Thursday, September 27, 2007

Chevrolet Volt, a plug-in polluter

In January, 2007, General Motors announced an electric vehicle, the Chevrolet Volt. At that point, GM had no working product, just a concept with a set of performance estimates. The vehicle was to be powered by a 121 kW peak (162 HP peak) electric motor drawing from a 16 kWh battery that could be discharged up to 70 percent. GM estimated that the vehicle would travel 40 miles at highway speeds on one such battery charge. Batteries can be charged from the electrical power grid. The vehicle was also to include a 53 kW (71 HP) gasoline engine driving a generator, estimated to yield 50 miles per gallon when it supplies the electricity to power the vehicle.

From these performance estimates, one can compare fuel costs and greenhouse gas emissions powering this vehicle from the gasoline pump versus the electrical power grid. Multi-stage inverters charging batteries from the electrical power grid are about 80 percent efficient in storing grid energy. Consider the estimated 40-mile range from one battery charge:

Gasoline using the generator: 40 miles / 50 miles per gallon = 0.8 gallon
Electricity using the power grid: 70 percent * 16 kWh / 80 percent = 14 kWh

For June, 2007, U.S. Department of Energy data show average retail prices of $3.06 per gallon for regular gasoline and $0.111 per kWh for residential electricity. Energy costs for 40 miles of travel:

Cost of gasoline: $3.06 * 0.8 = $2.45
Cost of electricity: $0.111 * 14 = $1.55

The U.S. Environmental Protection Agency estimates carbon dioxide emission from burning gasoline is 19.4 pounds per gallon and estimates U.S. average carbon dioxide emission from generating electricity is 1.37 pounds per kWh. Greenhouse gas emissions for 40 miles of travel:

Carbon dioxide from gasoline: 0.8 * 19.4 = 15.5 lb
Carbon dioxide from electricity: 14 * 1.37 = 19.2 lb

If this vehicle were driven 10,000 miles per year (about 40 miles per workday), powering it from the electrical grid instead of the gasoline pump would save about $200 per year in energy costs but emit about 925 pounds more carbon dioxide. The vehicle is estimated to cost about $5,000 to $10,000 more than a conventional gasoline-powered vehicle, so the cost recovery time from using household electricity would be at least 25 years.

Powering the Chevrolet Volt from the electrical grid is mainly burning coal instead of gasoline. The gasoline efficiency estimated for the vehicle is more than a match for the efficiency of electrical power generation, transmission and conversion. The energy cost difference comes mainly from not paying taxes on gasoline and instead burning largely untaxed coal. When powered from the electrical grid, the Chevrolet Volt becomes a small energy cost saver and a net polluter.