by Don Tuite for Electronic Design Magazine
One of the things the Smart Grid roadmap talks about is using the batteries in electric vehicles (EVs) to store energy produced during low periods in the diurnal demand cycle and making it available on the grid during periods of high demand. Whenever dynamic electricity pricing is introduced, this could make sense for the car owner who could set the car for “Okay to Charge” when rates were below a certain level, and “Sell down to [a pre-programmed] State of Charge” when rates were above another level. (The car would read the rates from the owner’s smart meter.)
That’s a business model. Ever since I heard of it, I’ve wondered, “Can that really work?” I mean, even ignoring the entropic losses, who would ever pump gas out of his car and sell it back to the gas station for a couple of bucks?
I could have asked people in the EV community, which is strong here in Silicon Valley. They have regular meetings in the Xerox PARC complex, near where Tesla Motors moved to, but I figured that would be like asking biplane enthusiasts how many wings an airplane should have. (“Two, one above the other” is the correct answer; the only guy who really liked the Fokker DR-1 was von Richthofen, and he was psycho.)
So I decided to run some numbers and see where they led me. Like any good engineer, I started with the wrong assumptions, but there things you can learn from wrong assumptions, so I’m going to repeat them below for the sake of showing a significant difference between conventional cars and EVs.
How Long, O Lord?
My first attempt was based on answering the charging-time question. My error was in thinking in terms of an EV with a “gas tank” equivalent to my Prius. I said, in effect, “It takes me three minutes to completely fill the 12-gallon Prius tank. Ignoring efficiency factors and energy storage technology, how long would it take to “pump” the same amount of energy from the grid into an EV?
If you look it up on-line and ignore winter-blend/summer-blend differences and octane ratings (higher octane = less energy), you can start with the assumption that one gallon of gasoline corresponds to about 35 kWh of energy, so after a fill-up, my Prius tank is brimming with about 420 kWh.
Given various single-phase and 3-phase combinations of line voltage and current capacity, it would take anywhere from “preposterously long” (days and days) using a120-v, 15-a extension cord thrown out the dining room window) to “still pretty long, compared to a gas pump” (with a 3-phase industrial drop). Since I was barking up the wrong tree at this point, I’ll spare you the details.
So I figured I’d better stop ignoring the vehicle power train and batteries and make some more realistic assumptions. From a Tesla owners’ site [] I got a performance number: 3.12 mi/kWh.
Converting that using my original 35 kWh/gal for gasoline, that’s definitely Prius-beating fuel economy:
3.12 mi x 35 kWh = 110 m/gal (equivalent)
1 kWh 1 gal

So forget that this is a highly engineered Tesla Roadster. Assume that a day will come when this performance is easily achievable. Let’s say this future EV is designed to give a 300-mile range, equivalent to what you’d expect from an ordinary car today. In that case, forget my Prius tank. You’d only need to store
300 mi x 1 kWh = 95 kWh
3.12 mi
That would still take over 50 hours from an extension cord thrown out the window, but a fill-up would be faster from a single-phase at 240 v, and faster yet with a full 3-phase industrial drop.
Faster yet, but still not even close to my Prius’s 3 minutes.
But now lets look at vehicle-owner psychology. Like most drivers who don’t have a gas pump at home, I don’t fill up every day. That would be too much of a hassle. Same for you, right?
What if you had a gas pump, even a relatively slow one, in your garage?
Consider that hypothetical car with the 300-mile range. (You’re going to want that range for big trips.) Let’s give you a long daily commute, 50 miles each way. Every night when you pull into the garage, you’ve only got to replace a little over 30 kWh. With a dedicated drop, you could do that in an hour or so.
Now I can imagine scenarios where you could be an electricity mogul. Sell some of surplus in your battery while electricity is dear, i.e., while everybody in the continent is cooking dinner, doing homework and watching plasma TV. Then buy it back cheaply later at night and still start the day with a car that has a 300-mile range. (Okay, you’re still paying something for the juice. There is no free lunch. But you’re paying less.)
Suddenly, the business model that I couldn’t buy into starts to make more sense. Of course, you would need some more infrastructure for remote charging. And it’s probably reasonable to keep the hybrid option for as long as possible. But the idea of using personal vehicles as a distributed energy storage pool, with a strong economic incentivizer, doesn’t seem so far-fetched.
Comments welcome. I’ve probably missed some important points pro and con. (And annoyed the Red Baron’s fan club.)