What *IS* This Nonsense? (Electric Cars)
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2011-05-16 11:33 by Karl Denninger
in Energy , 4 references Ignore this thread
What *IS* This Nonsense? (Electric Cars) *


Batteries are now "part of the clean-tech boom, with all the dewy and righteous credibility of thin-film solar and offshore windmills," Seth Fletcher asserts in "Bottled Lightning." Righteous? Surely. Credible? Maybe.

Uh, credible, no.

Some commentators worry that we're going to replace our dependence on foreign oil with a dependence on foreign batteriesand foreign lithium. "Bottled Lightning" alleviates at least one worry: By taking us to the salt flats of the "Lithium Triangle" in Chile, Bolivia and Argentina, Mr. Fletcher shows us the abundance of the metal and puts to rest any fears of "peak lithium."

Mr. Fletcher is in love with the Volt. After a test drive, he gushes: "The car, in short, is fantastic." And it is technically sweet. But at $41,000 per copy, will it interest American drivers?

Hypesterism is not scientific evidence or supportable.

Look, I'd love to find a solution that works in the "battery" realm.  But Seth (and everyone else!) has two problems he has to deal with (and hasn't):

  • Charge acceptance.  That is, how fast can you stuff energy into the battery.  This is largely a function of the battery's effective series resistance while being charged; the more of it the more energy gets dissipated as heat in the battery rather than being stored chemically.  Lithium batteries can be charged at higher rates than other chemistries, but the practical maximum is "2C", or double the amp-hour rating.  Going beyond that tends to do a lot of damage to the cell in a big hurry, reducing capacity dramatically, and this assumes you can dissipate the heat (if you can't you get a fire, which of course is very bad!)  As a practical matter this means that while a 30 minute charge is possible assuming you can find a plug that can deliver the amps necessary to do so, the expected "5 minute fillup" is NOT.  Note that the Chevy Volt has a 16 kWh battery pack in it but can only realistically draw down the pack to 30% before protective actions limit further discharge (cell damage occurs below this level.)  That is, we have about 11kWh usable in the pack, so to recharge it in 30 minutes (assuming "2C" can be done) we'd have to source 22 kW before losses.  That's about 100 amps @ 240V.  That's bad news but it in fact gets significantly worse because as batteries go over about 80% charge their acceptance goes down materially, and as a consequence trying to get the last 20% into them on a "rapid charge" is going to both decrease efficiency significantly and increase the heat dissipation problem.  As a result with losses we probably need around 125 amps @ 240V and we can only realistically charge for 25 minutes, leaving us 15-20% short of "full."

    Note that if you have a larger battery, allowing a longer range, in order to be able to charge it in 25 minutes or so your power requirement is going to go up a lot.  Let's assume that we want not 40 miles of range but two hundred miles, and we will accept a 30 minute charge after that (that is, we'll travel for three hours @ 70mph and will accept a 30 minute layover after those three hours.)  Note that this is quite conservative - the average modern car can travel about 400 miles before refueling, so a 200 mile range is actually quite a decrease.  But now we need five times the electrical delivery rate, or over six hundred amps of 240V power.  That's three times the total electrical capacity of a modern home's power feed - per vehicle that is charging at one time.  Exactly how many cars did you say that "filling station" was going to be able to support?

  • Energy density.  Batteries are chemical devices; they perform a chemical reaction called a "redox" reaction, or reduction + oxidation.  But unlike combustion (e.g. a gasoline engine) a battery has to carry its oxygen inside the case where a hydrocarbon fueled engine gets the oxygen from the air.  In the case of burning natural gas, for example, you have CH4 + 2O2 -> CO2 + 2H2O.   The total mass of the reactants for this chemical reaction is 12 + 4 + 64 or 80 amu of which 64, or 80% of them, come from the atmosphere rather than being carried in the vehicle. 

    In the case of the battery all of the reactants are in the case and the cell has to contain the products and have the other half-reaction (reduction) present so the discharge of the battery can be reversed.  This produces a huge disadvantage for the battery in terms of the amount of energy per unit of mass (and usually volume) for the battery that cannot be reasonably overcome.

These are the realities of chemical reactions folks.  I know there are a lot of people who would love to find a way to "replace" liquid hydrocarbons, but the fact remains that we don't use them due to some conspiracy.  We use them because they pack a lot of energy into a small space and the majority of their reactant mass comes from the atmosphere.

There's no getting around these facts.  Better technology will, over time, improve charge acceptance, but it is going to be hard-pressed to do much for density problem which comes about from carrying the necessary reactants in the battery's case.

Hype must give way to physical and chemical reality.