Despite the enormous potential of harnessing the movement of the tides to provide electrical power, the technology for tidal power generation has failed to take off. Only a handful of generating facilities exist worldwide, and half of those are tiny “test” projects. One of the largest projects is located in the La Rance estuary in France. Completed in 1966, the La Rance estuary project generates 540 million kilowatt hours per year. A tidal plant in the U.S. is located in Annapolis, Maryland, and generates a modest 30 million kilowatt-hours per year. The state of Oregon has exhibited significant interest in exploring tidal power.
The main benefit of tidal power, in comparison with other forms of renewable energy, is its predictability. The timing and force of tides can be predicted with great accuracy, and thus so can the power produced by a plant. The main drawback of this power source is its high initial cost, which runs many times that of conventional power sources, even though there is little maintenance or operation cost after instillation.
In a traditional tidal energy plant, a dam is constructed that captures tides as they flow inward. When the tide goes out, water from the dam is released which powers a turbine in a manner similar to traditional hydroelectric power generation. These systems work best when there is a dramatic difference, at least 16 feet, between low tide and high tide. An excellent example is Canada’s Bay of Fundy, with some of the highest tides in the world.
A 2007 report titled “Tidal Power in the U.K.” is calling for significant investment in tidal generation. In particular, the Severn Estuary could generate a large amount of the U.K.’s total electric needs, perhaps as much as 5%. Meanwhile, the government has given approval for a major wave generation project off the north coast of Cornwall. The intent is to develop a “Wave Hub” about 10 miles offshore that will act as a receptor for wave generation farms, gathering electricity and sending it ashore to be distributed in the grid.
Promising news for tidal power is coming from a radical design—a tidal mill that looks a lot like a land-based windmill. The tidal mill consists of three 30-foot long blades and weighs 180 tons. This design can offer several benefits, including minimal interference with sea life. Hammerfest Stroem, the electric company in Hammerfest, Norway, has constructed a 20-tidal mill site with a capacity of 32 gigawatt hours, built at a cost of $100 million.
Another tidal power development with real potential is the Archemedes Wave Swing (AWS), a large, submersed telescopic cylinder filled with air. Inside is a “floater” that moves up and down as pressure surrounding the cylinder changes due to waves. That movement, which corresponds with the ebb and flow of the water, is converted to electricity via a linear generator. Each AWS unit is about 39 feet in diameter and has an average output of 2.5 megawatts in a rough sea (producing about 5 gigawatts per year). The system has been tested in a pilot plant off the coast of Portugal. A company called AWS Ocean Energy Ltd. (
www.awsocean.com) hopes to deploy a pre-production model in early 2008, and have an operational commercial farm of AWS units later that year.
Another ocean driven technology uses wave power generators, which are currently being tested in waters near New Jersey, Hawaii, Scotland, England and Western Australia. The generators, known as wave energy converters, are semi-submerged cylinders of almost 400 feet in length and more than 11 feet in diameter. The cylinders are jointed and undulate in wave action like snakes. The energy of the wave action is resisted by hydraulic rams in the joints. The rams then pump high-pressure fluid into chambers that feed the fluid to a motor. The motor, in turn, drives a generator that creates electricity. Power from all the joints is transported down an umbilical cable connecting the cylinder to a junction on the sea floor that consolidates the power and sends it to shore via another cable connection. The cylinders are designed to work in concert connected by mooring lines, forming a wave “farm.”
A working example of a wave energy converter is the Pelamis, built by Pelamis Wave Power (formerly Ocean Power Delivery, Ltd.),
www.pelamiswave.com, an Edinburgh technology company focused on ocean wave power generation. Three Pelamis converters were first tested off the coast of Portugal. In September 2006, the company, in partnership with renewable generator company E.ON UK (www.eon-uk.com), announced plans to develop a five-megawatt wave power project off the coast of Cornwall called WestWave. It would consist of up to seven Pelamis converters connected to an existing offshore electrical “socket” called Wave Hub. WestWave could be operational by 2008, pending successful testing.
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