Re: Electric Car? How about a Compressed Air Car?

Larry Dighera <LDighera@xxxxxxx> wrote:

From the research below, it does indeed seem that the lithium-oxygen
battery offers the highest energy density that the laws of physics
permit.

http://www.polyplus.com/technology/laircell.htm
The high specific energy of the Li-Air couple is close to that of
liquid hydrocarbons such as gasoline, and much higher than that of
methanol.

That is wishful thinking. Gasoline has a gravimetric energy density
about 13,000 Wh/kg, while the theoretical maximum for Li-air is about
5,000 Wh/kg. Note the word theoretical. Even if you could get to the
theoretical maximum, a battery would be still over twice as heavy as
gasoline with the equivalent energy content. To put it in perspective,
consider what an extra 500 lb. of batteries over the weight of a fully
fueled 172 would do to your range and useable payload. There is also no
such thing as saving weight by partially fueling, unless you physically
remove some of the batteries.

Then consider that the actual energy density of every other battery
technology is only about 60% of the theoretical maximum. The best
lithium air batteries today are just getting to that point, with about
3,500 Wh/kg, and even then only in batteries with low current draw. With
those batteries you would need at least four times the weight of gasoline
to get the same amount of energy.

Finally, consider that Lithium-air batteries have a huge efficiency loss
when high current is pulled from them. Some consider the practical
density of lithium-air cells to be less than 2,000 Wh/kg.

I haven't even talked about the amount of space that the batteries would
require. I wonder where you would put them all?

I've ignored the difference in efficiency between an internal combustion
engine and electric motors, since while it is real, it still won't make
up for the huge difference in practical energy density of the two energy
sources.
.

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