Wave-energy device adapts its set-up from calm to rough seas

Danish engineers have demonstrated a wave-energy conversion device that is able to adapt its structure from calm to rough seas.

The Weptos device expands on the ‘Salter’s Duck’ principle of wave generation, devised in the 1970s by the eponymous Scottish engineer, which uses ‘bobbing’ tear-shaped buoys.

Back then, the original idea was to have single double-decker-sized buoys, but Weptos uses an array of 20 buoys — or rotors — facing perpendicularly along two arms joined at the ends to create a V-shaped set-up.

The 20 rotors of each arm drive a single axis and, with the use of a ratchet mechanism, are collectively able to keep the axle turning at a speed of 30–94rev/min.

‘The power is fed in on the upstroke to achieve a relatively constant rotation on the common axle,’ said Prof Jens Peter Kofoed of Aalborg University, which is a partner in the project along with Danish companies Energinet and Zacco.

A gearing mechanism then increases the torque to around 90Nm, which is utilised by generators at the intersection between the arms. 

‘We can lift in the very same generators solutions you already have in wind turbines today — that is very important for Weptos that we can use this tested technology without any special expertise,’ said Tommy Larson, chief executive of the newly created Weptos company.

One of the key features of the device is the ability for the arms to spread out in calm waves, maximising any potential energy generation, then fold in at a tighter angle in rough seas, essentially creating a ‘hull’ that can cut through turbulence. The device is able to maintain its position relative to the direction of the waves by being tethered to the sea floor.

‘You still keep a full utilisation of the generators without overloading it and also when the wave length grows you need a longer projected length of the device to keep it stable in the sea, to avoid it pitching,’ said Kofoed.

A Weptos device with 8m-long arms was recently tested at Santander in Spain, which has the largest wave laboratory basins in Europe. The Weptos consortium now plans to build a device 10 times larger and deploy it for a year in the Kattegat sea between Denmark and Sweden.

‘It is relatively sheltered conditions, but we can gain confidence and provide demonstration, then move that to a near-shore position on the west coast in the Danish North Sea,’ said Kofoed.