The Point Absorber



Sometimes the simplest designs are the best. The point absorber is a floating buoy that rises and falls with each passing wave, converting mechanical motion into electricity. As the buoy heaves up and down the movement is used to pump hydraulic fluid in a fixed cylinder that lies beneath the surface. The pressurised fluid spins a generator within the device, creating electricity that is transferred to shore via underwater transmission lines. Ocean Power Technologies of Pennington, New Jersey is currently testing its PowerBuoy – which measures 41 metres tall and 11 metres in diameter – in Scotland. It is expected to produce 150 kilowatts of continuous power.



(Image: Ocean Power Technologies)

The Attenuator



Attenuators are long floating devices that run perpendicular to oncoming waves and harvest energy from tip to tail. As waves pass along the length of the machine, individual sections heave up and down relative to each other creating mechanical movement that is converted into electricity. Pelamis Wave Power of Edinburgh, UK is currently testing its second-generation commercial-scale attenuator at the European Marine Energy Centre off Scotland’s Orkney Islands. The device, known as P2, is 180 metres long, 4 metres in diameter, and produces 750 kW of electricity.



(Image: Pelamis Wave Power) Advertisement

Oscillating Wave Surge Converter



Instead of bobbing up and down, oscillating wave surge converters pitch back and forth with each passing wave. The devices consist of massive hinged flaps attached to the sea floor in shallow waters. As waves pass the device they push the flap closed. That’s how the Oyster 800, built by Aquamarine Power of Edinburgh, works. But instead of generating electricity at sea, the device harnesses mechanical movement to pump pressurised water to an onshore facility. There the water drives a hydroelectric turbine that can easily be maintained from shore. Aquamarine will soon begin testing its commercial-scale device, Oyster 800, an 800 kW device that is 26 metres wide and stands 12 metres tall.



(Image: Aquamarine Power)

Rotational Wave Energy



A rather unusual form of wave energy turns energy-harvesting devices in circles like a spinning top. Rotational wave energy employs an eccentric, or off-centre, weight that is sealed inside a buoy or ship hull and spins in rough seas. Finnish company Wello will soon begin testing the Penguin, a 500 kW rotational wave energy device. The Penguin has an asymmetrically shaped hull that causes it to roll, heave, and pitch – much like the stilted stride of a penguin – with each passing wave. The movement is used to accelerate a 95-tonne weight that spins inside the hull and drives an electric generator to produce electricity.



(Image: Wello)

Oscillating Water Column



Oscillating Water Columns (OWCs) harness the rise and fall of passing waves to compress vast air reserves that drive a turbine to produce electricity. The machine sits semi-submerged on the ocean surface with a large hollow cavity open to the sea below the water line. Passing waves cause the water level inside the cavity to rise and fall, which compresses and decompresses air trapped inside the cavity above the water line.



Air flows out of the device through a vent on the surface when compressed and back in through the vent when decompressed. This moving air spins a bidirectional turbine that produces electricity regardless of direction. On 8 July this year, Voith Hydro Wavegen of Invernesss, UK completed the installation of a 300 kW OWC in Mutriku, Spain (shown here). The device, which also acts as a breakwater, is the first of its kind – a fully commercial wave energy device that is connected to the grid and selling power.



(Image: Voith Hydro Wavegen)