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This State of the Art report will focus on the concept of tidal power generation. First, tides will be defined, followed by a technical description of relevant engineering and scientific principles involved in power generation. Next, industry benchmarks, design failures and lessons learned, environmental impacts, and lastly the advantages or disadvantages of the current technologies leading the industry will be reported.
Tidal power generation requires the use of many engineering and scientific principles. Tides are the periodic rise and fall of the waters of the ocean and its inlets, produced by the attraction of the moon and sun (15). The gravitational forces, mostly between the moon and water bodies cause high tides on the earth's sides closest to and furthest from the moon (10). The tides also cause tidal currents, or the horizontal movement of water, which they can range from zero to 3 m/s, accelerate, decelerate and change direction once or sometimes twice daily (11). An example is the Messina strait between Sicily and the Italian mainland which has 2.4 m/s currents with negligible rise and fall of tides (3). Lastly, tidal energy is an attractive alternative green technology because of the predictability of tides and its huge potential which worldwide is estimated to be about 500-1000 TW h/yr (5).
The kinetic energy of moving tidal currents and can be used to generate electricity. Tidal streams can be harvested for power with "tidal stream turbines". These are stand alone turbines similar to modern wind turbines and for both, there is a cubic relationship between power and velocity of the median. Pts = Â½ ÏA v^3; where, Pts is the tidal power content in a tidal stream, Ï is the density of water. Its value for sea water is approximately 1025 kg/m3, A is the cross-sectional area perpendicular to the flow direction and v is the velocity of the tidal stream (12)(11)(3). This relationship can be improved by ducting turbines to eliminate the so-called Betz limit, which defines an upper limit of 59.3% efficiency in transference of kinetic energy (3). This will be further discussed later in the report. Moreover, water is more than 800 times denser than air and a typical water flow speed contains eight to ten times the kinetic energy of air. Tides only need to move at around 2.5 meters per second to generate power (4).
Differently, the potential energy generated between the high and low tides can be captured. The basin technique implements impoundments installed across estuaries or bays "catching" water in its elevated state at high tide by closing off entrance slates, creating hydroelectric reservoirs. Then, similarly to a hydroelectric dam, the water is drained back out spinning turbines at low tide, only to refill and repeat. This method is called "ebb flow generation" because it only generates power when the water is flowing in the ebb, or out direction. In some applications, pumping water above the high tide mark can create greater head, the potential energy inherent in a difference in water surface elevation, producing more power on drain than the initial pumping requires (3). Some barrages utilize both the ebb and flood flow, spinning generators as water flows into the estuary as well. Next is one of many equations that represent the potential energy of a "caught" basin of water. E max = Dg * R^2S, where R is the tidal range in meters, D is water density, g is gravitational acceleration, and S is the enclosed area of the basin. It has been proven however that only 30% of this energy can be retrieved (10).
Many other scientific principles are used to transform the kinetic energy of water into electricity, primarily Faraday's Law of Induction. All techniques of producing electricity covered in this report do so by use of a generator. Moving water spins through or across a blade or turbine system that turns a drive shaft connected to generator, usually by means of a gearbox. In some applications however, rotational force is transferred by hydraulic pumps; one pump is turned by the turbine, which is hydraulically connected to others that spin generators (12). The conversion of mechanical energy to electrical energy is done by inducing voltage into a wire by passing it through a magnetic field. In a generator, the armature (loops of wires), is spun by an external force (water) in a magnetic field causing the wires to cut through lines of flux, and by Faraday's Law of Induction, voltage is produced (9). Depending on the type of generator, current passes through brushes to a slip-ring configuration, finally into a stationary cable. To make the power useful, the electricity must pass through a transformer before connection to the grid (12).
Currently there are few commercial tidal power plants in operation, and tidal turbines are still in their infancy. Marine Current Turbines Limited currently leads the turbine division and claims their turbine "SeaGen is by far the largest and most powerful tidal turbine in the world with twin rotors each sweeping over 200 square meters of flow" (7). SeaGen was installed in May 2008, operates in the Strangford Narrows, Northern Ireland, using twin 16m diameter rotors to develop a rated power of 1.2MW at a current velocity of 2.4m/s(7). The twin rotors are connected to a "wing-like" concrete cross-member that can be raised and lowered along a single monopole to facilitate maintenance. Marine Current Turbines estimates that SeaGen generates electricity at about $5.5 million per megawatt installed, close to double the cost of offshore wind energy (8). In a New York Time article, Martin Wright, the company's managing director showed his optimism. "How many millions of kilowatts of wind are installed already, and here I've got one machine and I'm one small firm and I'm only twice the price? We haven't even started going down the cost-reduction curve yet" (8).
Around the same time similar wind-mill like designs were implemented in the US. Verdant Power installed six five meter 3 blade turbines in New York's East River, each capable of generating 35 kilowatts of power by April of 2007. This was the first major tidal-power project in the United States (2).
The first and still the largest operational tidal barrage plant in the world was built in the early 1960s (5). The La Rance plant on the Brittany coast of northern France has been generating 240 MW of electricity since 1966, when it became fully operational (2). The La Rance Barrage crosses a 750 meter width of the estuary and generates electricity in the both the ebb and flood flows of the tides using 24 10MW turbines, each 5.35m in runner (propeller) diameter (13)(10). It also pumps in both directions to raise (around high tide) or lower (around low tide) the basin level to add to the energy potential of the subsequent generating phase, and tide levels can exceed 13 meters (13). The barrage also allowed for a connecting 14 meter wide road across the Rance, cutting 30 minutes travel time between the towns on either side (10). Other operation tidal plants exist at Kislaya in Russia, Jiangxia in China, and Annapolis in Canada (5).
Many companies are developing new methods for tidal power generation. The next method I will present is a tidal fence. This approach consists of many turbines crossing the whole width of a water passage forming a fence like grid of turbines. A Canadian company, Blue Energy is at the forefront of this technology. They claim power yields of potentially 400% to 1000% over that of bottom mounted free stream methods, like that of SeaGen (6). Blue Energy's ocean turbine consists of three or four hydrofoil blades connected to a vertical axis, mounted in a concrete venturicle marine caisson, which directs flow through the turbine and supports the generator and gearbox above (11). The hydrofoil blades employ the hydrodynamic lift principle causing the blades to move faster than the passing water in either the ebb of flood tides. By stacking these modular turbines like building blocks across a water passage, a tidal fence is created, or as Blue Energy calls it, a Tidal Bridge (6). As the fence protrudes out of the water, a road can be constructed across the top, thus forming a transportation bridge. Blue Energy reports a fifty percent blockage ratio, increasing the head on one side of the bridge, and elimination of the betz affect as water must pass through the turbines (6). There are claims of many advantages including environmental and mechanical, which will be discussed later.
Tidal power uses many existing technologies so many design failures have been avoided. Tidal turbines are very similar to windmills and the shipping as well as off shore oil industry have provide many years of experience (material selections, propeller design, windmill design) to reduce risk. Time will only tell how well SeaGen and others perform, and tidal fences have yet to be actualized. As of January 10th 2010, SeaGen has been successfully generating 1.2 megawatts of electricity since April 2008 (8). When it comes to tidal barrages, much has been learned from the construction and maintenance of hydroelectric dams, including sharing many of the same components. La Rance Power Plant has operated since the 1960's and the original blade machinery has lasted for more than 40 years (13). It has delivered reliable, sustainable and highly useful power while the performance, improvements and updating have been consistently reported (10). It is also noted that these plants typically last between two and three times longer than nuclear or other typical power plants (10). Construction methods learned from the offshore oil industry today would have changed the way the barrage was constructed. It would not have been necessary to isolate the basin area from the sea (and hence also its tidal effects) whilst the main construction work was carried out (13). Doing so affected the ecology of the waterway some years.
As tidal power is a green energy alternative, it is important to understand the impact these technologies have on the environment. It is common to regard tidal barrages as the least environmentally friendly option (10)(2)(3)(5). There is mixed reviews though on the La Rance Power Plant. One author notes that decreased current velocity in the estuary above the barrage has increased fine sediment deposits. These deposits are reportedly ecologically beneficial, being responsible for much increased biological productivity. Increases in production have also been recorded in the Estuary seaward of the barrage (13). The basin is now considered a nursery for invertebrate fauna, which are more prolific and varied than in pre-barrage conditions. A greater increase in Shorebirds has been recorded in the basins, the fish fauna has developed to be much more abundant and diverse and fish mortality due to the turbines is reported to be very low (13). The fact is inevitable changes to the ecosystem will occur with the construction of a barrage, whether for the better or worse.
According to Verdent Power, the main reason for the development of tidal stream turbines is their very small environmental footprint (2). Some authors even claim that "water current turbines in open flow can generate power from flowing water with almost zero environmental impact" (3). Before SeaGen could be fully operational, environmental scientists onboard and onshore insured an initial requirement under the licensing arrangement; that SeaGen did not adversely affect the marine mammals that are a protected feature of the local waters. Observations to date suggest the seals and porpoises of these waters are not at any significant risk (7). The predecessor of SeaGen, Seaflow was subject to an independently completed environmental impact study. No harmful effects were detected, underwater acoustic measurements indicatde that noise levels and frequencies are unlikely to be disturbing for the marine fauna, there were no known leaks of pollutants, the seabed appears unaffected and wake measurements confirmed that the turbine became undetectable at a distance of 200Â m downstream of the rotor (14).
Lastly, tidal fences have not been tested yet as they have never been actualized, however early test on a 1983 install of blue energy's ducted turbine on the Seaway of Cornwall recorded zero fish fatalities (5). Blue Energy also claims that larger marine mammals will be prevented from contact with the rotary foils by a protective fence, and further protected by a backup auto-breaking system controlled by sonar sensors (6).
There are advantages and disadvantages to the different methods of energy capture. First, tidal barrages have proven to work and generate power competitively with other power generation plants (10). La Rance Tidal Plant generates as much power as many natural-gas-fired plants (2). Tidal stream turbines have the advantage of eliminating the high capital costs of large civil construction projects, little disruption to the environment and many locations can be used where there is no need for large tidal rise and fall (3). Ducted tidal stream turbines boast improved safety, protection from weed growth, increased speed and reduced turbine and gearbox size for a given power output. Maximum efficiency of energy conversion is not subject to the Betz limit of 59.3% of the energy incident on the swept area of an open turbine, since duct can draw in flow from a larger area and increase the available pressure drop across the turbine, generating about three times more power (3). Blue energy's tidal bridge is made of many modular ducted turbines and they have claimed tests conducted between 1981 and 1982 in the National Research Council Hydraulics Laboratory test flume in Ottawa showed an almost 5 time power increase versus a non-ducted turbine (11). They also claim that capital cost can be subsidized by transportation projects, as a road can be constructed atop (6). Moreover, repairs and maintenance are simplified by use of cranes from atop the bridge and because of modular construction (6). One problem may be finding a passage with sufficient flow and a desire to construct a transportation bridge along it as well.
Electric Power Research Institute, an industry R&D consortium noted tidal power is already economically competitive, producing electricity at prices similar to wind power (2). In some locations tidal power would be cheaper than transporting coal or a coal burning power plan (5). At the moment, although an environmentally friendly option, tidal power generation on the whole is still long from elimination other less friendly methods of power generation.