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Current Affairs

David Schneider

A curious form of renewable-energy generation may be on the horizon—or rather beneath the horizon: underwater turbines. These units harness the power of tidally driven currents, which flow back and forth like clockwork, making it possible to generate electricity on a predictable schedule. In this respect, underwater turbines are more attractive than their wind-driven counterparts, which are now employed widely to help power electric grids. Underwater structures are also less prone to damage from violent storms, which, as any submariner will attest, have little effect on submerged objects. And using underwater turbines sidesteps the common objections to conventional hydropower—that damming a river stops migrating fish and inundates land upstream.

Underwater turbines have long been used on a small scale (for example, to power irrigation pumps on a river). Now this technique is undergoing a renaissance, with proponents hoping soon to construct large systems that produce megawatts of electric power. Alexander Gorlov, a mechanical engineer at Northeastern University in Boston, is pushing a design with three blades that corkscrew around a vertical axis. This is a variation on the so-called Darrieus turbine, which the wind-power industry tried and then largely abandoned. Gorlov's helical configuration avoids a problem that arises with many Darrieus turbines: asymmetric loading forces, which stress the supporting structure and lead to unwanted pulsations in the output of the generator. Gorlov and GCK Technology, the Texas company he is working with, have produced relatively small turbines (a meter or so in diameter); scaling up is merely a matter of linking many of them together. The South Korean government, for example, is now testing Gorlov's helical turbine, with plans to install a series of them in Uldolmok Strait, a waterway near the south end of the Korean peninsula, where currents reach 12 knots.

Another project to harness the tides is under way off the coast of Norway, near Hammerfest. In choosing a design, the Norwegians were heavily influenced by the wind-power industry. Per Egil Skåre, a hydrodynamicist at SINTEF, a Norwegian contract research institute, has been involved in the planning for this project. He notes that almost every wind turbine now in use has three blades spinning around a horizontal shaft, adding, "We hope that the experience they have is also valid under water." Skåre anticipates that the 20-meter–diameter prototype soon to be installed will operate at about 40 percent efficiency, beating the 35 percent figure available from Gorlov's helical design.

A similar project is under way in the United Kingdom, where Marine Current Turbines, Ltd., will be installing an undersea turbine. Unlike the Norwegian unit, the British one will be mounted to a pile set permanently into the seabed and reaching all the way up to the surface. This, and some clever engineering, allows for servicing by raising the generator up the pile.

ôStingrayö tidal current generator . . .Click to Enlarge Image

Yet another type of underwater generator, dubbed "Stingray," is also being developed in the United Kingdom, this one by The Engineering Business, Ltd., a company involved in various offshore construction activities. The physical configuration of Stingray is dramatically different from that of the other underwater generators in the works. Rather than having a rotating turbine, Stingray sports a horizontal wing. Alternating the angle of attack allows the prevailing current to drive the wing up and down, which forces hydraulic fluid through a small internal turbine, which in turn generates electricity. Toby Bailey of The Engineering Business says that they adopted this curious geometry for their unit because they wanted to avoid the need for an underwater gearbox, which would otherwise be needed to connect a slowly spinning rotor to an electric generator. He explains further that their approach allowed them to design a unit that they could easily scale up by extending the length of the wing. (Underwater rotor blades run into problems as they become larger and the tip speed increases.)

Other schemes for tidal-current power generation are also being pursued. Presumably, the company that develops the first commercially successful unit will gain a significant edge on its competitors. So the innovative engineers at these companies must indeed be hurrying to refine a winning design, recalling, as another great inventor once said, that "time and tide wait for no man."—David Schneider

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