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Feature

Tidal turbines harness the power of the sea


George Marsh

Just as there were once 10 000 windmills across Europe, countless water mills helped power the rural economy and then the industrial revolution. And, just as reinforced plastics have supported the 21st century reincarnation of windmills as wind turbines, so they are beginning to contribute to the next likely large-scale commercial renewable energy source – water current power.

Harnessing power from flowing water in the 21st century may be anything between one and two decades behind wind power, but the technology has moved beyond the conceptual stage, has shown that it works in pilot installations and is poised to progress to initial commercial exploitation. This is the view of Marine Current Turbines Ltd (MCT), a UK company that is arguably at the forefront in this field.

MCT believes that current turbines can become commercially viable and has developed a pilot-scale system that is working successfully off the coast of north Devon, UK. Encouraged by a year's track record for its ‘Seaflow’ prototype, MCT is now embarking on a full-scale commercial system. Reinforced plastics are turning out to be a key enabler for these systems – just as happened with wind turbines.

Wind versus sea

The utilisation of composites is not the only similarity between wind and water turbines. Both convert kinetic energy from a moving fluid (or gas, in air's case), first into mechanical energy and then electrical energy. Both have a rotor with two or more blades driving a generator via a shaft and gearbox. Most ‘windmills’, and so far ‘watermills’ too, are horizontal-axis machines. In both cases, the cube law operates, power extractable being proportional to the velocity of the fluid medium passing through the rotor, raised to the power of three. Although the rotors may visually suggest scaled aircraft propellers, they turn relatively slowly, are driven by the flow rather than driving it, and operate in very different order of flow speed. One similarity, however, is that it is important to optimise the blade's aero/hydrodynamic shape for efficient operation. Both wind and water rotors require pitch control so that their blades can be adjusted to flow conditions. Both also need support structures so that the rotors can be positioned in a favourable flow regime.

Marine current turbines are designed to capture the power from tides, which flow in just two directions.

But there are also great differences. One is that marine current turbines are designed to capture the power from tides, which flow in just two directions. Since tides are lunar driven and reverse their direction every six and a quarter hours, rotors may simply be reversed to face the new current, or accommodate the change with a blade pitch ‘reversal’. Wind rotors, on the other hand, must generally be yawed so as to face the wind from any direction.

The most striking dissimilarity, though, is due to water's massively higher density – 832 times that of air. In the long, slender blades of wind turbines, centrifugal loads dominate, and these tend to restrict bending. In water turbines, however, because they operate in a much denser medium, bending loads dominate by a large margin. MCT's technical director Peter Fraenkel explains that a 1 MW tidal flow machine might have anything up to 100 tonnes of thrust on it, about double what it would be for a similarly rated wind machine. To keep bending loads manageable, marine turbine blades are relatively short and fat. Because marine rotors are of limited diameter and rotate slowly, there is little centrifugal force to react the bending.

Water flows at a lower rate than wind – for example, a tidal turbine might be rated to reach maximum power in a current of 2.3 m/s (or 4.5 knots), compared with a typical 12-13 m/s flow for wind. But this in no way compensates for the density difference. Whereas a 1 MW wind turbine rotor would have some 30-40 tonnes/s of air passing through the cross-section of its rotor when at full power, a 1 MW water turbine operating in a 4.5 knot tide ‘sees’ over 900 tonnes/s of water pass through its rotor. Designing to accommodate these flow rates over a lifetime of 20-30 years is a challenge. Structures for marine turbines must also be stressed to withstand transient forces caused by turbulence as well as by passing surface waves.

“At least we do not have to contend with an underwater equivalent of the atmospheric hurricanes that wind turbines are occasionally subjected to,” Fraenkel remarks.

Materials choice

Sub-marine structures also have to withstand the notoriously aggressive marine environment with its corrosive salt water, fouling growth and abrasive suspended particles.

Designers first considered producing the required stiff, unyielding marine rotors in steel. However, achieving the necessary compound-curved profile in steel proved to be expensive. Moreover, steel is heavy, prone to fatigue and susceptible to corrosion induced by salt water.

These disadvantages prompted a decision to adopt composites instead. Plastic-based materials ease the fatigue problem, both through their inherent fatigue tolerance and by reduced blade weight. Calculations also showed that, appropriately applied, they could deliver the required stiffness. However, whereas some wind blades rely solely on the form stiffness of a sandwich envelope structure, a robust internal ‘skeleton’ was considered essential for their marine counterparts.

A two-blade configuration was chosen for the 11 m diameter rotor of the Seaflow prototype. This was partly for practical reasons – a two-blade rotor attached to the turbine shaft and drive train forms a flat ‘T’ that can be readily laid on the deck of an installation barge and handled by cranes – and partly because it was the most cost-effective configuration. To fabricate the blades, MCT selected Berkshire, UK-based Aviation Enterprises due to its blend of composites, engineering and aerodynamic experience (it has even developed its own all-composite light aircraft, the Magnum).

Proprietor Angus Fleming told Reinforced Plastics that each blade derives most of its stiffness from a robust carbon fibre main spar. To this is bonded a glass fibre composite envelope that gives the blade its hydrodynamic shape. Carbon-reinforced ribs, attached perpendicularly to the spar, help transmit loads from the envelope to the spar. Glass was chosen for the envelope because of its affordability, and combination of modulus with impact resistance. Marine Current Turbines and Aviation Enterprises were, understandably, prepared to discuss their technology only in outline, but the following is known.

The carbon spar transitions from a circular section at the root end to a smaller box section at the tip. Aviation Enterprises laid up the spar by hand over a shaped rigid closed-cell foam core, a method considered cost-effective for a one or two-off quantity. Pre-impregnated carbon fibre/epoxy cloths were used for forming skins. Because water current turbine manufacturers are likely to face the same economic pressures as wind turbine manufacturers to minimise production costs, it was important to select an affordable fabrication technology.

“The prepreg resin cures at a low enough temperature to permit use of a normal industrial-grade oven rather than a high-specification oven or autoclave,” explains Angus Fleming. “Our cure cycle results in a glass transition more than adequate for the turbines' intended under-water use. The laminate was consolidated at atmospheric pressure under vacuum bagging.”

The envelope was produced in two halves, which were subsequently bonded together. Operatives laid up the glass/ epoxy laminate wet, by hand. Female moulds for the skins were produced from a male plug shaped to the required hydrodynamic profile supplied by MCT (actually a modified NACA section, familiar to aerodynamicists). Aviation Enterprises produced its own moulds inexpensively in composites, since further use after the prototype turbines was not expected. For future production runs of larger blades, this approach is likely to be modified, but it has proved more than satisfactory for the experimental prototype.

The blistering and osmosis that can attack marine composites are a major issue on small boats because they operate at the air/water interface where sunlight can penetrate. Turbines, in contrast, are located well below the surface where there is little air or sunlight. Neverthe-less, keeping blistering at bay for decades might be difficult and a combination of material quality control and surface protection is required to deal with it. A high quality laminate with low void content and incorporating non-hydrophylic epoxy resin is a good starting point, while innovative finishes that also resist marine growth are being considered.

The Seaflow blades are fitted with a pitch change mechanism that MCT adapted from a wind turbine product. The blades can be rotated through a large arc so that they may be driven with equal efficiency in either of the tidal flow directions.

Maintenance, both planned and unplanned, will be required over a turbine's life. This is allowed for in MCT's patented design, in which the normally submerged turbine can be raised to the surface by pulling it up the monopile on which it is mounted.

The monopiles, which project above the water's surface, are robust structures that are lowered into holes drilled in the sea bed and then grouted in place. They must resist cyclic tidal forces and marine environmental degradation for many years. Each pile must also support its weighty rotor and power train combination rigidly so that the rotor remains correctly aligned to the flow and disturbing resonances are not created.

Although steel is the traditional pile material, and is used for the Seaflow installation, ferrous metal is vulnerable to the marine environment. Concrete piles would have to be reinforced, probably again with ferrous metal. This has prompted suggestions that composites could be used instead. In the USA, some wind turbine support columns intended for offshore use are already being fabricated in composite materials. If the same solution were adopted for monopiles, such use could potentially constitute a substantial bulk application. Other uses for composites might include the generator enclosure (nacelle), which has to be well sealed against water ingress, the hub fairing, and parts of the access platform and ancillary equipment at the top of the pile.

Trial

MCT's 300 kW Seaflow installation has been operating off Lynmouth, north Devon, UK, since May 2003. Although this single turbine dissipates its energy into a resistive dummy load, in commercial devices electric current would be fed to a transformer on the pile top, and thence ashore via a submarine cable. Peter Fraenkel says the installation has operated without major problems and has exceeded its design performance. On its best day so far, it delivered 27% more power than was expected.

Partners in the £3.4 million pilot project include MCT's former parent organisation, Hampshire-based IT Power, offshore engineer and MCT shareholder Seacore Ltd, EDF Energy (formerly London Power Co), Bendalls Engineering, ISET from Kassel in Germany which designed the electrical system, Jahnel-Kestermann which designed the gearbox, and Corus (formerly British Steel) which supplied the steel. The demonstration is part-funded by the UK's Department of Trade and Industry (DTI), the European Commission (EC)'s former ‘Joule’ energy programme and the German government, as well as by industrial partners. The EC part of the programme is now complete and the DTI is maintaining support until August this year, after which the partners must decide whether to keep the trial going for longer, as a test and demonstration system, or whether to decommission.

Meanwhile, work has started on a 1 MW commercial successor to Seaflow in which twin 500 kW turbines would be cantilevered out on either side of a monopile. A twin-rotor solution could, it is estimated, harness twice as much energy as a single rotor of the same size, at considerably less than twice the cost. MCT hopes to locate the first grid- connected installation in Strangford Narrows and has applied to the Northern Ireland regulatory authority for permission. Current flows through the Narrows at up to 7 knots, making the site an excellent location for testing a prototype system. However, industry opinion suggests that tides flowing at peak velocities of 5 knots and above can be harnessed economically and there are hundreds of locations offering suitable conditions. Unlike wind, tidal flow is predictable and can be relied upon. Proponents suggest that a load factor of better than 40% can be achieved at a good site, rather better than for most wind sites.

Aviation Enterprises will produce the blades for the twin 16 m diameter rotors needed for MCT's new system. These will have to be fabricated to standards consistent with the intended 20- 30 year life that will be expected of marine current turbine clusters if they are to be commercially viable. Composites might, says MCT, also be considered for the cantilever ‘wings’ that support the turbines. High fatigue resistance will be a prime requirement for these substantial structures, giving composites a potential edge over conventional steel construction.

Although tidal turbines may be first, they are not the only rotationally-based possibility for utilising tidal power. US researchers, notably in Florida, have been studying various horizontal and vertical axis water current turbine concepts since the 1970s. The Blue Energy Power System in Canada utilises vertical axis rotors analogous to Darrieus-type wind rotors. Another possibility is to capture wave energy by means of the oscillating water column invented in Japan. This system, a chimney-like column standing on the seabed, admits waves through an opening near its base. The water inside the column rises and falls as the water level outside rises and falls due to the waves. In rising, it forces air up through a turbine which drives a generator. In falling, it sucks air back through the turbine, again driving the generator. Turbine/impellers and columns are structures for which composites could well be suited.

Now that wind power has shown its commercial exploitation potential, it looks as though the use of rotating machinery to extract a fraction of the immense power present in tidal currents could represent the emerging next wave in the effective harnessing of renewable energies. As more companies join pioneers like Marine Current Turbines and Aviation Enterprises in this emergent field, composites will be there to help.

 

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Comments

CONNIE GAL said

23 April 2012
Great article on the turbines. I'm wondering what surface protection will be used on the composites to keep them free from barnacles and other sealife that wants to cling to the surface of the blades. Additionally they will need to be coated with a protectant that is not toxic to the marine environment. http://buildsitepro.com/

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