This is the third part of the first portion of a multi-part series featuring papers on OE-related topics by students from the 2.65 class. The series is intended to raise interest and awareness of ocean energy research at MIT and in the world. This first section is on Tidal Power generation and is written by James Modisette, a graduate student in course 16. His sources and the full-text with citations are available upon request.
Current Installations
Current tidal power installations utilize one of two generation schemes: the barrage style, built mostly around the 60’s, and the stream generator, deployed over the past ten years. A large number of “prototype” barrage style tidal power generators are installed around the world. The main characteristics of the four largest tidal barrages are shown in table 1.
Table 1: Main characteristics of large barrage tidal power plants.
The most successful barrage style tidal power generator is La Rance, in France, shown in figure 5. It was built during the mid-60’s and other than a seven-year period in the late 70’s it has been consistently generating 240 megawatts from 24 turbines. Although it is only a single-basin scheme, the generators are reversible bulb turbines capable of turbining or pumping in both directions. This added complexity was included in the prototype plan to increase flexibility and provide as much data as possible for the development of tidal technology. The La Rance location has a maximum head of eleven meters, a minimum head of three meters, and an average tide of eight and a half meters. It also benefits from having no major seasonal changes in the tides, just a standard fourteen day recurring cycle.
Figure 5: Area view of the La Rance power plant.
La Rance has had only one major mechanical issue in its 40 years of operation which,an impressive feat for a prototype facility. In 1975 it was discovered that the stresses from the start-up for pumping had been underestimated and that every stator had to be rebuilt. This was concluded by 1982 with little loss (<>
were used. In 1979, after thirteen years of use, all of the blades were still in excellent condition with some of the original paint still on them.
Although La Rance has been generating “clean power” since 1969, it has caused environmental damage to the estuary closed off by the barrages. The closing of the estuary with the cofferdam caused almost complete destruction of the marine flora and fauna. The estuary had to be re-stocked with aquatic life, and after fifteen years it flourished again, but not with the same ecosystem. The basin was smaller and the tidal flow was broken up between the sluice gates and the turbines. These changes led to the survival of different species. While the original ecosystem was essentially replaced, it does not appear that the barrages accelerated silt growth in the estuary, but it has led to simple sedimentary redistribution. La Rance also came with some unforeseen advantages. Tourism in the Bretagne region received a major boost and maintains a level that never existed before the power plant was constructed. Additionally, the most positive outcome of La Rance is that the average cost of tidal energy generated in 1976 was equal to the average cost of nuclear kilowatt hour and also equal to the average system thermal kilowatt hour cost. Other than operation and maintenance costs there have never been any fuel costs at La Rance.
Other barrage style tidal generators include a facility in the Annopolis River in Novia Scotia generating (20 megawatts), the Kislaya Bay in the Russian Arctic (400 kilowatts), and more than a hundred Chinese plants (total capacity of 7.5 megawatts). All of these facilities were much smaller than La Rance and were trial facilities for proposed larger tidal generation installations. Different turbine technologies, pumping strategies, and construction techniques were tested and a great deal of information about barrage style tidal power generation developed. For the most part, all of the prototype facilities have been successful and yet none of them have led to large-scale commercial barrage style tidal power generation. The necessary capital costs and extremely slow start up times continue to scare away politicians and investors.
More recently, following the advances in technology for wind turbines, kinetic energy turbines have started to emerge around the world. The stream turbines benefit from the fact that they are significantly smaller installations, can be fully submerged in a tidal stream without disrupting shipping traffic, and do not require the same massive initial capital investment.
There are many new tidal stream turbines around the world that have been set up to test capabilities in preparation for larger developments. Two of particular interest are Verdant Power and SeaGen. Verdant Power has six turbines in the East River between Manhattan and Roosevelt Island. SeaGen has a Marine Current Turbine off of Northern Ireland capable of generating what they claim as the first “commercial” amount of power. Verdant Power is the first major tidal-power generation project in the U.S.. The project was initiated in 2002 and by December 2006 two turbines had been installed in the East River. One of the turbines has been delivering a maximum of 35 kilowatts to the grocery store and parking garage on Roosevelt Island while the other one has been delivering performance data. In May 2007 another four turbines were installed. The six turbines will need to be monitored for eighteen months to address environmental concerns.
In particular, the fear exists that the rotor blades could chew up the river’s fish as the rotor tips travel up to nine meters per second. As with any new technology Verdant Power has suffered some set backs. In September 2008 it had to replace all of the original rotor blades because the design was faulty and some had broken off. The original rotors were made from fiberglass with a steel skeleton while the new rotors were made from aluminum and magnesium. Although they have suffered some set backs, Verdant Power is optimistic and plans on installing enough turbines to produce 10 megawatts.
Marine Current Turbines of Bristol, England, has also tested an eleven meter turbine capable of producing 300 kilowatts off the coast of Devon, England, for four years. After the success of Marine Current Turbines’ test, SeaGen [3] selected them to produce what it is calling the world’s first tidal turbine capable of generating “commercial” amounts of energy or 1.2 megawatts. As seen in figure 3 (b), the SeaGen installation works by using two rotating blades that turn nominally at fourteen revolutions per minute. The two blades drive a gear box before the power reaches the generator. SeaGen was built at Belfast’s Harland and Wolff’s shipyards and is bolted to the sea floor. The process of bolting it to the sea floor encountered unexpected complications and it took fourteen days as the installation team had to fight the very tides that they hoped to harness. But, the ability to manufacture the stream generators on-shore, as SeaGen was, will go a long way to ensuring their economic viability.
Currently, the SeaGen power generation unit is designed to break the surface of the water with a column structure. This allows for the rotor assembly to be lifted out of the water for maintenance and inspection without the need for divers swimming in tricky fast-moving water. This characteristic may only be desirable in some locations and Marine Current Turbines has already conceptualized a fully submerged second generation design.
Be sure to keep following this series, the next portion of this paper will focus on the challenges facing tidal power generation..
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