Tidal power tested off the coast of Oregon
Posted by: isiria, in SUSTAINABLE LIVING, technology
Finavera Renewables, the Canadian company that wants to harness wind and wave power, has successfully deployed a prototype off the coast of Oregon (Finavera is also building wind farms in Ireland). The Oregon half-size prototype does not generate power yet, but is mostly used to study how much pressure can be generated and captured. The company will use the performance field data to design a larger version for commercial deployment. It should be capable of generating 250 kilowatts, enough electricity for nearly 100 homes, and be ready in 2008. Finavera hopes to be running wave power farms and generating electricity, probably off the northern Pacific coast, in 2010.
Finavera’s offshore power plants consist of patented wave energy converters. Clusters of these small, modular devices, called AquaBuoys, are moored several kilometers offshore where the wave resource is the greatest. The potential for 
wave power is enormous, say proponents. Water is 800 times denser than air at sea level; thus, waves and tides can generate more power in less space than wind turbines. Skeptics, though, note that installing infrastructure in the sea is fraught with difficulty and is expensive. Neptune’s fury is not to be messed with.
Waves push the AquaBuoy up and down in the water. Energy transfer takes place by converting the vertical component of wave kinetic energy into pressurized seawater by means of two-stroke hose pumps. Pressurized seawater is directed into a conversion system consisting of a turbine driving an electrical generator. The power is transmitted to shore by means of a secure, undersea transmission line. (The fact that the hydraulic fluid inside the AquaBuoy is seawater is also an environmentally sensitive solution; if one breaks, nasty oils don’t escape into the environment.)
Since these power plants are scalable from hundreds of kilowatts to hundreds of megawatts, they can provide clean, renewable energy for large population centers; they are suitable as distributed generation and load balancing at coastal transmission points.
Buoys bobbing far offshore are less visible than solar power farms or turbines. A cluster of AquaBuOYs would have a low silhouette in the water. Located several miles offshore, the power plant arrays would be visible to allow for safe navigation and no more noticeable than a small fleet of fishing boats.
The video is promotional but it explains a little how the AquaBuoys work.
Wave and tidal power are primarily in the prototype and experimental stage, but several companies are ramping up prototypes and test vehicles. Marine Current Turbines, one of those comapnies, hopes to put in a tidal turbine in the water off Northern Ireland later this year.
Marine current turbines work, in principle, much like submerged windmills, but driven by flowing water rather than air. They can be installed in the sea at places with high tidal current velocities, or in a few places with fast enough continuous ocean currents, to take out energy from these huge volumes of flowing water. These flows have the major advantage of being an energy resource which is mostly as predictable as the tides that cause them, unlike wind or wave energy which respond to the more random quirks of the weather system.
Artist’s impression of MCT Seagen pile-mounted twin rotor tidal turbine
The technology under development by MCT consists of twin axial flow rotors of 15m to 20m in diameter, each driving a generator via a gearbox much like a hydro-electric turbine or a wind turbine. The twin power units of each system are mounted on wing-like extensions either side of a tubular steel monopile some 3m in diameter which is set into a hole drilled into the seabed.
The technology for placing monopiles at sea is well developed by Seacore Ltd., a specialist offshore engineering company (and MCT’s largest shareholder) which is co-operating with MCT in this work. The patented design of MCT’s turbine is able to be installed and maintained entirely without the use of costly underwater operations. A unique, patented feature of MCT’s technology is that the turbines and accompanying power units can be raised bodily up the support pile clear above sea-level to permit access for maintenance from small service vessels. This is an important feature because underwater intervention using divers or ROVs (Remotely Operated Vehicles) is virtually impossible in locations with such strong currents as are needed for effective power generation. The artist’s impression indicates a row of turbines and shows one raised for maintenance from a small workboat.
The submerged turbines, which will generally be rated at from 750 to 1500kW per unit (depending on the local flow pattern and peak velocity), will be grouped in arrays or “farms” under the sea, at places with high currents, in much the same way that wind turbines in a wind farm are set out in rows to catch the wind. The main difference is that marine current turbines of a given power rating are smaller, (because water is 800 times denser than air) and they can be packed closer together (because tidal streams are normally bi-directional whereas wind tends to be multi-directional).
Environmental Impact Analyses completed by independent consultants have confirmed MCT’s belief that the technology does not offer any serious threat to fish or marine mammals. The rotors turn slowly (10 to 20 rpm) (a ship’s propeller, by comparison, typically runs 10 times as fast and moreover our rotors stay in one place whereas some ships move much faster than sea creatures can swim). The risk of impact from our rotor blades is extremely small bearing in mind that virtually all marine creatures that choose to swim in areas with strong currents have excellent perceptive powers and agility, giving them the ability to successfully avoid collisions with static or slow-moving underwater obstructions.
Another key advantage of MCT’s technology as a future large scale generating technique is that it is modular, so small batches of machines can be installed with only a short period between investment in the technology and the time when revenue starts to flow. This is in contrast to large hydro electric schemes, tidal barrages, nuclear power stations or other projects involving major civil engineering, where the lead time between investment and gaining a return can be many years.
Artist’s impression of a row of tidal current turbines,one of them raised for maintenance
It is expected that MCT’s turbines will generally be installed in batches of about 10 to 20 machines. Many of the potential sites so far investigated are large enough to accommodate many hundreds of turbines. It is worth noting that as a site is developed, the marginal cost of adding more turbines and of maintaining them will decrease, so considerable economies of scale can be gained as the project grows.
The design life for MCT’s tidal turbines will exceed 20 years and we believe the main monopile support structure can be designed to survive for many decades (the track record of steel offshore structures, providing they are properly protected, is excellent - many offshore oil and gas structures have lasted upwards of 40 years). The steel pile and other main structural elements in an MCT tidal turbine have cathodic protection and the rotor is constructed from glass and carbon fibre reinforced composite materials which are not significantly effected by contact with seawater.
[sources: Crave; Finavera Renewables; Marine Current Turbines]
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