Modelling the Impact of PHEVs on Ozone in Denver

Researchers at the University of Colorado, Boulder and the National Renewable Energy Laboratory (NREL) modeled the emissions impact had plug-in hybrid electric vehicles (PHEVs) replaced light duty gasoline vehicles in the Denver, Colorado area in summer 2006.

Denver represents an interesting case study for exploring potential PHEV impacts because it violates the federal air quality standard for ozone and because several large EGUs [electricity generating units] are located in or near the urbanized area.

—Brinkman et al.

This study models a single urban area with a finer grid resolution (4 km) than has been considered in previous PHEV studies, and also incorporates plume-in-grid treatment for improved tracking of plume dispersion. The ozone response to PHEV penetration was modeled under a wide range of meteorological conditions, as experienced during July 2006.

In their study, published online 15 June in the ACS journal Environmental Science & Technology, they found that with 100% PHEV penetration, nitrogen oxide (NOx) emissions would have been reduced by 27 tons per day (tpd) from a fleet of 1.7 million vehicles and would have increased by 3 tpd from power plants. VOC emissions would have been reduced by 57 tpd.

A unit commitment and dispatch model was used to estimate the charging patterns of PHEVs and dispatch power plants to meet electricity demand. Emission changes were estimated based on gasoline displacement and the emission characteristics of the power plants providing additional electricity.

These emission changes reduced modeled peak 8-h average ozone concentrations by approximately 2-3 ppb on most days. Ozone concentration increases were modeled for small areas near central Denver.

The Comprehensive Air Quality Model with extensions (CAMx) was used to simulate the effects of these emissions changes on ozone concentrations. Natural gas units provided most of the electricity used for charging PHEVs in the scenarios considered.

This study for Denver focused on impacts on ozone concentrations, due to Denver’s nonattainment status and interest in the potentially nonlinear responses this pollutant can exhibit. For other areas, where PM2.5 concentrations are higher than in Denver and where coal-fired power plants may contribute more of the marginal electricity supply, modeling effects of PHEV use on PM2.5 may also be warranted, especially if emissions caps are not in place to mitigate this impact.

In future work, additional research is needed to refine estimates of PHEV emissions, including understanding the importance of cold-start conditions. Further research is also needed to forecast the timing of PHEV penetration and better anticipate the characteristics of the power plant and vehicle fleets that might be in place when significant PHEV use might occur. Such projections will have high uncertainty, because without having PHEVs in mass production it is difficult to estimate their emission characteristics and added upfront costs, including the cost of battery production. Over the long-term, it is possible that significant PHEV penetration could affect how the generation fleet develops, possibly by leading to more base load coal capacity. Finally, future studies should pursue dispatch modeling that extends beyond the service territory of a single electric power utility to examine the effects of more widespread introduction of PHEVs.

—Brinkman et al.


  • Gregory L. Brinkman, Paul Denholm, Michael P. Hannigan and Jana B. Milford (2010) Effects of Plug-In Hybrid Electric Vehicles on Ozone Concentrations in Colorado. Environ. Sci. Technol., Article ASAP doi: 10.1021/es101076c

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