Saturday, November 15, 2008

Emissions from using electric vehicles

The U.S. Environmental Protection Agency is currently constructing "miles per gallon" ratings for electric vehicles, in what has become a political rather than a scientific exercise. While we cannot evaluate electric-powered and hybrid-electric vehicle efficiencies and gasoline-powered vehicle efficiencies on exactly the same basis, we can compare carbon dioxide emissions caused in powering them.

The hucksters for electric vehicles encourage people to think of them powered by low-polluting sources. Decades from now they might be. But today, at any given time, the U.S. electrical grid rarely has spare capacity from nuclear, hydro or renewable power. Added demands for electricity, demands that might have been met with some other form of energy, will usually cause coal-fired generators to come on line and run harder. Vehicles that might have been powered by burning gasoline will instead be powered by burning more coal.

The 2009 Tesla Roadster is rated as consuming 28 kWh of external electrical energy to travel 100 miles [1] on an EPA combined-cycle test. The 2010 Chevrolet Volt was measured to consume 8 kWh of internal electrical energy to travel 40 miles [2] under unstated conditions, or 20 kWh per 100 miles. We do not know the added amount of external energy consumed by charging and battery losses.

The U.S. government most recently rated average carbon dioxide emissions for coal-fired power [3] at 2.1 lb/kWh. This yields carbon dioxide emission estimates for the Tesla and for the Volt, when battery powered, as follows:

Tesla 2.1 * 28 / 100 = 0.59 lb/mi carbon dioxide
Volt 2.1 * 20 / 100 = 0.42 lb/mi carbon dioxide, plus losses

The Honda Civic is a popular, efficient, gasoline-powered vehicle. Carbon dioxide emissions have been measured for the 2009 Hybrid model at 109 g/km [4] and for the 2009 conventional Civic at 159 g/km [5] or, in U.S. units:

Civic 1.609 * 159 / 453.6 = 0.56 lb/mi carbon dioxide
Hybrid 1.609 * 109 / 453.6 = 0.39 lb/mi carbon dioxide

Thus, when powered by the U.S. electrical grid, those electric vehicles, while avoiding use of gasoline, result in more carbon dioxide emissions than efficient gasoline-powered vehicles.

[1] Tesla Motors, 2008, at

[2] General Motors, 2008, at

[3] U.S. Office of Energy Efficiency and Renewable Energy, Carbon Dioxide Emissions from the Generation of Electric Power, 1999 (Table 1), at

[4] ZerCustoms, 2008, at Also rated by GreenConsumerGuide, 116 g/km, 2008, at

[5] WhatGreenCar, 2008, at

There's much wonkery around, strong on philosophy. What comes out the smokestacks pays no heed. As demands come to the grid, interconnects respond. Renewables are maxed out by regulation, Nuclear and hydro are almost always saturated, priced for base load. In a few areas combined cycle sometimes has capacity but depends on imported gas. Almost always the low-bid reserve is coal, so what comes out the stacks is more coal smoke. Build millions of windmills, and at some point we see nuclear shutins and wind reserve, maybe in 50 years. The promise of electric vehicles that can be operated now is not to improve the environment but to replace imported fuels.


Tom Saxton said...

I see you have corrected the math errors in the original version of this post that had the CO2 emissions from the two Civic models too low by a factor of 5, so you had to scale back your conclusion from "EVs produce four to seven times" the emissions to simply "more."

That was a nice improvement.

However, you still have a large error in your analysis: coal is not the only source of incremental power on the US grid. Natural gas is also a big contributor and 40% cleaner than coal.

A more careful DOE-sponsored analysis found that there is enough excess off-peak capacity in today's electrical grid to power the conversion of "up to 84% of U.S. cars, pickup trucks, and sport utility vehicles (SUVs)" to plug-in hybrids with a 33-mile pure electric range. That's about 200 million vehicles.


Under that scenario, the study found that overall greenhouse emissions would decrease by up to 27%. Emissions of volatile organic gases and carbon monoxide would drop over 90%. Without improving generation technology, particulate and SOx emissions would increase, but we have a lot of time to solve that problem. It's a lot easier to solve a pollution problem at a few hundred power plants than it is with hundreds of millions of tailpipes.

Even with full commitment by the world auto industry, it would take decades to reach that level of conversion to EV/PHEV drive vehicles. During that time, our electrical grid will surely evolve to greener energy and electric vehicles will become greener in lockstep with those improvements.

If we stick to gas-powered vehicles, we'll never be able to power them from clean renewable energy sources like wind, solar and geothermal. We really don't want to be stuck with that problem forever.

We need to start the slow conversion to electric vehicles now, so that by the time our grid is cleaned up, our transportation sector will be also. The modest benefits we get now are just icing on the cake, an extra incentive to do the right thing for future generations.

Plus, electric vehicles are a blast to drive.

Tom Saxton said...

The URL for the DOE-sponsored study on powering EVs and PHEVs with today's US electrical grid got messed up.

Here are links to the summary and the full report.

I apologize for the mistake.