Tuesday, August 25, 2009

Gallons in an electric car?

MPGe and gallons equivalent is a convenient metric to compare electric and conventional cars, but it's pretty evident that an electric vehicle does not burn gasoline. What does an electric car really use?

We monitored the charging behavior of the car closely and matched this information with the hourly data from ISO New England (wikipedia: about).
Here is a plot of electricity demand in New England with the profile of car charging overlayed. Both axes are in the units of power, MW stands for a megawatt (1 million Watts), and kW is 1000 Watts. An average incandescent light bulb is 60 Watts, an average compact fluorescent bulb is 20 Watts, and an average car has about 160 horsepower, equal to 120 kW. That makes 6,000 CFL bulbs in an average car.

The electricity demand follows a fairly typical summer profile. We use much more electricity in the afternoon (peak demand) than at night. The main use during the day is air conditioning.

The car charges whenever it is plugged in. Most electric cars will be charged at night in garages, on driveways, or on public charging spots. Just like we charged at our friendly hosts, Ford of Greenfield. We also charged during the day, in fact, during peak demand. Did it matter for the utility company? We'll see shortly.

United States fuel mix for electricity generation

United States electricity grid consumes about 50% of coal. Electricity produced with coal is particularly "dirty". Yet all states are quite different. Here is a fuel mix used in the New England: coal is only 11%, and renewables (including hydro) is greater than that. As you can see, the electricity demand on August 19th and August 20th were less than maximum capacity, so we did not run the dirty diesel generators.

The car charges "on top" of the demand bar, so we used capacity from a combined cycle natural gas plant. For this race, our Porsche was actually powered by the natural gas. It is a fossil fuel, yet its source is all domestic.

We were concerned whether our electric car mattered for the utility since it was being charged at peak. For the comparison, we looked at the difference between two consecutive days, Wednesday and Thursday. Wednesday was slightly hotter, so consumers used a bit more electricity. A difference between Wednesday peak and Thursday peak is 871 MW sufficient to recharge 414,762 electric cars like our Porsche

We will continue with the summary of CO2 emissions from the race.

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