Life + Outdoors » Garlick's Notebook

Can We Extract Energy from Waves?



PG&E is seeking permits to investigate the feasibility of producing energy from waves off Humboldt Bay. The potential is high off our coast because of our winter storms and summer winds.

Waves approaching shallow water begin to break and lose energy, so most schemes target waves in water deeper than the “wave base” (about half the wave length) but close enough to shore to minimize the cost of transmitting power via submarine cables.

Some schemes are easily understood: One involves focusing waves into a chimney where air is trapped and compressed. That air turns a turbine attached to a generator. Another method involves floating a string of pontoons hinged together by actuator arms that compress hydraulic fluids as the string bends back and forth. The fluids run hydraulic motors attached to generators. Then there are the “oscillating point absorbers” — aka buoys — that respond equally to waves from any direction.

A simple experiment is recommended: Tape a quarter to the end of a wine cork. Float this buoy in a glass of water and observe its behavior as you accelerate the glass up and down. It behaves as though set in concrete! Even a submarine’s periscope would not submerge if an earthquake were to suddenly lift the sea floor many meters. That’s because the inertial mass of a buoy or submarine is exactly equal to that of the water it displaces.An obvious way to extract energy from the buoy is to connect the bobbing buoy to a stationary “object” and use their relative motion to generate energy. The stationary “object” could be the sea floor, but a more resilient “object” would be the water mass below wave base. One could use a deeply submerged, neutrally buoyant tank to push and pull a magnetized rod through a coil fixed in a bobbing buoy.

The method that is most intriguing, and already deployed for testing off Oregon’s coast, is the AquaBuOY system. Attached to the buoy is a cylinder, open at both ends, ideally extending down to below wave base. Within the cylinder is a piston to which are attached two steel-reinforced rubber hoses. When the buoy moves up, the piston is held down by the inertia of water within the large cylinder. The upper hose stretches and squeezes its contained fluid to high pressure. (I confirmed this “hose pump” effect by stretching water-filled surgical tubing.) Meanwhile, the lower hose relaxes and refills with fluid which is later expelled when the buoy moves down, stretching the lower hose. Each 5-meter diameter AquaBuOY, operating in our average 35 kW/m waves, could produce 55 kW of power, enough for 50 homes (according to Finavera Renewables).

Energy can certainly be extracted from waves, but whether or not that happens here depends upon the involvement of interested stakeholders, including fishermen and public agencies.

Don Garlick is a geology professor retired from HSU. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can.E-mail [email protected].

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