Marine Renewable Technology

February 1, 2013

Rube Goldberg had nothing on these guys. The American engineer, inventor and cartoonist most famous for drawing outlandish machines would have smiled at all the ideas for capturing ocean wave and tidal energy.

According to the European Marine Energy Center, there are at least eight design types for waves, and six for tides. The US Bureau of Ocean Energy Management (BOEM) counts four for waves, one for tides. All of them have been tried somewhere, by somebody.

The goal for each design is to apply Faraday’s Law by moving magnets through a coil of wire to generate electricity. Power takeoffs – they can be direct, geared, or hydraulic – use wave or tidal energy to create this movement. Water is 832 times denser than air, so slower currents generate as much power as high wind speeds.

“The equipment just has to be robust,” says Paul Jacobson of Electric Power Research Institute. “Water is powerful. The forces, turbulence create challenges. The ocean is corrosive. The cost of getting out there and maintaining and repairing is greater when you’re working in water.”

EMEC’s Neil Kermode, adds: “With wave, the big challenge is making sure you can be there after the big storm. Station-keeping, making that right, is something we’ve yet to deal with.

“On the tidal side, it’s mainly about getting the machine in the water at the start, fixing things down in tides that are pretty unrelenting.”

He quoted a Marine Current Turbines engineer who said that installing its tidal turbine in Northern Ireland was “pretty much equivalent to putting a wind turbine up and it withstanding Hurricane Katrina”.

Wave designs

1. Attenuator: a hinged pair of floats that ride the waves, capturing energy from the relative motion of the two floats as a wave passes. (Pelamis Wave Power)

2. Point Absorber: a floating structure with a less-buoyant base, converts the motion of the buoyant top, relative to the heavier base, into energy. (Wave Energy Technologies-New Zealand, Resolute Marine Energy, AWS Ocean Energy, Columbia Power Technologies, Ocean Power Technologies, WaveBob)

3. Rotating mass: a floating structure with a rotating internal weight that rocks on the waves and spins a generator, like a self-winding watch. (Wello Oy)

4. Oscillating Wave Surge Converter: a flap perpendicular to the waves, anchored on the bottom, that extracts energy as it moves back and forth with the waves. (Aquaramine Power, Neptune Renewable Energy)

5. Oscillating Water Column: a partially submerged hollow structure, with a column of air trapped above a column of water, when the water moves, it compresses the air, driving a turbine. (Dresser Rand, Oceanlinx, RWE, Voith)

6. Overtopping Terminator Device: catches water as waves break over a storage reservoir, drains the water back into the sea through a low-head turbine. (Wave Dragon)

Tide designs

1. Horizontal Axis Turbine: like a wind turbine with blades underwater, with the axis parallel to the current; extracts energy from water as it passes. (Atlantis Resources, Hammerfest Strom, Marine Current Turbines, OpenHydro, Verdant Power)

2. Archimedes Screw: a corkscrewshaped device that draws power as the tidal stream moves through the strakes (spiral blades), driving turbines. (Flumill)

3. Cross-flow Turbine: horizontal to the seabed, it looks like a hollow kitchen dough roller, with foils spiraling around an axle that turns a generator as water passes through. (ORPC)

See main story: "Our Crowning Glory?"

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