Q: Would a large SUV that is 50 percent electric, 50 percent gas powered use more resources (both from electricity if it's generated by a non-renewable resource, and from the gas itself) than a small station wagon (such as a Volvo)?
Asked by Amanda Schlott Lietman, ’88, Chagrin Falls, Ohio
Sometimes even straightforward-seeming questions require a good bit of number crunching, so I will spell out the details of the calculations. (Engineers, you are back in the game!) Hybrid SUVs on the market do have stated fuel efficiencies, but owners of these vehicles often complain that the vehicle rarely achieves these mileages and there has been controversy over the accuracy of the mileage ratings. So we won't take their word for it, and instead calculate the resources used in a hypothetical, plug-in hybrid SUV from scratch.
In this question we are considering two different vehicles, of two different makes, sizes and propulsion systems. So to make a comparison, we need to standardize "resources used" into something common between both machines. A good way to do this is in terms of pollution—specifically, CO2 emissions. How much pollution is produced for every mile that vehicle travels?
To find this "resource intensity" for an all-electric vehicle, I used the energy requirements of a Tesla Roadster as a model. The Tesla Roadster weighs about 2,600 pounds. It has a maximum range of 250 miles on one charge. And it requires 70-kilowatt hours of energy to fill that charge. If we divide backwards, we see that the Tesla Roadster requires about 1.07*10-4 kWhr/lb./mile. That means it takes about one-tenth of a percent of a kilowatt-hour to move each pound of vehicle one mile.
The 2010 Chevy suburban weighs around 6,000 pounds, so to move all that weight, the electric motor in a hybrid Chevy suburban would consume about 1.07*10-4 * 6000 = .749 kWhr/mile, or about three times more than a Tesla. If you were traveling at highway speeds, you can imagine that every minute this Suburban would be using the same amount of energy as 500 light bulbs! And, remember, electric vehicles produce carbon dioxide, too. Under the current mix of coal, natural gas and renewable energy in our electric grid, generating one kilowatt-hour of electricity produces .605 kilograms of CO2. Multiplying, we get that the electric motor in a suburban would produce about .453 kg of CO2 per mile traveled.
In reality, the suburban would likely require even more energy than this because it is far less aerodynamic than a Tesla, and the wind resistance would force the motor to work harder, especially during acceleration—but for now we will go easy on the hybrids.
Because there is little acceleration on the highway, and at high speeds most of the engine's energy is used to counteract air resistance, the gasoline motor of the hybrid SUV will maintain a similar mileage to the stated mileage of a gasoline-only Chevy Suburban—13mpg on the highway. We use the higher, highway efficiency because we assume that this hybrid vehicle uses 100 percent electric power in the city and 100 percent gasoline power on highways. Again, this is likely overestimating the fuel efficiency of the suburban hybrid somewhat. Every gallon of gasoline produces 8.81 kg of CO2, so the suburban will produce .734 kg CO2/mile while driving on the highway.
The EPA assumes in its mileage averages that a car will drive 45 percent of the time on highways and 55 percent in cities. So, we will veer a little from the straight 50-50 stipulation of the question and use this relationship to average the pollution produced by the hybrid SUV.
55 percent of the time, the Suburban produces .453 kg CO2/mile.
45 percent of the time, the Suburban produces .734 kg CO2/mile.
On average, the Suburban produces .593 kg CO2/mile.
So what did the marine biologist call slow drivers?
Answer: Plankton
I said the Volvo (I assumed a 2010 V50 station wagon) is much easier on the environment—and it is also much easier to figure out why. We can simply take the stated fuel efficiency of 19 mpg (.463 kg CO2/mile) in the city and 31 mpg on the highway (.284 kg CO2/mile) and apply the same driving patterns as we did for the Suburban.
55 percent of the time, the Volvo produces .463 kg CO2/mile.
45 percent of the time, the Volvo produces .284 kg CO2/mile.
On average, the Volvo produces .382 kg CO2/mile.
Voila! The Volvo uses only 65 percent, or about two-thirds, of the energy as our plug in hybrid Chevy Suburban. Even with the simplifying assumptions I used that played in the hybrid's favor and the complete disregard of the massive amounts of embodied energy that you bring along for the ride when you drive a massively built hybrid SUV, the resource efficiency of the Volvo beats that of the Suburban.
Calculating exact numbers for embodied energy is a far more nebulous affair. One must take into account the energy cost of mining and processing the raw materials, the energy used to transport those raw materials to the manufacturing plant, the energy used on the assembly line to put those materials together, and the energy used to ship the vehicle to the dealer's lot. So while numbers for this are frustratingly ballpark-ish, they are also very significant. For example, on average, every kilogram of steel you add to a vehicle will add about 5 kg of associated carbon emissions. If you add 1,000 kg of steel to the weight of a car, you are effectively emitting about another 15,000 miles worth of carbon!
Total embodied energy can account for 15 to 30 percent of a vehicle's total emissions over its lifetime. Embodied energy is an extremely important concept that is too often overlooked. Your hybrid SUV has a huge carbon footprint even if you never drive it. In fact, an eco-conscious consumer should always be looking at the world through the lens of embodied energy. But in the world of transportation there is no way to completely avoid it . . . unless, perhaps, you begin to run to work in the morning! It would sure help us all shed some of our unnecessary mass, but who has the energy?
| Material | Per Embodied Energy MJ/kg |
|---|---|
| Plastics – general | 90 |
| PVC | 80.0 |
| Synthetic rubber | 110.0 |
| Glass | 12.7 |
| Aluminum | 170 |
| Copper | 100 |
| Galvanised steel | 38 |
Chris Fedor, ’10, is an earth systems major.
