UC Davis Study Finds That Near-Term Marginal Electricity Mix in California for Plug-in and Fuel Cell Vehicles Will Result in Fuel With Carbon Levels More Than 60% Higher Than Estimated in the LCFS
|Well-to-wheels vehicle emissions (gCO2 equiv.km-1) by energy source, vehicle energy intensity (MJkm-1), and fuel carbon intensity (gCO2 equiv. MJ-1) by vehicle pathway and timing profile. Source: McCarthy et al.Click to enlarge.|
A study by researchers at the UC Davis Institute of Transportation Studies suggests that the near-term marginal electricity mix for plug-in electric and fuel cell vehicles and fuels in California will come from natural gas-fired power plants, including a significant fraction (likely as much as 40%) from relatively inefficient steam- and combustion-turbine plants. The marginal electricity emissions rate will be higher than the average rate from all generation—likely to exceed 600 gCO2 equiv.kWh-1 during most hours of the day and months of the year—and will likely be more than 60% higher than the value estimated in the Low Carbon Fuel Standard.
The study also concluded that despite the relatively high fuel carbon intensity of marginal electricity in California, alternative vehicle and fuel platforms still reduce emissions compared to conventional gasoline vehicles and hybrids, through improved vehicle efficiency. The study will be published in the April 2010 issue of the Journal of Power Sources, and is currently available online.
This study used an hourly electricity dispatch model with plant-level detail—the Electricity Dispatch model for Greenhouse gas Emissions in California (EDGE-CA)— to simulate grid response to added vehicle and fuel-related electricity demand in the state in 2010. The authors developed hourly electricity demand profiles for seven vehicle and fuel pathway scenarios. Conventional ICEs (internal combustion engines) and HEVs (hybrid electric vehicles) are compared to PHEVs (plug-in hybrids), BEVs (battery-electric vehicles), and FCVs (fuel cell vehicles). Fuel cell vehicle pathways include hydrogen produced at refueling stations from either electrolysis or SMR. The model identifies the “marginal electricity mix”—the mix of power plants that is used to supply the incremental electricity demand from these vehicles and fuels—and calculates greenhouse gas emissions from those plants.
It also explores sensitivities of electricity supply and emissions to hydro-power availability, timing of electricity demand (including vehicle recharging), and demand location within the state.
Because electricity cannot be practically stored in significant quantities, the grid has evolved to meet continually changing electricity demands by using a suite of power plants that fulfill various roles in the grid network. Each type of power plant operates differently—using different size, technology, or energy resources to satisfy its function—and as a result, each has unique cost and emissions characteristics.
...Electricity generation must match demand continuously, and adding electricity demand from vehicle recharging or hydrogen production and refueling will require additional power to be generated. The key to identifying the marginal mix of electricity for vehicles and fuels is to understand which power plants will generate this additional electricity.—McCarthy et al.
Overall, the study found that electricity demand from these vehicles would have a minor impact on overall demand. If 1% of VMT were to come from FCVs using grid electrolysis—“an unlikely near-term scenario”—total electricity demand increases by 0.7% and peak demand increases by 1%. Demand impacts from the other profiles are much smaller.
Among the findings on vehicle emissions were:
All of the pathways except for FCVs using hydrogen from electrolysis reduce GHG emissions compared to ICEs and HEVs.
Fuel cell vehicles using hydrogen from SMR (steam methane reforming) and BEVs recharging according to the load-level profile reduce emissions the most, by more than 25% compared to HEVs.
Battery-electric vehicles recharging according to the Offpeak profile reduce emissions by 21% compared to HEVs.
Driving a PHEV20 offers little emissions improvement compared to HEVs, only 3% in the Offpeak profile and 6% for the load-level profile.
The reduction in emissions from advanced electric-drive vehicles in the near-term is a result of improved vehicle efficiency, rather than reduced carbon intensity of fuel. None of the pathways here use “low carbon fuel,” compared to gasoline in the near term (although there is potential to do so in the future).
In the base case of BEVs recharging according to the Offpeak profile, for example, the carbon intensity of electricity is 80% higher than that of gasoline, but BEVs use less than half as much energy, and are lower emitting than HEVs.
These findings counter the assumptions for marginal electricity included in the LCFS rulemaking. The statue assumes that marginal electricity comes from NGCC plants (79%) and renewable power (21%), with a GHG emissions rate of 104.7 gCO2 equiv. MJ-1, or 377 gCO2 equiv.kWh-1.
But in the near-term, the likely marginal mix and GHG emissions rate will be quite different. Renewable power does not operate on the margin and marginal generation from dispatchable power plants is unlikely to come entirely from NGCC plants operating with average heat rates. Rather, NGCT plants will supply an important fraction of marginal generation, and when NGCC plants do operate on the margin, they will likely have a higher heat rate and GHG emissions rate than average NGCC generation.
Assuming that the Offpeak profile represents likely near-term charging, the results here suggest that the marginal generation mix will be about 63% from NGCC plants and about 37% from NGCT plants, and marginal emissions rates will be more than 65% higher than in the LCFS. Vehicle emissions, then, are underestimated by a similar fraction for BEVs, and by 11–25% for the PHEV pathways. These findings, as discussed, are sensitive to a number of parameters.
...The results presented in this paper describe the emissions implications of using electricity as a fuel or as an input for hydrogen production from the current grid. Over time, the carbon intensity of the grid will decrease, as energy policies promote renewable generation or impose costs on GHG emissions, and as older power plants are retired and replaced with newer, more efficient ones. In the future, the carbon content of electricity supplying vehicles and fuels could be much lower than it is currently.—McCarthy et al.
McCarthy, Ryan W. and Christopher Yang (2009) Determining marginal electricity for near-term plug-in and fuel cell vehicle demands in California: Impacts on vehicle greenhouse gas emissions. J. Power Sources doi: 10.1016/j.jpowsour.2009.10.024