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Fermi problem: embedded energy of cars

In online debates, I've often seen some doom-laden people say "maybe solar can meet our day to day needs, but there's all the embedded energy in stuff, we'll never make that up." The classic example is the claim that solar panels don't pay for their own creation, which I'm fairly sure is false by now, if ever true. But today I'll talk about cars: how does the energy of a car compare to moving it around?

Say the cas lasts 100,000 miles, and is particularly efficient at 40 mpg, so uses 2500 gallons in its lifetime. A gallon is about 4 liters or 4 kg of water, so we're talking about 10,000 kg of gasoline. The car probably weighs 1-2 tonnes, toward the lower end if it's getting 40 mpg. Even if the car were made of air-synthesized gasoline, it'd still be a small fraction of the lifetime energy cost, and it's not, it's made of stuff like "turn iron oxide into iron". Ideally speaking, nothing in a car is going to compare to the energy density of gasoline, though it's possible processes aren't ideal.

A factor of 10 is close enough to be worth more precision. A gallon of water is actually 3.7 kg, and oil is lighter than water -- 70% the density, even. So 2.6 kg/gallon gasoline, and 6500 kg for the total. And this gives 1.3-1.6 tonnes for the weight of compact to midsize cars. So, the lifetime gasoline weighs 4.3x as much as the car, and is still probably a lot more energy intensive.

Another thing that gets brought up is moving parts around in shipping and manufacture, how components of something might have made a few trips across the Pacific or world among them. Mexican iron ore to China to become steel to become a car in the US, say. For other goods that might be significant, but here we're talking about a car, which by assumption already moves 100,000 miles under its own inefficient power. If we grant a round trip across the Pacific, of maybe 19,000 miles, that's still 1/5 the distance, and container ships are far more energy efficient than a car's engine. I'd guess 10%, making the parts-transportation energy more like 2% of the car's lifetime total. Which suggests it might be significant for things that aren't cars. Then again, Without Hot Air says shipping can be 1.5% the energy of road transport, far smaller than even my guess.

Wait! One last check. Without Hot Air again says a car's embedded energy is 76,000 kWh, which would be 2.7e11 Joules, or the equivalent of 6750 kg of gasoline. That's way more than my estimate, directly comparable to the amount burned moving the car around. Wikipedia says the same thing, but it's quoting the same source (Treloar et al.) OTOH, says "the Union of Concerned Scientists (UCS), pointed out that a common life-cycle assessment calculation is that 85 percent of embodied energy use associated with a conventional vehicle’s life cycle is attributable to operation and 15 percent is attributable to manufacturing and disposal". This says "on average, every kilogram of steel you add to a vehicle will add about 5 kg of associated carbon emissions." and "Total embodied energy can account for 15 to 30 percent of a vehicle's total emissions over its lifetime." and gives a table of energies, such as 38 MJ/kg for galvanized steel, which is pretty much the same as gasoline. Aluminum and plastics or rubbers are even more. And this gives a longer table, with slightly but not hugely different numbers.

So that's a range of estimates, from maybe 1:6 to 1:1. Looks like my naive chemistry failed and stuff is a lot closer to gasoline density in manufacture than I thought. Not surprising for plastic (it's solid oil) but I thought steel would be cheaper than that. Of course, at less than 40 MPG the notional car would be burning more gasoline, but still; for cars of that weight that's likely no more than a factor of 2.

Except for one last complication: I was using the chemical energy density of the physical gallon of gasoline. Total energy cost of operating a car would include mining the crude oil and refining it into gasoline, which adds at least a bit.

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Damien Sullivan

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