Nissan said the Leaf hatchback would be the world's first affordable, mass-produced, zero-emission car.

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One example was a tax for not purchasing emission control equipment for a car.

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Even if the electric car is driven for 90, 000 miles and the owner stays away from coal-powered electricity, the car will cause just 24% less carbon-dioxide emission than its gas-powered cousin.

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Finally, Declan looked at the BMW Hydrogen 7 - its only emission is water vapour but the technology for this type of green car is still being developed and currently, there are no models on sale.

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In the daytime, when the sun is at its strongest, solar panels like these and electricity stored in car batteries will be able to power the grid with affordable, emission-free energy.

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Tesla's biggest windfall has been the cash payments it extracts from rival car makers (and their customers), via its sale of zero-emission credits.

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So, a hydrogen fuel-cell car even running ... on hydrogen derived from natural gas cuts that carbon emission by 60%.

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