Sustainable Mobility: Efficiency, Electricity and CO2

When driving in heavy traffic or sitting on the freeway with cars backed up for miles, the phrase “sustainable mobility” might seem like an oxymoron, but the Challenge Bibendum event which takes place in Rio de Janeiro at the end of this week will seek to prove otherwise. Much of the industry will be there to show that not only are they alive and well after the severe economic downturn but that a new range of products and services is on its way to trigger what may be the biggest change in mobility since the Model-T.

But change often brings challenge and always raises questions and draws comparisons with the status quo. One such area of discussion is the determination of the efficiency of electric cars. How do such vehicles compare with their internal combustion equivalent and therefore which is the better mode of transport (at least as far as efficiency is concerned). This isn’t a simple determination, particularly given that there really aren’t any cars to compare, but some data is starting to appear, notably on the website of the upcoming Nissan Leaf electric car .

According to Nissan, the Leaf has a 24 kWh lithium-ion battery, with a range of 100 miles (based on the USA EPA LA4 City Cycle]. This means that the vehicle consumes 0.24 kWh/mile or 0.864 MJ/mile – at least as far as electricity goes. But electricity has efficiency implications as well. If the source of electricity is a current “average” coal fired power station, then the efficiency shifts to 2.5 MJ/mile. By contrast, if the source is a Combined Heat and Power Natural Gas fired power station with an efficiency as high as 75%, then the efficiency of the same vehicle becomes 1.2 MJ/mile.

By contrast, compare this with a similar internal combustion engine vehicle. A good comparison vehicle for the Leaf might be a Toyota Yaris (diesel). The vehicles are about the same size, although the Yaris has a range of many hundreds of miles. According to the Toyota website the efficiency of the vehicle is about 70 mpg (UK gallon), which equates to some 2.3 MJ/mile. This means that for one particular source of electricity the Yaris is more efficient than the Leaf. But then of course there are considerations relating to the refining of the petroleum, the mining of the coal or extraction and transport of natural gas, transmission losses for electricity and so on.


Going a step further and comparing the CO2 emissions of the vehicles is equally if not more complex. On the face of it the Leaf has no emissions, but again it all depends on the source of the electricity. A Leaf in the EU utilizing the grid average electricity (358 gms CO2/kWh) will therefore emit 58 gms CO2/km, whereas the Yaris is listed on the Toyota website at 109 gms CO2/km. But a Leaf in Poland, which has a significant coal base, will be about the same as the Yaris.

Key to the impact that electric vehicles will have on our use of energy and the emissions that result will be the impact they have on the marginal kWh of electricity generation. For example, France exports some electricity to Germany. So a Leaf in France may well benefit from clean nuclear electricity, but that could result in less export to Germany and therefore more coal or gas used in that country. Equally, the demand may come at a time when Germany can make good use of available wind capacity.

As the use of electricity in the transport sector evolves, so too must our understanding and use of the energy systems that will feed the vehicles. Tools such as Smart Grids will be essential. But with both energy and emissions now having a clear price attached to them (at least in the EU for the CO2), perhaps the simplest way to follow the impact will be the way we have always made such judgments, the bottom line, although even that will require some form of smart metering. What is clear is that the days of simply looking at the sticker in the showroom and making a judgment that way are probably nearing an end.

P.S. A recent green car award went to the Volswagen BlueMotion line of vehicles – not electric, not hybrid, but diesel!! Meanwhile at the Shell Eco-Marathon earlier in May, the winning car in the urban concept category achieved 747 km/litre and the overall winning vehicle recorded 4896 km/litre with its hydrogen fuel cell based motor.