The pair used an electric power system model to explicitly evaluate the change in generator dispatches resulting from PHEV deployment in the Texas grid, and applied fixed and non-parametric estimates of generator emissions rates, to estimate the resulting changes in generation emissions. They found that by using the flexibility of when vehicles may be charged, generator efficiency can be increased substantially.

For the study, they assumed each PHEV battery had an energy storage capacity of 9.4 kWh, corresponding to an electric-only driving range of about 35.9 km (22.3 miles), depending on the vehicle class. Average energy use over drive cycle was assumed to be 23 km/l and 59 Wh/km (54 mpg and 95 Wh/mile). Charge-depleting electric energy use was assumed to be 0.183 kWh/km (0.295 kWh/mile).

In this analysis we focus on the use of PHEVs to provide spinning reserves, capacity from generators that are online but reserved specifically to respond to unforeseen increases in electricity demand or generator outages. 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. Our analysis assumes that the power system includes smart grid controls which will charge and discharge PHEV batteries depending on the cost of conventional generation and the need for ancillary services.

—Sioshansi and Denholm (2009)

Their analysis focused on the three pollutants CO₂, SO₂, and NO_x. Emissions of CO₂ and SO₂ are tracked on an annual basis, while NO_x emissions are tracked during two periods: an ozone season (May through September) and a non-ozone season (the remaining months).

They broke down generation-related emissions into generator emissions and upstream emissions from fuel extraction and transportation. Vehicle emissions were broken down into tailpipe emissions and upstream refinery emissions. Tailpipe emissions of CO₂ and SO₂ are determined based on the carbon and sulfur content of gasoline. While the carbon content of gasoline is fixed, the sulfur content depends upon the refining process and is generally subject to environmental regulation.

Sioshansi and Denholm used EPA’s Tier 2 requirement that gasoline sulfur content be below 30 ppm to estimate the tailpipe emissions rate of SO₂. In comparing tailpipe emissions of NO_x from PHEVs to CVs and HEVs, they assume that CVs and HEVs will be designed to meet the Tier 2 NO_x requirements (< 0.07 g/mile, or 0.043 g/km).

Without V2G services, they found that PHEV charging loads increase generator emissions of CO₂ and SO₂, with marginal CO₂ emissions rates of between 582 and 935 kg/MWh and marginal SO₂ emissions rates of between 0.9 and 1.2 kg/MWh. NO_x emissions from generators decrease during ozone season, however, due to the load-shifting and generation efficiency improvements caused by the flexibility in PHEV charging.

Adding V2G services reduces generator emissions of CO₂ and SO₂, and can also reduce generator emissions of NO_x beyond the reductions achieved through load-shifting.

At the 1% PHEV level in the fleet, V2G services eliminate more than a quarter of generator emissions of CO₂ from introducing the PHEV fleet without V2G services. The reduction of CO₂ and NO_x emissions is much larger than that of the SO₂ emissions. The reason for this, the researchers said, is that without V2G services, spinning reserves are typically provided by natural gas-fired generators, since their generation is more expensive than coal-fired generation. As such, if both a coal- and natural gas-fired generator have capacity available, it is more economical to reserve the capacity of the natural gas-fired generator and use the coal-fired generator to provide lower-cost energy.

When PHEVs provide spinning reserves, they tend to reduce the need to keep natural gas-fired generators online. The low sulfur content of natural gas implies that V2G services will have more of an impact in reducing CO₂ and NO_x emissions as compared to SO₂.

The results of this analysis suggest that PHEVs can play a role in decreasing transportation-related emissions by using electricity as a source of energy, while the provision of V2G services can result in even more substantive emissions reductions. Moreover, the flexibility in choosing when to recharge PHEV batteries can have a noticeable impact on generator emissions—in the case of Texas reducing generator emissions of NO_x below the levels there would be without any PHEVs, despite the fact that generating loads are higher.

Even more importantly, this reduction in NO_x emissions takes place during ozone season, when the environmental impact of NOx tends to be highest. Our results showed that because coal-fired generation served at least a fifth of the PHEV charging loads, and due to the high SO₂ emissions rates of Texas coal generators, the net impact on SO₂ emissions would be an increase above emissions from CVs and HEVs.

This shows that the emissions impacts of PHEVs will be highly sensitive to the generation mix, and it may be prudent for future vehicle charging loads to be taken into account when generation investment is undertaken.

...Our analysis showed that V2G services can reduce generator emissions and make PHEVs more environmentally attractive in terms of total vehicle emissions. 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. The impact of V2G on SO₂ is less than on CO₂, since most of the effect of V2G is to reduce the system’s reliance on gas-fired generators, which have low SO₂ emission rates.

—Sioshansi and Denholm (2009)

Resources

• Ramteen Sioshansi and Paul Denholm (2009) Emissions Impacts and Benefits of Plug-In Hybrid Electric Vehicles and Vehicle-to-Grid Services. Environ. Sci. Technol., Article ASAP doi: 10.1021/es802324j