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Energy Numbers in Transportation

Energy Numbers in Transportation

Energy (and greenhouse gas) calculation in transportation is hard. There are many forms of transportation, and many forms of energy. One top standard to work for is the "well to wheels" standard, which tries to account for all energy consumed, and emissions made, from fuel in the ground all the way to a moving vehicle.

That's not easy though, and in many ways it's not enough. For it is also fair to consider the energy cost of making vehicles -- which we can then ammortize over a vehicle's lifetime miles, perhaps -- and even the energy cost of making roads or tracks. There are a lot of factors.

An Electric Train or Electric Car

  1. Coal, most commonly, is mined out of the ground. Alternately natural gas is extracted from the ground.
  2. The coal is shipped to a power plant, or gas is sent by pipeline. That shipping uses transportation energy.
  3. In the power plant, fuel is burned, and we know how many BTUs that generates. This turns a generator to make electricity. Power plants range from 25% to 55% efficient at doing that, and the DoE puts the average at under 40%.
  4. The electricity is then sent over wires. Losses here average about 7%. For electric cars, the energy then goes through a charger and is stored in a battery, which incurs additional loss, but not a great deal.
  5. The electricity enters an electric train and powers its motor. That motor is pretty efficient at turning the resulting electicity into motion.

A Gasoline Car

  1. Oil is pumped from the ground and transported to a refinery.
  2. The refinery cracks the oil into various components, including those that are mixed to make gasoline.
  3. The gasoline is then piped or trucked to gas stations. The total for these three steps, sources report, delivers 82% of the well energy to the gas station.
  4. In the car, the gasoline is burned to turn the engine. This is less efficient than the big electric generator, but it turns the wheels.
  5. In a hybrid car, some of the energy charges a battery that later turns an electric motor. In a plug-in hybrid, grid energy from the process above goes into the battery.

My essay uses a less pure "tank to wheels" which examines the fuel burned in a liquid fuel vehicle and the fuel burned in an electric power plant. So I am starting at step 3. The oil-to-gas refine and deliver process is 82% efficient. I don't have a number for the mine-to-power plant efficiency of coal, but I would suspect it's better. However, the oil-to-gasoline process is efficient enough that this doesn't make a big difference the way many people think. I thus think it's fair to compare BTUs of fuel burned (in a gas tank or power plant) per mile of transport as a result.

I would like to get reliable analysis from well to wheels, but I am comfortable that it doesn't alter the numbers by more than about 15-20%. (Some gasoline, such as from syncrude from the Athabasca tar sands, has a much lower well-to-tank efficiency. We don't use too much of that, yet.)

I don't yet analyse vehicle construction energy. I've seen figures suggesting around 120 million BTUs to make a typical car. Cars contain a lot of steel. They tend to go about 150,000 miles in their lives, so that's about 800 BTUs per mile or 500 per passenger mile. Not trivial, but again at most only a 15% increase. I would be interested in figures on the energy requirements to build train cars and buses, and their lifetime passenger miles, but even if we assume it's all zero, the whole balance is only shifted up 15% for the cars.

I don't have any figures on energy consumption to build roads and the passenger-miles that roads deliver in their maintenance lifetimes. Ditto for rails. Of course city buses and street cars need roads (or roads plus rails).

Other sources of electricity

By comparing energy per passenger mile, I am not considering that about 30% of the U.S. electrical grid does not emit greenhouse gases. Most (2/3) of that green 30% is nuclear. It's beyond the scope of this article how green you judge nuclear to be. Some think it's the green answer, others think it's the worst horror in energy. The rest of that green power is hydro. Again, some think hyrdo is wonderful green power, others think it is, at least at many sites, a valley and wildlife destroying ecological catastrophe.

Sadly, very little of U.S. electricity comes form the more purely green sources like solar, wind and geothermal.

Depending on what you think of nuclear and hydro, you can increase or decreaase how much favour you apply to the electric forms of transport over the fossil fuel burning ones. But at least on average, 70% of electric transport has to be viewed as fossil fuel burning -- and in fact with coal, burning the nastiest of the fossil fuels. Coal burning emits all sorts of nasty pollutants, including radioactive ones, and coal mining kills a fair number of people each year, too.