Consuming to Curb Consumption: the Case for a new Prius

Greetings, fellow humans.

Today's post is by request. One of my readers who is interested in minimizing his environmental impact asked about whether the energy costs of manufacturing a new car outweigh the energy costs of running an older, less fuel-efficient vehicle. The reader in question bikes to his law firm in all weather but snow, so he's already taken the cheapest (and probably most significant) step towards reducing his transportation-related energy consumption. However, many of us need cars at least once in a while, so it will be fun figuring out how many miles of driving you'd have to do to make buying a new car worthwhile.

Manufacturing a New Car

The Internet's too powerful these days. I thought I'd have to sift through details about modern steel-making techniques to get an estimate of how much energy goes into making a new car. It turns out that Google Answers beat me to it though: the average energy consumption associated with making a new car is 73 Gigajoules. Given that a liter of gas has about 32 Megajoules of energy, that means the energy content of manufacturing a new car is equal to the energy content of about 610 US gallons of gas. Since fossil-fuel-burning power plants are only about 40% efficient, the energy cost of making a new car is equivalent to that of burning about 1500 gallons of gas. (Aside: making cars from recycled steel reduces this energy cost by about 20%.)

Comparing Manufacturing Energy to Use Energy

Now that we know how much energy it takes to make a car, let's see how much you would have to drive a new, fuel-efficient car to make up for the extra energy used in producing it. Suppose that your new car gets about 45 miles per gallon while the old one got only 30. Then, for every 90 miles you travel, you'd save one gallon of gas from the fact that you bought a new car. Since making the new car consumed the equivalent of 1500 gallons of gas, you'd have to drive 135 000 miles to get to the break-even point, energy-wise.

Adding Emissions to the Mix

One thing I haven't factored into my account is the fact that power plants tend to have lower emissions than vehicles, since some power plants are zero-emission and others may have scrubbers (i.e., they may clean their exhaust of the worst polluting chemicals before dumping it into the air). In summary, this report says that 68% of the CO2 emissions from the life cycle of a typical car come from fuel consumption, 21% come from fuel processing and only 11% come from vehicle manufacturing, based on a vehicle lifetime of 120 000 miles. That means that, from an emissions standpoint, you have to drive your new hybrid only about 15 000 miles to reduce your net CO2 output.

Conclusions

I guessed that the energy cost of operating an old vehicle would be much greater than the cost of making a new, fuel-efficient one. The marketing behind new, hybrid cars is slick: it had me thinking about ditching old clunkers in the name of environmental responsibility. It's almost as if there's no corporate muscle behind the message "don't buy a new car while your old one still works." I guess that commercial culture will never miss a chance to tell us to buy something new, even when hiding behind the message "consume less!"

It's true that many new cars will probably make it beyond the 135 000 mile mark, meaning that you could ditch your old car for a new hybrid and rest assured that probably your net energy usage would go down eventually. It's also true that if you're worried about emissions as well as consumption, you would have to drive only about 15 000 miles to break even. Still, the environmental impact of buying additional vehicles, even if they're hybrids, is not insignificant, and should be factored into any decision over "going green" by ditching an old but still usable car.