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    Hydrogen, one of earth's most abundant elements, once was seen as green energy's answer to the petroleum driven, easy to produce, available everywhere and nonpolluting when burned. Hydrogen energy was defeated by a mountain of obstacles, the fear of explosion by the highly flammable gas, the difficulty of carrying the fuel in large, heavy tanks in the vehicle and the lack of a refueling network. Automakers turned to biofuels, electricity or the gas-electricity hybrid.

    But hydrogen, it turns out, never was completely put of the race. Now Israeli scientists and entrepreneurs claim to have brought hydrogen energy a step closer by putting it in much smaller, lighter, containers. Rather than using metal or composite cylinders of compressed gas that look like bulky scuba gear, hydrogen is packed into glass filaments which, once out of the lab, will be only slightly thicker than a human hair. These 370 glass capillaries are bundled into a glassed tube called capillary array, about the width of a drinking straw. The scientist say 11000 such arrays will fuel a car for 400 kilometers, take less than half the space and weight of tanks currently installed in the few hydrogen cars now available.

    The scientist make no attempt to improve the standard fuel cell, which is not much different today from when it was invented more than 150 years ago. A fuel cell makes electricity from chemical reactions involving hydrogen and oxygen, producing only water vapor as a byproduct. The fuel cell can be compared with a standard car's engine, while the capillary arrays would be comparable to the gasoline tank.

    While its backers call the technology a breakthrough, it is unlikely to gain traction without a large injection of capital to scale up development. It also would need a distribution system and the support of major car companies, which have poured billions of dollars into their own closely guarded research programs. Like electric cars, the driving force behind hydrogen research is the need to break away from oil and rein in emissions of greenhouse gases blamed for climate change, especially carbon dioxide from industry and transport. Transportation adds about 13 percent of manmade carbon to the atmosphere. Hydrogen boasts zero emissions. It can be produced from water through electrolysis, or harvested as the waste product of nuclear reactors and chemical slants. Within few years, perhaps a decade, hydrogen fuel will shift the world's energy balance away from oil.

According to the passage, why was the hydrogen less preferable energy source to petroleum?