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25- www. energy- future. com 2.1- Transportation they'd be able to cope with millions of driv-ers plugging in their vehicles at the same time. And unless the electricity comes from a low- carbon source, such as solar panels, wind turbines or biogas, then it won't mean mobility without emissions. Lithium- ion batteries, which are smaller and have a greater energy density than the nickel- metal- hydride batteries used in hybrid cars such as the Toyota Prius, have the po-tential to improve range. However, it won't necessarily be a smooth journey. Lithium ion batteries can explode if they overheat: not a comforting thought as you're belt-ing down the freeway. They're also expen-sive, costing four to five times more than the nickel- metal- hydride variety, and gradually degrade with use. Tesla, a US electric- car manufacturer, says it's getting round the overheating prob-lem with a special cooling system for the 6,831 lithium- ion cells under the hood of its $ 100,000 Roadster. There are other reasons for the Tesla Roadster's high price tag - and they're more encouraging for the electric- vehi-cle industry. With acceleration from 0 to 60 miles per hour in 3.7 seconds and a top speed of 125 miles an hour, it is a high-performance vehicle - a rival to gasoline cars. It has a US Environmental Protection Agency efficiency rating equivalent to 135 miles per US gallon, making it almost three times as efficient as a hybrid vehicle. And one full charge should last something like 350 kilometres. One disadvantage is that, from dead, it takes about three and a half hours to charge the battery - not as quick as pumping 50 litres of gasoline into a tank. Tesla says it's rarely likely to take that long because the battery will seldom run down completely before its overnight recharge. While that may be the case for everyday urban use, the car would certainly not be as practical for long journeys or to places without access to electricity. The Tesla has another advantage: it looks like a car you might want to drive. Electrically powered milk floats may have been on the road for decades - since 1920s in the UK - but you wouldn't want to go to work in one, unless you were a milkman ( even then you might not - editor). ?? Unless the electricity to charge vehicle batteries comes from a low- carbon source, then electric cars won't mean mobility without emissions You wouldn't go to work in this ... . but you might in this: Tesla's Roadster

26- www. world- petroleum. org 2.1- Transportation Hydrogen power: pros and cons As an electricity- generation system, the great advantage of a hydrogen fuel cell is that its waste product is water, so in the-ory it's zero carbon. But things aren't that straightforward. Hydrogen doesn't occur naturally in great quantities so it has to be manufactured. Energy used in the manu-facturing process could generate emissions and if it's reformed from a fossil fuel, then that would add to the carbon footprint. There are other drawbacks. Fuel cells aren't yet robust enough to tolerate the rough treatment they'd get in a car. They're heavier than batteries and more complex - so more can go wrong with them and they're more expensive to buy and maintain. Hydrogen, meanwhile, is a volatile gas, making storage and transportation tricky. One possible way around the problem is to fill the tank with natural gas and reform it on board - safer than carrying round a tank of com-pressed hydrogen. But an on- board reformer is a complex piece of kit - like driving around with a refinery in the back of your car - which adds to the up- front and running costs. Also, there are almost no refuelling sites - so they would need to be built, which would be expensive. Nonetheless, despite the bar-riers to successful commercial development, the technology works and there are several fuel- cell pilot programmes under way. Japan's Honda, for example, began leasing a limited number of its FCX Clarity hydrogen fuel cell cars last year and sees hydrogen as one of the long- term alternatives to gasoline. ?? Biofuels: pros and cons One of the big advantages of biofu-els is that they can be used in exist-ing car engines, so increased use of biofu-els wouldn't require the renewal of the car fleet. Distribution networks wouldn't have to change radically either: they are already handling large volumes of biofuels. Ethanol is in widespread use around the world as an alternative to gasoline. It's been especially successful in Brazil, where all gasoline contains a 25% ethanol blend. In addition, the widespread use of flex- fuel cars, which can run on any mixture of eth- Biofuels: big business in Brazil