New wave

Visitors to the UK’s Atlantic coast cannot fail to be impressed by the rolling ocean waves. And watching the breakers, it’s easy to understand how engineers are excited by the possibilities of harnessing the power of the waves.

After a long lull, design of new wave energy devices is gathering pace and it may not be long before arrays of wave parks become a relatively common sight off the west coast and northern Scotland. But one device, soon to begin coastal testing off Orkney, will not be visible.

Most wave energy devices fall into two groups: those that float on the surface of the water, moved up and down and side to side by the waves or allowing the waves to break over them; or coastal devices, which use water forced into them by breaking waves to operate generators.

The Archimedes Wave Swing (AWS), however, floats below the sea surface, anchored to the bed on a flexible stalk and resembling a giant aquatic mushroom. The design, being developed by marine renewables company AWS Ocean Power, based in Ross-Shire near Inverness, is claimed to have significant benefits over surface-based wave energy converters.

‘There are no other designs that operate completely under the water,’ said AWS chief executive Simon Grey. Rather than moving with the motion of the sea, the AWS expands and contracts in response to the changes in water pressure caused under water by waves passing overhead. ‘To my knowledge, there are no other wave energy converters that work by changing volume,’ Grey added.

The main body of the AWS is in two parts. The floater, shaped like an inverted cup, sits on top of the can-like base unit and is joined to it by a flexible rolling membrane seal, made from a composite including Kevlar and rubber. The space between the floater and the base is under a partial vacuum, which pulls the two parts together. To counteract this, the floater is held away from the base by one or more large hydraulic rams containing high-pressure oil, which act like springs under compression, pushing the floater upwards.

Under a flat, calm surface of water, the hydrostatic pressure exerted by the water and the force of the hydraulic rams cancel out the force of the vacuum inside the device, and the floater sits motionless on top of the can. But when a wave passes over the device water pressure increases, then decreases, and the machine has to contract and expand to maintain the equilibrium between its interior and exterior.

The device is held, with the crown of the floater some 6m below the surface at low tide, by a tension leg up to 100m long, attached to a sea-bed anchor — a concrete block or steel plate, depending on the conditions — by a universal joint. This allows the device to be winched down to position and snapped into place in a quick, simple operation. The power generation equipment is separate from the device, mounted on a sea-bed skid: the most likely option for this is a hydraulic motor operated by the high-pressure hydraulic oil, Grey said, although linear generators might also be an option.