Stephen Salter: pioneer of wave power

Wave power is on the rise. As concerns grow over the need for diverse ways to generate electricity without boosting greenhouse gas emissions, dozens of new devices are taking shape in wave tanks and slipping into the water for testing.

But wave power’s history dates back more than three decades and, if anyone can claim to be its founding father, it is veteran design engineer Stephen Salter. His Salter’s Duck is still the most efficient wave power converter. Now an emeritus professor at Edinburgh University, Salter is still working on ambitious designs for marine power systems — some of which may help reduce global warming in unexpected ways.

According to Neil Kermode, director of the European Marine Energy Centre in Orkney (The Engineer, 26 March), Salter can claim much of the credit for the resurgence of wave power.

‘Stephen was a lone voice in the wilderness as far as wave power was concerned,’ he said.

‘I remember seeing him on TV when I was at school; he was very inspiring. He had such a clever concept that was so unusual — the words ‘wave’ and ‘energy’ were never associated in the public mind before that. Waves were something you went surfing on. Energy was something that came out of a socket. But after Stephen spoke about it, it became something people connected with, and that’s never really gone away.

‘Now there’s a change in the political climate and Stephen and the people who were associated with his research are forming a critical mass of expertise.’

Born in South Africa just before World War II, Salter’s career as a design engineer has led him into several unexpected places. He was an instrumentation engineer on the first hovercraft and on Black Knight, the UK’s rocket project of the early 1960s. Later he moved to the University of Edinburgh to work on robot mechanics and electronics in the School of Artificial Intelligence.

Soon after moving to Scotland, Salter became concerned about the finite nature of oil resources. He turned his attention to renewable energy and wave power in particular.

Forming his own research group, he designed the first generation of his now-famous wave energy converter. Shaped like a flattened cam with the narrow end facing into the waves and the rounded end rotating around a flexible spine, the device resembled the head of a duck, nodding up and down as the waves passed underneath it. Salter’s Duck was born.

To develop the Duck, Salter designed a revolutionary wave tank whose wave maker was driven by digital drives and incorporated force feedback and water velocity sensors.

The Duck works by absorbing about 90 per cent of the energy from incoming waves, leaving a calm sea behind the cam. The nodding motion of the cam operates pistons that compress hydraulic oil. Once pressure has built up, the pressurised oil is released through a hydraulic motor that generates electricity, converting 90 per cent of the captured power. To date, it is the most efficient wave-energy converter ever designed.

Initially, Salter said, enthusiasm for the Duck was high. ‘Cost predictions were impossibly high but the people in charge of the programme were very optimistic. Their enthusiasm diminished as costs came down.’

In the early 1980s, the government closed the wave-energy project. Salter thinks this was politically motivated; the nuclear industry was heavily represented on the panel awarding energy grants and its members became concerned as the cost projections came down. ‘When the programme manager predicted that with development, the cost would be 3.3p/kWh, which was getting close to being economically viable even then, they excluded him from the next important meeting of the key committee,’ said Salter. ‘They basically killed the project because it was going to threaten the expansion of the nuclear industry,’ he claimed.

Salter agrees that initial costs for a Duck system would be high, and that his proposals for a functional string were ambitious. ‘We’d been told to build a 2,000MW station, equivalent to two really big power stations, in the first go, so we designed something exactly aimed at that. It’s too big an initial investment. The very smallest system we could do would be about 600MW, and that would be 10 Ducks, each 45m wide, with a gap of 60m between them — about 600km of sea. They’re set in a row and the longer the row the more power you get, because they also assist each other in a variety of nice, complicated ways.’

However, such a large, complex system was ahead of its time. ‘It’s a bit like somebody saying in 1905 that they had a really good idea for a huge aircraft like the Airbus A380 when people didn’t believe that biplanes would fly.’

Salter is encouraged by the current high profile of wave power, particularly with the Pelamis device, which was developed by one of his former students, Richard Yemm, at Edinburgh’s Ocean Power Delivery (OPD).

Pelamis uses a similar, high-pressure, oil-based system to generate power but rather than using a wide, flat cam to absorb the power of the waves it is based on a long, narrow tube, which flexes with the waves, absorbing less energy. The trade-off, OPD believes, makes the device more able to survive in high seas than the Duck, which was believed to be vulnerable to very large waves (although Salter disputes this).

‘OPD has done a really thorough job, and started in our wave tank,’ Salter said. ‘It’s done some fantastic computer modelling of how Pelamis behaves, and I’m full of admiration and very proud of it.’

However, he sees Pelamis as an intermediate technology to restore faith in practical wave power; if it succeeds, he believes, people will look at the more efficient version of the technology. ‘When you look at the power per unit mass [from Pelamis] it’s very nice but it doesn’t make good use of the wave front. It’s the right thing to do at this stage but we’ll start looking at Ducks again when we think “There’s only 400km of sea here, we would get a bigger energy resource if we used it more intensively”.’

Salter remains convinced of the potential for marine renewables. ‘You could run continents on this sort of power,’ he said. ‘The long-term dream for the Duck stream is that you run a long line of them from the Hebrides down to the west coast of Ireland, with a break to allow shipping through, then you build out from Cape Wrath [the most westerly point on the northern coast of Scotland], past the Faroes and all the way to Iceland. You can use hydroelectrics and the Icelandic geothermal to back that up when there aren’t any waves.

‘So you get a very high-capacity factor of wave power coming into Scotland and Norway and feeding on down into the rest of Europe. That’s a really enormous resource.’

The prospects for tidal power are equally large, he said, particularly from the Pentland Firth, the channel between Scotland and Orkney that is estimated to hold half of Europe’s tidal power resource. ‘I think we could get 10-20GW out of Pentland, using tidal stream turbines,’ he said, which is more than is generated in the UK by nuclear power stations. ‘You could run a distribution system down the east coast of the UK, with spurs going off it for Newcastle, York and Birmingham down to London. Then you’d link the Iceland string into the top.’

Now approaching 70, Salter is still active in engineering research and his concepts are as ambitious as ever. His current project, in collaboration with atmospheric physicist John Latham of the US National Centre for Atmospheric Research, is to design a fleet of renewably-powered, remote-controlled yachts that would spray a mist of seawater into the air to create low-level clouds with a high albedo (reflectivity). These clouds, Latham believes, would reflect solar energy away from the Earth and provide a global cooling effect to counteract the warming caused by greenhouse gases.

Salter has developed plans for the yachts, which would be propelled by 60ft high vertical rotors. Known as Flettner rotors, these interact with the wind to propel the yacht, acting like sails, but with far greater efficiency. The yachts would drag two 3m-diameter turbines that would generate the electricity to spin the Flettner rotors and spray water droplets from their tops. He and Latham believe a fleet of 50 such yachts, guided to the required locations by GPS, would be needed to produce an appreciable effect.

‘There’s no funding for this, but we’re going ahead quite quickly. We’re working on the full-scale design and we’ve got work going on predicting how much we need to spray — it’s an amazingly small amount,’ said Salter. Calculations indicate a spray-rate of about 10kg of water per second. ‘But we do need to get information on atmospheric conditions and cloud-cover fractions.’

Despite the difficulties, Salter is driven by the need to ameliorate the effects of climate change — as he has always been. ‘We’re in a massive state of denial about this,’ he said. ‘And we need to take action.’