A Comparison of Carbon Dioxide Emissions from Electric Vehicles to

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A Comparison of Carbon Dioxide Emissions from Electric Vehicles to Emissions from Internal Combustion Vehicles Daniel J. Berger*,† and Andrew D. Jorgensen‡ †

Department of Chemistry and Physics, Bluffton University, Bluffton, Ohio 45814, United States Department of Chemistry and Biochemistry, University of Toledo, Toledo, Ohio 43606, United States

‡

S Supporting Information *

ABSTRACT: Increased use of electric vehicles has been proposed as a means of reducing anthropogenic carbon dioxide emissions. It is therefore reasonable to compare emissions associated with generating the electricity used by electric vehicles with the emissions associated with internalcombustion vehicles, including hybrid-electric vehicles. The student exercises in this activity will permit students to understand the trade-offs involved in making a choice of vehicles. When emissions from electrical generation are taken into account, electric vehicles that obtain power in most U.S. states do have lower emissions than gasoline-powered vehicles, but in some states the heavy use of fossil fuels for electric generation causes electric vehicles to do no better than gasoline-powered hybrid vehicles. In a few states, electric vehicle emissions are worse than those from comparable hybrid vehicles. KEYWORDS: General Public, First-Year Undergraduate/General, Environmental Chemistry, Problem Solving/Decision Making, Public Understanding/Outreach

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BACKGROUND It is well-known that carbon dioxide is a greenhouse gas,1−3 and the most recent report of the Intergovernmental Panel on Climate Change states that it is almost certain that human greenhouse gas emissions are largely responsible for the significant rise in global average temperatures4 during the past 60 years: It is [a 95% probability] that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period.5 Battery-electric vehicles (EVs) have been introduced as a way of reducing anthropogenic greenhouse gas emissions, and as a way to make personal transportation more efficient. They have so far been less than successful in the marketplace, but still deserve a look given the possible positive aspects of these vehicles. The U.S. automobile market includes a number of EVs that are sold nationwide as personal vehicles. Some have been designed from scratch (the Tesla Model S), while others are based on production internal-combustion vehicle (ICV) models−for example, the Honda Fit and Toyota RAV4 EVs. Many ICV models have hybrid electric vehicle (HEV) versions, which are powered by an internal-combustion engine and a battery-powered electric motor. Some models are configured as EVs but contain internal-combustion engines for longer trips; these are known as plug-in hybrid electric vehicles (PHEVs). © XXXX American Chemical Society and Division of Chemical Education, Inc.

In this exercise, students use publicly available data to compare the carbon dioxide emissions associated with ICVs, including diesel ICVs and HEVs, to emissions associated with the generation of the electricity required for EVs. PHEVs are classed as EVs for the purposes of this analysis; only three PHEVs were listed in the database we used. Depending on the level of analysis, this exercise could be used for students in introductory engineering, chemistry, or environmental science courses, whether for majors or nonmajors. Other detailed analyses regarding this topic have been published, but are either more specific6 or do not address quite the same questions7,8 as this more general analysis. We will address the carbon dioxide emissions associated with electrical generation in order to compare EVs with the direct emissions from ICVs. However, in order to keep things as simple as possible, we will not address emissions associated with gasoline or diesel fuel production; nor will we address the emissions associated with the production of coal or natural gas. Except in a cursory way, we did not account for the emissions associated with vehicle production; these emissions are significantly higher for EVs than for ICVs.6 We believe that the present level is appropriate for student work and is internally consistent. Previously published student exercises have explored carbon dioxide emissions primarily to teach stoichiometry,9,10 as a tiein to infrared spectroscopy11,12 or through more general discussions of the atmospheric greenhouse effect.2,3,13

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database includes such information as the displacement and number of cylinders in each vehicle’s engine, estimated carbon dioxide emissions in grams CO2 per mile driven, and each vehicle’s miles per gallon (MPG) or miles per gallon equivalent (MPGe). For EVs it also provides kilowatt-hours consumed per 100 mi. MPGe values, which are given for both EVs and PHEVs, are calculated by the EPA using a value of 33.7 kW·h per gallon of gasoline, based on the generally accepted heat content of 115 BTUs per gallon of gasoline.14 To compare emissions from EVs with those from ICVs, we calculate the carbon dioxide emissions for each EV since the EPA does not include sources of electricity in its emissions calculations. The International Energy Agency (IEA) provides data on carbon dioxide emissions per kilowatt-hour for every country in the world, as the national average for all fuel sources, and the national averages for each type of fossil fuel−coal, petroleum and natural gas.15 To compare EV carbon dioxide emissions from state to state within the United States, we used a state-by-state breakdown of different energy sources in installations generating at least 1 MW, compiled from data from the U.S. Department of Energy.16 These data include the number of annual megawatthours for each energy source, but not carbon dioxide emissions; such emissions can be calculated by combining state-by-state generation information with the IEA’s data on carbon emissions for each type of fossil fuel, while assuming that nonfossil fuel sources, such as wood or other biomass, have zero net emissions.

The goal of the exercise is to permit students to directly compare the emissions from conventional ICV automobiles with those associated with the electricity needed to power EVs. Student involvement can vary according to the instructor’s preference and the academic level of the student. We present the results of a student activity carried out with nonscience majors in a liberal-arts chemistry course. The approach could be modified depending on the audience. Some examples include the following: The first step of the analysis could be thermodynamic calculations of the heats of combustion for methane as a model for natural gas electric generation, octane as a model for gasoline combustion, and graphite as a model for coal electric generation. This last calculation provides a value that is slightly higher than the top of the wide range of heats for the various types of coal. The result of the calculation clearly shows the greater energy produced from the combustion of methane compared to liquid and solid fuels per mole of carbon dioxide produced, which is the chemical basis for the value of natural gas in generating electricity. This simple analysis would be appropriate for general chemistry students as an introduction to the topic. The instructor could present the data graphically as given here, explain where it comes from and ask students to analyze the graphic data and draw conclusions. With the data analysis provided in these graphs and the thermodynamic considerations, general chemistry students would gain insight into the problem. The chemistry could be presented in a part of one class. Students could then use the time between class periods to review the graphs and make statements about their conclusions. More advanced students, such as those in an environmental chemistry class, could be presented with the data in the spreadsheets found in the Supporting Information, along with required or suggested ways to analyze it to produce the graphs in this paper. They could also consider other assumptions in their analyses. Such an exercise would require several hours by the students, so allowing a week for its completion is reasonable. Students could be asked to suggest additional questions to be addressed, then use the database to address them under the guidance of the instructor. We used published data from sources including the U.S. Government, the International Energy Agency, and the Edmunds automobile database to determine whether and under what circumstances driving an EV will generate lower carbon dioxide emissions than driving an HEV or ICV. We did this by determining the carbon dioxide emitted by generating the electricity required for the EVs in our database, under different mixes of electrical generation fuels, and comparing these emissions to those cited by the U.S. government for HEVs and ICVs. We first compare emissions from ICVs and HEVs to those from the production of electricity required to power EVs, averaged over the entire U.S. electrical generation grid. We then compare EV emissions depending on the source of electricity, and finally compare EV emissions from state to state, based on each state’s mix of electrical generation fuels.

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Calculations

For both gasoline and diesel ICVs, including HEVs, the EPA provided an estimated value of grams of CO2 emitted per mile.14 However, there are two complicating factors. First, EVs are listed by the EPA as having zero CO2 emissions. This is formally true, but does not take into account emissions associated with the generation of electricity. Therefore, we combined EPA values of kilowatt-hours consumed per 100 mi with the IEA’s values for CO2 emissions per kilowatt-hour15 to obtain the grams of CO2 emitted per mile for EVs. Second, while the EPA provides carbon dioxide emission numbers for PHEVs, it is unclear how those numbers are obtained. We therefore treated PHEVs as EVs, back-converting the EPA values for MPGe into kWh/100 miles using the EPA’s conversion factor of 33.7 kWh per gallon of gasoline.14,17 This allowed us to calculate realistic values of grams CO2 per mile for PHEVs operating as EVs. Students carrying out an analysis parallel to that presented here will need guidance on this assumption and the procedure used to make the given comparison. To account for differences in vehicle size, we plotted carbon dioxide emissions per mile against vehicle curb weight in pounds as a proxy for the complex interaction of body aerodynamics, momentum and road friction. Curb weights were obtained from the Edmunds automobile database18 or from the Web site of the appropriate manufacturer.

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DATA AND CALCULATIONS

RESULTS AND ANALYSIS

Figure 1 combines emissions data for a large number of vehicles selected from the EPA database.14 EV emissions are calculated using the weighted average of CO2 per kW·h for all methods of electrical generation in the U.S.A.15 When comparing vehicle types, it is clear that−using our assumptions−EVs that draw

Sources of Data and Assumptions

Our analysis used a database of fuel mileage data provided by the U.S. Environmental Protection Agency (EPA).14 This B

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But the sources of electricity vary widely from state to state in the United States. Figure 3 is a chart of calculated EV emissions

Figure 1. Plot of grams of CO2 emitted per mile driven vs vehicle curb weight in pounds. This graph comprises data from 510 gasoline and 16 diesel ICVs, 43 HEVs, 11 EVs, and 3 PHEVs; EVs and PHEVs are combined into a single category. EV emissions are calculated using average emissions per kilowatt-hour for the entire United States.

Figure 3. Average CO2 emissions for fossil-fuel-powered vehicles are shown in gray, at left. EV emissions for the particular mix of electrical sources in the USA as a whole, and in each of the 10 most populous states, are shown in black. Before averaging, all emissions values were divided by curb weight.

their power from the U.S. electrical grid as a whole emit at least 30% less CO2 than comparable ICVs, whether gasoline- or diesel-fueled. For example, 240 vs 169 g CO2 per mile is a 30% difference between the Scion iQ ICV and the Smart ForTwo EV, which have comparable curb weights. However, Figure 1 shows that the best-performing HEVs have emissions comparable to EVs under the current U.S. mix of electrical generation. When sources of electrical energy are compared,16 it becomes clear how dependent EV emissions are on the source of their electricity. In Figure 2, emissions values are shown

for the 10 most populous U.S. states, including Ohio, the home state of the authors of this paper. These emissions values are based on the average value of kW·h per mile, per pound of curb weight, for EVs, and the mix of electricity generated in each state. This mix will not be identical to the mix of energy used in the state due to importing and exporting electricity, but it should be a reasonable approximation.16 Average emissions per mile, per pound of curb weight, from ICVs and HEVs are included for comparison. One common criticism of EVs is that they simply transfer carbon dioxide emissions from the vehicle tailpipe to the electrical power plant. This is true; therefore, we took into account the emissions associated with sources of the electricity used for EVs. There is a stark contrast between EVs used in states such as California or New York, with low-carbon electricity, and states such as Ohio, with high-carbon electricity. An EV in Ohio emits more CO2 per mile than a typical HEV of the same size, while an EV in California undercuts the emissions of a typical HEV by almost 70%.19 It should be noted that CO2 emissions associated with the manufacture of EVs are estimated to be between 25% and 75% higher than emissions associated with the manufacture of ICVs, when normalized for the estimated distance each type of vehicle will be driven before being scrapped.6 Taking this into account means, for example, that−rather than being about equal to HEVs in emissions−EVs in the state of Ohio, which has highcarbon electricity, have total CO2 emissions that are probably no better than a gasoline ICV. More generally, to have a significant advantage over HEVs, an EV would have to be driven in a state with low-carbon electricity such as New York or California. This does not change the fact that in the U.S. as a whole, the EV has significantly lower CO2 emissions than gasoline-powered ICVs.

Figure 2. EV carbon dioxide emissions, in grams of CO2 per mile driven, are plotted against vehicle curb weight. For clarity, data points for ICV and HEV emissions, shown in Figure 1, have been replaced with trendlines for each data subset.

using U.S. average emissions from electrical generation, for U.S. average coal-fired electrical generation, and for U.S. average natural-gas-fired electrical generation. EVs powered by coalfired electricity emit about the same amount of carbon dioxide as diesel ICVs; changing to lower-carbon sources of electricity such as natural gas substantially improves the EV emissions picture.

Student Exercise

Students in a course for liberal arts majors at the University of Toledo were given a two-part assignment that had the purpose of enlightening them on aspects of electric vehicles, especially C

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CONCLUSION We have presented an exercise in which students can (a) compare carbon dioxide emissions of gasoline and diesel ICVs to HEVs. (b) compare carbon dioxide emissions of petroleum-fueled vehicles to those associated with the electrical generation needed to power EVs. (c) compare EV emissions to those of petroleum-fueled vehicles using the mixtures of electrical generation found in different U.S. states. These exercises allow students to discover that EVs have the potential to significantly reduce CO2 emissions from personal vehicles, but only if their sources of electricity are other than the coal-fired power plants that, as of 2011, provided some 40% of all U.S. electricity.16 In U.S. states that depend heavily on coal, HEV emissions compare favorably with those associated with the electricity required to power EVs. Furthermore, due to shorter road lifetimes and the need for more energy-intensive materials for batteries and electric motors, EVs are estimated to have substantially more CO2 emissions associated with their manufacture than ICVs.6 When the higher emissions cost of manufacturing EVs is taken into account, reducing the carbon footprint of electrical power generation becomes even more important in order to realize potential emissions reductions from EV use.

carbon dioxide emissions. It was designed for them to utilize chemical principles that they learned in the course up to that point (e.g., energy changes, climate change, air quality), as well as to provide an application for analyzing graphical information using real-world data. Part 1 of the assignment asked students to prepare a brief essay on their perceptions of pros and cons of electric vehicles before doing any research on the topic. Part 2 used a guided inquiry method that posed several questions based on students’ previous work as well as additional information, including that shown in Figures 2 and 3. The information was sufficient for nonscience majors to understand some aspects of electric vehicles that they had not previously considered, as evidenced from the Part 1 essays. This was particularly true regarding indirect carbon dioxide emissions produced by the generation of electricity; a number of students commented favorably on having that brought to their attention. A post-assignment anonymous survey of six multiple-choice and three open-ended questions revealed several conclusions about the value and utility of the lessons. A total of 36 valid surveys were received. After completing the assignment 19% of the students judged their understanding of the pros and cons of electric vehicles to be “Significant” and 56% indicated a “Medium” level. The exercise was rated as “Very Valuable” by 28% of responding students, and another 39% indicated that it was “Somewhat Valuable.” Open-ended comments frequently expressed surprise at the variation of emissions by state as well as issues on the limited range of electric vehicles. In general they were favorably impressed by the opportunity to learn objective information on this timely and practical issue. Two telling responses were: “It was cool to see the graphs and use logic and problem solving to discover trends and figure out the reasons for those trends” and “I learned that electric vehicles still produce a small amount of CO2 and that electric vehicles that run on electricity from coal produce a significant amount of CO2.” Clearly the assignment achieved its primary purpose of introducing an aspect of sustainability into an introductory level course. While improving students’ views of EVs was not the aim of the project, 44% indicated that their view of electric vehicles was “More Favorable” after their work while 50% indicated no change. The text of the assignment, modified for clarity based on student comments, and the survey questions are included in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Source data from references 14 and 16, edited for student use, and suggested student exercises using the data to perform the analysis we present in this paper. One suggested student exercise uses the thermochemistry of proxy compounds (e.g., graphite for coal, octane for gasoline) to estimate the energy-tocarbon dioxide ratio of different fuels. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: bergerd@bluffton.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.J.B. gratefully acknowledges Bluffton University for sabbatical support, and the faculty of the Department of Chemistry at the University of Toledo for hosting and many interesting discussions.

Additional Exercises

The data used for this analysis can support other student exercises. For example, high-powered automobiles with larger engines are expected to use more fuel than automobiles with smaller engines; students can graphically determine whether this is true by comparing ICVs by the number of cylinders in the engine, or by engine size, or both, while normalizing for curb weight. Students can also compare vehicles in which the same model has versions with different power trains. For example, the Ford Focus has ICV and EV versions, and the Volkswagen Jetta has both diesel and gasoline ICV versions as well as a gasolinepowered HEV version. This allows students to address the question of whether a particular type of power train has a similar effect on fuel efficiency for different automobile makes and models.

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REFERENCES

(1) Science Service. “Carbonic acid cause of earth’s ice age.” J. Chem. Educ. 1931, 8, 1855. (2) Meserole, C. A.; Mulcahy, F. M.; Lutz, J.; Yousif, H. A. CO2 Absorption of IR Radiated by the Earth. J. Chem. Educ. 1997, 74, 316− 317. (3) Trogler, W. C. The Environmental Chemistry of Trace Atmospheric Gases. J. Chem. Educ. 1995, 72, 973−976. (4) NASA. Goddard Institute for Space Studies. “GISS Surface Temperature Analysis.” data.giss.nasa.gov/gistemp (accessed 12 Dec 2013). (5) IPCC (Intergovernmental Panel on Climate Change). Climate Change 2013: The Physical Science Basis. http://www.ipcc.ch/report/ ar5/wg1/ (accessed Apr 2015). D

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(6) Wilson, L.. “Shades of Green: Electric Cars’ Carbon Emissions around the Globe.” Shrink That Footprint, 2013. http:// shrinkthatfootprint.com/electric-car-emissions (accessed Apr 2015). (7) Crist, P. “Electric Vehicles Revisited−Costs, Subsidies and Profits.” International Transport Forum Discussion paper, 2012. http:// www.internationaltransportforum.org/jtrc/DiscussionPapers/ DP201203.pdf (accessed Apr 2015). (8) Elgowainy, A.; Burnham, A.; Wang, M.; Molburg, J.; Rousseau, A. Well-to-Wheels Energy Use and Greenhouse Gas Emissions: Analysis of Plug-in Hybrid Electric Vehicles. Argonne National Laboratory, publication ANL/ESD/09−2, 2009. http://www.transportation.anl. gov/pdfs/TA/559.pdf (accessed Apr 2015). (9) Oliver-Hoyo, M. T.; Pinto, G. Using the Relationship between Vehicle Fuel Consumption and CO2 Emissions To Illustrate Chemical Principles. J. Chem. Educ. 2008, 85, 218−220. (10) Treptow, Richard S. Carbon Footprint Calculations: An Application of Chemical Principles. J. Chem. Educ. 2010, 87, 168− 171. Correction: J. Chem. Educ. 2010, 87, 679. (11) Elrod, M. J. Greenhouse Warming Potentials from the Infrared Spectroscopy of Atmospheric Gases. J. Chem. Educ. 1999, 76, 1702− 1705. (12) Kauffman, J. M. Water in the Atmosphere. J. Chem. Educ. 2004, 81, 1229−1230. (13) Dunnivant, F. M.; Moore, A.; Alfano, M. J.; Brzenk, R.; Buckley, P. T.; Newman, M. E. Understanding the Greenhouse Effect: Is Global Warming Real? An Integrated Lab-Lecture Case Study for Nonscience Major. J. Chem. Educ. 2000, 77, 1602−1603. (14) U.S. EPA. “Vehicle mileage and other information for vehicles in the 2013 model year.” http://fueleconomy.gov/feg/EPAGreenGuide/ xls/all_alpha_13.xlsx (accessed Apr 2015). This spreadsheet is found at http://fueleconomy.gov/feg/download.shtml (accessed Apr 2015) and is provided in the Supporting Information. (15) International Energy Agency. CO2 Emissions from Fuel Combustion2012. http://www.iea.org/publications/ freepublications/ (accessed Apr 2015). (16) Martner, B.; Ellingson, R. Geography of Electricity Generation in the United States2011. http://astro1.panet.utoledo.edu/ ~relling2/geography_of_2011_US_electricity_generation-martner.pdf (accessed Apr 2015). Data are provided as a spreadsheet: http:// astro1.panet.utoledo.edu/~relling2/fuels_2011_allstates.xlsx (accessed Apr 2015). (17) For example, if the EPA reports 100 MPGe (miles-per-gallon equivalent), this is equivalent to the vehicle driving 100 miles on 33.7 kW·h, and so the vehicle’s electrical consumption is 33.7 kW·h per 100 mi. In other words, to convert MPGe to kW·h/100 mi, divide 33700 (33.7 × 100) by the MPGe value. (18) Edmunds. New Cars. http://www.edmunds.com/new-cars/ (accessed Apr 2015). (19) HEV emissions average 73 mg CO2 per mile, per pound of curb weight. EVs in California, by our analysis, have average emissions of only 20 mg CO2 per mile, per pound of curb weight. This is because electricity generated in California is only 47% from fossil fuels, with only 1% from coal, while that generated in Ohio is 78% from coal alone, with another 10% from other fossil fuels. See reference 16.

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DOI: 10.1021/acs.jchemed.5b00125 J. Chem. Educ. XXXX, XXX, XXX−XXX