Secondary Emissions

Can an EV in the Garage Save You Money While You Sleep?

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 voltag

TAGS: Power
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