Driving further on natural gas

BPL077 Natural Gas Kms v4 Web ThumbAn electric car drives nearly three times as far as a natural gas powered internal combustion car, for the same amount of natural gas consumed.

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As petrol prices continue to rise over time and future oil supply becomes less certain, governments across the world are seeking alternatives to power their vehicles. Diesel and liquid petroleum gas (LPG) are already in use in some cars, and natural gas, another fossil fuel, has also been proposed for broad adoption in the next ten years. Natural gas, also known by its chemical name of methane, is widely available and relatively easy to transport. However, it’s also a limited resource that needs to be carefully managed. The question must be asked: which is more efficient? A given amount of natural gas used to generate electricity for a battery electric car, or the same amount directly burned in an internal combustion engine car? Which car will drive further?

           

 

 

 

 

 

Efficiency Comparison

Comparing two similar-sized cars with alternative configurations:

1. A conventional car powered by compressed natural gas (CNG) instead of petrol:

• Honda Civic GX automatic

• Tank contains 30 litres of petrol equivalent (GGE), providing a range of 351km.1

2. An electric car, using electricity from a power station fired by natural gas:

• Renault Fluence Z.E.

• The battery stores 22 kWh of electricity for a range of 185km.2

Other configurations such as fuel-cell hybrid cars are possible, but are not discussed in this technical note as they are likely to be very expensive, and are not yet commercially available. To meaningfully compare fuel efficiency, the complete life cycle of extraction from the natural gas well, to delivery of mechanical energy at the car wheels, (“well-to-wheel” efficiency) must be considered. This technical note examines new car configurations, therefore the analysis that follows assumes state-of-the-art technologies.

Natural Gas Car Well-to-Wheel Efficiency

In the United States, the efficiency of producing and distributing natural gas is about 95%,3 and would be expected to be very similar in Australia. The energy used to compress the natural gas to fill the car tank is about 9% of the energy value of the gas itself, resulting in 91% efficiency for this process.4 Thus, the well-to-tank efficiency is 0.95 x 0.91 = 86%. In the Honda Civic GX, the compressed natural gas is burned in an internal combustion engine. The natural gas stored in a full tank contains 261 kWh of chemical energy,5 and is sufficient to drive the car 351 km. Thus, the tank-to-wheel fuel efficiency is 351/261 = 1.34 km per kWh of chemical energy content of the natural gas. Given that the well-to-tank efficiency is 86%, the overall well-to-wheel fuel efficiency is 0.86 x 1.34 = 1.15 km per kWh of chemical potential energy contained in the natural gas.

 

natural gas graph 1

Electric Car Well-to-Wheel Efficiency

Modern natural gas-fired power generators operate at 60% efficiency.6 Approximately 4% of the plant’s output is spent running the power plant itself,7 and a further 6% of the dispatched electricity is lost in transmission and distribution.8 Thus, the generator-to-electrical outlet efficiency is 90%. In the Renault Fluence Z.E., the energy is stored in a battery. How much electricity must be generated in order to fully charge the battery? 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 in efficiency in the DC-DC converter9 and a 5% loss from intrinsic in efficiency in charging the battery.10) As noted above, the generator-to-electrical-outlet efficiency is 90%. Thus, the energy that must be generated at the natural gas power station in order to charge the 22 kWh battery in the Renault Fluence Z.E. is 22/(0.87 x 0.90) = 28.1 kWh of electrical energy. Given that the efficiency of the natural gas generator is 60% and the well-to-generator efficiency is about 95%, the amount of natural gas chemical energy required to produce 28.1 kWh of electrical energy is 28.1/ (0.60 x 0.95) = 49.3 kWh of chemical energy content of the natural gas. The range of the Renault Fluence Z.E. is 185 km, so the well-to-wheel fuel efficiency is 185/49.3 = 3.75 km per kWh of chemical potential energy from the natural gas.

natural gas graph 2

Impact on Carbon Emissions

Since nearly all the carbon in natural gas is converted to carbon dioxide on combustion (either in a power station or in a car), this modelling indicates that the CO2 emissions of the electric car powered by electricity from natural gas are about a third of the emissions from the equivalent car powered by burning compressed natural gas in an internal combustion engine. The Civic GX produces 157 grams of CO2 for every kilometre driven, whilst the Fluence Z.E. only contributes 49 g/km when powered by gas-fired electricity generation.11 When using 100% renewable energy, the emissions from an electric vehicle are zero. All cars on Better Place’s network will be powered by 100% renewable energy.

 

 

 

 

CONCLUSION

The internal combustion powered car fuelled by compressed natural gas has a well-to-wheel fuel efficiency of 1.15 km per kWh of chemical energy content of the natural gas. The electric car “fuelled” by electricity from a modern natural gas power station has a well-to-wheel fuel efficiency of 3.75 km per kWh of chemical potential energy from natural gas.

In summary, an electric car drives approximately 3.3 times further than a natural gas powered internal combustion car, for the same amount of natural gas consumed. Natural gas is a limited resource, and as such must be used in the most efficient way possible. The efficiency comparison above shows that if natural gas is to be used for transport as an alternative to oil, it should be used to generate electricity for battery electric cars. Using it instead in internal combustion cars will deplete this limited resource at a much more rapid rate, and result in much higher greenhouse gas emissions.

 

References

  1. Based on the US EPA 5-cycle test result of 28 miles/GGE for combined city and highway driving. Real-world fuel e¬ciency is likely to be poorer. For Honda GX specifications: http://automobiles.honda.com/civic-gx/specifications.aspx
  2. See Renault web site: http://www.renault-ze.com/en-ie/gamme-voitures-electriques-renault-z.e./fluence-z.e./presentation-2485.html
  3. Federal Register, Department of Energy, O¬ce of Energy Efficiency and Renewable Energy. 10 CFR Part 474. Electric and Hybrid Car Research, Development, and Demonstration Program; Petroleum-Equivalent Fuel Economy Calculation; Final Rule. June 12, 2000
  4. The compressor supplied by Honda to buyers of its Civic GX car (See http://www.tulsagastech.com/files/civicvra07.pdf and http://www.brcfuelmaker.it/ing/specificheUsa.asp ) operates at 240 V and draws 7.5 A, and compresses 1.0 GGE of gas per hour. A full tank would require 240 V x 7.5 A x 7.8 GGE = 14.0 kWh of electricity. The generator-to-electrical-outlet efficiency is 90%, thus the energy that must be generated at the natural gas power station is 14.0/0.9 = 15.6 kWh. The chemical energy of the natural gas consumed at the power station to produce this electrical energy is 15.6/0.6 = 25.9 kWh. The full gas tank contains the equivalent of 261 kWh as chemical potential energy (see reference 5), thus the compression loss is 25.9/(261+25.9) = 9%.
  5. The Honda GX tank holds 7.8 gasoline gallon equivalents (GGE). Each GGE of natural gas occupies 126.67 ft3 under standard conditions, at 900 BTU / ft3. Thus, 1 GGE possesses 114,000 BTU of chemical energy, equivalent to 33.4 kWh. The tank therefore contains 7.8 x 33.4 = 261 kWh of chemical energy.
  6. H System gas turbines from General Electric. See the “Gas Turbines and Combined Cycle Products” brochure from GE Energy, Atlanta Georgia, GEA 12985G (05/07)
  7. Final Report, “Fuel resource, new entry and generation costs in the NEM”, prepared for the Inter-Regional Planning Committee, ACIL Tasman Pty Ltd by April 2009. See Table 32 for the auxiliary loads of combined cycle gas turbines, in the range 1% to 4%. Here we have used the more conservative 4% value.
  8. Australia: Garnaut Climate Change Review, Section 19.2.1, footnote 3 (5.9%). USA: United States Energy Information Administration, Table 10 of the United States Electricity Profile 2008 Edition, DOE/EIA-0348(01)/2. Transmission and distribution percentage loss calculated from total disposition minus estimated losses is approximately 6%. See http://www.eia.doe.gov/cneaf/electricity/st_profiles/us.html
  9. The NLG5 on-board battery charger (manufactured by BRUSA Elektronik AG) has an electrical efficiency of 92%.
  10. 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.
  11. CO2 emissions were calcualated as follows: 1 GGE of methane = 2.567 kg (by NIST definition.) When burned, it produces 2.567 x (44.0/16.0) = 7.059 kg of CO2 (ratio of molar weights). Thus, one Civic GX’s full fuel tank holds enough natural gas to produce 7.059 x 7.8 GGE = 55.062 kg of CO2, and on which it can travel 351 km. 55.062 kg / 351 km = 156.9 g /km. For the Fluence ZE: 156.9 multiplied by the ratio of the cars’ efficiencies (1.15 / 3.25) = 55.5 g /km.