In April, 2012 the Union of Concerned Scientists (UCS) published a report entitled “State of Charge: Electric Vehicles’ Global Warming Emissions and Fuel-Cost Savings across the United States”[1]. The report discussed the electrical generation and distribution system in this county, that is, the “grid”, relative to plug-in cars. Early on in the report the power grid network is shown divided into 26 grid regions (Figure 1). UCS grades the 26 regions relative to the efficiency of electric cars because different areas of the country have different fuel sources for their electricity, resulting in different amounts of CO₂ emissions for the same amount of electricity generated. For example, coal generates more CO₂ for a kilowatt/hour of electricity than natural gas. In areas with a high percentage of hydroelectric power, for example the Pacific Northwest, there is much less CO₂ generated for a kWh of electricity then in the rest of the county.

Figure 1: U.S. 26 Electricity Regions

The report divides the county into good, better, and best categories. The middle of the country is mostly designated good (purple color), the south and mid Atlantic is mostly designated better (dark blue color), and the two coasts are mostly designated best (light blue color). Forty five percent of the population lives in these two regions where CO₂ emissions from a plug-in car would be less than the most efficient gasoline hybrid, the Prius.

Another graphic from the UCS report (Figure 2) shows there is a wide range of emission rates for the 26 regions of the country. The highest emission rate is more than twice the lowest emission rate.

Figure 2: Grid Regions Comparisons of Emissions Rate

Americans are used to a simple mile per gallon (MPG) metric for fuel economy. The idea of different emissions and different miles per gallon for different regions of the country may be a new idea. Thus a plug-in vehicle may have an advantage or a disadvantage compared to the conventional car, based on the owner’s location in the country.

Calculating the fuel economy of an electric vehicle using MPG has proved difficult. The Environmental Protection Agency (EPA) developed a new designation of MPGe which stands for MPG equivalent. The EPA’s methodology is described later in this report.

In the UCS report, the term MPGghg is introduced, which is analogous to MPGe in that it estimates mileage from some other measure.  MPGghg stands for MPG greenhouse gas equivalent. Since the formula for converting the CO₂ emissions to MPGghg is not explained in the UCS report I requested it and co-author, Don Anair[2] provided the following information:

MPG equivalent (MPGghg)  = (1/Emissions intensity)*(1/electric vehicle efficiency) *grams of CO₂ equivalent per gallon of gasoline.

Two parameters that are necessary to evaluate this formula are:

Electric vehicle efficiency = 0.34 kwh/mile (Nissan Leaf)

Grams CO₂ eq./gallon of gasoline = 11,200 (GREET*)

(*GREET stands for the “Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation Model” developed by Argonne National Laboratory.)

Don also provided the national average Emissions Intensity for all regions which is  684 g_CO2/kwh. This converts to about 48 mpg using the formula above. He also explained that the 11,200 grams of CO₂ per gallon of gasoline includes the extraction and refining of oil as well as the transport of oil and gasoline. A direct comparison of CO₂ from a gallon of gasoline not considering these factors is 9,300 grams/gallon rather than 11,200.

Table 1 is taken from the report. It shows the information in Figure 1 and Figure 2 in a tabular format. I have added a column to this table at the far right that illustrates how the MPG (really MPGghg) in Figure 1 was derived for the 26 different regions. This column, labeled “MPGghg 33000/EI” (Emissions Intensity), uses the number 33,000, which is derived by dividing 11,200 (grams of CO₂eq in a gallon of gasoline) by .34 (electric vehicle efficiency measured in kwH per mile for the Nissan Leaf). Thus the new entries in the added column are obtained by dividing 33,000 by the value in the Emissions Intensity column. For example the 46 MPGghg for the first entry is calculated by dividing 33,000 by 717. I also added a row at the bottom of the table that shows the national average for Emissions Intensity provided by Don Anair.

Note that this table is for a single car, the Leaf, and a different table would have to be generated for cars with different electric vehicle efficiency. Such tables could be made simpler, for example, by eliminating the distribution of fuel sources.

Table 1: Table from UCS Report for Nissan Leaf

 There are important concepts and useful information in the UCS report, such as the new MPGghg. The maps of the country also provide a quick look at how effective a plug-in would be in different parts of the county. The algorithm provided by one of the authors for obtaining MPGghg is also important.

Comparing the Prius and the Leaf

The UCS report concentrates on comparing electric cars, particularly the Leaf, to a conventional car that gets 27 MPG and generates 415 grams of CO₂ per mile (g_CO2/mile). The Prius and other conventional hybrids are discussed but much of the financial advantages of plug-ins in the report are derived from comparisons to a basic 27 MPG car rather than a 50 MPG car like the Prius. In other papers I have pointed out that the main competitor to plug-ins are conventional hybrids, not conventional gasoline cars. Over half the hybrids that have been sold in the last thirteen yours have been the Prius model. Thus a more detailed comparison to a Prius would be useful. The UCS Executive Summary provides a useful graphic (Figure 3) showing how the Prius relates to the cleanest and dirtiest regional grids.

Figure 3: Best, Worst and Prius

The EPA fueleconomy.gov website recognizes that the total emissions for a car must consider the “upstream” as well as the “tailpipe” emissions. The term “upstream” is equivalent to the emissions from a power plant or an oil refinery along with the transmission of electricity or transportation of gasoline. The “tailpipe” emissions correspond to the emissions generated by driving the car. According to the EPA, the Prius has a CO₂ emission rate of 222 g_CO2 /mile (see Figure 4). (Both EPA and UCS use “g/mile” but I prefer to add a “CO₂” subscript as a reminder of what is being measured.) As a check if one uses the 50 mpg of the Prius from fueleconomy.gov and divides that into 11,200, the result is 224 g_CO2 /mile, very close to the number in Figure 4.

 Figure 4:  Prius from MPG and CO₂ – fueleocnomy.gov

The emissions for the 50 MPG Prius are taken from the main page of the fueleconomy.gov website. The CO₂ emissions for the LEAF and other plug-in cars are found at a special part of the fueleconomy.gov website[3]. From the homepage, this is accessed first by selecting “About EPA Ratings” from the horizontal menu across the top. When the drop-down menu appears, the option of “Beyond Tailpipe Emissions” is displayed at the far left under the heading “New Window Stickers”. Clicking this option displays the “Greenhouse Gas Emissions for Electric and Plug-In Hybrid Electric Vehicles” choices. This part provides data on several plug-in cars, including the Leaf.  The result for a Leaf is shown in Figure 5. Note that the emission rates includes a national average as well as the value for a particular region. This is the same approach used by the UCS except EPA uses the ZIP code and UCS uses the 26 grid regions.

 Figure 5: Leaf EV CO₂ Emissions from fueleconomy.gov

 The UCS report includes another table which is a summary of the MPGe from the EPA (see Table 2). I have previously shown that the Prius and Leaf are almost the same relative to CO₂ (230 versus 222 g_CO2/mile). But Table 2 shows that the Leaf fuel economy measured by MPGe is about twice the Prius (99 MPGe versus 50 MPG). The reader might be puzzled by the fact that Leaf gets 48 MPGghg based on CO₂ emissions while it gets 99 MPGe based on electricity consumed.

Table 2: EV MPGe from UCS Report [4]

The difference, which I covered extensively in a white paper History and Status of the Battery Electric Vehicle [5], comes from the EPA’s method of deriving the MPGe for a plug-in vehicle. The EPA’s formula uses a value of 33.7 kWh per gallon of gasoline which does not account for the energy burned at the power plant and energy lost in electricity transmission. The appropriate value is 12.3 kWh per gallon of gasoline as explained in my white paper. To determine the more accurate measure, one need only multiply the EPA MPGe by a correction factor of .36 (12.3 kWh per gallon / 33.7 kwH per gallon). Table 3 shows the application of this correction factor to the 99 MPGe resulting in a more accurate 36 MPGe.

In the same report, the Prius MPG is corrected to take into account the upstream use of energy similar to the method used by the UCS in its report. This correction factor is also explained in the white paper. To determine a more accurate measure, the EPA MPG for conventional cars and hybrids must be multiplied by a correction factor of .83 (9,300 grams of CO₂ per gallon / 11,200 value grams of CO₂ per gallon). This will give a MPGe for gasoline and diesel cars that reflects refinery and petroleum energy losses. The UCS approach is more accurate from a scientific perspective than the EPA plug-in fuel economy methodology. Table 3 shows a corrected MPGe from the referenced report.

Table 3: Leaf and Prius MPGe Comparison

In summary, the Prius has roughly the same MPGghg as the Leaf while its corrected MPGe is somewhat better than the corrected MPGe for the Leaf.

The UCS report has provided two useful tools, the first being an algorithm for determining MPGghg which is similar to the EPA MPGe but more accurate in that it accounts for the emissions from energy used to obtain gasoline. The argument about efficiency of the power plants is overlooked in the EPA analysis since it does not do the equivalent analysis of the UCS, thus providing misleading measures. Secondly the report shows the map of the country with its regions including the MPGghg. Unfortunately, the UCS does not carry the MPGghg designation throughout its report. But the visual representation for the country in Figure 1 is very useful.

On a national basis, the Leaf has no advantage over the Prius relative to CO₂. But there are major sections of the country with large populations where the Leaf does have a significant advantage, since much of the electricity comes from hydroelectric sources rather than coal or natural gas. The report adds a key dimension to the HEV vs. plug-in debate by showing that regional considerations must be evaluated. For a student or concerned citizen of energy and CO₂, this report is extremely useful.

Finally I have not addressed here the question of capital and operating cost, which is covered extensively in the UCS report. I may analyze this in a future blog.

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