Promise and Problems with Electric Vehicles

What’s the big deal with electric vehicles, like the Tesla or the Leaf or the Chevy Volt? On the plus side, they use a lot less energy to drive a mile than a comparable fuel-powered vehicle. On the minus side, they are expensive, they don’t go very far on a “tank” of kWh, and they take a long time to charge.

Let’s start with efficiency. According to the U.S. DOE, gasoline engines deliver at most 25% of the energy in the fuel to the wheels. A more typical number is around 20% (diesels are slightly better). Most of the loss is “thermodynamic” loss as determined by the second law of thermodynamics. Part of the loss comes from driving the complex machinery of pistons, cranks, and gears that get energy from the burning fuel to the wheels. Hybrids do a little better because they typically have smaller engines and they can recover much of the energy lost from braking. Electric vehicles have little thermal loss and use regenerative braking, so they can get over 60% of the energy in the battery to the wheels, roughly 3 times better than what a gasoline engine can do.

What about CO2 emissions? When you burn gasoline, you release about 9 kg of CO2 for every gallon burned, so a car that gets 30 mpg releases about 0.3 kg/mile, and costs $.10 / mile at $3/gallon. A Tesla S goes about 100 miles on 33 kWh, or 3 mi/kWh, and costs about $.03/mile at $.10/kWh. When we make electricity from coal, we emit about .95 kg of CO2 for every kWh produced. If all your electricity comes from burning coal (it does not), then the Tesla is “responsible” for about the same level of CO2 emission as the gasoline car. But the overall average CO2 emission for electricity generation in the U.S. is below .7 kg/kWh and dropping, so the Tesla comes out ahead. Of course you can charge it with your solar panels and get to zero.

Why does it take so long to recharge an electric vehicle? You can put enough fuel in your tank to go over 300 miles in under 2 minutes. That’s 9000 miles of range in an hour, if your tank was big enough. The standard 40 amp, 240 volt charging setup for a Tesla gives you 29 miles of range in an hour. The gasoline vehicle “charges” 300 times faster. That’s because the fuel is coming from a local tank, and the hose is delivering energy to the car at a rate of nearly 10 MW, while the charging setup for the Tesla delivers energy 1000 times slower at the rate of 10 kW. Most modern houses in the U.S. have electric grid hookups that can deliver at most 44 kW (200-amp service), and have no appliances that can use more than 5 kW.