ODAC Guest Commentary - How green are electric cars?
I have been reading and watching with some bemusement a number of stories appearing in the British press and on television this past week on the subject of electric cars. The media interest is largely a reaction to the UK government’s recent announcement of plans to provide cash incentives to buyers of plug-in vehicles, designed to stimulate the market for highly efficient vehicles. A number of articles, some of which have hot-links from the ODAC website, have ‘experts’ variously dismissing the environmental benefits of electric cars as fiction, claiming their mass adoption will cause blackouts, or accusing the government of a cheap gimmick. Whatever the rights and wrongs of the proposed stimulus package, its lack of sophistication should not be allowed to undermine the fact that electric cars are fundamentally a good idea. Shifting transport away from liquid hydrocarbon fuels towards electricity can make a significant contribution to the twin challenges of climate change and energy security.
Frequently repeated is the lazy sound bite that “electric cars are only as green as the electricity they run on”. Sounds obvious, doesn’t it? But it neglects the fact that based on today’s UK electricity mix – still heavily reliant on natural gas and coal – electric cars can cut CO2 emissions in half compared with conventional mechanical vehicles running on petroleum. Even taking into account transmission and distribution losses, it is always more energy efficient to burn carbon-based fuels – coal, oil, gas, and biomass – in large stationary power plants running at constant load than it is to waste additional energy converting them into liquid transport fuels and then burning them in small mobile internal combustion engines running at variable speeds.
In a Daily Telegraph article, one expert was quoted as saying that modern diesel engines can achieve 45% efficiency. This is an extraordinarily optimistic estimate, especially considering that automotive engines are seldom running at optimal efficiency but instead are subject to cold start energy losses, frequent short journeys, stop/start urban driving conditions, idling at traffic lights and in queues, fast acceleration and hard braking, all of which combine to reduce the practical efficiency of the mechanical powertrain to around 20%.
The electric motor is a vastly more efficient – and reliable – device in principle than the internal combustion engine. To get the picture, we need to compare two vehicles sharing the same platform but utilising different powertrains. This way, we can eliminate variables such as vehicle size and aerodynamics which complicate comparisons from one vehicle platform to another. I reviewed the US Department of Energy website devoted to vehicle fuel economy (www.fueleconomy.gov), and found that in 2003 the electric variant of the Toyota RAV4 was 4.9 times more energy efficient over the standard test cycle than its petroleum-powered equivalent. 4.9 times! Note also that Toyota’s aim was not to build an energy efficient vehicle per se, but to comply with California’s “Zero-Emissions Vehicle Mandate” (the RAV4-EV used nickel metal hydride batteries, which are less efficient than modern lithium batteries that will power the new generation of electric cars). In other words, Toyota achieved this factor ~5 efficiency advantage almost by accident!
Putting this efficiency advantage into context, we can apply the carbon intensity of any given energy source to see what the effective life-cycle emissions would be. Imagine a run-of-the-mill pulverised coal plant generating power with approximately 1,000 gCO2/kWh. Factor in grid losses of around 6%, and the electricity at the plug socket contains roughly 1,064 gCO2/kWh. Meanwhile, petroleum-based fuels contain around 300 gCO2/kWh, taking into account the efficiency of a typical oil refinery. On this basis it looks as though petrol is better for the environment than coal-fired electricity. But when you apply the energy efficiency advantage of the RAV4-EV (i.e. 1,064 divided by 4.9), the relative carbon intensity of energy at the wheels is 28% less than the petrol version. Diesel engines are typically around 25% more efficient than petrol engines, all else being equal. This means the RAV4-EV charged with electricity from a run-of-the-mill pulverised coal plant would still be marginally better in terms of CO2 emissions than its diesel-powered equivalent.
But no country, not even China, has exclusively coal-fired electricity. In Britain, a diverse range of power generating technology means that electricity drawn at the domestic socket emits around 520 gCO2/kWh on average. On this basis, an electric RAV4 would produce two-thirds less CO2 per mile driven than the petrol version, and half as much as a comparable diesel.
Furthermore, once all those CO2 emissions have been concentrated from millions of vehicle tailpipes into a relatively few stationary point sources, then they lend themselves to a future in which we can capture and lock away the CO2 underground. Personally, I cannot imagine carbon capture and storage (CCS) from moving car tailpipes, but I can envisage CCS from large stationary power plants situated near suitable geological storage locations.
Further still, electric vehicles can actually help to accelerate the penetration of renewables such as wind and solar power, because one of the limits to renewable electricity generation is storage of energy from intermittent sources. With millions of electric vehicles connected to the grid we will have created a massive distributed energy storage facility, in the form of automotive batteries.
The more important point is this: if we are to avert catastrophic climate change, then the power sector will need to steadily decarbonise because it represents the single largest source of CO2 emissions. The good news is that we know how to decarbonise the power sector; we have a range of technologies and policy measures at our disposal and all that’s lacking is a globally inclusive international treaty to put an effective cap on emissions. In this respect, it is sensible to take: “Power decarbonisation over time” as one of our starting assumptions.
Contrast this with the liquid fuels sector, in which the carbon intensity is heading northwards as oil companies are forced to exploit more energy-intensive forms of liquid hydrocarbon (e.g. oil sands, oil shale, coal-to-liquids, etc.). Biofuels – even when produced sustainably with real greenhouse gas benefits – will struggle to make up the difference. Oil is going to get dirtier. And if the worst of electricity (i.e. pulverised coal) compares favourably with the best that petroleum has to offer (i.e. conventional diesel), then over time the advantage of electric vehicles can only increase.
Finally, there is much to be done in redesigning the entire transport paradigm, e.g. through modal shift from private cars to mass transit, encouraging more walking and cycling, and improving urban planning practices to eliminate demand for transport. Electric vehicles are not a panacea to cure all transportation ills. However, the clear energy efficiency advantages of electric vehicles, not to mention the crucial energy diversification potential (energy security frequently trumps environmental security in policy discussions), make them a very important part of the solution as we move toward a sustainable energy future.
Gary Kendall is author the WWF report Plugged in: The end of the oil age
What do you think? Leave a comment below.
Sign up for regular Resilience bulletins direct to your email.
This is a community site and the discussion is moderated. The rules in brief: no personal abuse and no climate denial. Complete Guidelines.