The idea that hydrogen is a source of energy is really a bit misleading. One doesn't find free hydrogen lying around that we can burn to get energy. Essentially all the hydrogen on this planet is bound up with other elements.

When hydrogen atoms bond to other elements (for example, when two hydrogen atoms bind to an oxygen atom to make water), energy is released. To get the hydrogen back out, energy has to be put in, and that energy has to come from somewhere. For example, an electrical current can split water into hydrogen and oxygen, but energy is needed to make that electricity. It might come from nuclear, solar, wind, geothermal, or whatever, but it has to come from somewhere.

It would really be better to think of hydrogen as a means for storing and transporting energy than as a source per se. For example, if you have a generating station you could use the electricity locally to split water into hydrogen and oxygen. The hydrogen you stored up could be burned at any time in the future to provide energy, and that burning could happen anywhere you shipped the hydrogen to. That said, you would never get any more energy out of burning the hydrogen than it cost you to make it in the first place.

There is an exception to the above argument, and that's if you use hydrogen in fusion. In this case you push hydrogen atoms together to make helium plus a lot more energy than you used to get the hydrogen in the first place. This is not easy to accomplish. The sun does it, and so do hydrogen bombs, but we don't really know how to make it go at a nice rate for commercial applications - yet!

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

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