With the threat of global climate change casting a dark cloud over the future of fossil fuels, engineers have turned increasingly to technologies that harness clean energy from renewable natural sources such as the sun, the wind, and the ocean. But while solar and wind power projects have taken off in recent years, the potential of Earth's vast system of oceans has remained largely untapped.

And that potential is huge. The ocean produces three kinds of renewable energy: thermal, tidal, and wave. Ocean thermal energy conversion (OTEC) systems pump cold water up from the ocean's depths to the sun-warmed surface, where the resulting heat exchange powers turbines that produce electricity. Tidal energy systems control the kinetic energy of the ocean, either with free-flow devices, which as their name implies convert tidal currents into energy without impeding their flow; or with tidal barrages, which trap water at high tide and later release it through turbines. Wave-based systems also take advantage of the ocean's movement in this case, the incessant up-and-down bobbing familiar to anyone who has been on a small boat by channeling it through some type of device, usually made of concrete or steel, that is anchored near the shoreline and connected to a land-based generator.

Taken together, these three kinds of ocean-based power add up to an energy source that is both clean and unlimited. The main barrier to implementing them is not technological but economic: None of these systems can currently produce enough power at a low enough cost to make them competitive with their conventional rivals. Even wave energy technology, the most promising of the three, has not yet made much of an impact worldwide.

But the tide may be turning. The most recent evidence of this sea-change is a massive wave farm under construction off the coast of Portugal. Designed by a Scottish engineering firm, the project will use huge steel tubes, linked in parallel rows and anchored to the ocean floor. This system named "Pelamis" after the sea snake it resembles may eventually produce enough electricity to run 15,000 households. Although no one expects the wave farm to turn a profit any time soon, the Portuguese government has agreed to subsidize it as an investment in a greener future.

And Portugal is not alone: the United Kingdom and Spain reportedly have similar projects on the drawing board. Will the U.S., with its thousands of miles of coastline, follow suit? Only time and the tide will tell.

Learn more about this author, Denise Marie Anton.
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