Stephen Salter is Emeritus Professor of Engineering Design at the University of Edinburgh, Scotland. In the 1970s and 80s he pioneered the first viable design for producing wave energy, called Salter’s (or the Edinburgh) Duck. Some of the innovative technology is now being used in the latest wind turbines.

© Aquamarine Power

Wind and wave power technology were both starting out in the 1970s, when you invented the ‘Duck’ design for producing wave energy. But today wind energy is a mature technology, while wave energy is about 20 years behind. The Duck is still probably the most efficient source of wave power. What happened? Why hasn’t it taken off?

Wind took off because the investors were there. Back in the 1970s there were tax incentives in the United States for investment in wind energy, even though the machines didn’t ever generate any energy, had absolutely awful reliability and only about 5% efficiency. Meanwhile, in Denmark things were better organised, with local ownership of single turbines. But in the UK, wave energy was under the control of the Atomic Energy Authority, which was basically hostile to the whole concept. So there was a benign government in Denmark supporting wind and a hostile control of information flow in the UK that held wave power back.

The only problem with the Duck is that it produces far too much energy, because you need to have a lot of them to take an average of the wave amplitudes. You need lots of phases of waves. Originally, we thought that a 250 MW system was about right. We then tried to do it with small ones in the wave tank. We think that an absolute minimum is about 10 Ducks, which means you’d be developing about 60 MW. And that’s far too big for the present market. Investors want about 1 MW. They are rather cautious and want to try a small one before they try a big one. That’s why Ducks can’t raise any money for investment.

The second thing is that to make the Ducks work we needed to invent all kinds of new hydraulic technology and make it compatible with digital control. The pumps and motors at the time didn’t have good enough part load efficiency, were hard to control and couldn’t combine energy from different places. The consultants who were selected by the Atomic Energy Research Establishment in Harwell to scrutinise the work back then didn’t have any feeling for electronics at all. They were basically engineers and thought that computers were great big things in special rooms with air conditioning and men to look after them. They didn’t realise they could be available very cheaply. Now I think the chip in the digital hydraulics controller costs less than £2.

© Jamie Taylor

But this situation has changed. In Edinburgh, we managed to get a little spinoff company going, called Artemis, to develop the digital hydraulics. Artemis was recently bought up by Mitsubishi. They are fitting digital hydraulics into wind turbines for power conversion and should be installing them off of Fukushima, of all places. They offer great savings in weight and also improvements in efficiency. If this builds confidence in digital hydraulics they might be tempted to do it in tidal stream technology and then in wave energy. I’m quite happy to leave the Ducks on the back burner until we really believe in the digital hydraulics.

So tidal stream is likely to be commercially viable first?

It’s more marketable, because it’s more predictable. In the spot market, every half hour the electricity producers bid for how much they can produce in a given time for an agreed price.  Knowing when you’d be able to deliver makes bidding a lot easier.  Also, tidal is usually near to shore. In the UK there is already a 250 MW connector from Dounreay. So everything needed for tidal is already there. 

So, is synergy between wave, tidal stream and wind the best way forward?

Well yes, because the waves come a day or so after the wind. There’s a phase lag, so the two are less intermittent in combination with each other.  Tidal streams have their own very predictable cycle.  And both wind and wave are more plentiful in the winter, when the demand is greater and that’s a good thing.

But if we did all three together, there would be times when there was too much electricity. So you’d have to find a way to use the electricity that you can’t put into the grid. One way to do this is to use synthetic chemical processing to make liquid fuels. We’re going to be short of liquid fuel and we have too much electricity. There is a chemical process, called the Fischer Tropsch process, to do this that’s well established. It needs a source of carbon, electricity and a catalyst. If you used carbon from municipal garbage you could get a credit on landfill.  Anything with carbon in it can be used, especially if it has a negative cost. You can then use it to make a fuel that would substitute for diesel or natural gas. Anything that’s not used can be recycled back into the system.

Isn’t there another issue, in that the grid interconnections off northwest Scotland, where wave energy potential is greatest, are not yet adequate even for wind?

The UK is building a DC bus down the West coast from the Hunterstone area and I’d like to have another one that went from the Pentland Firth down the east coast. You could get into it from Aberdeen, Edinburgh, Newcastle and all the way down to London. Also, there are plans for a complete joining up of the whole of Europe in one grid. One idea is to produce solar energy in North Africa and bring it up to Norway. If you produced a single European grid, a lot of the worries about intermittency vanish.

What are the next steps?
I want to see successful digital hydraulics in wind followed by digital hydraulics in tidal streams. That’s going to give us a lot more experience and confidence. Then we can go back to the Ducks.  That’s why they’re on the back burner.  The quieter we leave them the better.  However, Richard Yemms’s Pelamis wave power design basically started with the work on the bending sections on the spines of the Ducks. Pelamis is essentially Duck strings without any Ducks on. It’s teaching them a great deal about what they need to know about bearings and joints and rams and hydraulics.   It’s very nice and a good, cheap source of electricity. But it’s wasteful of sea front. We haven’t got an infinite amount of sea front; Scotland has probably got about 400 km of sea front and each Pelamis, I think, takes about 500 metres of width. The Duck strings use every millimetre.

And what are the main remaining obstacles?

Well, apart from investment, we still need to build a component test raft. Technology that costs EUR 20 million to put in is still failing because of an off-the-shelf component that costs EUR 2 that the manufacturers thought was OK. They’re finding that this is costing them a year’s development and lots of electricity not generated. It would be nice to have individual components being tested in large numbers in the right chemistry and biology but they don’t do it. It wouldn’t cost much. The things we’re testing aren’t expensive. What you need to do is test lots of them in parallel and find out which ones work and which don’t.

You’d want all the information about this, – how long seals work and under what conditions and so on. Everyone would want this information. This happens with aeroplanes. You get type approval for components that go into aeroplanes and if anything doesn’t work there’s an air crash and everybody in the aircraft industry in the whole world is told about it.  At the moment things are going wrong with wave energy devices and it’s all hushed up. They’re risking the whole credibility of the technology but also risking causing shipwrecks. That’s the first thing I’d try to sort out.