The Tata AirPod: a car that runs on compressed air w/150-200km range
stoweboyd.com
stoweboyd.com
Wikipedia has good articles on compressed-air engines and cars. Here: http://en.wikipedia.org/wiki/Air_engine#Automotive and http://en.wikipedia.org/wiki/Compressed-air_vehicle
Tata has been a licensee of the technology since 2007 (http://www.gizmag.com/tata-motors-air-car-mdi/22447/). When they go into production it will be interesting news.
So back in 2007 I saw that Tata had licensed the technology for using the air-motor in its cars. As an infrastructure guy I thought "Hmmm, if this stuff works, I could use it in a data center." So I tried to do that.
Here was the plan, using a windmill you could compress air, it's a pretty simple mechanical linkage, run a pump off the rotating vanes, has been done for hundreds if not thousands of years. But instead of water, you pump air into tanks. This generates heat because you are compressing the air of course but its windy so you've got heat sink vanes in the air flow being cooled by convection. Now at some point your data center wants a bit more juice, so you dump air through these MDI engines, which are running electrical generators. But here is the thing, when you dump the air it gets colder, a lot colder and the pipes can freeze up, but this is a datacenter and you run the pipes through the data center and cool it epic win! Power and cooling for free, the more power you use the more cooling you get, how neat is that?!
Except there was no amount of money I could give MDI in exchange for engines. They don't make the engines, they don't have a plan to make the engines, they won't let someone build the engines and sell them at a profit because they won't license them to do so. I was a classic case of an inventor being so unrealistic about what the value of their invention is, they prohibit the creation of a market around it.
The trick is that markets have choices, and an air engine would be a good choice, but not if it is not cost competitive with high efficiency diesel, or cheap (and dirty) two stroke gas engines. After I gave up (by mid 2008) trying to get them to see the error of their ways I figured it would be 20 years until we saw a credible use of this technology because all the patents would finally expire, and people would start to make them, and probably curse the folks at MDI for their short sightedness.
You could do the same thing with the exhaust heat from the compressor to heat the restroom water for the data center's bathrooms.
Not exactly super exciting to most, but for whatever reason I think stuff like this is the coolest thing ever.
It's not in the slightest free. If you attached your windmill to a regular electrical generator, then used the electrical power as needed, plus a standard A/C you would come out ahead vs first compressing air, which is very wasteful.
The reason it's so wasteful is the "you've got heat sink vanes in the air flow being cooled by convection". You are throwing away energy there.
Generally when the wind was blowing (and it blows a lot along the Columbia River Gorge) there is no need for additional cooling. When there is no air movement and the outside temperature is high, the demand for cooling increases requiring a more 'active' engagement (sorry but I can't be more specific than that).
So realistically I was looking to time shift the excess cooling capacity that was available during windy times to the times when there was little wind and excess heat. Compressing air has some advantages in that the amount of energy you can get out vs the amount you put in is comparatively efficient to battery power, and using / exploiting the cooling effect of the expanding air, and the kinetic energy of its expansion to generate electricity, was actually more efficient than other methods. Maintenance burdens were also lower and municipal permitting was made easier by the lack of 'dangerous' chemicals or catalysts.
These advantages are perhaps uniquely suited to data centers. Of course I didn't get to actually build it because MDI wouldn't participate. That was too bad but certainly well within their rights.
I'm not convinced it was worth it. You anyway are collecting energy from the wind, and instead of storing it as energy you stored (part of) it as (potential) cold.
But in order to do that you threw some of the energy away - I suspect that if you stored all the energy directly, and then created the cold on the spot as needed you would have higher efficiency.
If I did want to store cold I would not do it using compressed air, I would use a water tank - cool the water to ambient (no active cooling, just some pumping) then dump heat into the water when necessary.
You are correct that my concept 'threw away' the heat generated by compressing the air. However, since any energy stored at all was collected from a previously unharnessed source (the wind blowing past the data center) it might be more accurate to say that it would not collect all that it 'could'. But here is a very important distinction, the heat from air compression is 'low grade' heat, which is to say the delta between it and ambient is small enough that harvesting it in meaningful amounts is quite difficult. You also need to consider that this thing is operating next to a data center which is running an evaporative cooler 24/7 to pull excess heat from inside the box and dump it into the air outside the box. Basically spending energy to move the heat outside.
So from this side of the screen, to accept you point that not capturing the heat generated from compression was 'throwing it away' I would need to have some credible way of utilizing the captured heat.
Looking at it from the overall energy exchange picture, you've got the kinetic energy of the wind, with is 1/2mv^2 where m is the mass of the wind moving pass the windmill. Some percentage of that you convert to mechanical energy which runs the air pumps and some of that becomes heat in the bearings and linkages. The mechanical energy then compresses the air which by the ideal gas law goes up in temperature proportional to the change in volume. Just sitting there, the tanks holding the air cool off, losing this heat to the surrounding atmosphere. We could insulate the tanks to keep it inside (and pressures up) but it turns out that we'd like to harvest heat energy out of the data center later so we let this heat leave by convection. The tanks then go down in pressure thanks to that same ideal gas law.
So when we use this air, we decompress it inside our data center. This allows heat in the data center to be absorbed by the expanding air, which saves us the energy we would have been using to run the evaporative coolers to pull it out and increases this energy available in the air as it increases that static pressure differential between ambient and the decompressed air. So we 'get it back' as it were, in a positive way. Running it through the engines allows us to then harness the pressure differential, and if it comes out of the engines still 'colder' than the data center air we can inject it right into the regular atmosphere of the data center to offset warm air that is already there.
[1] Facebook also has a data center up there in the region and if someone there wants to pick up the ball and run with it that would be pretty cool to. You could write it up for the open compute stuff. Of course you still have to figure out how to get an MDI engine. If Tata ever shipped I considered buying cars, throwing away the body and just pulling out the engines. That would work but requires Tata to actually have a product you can buy.
Just curious, it seems like with prior art, MDI would not have exclusive rights to the motor, right? Also their Wikipedia page says that they are currently using a design that was patented in 1990. Did you abandon this idea because of patent issues, or was it merely the lack of access to a proven/tested motor that stopped you?
You can of course run an existing steam engine on air pressure (I've done that with models) but they are optimized for a higher static pressure than the MDI engines target.
If you're still interested in a real company that's building highly efficient air engines (compression and expansion) for this, and other purposes, send us a message. We have a legitimate approach and will, in fact, sell people things to work real applications. We've been in talks with the big guys to power their data centers. But you could be among the first.
dfong@LightSail.com
If they say they have an air-powered car, they have an air-powered car. This isn't just some dude in their garage.
Because AFAIK the effective energy density of CAES (Compressed Air Energy Storage) is 40~100kJ/kg depending on the tank material, with variable pressure. The upper range is roughly that of a standard lead-acid battery (except the battery has roughly constant voltage) and it gets completely blown away by e.g. li-ion (360~900kJ/kg)
Though the comments suggesting the electric engine is a compressor to "refill" the bottles on the go could need that reality check.
Let's be very, very generous and put a cubic meter of compressed air in that car. That would be, rounding up at every step, 30 kWh of energy in the air. If you manage to transfer all of it to your wheels, a small engine might run for an hour on that (http://en.wikipedia.org/wiki/Tata_Nano has a 28 kWh engine) at full speed.
So, with lots of hand waiving, this seems doable, if you are willing to build a 1000 liter tank for this thing, and manage to keep the car and its tank within the Nano's weight. With a more realistic but still IMO very generous 250 liter (about twice the Nano's trunk), you already are at least a factor three of (rounding generously as in every step before). Oh, and did I mention that that Nano will drive about 50 miles in that hour, if driven at full speed, not the 125 claimed?
http://www.tribology-abc.com/abc/thermodynamics.htm
gives just 2kwh for 250L tank at 150atm. Which is at most 10 miles of radius, probably less, and efficiency of about 40%
Say the energy output is equivalent to running a 50-horsepower engine for an hour. 50 HP-hours = about 35000 watt-hours, or 250 megawatt-seconds (MJ).
A stick of dynamite produces about 2 MJ. So that tank of compressed air, if it ever ruptures, will yield the equivalent explosive power of more than 100 sticks of dynamite.
Gasoline is a safe way to store this much energy because a tankful of gas can't be oxidized rapidly enough to do much damage, at least not by accident. Same is true for hydrogen. But compressed air, where the released energy is purely mechanical, is a bit more interesting.
Never mind the extremely high thermodynamic losses associated with compressing (and expanding) that much gas. They're probably on the same order as internal-combustion efficiency, or lack thereof.
Sorry, but the physics just don't work as far as I can see. It really does smell like a scam.
The LFL/UFL (Lower/Upper flammability limit percentage - the min/max amount of fuel vs air you can have and still burn) for gasoline is 1.4/7.6 - that's a pretty narrow range. But hydrogen is 4/75, so hydrogen will burn in virtually any circumstance.
Hydrogen is, however, lighter than air, so it tends to escape upward before, or while, burning, which limits damage tremendously (except in a tunnel).
The range sounds implausible to me at any reasonable speed on compressed air alone. How is the electric motor involved? Is the air a backup for the battery or vice-versa?
BTW - This story has been all over the I-Net the last days (again). A link to the actual manufacturer / technology company behind that. http://www.mdi.lu/english/
If such energy storage technology becomes available at low prices lots of additional "synergies" can be envisioned - e.g decentralized & cheap storage for small solar power (e.g. 50KW) - seen some DYI solutions for that with compressed air - overall the issue with compressed air storage is the low energy density.
They are not interchangeable, and having one doesn't help you with the other since the pressures are so different.
http://www.practicalenvironmentalist.com/eco-gadgets/air-bik...
Regardless, you can compress air yourself all day long using solar panels and/or wind turbines, and even if you use the grid, it'll still come out a lot cheaper...
Perhaps Tata is experimenting with an electric/pneumatic hybrid (so electric, but compresses air when braking, and consumes it when accelerating). This might work, but doesn't help with infrastructure rollout.
Joule for joule, it's not. The best compressed air tanks you'll find roughly reach lead/acid batteries in energy/weight (~100kJ/kg). Lithium-Ion batteries have 4 to 9 times the energy density.
My only worry (confusion?) is if I brought this car into a small town and the car runs out of air, where can I fill it up? Can I just go to any car-repair shop that has a compressed air line (e.g. for hydraulics) and use their equipment?
I'm not even sure where to start with this.
Say, for instance, you need a car. For me, if a car costs more than $10k, I really have to think about how reliable it is, what features it has, and generally if I am getting value for my money. This is all because anything over $10k is going to take longer for me to recoup. This is what I'm defining as my impulse buying threshold.
For reference, I put about 40 psi in my car's tires.
I'm probably being naive here...I have no knowledge on this subject.
Not sure it's sufficient considering the kind of pressure you'll need in the tank to store an amount of energy worth using.
Imagine getting stuck in traffic in such a car. What powers the A/C?
Releasing pressurized gas absorbs heat so simply running the released gas through a set of A/C coils would provide indirect A/C or you could merely vent the "exhaust" (it is air, after all) into the passenger compartment since that "exhaust" should be very cold.
No you are not. The waste gas vented from the system is very cold, using it as A/C costs nothing.
Although if you are sitting in traffic you are not moving, and have no waste gas.
Guess I must have been asleep during my Dynamics classes...
This seems to be the official teletubbie's car.
People wants something more akin a kiss ass compress air-electric kart to move in the city. Something simple, that does its job well, and fast when you need to accelerate in a hurry.
Cars are empty in the cities most of the time, make them unipersonal.
Exactly. And isnt it what this thing is trying to do ?