Composite blades turn the tide

Composite materials have a number of advantages for use in the aggressive subsea environment of tidal power generation. Dr Neil Calder learns more about Gurit’s work with Andritz Hydro Hammerfest to produce a set of blades. The inhabitants of the Cromarty Firth on the North East coast of Scotland are used to seeing the construction and repair of offshore structures in their own backyard. These have traditionally been fabricated steel structures, but that is all changing as tidal flow turbines are starting to make their mark and advanced composites are finding increasing use in these structures.

There are opportunities here for the composites value chain which could, as Brutus foretold in William Shakespeare’s Julius Caesar, lead on to fortune: “There is a tide in the affairs of men. Which, taken at the flood, leads on to fortune; Omitted, all the voyage of their life. Is bound in shallows and in miseries.”

The waters around Scapa Flow use to hold much of the naval power of the British Empire, but now a different sort of power is being derived from these waters. At the European Marine Energy Centre (EMEC), a number of different tidal and wave power devices are being evaluated for their ability to harvest the bounties of marine energy sources, and many of these will rely heavily on the benefits of advanced composites structures.

What lies beneath

Composite materials have a number of inherent advantages for use in the aggressive subsea environment of tidal power generation. Superior properties in corrosion resistance, excellent fatigue performance, and reduced density or improved buoyancy compared with traditional marine steel fabrications, make these attractive for many subsea applications. As with the wind turbines, which we’re becoming used to seeing above the waves, composites are not used for all parts of the structure. It is in the manufacture of blades that composites are finding their applications. These are structurally demanding due to the geometry requirements for hydrodynamic efficiency, and savings in the rotating mass can have very significant positive knock-on effects in the design and operation of the rest of the powertrain.

Although hydrostatic forces support the blades in the water, there are many other aspects which introduce some significant design challenges over and above those with which wind turbine blade manufacturers are familiar. The need for thin and efficient blade geometries introduces some quite considerable stresses. Although the velocity of sea water in a tidal flow is less than typical wind speeds for effective power generation, the fact that it is around a thousand times denser than air means the kinetic energy is vastly greater. The 9m blades of this 1MW machine are extracting energy at around 10 times the density of similar wind turbines, with the design case indicating some 4kW/m² of the swept area of the turbine, as opposed to around 400W/m² for wind turbines. This means that there is a need for the use of carbon fibre in the most highly loaded parts of the blade.

Composite materials and production technology company, Gurit, has been working with Andritz Hydro Hammerfest to produce just such a set of blades for the HS 1000 1MW equipment, one of which has been installed at EMEC for over a year now. These blades have been made using epoxy resins for the ultimate in long-term performance in a subsea environment. Glass fibre has been widely used in the blades, with carbon employed in more structurally demanding areas. As usual there has been a trade-off between material cost, manufacturing labour costs and component geometry.

Degradation in structural materials of some form in any subsea environment is inevitable and it is very important to be aware of the effect of that degradation on the structure, not just within the lifetime of a prototype commercial demonstrator such as has been installed at EMEC, but also in preparation for rollout with fully-commercial arrays of devices. Over the last few years Gurit has been running a test programme to determine the long-term performance of suitable materials saturated with seawater. This ongoing test programme means that its in-house structural engineers have an excellent understanding of not just the critical properties during initial manufacture and operation, but also the predicted reduction in performance during the lifetime of a component.

The solution which has been evolved for the HS1000 blades has been a manifestation of the eternal triangle of the interaction between process, material and product, where the final outcome is both an optimisation method for producing effective blades and the knowledge implicit in the engineering value chain to make this happen. As the design of the blades will have to be optimised for the tidal conditions of each individual generating site, this knowledge is an important part of the value proposition from the Andritz/Gurit partnership.

The generation game

One of the advantages of tidal flow power in comparison with wind power is the ability to predict, over many decades, exactly how much energy will be generated from the units. Given the constraints of water depth in tidally active areas, it is most likely that the one megawatt class will be a typical practical size of the devices in the schemes consisting of some hundreds of individual turbines.

Because of the amount of accessible tidal energy present in the waters around the UK, which the Crown Estate (the body which grants licenses to operate in UK waters) estimates at 153GW, the UK has secured a leading position in the worldwide progression towards realistic commercial scale tidal flow power generation. Notably, though, there is considerable activity in South Korea, which has a similar north-south coast line and corresponding tidal flow.

Expansion of the local UK market into a global one requires a number of things to fall into place. Firstly, tidal turbine developers need to continue recent successful deployments to improve confidence in the sector. They also need to build a convincing pathway to lower costs and a reduction in the dependency on subsidies. Secondly, local government needs to create environments where turbine developers and utilities can feel confident in the long-term investments needed to enable growth in this sector. The final important point is that equipment developers need to have the supply chain lined up to roll the technology out globally and in greater volume. To do this they must spend time selecting development partners and suppliers carefully and ensuring that the long-term requirements are understood.

As King Canute proved some thousand years ago, the tide of progress is as unstoppable as the progress of the tide: an irresistible force of nature.

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