Hacker News new | comments | ask | show | jobs | submit login
The Tata AirPod: a car that runs on compressed air w/150-200km range (stoweboyd.com)
126 points by masnick on Aug 17, 2012 | hide | past | web | favorite | 77 comments

The company behind this invention has been talking about the roll out of these cars for many, many years (http://en.wikipedia.org/wiki/Motor_Development_International). And this particular blog post doesn't give any details of when Tata is going to actually produce a car. I recall a long time ago that these cars were going to be introduced in Mexico (that was one of many things that didn't happen).

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.

Taking your idea a step further... When I did an internship at an architecture studio, the design of the central plants that provide hot/cold water/air to their surrounding buildings would use the exhaust heat of say, a water chiller for the air conditioning unit, to help heat the hot water flow, taking load off of the water heaters. In one case, they had enough "excess" heat available from doing this that a school got a heated pool for "free."

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.

> Power and cooling for free

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.

You are pedantically correct sir, but there is a larger context. So in that larger context I was working on this plan when working for a company that had a data center near the Columbia River and 'industry leading' PUE numbers.

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.

Essentially what you are saying is that your want to (also) store cold rather than store just energy.

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.

Perhaps when I get a chance to build my own data center for Blekko I'll be able to actually run the experiment [1]. Then I'll have a better answer. But would like to share a bit my reasoning.

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.

This is an awesome idea by the way. I work in the energy industry and haven't heard about anything like this.

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?

It was getting reasonably efficient compressed air motors of decent size. The MDI docs claim they do about 50hp (or about 37kW). You'd probably want five or ten 'units' at that level (assuming the data center is 10 or 20MW like the ones Apple, Facebook, and Amazon have built).

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.

Not for all possible thermodynamic cycles -- there is no fundamental thermodynamic irreversibility due to compression or expansion.

Hi Chuck,

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.


I've passed it along to my previous employer :-) But don't get your hopes up, they weren't nearly as enthused about it as I was. It really is an incremental PUE change vs a game changer but my thinking is every 10 or 20 basis points at that scale is real money.

No problem; we're pursuing other leads in the data center space of course :-)

this is a scam which has been explained a lot of time. energy storage density of compressed air is extremely small and will be enough for only a few miles of range, no way for improvement here, and efficiency is very, very low, way lower than batteries because of the energy inevitably lost when the gas heats due to compression, then dissipates heat. forget it.

And yet Tata are a huge, well-known international.

If they say they have an air-powered car, they have an air-powered car. This isn't just some dude in their garage.

The car apparently also has an electric engine. Did tata actually say the car ran on compressed air for the range specified in TFA, or is it just TFAA bullshitting?

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)

worse than that, 'charge-discharge' efficiency is terrible compared to any kind of battery. it won't save any energy vs conventional car as measured in well-to-wheel efficiency.

Yes absolutely, I wasn't putting that in because the case was damning enough without involving the energy loss in charging the thing.

Though the comments suggesting the electric engine is a compressor to "refill" the bottles on the go could need that reality check.

So what if it's Tata? They may be a huge, well-known multinational, but the laws of thermodynamics apply to them too.

Do you have any sources for the technical arguments to why this is not feasible?

If for no other reason, consider the amount of energy that would be released if a tank with enough compressed air to propel an automobile for 50 miles were released in a collision over the course of a few hundred milliseconds.

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.

Gasoline is hard to oxidize fast enough, but hydrogen is not.

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).

http://www.combro.co.uk/nigelh/diver/tank.html claims about 1/3 kWH in a 230 bar 12l scuba tank. http://www.tinaja.com/glib/energfun.pdf claims 17 Wh/l for compressed air (without mentioning pressure, but that is in the same ballpark if is 150 bar, as I guess from the text).

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?

You are terribly overestimating.


gives just 2kwh for 250L tank at 150atm. Which is at most 10 miles of radius, probably less, and efficiency of about 40%

> a city car running on compressed air (as well as a battery-powered electric motor)

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?

Great when they finally will start mass producing this technology - perfect for large cities in India, Argentina and Brazil e.a where already lots of small vehicles run on compressed natural gas (CNG).

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.

CNG and compressed air are quite different. One burns to provide energy, the compression is just for storage. The other uses the compressed air as the power source.

They are not interchangeable, and having one doesn't help you with the other since the pressures are so different.

I hope this works - a compressed-air bike design from here (Uruguay) from 2005 failed to be funded, probably because the inventor was a bit of the crackpot type, but it did work.



That's pretty awesome, though I wonder how they achieved that efficiency from simple compressed air (surely a pack of Li-Ion batteries of the same volume would do better?).

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...

The proposed benefits seem to mainly be in the ease of deploying the recharging infrastructure, not greater efficiency. Here's a paper that's skeptical of the overall efficiency: http://iopscience.iop.org/1748-9326/4/4/044011/fulltext/

Interesting paper, and it suggests some other possibilities, including a Pneumatic/Combustion hybrid (i.e. using a compressed air "flywheel") which seems to have greater benefits than an Electric/Combustion hybrid (a tank of compressed air is lighter than batteries).

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.

> a tank of compressed air is lighter than batteries

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.

Indeed. Also the mechanism to capture compressed air when braking would be in addition to the electric motor, which is needed in any event, and can charge batteries when braking. So yeah, upon reflection, just a Bad Idea.

I love this idea, especially for short city-style commutes. The price ($10k) is good enough that it's under my impulse buying threshold for cars.

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?

> The price ($10k) is good enough that it's under my impulse buying threshold for cars.

I'm not even sure where to start with this.

I hope it's just my wording that is throwing you off.

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.

Not really that weird at all. Cars maintain a healthy resale market so if you buy a car with a couple of owners and a lot of miles for $8,000 and drive it for a few months, you can often sell it for the same, or just a little less, than you paid for it. It's not an $8,000 gamble, it's more like a $500 gamble.

Your impulse threshold is about 3x my ceiling price for a car.

If your ceiling is $3333, maybe you should stick to riding a bus. Do you realize how much it costs to run one car for a year? Gas, registration, insurance, oil changes, maintenance, it adds up pretty quickly.

What would be interesting is if it could carry a small electrically powered compressor that you could plug in for emergencies.

I think this is exactly what the electric motor does: compress the air if you run low.

Wait - you want to use a battery, to power a motor, to compress air, to then power another motor?

No - I meant powering a compressor using mains power.

Most petrol stations in the UK have an air compressor that you can use to top up your tyres. I wonder if these could be re-purposed.

I'm pretty sure you don't top up your tyres to 20MPa (~200 bar)

Yep, 2900 psi; and I just read that they plan to upgrade to 4400 psi tanks.

For reference, I put about 40 psi in my car's tires.

These would be great for city travel. The two main issues would be safety and parking. They'd have to make a strong case for the vehicle being safe in the case of an accident, not unlike the challenge the Smart Car had. For the issue of parking it would be unlikely that a city would set aside real estate just for these cars as they'd have to do it all over the city and it would be relatively expensive. I think a better solution would be a car-sharing one like Zip Cars. I can pay a monthly fee to have access to the cars at certain spots and if I want to leave in rush hour, I'd pay a premium, or something to that extent. It's a really interesting idea, I live in NYC and I'd love it here.

Did I miss something, why can't they use normal parking spaces?

I assume because the cars need to be charged while parked so you can get home.

Mechanical simplicity and no expensive batteries are pluses.

I agree with you, to a point. I wonder about the mechanical simplicity of having air drive pistons in the engine. It's yet another point of failure and, while similar to combustion engines, is there no other way to have compressed air drive a car other than with a piston engine?

I'm probably being naive here...I have no knowledge on this subject.

You could use a Quasi-Turbine. http://www.howstuffworks.com/quasiturbine.htm

There are a lot of different ways to have high-pressure gas push solid objects around. Piston engines, turbines, and rockets are the three most common, but there are also Wankel engines, rotary vane motors, and more exotic designs like the Quasiturbine and the dozen or so other kinds of positive-displacement pumps.

What happens in a car accident? I can't imagine the sudden expulsion of all that compressed air is going to act like anything other than a small bomb going off.

Some sort of quick-release valve as with LPG?

Not sure it's sufficient considering the kind of pressure you'll need in the tank to store an amount of energy worth using.

It's not hard to make a tank like that strong enough for a car accident. It already needs to be very strong just to handle the pressure.

These have been cooking for a long time. I was on the MDI mailing list back in 2000/2001 when they were supposedly coming to market the following year. It's cool to finally see them in production - I thought it might never happen.

Leaving aside the laws of thermodynamics, well-to-wheel efficiency, etc.

Imagine getting stuck in traffic in such a car. What powers the A/C?

That's easy - the expanding gas!

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.

Releasing pressurized gas is cold when there is a phase change, i.e. from liquid to gas. Going from high pressure gas to low does involve a temperature drop but it's not going to be anything like a real air conditioner.

Have you ever fired a PCP(pre-charged pneumatic) air gun? No phase change; much icing. Yes, the latent heat of phase change makes an enormous difference, but it doesn't mean that the pressure drop alone won't cause a big enough temperature differential, and since you're not in a closed compression/decompression loop, you wouldn't be particularly worried about the efficiency -- you're just using a byproduct of the propulsion system that would otherwise be wasted.

It's a cute answer, but very inefficient. You're throwing away the mechanical energy. Also bear in mind that "refueling" with compressed air will heat up your car.

> You're throwing away the mechanical energy.

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.

The system overall may be inefficient, but using the cold exhaust from the engine as A/C doesn't make it any more inefficient.

In fact, if the air conditioner coils are between the air tank and the engine, the air conditioner could make it more efficient, if you don't have some other kind of regenerator-in-a-dewar trick going on.

It looks like this is getting completely owned by HN. Does anyone have a mirror?

I doubt it, it's hosted on tumblr. It handles far more traffic than HN.

I wish they published energy storage capacity in KW-hr. Theoretically a car can run for infinite distance once it has been accelerated to its desired speed.

No, a car can't run for infinite distance even once at its final speed, unless you're talking about the kind of theory that doesn't involve friction, drivetrain losses, aerodynamic resistance, or even things as simple as having a radio turned on (not to mention headlights)

Sure. These are practical things. And practically it can't.

Friction forces have been core theoretical concepts since many centuries.

Friction, mechanical losses and drag aren't concepts covered by physics theory?

Guess I must have been asleep during my Dynamics classes...

You forgot to account for the ramjet.

Theoretically an electric circuit can operate with zero heat output using perfect conductors.

It looks lame, like the Segway.

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.

"Something simple, that does its job well"

Exactly. And isnt it what this thing is trying to do ?

Applications are open for YC Summer 2019

Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact