Compressed air car
From Silent Revolution
Topic: Compressed air cars are a technological non-starter
While hydrogen cars fail to beat electric cars on many fronts, compressed air cars fail on virtually every front in comparison to everything else.
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[edit] Efficiency
Motors running on compressed air are reasonably efficient. However air compressors are extremely inefficient, generally 10-20%[1][2][3]. This comes on top of generating electricity losses (typically 40-50% efficiency) and transmission losses (typically 92.8% efficient), for a net total of ~5% efficiency, respectively. This compares with ~20% for hydrogen and gasoline, and ~30% for EVs. To even compare on efficiency to their hydrogen and electric bretheren, home charging is right out (far, far worse system efficiency than gasoline) and charging at dedicated charging stations with the most efficient regenerative compressors on the market still lags behind electric vehicles in terms of efficiency.
Given that energy efficiency is currently directly tied to greenhouse gas emissions, anything on par with gasoline isn't helping with global warming. Energy efficiency is critical in a "green" vehicle. See the article on hydrogen cars for more details. Compressed air cars just don't cut it.
[edit] Range, Charge Time
Compressed air vehicles have reasonable refill times at commercial stations, but poor refill times at home unless you had an overpressurized (extra wasteful) tank at home. As for range, however, they significantly underperform without an auxiliary power source (akin to a plug-in hybrid electric). Compressed air at 300 bar has an energy density of 4 MJ/kg and 0.14MJ/L. This compares to gasoline at 46.9 MJ/kg and 34.6 MJ/L. Even with higher pressure and taking into account the low efficiency of gasoline engines (scaling by efficiency, the gasoline energy densities are more like 12 MJ/kg and 9 MJ/L), there's no comparison. Compressed air vehicles simply have, and will continue to have, poor range.
Even when the comparison is with electrics, it's still a poor comparison. Lithium ion batteries, ignoring the significant Stanford advances, provide 0.54-0.9 MJ/kg and 0.9-1.9 MJ/L. While the energy density per kilogram is better for compressed air storage than lithium-ion (due to being mostly hollow space), the real problem is that there's just not enough room in a vehicle for a relevant amount of compressed air storage (the extremely low 0.14MJ/L number). When you include the Stanford advances, you're looking at ~2 MJ/kg and ~4 MJ/L without a cathode improvement and ~6 MJ/kg and ~11 MJ/L with a cathode improvement. An EESU (barium titanate ultracapacitor in development by EEStor) would also improve the electric numbers in question. Unfortunately for compressed air storage, it requires radical advances in materials technology to improve its range -- something that is slow to happen. And even with stronger materials, there may be hesitation to move to higher PSI due to increased consequences of rupture.
The longest range non-boosted (no hydrocarbon fuel) air cars are MDI's series to be built by Tata Motors. The cars are too fragile to meet US safety requirements[4], and production has been pushed back by two years, leading to questions as to whether they'll ever be produced at all.[5]. France's prestigious L'Auto Journal released a highly skeptical article on their plans.[6]
Needless to say, these numbers are not impressive compared to those for electric cars. Yet, electric cars have huge room for improvement. With air cars, the only choices are increase the PSI (which affects safety and stretches material technology and price plausibility) or increase the volume (not really realistic).
[edit] Safety
As compressed air is all about putting maximum pressure into a tank inside the vehicle, the occupants are quite sitting on top of a tremendous amount of energy that wants to be released all at once. While the name of the game is to make the vessel as strong and damage-resistant as possible (as in a hydrogen vehicle), that doesn't change that the passengers are still sitting on top of a tank that by its nature must contain all of the energy to get the vehicle across its entire range. To try and make it so that the pressure vessel doesn't rupture in an accident, safety codes limit the percentage of the tank's capacity that can be used -- typically 40% for steel vessels and 20% for fiber ones. This further lowers the energy density.
[edit] Other issues
Compressed air vehicles have ample torque when just filled; however, this falls off the further the vehicle travels. The closer to the end of its range, the worse the acceleration and top speed of the vehicle.
[edit] Myths
The Discovery Channel Futurecar implied that a compressed air vehicle could compress its own air with the power from its engine (that runs on compressed air). This is, of course, a perpetual motion machine and is in violation of the laws of thermodynamics.
