New wave

An original method of harnessing wave power using underwater devices claims significant advantages over surface-based systems. Stuart Nathan reports

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.

Out of sight: the Archimedes Wave Swing floats beneath the surface of the sea, anchored to the sea bed on a flexible stalk

AWS was invented by a Dutch commercial R&D company, Teamwork Technology, in 1995. AWS Ocean Systems bought the intellectual property rights to the device in 2004 and its original inventor, Fred Gardner, now sits on AWS’s board with his fellow Teamwork director, Hans van Breugel.

‘At the stage we bought the IP, the concept was quite different,’ said Grey. Although the device looked much the same and the volume-changing concept still underpinned the mechanism, the components were different. The floater and can were held apart by a low-pressure air spring, with power offtake by direct generation with a linear generator mounted inside the machine.

The changes resulted from a technical optimisation exercise, said Grey. ‘We found that the original configuration required a very large volume of subsea gas and extremely large machines,’ he said.

Switching from air spring to high- pressure hydraulics allowed AWS to cut the size and the cost of the machine. A 500kW AWS is about 8m in diameter with an 8m stroke. ‘That’s still quite large, but we’ve done a detailed exercise looking at swept volume and the total volume of the device, and it’s smaller in terms of total volume/kW than any other wave device out there,’ claimed Grey.

Switching to hydraulics also gave more options for power take-off. ‘As soon as you put in a high-pressure oil spring you have the option of taking off power directly using hydraulics,’ said Grey. ‘Linear generation is something that might be viable, but it’s extremely expensive and needs quite a bit of product development before it’s a serviceable product for the marine environment.’

The rigours of the marine environment are a crucial aspect of design for any wave energy device, and AWS believes that positioning the device under the water gives it a crucial advantage for maintenance. ‘There’s a lot of misunderstanding among people who haven’t been to sea over what you can do there and where you can do it,’ said Grey. It might seem more difficult to operate under water than on the surface but the reverse is true. Grey uses an analogy from scuba diving: no matter how much you’re thrown around in a boat on the surface, once you get under the water, things seem peaceful.

This is particularly apparent in maintenance, which Grey thinks is a major, and underestimated, issue for marine energy devices. ‘Surface devices can only be maintained at sea in very calm conditions,’ he said. According to HSE regulations, offshore wind turbines can only be maintained in sea states with a maximum of 1.5m of swell, and as most wave energy converters are designed to amplify the movements of the waves, about a metre of swell is likely to be the limit. And in regions where the waves are big enough to make wave energy converters worthwhile, such conditions are rare.

This means most wave energy converters will have to be disconnected and towed back to shore for maintenance. ‘Then you have to fix it, wait for another sea state of a metre of swell to tow it back and reconnect it,’ Grey said. For a location off the Isle of Lewis, such sea states in the winter — the peak time for wave energy generation — occur about every 45 days. That would mean a three-month outage, ‘and by that time, the winter’s gone and so has all the revenue for your device’.

By contrast, the mechanical simplicity of the AWS means it can be maintained by remote-operated vehicles (ROVs). Because of the gentler conditions under water, these can operate in much larger swells than human engineers on the surface. ‘For a relatively small and cheap ROV, you’re talking about a sea state of 2.5m; for larger and more expensive vessels, which you might well own if you’re operating a multi-million pound plant of 100 or more devices, you have an intervention limit of 5m of swell.’ These conditions occur every three or four days in the Atlantic offshore, meaning outages will be short.

‘The key thing about these things is that they’re cash machines; they’re there to make revenue, and if they aren’t up and running, you’ll make no money,’ said Grey. ‘People just aren’t looking at these issues at the moment; there’s a huge naivety in the business.’

The original Teamwork configuration was tested at full scale off the Portuguese coast in 2004, which proved the concept of the motion capture technique, said Grey. AWS has now built a scale model for testing in a wave tank flume to confirm the machine and its anchors can survive pitch motion in extreme seas. These tests will be complete in November, and the company has signed up for a berth at the wave energy testing station at the European Marine Energy Centre in Orkney, where it plans to test its prototype by mid-2009.

The Scottish government has awarded the company a £2.1m grant to build the full-scale test device, and the Carbon Trust has accepted the project on its Marine Accelerator programme, which will provide further funding.

The AWS is an expensive device compared with other wave energy converters, Grey admitted. However, he believes the advantages incurred by the under-water environment give it an unassailable advantage. ‘It’s better to be expensive and be able to generate electricity, than not to be able to do it at all, no matter how affordable you are.’