The tide moves a huge amount of water twice each day, and harnessing it could provide a great deal of energy. Ocean currents are found along coastlines beneath the oceans surface at depths of 80 to 230 feet (25 to 70 meters) and with flow rates of 5 to 10 feet per second (two to three meters per second). Horizontal or vertical axis turbines, much like underwater wind turbines, can be used to capture the energy from ocean currents or tidal flows. They can be installed so as not to interfere with fishing or the passage of vessels. The slow speed of the turbines does not interfere with aquatic life.
The potential for tidal power is large and a numerous potential sites have been identified. However there are not nearly as many potential sites as there are for wave power. The technology is so similar to that of wind turbines that it could be expected that a wind turbine manufacturer could jump into this market after it starts to develop.
The gravitational pull of the moon causes water to flow in from the ocean on the flood tides, and outward during ebb tides. However, tides are totally predictable, so other power stations can be generating at times when the tidal station is out of action. This energy source is independent of weather and climate change and follows the predictable relationship of the lunar orbit that is known many years in advance. In order for this to work well, large changes in the height of tides are needed.
Tidal dams are based on using a barrage, a damlike structure with openings allowing water to flow up and down an estuary as the tides move in and out. They are located at a bay or estuary with a large tidal range. As the tide flows in, water is allowed to flow up through openings in the barrage, into the estuary, then sluice gates on these opening are closed trapping the water above the barrage. The sluice gates on the turbines are then opened, allowing the water to flow out through the turbine, generating electricity.
Tidal dam technology is well established as evidenced by the installation at La Rance, France where a 240 MW plant has operated since 1966. A 20 MW facility has also been operating at Annapolis, Nova Scotia since 1984. The Alternative Energy Blog reported, on a September 30, 2004 announcement of the construction of the worlds largest tidal power plant in South Korea. The 254 MW plant will be larger than the plant in France. However, estuaries are amongst the world’s most productive and sensitive ecosystems, and the flooding by these by barrages causes a great disruption to their natural processes.
Tidal Electric, London, UK propses to overcome the problems with tidal barrages by using tidal lagoons to site its turbines. Instead of building a barrage across a river, they are building a man-made lagoon just offshore. The walls of the lagoon would rise from the seabed and extend about one meter above the high tide level. The turbines are located in the wall near the seabed. At high tide the sea rushes in through the turbine and fills the lagoon. At low tide the water flows out turning the turbine in the opposite direction thus generating electricity in both directions in four cycles each day. The best location for the lagoons is where the height difference between high and low tides is the greatest. Cost of electricity from theses plants is estimated by Tidal Electric to be about 2 pence ($0.036), about the same as coal fired plants in the U.K. (Sounds low to me)
The company is planning to build the first plant 2 km off the coast of Wales, near Swansea. This plant would generate 60 MW at a capital cost of 1.3 million pounds ($2.3 million). The next power station would be built along the northern Welsh coast and would generate 432 MW. Tidal Electric has also signed a agreement with the Chinese government to cooperate with the development of a 300 MW offshore tidal lagoon near the mouth of the Yalu River.
To further overcome concerns about barrages ocean turbines have been developed which are installed in independently so as not to interfere with the passage of ships or intrude on ecosystems. Narrow and shallow constrictions produce the fastest and most powerful movements of current, whose energy can be harnessed using ocean turbines. There are also considerable locations where offshore currents are rapid enough to use ocean turbines. Several manufacturers of ocean turbines have developed, their technology and projects are described below:
Blue Energy is a Vancouver, BC, Canada company. Four fixed hydrofoil blades of a turbine are connected to a rotor that drives an integrated gearbox and electrical generator assembly. The turbine is mounted in a durable concrete marine caisson which anchors the unit to the ocean floor, directs flow through the turbine further concentrating the resource, and supports the coupler, gearbox, and generator above it. The later sit above the surface of the water and are readily accessible for maintenance and repair. Prototypes of turbines ranging in size from 4 kW to 100 kW have been built and tested. Blue Energy has installed several 5 kW to 25 kW systems to remote domestic consumers who were too remotely located for connection to the grid. It is preparing to install a 500 kW system using two 250 kW turbines.
Marine Current Technologies is developing new technology, backed by the U.K. government, to exploit tidal currents for electrical power generation. The large turbines are best suited to installations where the water velocity is greater than 2.25 m/sec with a depth of 20 to 30 meters. The first phase of their development is nearing completion. A 300 kW commercial scale monopile mounted 11 meter diameter rotor turbine was designed, built, installed and tested for two years. Currently a 1 MW twin rotor system is being developed. It will have rotors 15 to 20 meters in diameter operating at 10 to 20 rpm. 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 turbines and associated power units can be raised up on the monopile to above the sea level for maintenance. Installation of systems is expected to start in the 2006 to 2007 time frame.
SMD Hydrovision is developing theTidEl system, a moored free stream tidal turbine system. TidEl uses a pair of buoyant turbines anchored to the sea bed by mooring chains, to extract power from tidal flows. The generators use variable speed drives which allows the turbines to operate at varying speeds. This allows the optimum hydrodynamidc efficience to be achieved for any tidal current velocity. The mooring arrangement allows the system to generate electricity in both directions, simply by flipping over as the tide changes direction; making it extremely efficient and easy to install and support.
A one-tenth scale model has beeen built and undergone testing. The complete system, consisting of buoyant dual generator, spread mooring and grid integration controls were tested in a wave tank for two months. They have received a grant from the Department of Trade and Industry to develop a 1 MW, grid connected system, with blades 15 meters long, to be installed at Eday, Orkney in 2006.
Verdant Power, Arlington, VA, was formed in 2000 to conduct research and development of a "free-flow" unducted, axial-flow rotor turbines that utilize the water stream's natural pathway. The design of these turbines consist of a concentric hub with radial blades, similar to that of a windmill. Units convert kinetic hydro energy to electric power in ranges from 25 kW to 250 kW depending on model size and water flow velocities. They are designed to be deployed in water currents with flow velocities of five feet per second (3 knots) or more, and in depths of at least six meters (20 feet). The turbines are anchored to water bottoms, either by pylons, concrete bases, or other site specific anchoring devices.
Mechanical power is applied directly through a speed increaser to internal electric generator, or through a hydraulic pump that in turn drives an onshore electric generator.
In December 2002 and January 2003, Verdant Power successfully deployed a prototype turbine system in the East River in New York City. This has been followed by a contract to design, fabricate and deploy six more turbines. Deployment of the first two turbines was underway in January of 2005 for an 18 month test. Each 36 kW turbine has three slow-turning composite blades to form a rotor 16-ft in diameter which rotate at 36 rpm. A yaw system allows the turbines to rotate on a vertical axis so they can capture energy from both the ebb and flow of the river tides. The project's ultimate goal is to construct a 5 to 10 megawatt power field. It will be populated with several hundred turbine units, mounted on mono-piles affixed to the bottom of the tidal basin. They also have a contract to develop a tidal site in Massachusetts. There they will use the Gorlov Helical Turbine, from GCK Technology in conjunction with an operating platform - including drive-train, generators, and power conditioning systems - developed by Verdant Power.