This week David asks “how much more energy efficient does a new car have to be to make up for the energy of production vs a used car? For example, if someone was considering buying a used car that gets 18 mpg vs. a new car that gets 30 mpg. At what point in driving would that increase in mpg make up for the energy of production of the new vehicle?” Read on to find the answer in this week’s AskPablo.
The Argonne National Lab has done a great job in analyzing the material intensity and energy consumption in manufacturing vehicles and vehicle fuels. Their work is packaged in the GREET 1.7 and 2.7 models (Excel-based and available at: http://www.transportation.anl.gov/software/GREET/). According to the assumptions in their model the average conventional internal combustion engine vehicle (ICEV) is made up of 61.7% steel, 11.1% iron, 6.9% aluminum, 1.9% copper/brass, 2.9% glass, and around 13.6% plastic/rubber. This data will help us determine the energy required to produce a vehicle. We will also have to look at the energy used in operating a vehicle. To help us along in our analysis we will look back at AskPablo: Lighten the Load to find the relationship between vehicle weight and fuel efficiency.
According to the GREET model it takes 100.391 mmBTU (million BTU) to make the vehicle, batteries, and fluids in an average 3,201 pound vehicle. This comes out to 31,362 BTU/lb. The obvious lesson in this is that heavier vehicles require more energy to make than lighter ones, in general. There has been a study circulating that states that hybrids are more environmentally damaging than Hummers because of the battery production but this has been widely disputed. According to the GREET model a Hybrid Electric Vehicle (HEV) that weighs 2,632 pounds requires 101.726 mmBTU to make, or 38,650 BTU/lb. As we will see, this small difference in production energy becomes negligible when you factor in the increased fuel efficiency.
Using the GREET assumptions I will compare several vehicles, a Hummer H2, a Toyota Prius, and the Toyota Highlander (standard and hybrid). I will use the vehicle’s published curb weight to determine the energy used in manufacturing, based on the mmBTU/lb factors above and I will use the average MPG (city and highway) to estimate fuel usage over a 160,000 mile lifespan. The energy required to manufacture the vehicles is:
- Hummer H2: 200.717 mmBTU
- Toyota Prius: 113.322 mmBTU
- Toyota Highlander: 107.133 mmBTU
- Toyota Highlander Hybrid: 155.18 mmBTU
Gasoline contains 113,500 BTU (0.1134 mmBTU) per gallon. By dividing the expected lifespan of a vehicle (160,000) by its average MPG we can determine the gallons of gasoline used over that lifetime. We can also multiply this by the energy content of the fuel to get the total energy used. The gallons used during a 160,000 mile lifespan and the energy contained therein is:
- Hummer H2: 13,913 gallons ($44,800 at today’s prices!), 1579.13 mmBTU
- Toyota Prius: 2,883 gallons, 327.207 mmBTU
- Toyota Highlander: 6,400, 726.4 mmBTU
- Toyota Highlander Hybrid: 5,424, 615.593 mmBTU
So, in comparison, 89% of the energy consumed by a Hummer H2 is in burning fuel, whereas the Toyota Prius uses 74% of total energy on burning fuel. This means that, in relation to weight, the Prius requires more energy to manufacture, but the Hummer uses far more energy to operate. What we also learn (click on the graph to enlarge) is that a Hummer H2 uses more energy in the first 24,000 miles (roughly 2 years) than the Prius will in its entire lifetime.
Here is my advice, David: Continuing to drive an older car with poor fuel economy is less environmentally friendly than getting a new car that gets drastically better fuel economy. You can take my factors above and calculate the exact energy use for your old vehicle and a new vehicle to see the comparison. Keep in mind that these results are for the energy used, not the carbon dioxide emissions, but the two are highly correlated since most of our energy comes from fossil fuels.
Pablo P√§ster, MBA