How do the CO2 emissions from an electric car compare to a petrol car?

Brief How do CO2Operating on grid-average electricity, an electric car emits 27% less CO2 than an equivalent petrol-powered car. Using green power, there are neither tailpipe nor upstream emissions from an electric car.

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The easiest and most cost-eff­ective contribution that an individual can make to reducing greenhouse emissions is to replace their fossil- fuelled car with an electric car powered by renewable energy (“green power”). Better Place Australia is committed to using 100% green power in our electric car network. This technical note compares the greenhouse emissions profile of two similar cars: one with a petrol engine and the other with an electric motor.

The comparison looks at the carbon dioxide (CO2) emissions profile when the electric car is supplied with either green power or conventionally-produced electricity. The Renault Fluence is a mid- sized sedan, of a similar size to the Toyota Camry or the Honda Accord. The petrol version incorporates many state-of-the-art fuel efficiency measures, using 7.8L/ 100km – significantly less than the average car on Australian roads which uses 11.1L/100km.3,4

As petrol-powered vehicles dominate the Australian market, other important vehicle types such as diesels and hybrids are not discussed here. It should also be noted that CO2 emitted during the vehicle manufacturing process (“embodied energy”) is not considered here either, however it is explored in our briefing note titled, ‘Are electric cars really better for the planet?’5.

CO2 note car picture

The efficiency comparison

To meaningfully compare greenhouse gas emissions from driving, the complete life cycle of the fuel, from extraction from the well or mine to delivery of mechanical energy at the vehicle wheels (“well-to-wheel”) must be considered. We will examine this, first with the petrol car and then with the electric car, using both grid average and green electricity.

Petrol efficiency and emissions

The energy lost (and associated emissions) extracting oil, refining it and transporting it to the petrol station is around 18% in North America,6 and is approximately the same in Australia.7 Thus, the well-to-tank efficiency is 82%. The vehicle CO2 emissions figures quoted by the manufacturers are tank-to-wheel figures. Taking into account the well-to-tank efficiency, the total emissions are 184/0.82 = 224 g/km.

Emissions from grid-average electricity

The carbon emissions produced during electricity generation are well documented. Australia currently generates 228 million MWh of electricity annually, resulting in 203 million tonnes of CO2 emitted. This equates to 891 grams of CO2 per kWh produced in electricity generation.8 Approximately 9% of the plant output is spent running auxiliary loads within the power plant itself,9 and the losses incurred transmitting and distributing the available electricity to the point of use are 6%10, so the total generator-to-electrical- outlet efficiency is 0.91 x 0.94 = 86%.

The efficiency for converting electricity from the electrical outlet into energy stored in the car battery is about 87% (a combination of an 8% loss due to inefficiency in the AC-DC converter11 and a 5% loss from intrinsic inefficiency in charging the battery12). The generator-to-electrical outlet efficiency is calculated above as 86%. Thus, the energy that must be generated in the grid in order to charge the 22 kWh battery in the Renault Fluence ZE is 22/(0.87 x 0.86) = 30 kWh of electrical energy. The distance driven per generated kWh is thus 160/30 = 5.4 km per kWh. Thus, from the national average grid mix, the emissions per km are 891/5.4 = 164 g/km. This is 164/224 = 73% of the emissions of a comparable petrol car. That is, 27% less CO2 is emitted by the electric car.

Emissions from green electricity

Electric cars are significantly cheaper to run and operate per kilometre than petrol vehicles. For an increased cost compared to “black electricity” of around one cent per kilometre, an electric car battery can be charged using 100% green power. In this case, there are no associated emissions.

Conclusion

Even using grid-average electricity in Australia, an electric car emits significantly less CO2 than its petrol-powered equivalent: 27% less in the example in this technical note. Looking to the future, grid-average emissions will decrease as Australia’s power production migrates from coal to lower emissions sources under the Renewable Energy Target scheme, thereby steadily increasing the CO2 emission reduction benefits of electric cars. Of course, to realise the maximum environmental benefit of an electric car, its electricity should be generated from zero-emissions sources. That is why Better Place Australia is committed to using zero-emissions renewable energy in our charging infrastructure.

References

  1. Australian Government Green Vehicle Guide: http://www.greenvehicleguide.gov.au/GVGPublicUI/home.aspx
  2. See Renault web site: http://www.renault.com/en/Vehicules/Pages/renault-fluence-ze.aspx
  3. Australian Government Green Vehicle Guide: http://www.greenvehicleguide.gov.au/GVGPublicUI/home.aspx
  4. Australian Bureau of Statistics, Survey of Motor Vehicle Use: An investigation into coherence, September 2006
  5. See Better Place Australia technical note: Are electric cars really better for the planet?, June 2011.
  6. “Well-to-Tank Energy Use and Greenhouse Gas Emissions of Transportation Fuels – North American Analysis”, Vol. 3, Page 59; June 2001; General Motors Corporation, Argonne National Laboratory, BP, ExxonMobil, and Shell.
  7. This is consistent with the 15-20% quoted in the CUSP institute discussion paper (“Environmental Attributes of Electric Vehicles in Australia”; July 2009; Andrew Simpson, Curtin University Sustainable Policy Institute; http://sustainability.curtin.edu.au/local/docs/0907_Environmental_Attributes_EVs_Australia.pdf), determined separately by Simpson and CSIRO.
  8. From the CARMA Carbon Monitoring database (January 2010 data): http://carma.org/region/detail/18.  224m US tons x 0.907 = 203m metric tonnes.
  9. Final Report, “Fuel resource, new entry and generation costs in the NEM”, prepared for the Inter-Regional Planning Committee, ACIL Tasman Pty Ltd, April 2009. See Table 32 for the auxiliary loads of combined cycle gas turbines and water cooled coal fired stations. The auxiliary loads range from 1% to 9.5%. In this technical note we conservatively estimate that the average auxiliary load is 9%.
  10. Garnaut Climate Change Review, Section 19.2.1, footnote 3 (5.9%).
  11. The NLG5 on-board battery charger (manufactured by BRUSA Elektronik AG) has an electrical efciency of 92%.
  12. Though the charge/discharge efficiency of modern Li-ion batteries is often claimed to be approaching 100%, the evidence for this is still unclear, and so we have conservatively assumed a 5% loss in this case.