money from selling crafts to tourists, so they have a vested interest in maintaining the "image" that the tourists expect. They are known to not use electricity, but lately they have been using solar panels to charge batteries that power LED lights at night. Their main philosophical theme has actually been to remain independent. Therefore...buying electrical appliances that require them to be tied to the grid violates their philosophy, but solar panels and LED lights do not. A funny twist to this is that most of the LED are type that purposefully "flicker" in order to mimic the DIY candles they are known for (Gotta keep up appearances!)

What I'm getting at is that the Amish use a lot of air tools, such as a drill, and other shop tools that either already are air-driven, or were converted to air.

For air generation, a high solidity turbine has fairly low RPMs, but decent torque (bad for electric generation, good for water-pumping, which is what the iconic farm wind turbine is designed for).

As far as using compressed air to generate electricity, I agree with Luke that it is a poor choice. If you are committed to making a prototype, study and copy the engine in the tiny air-car...turbines would be too wasteful compared to a piston engine, which is bad enough. A workable piston engine could be made, but a 14,000-RPM turbine would require tools, skills, and materials that are significant. If you are committed to making a desktop turbine, I suggest starting with compressed air bearings. For the electrical generator, I've read that Litz wire is good for high RPMs, but I don't know how to make one of those.

 

[swbluto]

I'm not really committed to a turbine, I'm right now trying to figure out the most efficient kind of "air engine" for, like, 200PSI of 60F air.

Turbines are more efficient the higher the input pressure(PSI), and the hotter the gases. That's why the navy uses steam turbines, with super-hot steam at super high pressures.

Therefore, I don't know if turbines are the best choice for this kind of compressed air (Lower pressure, room temperature air.).

So, back to the topic's headline, what is the most efficient compressed air turbine/motor for 200PSI 60F air? And, what kind of power/efficiency am I looking at with whatever that ideal air motor/engine is?

I've found compressed air motors (pneumatic motors) on ebay, but I didn't see a datasheet on the manufacturers product page, so I don't know how efficient these things are. I get the sense, however, they are not 'maximally efficient', given their seeming lack of adequate expansion volume to take maximum advantage of the energy stored in the compressed air. They are, however, commercially available unlike desktop turbines. And if 15,000+ RPM is necessary for some reason, then I have a feeling you'd need metal blades and that drives the cost up /immensely/. Subtractive processes(CNC) are intrinsically expensive, and casting is also intrinsically expense(Energy costs for 1900+F and sheer equipment costs.)

However.

I do wonder.

If I could cast some 3"-4" blades in those desktop electric kilns. I wonder what kind of metals I could cast in those things.

Seems those electric crucibles used for melting gold can get upto 2050F according to specs.

 

[liveforphysics]

But, air compression system that can last over 10,000+ cycles and many /decades/ has some longterm cost advantages over modern batteries. They can usually be used to 100% DOD with no affect on the longevity of the storage system.

Compressor and turbines have moving parts. Batteries have moving parts too, but they don't carry much kinetic energy to wear themselves out.

 

[nutnspecial]

So how many amp hours would you like? 800+? Lead acid can be around 100$ per 100ah right?

For a little extra energy that's not dependent on sun, wind, or other fuels, I'd like to try the 'grandfather clock' approach, if I couldn't use moving water. You could set it up to hand winch with platform too, but the general idea is you come home and park your vehicle on a pad that will drop slowly like the weights of a clock, giving alot more energy back than it took to coast the vehicle up the slope.

Besides a car and winch, there are other possibilities for it's use too, like water and stuff I'd think. I don't know what they're called or fully consist of and/or can output, but I thought the concept was interesting. The solar could also be sequestered by driving via worm gear alot of weight up in the air. Still, I like how plants sequester the sun's energy best. Harvest the wood sustainably and expedite the decomp process the wood will go thru anyway, while getting some decent energy for yourself. (gasifier running an ic engine), or perhaps steam?

On turbines, some say the tesla turbine is nice. People only ever seem to make the model scale though.

 

[bigmoose]

NASA did a lot of research in the late 70's and early 80's into alternate energy systems and storage. Much progress was being made as the DOE didn't have research centers at the time. The typical bureaucracy came into DOE, they threw a hissy fit, had congress build them research centers and take the work from NASA. The results? I really have not seen much relevant work out of DOE since they regained control of energy research.

Google/search for NASA and stuff like compressed air storage. Lots of work was done associated with diurnal cycle storage for PV and other systems. Also power peaking for wind farms.

Here are examples
http://ntrs.nasa.gov/search.jsp?R=19740008672
http://ntrs.nasa.gov/search.jsp?R=20000045699
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790005326.pdf
https://archive.org/details/NASA_NTRS_Archive_19740008672

This stuff is going to be utility size, but it should give you ideas. My memory of this is very old... from 35 years ago! But I remember standard compressed storage as a non starter. The only concept that made sense was pressurizing salt domes then re heating the air on discharge through a turbine generator. Basically a gas turbine engine without the compressor.

Pumped hydro storage works. If you can get a 100 foot or more head. Small scale water turbines are easily built and centrifugal pumps are available in the size you would need.

Lots of conceptual design done back then, and if you can get them, the reports had real data and real calculations in them. Also since super computers didn't exist then, there is none of this fancy multivariate system optimizations that no one could do at home now.

 

[swbluto]

The solar could also be sequestered by driving via worm gear alot of weight up in the air.

Using gravitational potential energy is interesting. But, it takes a lot of height and weight to store appreciable energy.

The standard formula is E = mgh

But, since we care about wh and not joules, let's make that watthours = mgh/3600.

Since meter is just about the same as yard, we can just imagine yards for approximation.

Since there's 2.2 lbs per kg,

we could make it..

watthours = pounds*g*h/1636

Since G = 9.8, we could make that..

watthours = pounds*h/182.

So, how much weight would be needed to lift up a 15 meter tree for 1000wh?

pounds = 182*watthours/h = (182*1000)/15 = 12,133 lbs.

Hmm... well, this actually doesn't look impossible with multiple trees available. I really do wonder what form the 'climber' would take, and how one would convert gravitational potential energy into electric, that is, converting a falling weight into electricity.

I also wonder about digging holes. It seems like digging 100 foot holes would be easier than finding suitable 100 foot tall trees (Depending on the water table).

 

[swbluto]

And, by the way, when NASA was doing the calculations for air compressors, they were doing a simple ENERGY IN / ENERGY OUT efficiency calculation.

If you do that way, why yes, if you lose heat during the compression stage (The air cools sitting in the tank), you lose a bunch of energy, thus overall EIEO goes down.

That's not really what I care about, though. I care about being to store /any/ excess energy, instead of just letting it completely go to waste. So, ENERGY IN doesn't matter to me; If I get 10000wh of excess solar and store 3000wh of it, great! I don't care that I lost 70% of the input energy because it would've been otherwise 100% wasted (As the batteries are already fully charged), I only care that I have 3000wh on tap now for use at some later date. I care much more about getting the ENERGY OUT, thus I only care about turbine (Or whatever air motor/engine) efficiency for something like 200PSI 60F AIR.

So, Nasa's EIEO calculation has little relevance as to it being a 'non-starter' for my purposes. I care much more about EO, and that depends on the turbine efficiency; And that, in turn, determines what kind of compressed air storage costs I'll have to pay for, the most significant cost by far.

 

[spinningmagnets]

Whatever you come up with...if it actually performs work, then it will be large compared to the amount of work it accomplishes. So, I'd say "efficiency" might be very low, however...if the fuel is free? (wind-generated compressed air 24-hrs a day). After the large and somewhat expensive system is built, it should last decades.

You can use a large lower-pressure tank to feed a smaller tank through a ball valve and a check-valve, and then heat the small tank with solar energy (whether piston or turbine, heating the stored air would double the work it was capable of).

Using compressed air to run a turbine (or 2-stroke piston engine), it's kind of like idling a big V8 to charge your cell phone and laptop through the 12V socket, during a power outage. Does it work? Clearly it would "work", but...idling a V8 to charge those two items would be horribly inefficient.

Efficiency comes up in wood heating discussions. Even if the wood is free, an efficient system will require less chopping, gathering, and stacking of wood.

 

[liveforphysics]

Hoisting 1,000,000kg (2.2Million lbs) to 100m (330ft) up, stores 272kWh of potential energy assuming a frictionless mechanism and 100% efficient generators/conversion.

The same energy storage could fit into a closet or basement using batteries that if done life would function maintenance free for decades.

If you have a property that happens to already have a lake at a +50-100ft above a high volume capable drain, using kinetic energy storage would make so much sense if you didn't mind maintenance of drain screens and turbines and generators occasionally.

If you have to build the structure yourself, I bet it's much lower effort and cost to just implement full solar using recycled Tesla modules or the cheap recycled GM Volt packs or LEAF packs. It's not uncommon to see ~$100-200/kWh in something that if kept cool and dry and cycled from say 20% SOC to 80% SOC should last for at least a decade of service if not more. By the end of a decade, replacement EV packs to swap in its place will cost even less and perform even better if not last the rest of your life.

ATB,
-Luke

 

[jonescg]

Find a suitable part of the world and use pumped Hydro. You'd need a bunch of excess energy from somewhere and a huge body of fresh water to pump, but it can, and is done around the world. Best of all, if it rains over the catchment you get bonus stored energy!

Otherwise, yeah batteries are pretty darn good.

 

[swbluto]

Hang on guys, I'm going to run some efficiency calculations on 200PSI 60F air. I suspect it'll be low, but a reduction from 200 PSI to 14.7 PSI(air pressure at sea level) seems like it could be high.

I do know if you compress air, the air will heat up. When you decompress it, it will cool down. But, I think due to nature of a turbine in how volume decreases as the air moves along between the different stages, I don't think it cools down /that/ much, because of the reduction in volume.

https://www4.eere.energy.gov/manufacturing/tech_deployment/amo_steam_tool/equipTurbine

Apparently this is calculating 95% efficiency, not including the generator efficiency. /BUT/, I don't really know if the outlet temperature is actually going to be the same as the input temperature, so my figure could be off. I just assumed it would be. I also don't know if the outlet pressure is going to be 14.7 PSI(Sea level pressure), it could be higher depending on the amount of expansion in the turbine. I'm guessing it will probably will be, but I don't know how much, lol.

And, this calculator is warning mean about steam condensate. Yes, the compressed air will have some moisture in it (It's compressing 60% humidity air), but I really don't have to worry about that, right? I really don't want to implement a steam trap, because I didn't get that far in the nuclear pipeline, lol. (And not because of academics; I'll have everyone here know, I was actually in the top 10% of the class academically. But, to quip a Linkin Park song, "I tried so hard and got so far, but in the end, it doesn't even matter....", woe is me, lol.)

 

[swbluto]

This digging holes idea seems kind of intriguing. I wonder what the mechanism would operate like?

So, you dig a hole and you install some piping to preserve the hole.

Okay, assuming one just has weights hanging around.

The lowering of the weight using a winch (Or similar, essentially a motor attached to a reel) would generate electricity. It would probably have to be one heavy weight and a super low KV (High turn) motor if one wants the 'falling action' to last more than a couple of seconds/minutes.

Then one stores electricity by powering the winch, which lifts the weight back up.

This system would obviously require active operation, to load the weights and lower them into the hole, as well as hooking the weights sitting at the bottom.

Installing 100 holes/pipes on a small property doesn't seem infeasible, so I suppose that might ease the operation requirements somehow. (Perhaps some holes could be lifters, other droppers, so to minimize the overall amount of weight-change-overs/fishing events.)

For 100 feet, one would need to use around 6000 lbs to store 1000wh of raw energy. The generator efficiency might be ... 80%? Not sure what kind of efficiency super low KV motors have at low RPMs. I know ordinary motors normally need to be running at high RPMs to run efficiently(Conversely, generate efficiently).

I suppose one could use lead weights, but it seems like some form of weight using sand or similar might be more cost effective.In case a bag breaks or some such, I suppose one would need to vacuum the sand at the bottom somehow. Perhaps sand could be stored in slightly smaller pipes.

 

[liveforphysics]

For 100 feet, one would need to use around 6000 lbs to store 1000wh of raw energy. The generator efficiency might be ... 80%? Not sure what kind of efficiency super low KV motors have at low RPMs. I know ordinary motors normally need to be running at high RPMs to run efficiently(Conversely, generate efficiently).

PE =
2723.5093325586463
kg x 9.8 m/s2 x
30.48
m =
813523.131672598
joules.
PE =
6000
lbs x
100
ft =
599999.9496063035
ft lb.

6000lbs and 100ft is actually 225Wh of energy storage with frictionless 100% efficiency generators and mechanisms.

The same energy storage of ~16-20x 18650 cells.

If I remember, you had dealings with early lithium cells which sucked badly. If you find something like modern GM volt batteries or Tesla modules, you will store more in a cooler sized maintenance free space than you would store if you dedicated your whole property to various towers and holes with weights hanging from them.

 

[swbluto]

Did a quick check, you're right. One of the equations got the coefficient mangled somewhere.

Oh, I see. The equation was supposed to be multiplied by 2.2, not divided by. The correct equation should be...

watthours = pounds*h/802.256

Or

watthours = pounds*feet/2631.4

Double check

(6000*100)/2631.4 = 228.1 wh. It works

Okay, so one would need...

pounds = 2631.4*watthours/feet = 2631.4*(1000)/100 = 26313 pounds.

That's starting to look kind of impractical, lol.

If only I could dig a hole 1000 feet long! lol, I'm sure i'd hit the water-table somewhere around 200-300 ft.

I'm not quite sure how you'd install a pipe in the ground going that far down. It just seems like you could.

 

[Chalo]

[...]the general idea is you come home and park your vehicle on a pad that will drop slowly like the weights of a clock, giving alot more energy back than it took to coast the vehicle up the slope.

Umm, what?

W = F * D

Same mass, same change in height, same work/energy. Minus losses, of course.

 

[bigmoose]

Sorry to have sent you on a dead end...

BTW the Amish east of me use large propane tanks that have failed certification for their air storage tanks. One settlement that I was able to tour due to a friend on the "inside" had a centralized compression facility and piped the compressed air to homes and to workshops. They used a large caterpillar diesel compressor, and got a "special dispensation" from the bishop to use electric start on the massive diesel.

 

[nutnspecial]

Yes chalo I worded that wrong, or at least confusingly. Technically nothing 'gives' energy does it- just transforms.

If i'm reading the equations correctly, a car dropping one story only can give like 22wh max? That doesn't seem right. . . as I think about the damage that could be done if the kinetic force wasn't safely used. There is no account for time in the equation?

I imagined a grandfather clock stored even 22wh, but perhaps it's just very efficient.
_______________________________

Using a Clock Pendulum to Generate Electricity.
Without knowing of the work of Veljko Milkovic, Bill came up with a mechanical-electrical system that may employ a variation of the secondary oscillation effect described by Milkovic.

http://pesn.com/2007/03/24/9500464_BillMahess_pendulum_clock_generator/
Hmm, well that's interesting!

_______________________________

Having seen some amish in action I have to agree with bigmoose that they have alot of stuff refined for 'off grid'. They run quite alot of things from air and/or hydraulic. I think it would be neat to have a big windmill that drives a compressor head keeping a large tank bank pressurized. Or that excess solar.

Back to harvesting the pressure? If air is so inefficient compared to water (and you don't have much elevation to play with, or ground water), why not use the excess solar to pump water into a tower? Then make a mini closed system hydroplant. If it's potable you'll always have great pressure in the shower too

 

[liveforphysics]

Back to harvesting the pressure? If air is so inefficient compared to water (and you don't have much elevation to play with, or ground water), why not use the excess solar to pump water into a tower? Then make a mini closed system hydroplant. If it's potable you'll always have great pressure in the shower too

Again as mentioned above, you can spend 10s of thousands of dollars on the tallest tower with the greatest sized tank at the top, and this stores the energy of a shoebox of 18650s, while wasting at least 30% of the energy it stores, and another 30% wasted on recovery.

 

[Punx0r]

This isn't problem solving, it's starting with a pre-conceived "solution" and trying to justify it...

There's little material salvage value in li-ion packs - in 10 years time they'll be practically giving away EV packs at 70-80% remaining capacity.

 

[nicobie]

I agree, what's the problem with just using batteries?

 

[swbluto]

This isn't problem solving, it's starting with a pre-conceived "solution" and trying to justify it.

Uh, here's an idea for all the "give uppers" in the world. It's following through each idea to their very end, so as to leave no stone unturned.

 

[swbluto]

[pre][/pre]

I agree, what's the problem with just using batteries?

Mainly they die like every 1-3 years with daily usage. Other energy storage systems have the potential to last forever, thus potentially driving down the amortized costs.(I.e., "the monthly cost", which is (lifetime costs + maintenance costs)/lifespan.)

Note, I'm much more concerned about the lifetime monthly costs than energy density. This is for a stationary application.

However, if salvaged tesla/volt packs really do have the ability to last 10+ years, then that's a harder possibility to ignore when it comes to putting money down on the table.