Swell development

It was while sailing in the Caribbean that James McCague, technical director of Ocean Navitas, hit upon the idea of developing a new type of wave-energy device.

Returning to the UK, McCague conducted a review of existing wave-energy-conversion devices before concluding that his method of extracting electrical energy from the waves would provide genuine advantages over existing systems. The result of his efforts is a renewable-energy system that will generate power at its maximum rated capacity in sea states of 3.5m in coastal regions around the UK.

In the Ocean Navitas system, a buoyancy float is carried up and down as it reacts to ocean swell and the effect of gravity. The motion of the float is then transferred through a set of bespoke gearing to a standard permanent magnet alternator that creates electrical power.

At the heart of the system is the Aegir dynamo mechanical drive. This is responsible for converting the energy captured from the upward motion of the waves and the downward motion of gravity into singular directional rotational energy.

The dynamo is comprised of a linear double-sided rack that links up to two gear sets that alternately transmit power into a single-gear system.
When the float moves in the upward direction, one set of gears drives the central gear system while the second set freewheels. When the float moves downward, the gear set that was previously freewheeling drives the central gear while the other freewheels.

‘Because of the design of the Aegir dynamo, it is irrelevant how much the buoy moves up and down - it will still spin the dynamo,’ said McCague. ‘The actual displacement of the float in the water isn’t related to the amount of power that is transmitted from it. It works in any condition where the buoy is moving up and down.’

To test out the concept, the Ocean Navitas team placed a small-scale prototype of the system in a large, custom-built wave tank. ‘We used a proof-of-concept model in the wave tank to demonstrate that the idea would work,’ said McCague. ‘To generate power from the unit, we used a small, permanent magnet alternator from a wind turbine that had a 1.5kW capacity. When we simulated waves with up to 0.5m displacement, we found that the system could produce around 0.3kW.’

A hydraulic wave simulator was then constructed for testing the power take-off system in laboratory conditions and the system was subjected to wave patterns that had previously been captured by the company at various coastal locations around the UK. This enabled McCague and his team, together with consultants from Nottingham Trent University, to assess the performance of the system under the conditions that it would actually encounter at sea. The tests confirmed that the technology would be viable in locations with relatively small waves and also that when it was scaled up it could effectively generate more power (up to 1.4MW at maximum scale) in coastal conditions where waves could be up to 3.5 m.
Ocean Navitas hopes to develop bespoke wave-energy-conversion devices for UK waters later in 2010. ‘While wave-energy devices such as the Pelamis Wave Energy Converter operate at an optimum efficiency in 5.5m waves, our Aegir system would be better suited to locations that have significantly less resources,’ said McCague.

He added that the optimum way to deploy the system would be in multiple units. These would be less expensive to produce than one larger unit, yet deliver a similar power output. The units’ cost could be reduced further if the company chose to mass produce them.
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