Not surprisingly, coal without carbon capture is the worst power source, and even "advanced coal burning" isn't much better. And as the source of 43% of U.S. electricity, it must be taken seriously.
Two Reasons for Hope: Night Charging & Other Power SourcesA study on plug-in hybrids by the Oak Ridge National Laboratory shows that if charged at night, off peak, the vehicles might not put much additional strain on the grid. That's because coal plants cannot be shut down and restarted rapidly, so they generally don't stop producing at night. So while it is true that a plug-in hybrid charged by coal-generated electricity would generate more CO2, the alternative might be to burn burn that coal anyway and get zero miles out of it. Plug-ins could actually make the grid more efficient by putting more of the electricity generated to productive use.
The second reason is that if coal represents 43% of production in the U.S., that means that there's 57% of things that aren't as bad as coal. A lot of it is natural gas, nuclear, and hydro. Wind power, solar and geothermal still represent a small fraction of the total, but they are the fastest-growing energy sector.
This means that plug-ins not charged with coal power would still be doing quite a bit better than regular hybrids, especially if you drive less than the electric range between charges, and as time goes on more of the grid should get power from clean sources. There is also a benefit to air quality when you move the emissions from millions of tailpipes that are close to people's lungs to a few centralized power plants. Ideally all energy would come from renewables, but even in the meantime, we might get clean air benefits.
Outside the U.S.
It also means that the environmental performance of plug-in hybrids will vary from country to country, and even from region to region. You can find out about where your power comes from by contacting your local utility, and in many places even if your local grid is dirty, you can switch to green power.
Plug-in Hybrids Will Keep Getting Better
Another question we might ask ourselves is: What model of plug-in did they base the chart on? Chances are, as time goes on and as oil prices rise, batteries and ultracapacitors will improve, more advanced light materials will be used (such as carbon fiber), there might be breakthroughs in nanotechnologies, aerodynamics will improve, etc. So future plug-in hybrids might be quite a bit more efficient than the one this chart is based on.
And for those worried about all these battery packs from plug-in hybrids, know that they can be made from recyclable and non-toxic materials.