There have been attempts to harvest the power of waves through tidal barrages, such as the one in use at the LaRance Tidal Power Station in France, but these have posed environmental problems, and proved costly to build and operate.

Now, companies in the United Kingdom and Canada are trying to harvest the power of sea current through a new application of an old technology: turbines. These current, or sea, turbines function much like wind turbines. Instead of being driven by wind, though, they derive energy from coastal currents. Seawater is 832 times as dense as air, so the kinetic energy available from a 5-knot ocean current is equivalent to a wind velocity of 270 kilometers per hour.

Neither IT Power of Hampshire, England, nor Blue Energy Canada of Vancouver, British Columbia, has progressed beyond theoretical work and small-scale testing, but both claim that the technology will be ecologically and economically sound.

IT Power will be conducting its first major test in the coming months. The company will deploy a prototype 300-kilowatt turbine off the North Devon coast of southwest England, according to senior engineer Jeremy Thake. In a commercial application, the turbine could be connected to the power grid via a submarine cable. In this test, the turbine will have its own resistive load.

IT Power is using technology from its spin-off company, Marine Current Turbines, also in Hampshire. The technology consists of a pair of axial flow rotors that are roughly 50 to 65 feet in diameter. Each drives a generator via a gearbox, much like a wind turbine.

The proper conditions for current turbines call for concentrated tides at a minimum of 4 knots. Faster currents are even better.

The power unit is mounted on an 8- to 10-foot-diameter tubular steel monopile, which is set into a hole drilled into the seabed. The technology for placing these monopiles was developed by Seacore Ltd., an offshore engineering company and MCT's largest shareholder.

The turbine is connected to the shore by a marine cable lying on the seabed, which emerges from the supporting pile. For commercial use, turbines could be grouped in arrays similar to wind turbines in a wind farm. IT Power expects these farms to have a generating capacity of at least 20 megawatts.

The MCT design used by IT Power allows the turbine to be lifted out of the water for maintenance, rather than requiring divers to service the units, Thake said.

MCT has designed variable pitch blades for its system and these will be used on the 300-kW test. Variable-pitch blades will provide an increase in the turbine rotor's hydraulic efficiency over that possible with fixed blades, Thake said.

IT Power expects to conduct pre-commercial prototype tests within the next four to five years. Ultimately, the system will include twin rotor generator units on a single vertical monopile.

According to Thake, the Devon coast site was selected for this test because it possesses the "ideal" conditions for operating a tidal turbine. It is relatively deep, somewhat sheltered, offers good, strong currents, and is relatively close to land.

The turbine being used in the test requires water depth of 40 to 45 meters (roughly, 130 to 150 feet). The goal is to one day be able to run the turbines in water that is a minimum of 20 meters, or 66 feet, deep. The currents off the Devon coast average 5 knots; the system has a 4-knot current speed minimum. There is no maximum current speed for the system, and the faster the currents, the more economical the system is to run, Thake said.

Building a Bridge

Blue Energy Canada is also working the currents. Its approach differs from that of IT Power in two significant ways: orientation of the turbine blades and their arrangement.
Instead of using horizontal axis turbines, Blue Energy's system relies on the ducted vertical axis Davis Hydro Turbine, which is technically very similar to the large vertical axis Darrieus wind turbines, according to a company spokesperson. The Canadian National Research Council funded the turbine's early development, but ended its support in the late 1980s.

Four fixed hydrofoil blades are connected to a rotor that drives an integrated gearbox and electrical generator assembly. The turbine is mounted in a concrete marine caisson, which anchors the unit to the ocean floor, directs the water flow through the turbine and supports the coupler, gearbox, and generator above. The hydrofoil blades use a hydrodynamic lift principle that causes the turbine foils to move proportionately faster than the speed of the surrounding water. BEC foresees operating the turbines in tidal fence and tidal bridge configurations.

IT Power expects to deploy its turbines in a farm, with the turbines lifting out of the water for maintenance.

Whether farms or fences are the better configuration for generating power remains to be seen. The true cost of these systems and their environmental impact have yet to be proven conclusively. Hundreds of sites throughout Canada, the United Kingdom, Europe, and the Philippines have been identified as having the right conditions to house a current generating facility, but there's little hard science to back up these claims.

A study conducted in 2001 by Triton Consultants, based in Vancouver, B.C., on behalf of BC Hydro (one of the largest electrical utilities in Canada), found that the cost to develop a current turbine site is rather high, but the cost of annual power generation would be low. The study considered a site at the Discovery Passage in British Columbia, which it speculated would run 794 1-MW Marine Current Turbines spread over roughly 3,922 acres (1,587 hectares). The study found that the cost to develop this site would be roughly $900 million U.S. ($1.4 billion Canadian), including an interconnection and strengthening of the power grid. The annual power generation from the facility would be 1,390 GW/h per year at a cost of 7 cents U.S., or 11 cents Canadian, per kilowatt-hour.

Both this study and early research by IT Power indicate that the environmental impact of such an installation would be low. The BC Hydro study foresees little meaningful change in tidal height and its timing. According to the study, immediately downstream of the current generating farm there would be about a 10 percent decrease in the velocity of the current, but this shouldn't pose any real problems to the environment.

IT Power's Thake says that because the rotors of the turbines are relatively slow-moving, they pose little threat to fish and other small marine life. "Fish will likely be accelerated by the movement of the rotors, but not cut up by them," Thake said. "It's like moving through a revolving door."

Still, these claims are all theoretical. So while sea turbines hold promise as a source of renewable energy, they're a long way from being a viable technology. According to Thake, IT Power's Devon coast test is as much about "convincing people that current turbines work as it is about testing the economics and environmental impact."

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