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If hydrogen could be used in cars, it would be important in establishing the [[hydrogen economy]].

If hydrogen could be used in cars, it would be important in establishing the [[hydrogen economy]].

As free hydrogen does not exist on Earth, hydrogen is not actually a source of energy but rather a carrier of it: its value as a fuel is that it provides a clean and efficient way to use energy produced by some other source. Due to the Second Law of thermodynamics the energy used to produce hydrogen must be at least equal to the energy contained within it, meaning that at present it is not a viable fuel for most applications. Potential solutions include the use of bacteria that produce hydrogen through photosynthesis, which would effectively allow solar energy to be stored in hydrogen and used as fuel.<ref>http://www.making-hydrogen.com/hydrogen-from-bacteria.html</ref>

As free hydrogen does not exist on Earth, hydrogen is not actually a source of energy but rather a carrier of it: its value as a fuel is that it provides a clean and efficient way to use energy produced by some other source. Due to the Second Law of thermodynamics the energy used to produce free hydrogen must be at least equal to the energy contained within it, meaning that at present it is not a viable fuel for most applications. Potential solutions include the use of bacteria that produce hydrogen through photosynthesis, which would effectively allow solar energy to be stored in hydrogen and used as fuel.<ref>http://www.making-hydrogen.com/hydrogen-from-bacteria.html</ref>

==Notes and references==

==Notes and references==

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