Carbon Footprints ‐ Combustion Versus Electric

Which has a smaller carbon footprint, a gas powered vehicle or an electric car?  Energy in the form of electricity is more valuable than energy in the form of heat.  According to the second law of thermodynamics, all the heat energy in a combusted fuel can not be turned into mechanical energy or a form which is equivalent to electrical energy.  Power plants that burn fossil fuels typically turn 1/3 of the energy in the fuel into electrical energy.  Several articles I have read on electric cars equate heat energy and electrical energy.  They assume all the energy in a gallon of gasoline can be turned into electricity, which can not be done by an engine incorporating combustion.

Fuel cells have the potential of changing a much higher percent of a combustible fuel into electricity.    The energy content of hydrogen for example could be converted to electricity either in a fuel cell or in a combustion engine.  Conversion in a fuel cell would be much more efficient.  Unfortunately only a limited number of fuels can be used in a fuel cell and a number of practical problems with fuel cells are awaiting solutions.

A straight forward way to compare the carbon footprint of a combustion powered and battery powered electric vehicle is to look at how far a car can travel on a gallon of gasoline.  For the electric vehicle, the gas could be combusted in a conventional power plant and 1/3 of its energy could be converted to electricity.  The gallon of gas will produce 11.7 Kwh’s of electricity.  A typical electric car will travel about 3 miles on a Kwh of electricity.  From data given by Nissan, it appears that the Leaf traveling at 55 mph will consume 1 Kwh for 3 miles of travel.  The Leaf will then travel 35 miles on the energy produced from a gallon of gasoline.  A Prius would travel about 50 miles.

A number of factors could affect the relative advantage of the Prius.  An advantage could go to the electric vehicle if cogeneration was used to generate electricity.  With cogeneration, the waste heat generated at the power plant is utilized.  An automobile could also incorporate cogeneration if the driver could store and use the heat generated in the radiator.  Perhaps this heat could be used for space heating or domestic hot water when the driver gets home.  The details would have to be worked out.

In a fossil fuel power plant gasoline is not the typical fuel.  Natural gas is typically used about 30% of the time and coal about 50%.  When the energy equivalent of 1 gallon of gasoline is combusted in the form of natural gas, 32% less CO₂ is put into the air.  When coal is combusted, its one gallon equivalent produces 10% more CO₂ than gasoline.  In the United Sates electricity is also produced with hydro and nuclear power, which produce little CO₂ directly.  The contribution of other renewable sources of energy is very small.  Considering the overall mix of fuel used in the US to produce electricity, an average Kwh of electricity would produce 29% less CO₂ than a power plant burning gasoline.

In the previous example, if the 3 mile / Kwh electric car is plugged into an outlet where the utility has the average mix of generators, the electric car and the Prius will produce about the same amount of CO₂ per mile traveled.  There is currently no clear winner when it comes to a carbon footprint.  Future technological improvements could change this balance to one or the other technologies.

A home owner with a grid tied photovoltaic (PV) system could increase the size of the PV system and generate the electricity needed to run the car renewably.  To produce 1 Kwh/day, the investment in the PV system would be about $2000.  For a 3 mile/Kwh electric car, an investment of $667 would be required to run the electric car 1 mile every day.  If you drive 25 miles per day, the investment in the PV system would be about $17,000.  This cost could be reduced by tax credits.

Instead of using the excess installed capacity to run an electric vehicle, the excess electricity cold be fed into the grid.  In addition, if a vehicle similar to the Prius is purchased, the net effect on global warming will be the same in both cases.  However, the electric vehicle will be a better investment because of the relatively low cost of electricity and the lower taxes on electricity.

Larry Schlussler, Ph.D.