Last week we saw that doing right by the environment is more complicated than simply downsizing the carbon footprint. Shifting to soot-free power sources will reduce mortality in cities, we noted, while increased use of variable wind power could jack up emissions of smog-forming NOx from the ‘peaking power’ plants that ramp up and down to balance electrical supply and demand.

Now power grid modelers at the National Renewable Energy Laboratory add another twist to this story with a report that electric vehicles (EVs) plugged into the grid can reduce NOx emissions and possibly more. Their report “Emissions Impacts and Benefits of Plug-In Hybrid Electric Vehicles and Vehicle-to-Grid Services” appears in the January 22 issue of the journal Environmental Science & Technology.

Ramteen Sioshansi (now at Ohio State University) and NREL’s Paul Denholm found that putting electric vehicles on the grid had the opposite effect of adding variable renewable energy sources: whereas wind turbines put extra stress on the power plants balancing the grid’s supply and demand, electric vehicles can act as shock-absorbers.

Plug-in hybrid vehicles accounting for just 15% of the U.S. car fleet, for example, could provide enough buffer in the electrical signal to cut NOx emissions during the summer ozone season. And that despite the need to generate extra electricity to recharge the plug-ins. As the authors put it, the PHEVs can behave like extra ‘spinning reserves’ to take strain off the peaking plants:

When PHEVs act as a source of spinning reserves, they allow the system to operate more efficiently, decreasing the emissions from peaking units and partially loaded power plants currently used to provide ancillary services.

Adding more sophisticated vehicle-to-grid (V2G) controls that enable the grid to direct charging and discharging of EVs can cut NOx emissions further and also reduce emissions of CO2 and sulfur dioxide, by enabling grid controllers to dial-in use of the most efficient power plants.

V2G services can substantially reduce generator emissions of CO2, in some cases eliminating more than 80% of the increase in generator emissions of CO2 from introducing the PHEV fleet.

One caveat, just to make life more complicated: the authors predict that emissions impacts of PHEVs will be highly sensitive to the mix of power generators on a given section of the grid. The results above are for Texas.

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This post was created for Energywise, IEEE Spectrum’s blog on green power, cars and climate