- Email: [email protected]
New Directions: The electric car and carbon emissions in the US
Atmospheric Environment 44 (2010) 733–734
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
New Directions: The electric car and carbon emissions in the US
With the great emphasis being placed on the development of the electric car, I am often asked if converting US cars to battery power would result in a net reduction of CO2 emissions. After all, the electricity needed to recharge tomorrow's vehicle batteries will be generated at power plants which themselves have a carbon footprint. Put another way, will the recharging of vehicle batteries from the electrical grid simply transfer the emissions of CO2 from the vehicle tail pipes to the stacks of power generating stations without effecting a net reduction in emissions? Here are the numbers: US cars and light-duty trucks traveled 2,773,024 million miles in 2006 consuming 135,694 million gallons of fuel (National, 2009). In the US, 71% of the electricity is currently being generated with coal, natural gas and fuel oil. Coal accounts for 50%, natural gas 18% and oil 3% of the fuel mix (USEPA, 2006). In the combustion process, each fossil fuel emits CO2 at different levels: coal: 992 kg CO2 (MWh)1, natural gas: 539 kg CO2 (MWh)1 and oil: 735 kg CO2 (MWh)1 (Arar and Southgate, 2009). We can derive an emission factor for electricity generated with fossil fuels, including the lifecycle non-combustion emissions of 11% for coal, 14% for natural gas (Dones et al., 2003), and 17% for oil (DOE, 2000) as follows:
0:53ð992=0:89ÞD0:173ð539=0:86ÞD0:033ð735=0:83Þ [ 690 kg of CO2 ðMWhÞL1
(1)
To account for CO2 emissions produced as a result of electricity generated with the non-fossil fuel sources, we add the contribution of the nuclear and hydroelectric components. Twenty percent of electricity is generated with nuclear energy and 7% with hydropower (USEPA, 2006). Nuclear lifecycle CO2 emissions are estimated at 30 g CO2 (kwh)1, hydroelectric emissions at 20 g CO2 (kwh)1 (Jacobson, 2009). Biofuels represent a small portion of the energy mix (1%) and are not included in this analysis. We derive an emission factor for electricity generated with nonfossil fuels.
0:2330D0:07320 [ 7:4 g CO2 ðkwhÞL1 [ 7:4 kg CO2 ðMWhÞL1 (2) And then derive an overall CO2 emission factor for electricity generation in the US by adding the results of equations (1) and (2): L1
690D7:4 [ 697:4 kg CO2 ðMWhÞ
(3)
Since the emission factors cited for electricity generation are empirically derived, they include an electricity generation efficiency factor. In other words, the numbers already incorporate the energy losses in converting fuel to electricity. They do not however incorporate the 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.09.042
electricity transmission efficiency of 0.924 (DOE, 2000). Applying this efficiency to the results of equation (3) we derive an emission factor of:
697:4 kg of CO2 ðMWhÞL1
0:924 [ 755 kg of CO2 ðMWhÞL1 (4)
So, how much electricity does an electric car consume? The Tesla Motors Company reports a vehicle efficiency of 2.18 km MJ1 for its electric vehicles (www.teslamotors.com/efficiency/well_to_wheel. php). This is equivalent to 0.128 (kwh) km1. General Motors predicts an efficiency of 25 kwh per 100 miles for the Chevy Volt (www.chevrolet.com/electriccar). This is equivalent to 25 kwh/ (100 miles 1.61 km mile1) ¼ 0.155 kwh km1. Applying an approach detailed by Jacobson (Jacobson, 2009), and using the actual 2006 fuel consumption of 135,694 106 gallons for 2006 (National, 2009), we can also derive a theoretical electricity requirement to generate the energy needed at the wheel.
A [ Energy needed to power gasoline passenger vehicles and light duty trucks in 2006 MJ yrL1
(5)
[ 135; 694 gals3106 3125 MJ galL1 [ 16:9631012 B[Netenergyto powerthosevehicles MJyrL1
(6)
[A3gasolinevehicle efficiency[A30:17[2:8831012 Netenergytopowerthose vehicles[B= MJðkwhÞL1
(7)
[B=3:6[8013109 kWh Or 8013109 kwh= 27730243106 miles31:61kmmileL1 [0:18kwhkmL1
(8)
Since not all cars are/will be as small as the Tesla or the GM Volt, we will use the higher theoretically-derived value. If all distances driven in 2006 had been powered by batteries, the electricity requirement would have been:
[ 2; 773; 024 miles3106 31:61 km mileL1 30:18 kwh kmL1 [ 803; 6223106 kwh [ 8043106 MWh (9) Resulting in CO2 emissions of:
8043106 MWh3755 kg CO2 ðMWhÞL1 [ 6073106 tons CO2 ; or 1663106 tons C
(10)
The additional load on the electricity generating infrastructure would have been
734
J.I. Arar / Atmospheric Environment 44 (2010) 733–734
804=4065ðEIA; 2009Þ [ 20% Now let us compare the above scenario to one where vehicles are powered exclusively with gasoline and diesel fuel. To achieve a meaningful comparison between battery powered vehicles and fossil fuel powered vehicles, we must include not only the vehicle efficiency in converting fossil fuel to work, but also the well-to-station efficiency in the production and transport of petroleum products. Using a well-to-station efficiency of 83% (DOE, 2000), the 2006 fuel consumption of 135,694 106 gallons, and an emission factor of 8.8 kg CO2 gal1 (USEPA, 2005), we derive the CO2 emissions for fossil powered vehicles as follows:
135; 6943106 gals38:8 kgs CO2 galL1
0:83
[ 14393106 tons of CO2 ; or 3923106 tons C
(11) References
Comparing the two scenarios shows that conversion of the US fleet of passenger vehicles and light-duty trucks to battery power would result in an emission reduction of:
ð392L166Þ=3923100 [ 58%
annually then if conversion to electric vehicles had not taken place and that cumulative emissions would be 3651 millions tons carbon under the fossil fuel powered scenario and 2820 million tons carbon under the gradual ten-year transition scenario, representing a 23% (21–28) reduction in ten-year cumulative emissions. To account for the uncertainties regarding future improvements in gas mileage, in efficiencies of electrical vehicles, in rates of fleet conversion to battery power and in the future composition of the fuel mix used for electricity generation, I calculated uncertainty ranges which I showed in parentheses. So when asked: ``Will CO2 emissions be reduced with the adoption of electric cars in the US'', I answer ``yes, and here is why .''.
(12)
But we're not quite finished! These results assume an instantaneous conversion of the US fleet and therefore they do not represent a plausible scenario. A more realistic scenario would be for the transition to electric vehicles to occur over a period of time. For example, the transition could realistically be expected to occur gradually over the ten years following 2010, at a conversion rate of 10% per year. Assuming the historically-derived 2.3% growth in ``miles traveled'' (National, 2009), starting with 3 1012 miles for 2011 (2006 figures projected to 2011), accounting for a vehicle fuel efficiency improvement from 35 km gal1 (22 mpg) to 50 km gal1 (31 mpg) over the 10-year period and with no change in the fuel mix used for electricity generation, I projected CO2 emissions over the 2011 through 2020 time horizon. I concluded that by the end of the tenyear period, vehicles would be emitting 36% (31–45) less CO2
Arar, J., Southgate, D., 2009. Evaluating CO2 reduction strategies in the US. Ecological Modelling 220, 582–588. DOE, 2000. Electric and Hybrid Vehicle Research, Development, and Demonstration Program: Petroleum-Equivalent Fuel Economy Calculation. Final rule, 10 CFR Part 474. Dones, R., et al., Greenhouse gas emissions from energy systems: comparison and overview. PSI Annual report, 2003. Annex IV. Jacobson, M., 2009. Review of solutions to global warming, air pollution, and energy security. Energy and Environmental Science 2, 148–173. National Transportation Statistics, 2009. Bureau of Transportation Statistics, Table 1–32, 4–11 and 4–12. Available at: http://www.bts.gov/publications/national_ transportation_statistics/html/table_01_32.html. USEPA, 2005. Emission Facts: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel. Available at: http://www.epa.gov/OMS/climate/ 420f05001.htm#calculating. USEPA, 2006. E-Grid. Available at: www.epa.gov/cleanrgy/egrid/index.htm.
Joseph I Arar* RCP Inc., 8082 Luckstone Dr. Dublin, OH 43017, USA Tel.: þ1 614 763 0400; fax: þ1 614 763 0403. E-mail address: [email protected] 13 July 2009
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
New Directions: The electric car and carbon emissions in the US
With the great emphasis being placed on the development of the electric car, I am often asked if converting US cars to battery power would result in a net reduction of CO2 emissions. After all, the electricity needed to recharge tomorrow's vehicle batteries will be generated at power plants which themselves have a carbon footprint. Put another way, will the recharging of vehicle batteries from the electrical grid simply transfer the emissions of CO2 from the vehicle tail pipes to the stacks of power generating stations without effecting a net reduction in emissions? Here are the numbers: US cars and light-duty trucks traveled 2,773,024 million miles in 2006 consuming 135,694 million gallons of fuel (National, 2009). In the US, 71% of the electricity is currently being generated with coal, natural gas and fuel oil. Coal accounts for 50%, natural gas 18% and oil 3% of the fuel mix (USEPA, 2006). In the combustion process, each fossil fuel emits CO2 at different levels: coal: 992 kg CO2 (MWh)1, natural gas: 539 kg CO2 (MWh)1 and oil: 735 kg CO2 (MWh)1 (Arar and Southgate, 2009). We can derive an emission factor for electricity generated with fossil fuels, including the lifecycle non-combustion emissions of 11% for coal, 14% for natural gas (Dones et al., 2003), and 17% for oil (DOE, 2000) as follows:
0:53ð992=0:89ÞD0:173ð539=0:86ÞD0:033ð735=0:83Þ [ 690 kg of CO2 ðMWhÞL1
(1)
To account for CO2 emissions produced as a result of electricity generated with the non-fossil fuel sources, we add the contribution of the nuclear and hydroelectric components. Twenty percent of electricity is generated with nuclear energy and 7% with hydropower (USEPA, 2006). Nuclear lifecycle CO2 emissions are estimated at 30 g CO2 (kwh)1, hydroelectric emissions at 20 g CO2 (kwh)1 (Jacobson, 2009). Biofuels represent a small portion of the energy mix (1%) and are not included in this analysis. We derive an emission factor for electricity generated with nonfossil fuels.
0:2330D0:07320 [ 7:4 g CO2 ðkwhÞL1 [ 7:4 kg CO2 ðMWhÞL1 (2) And then derive an overall CO2 emission factor for electricity generation in the US by adding the results of equations (1) and (2): L1
690D7:4 [ 697:4 kg CO2 ðMWhÞ
(3)
Since the emission factors cited for electricity generation are empirically derived, they include an electricity generation efficiency factor. In other words, the numbers already incorporate the energy losses in converting fuel to electricity. They do not however incorporate the 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.09.042
electricity transmission efficiency of 0.924 (DOE, 2000). Applying this efficiency to the results of equation (3) we derive an emission factor of:
697:4 kg of CO2 ðMWhÞL1
0:924 [ 755 kg of CO2 ðMWhÞL1 (4)
So, how much electricity does an electric car consume? The Tesla Motors Company reports a vehicle efficiency of 2.18 km MJ1 for its electric vehicles (www.teslamotors.com/efficiency/well_to_wheel. php). This is equivalent to 0.128 (kwh) km1. General Motors predicts an efficiency of 25 kwh per 100 miles for the Chevy Volt (www.chevrolet.com/electriccar). This is equivalent to 25 kwh/ (100 miles 1.61 km mile1) ¼ 0.155 kwh km1. Applying an approach detailed by Jacobson (Jacobson, 2009), and using the actual 2006 fuel consumption of 135,694 106 gallons for 2006 (National, 2009), we can also derive a theoretical electricity requirement to generate the energy needed at the wheel.
A [ Energy needed to power gasoline passenger vehicles and light duty trucks in 2006 MJ yrL1
(5)
[ 135; 694 gals3106 3125 MJ galL1 [ 16:9631012 B[Netenergyto powerthosevehicles MJyrL1
(6)
[A3gasolinevehicle efficiency[A30:17[2:8831012 Netenergytopowerthose vehicles[B= MJðkwhÞL1
(7)
[B=3:6[8013109 kWh Or 8013109 kwh= 27730243106 miles31:61kmmileL1 [0:18kwhkmL1
(8)
Since not all cars are/will be as small as the Tesla or the GM Volt, we will use the higher theoretically-derived value. If all distances driven in 2006 had been powered by batteries, the electricity requirement would have been:
[ 2; 773; 024 miles3106 31:61 km mileL1 30:18 kwh kmL1 [ 803; 6223106 kwh [ 8043106 MWh (9) Resulting in CO2 emissions of:
8043106 MWh3755 kg CO2 ðMWhÞL1 [ 6073106 tons CO2 ; or 1663106 tons C
(10)
The additional load on the electricity generating infrastructure would have been
734
J.I. Arar / Atmospheric Environment 44 (2010) 733–734
804=4065ðEIA; 2009Þ [ 20% Now let us compare the above scenario to one where vehicles are powered exclusively with gasoline and diesel fuel. To achieve a meaningful comparison between battery powered vehicles and fossil fuel powered vehicles, we must include not only the vehicle efficiency in converting fossil fuel to work, but also the well-to-station efficiency in the production and transport of petroleum products. Using a well-to-station efficiency of 83% (DOE, 2000), the 2006 fuel consumption of 135,694 106 gallons, and an emission factor of 8.8 kg CO2 gal1 (USEPA, 2005), we derive the CO2 emissions for fossil powered vehicles as follows:
135; 6943106 gals38:8 kgs CO2 galL1
0:83
[ 14393106 tons of CO2 ; or 3923106 tons C
(11) References
Comparing the two scenarios shows that conversion of the US fleet of passenger vehicles and light-duty trucks to battery power would result in an emission reduction of:
ð392L166Þ=3923100 [ 58%
annually then if conversion to electric vehicles had not taken place and that cumulative emissions would be 3651 millions tons carbon under the fossil fuel powered scenario and 2820 million tons carbon under the gradual ten-year transition scenario, representing a 23% (21–28) reduction in ten-year cumulative emissions. To account for the uncertainties regarding future improvements in gas mileage, in efficiencies of electrical vehicles, in rates of fleet conversion to battery power and in the future composition of the fuel mix used for electricity generation, I calculated uncertainty ranges which I showed in parentheses. So when asked: ``Will CO2 emissions be reduced with the adoption of electric cars in the US'', I answer ``yes, and here is why .''.
(12)
But we're not quite finished! These results assume an instantaneous conversion of the US fleet and therefore they do not represent a plausible scenario. A more realistic scenario would be for the transition to electric vehicles to occur over a period of time. For example, the transition could realistically be expected to occur gradually over the ten years following 2010, at a conversion rate of 10% per year. Assuming the historically-derived 2.3% growth in ``miles traveled'' (National, 2009), starting with 3 1012 miles for 2011 (2006 figures projected to 2011), accounting for a vehicle fuel efficiency improvement from 35 km gal1 (22 mpg) to 50 km gal1 (31 mpg) over the 10-year period and with no change in the fuel mix used for electricity generation, I projected CO2 emissions over the 2011 through 2020 time horizon. I concluded that by the end of the tenyear period, vehicles would be emitting 36% (31–45) less CO2
Arar, J., Southgate, D., 2009. Evaluating CO2 reduction strategies in the US. Ecological Modelling 220, 582–588. DOE, 2000. Electric and Hybrid Vehicle Research, Development, and Demonstration Program: Petroleum-Equivalent Fuel Economy Calculation. Final rule, 10 CFR Part 474. Dones, R., et al., Greenhouse gas emissions from energy systems: comparison and overview. PSI Annual report, 2003. Annex IV. Jacobson, M., 2009. Review of solutions to global warming, air pollution, and energy security. Energy and Environmental Science 2, 148–173. National Transportation Statistics, 2009. Bureau of Transportation Statistics, Table 1–32, 4–11 and 4–12. Available at: http://www.bts.gov/publications/national_ transportation_statistics/html/table_01_32.html. USEPA, 2005. Emission Facts: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel. Available at: http://www.epa.gov/OMS/climate/ 420f05001.htm#calculating. USEPA, 2006. E-Grid. Available at: www.epa.gov/cleanrgy/egrid/index.htm.
Joseph I Arar* RCP Inc., 8082 Luckstone Dr. Dublin, OH 43017, USA Tel.: þ1 614 763 0400; fax: þ1 614 763 0403. E-mail address: [email protected] 13 July 2009