Currently, as many as 24 tidal or current turbines are in development, primarily in model-scale to subscale test sizes (20 kW to 300 kW capacity). Many are built of conventional materials, such as steel, none have progressed to commercial scale and most have yet to announce plans to do so. There are, however, at least three notable exceptions to the latter, all of them featuring rotor blades manufactured with composites. Despite their small number, these three quite different approaches indicate that undersea turbines hold significant promise.
CURRENT TURBINES GEARED FOR GIGAWATT PRODUCTION
OpenHydro Group Ltd. (Dublin, Ireland) recently signed an agreement with Alderney Renewable Energy Ltd. for a project in the Channel Islands, a British protectorate off the coast of Normandy in France. OpenHydro will install a series of its unique Open-Centre Turbines (top photo, p. 29) off the coast of the island of Alderney, where tidal flows are reportedly among the world's strongest. The multimillion euro project will be complete in 2009. Developers hope to tap a potential 3 GW (gigawatts) of tidal energy, enough for 1 million households when connected to the European power grid. Developed in the U.S. by Hebert Williams between 1996 and 2004, the turbine concept was acquired by OpenHydro with the purchase of William's company, Florida Hydro and Power, in 2006 (see "Learn More").
OpenHydro also has attracted interest from Nova Scotia power, a Canada-based utility, for a project that would install turbines in the Bay of Fundy, known for having the world's greatest difference in water level from high to low tide.
Conventional Rotor in Nyc'S East River
Meanwhile, Verdant Power (Arlington, Va.), in collaboration with the New York Power Authority and New York University, has developed a three-blade turbine design similar to those used in wind farms. After a full-scale design was tested in Pakistan, Verdant worked with the New York State Energy Research and Development Authority (NYSERDA, which pro-vided grant money) on a $1.5 million, three-phase project intended to moor a cluster of turbines off Roosevelt Island in New York City's East River. The company completed the second phase in April, placing six 35-kW turbines with 16-ft/4.9m-diameter rotors and a yaw system, that permits the turbine to reverse and capture tidal energy during tidal ebb and flow. (First-phase turbines with 10-ft/3m-diameter rotors each generated 16 kW.) Verdant plans to monitor second-phase performance for 18 months, then embark on a more ambitious effort in the East River that will involve at least 100 turbines.
Crafting Turbines for Commercial Viability
In 2003, Marine Current Turbines Ltd. (MCT, Bristol, U.K.) and its partners developed Seaflow, the world's first "full-size" tidal turbine. The 300 kW axial flow rotor system - large enough to expose key issues without undue expense - was installed off the coast of Devon, U.K. The turbine, however, will be scaled up significantly for commercial use. "Our research has shown that tidal turbines must provide at least 1,000 kW of power to be commercially viable in order to overcome the costs of installation and connection to a power grid as well as typical maintenance overheads," explains MCT technical director Peter Fraenkel.
Seaflow's 11m/36-ft-diameter propeller-like, two-blade rotor was mounted on a 3.1m/10-ft diameter steel monopile. This design was chosen after MCT engineers observed that the wind turbine industry, in which every conceivable form of rotor has been attempted, had converged on a tubular tower supporting an upwind, pitch-regulated, axial-flow rotor.
According to Fraenkel, the MCT turbine concept differs significantly from the widely publicized ENERMAR tidal-turbine, which was moored in Italy's Straits of Messina, off the shore of Naples in 2001. ENERMAR was a much smaller foray into marine turbine technology, generating only about 20 kW of energy. The ENERMAR project used a vertical-axis Kobold-type turbine that features three straight, vertical blades attached to the axis via horizontal support arms (see "Learn More," p. 30), a design Fraenkel believes may be impractical for utility-scale installations. "This was actually a vertical-axis Darrieus turbine and, in my opinion, cannot be scaled up very much," he notes. (See "Urban Turbine," on p. 30, which features a Darrieus-type application with an illustration.)
MCT refined its turbine's single steel piling design and subsea installation to provide a cost-effective and reliable support in strong tidal currents, liaising with marine classification society Det Norsk Veritas (DNV) to ensure compliance with the structural standards, which eventually will be needed for insurance and type certification. Originally, the system was designed to reverse the rotor orientation, allowing it to face either incoming or outgoing tides. But MCT patented a simpler and less costly solution to bidirectional flow, using electrical pitch control servos built into the rotor hub, which rotate each blade 180° on its axis.The rotor was first attempted in steel to control cost, but fatigue issues prompted a composites-based solution. MCT completed hydrodynamic analyses and developed the blade profile and curvature using its own rotor modeling software. Aviation Enterprises Ltd. (AEL, West Berkshire, U.K.), a specialist in carbon fiber composites, was asked to develop the blade.
Although AEL was experienced in small wind blade design, managing director Angus Fleming points out, "Tidal turbines are much different from wind turbines. Even though horizontal-axis wind turbines and horizontal-axis tidal turbines use similar propeller-type designs, wind turbines rotate faster and exhibit centrifugal release, where centrifugal force pulls the blade back, relieveing the bending stresses." Also, tidal currents apply much greater loads to turbine blades than do wind currents, because air is not a high-intensity energy source. "Tidal rotors are relatively slow moving, feature short blades and have no centrifugal release. Therefore, bending is the primary load case. For that reason, he explains, "We started from scratch and developed a composite structure based on MCT's requirements."
The composite rotor features a 65-mm/2.56-inch thick carbon fiber-reinforced spar bonded to fiberglass ribs and sheathed with a fiberglass-reinforced skin, all using a marine-quality epoxy resin matrix. Fleming explains, "Fiberglass offers needed impact strength vs. carbon fiber, as these blades could possibly encounter foreign objects floating in the current - even steel containers." The spar is made using proprietary VTM264 prepreg developed by Advanced Composites Group Ltd. (Heanor, U.K.), which can be vacuum-bagged and cured in an oven at 75°C (167°F), a less expensive process made possible because high-temperature properties are not required. Currently, the fiberglass ribs and skin are made by a combination of wet layup and prepreg, although Fleming expects all-prepreg construction will be used in the future.
Seagen will be the world's first megawatt-sized (1.2 MW) tidal energy system connected to a commercial grid, when it is installed in Northern Ireland's Strangford Lough later this year. A 10-MW tidal farm off the coast of Wales, near North West Anglesey, is in planning now and could be operational by 2009. It will most likely use 7 to 10 Seagen-type units with the capacity to provide energy for 4,000 to 6,500 homes.
An unusual aspect of the design is that, in operation, the rotor blade is flooded with water. MCT developed and patented the flooded-blade concept in the U.K. and internationally. Fleming explains, "At a subsea pressure of 3 bar [43.5 psi], filling the blade with foam would not only require more structure and material cost, but could also cause fatigue issues as you would have an edgewise load reversal due to the buoyancy." Fleming explains that no one has tested what the effect of 1 bar/14.5 psi reversal pressure would do to foam-filled blades when subjected to 1 million cycles over 25 years. Flooding the blade, however, equalizes the internal and external pressure and, because salt water won't corrode the composite, does not reduce blade service life.
Due to the success of Seaflow, which is still in operation after several winters in rough seas, MCT has received funding for Seagen, a 1.2-MW tidal energy device. Seagen will be the world's first megawatt-sized tidal energy system connected to a commercial power grid. It will be installed later this year in Strangford Narrows, Northern Ireland, which offers maximum currents of 7.5 knots or 3.6 m/s (11.8 ft/s) as well as high ground on either side to minimize weather risk, permitting easy service via a rigid inflatable (RIB) workboat.
Seagen uses twin, two-bladed 16m/52.5-ft-diameter rotors, which MCT says are more cost-effective than those with three blades, and are more easily lifted clear of the water for service. Unlike the straight-edged Seaflow blade, the Seagen blade has a fully optimized geometrical shape, with a compound curvature and twist that raise its performance coefficient (i.e., efficiency) to 45 percent, compared with 37 percent for Seaflow. However, as was observed in the Seaflow installation, Seagen's siting in close proximity to the seabed and surface (vs. the design model's infinite boundary conditions) will concentrate water flow, reportedly increasing the rotor's efficiency to 50 percent.
AEL has delivered the 7.5m/24.6-ft blades to MCT and is in the process of developing blade designs for the next generation of Seagen rotors that will span 18m to 24m (59 ft to 79 ft). Seagen will have other composite components, all made from glass-reinforced plastic, including a housing on top of the monopile and drag-reducing fairings on the horizontal "cross-arm" that supports the rotors. MCT wants to use composites for the entire cross-arm eventually, but this will require significant R&D and investment in molds. The company hopes to install and commission Seagen by third-quarter 2007.
A proposed 10-MW tidal farm off the coast of Wales, near North West Anglesey, will feature 7 to 10 Seagen-type units connected to the local electricity network and will be capable of providing electricity to 4,000 to 6,500 homes - 10 to 15 percent of the island's demand - sometime in 2009.
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