Wednesday, May 18, 2011

Cars and Energy, part 1

I have heard it claimed that it takes more energy to build a car and destroy it, than it uses throughout its life.

These kind of statements immediately get my bogosity antennae quivering, so I've finally decided to do a bit of research.

Energy use by a car for a year is pretty easy to calculate. You find out how many litres of gas you use in a year and multiply by the energy content of a litre of gas (which is, according to the Interwebs, 35MJ / l). So if I drive 20,000 km in an average year and my car gets 10 km/l, that means I will use 70 GJ (that's Giga-Joules) of energy in a year.

Now what about fabrication of that car? This is where things get a little tricky. Do I include the energy used to mine the metal that goes into the engine? Or only in its manufacture? A quick hit o' the Google brings up several densely analytical papers concluding that the average car takes about 70 GJ to make and 5 GJ to dispose of (recycling the metal and glass).

As far as I can tell, these analyses don't include the energy used in mining, just in shaping the metal sheets. So maybe we can bump the number up a bit if we want to account for the mining and casting. Thanks to Google again, it takes 25MJ/kg to make steel, so if you assume the worst case, that a typical 1500 kg car is 100% steel, then you can add a further 37.5 GJ to the energy cost of manufacture - for a total of about 110GJ. (note: if you use recycled aluminum and recycled glass in your car, it'll take less energy to make it).

So t's pretty clear that the main energy use of the car is in driving it around, not in making it. So, it follows that any efficiency you can get out of your car, using less energy (gas) will have a bigger impact on the total energy use than by scrimping during its manufacture.

On a final note, electric motors are much more efficient than gasoline engines - about 3x more efficient. This means that you can go 3x farther on your GJ of energy in an electric car. Or put another way, if you need to still drive 20,000 km, you'll only need 23 GJ of energy to do it. So maybe we should be electrifying our transportation network?

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