MAIL: Hybrid Footprint


In regards to the article “Plug-in Hybrids” in HP121, the author points out that it is more economical to drive a vehicle with batteries recharged with utility electricity than to drive a gas-consuming vehicle. But being more economical does not mean you have a smaller carbon footprint or that you’re burning less primary fuel (usually combustible fossil fuel). Because of inefficiencies in heat engines and distribution losses, typically only about one-third of the energy in a combustible fuel is turned into electrical energy.

The article states that 1 gallon of gasoline is equivalent to 34 KWH of electricity. In practice, 1 gallon of combusted gasoline will produce 11 KWH of electricity, not 34 KWH. One of the comparisons the article makes is between an electric car and a hybrid Prius. In the author’s comparison, an electric car can travel 110 miles on 34 KWH, assuming 34 KWH is the amount of heat energy in a gallon of gasoline. But typically only one-third of this energy can be converted to electricity. The electric vehicle’s mileage is then 110 divided by 3, or 36.6 miles per gallon. The author points out that the Prius at 50 miles per gallon costs twice as much to operate per mile.

With the author’s assumption that most of the electricity generated in the United States is produced using fossil fuels, the Prius will consume 35% less primary energy than the electric car. However, because of the price structure, the electric car will cost less to operate.

The amount of carbon put into the air while generating electricity will depend on the type of fossil fuel consumed at the power plant. Per unit of energy, coal produces 66% more carbon dioxide (CO2) than gasoline, and natural gas produces 20% less. The article points out that half of our electricity comes from coal. If more of our electricity is produced by renewables, the primary energy consumption and carbon footprint will be reduced for the electric car.

One reason for an electric car’s economic advantage is that there are fewer taxes on electricity than gasoline. As a consequence, taxes will go up someplace else to pay for our roads. In the article on my house in HP112, this point is discussed further in regards to gas and electricity consumption for a home. For a home generating more electricity than it consumes, the article looks at the relative merits of feeding this electricity into the grid or using the excess electricity to reduce natural gas consumption. In California, if you generate more electricity than you consume on a yearly basis, you are not paid for the excess electricity you feed into the grid. In my home, I decided to feed my excess electricity into the grid. This further reduced my carbon footprint but resulted in a larger utility bill. My point is that the price structure does not always encourage the most efficient way to consume energy.

Larry Schlussler • Arcata, California

The efficiencies of creating electricity depends on the generator used. A large-scale power plant will be more efficient than a small engine. According to the U.S. Department of Energy, well-to-wheel efficiencies (including all transition stages, from raw materials to motive power at the drive wheels) are 11% for gasoline-fueled vehicles versus 17% for electric vehicles. So whatever portion of its energy a plug-in hybrid took from the grid would be half again as efficient as the gasoline burned in an internal combustion engine (ICE) vehicle.

If you are going to look at the carbon footprint of the grid-supplied electricity of the plug-in hybrid and include the power plant, then you will also need to include the carbon footprint of the gas refinery in your calculations for an ICE car, as well as the tanker trucks that deliver it. Keep in mind that gas refineries use an enormous amount of electricity, so some carbon impact from the electric power plant would have to be applied to the ICE vehicle as well. It is inherently more efficient to use the electricity to directly power a vehicle than to power a gas refinery to make gasoline to power a vehicle.

The scale for greater efficiency (therefore, lower pollution and carbon footprint) proceeds in this order: gas vehicles to hybrids to plug-in hybrids to grid-charged pure EVs to renewable-energy-charged pure EVs.

Shari Prange, Electro Automotive • Felton, California

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