Way back in Napoleonic Paris, a Monsieur Girard had a novel idea about energy: power from the sea. In 1799, Girard obtained a patent for a machine he and his son had designed to mechanically capture the energy in ocean waves. Wave power could be used, they figured, to run pumps and sawmills and the like.

These inventors would disappear into the mists of history, and fossil fuel would instead provide an industrializing world with almost all its energy for the next two centuries. But Girard et fils were onto something, say a growing number of modern-day inventors, engineers, and researchers. The heave of waves and the tug of tides, they say, are about to begin playing a significant role in the world's energy future.

In the first commercial-scale signal of that, last October a trio of articulated, cylinder-shaped electricity generators began undulating in the waves off the coast of Portugal. The devices look like mechanical sea snakes. (In fact, their manufacturer, Scotland's Pelamis Wave Power Ltd., takes its name from a mythical ancient Greek sea serpent.) Each Pelamis device consists of four independently hinged segments. The segments capture wave energy like the handle of an old fashioned water pump captures the energy of a human arm: as waves rock the segments to and fro, they pump a hydraulic fluid (biodegradable, in case of spills) powerfully through a turbine, spinning it to generate up to 750,000 watts of electricity per unit. Assuming the devices continue to perform well, Portuguese utility Energis expects to soon purchase another 28 more of the generators.

The completed "wave farm" would feed its collective power onto a single high voltage sea-floor cable, adding to the Portuguese grid about 21 megawatts of electricity. That's enough to power about 15,000 homes.

In a world where a single major coal or nuclear plant can produce more than 1,000 megawatts of electricity, it's a modest start. But from New York's East River to the offshore waters of South Korea, a host of other projects are in earlier stages of testing. Some, like Pelamis, rely on the motion of waves. Others operate like underwater windmills, tapping the power of the tides.

Ocean-powered technologies are in their infancy, still technologically well behind such energy alternatives as wind and solar. Necessarily designed to operate in an inherently harsh environment, the technologies remain largely unproven and — unless subsidized by governments — expensive. (Portugal is heavily subsidizing the Pelamis project, with an eye to becoming a major European exporter of clean green power in the future.) Little is known about the effects that large wave or tide farms might have on marine ecosystems in general.

Despite the uncertainties, however, proponents say the potential advantages are too striking to ignore. Eight hundred times denser than air, moving water packs a huge energy wallop. Like solar and wind, power from moving seas is free and clean. But sea power is more predictable than either wind or solar. Waves begin forming thousands of miles from coastlines and days in advance; tides rise and fall as dependably as the cycles of the moon. That predictability makes it easier to match supply with demand.

Roger Bedard, who leads ocean energy research at the U.S. utility-funded Electric Power Research Institute (EPRI) in Palo Alto, says there's plenty of reason for optimism about the future of what he calls "hydrodynamic" power. Within a decade, he says, the U.S. could realistically meet as much as 10 percent of its electricity needs from hydrodynamic power. As a point of reference, that's about half of the electricity the U.S. produces with nuclear power today. Although he acknowledges that initial sea-powered generation projects are going to be expensive, Bedard believes that as experience grows and economies of manufacturing scale kick in, hydrodynamic power will follow the same path toward falling costs and improving technologies as other alternatives.

"Look at wind," he says. "A kilowatt hour from wind cost fifty cents in the 1980s. Now it's about seven cents." (That's about the same as producing electricity with natural gas, and only about three cents more than coal, the cheapest — and dirtiest — U.S. energy choice. Any future tax on carbon emissions could narrow that gap even more, as would additional clean-power subsidies.)

For some nations, wave and tide power could pack an even bigger punch. Estimates suggest, for instance, that the choppy seas surrounding the United Kingdom could deliver as much as 25 percent of its electricity. British alternative energy analyst Thomas W. Thorpe believes that on a worldwide basis, waves alone could produce as much as 2,000 terawatt hours of electricity, as much as all the planet's major hydroelectric plants generate today.

Although none are as far along as Pelamis, most competing wave-power technologies rely not on the undulations of mechanical serpents, but instead on the power captured by the vertical bobbing of large buoys in sea swells. Ocean Power Technologies (OPT), based in New Jersey, drives the generators in its PowerBuoy® with a straightforward mechanical piston. A stationary section of the mostly submerged, 90-foot buoy is anchored to the ocean floor; a second section simply moves up and down with the movement of sea swells, driving pistons that in turn drive an electrical generator. The Archimedes Wave Swing, a buoy-based system developed by Scotland's AWS Ocean Energy, harnesses the up-and-down energy of waves by pumping air to spin its turbines. Vancouver-based Finavera Renewables uses seawater as its turbine-driving hydraulic fluid.

Although Pelamis beat all of these companies out of the commercialization gate, OPT appears to be right behind, with plans to install North America's first commercial-scale wave power array of buoys off the coast of Oregon as early as next year. That array — occupying one square-mile of ocean and, like other wave power installations, located far from shipping lanes — would initially produce 2 megawatts of power. OPT also announced last September an agreement to install a 1.4-megawatt array off the coast of Spain. An Australian subsidiary is in a joint venture to develop a 10-megawatt wave farm off the coast of Australia.

Meanwhile, Pelamis Wave Power plans to install more of its mechanical serpents — three megawatts of generating capacity off the coast of northwest Scotland, and another five-megawatt array off Britain's Cornwall coast.

The Cornwall installation will be one of four wave power facilities plugged into a single, 20-megawatt underwater transformer at a site called "Wave Hub." Essentially a giant, underwater version of a socket that each developer can plug into, Wave Hub — which will be connected by undersea cable to the land-based grid — was designed as a tryout site for competing technologies. OPT has won another of the four Wave Hub berths for its buoy-based system.

Other innovators are trying to harness the power of ocean or estuarine tides. Notably, in 2007, Virginia's Verdant Power installed on the floor of New York's East River six turbines that look, and function, much like stubby, submerged windmills, their blades — which are 16 feet in diameter — turning at a peak rate of 32 revolutions per minute. The East River is actually a salty and powerful tidal straight that connects Long Island Sound with the Atlantic Ocean. Although the "underwater windmills" began pumping out electricity immediately, the trial has been a halting one. The strong tides quickly broke apart the turbines' first- (fiberglass and steel) and second- (aluminum and magnesium) generation blades, dislodging mounting bolts for good measure.

Undeterred, in September Verdant Power began testing new blades made of a stronger aluminum alloy. If it can overcome the equipment-durability problems, the company hopes to install as many as 300 of its turbines in the East River, enough to power 10,000 New York homes.

A scattering of similar prototype "underwater windmill" projects have been installed at tidal sites in Norway, Northern Ireland, and South Korea. (In addition, interest in moving into freshwater sites is growing. Verdant itself hopes to install its turbines on the St. Lawrence River. At least one other company, Free Flow Power of Massachusetts, has obtained Federal Energy Regulatory Commission permits to conduct preliminary studies on an array of sites on the Mississippi River south of St. Louis.)

The environmental benefits of hydrodynamic power seem obvious: no carbon dioxide or any other emissions associated with fossil-fuel-based generation. No oil spills or nuclear waste. And for those who object to wind farms for aesthetic reasons, low-profile wave farms are invisible from distant land; tidal windmill-style turbines operate submerged until raised for maintenance.

There are, however, environmental risks associated with these technologies.

New York state regulators required Verdant Power to monitor effects of their its turbines on fish and wildlife. So far, sensors show that fish and water birds are having no trouble avoiding the blades, which rotate at a relatively leisurely 32 maximum revolutions per minute. In fact the company's sensors have shown that fish tend to seek shelter behind rocks around the channel's banks and stay out of the central channel entirely when tides are strongest.

But a host of other questions about environment effects remain unanswered. Will high-voltage cables stretching across the sea from wave farms somehow harm marine ecosystems? Will arrays of hundreds of buoys or mechanical serpents interfere with ocean fish movement or whale migrations? What effect will soaking up large amounts of wave energy have on shoreline organisms and ecosystems?

"Environmental effects are the greatest questions right now," EPRI's Bedard says, "because there just aren't any big hydrodynamic projects in the world."

Projects will probably have to be limited in size and number to protect the environment, he says – that's a big part of the reason he limits his "realistic" U.S. estimate to 10 percent of current generation capacity. But the only way to get definitive answers on environmental impact might be to run the actual experiment — that is, to begin building the water-powered facilities, and then monitor the environment for effects.

Bedard suggests that the way to get definitive answers will be to build carefully on a model like Verdant's: "Start very small. Monitor carefully. Build it a little bigger and monitor some more. I'd like to see it developed in an adaptive way."

• This article was shared by our content partner Yale Environment 360, a member of the Guardian Environment Network.