Air Car Introduction

An 'air car' sure sounds like an environmentally friendly mode of transportation.  After all, air is pretty clean!  But it takes energy to compress air, so the question becomes, how do compressed air cars stack up against electric vehicles and hydrogen cars?

How green are they?

One example of a compressed air car is the Zero Pollution Motors (ZPM) MDI Air Car.  This car is a hybrid which operates on compressed air alone under 35 mph, and uses gasoline to compress more air above 35 mph.  The company claims their air car is "the worlds cleanest car" with "half the CO2 per mile as a Toyota Prius" at speeds over 35 mph.  However, both of these claims are based on just the gasoline emissions and ignore the emissions associated with compressing the air in the first place.  A bit sketchy.

So how much energy and CO2 emissions are associated with compressing the air?  Well, according to the car's specifications, it has a 5.5 kW charger which takes 4 hours to fully refill the tank.  This means it requires roughly 22 kWh to replaced the compressed air.  The company also claims the car has an 848 mile range on both a full tank of compressed air and 8 gallons of gasoline, and that the gasoline emits 0.141 lbs/mile of CO2 from just the gasoline.

Over those 848 miles, it's also responsible for the 22 kWh of energy required to replace the compressed air.  So how much CO2 is associated with that energy?  Well, of course it depends where you get it from.  According to the EPA, the average CO2 emissions from power generation in the US are 1.363 lbs/kWh.  So on average, you're creating 30 lbs of CO2 emissions over those 848 miles, or a further 0.035 lbs/mile for a total of 0.176 lbs/mile of CO2.  This is roughly half the CO2 emissions associated with a Toyota Prius.  If the compressed air is powered by a more environmentally friendly energy source, it will produce even lower emissions.  For example, Pacific Gas & Electric (PG&E) in California produces 0.52 lbs CO2 per kWh, which would bring the ZPM down to 0.155 lbs/mile of CO2.

Air Car vs. Phoenix SUT, Tesla Roadster, and Aptera typ-1e

So how does this compare to electric cars?  The Phoenix Electric SUT goes 3.7 miles/kWh, which equates to 0.37 lbs/mile for the average US power grid mix, and 0.14 lbs/mile for the PG&E mix. 

The Tesla Roadster uses 0.11 kWh per km, or 5.6 miles/kWh.  This creates 0.24 lbs/mile of CO2 for the average US power grid mix, and 0.093 lbs/mile for the PG&E mix.

The Aptera typ-1e has a 10kWh battery pack and range of 120 miles, so it travels 12 miles/kWh. Thus the Aptera creates  0.114 lbs/mile of CO2 for the average US power grid mix, and 0.044 lbs/mile for the PG&E mix. 

So in most instances, the ZPM will produce lower emissions than the Phoenix and Tesla, but not when the power comes from low emissions sources like PG&E's.  The Aptera typ-1e produces the lowest emissions by far.

In terms of size, the ZPM is 1874 lbs while the Phoenix SUT is 5280 lbs, so the light weight explains why the ZPM can create lower emissions than the Phoenix despite using a less efficient process (using electrical energy to compress air and push a piston rather than storing it in a battery to power an electric motor).

The Aptera weighs 1480 lbs and uses a more efficient process than the air car, which accounts for its extremely low emissions.

How much do they cost?

The ZPM is expected to cost approximately $17,800, similar to the Green Vehicles Triac at $19,995 and the Toyota Prius at $21,500.

In terms of fuel, the ZPM will use 22 kWh and 8 gallons of gas over 848 miles.  Assuming electricity costs 10 cents/kWh and gasoline costs $4.50/gallon, refuelling the ZPM will cost 4.5 cents per mile, the Prius will cost 9 cents per mile, and the Triac will cost roughly 2.5 cents per mile.  The Prius will therefore be the most expensive of the three, while the Triac will become cheaper than the ZPM after 110,000 miles, but the two will be comparable.

Pros vs. Cons

There are many advantages to the ZPM air car.

Pros:

• Its estimated 848 mile range exceeds that of essentially all cars currently on the road, and even plug-in gas/electric hybrids like the Chevy Volt. 
• It can be refueled quickly, taking approximately 3 minutes to refill the compressed air tank at a service station, and little time to refill the 8 gallon gasoline tank.  Refilling the tank by plugging in the on-board air compressor will take a reasonable 4 hours.
• It will produce roughly half the CO2 emissions of the Prius, the most environmentally friendly mass produced car currently on the road.
• It will be affordable, selling for less than a Prius or any highway speed electric car, and having a low refueling cost.
• The ZPM will have 6 seats.

Cons:

• The process of using electrical energy to compress air to mechanically push pistons is less efficient than storing the electrical energy in a battery and using it to power an electric motor (approximately 60% grid-to-motor efficiency for the air car vs. approximately 85% for an electric car with lithium ion batteries).  Thus an electric car of similar size and weight to the air car (like the Aptera typ-1e) will be more efficient.
• Because it must be relatively lightweight, there are some concerns that the air car will have trouble passing US crash and safety tests.  However,  Shiva Vencat (CEO of ZPM) has stated that their air car will be of similar construction to the SmartCar (currently available in the US), but larger.  Additionally, obviously the car won't be allowed on the road until it passes the crash tests.
• As a hybrid, the air car still relies on fossil fuels, although to a much lesser degree than even today's best gas/electric hybrids.
• Even if there are no problems passing safety tests, the ZPM will not be available in the US until late 2010.

How do they work?

A compressed-air car uses the force of super-compressed air to move the engine’s pistons up and down, as opposed to explosions produced from injecting a small amount of fuel.

(Diagram courtesy of Zero Pollution Motors)