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Wonko the Sane's Journal: Batteries, Gasoline and Math 2

Journal by Wonko the Sane

I got some flack on the article about the Teals Roadster for the statement that batteries need to improve by about a factor of 20 before electric cars will have the same long distance capabilities as gas powered cars so I've decided to publish a more complete explanation.

For the purposes of this exercise we'll consider a hypothetical 4-door sedan. This particular model seats 5 adults and has a large trunk capable of carrying the luggage that a family of five needs for a road trip. It has a 16 gallon gas tank and can travel 400 (highway) miles on a tank while carrying that family and their luggage. Finally, this car is equipped with a modular engine that can be removed and replaced with an electric propulsion system. Likewise the gas tank can be removed and replaced with a battery (we want to make this comparison apples-to-apples)

The efficiency of this car when powered by gasoline is about 20%. When it is powered by electricity it is about 90%. Efficiency in this context means tank(battery) to wheel. Since we are comparing the ability of these devices to store energy the efficiency of pulling crude oil out of the ground and getting into the tank as gasoline, as well as the efficiency of generating and transmitting electricity and charging the battery are outside the scope.

First let's figure our how much energy is necessary to move this family of five and their luggage 400 miles on the highway. We know that it the engine consumes 16 gallons of gasoline, which contains 1.9 gigajoules of energy. Since we also know that the engine wasted 80% of that energy the actual amount of work necessary to move this vehicle 400 miles at highway speed is 387 megajoules. In order for our car to make the exact same trip at the exact same speed on electric power, we need a battery that can store 426 megajoules (90% efficient).

How big will that battery be? To start with lets convert 426 megajoules to electrical units: 188 kilowatt-hours. Using the most optimistic numbers we have for lithium-ion batteries today gives us an energy density of 160 watt-hours/kg and 360 watt-hours/liter. Our battery will weigh 2,600 pounds (compared to 97 pounds for gasoline) and take up 138 gallons worth of space. Even if we assume that the frame of the vehicle can handle the extra weight of this battery there is no way you are going to fit it, five adults and their suitcases on your trip.

Are electric vehicles great for getting around in town? Yes. Are they ready to replace fossil fuels for long-haul travel? Not yet. When you see that batteries can store 4 kilowatt-hours per kg and 3 kilowatt-hours per liter then you'll know that batteries have caught up with gasoline in terms of vehicle energy storage.

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Batteries, Gasoline and Math

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  • What I keep telling everybody who'll listen: One of the biggest problems in Energy right now is that storage sucks. You can have all the alternative energy and smart grids you want, but without decent, reliable and cheap batteries, it will suck balls.
    • I love the idea of electric cars and I do plan on buying one in the future, but there's just no way to say that they are a 100% replacement for gasoline engines (yet). I'd actually prefer a capacitor powered vehicle with enough range to get me to and from work and to the grocery store and keep the car I have now for anything longer.

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