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What about improving the efficiency of cars? We can make cars at best an order of magnitude more energy efficient.

Ahh do believe that ahh detect th' aroma of fresh fertilizer. "An order of magnitude", you say... are you thinking binary? If not, your statement seems, to me, to require a second-law violation (unless you think that cars are < 10% efficient, as is). Or are you suggesting that we'll be able to use cars as breeder reactors?

Let's think about this, a bit... To approach the issue of efficiency logically, one could say that an internal combustion engine is a form of a heat engine, which could be described (in an ideal case) as a Carnot engine. Using the combustion temperature of gasoline ~ 2300 K, and the operating temperature of the engine as 100 C (373 K) (should be between 70 and 110 C, but we'll assume pure water, at one atmosphere of pressure as the coolant, for this case), then we get a [theoretical] maximum efficiency of (2300-373)/2300 ~= 84%. Some sources place the working temperature of the steel at 925K, bringing us into the realm of 60% efficiency.
IIRC, the best efficiency that has been managed, in a diesel engine, is around 52%. In modernOtto Cycle engines, one can expect to get an efficiency of somewhere around 32%. An "order of magnitude" improvement would put that at ~320%.

Think about what that implies.

[Of course, if you were thinking in binary, that would be 00 10 00 00 going to 01 00 00 00, putting us in the realm of 64%, which is theoretically possible.]/i

The second challenge there would be a power infrastructure capable of supporting many thousands of fast recharges like that.

Good question. Let's see. First, how much energy does a car use? (Warning, I'm using estimates geared towards someone in the US. Your milage (literally) may vary).

For a 400 mile trip, and 20 miles per gallon, we need 20 gallons of gas. Gas has 131 MJ/Gallon, and engines are almost 50% efficient, so that's 20 gallons x 131 MJ/gallon * 50% = 1310 MJ. Electric engines are much more efficient, at about 90%, we'll say 80% to be conservative. So we need 1310 MJ/80% = 1637.5 MJ. Let's say we want this recharge to take 5 minutes, so it's on the order of a fuel fillup.

That's about 5.5 MW. Damn, most places couldn't do one charge. So, let's say you have some heavy duty hookups, capable of 200 amps at 400 volts (200 amps choosen so your cables aren't so bulky they're too hard for a single person to hook up to a car, and 400 volts so you don't need to worry too much about arcing and the such). Thats nearly 5.75 hours. A long time if it's in the middle of your trip, but ok for an overnight kind of charge, and at a power level not much more than most homes are capable of providing. Proving that we not only need a better power infrastructure (at least at 'gas stations'), but new ways to safely (and temporarily) connect high voltage/current to a car and then disconnect when done.

Also, at an average price of 10 cents per kWh, it's not that far off current fuel prices, at $2.75 / gallon equivalent.

This actually isn't a bad suggestion, and it's one I'd never seriously considered. But it makes a certain amount of sense.

Hydrogen is really an energy storage and transportation medium. It's a way of getting energy from a power plant to where you want the work to be done (or close to it). The advantage, as I understand it, comes from the economy of scale at the power plant.

Suppose just for the sake of argument that you have a car that runs on petroleum and a power plant that also uses petroleum. Putting aside that the car's petroleum requires a lot more refining (gasoline or diesel as opposed to bunker oil or diesel) than the power plant's fuel, the power plant is probably going to be a lot more efficient at converting the stored chemical energy in the fuel to a usable form (electricity). I'm pretty sure that an average car is under 20% (allegedly the Prius is 37%), with a theoretical maximum to the Otto Cycle of around 53% (reference). Today's properly designed power plants can greatly exceed that, and can be over 50% if they use a combined cycle, which combines the Brayton cycle of a direct-combustion turbine with a Rankine cycle steam-boiler (theoretical efficiency >80%).

So obviously there is an advantage towards centralizing the fossil-fuel combustion, in the interests of efficiency. The big question is whether you can distribute that power in a way that preserves the efficiency gain. One way is just to string power lines; this doesn't really help the automobile situation (because electric storage batteries are heavy, etc.) though. One option is 'home generation' of hydrogen which you then load into your car, but this involves a number of transitions in which energy is lost. The power plant, the transmission lines, the hydrogen generator, the fuel cell, the electric motors. If anyone has efficiency numbers for those and wants to add them up, I'd be interested: I suspect the net is pretty dismal.

Producing the hydrogen right near the power plant and then physically transporting it seems to be a popular idea right now, although I'm not sure the numbers are going to pan out. Assuming your power plant has an efficiency of 50%, and the hydrogen production has an expended/produced ratio of 2:1, your net efficiency is 25% -- already lower than the "well to wheel" efficiency of a gasoline-powered hybrid car. And that's before you even get into the energy investment into transportation and distribution costs, etc. It seems entirely possible that a hydrogen-distribution infrastructure could end up being less efficient than one where the hydrocarbons are burned directly by the end users, especially if the energy density is lower (so that the per-unit-energy transportation costs are higher). Let's not forget also that in our current situation, the liquid-petroleum distribution network is essentially a 'sunk cost,' already there for us to use. Anything that requires us to change this has to offer enough gain to offset the expenditure involved.

Hydrogen is certainly a neat idea, but I think we risk getting ourselves into a bad situation in the future if we don't consider all the alternatives, including some form of synthesized hydrocarbon energy-carrier which could use the existing distribution network and only be converted to electricity (or directly to heat) at the point of usage. While the devices themselves might seem less "green," the larger system that they are part of might be more efficient, and thus a better option in the long run as we search for more sustainable sources of energy.

In other words, cautiously we project the current power consumption of all computers running somewhere in the order of at least 20 Hoover Dam power plants

If 9000Mw/hrs are the equivalent of 4 Hoover dams and current estimate is 20 Hoover dams, then current consumption by CPUs is around 45,000 Mw/hrs.

This site quotes 10.9 cubic meters of oil per megawatt/hour.

If my math and sources are right, then CPUs alone, worldwide consume the equivalent of nearly 500,000 cubic meters of oil each year.

According to this site, one American barrel of oil is 0.15899 cubic meters.

That means that the power consumption of all the CPUs in the world equate to over 3 million barrels of oil/year.

Perspective? The US currently uses a bit over 20 million barrels of oil/day. So CPUs worldwide are using around the equivalent of .04% of the annual US oil consumption.

Going from these sites http://pocketcalculatorshow.com/walkman/history.ht ml
http://ecen.com/eee41/eee41e/economical_indexes.ht m
and using Pe^(rt), I get that the real price increased from $70 to $115, while it goes for $350, giving it an investment value of 4.8 percent, which isn't great for a collectable, but if you used it and had it sitting in your attic a pretty decent return.

[1]Yes, I know about changes to H2 specific heat with temperature - orders of magnitude is all the precision we need here.

90% chemical-to-heat efficiency
30-40% internal-combustion engine efficiency
60-70% large scale turbine efficiency
70% lead-acid battery efficiency
Another reference for gasoline energy density

Which numbers were out to lunch in the real world, and what are the correct numbers?

Sorry to burst your bubble, but fuel cells do have the same limitations -- known as Carnot efficiency, btw.

No they don't - see here

You are talking drivel about the engine efficienies also - see here

My source for info? a good introductory thermodynamics class.

Introductory? A little knowledge is a dangerous thing.