Eco2 – committed to the future of renewable energy

Martin Murphy, managing director of Tidal Energy Ltd.

PROFESSIONAL ENGINEERING JUNE 2012

While it’s often said that Britain will become a world leader in marine energy, it is actions rather than words that will result in the commercial exploitation of the bountiful tides and waves around our shores.

To that end, there are dozens of small engineering-led companies that are designing and building prototype devices to put into the water. Marine energy might have huge potential, but it is only through the endeavours of such pioneering organisations that the promise will ever come to fruition.

Cardiff-based Tidal Energy Ltd (TEL) is a good example of a renewable energy firm that is moving steadily towards commercialisation of a promising device. TEL has been working for more than 10 years on the refinement of its DeltaStream tidal converter, which it hopes to test next May in the Ramsey Sound off the coast of west Wales. It has been a long journey so far, and there are still challenges to be overcome.

“Essentially, there have been three broad areas that have needed to be addressed to enable us to progress to where we are: access to finance, the design of the device, and marine conservation,” says Martin Murphy, TEL’s managing director. “We are now at a stage where we are planning to install a utility-scale DeltaStream prototype in Ramsey Sound next year. We’ve come a long way over quite a long period of time.”

Finance is a perpetual struggle for marine energy firms. There are pockets of cash available from organisations such as the Carbon Trust and the European Regional Development Fund but grants often have to be matched by private capital. TEL was fortunate: in 2007 it secured major backing from Eco2, a much larger renewable energy company that specialises in biomass and wind. This deal gave TEL the funding it needed to get where it is today. Without it, says Murphy, progress would have been far slower.

The technological challenge of designing a device that can operate reliably has also tested Murphy and his team. TEL has tried to keep things simple: its concept of a horizontal-axis turbine is quite straightforward, but its development has still required an enormous amount of consideration. “We will be putting the device into a very harsh environment where vast quantities of water will be moving very quickly. It’s not an ideal place to put something that you want to stay put and generate energy. The challenges associated with that are very considerable,” says Murphy.

And then there is environmental assessment. Potentially, the DeltaStream device could cause damage to marine species, the seabed and to water quality.

“Ramsey Sound is an area of marine conservation,” he says. “We have had to be careful when progressing the design to be sensitive to the environmental conditions. We have had to work with environmental regulators in seeking licences and consents to put the design in the water. And when the turbines are operating there will need to be careful environmental monitoring.”

The DeltaStream tidal converter is a lightweight gravity-based design with relatively slow rotational speed. Each device will have three tidal-stream turbines, made up of a nacelle, hub and blades.

DeltaStream features fixed-pitch blades and a yaw mechanism, rather than a controllable pitch system, to drive the nacelle around from one direction to another to face the oncoming tidal flow.

When the rotors are turned by the tidal flow, the combined generation capacity of the three turbines will be up to 1.2MW.

Murphy says the triangular design stems from a belief that multiple turbines on a single structure is the best way to move forward. “The energy generated is directly proportional to the swept area of the turbine. So the greater we can make the swept area for a given device, the greater the average energy output will be.

“We’ve been through a whole series of iterations to try and optimise where we think the design should be.”

The design criteria were a capacity of 1.2MW at peak output at a rated tidal current speed of 2.25m/s. DeltaStream’s operating principle is that as the tidal current accelerates from slack water to 0.5m/s, the turbines start to rotate, until they reach their full output at 2.25m/s. Thereafter, as the tidal current goes beyond 2.25m/s, TEL has chosen to limit the output power to prevent the device from being pushed along the seabed.

“We actually power shed the additional energy that might be available to us above 2.25m/s,” explains Murphy. “The reason we do that is that we can then design the whole structure on a gravity-based principle whereby the base will sit on the seabed secure under its own weight rather than requiring the need to drill or pile the structure into the seabed. There’s no anchoring device at all – that’s been a key part of our solution.”

This approach means the DeltaStream structure will weigh 350 tonnes, considerably lighter than many other marine turbines. When the time comes to install it, a barge will simply lower it into the water.

“Installation really is a dominant cost in the overall commercial feasibility of extracting energy from tidal resources,” says Murphy.

“We’ve tried to take a holistic approach – we have not only designed DeltaStream as a unit to extract energy from the sea, but have also taken account of getting the device in and out of the water as necessary.”

Once the Ramsey Sound trial begins, electricity will be brought ashore from DeltaStream by three subsea cables, which will be laid on the seabed as a single bundle. The cables will come ashore at St Justinian’s lifeboat station slipway.

Here the subsea cables will connect to the land cables at a connection box. The three land cables will be laid in the ground to a temporary power conversion compound to convert the raw generated power into “clean” power suitable for connection to the electricity grid.

So far 13 contracts have been placed for long-lead items such as blades, gear hubs, generators and electrical equipment for power conversion. TEL is assessing tenders for assembly of the structure and for installation. When built, the device will be fitted with a complex network of monitoring equipment to relay performance data for onshore analysis.

“The primary objective is to get the device into the water in May 2013, and the trial will last a year,” says Murphy. “Additionally, the Crown Estate has awarded us an ‘agreement for lease’ to build a 10MW commercial demonstrator array comprising nine devices in an area off St David’s Head in Pembrokeshire. Installation, subject to finance, will start in 2017.”

Looking forward, despite all the travails that companies such as TEL face in their attempts to commercialise marine energy, Murphy still believes the sector will eventually flourish. “Momentum behind the marine industry is picking up,” he says. “Large engineering firms such as Rolls- Royce, Siemens, ABB and others are coming into this space because they see an inherent pull-through of technology. With organisations like that behind it, I believe the marine industry will move forward.”

Will TEL be part of that bright future? “I hope so,” says Murphy. “We’ve adopted a careful engineering process that we hope will underpin our success. But, to be honest, we are not underestimating the challenges of the marine environment and there will be hiccups along the way.”