Tech

IT MIGHT have hogged the limelight at last week's Geneva Motor Show in Switzerland, but the most arresting detail on Porsche's latest concept car (pictured) was actually somewhat mundane: a wall plug. But over the next 12 months plugs will be increasingly appearing on production models from the world's biggest car makers. And as they do, electricity providers and governments will be scrambling to prepare for the as-yet-unknown effects of shackling our transport power needs to the electricity grid.

Plug-in cars come in two forms: electric vehicles fully reliant on a battery and the electricity grid, and plug-in hybrids that combine a smaller battery with a conventional engine.

When they start to appear in significant numbers, electric cars have the potential to drastically alter the demand patterns that our electricity infrastructure has been built around (see graph). The Nissan Leaf, a fully electric family car, will start to roll off production lines in October with a 24-kilowatt-hour battery pack. That sort of capacity is not far short of the average American household's daily consumption of electricity - 30 kWh, according to 2008 figures from the US Department of Energy.

Plug-in hybrids' batteries have lower capacities: 5 kWh in the case of the Toyota Prius and 16 kWh for the Chevrolet Volt, due out in November. These batteries can also draw charge from their gasoline-driven engines, but they will nevertheless consume additional mains power.

"The concern is that this new demand is potentially invisible to us," says David Densley, head of sustainability at UK energy supplier Scottish & Southern Energy, which is working with car-maker BMW on a trial of about 50 electric versions of its Mini, the Mini E, in southern England. "People could go and buy cars, plug them into their existing socket and the first thing we'll know is that the lights go out on the whole street."

Densley says a clustering effect is expected, where the appearance of one electric vehicle in a neighbourhood leads to a concentration of several on the same section of grid. "That could have a significant impact."

Results from the Mini E trial and others taking place around the world are being used by suppliers to try to predict how many cars can be supported without upgrading local networks.

Given that the specification of vehicles set to ship and the plug-point power are both known, basic forecasting of demand is possible. For example, charging a Nissan Leaf takes 16 hours on a standard US 110-volt supply, or 8 hours using the 240-volt supply which is standard in Europe and also installed in US homes for high-demand appliances like tumble-dryers.

Predicting the all-important behaviour of the drivers of these cars is more tricky, however. Extensive user testing has taken place in advance of the Leaf's launch, says Olivier Paturet, head of Nissan's European Zero-Emissions Mobility Program. "But we still don't quite know how they will be used."

A variety of studies, including the Mini E research, has found that "two-centre charging" is popular with drivers, combining an overnight charge at home using cheap power during the low-demand hours with a top-up at work during the day. The public charging networks being rolled out in cities such as London, Amsterdam in the Netherlands and Houston, Texas, will allow more options.

However they choose to charge, drivers will have to become used to power suppliers taking a strong interest in what they do with their vehicles, whether by needing to know when a new vehicle is bought, the pricing of peak power use, or through direct control of their car's connection.

"The electricity industry has to keep the lights on," says Joe DiNucci, a director of Coulomb Technologies in San Jose, California, which makes electric vehicle charging points. "They need to know what charging points are doing, and to have some control."

Coulomb's charging points are linked to the internet and can be monitored and even remotely controlled by a power utility, allowing them to slow down the rate cars are drawing power at times of high demand, for example. The firm is already operating chargers on the sites of Silicon Valley companies such as Apple, Pixar and Google, and in public for city authorities such as San Francisco and Houston.

The chargers being used in the UK's Mini E trial are less subtle. They deliver power only after 11 pm, when electricity is cheap, unless a "boost" button is pressed to trigger an hour of more costly charge at any given time. In future, the kind of nuanced monitoring offered by networked points like Coulomb's is likely to become standard for home chargers, says Denseley. As these "smart meters" are rolled out by the US, the UK and other western governments and start to appear in homes, electricity grids will become more efficient.

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