Wednesday, October 19, 2005

Compressed Air Cars

Even before the electric car fad was stillborn, aborted by high costs, short range, and slow recharging, I've thought there'd be a market for cars using compressed air energy storage. Now that gas is $2.55/gallon in my neighborhood, the opportunity seems even more interesting.

The advantages over electric cars are compelling. Compressed air, expanded near-isothermally, has a considerably higher specific energy than batteries (especially if one takes into account the loss of mass of the air as it is vented). Tanks seem likely to be cheaper and longer lived than sophisticated batteries. Power can be extracted from the air and supplied to the drivetrain without electric energy conversion.

So, I was pleased to see that a company in Europe, MDI, has designed two compressed air cars and is licensing their manufacture around the world. The designs reflect experience with prototypes over more than a decade of development and show some interesting refinements.

In these vehicles, air is stored at 300 bar (4500 psia) in carbon-fiber wrapped plastic tanks. The tanks are under the floor, and in tests fail at 740 bar -- not too bad a safety factor. The tanks can be refilled in a few minutes from a stationary compressed air source, or in 3 to 4 hours at home (on 240 V AC) using a built-in electric motor to drive the car's engine as a compressor.

The first car is air-only, and has a range of 200 km on a single charge, about twice the range of electric cars. The engine has three expansion units with heat exchangers after the first two to reheat the air using ambient atmospheric heat. In the second car, the air can be preheated by an external combustion heat source. This extends the range of the vehicle and also greatly increases the amount of work that can be produced per unit of heat energy, since, unlike in a normal engine, none of that work is used to compress the working fluid. However, since the car also has a compressor, it can also operate in a mode where the engine powers the compressor to refill the tanks, operating as a kind of external-combustion air engine. So, if you really need to drive long distances, the car naturally converts to fuel-burner with very long range (2000 km on 50 liters of gasoline). In this sense it's alot like a 'plug-in hybrid'.

The price appears to be very reasonable -- 6840 and 9460 euros, respectively. I'm looking forward to hearing more about these as they move into production. Assuming they're not too flimsy for US crash standards, maybe someone will sell them here.


Blogger Ivy said...

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5:47 PM  
Blogger Jon Goff said...

Interesting. I like the second one. If you can "recharge on the go" using a little gas (or some other fuel) operating just as a heater, and get that kind of mileage (about 94 mpg!), you could have a very interesting system. Question though. How safe is it to have a 4500psi air tank under your car in a collision? I know that pressure vessels are obscenely tough when under pressure (such as the Atlas balloon tanks), but do they have crash test data on this stuff?


11:56 AM  
Blogger Paul Dietz said...

I understand these tanks are supposed to fail gracefully, cracking and leaking before bursting. Venting the air would be spectacular, but probably safe if no shrapnel were produced.

The safety concern I'd have would be the low mass of the vehicles. Are they going to be crushed if hit by an SUV?

I'll be putting up another post soon on compressed air storage, for use in vehicles and stationary applications.

2:24 PM  
Blogger Iain McClatchie said...

Utilities are using Compressed air energy storage already. It's a gas turbine with a clutch between the turbine and compressor, and an electric motor on the turbine side. They compress air with either electric or turbine power when full output isn't necessary.

It seems like one big advantage of this system for cars is that when the engine runs on compressed air, it should have greatly improved peak power to weight, allowing the car to use a smaller displacement and lighter engine to get the same performance.

The utilities use internal combustion, which is more efficient than external combustion. Internal might be more light weight, since there is then no need for a heat exchanger.

So why are the car folks using external combustion for cars?

10:40 AM  

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