Vidi kôd stranice Theeconomist howgreen is yourhidrogen.html

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face="Arial, Helvetica, sans-serif" size=3><B>SCIENCE AND TECHNOLOGY</B></FONT><BR>
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      <DIV><FONT face="Arial, Helvetica, sans-serif" size=6>How green is your hydrogen?</FONT> </DIV>
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FUEL cells, which generate electricity by reacting hydrogen and oxygen together, and thus yield water as their exhaust, may sound like the answer to environmentalists’ prayers. Most big car makers certainly hope they are. They are spending huge sums of money on what may turn out to be the 21st century’s replacement for the internal-combustion engine. But just how clean a fuel cell really is depends on the answer to a dirty little question: where, exactly, did the hydrogen that powers it come from?
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That a fuel-cell-powered vehicle does not muck up its immediate environment is unquestioned—and it is a good thing in its own right. But the naive might assume that the lack of carbon in its fuel means that a fuel cell cannot contribute any carbon dioxide to the atmosphere, and thus will not have any effect on global warming. That is frequently wrong. 
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In an environmentally ideal world, the most elegant option would be to produce hydrogen directly by the electrolysis of water, using electricity generated from renewable energy. However, this is unlikely to be economic in the immediate future, except in a few countries, such as Iceland, that have abundant hydroelectric or geothermal power. 
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The alternative is to get the hydrogen by stripping it from hydrocarbon molecules of the sort found in fossil fuels. Current prototypes do this on board a vehicle in an apparatus called a reformer. But the chemical processes used in reformers release the surplus carbon as carbon dioxide. 
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That may, for the moment, be inevitable. But a report just issued by the Pembina Institute, a Canadian research group, suggests that the hydrocarbon fuels most likely to make it to market are also the ones that do the least to cut greenhouse-gas emissions. The report tries to calculate the “well to wheel” release of greenhouse gases (mostly carbon dioxide) for a variety of fuels—in other words, the amount of greenhouse gas released as a result of the extraction, refining and transport of the fuel, and its use to power a car for 1,000km. The smallest improvements over current technology came from so-called “clean” petrol (ie, with all nasties such as sulphur removed) and methanol, fuels favoured by the car and oil companies. The best result was obtained with natural gas. 
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That is not surprising. Natural gas is mostly methane, which has four hydrogen atoms for every carbon atom—the maximum that is physically possible for a hydrocarbon. And yet what is worrying is that the big firms are largely ignoring natural gas as a source of hydrogen. 
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It is also curious. One of the things that car companies fret about is how fuel will be delivered in a fuel-cell world. Hydrogen and methanol, at least, would require a whole new infrastructure. But natural gas would not. It is already widely on sale, although not at the petrol stations so dear to oil firms. 
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Gas is also easy and efficient to reform into hydrogen. That is not true for methanol. And it is cheap. According to Richard Stobart of Arthur D. Little, a consultancy that has investigated the matter, the running costs to a driver of a fuel-cell car that gets its hydrogen from natural gas would be comparable to those of one that used clean petrol. 
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The only serious objection to natural gas is that, even when compressed or liquefied, it takes up a lot of space. In the short run, that might restrict natural-gas use to vehicles such as lorries and buses, where space is at less of a premium (and which already run on a different fuel from most cars). In the longer run, however, it should be a soluble problem. 
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One promising idea is to extract the hydrogen from natural gas in large reformers located at filling stations, and then store it on board vehicles in solids known as metal hydrides. The latest versions of these, such as one recently unveiled by a Detroit-based firm called Energy Conversion Devices, soak up hydrogen so well that a tank of hydride can deliver as much energy as a same-sized tank of petrol. This technology might be ready for market within a couple of years. Or perhaps a few show-off greens will resort to an expedient that was seen in wartime Britain—huge gas bags on top of their vehicles. That way they will be able to proclaim their saintliness in a form that all can see.
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                     <p align="justify"><font face="Arial" size="2">  <FONT face="Times New Roman, Times, serif" 
            size=4><B>LINKS</B></FONT> <BR> <BR>
Energy Conversion Devices’s [http://www.ovonic.com/enstor.html Ovonic Battery Company] is developing Ovonic battery technology.  </font> <BR><BR>
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