Sea turbines

Tidal energy comes of age

The power of the ocean is apparent to all and we are now much closer to harnessing that energy to power our lifestyle.

According to the British Cartographic Society the length of coastline of Great Britain plus its principal islands is just shy of 20,000 miles, so it comes as no surprise that marine energy is considered to hold great potential as a future energy source.

According to research by The Carbon Trust – Technology Innovation Needs Assessment (TINA) – this has the potential to deliver more than 75TWh a year; over 10 per cent of the UK's predicted needs, by mid century. Predictions of how much of that energy can be harvested by 2050 vary from 20GW to zero. What is clear, however, is that it will not have any impact before 2020.

The same report pointed to the pricing pressures that marine energy will face. It predicts that it will need to reduce costs by 50-75 per cent by 2025 if it is to compete with offshore wind power. This level of cost reduction is ambitious but conceivable with significant economies of scale and innovation, combined with supply chain optimisation and appropriate financing.

None of this hides the fact that marine energy technologies are not yet commercial. At the last count there were only six devices – two wave and four tidal – that have made it through to full-scale demonstration, with an additional three devices expected to do so over the next year.

The fact that the UK government continues to pile incentives at the feet of marine energy companies bears witness to the myriad challenges that the industry faces on the road to commercialisation.

The early stages of development fall under the SuperGen Marine Programme and additional support is available from a variety of agencies including the Technology Strategy Board (TSB), The Carbon Trust and The Energy Technology Institute (ETI).

The ROC regime

Midway through last year the UK Department of Energy and Climate Change (DECC) pledged another £20m to cover the move from prototypes to offshore arrays. The scheme will support two projects to test wave or tidal stream energy prototypes in array formations – the final development stage in generating large scale electricity from marine power prior to commercial roll out.

"We are in a scenario where this industry is nascent – it is developing the banded Renewable Obligation Certificate (ROCs) that carries us forward into 2017 and the feed-in tariff," says Martin Murphy, MD of Tidal Energy. "As the industry moves forward and we drive down the cost of electricity production, it's natural to expect this subsidy regime to tail off.

"I'm comfortable with the ROC regime," he adds, "but a bit uncertaint about what happens around the 2017 regime – there's not a great deal of clarity at the moment about the feed-in tariff structure."

Among the portfolio of renewable energy technologies, tidal stream has one inherent advantage as it is entirely predictable. Once a developer demonstrates a power curve that will be the amount of power to be generated which is consistent against a given tidal flow profile. "Because we can forecast tidal current conditions to infinity, we are going to be able to predict consistently what generated power we will get from our device," Murphy says. "It's not quite the same as wind or solar, which are dependent on the wind blowing and strong light or sunshine for solar energy.

"I think that tidal stream can contribute to base load calculations of putting power onto the grid." Although tidal energy benefits from a predicable power delivery it has its challenges, not least the demanding operating conditions. By definition the devices will be located in regions that offer fast flowing waters, which inherently means high energy with lots of turbulence, the bane of engineers.

Embracing tidal energy

Tidal Energy, much like the industry itself, are relatively new having come into existence just ten years ago. However, it is only in the last three years that they have accelerated their progress. The catalyst for this was investment by Eco-2, a renewable energy project development company, who decided they wanted to venture into marine energy systems.

"In the last three years, our task has been two fold," Murphy says. "One is to take the design of our device from the concept stage to full scale design ready for installation as a full scale prototype, which will happen in 2013, and the second task associated with, sitting alongside and parallel with the engineering work, has been to develop and consent the site for this device."

Cranfield University is the research and development partner on the project. "They have excellent computational fluid dynamics (CFD) tool sets, which we have used to optimise the design of the turbine," Murphy explains. "We have taken and iterated through our CFD analysis with life tank testing in simulated flow conditions."

That design transitioned through one-30th and one-20th scale models in tank test facilities both in the UK at Cranfield and also at a facility in France, near Boulogne, where the flow characteristics and the physical size of the circulated water channel was ideally suited to the needs. Then it was on to the moment of truth with the construction and installation of a full-scale prototype.

Environmental test beds

As Tidal Energy is a Welsh company they were keen to support and develop marine renewable energy industry in Wales. With that in mind they spurned the traditional test beds of EMEC, off the Orkney Islands, where companies such as Tidal Generation, Open Hydro and Voith Hydro currently test devices and the Cornish Wavehub facility, where Ocean Energy, are operating, for their own site off the West Wales coast.

The selected site lies within a Marine Special Area of Conservation, which prompted the decision to apply for a single year operating consent. "Our grant has required us to go forward with an environmental monitoring package what we call a deploy and monitoring approach in terms of the conditional requirements and the consent," Murphy says. "We put the device in and we've got a suite of instrumentation, which will be monitoring the device and what's happening around it to see the impact it has on the environment."

The device has a 1.2MW capacity device and in very simple terms it consists of a triangular frame structure, which at the apex of each corner carries a tower and a cell. The cell is fundamentally a horizontal axis turbine, which faces the oncoming tidal flow. The device is totally submerged and rotates as the tidal current flows past it. From the turbine, driving through the gear hub, is a generator and from here the raw power is transferred to shore where the necessary power conversion and conditioning is carried out to make it power grid code compliant. The power is then fed into the grid at a connection alongside the St David Life Boat station at Ramsey Sound.

There are various options for securing the device – the two most popular being seabed or pile mounted. There are also floating devices that are secured by flexible or rigid moorings, or even on a floating structure. Tidal Energy opted for a gravity-based installation. "We decided to have no drilling or pylons in the seabed, which to our mind carries both financial penalties because of the cost of drilling and pyloning, but secondly the environmental impact is exacerbated by drilling and pyloning," Murphy explains. "Our technology is such that we have a relatively lightweight gravity-based structure with these three towers each with a 400kw cell on it and that gives us our 1.2MW capacity."

The torque of the town

Cooling is provided by the sea water flowing past the components in the cell for the circulating oil in the gear hub and for the heat losses in the generator.

The overall weight for the three devices is 350 tonnes, which is significantly less than other devices. With this light weight a novel technology was applied to assist the gravity location. "Our turbines not only create torque, which is what we want to generate power, but they also create axial thrust," Murphy says. "The balance for us is to create torque and not thrust, thrust will try to move the device along the seabed.

"With Cranfield University we've studied the characteristics of turbines very carefully and we've optimised our turbine to allow us to get maximum power at minimum thrust. There are techniques we've adopted to do that and we've been able to patent them. By minimising the thrust that enables us to keep down the weight of the device so we are saving weight in the structure. We have done many iterations on this thrust versus weight equation to get within appropriate safety factors."

Unlike the wind industry that has settled on a three-bladed vertical turbine, the tidal energy sector is developing a plethora of devices of varying styles. "What we are trying to do here in our industry is create an industry and there are different ways," Murphy says. "The principle is simple – to generate power from the rotation of a turbine, but there are different ways of approaching that problem and different developers have gone down different paths.

The holistic approach

"We've tried to keep our device as simple as possible, as robust as possible, given the'harsh environment into which it's going. Our design has not just focused on the device itself, but it's been a holistic approach. This takes into account the interface through the power conditioning system to the grid network and crucially how we are going to install it, because all these things add to the overall cost of electricity. We've tried at all times to optimise the design of the device taking into account these other factors.

"We've also kept it simple," Murphy continues. "For example we've gone for a fixed pitch turbine solution rather than controllable pitch because we didn't want the complexity of an underwater controllable pitch control system. We've got a lightweight gravity structure rather than drilling and piling into the seabed. We are hopeful that we will have a front to back solution, which is going to be very cost competitive when we get to commercial scale downstream from here."

The next stage for DeltaStream is to move from a single prototype to a commercial demonstration array where multiple devices will be interconnected sub-sea. Tidal Energy have secured an agreement lease with the Crown Estate to own a part of the seabed around the UK for a site for this array and that too is in West Wales, just two or three miles further up the coast from Ramsey Sound in Pembrokeshire off St David's Head.

"That lease is for a 10MW capacity site and, given where we are with our design, that would imply that we would install nine devices," Murphy says. "We go from single prototype to demonstration array of about 10MW capacity. That is the building block for full commercial development to arrays of 50MW plus in the appropriate locations of coastlines around the UK." 

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