Tidal energy: It's a shore thing ...

Paul Boughton

Louise Smyth meets the man aiming to harness the power of the sea and bring tidal energy to the UK market

One of the first things Martin Murphy tells you about himself is that despite a 30-year career spanning various facets of the engineering world from the navy to big corporations, the marine sector has been ‘the current that has flowed through’ this career path. The second thing he tells you is that he can’t resist a nice metaphor.

This is a busy time for Murphy. He’s the new president of the Institute of Marine Engineering Science and Technology (IMarEST) in London, the global home for almost 16,000 professionals working in the sector across a range of spheres encompassing everything from oil & gas to maritime services to marine leisure. As well as being deeply involved in the work the institute does to champion a sector that is clearly very close to Murphy’s heart, he has also spent the past six years working in the area of marine renewal generation. Murphy heads up an SME (currently 10 employees) called Tidal Energy Limited.

Explaining the work his company is focusing on, Murphy firstly mentions that the UK is blessed with a very valuable ocean resource due to its tides. “What we try to do is extract energy from the tidal systems,” he begins. “The beauty of it as a marine renewable energy is that it is entirely predictable - it has not got the intermittency issues that need to be addressed in other renewable forms, such as solar and wind. But that said, there are lots of challenges to face in terms of making a valuable contribution to the overall renewable energy portfolio. To harness the potential power involves putting devices into very harsh environments. The ability of these devices to withstand those conditions is a key part of the work that companies such as mine are doing to establish whether we can really take advantage of this resource.”

Murphy explains that if such work does prove fruitful, “It will be fantastic! If we can take the tidal resource just around the UK coastline, that could contribute 5 to 10% of the UK’s electricity requirements in future. If you add to that the potential for extracting energy from waves rather than just tides, then the number is closer to 20%.”

Go with the flow

Much work needs to be done before we can start to achieve the kind of results Murphy predicts. Unlike others in the sector that are focusing on what’s known as ‘tidal range’ – capturing tidal water in a lagoon or behind a barrage and generating power as the water flows in and out of the structure – Murphy and his team at Tidal Energy Limited are focusing on tidal stream.

“Around headlands and through narrow channels, as the tidal sequences naturally flow, water gets accelerated and what we are doing is putting an underwater windmill on the seabed to extract the vast energy that is carried in those fast flowing tidal streams,” he explains.

For his flagship project, Murphy and his team assessed the UK to see where the tidal stream had good potential and he settled on an area called Ramsey Sound off Pembrokeshire, Southwest Wales. As the water in the Sound changes direction – which happens four times a day here – it is accelerated through a channel. So a couple of years ago, Murphy set out to put a turbine right in the middle of that flow to try and extract the energy.

So, where is the project today? “Well, we’ve built the turbine,” reveals Murphy proudly.” We finished construction of the device, which we’ve named the DeltaStream, in late summer 2014, completed all the commissioning work for it in October and since then have been waiting for a suitable weather window to have the opportunity to now install this in Ramsey Sound.

“The way the device works is that as the tidal flow comes in one direction, it turns the turbine in one direction. As the tide reverses, we turn the turbine with (or nacelle) to face the flow in the opposite direction and generate power in that way. The electrical generator is in the nacelle and the power from is carried to shore by a subsea cable, where we connect it to the grid.”

Murphy reels off the statistics pertaining to the DeltaStream like a proud father discussing the birth-weight of his child. “The total unit is a 400KW machine at 4 knots flow speed. It weighs 150 tons. It’s a 12m diameter turbine, which has a 15m wide structure and is 20m from the foot up to the top of the rotation of the turbine.

“In Ramsey Sound this is sitting in a minimum depth of about 30m so there is some 10-15m clearance from the top of the turbine to the surface for small vessel traffic to pass over it. It will be completely invisible; it sits on the seabed.”

In September 2014, the team successfully carried out the first lift of the full structure in readiness to load it onto the barge that will take the device from Pembroke Dock (where it was built) round to Ramsey Sound – as soon as the optimal weather conditions present themselves.

Murphy recalls being very happy indeed with the results of this lift. “As well as being the first time we lifted the device, we also wanted to check the alignment of it in relation to the centre of gravity. We wanted about a 5° tilt on the tubular structure so that the structural tubes, which are hollow, can flood and drain freely as we put the device through the splash zone (the surface of the water). These impact on crane lifting limits and calculations associated with the speed of lowering the device up and through the water. Happily our estimate of weight was within half a ton.”

Next on the agenda

Murphy is obviously now itching to get the turbine into the sea. Tidal Energy Limited has been granted a licence (by the Crown Estate, which owns the seabed) for this device for a one-year operation period. He says: “This is the time when we determine whether there is any adverse impact on the environment. Provided we can demonstrate there is no adverse operating impact, we will extend the operations licence. We have a technology development plan that we want to pursue. We signed off the design of this device two and a half years ago and then went into the procurement process for the long-lead items such as the blades, the generator technology, the gearbox, etc. Since that time we’ve already come up with many ideas on how we can improve it. Our intention is to build a second pre-production prototype in 2016, then in 2018 we will be looking at the Ramsey Sound site as being our commercial-scale project objective.”

In terms of scaling this up to a commercial level, Murphy reveals that it won’t simply be a matter of building a bigger turbine. “We’ve built one turbine on a reduced size structure because this is most valuable step to prove the technology before we start scaling up. But we can scale up in three ways. Firstly, by having more of these devices on the seabed in that location. Secondly, by increasing the size of the ‘triangle’ of the structure to put two or three turbines onto the same structure. Finally, we can scale up the size of the turbine itself to create more output for a given tidal current flowstream. Until we’ve done the prototype evaluation work it’s difficult to say exactly how the scale-up will best be tackled but it’s likely to be a combination of those three elements.”

Recent Issues