Thursday, August 8, 2013

Rapid Charging Electric Cars

One of the things that comes up repeatedly with electric cars is how fast you can charge them. The current answer is “overnight” or, at least, on the order of hours. Some more bullish electric car proponents argue that charging stations could be built that would reasonably match gasoline filling speeds. I don't think that's plausible.

The EPA limits gasoline filling speeds to 10 gpm (gallons per minute), or 0.63 l/s (liters per second). The volumetric energy density of gasoline is about 36 MJ/l (megajoules per liter), which means that, at the gas station, energy is flowing into your tank at a rate of 36 MJ/l * 0.63 l/s = 22.7 MJ/s = 22.7 MW (megawatts).

Now, suppose that the battery-to-wheels efficiency of an electric car is five times the tank-to-wheels efficiency of a gasoline car, which is a fairly reasonable assumption. Then we only need a charging power of 22.7/5 =  4.5 MW. Generously supposing 90% efficiency from grid to battery, we need “only” draw five million watts off the power grid.

That is an absolutely vast amount of electric power: not much less than is needed to supply 4,000 American homes. This is not a plug-in device.

A reasonable rule of thumb for distribution level electricity supply is “100VA/kV” in other words, if you were to be a 10kV primary customer, you can expect to be able to draw at most 1 MVA (the difference between VA and watts is not important for the current discussion). On that basis, the 5MW charging station will need to be a 69 kV subtransmission customer of the local power utility. One charging station, not a gas station forecourt with 8 of them.

Now, let's talk about slew rate. You can't just turn on a 5 MW load (for want of a round number) like a 60W lightbulb. Call your local power company and ask them how quickly they would allow a subtransmission customer to turn on a 5 MW load. The answer cannot be faster than they can spin up a gas turbine. The slew rate of a “hot” GE gas turbine generator is, at most, 5%-per-minute. In other words, to be able to even ramp-up a 5MW charger to full power in one minute would take 100MW of spinning reserve, and only after that have you hit gas pump-equivalent power.

Let's look at that another way. Take the 85kWh (kilowatt hour) battery in the top-end Tesla Model S. 85kWh is 306MJ. Suppose you want to charge that sucker in a minute flat, which is not unreasonable given its range of 265 miles: one minute would give you ten gallons of gas, at least enough to run a modern luxury car for that distance. To supply 306 MJ in 60s is 306/60 = 5.1 MW. Pretty much the same number.

In synopsis, refuelling an electric car at the same rate as a gas pump, any way you look at it, requires something of the order of 5 MW of electric power. This is, practically speaking, impossible. Notice that I haven't mentioned cost. Economically speaking, it is utterly beyond any reason: the charger alone would cost millions. Recharging an electric car at even ten percent of gas-pump equivalent speeds (requiring “only” a half-megawatt charger), presents enormous technical challenges in electricity supply.

In short, the technical challenges of recharging an electric car on consumer-acceptable timescales is almost nothing to do with the car or its battery.

1 comment:

  1. I feel like this is a discussion that is desperately needed with all the bulls currently behind Tesla. Maybe you could do a write up on the metal air battery, many are thinking this will be available in a Tesla soon (3-6yrs). From what I've read, that's being enormously generous if we're talking implementing it in a safe way.