Greenhouse Gas Emissions from One Gallon Challenge

Just to complete the previous post, here a map of the United States describing the fuel mix used for electricity generation. Source: EPA eGrid page.



Emissions from tailpipe.

Gasoline and hybrid-electric cars emit through their tailpipes. They produce emissions and reduce air quality in the urban centers where population density is high. An electric car does not have a tailpipe. Here is a chart of CO₂ directly emitted by the cars during the One Gallon Challenge. [Based on DOE data, more info at Carbon Dioxide FAQ]


Emissions from well to wheels.

Even though an electric car does not have a tailpipe, an electric car produces greenhouse gas emissions where electricity is generated. These emissions are usually counted as "well to wheel" emissions, that is, a full life cycle of fossil fuels from their extraction to actual process of burning. They are differentiated from "tank to wheel" (or "plug to wheel" for electricity) emissions from the tailpipe.

Gasoline-powered cars, both conventional and hybrids, also produce upstream emissions. Most significant ones are emissions from the oil refineries that convert barrels of oil in gallons of gasoline. [Based on Argonne National Lab GREET]


A few comments:

1. Since we kept track of when we charged the car, we know that natural gas was used as a fuel to produce electricity for our drive. In general, you do not know which fuel was used to produce electricity for your air conditioner or flat screen TV. A common approach is to take US average grid fuel mix with 50% coal, which is almost twice as polluting as New England grid.
2. The 75 MPGe well-to-wheel number quoted in the AutoBlog was based on the US average grid. Had we taken New England numbers, we'd get above 100 MPGe. Had we charged our car with wind or solar power -- no fossil fuels used -- we would have infinite MPGe!!!

As you can see, an electric car is two and a half times cleaner than a gasoline-powered Prius with its strong 52.9 MPG. The main difference is the fuel. Our Porsche effectively used natural gas while Prius used gasoline. Had Prius used compressed natural gas (CNG), its greenhouse gas emissions would be probably about the same or even slightly lower than those from an electric car.

A conventional engine CNG vehicle, for example, the only mass-produced available in the states Honda Civic GX would not be able to rival an electric car because it lacks a hybrid option. There have been prototypes of CNG Hybrids, for example, Toyota Camry CNG Hybrid and Opel/(Saturn) Astra Caravan CNG Hybrid, demonstrated in the past year.

CNG Hybrids or Electric Cars?
As a society, should we focus on electric cars or CNG Hybrids?

Advantages of CNG Hybrids.

1. Price. Electric cars are quite expensive due to the cost of batteries. When our Porsche was built a few years ago, the battery cost for ~100 miles range was $36,000. If the car were mass produced today by a major automotive manufacturer, the battery cost would be closer to $15,000. It is still much more expensive than a hybrid powertrain.

Disadvantages of CNG Hybrids.

1. Leakage. Natural gas leaked from the refueling stations and from the car itself due to leaky hoses is a potent greenhouse gas, four times as damaging as CO₂. Because of that, developing a complex infrastructure for natural gas refueling may end up producing just as much harm to the climate as today's gasoline cars.
2. Renewable energy. Electric grid in the US will become greener with time. Emissions from the grid are already tightly controlled and budgeted (see RGGI, an initiative by ten Northeast and mid-Atlantic states). Addition of renewable sources, such as wind energy or solar power, to the fossil fuel powered grid will inevitably reduce well-to-wheel emissions from electric cars.
   An average car in the US lives for about 15-17 years. An electric car sold TODAY emits as much greenhouse gases as TODAY's CNG Hybrid car. In ten years, an aging CNG Hybrid car will emit the same amount of CO₂ per mile or even more, due to leaky hoses. In ten years, the same electric car will emit less CO₂ per mile because its electricity will be cleaner. Electric cars are future-proof. That is one of the reasons our team is the Electric Vehicle team.