By Charlotte Helston
Tidal power exploits energy drawn from the movement of ocean tides to produce electricity. There are two scenarios in which tides can be tapped for energy. The first is in changing sea levels. This phenomenon is responsible for the advancing and receding tides on shorelines. With the help of turbines, incoming tides can be manipulated to generate electricity. The second way to exploit tidal energy is by sinking turbines to the sea floor where fast-flowing currents turn generator blades much like wind does with a wind turbine.
Tidal energy is considered renewable because the tides move on a predictable, daily schedule, depending only on the orbits of the Earth, Moon, and Sun, and are essentially inexhaustible.
In the past, large-scale barrage systems dominated the tidal power scene. But because of increasingly evident unfavorable environmental and economic drawbacks with this technology, research into the field of tidal power shifted from barrage systems to tidal current turbines in the last few decades. This new technology leaves a smaller environmental footprint than tidal barrages, as turbines are placed in offshore currents avoiding the need to construct dams to capture the tides along ecologically fragile coastlines. Harnessing tidally-driven coastal currents cannot yet deliver the sheer amount of power that barrage style facilities can, like at the 240 MW barrage generating station at La Rance, France.
Canada hosts two test sites, one tidal barrage and one tidal current power station. With one new and one old, both a history and a newfound interest in tidal power is apparent. The Annapolis Royal tidal barrage built in Nova Scotia's Bay of Fundy in 1984, with its world-famous tides, operates as the third largest tidal power plant in the world, with 20 MW.
Barred from the mainstream by financial difficulties and environmental concerns, both tidal barrages and tidal current turbines face challenges in becoming major suppliers of energy in the 21st century. Recent emphasis on the potential of tidal current turbines, and their reduced effect on shoreline and aquatic ecosystems, suggests that they will replace tidal barrages as the preferred method of exploiting tidal energy.
Brief History of Tidal Power
The energy stored in tides been known to people for many centuries. The earliest records of tidal mills are dated back to the 8th Century CE.
Tidal energy harnesses the natural ebb and flow of the tides to produce power. Tides are created by the gravitational pull of the moon and sun, combined with the rotation of the earth.
A Tidal barrage is a close adaptation of conventional hydroelectric dam technology. This method blocks off an existing tidal estuary with a dam, or barrage. Movable flood gates, called sluice gates, on the dam allow incoming tidal waters to fill up in a reservoir. Once the water reaches its maximum level, the gates close and trap the water. The water in the artificial estuary is called
As the tide ebbs, a gradually increasing
A second way to exploit tidal power is through the use of tidal turbines to harness the energy found in tidal currents. Tidal currents are created by the
An extension of tidal turbine technology is found in tidal fences. A row of turbines is positioned as a "fence" through which water passes. Tidal fences can be constructed in channels between two landmasses. The energy potential depends largely on the rate of flow, which is unique for each location. Research has shown that little power is generated when only a few turbines are installed, whereas too many obstructs the flow, which also limits the power potential.
Tidal power technology is only useful if it is employed in a prime location. The success of all types — barrages, turbines, and fences — is contingent upon naturally occurring geographical elements. Although all tides produce power, there are only a few locations where tidal power can be harnessed. A suitable location must contain the following:
- The tide has to rise to unusual height. At least 7 meter tidal differences are required.
- Stable conditions for a barrier or turbine to be built into.
- Environmental disturbances must be reduced to a minimum.
Both tidal fences and tidal turbines depend on fast flowing water to generate power. Developers will seek locations that possess tidal streams — areas of quickly flowing water caused by the motion of the tides. Typically, tidal streams are found where underwater valleys force currents to constrict and speed up.
The United Kingdom island nation is seeking to take a lead in this field. These locations include the Pentland Firth, Irish Sea, North Channel, Alderney Race, Isle of Wight to Cherbourg, the Orkneys to Shetlands; and the Florida Current.
Major tidal currents also occur in the Arctic Ocean, Skagerrak-Kattegat, Hebrides, the Bay of Fundy, the gulfs of Mexico and St Lawrence, the Amazon and Rio de la Plata, the Straits of Magellan, Gibraltar, Messina, Sicily, and the Bosporus.
Tidal swell — the difference between the high and low tide marks — discerns the capabilities of the facility. High tidal swell locations provide the greatest potential for tidal development. Often, good sites are located in areas where incoming waters must funnel into narrow channels, including bays, river mouths, and fjords.
Not all coastlines feature the minimum 5 meter range needed to make ventures feasible. The world's greatest tidal range is found in Canada's Bay of Fundy, where tidal swell is over 15 meters.
There is untapped tidal energy in waters all around the world. For example, European waters are estimated to hold an exploitable 48 TW/year if put into service. Russia has a possible 90,000 MW. Canada is believed to have a potential 4,000 MW along the coast of British Columbia alone. In all, current studies suggest a potential 1,800 TWh/year globally.
The world's largest tidal barrage is the Lake Shiwa in South Korea. It has a peak capacity of 254MW and yearly production of 552.7 GWh.
The world's oldest and second largest operating facility, at La Rance, France, exhibits a peak rate of 240 MW capacity. With tidal ranges of about 8 meters, the facility generates about half a billion kWh annually using 24 low-head Kaplan turbines.
The third largest power plant, and the only one in North America, is Canada's Annapolis Royal tidal power plant. Located in Nova Scotia's famous Bay of Fundy, the plant exhibits a peak generating capacity of 20 MW with annual yield of 30Gwh/y.
The Jiangxia Tidal power station in China emits 3,200 kW from five experimental units. This is the only tidal power plant in China. China has raised several other small test plants, though about half of them are now shut down.
The Kislaya Guba Tidal facility in Russia is another pilot plant. Built in 1968, the plant has a small capacity of 400 kW. The plant has been mainly used to conduct research on the ecological safety of tidal barrage plants.
Another notable project is the Severn Barrage proposed for the Bristol Channel between Wales and England. The idea of a tidal power plant for this area dates back to 1925, though plans are hardly any more definite today. A combination of economic and environmental barriers has hampered development of the project that some say could have compensated for 5% of the UK's electricity.
Tidal current turbines have been on the minds of researchers and developers since the 1970s, though they have only recently been put into operation. The UK company Marine Current Turbines (MCT) paved the way for tidal projects with its unveiling of SeaGen in 2008. The 1.2 MW tidal energy converter, called SeaGen, is located in northern Ireland where it provides enough power for about 1, 000 homes.
Canada's potential tidal energy exceeds 42 GW; there have been 190 suitable sites identified, with BC having the most sites and Nunavut the greatest total potential.
The Bay of Fundy, which rests between New Brunswick and Nova Scotia, is Canada's — and likely the world's — most promising location for tidal power development. Each day, volumes of water in excess of 100 billion tons flow into the bay. That's more than all the world's freshwater rivers combined.
Aside from Nova Scotia, British Columbia is the only other Canadian province to have an installed tidal power system. In 2006, Race Rocks, BC, became home to a 65 kW tidal current turbine.
A 2002 study from BC Hydro estimated a potential 1,500 MW from identified sites. BC Hydro highlighted the passages between the Strait of Georgia and Johnstone Strait as the best prospects, due to their high-velocity tidal flows. Of 55 identified sites, 12 were isolated as the most feasible for development. These 12 sites harbor a potential energy production of 2700 GWh per year.
The recent Race Rocks demonstration site, off the coast of Vancouver Island, made the first splash in BC's tidal power scene. The 2006 micro plant of 65 kW replaced two diesel generators, and offers a prime testing site for tidal technology.
Planned projects include the 500 kW Canoe Pass Tidal project just north of Campbell River.
For years the high up-front costs of tidal power plants and lack of government support have deterred new projects from gaining ground. But, Nova Scotia's recently announced ComFIT program may turn the tides on financial matters. The province's Community
Government support was also seen at BC's Race Rocks demonstration site, where some funding was provided by Sustainable Development Technology Canada by way of a grant won by Pearson College project partners. The foundation, created by the government of Canada, controls a $550 million fund to assist the development and demonstration of clean technologies.
Large tidal barrages present several unfavorable economic factors: they have large capital costs and long construction times. This is somewhat balanced out by long plant lives of 100 years for the actual barrage structure, and 40 for the equipment, as well as low operating costs.
Much depends on the existing geographical and climatic conditions. A main investment is devoted to the development of the basin. Generally, costs increase for sites that experience violent winds and waves, as dykes must be built stronger and larger to withstand them.
Tidal energy generation is an emerging technology, yet in its infancy. With only four main tidal barrage plants operating in the world, clear capital costs are unknown. An estimate is given by researcher Eleanor Denny. Denny estimates that in order for a facility to be profitable, its capital cost should be less than €530,000 (~$700,000 USD) per MegaWatt which with the current technology is not a realistic goal, meaning that so far the industry produces negative net benefits.
With very few examples of tidal turbine and tidal fence power plant development, it is difficult to determine a typical cost. To provide a ball-park investment, it is possible to consider two existing tidal current installations.
Canada's Race Rocks site, where a single turbine generator converts 65 kW of energy, cost $4,000,000.
On the higher end of the dollar spectrum, we have Ireland's SeaGen, a 1.2 MW generator, driven by a pair of turbines. This plant produces about 100 times the power generated at Race Rocks. An investment of around €8.5 million ($11 million USD) made SeaGen a reality.
The 240 MW La Rance power plant provides electricity at 3.7 cents/kWh, which is much more reasonable than the 10.8 cents/kWh charged by thermal plants in the area.
BC Hydro's 2002 Green Energy Study for BC estimated the price of electricity from potential tidal developments to be in the range of 11-25 cents/kWh. This is a figure based on past and present technologies, and it is likely that as designs are improved, prices could fall considerably. The BC Sustainable Energy Association (BCSEA) notes that costs are expected to decline to around 5-7 cents/kWh.
An increase in tourism has been observed at Canada's Annapolis tidal plant, as well as at France's La Rance plant. More than 40,000 tourists visit the Annapolis facility each year.
On the other hand negative environmental effects on marine life can be detrimental to the fishing industry. Some fishermen have raised concerns over the fact that most identified sites for tidal power are also key migration routes for fish.
Tidal barrages and tidal current turbines each have their own set of environmental impacts. Best discussed separately, we will look first at barrages, and follow with a section on tidal current turbines.
Few studies have yet been done that fully analyze the impact of tidal power on local marine life. In all likelihood the diversity of marine ecosystems means that the effect of each tidal barrage or current turbine will be different. The environmental impacts of tidal barrage include hampered fish migration, forced water level changes on the basin behind the barrage, reduced salinity in the basin due to low quantities of ocean water, and reduced ability of currents to transport and suspend sediments.
A 2010 study examined ecological impacts at the Kislaya Guba tidal power plant in Russia. The 400 kW plant was completed in 1968 and continues to run to this day. Because of increased interest in tidal power, an ecological monitoring program was established there. An evaluation of the Kislaya site, sponsored by UNESCO, was conducted for the stages of formation, operation, and modernization. The site and environmental findings discovered there provide a good assessment of potential risks associated with tidal power plants.
Prior to development, Kislaya Guba Bay was a fjord with a rich array of marine life. During the four years it took to construct the power plant, the bay was closed off from the sea by a dike. Water exchange was massively reduced (to several percent of the natural exchange). The lack of moving water permitted the entire bay to freeze over in the winter, which annihilated coastal biota to a depth of 5m (15 m where oxygen was depleted and accumulated hydrogen sulfide contaminated the water). Evidence of ecosystem damage can be found in the abundance of dead mollusks in the bay.
Continuing impacts of operation include: "diminution of tides, diminution of sea swells, reduction in the flow of fresh water from the partitioned water area to the sea, and the mechanical effect of the turbine on plankton and fish.."
Though it is possible to use the Kislaya Guba power plant as an example, and perhaps use it as the basis for predictions of impacts at other sites, it is important to conduct site-specific analyses for each prospective location.
In general, tidal barrages reduce the tidal range by about half; diminishing the intertidal zone and instigating a ripple of effects through the coastal ecosystem.
- Study on tidal power's impact upon fish populations published in the Biological Journal of the Linnean Society
The macrotidal estuaries of the Bay of Fundy, for instance, are used by large numbers of migratory fish, including dogfish, sturgeon, herring, shad, Atlantic salmon and striped bass, as well as larger marine animals such as squid, sharks, seals and whales. Studies have shown that fish passage utilizing the Annapolis estuary has turbine related mortality of 20-80% per passage depending on fish species.
Since tidal current turbine technology is a relative new industry and applied in only a few locations, the research regarding environmental impacts is limited to hypothesis, modeling and lab experiments. The turbines are designed to turn at low rotation speeds which are considered unlikely to injure fish, marine mammals, or diving birds.
Tidal fences — rows of linked tidal turbines — on the other hand, raise several concerns. The effects of extensive development, including undersea cables as well as land-based, or floating facilities, can include displaced seabirds and marine creatures. The placement of undersea "fences" causes changes in the natural tidal range, with consequences onshore even when sites are far from the coast.
The operation of a tidal power plant is mostly emissions-free. As a general trend, as the capacity of tidal generation increases, it displaces
Case Study: The Severn Barrage Tidal Power Project, UK
The planned barrage is 16 km long and using a tidal range of 6-12 meters it is estimated to produce 5% of the UK's national electricity. The facility requires the installation of more than 200 large water turbines and generators and 166 water control gates. The accompanying table summarizes the total carbon dioxide emissions during the sourcing, manufacturing and transporting of the building materials (i.e. cement, steel etc).
Though operation is emissions free — substituting the burning of fossil fuels for the clean "fuel" of water — the construction phase leaves an unavoidable, though comparably small, footprint. A 2008 report assessing factors of the proposed Severn Barrage project in the UK, detailed the CO2 emissions and the total
The second table compares the average annual energy output and carbon dioxide emissions of the Drax coal-fired power station and the Severn Barrage. Assuming that there are no emissions once the facility starts operating, in less than six months the project can "pay back" its carbon cost by replacing the coal-fuelled power station operating in the area.
Although sustainable energy resources produce limited amounts of carbon dioxide emissions, they are, by nature, reliant on the natural environment and therefore are vulnerable to the effects of climate change. While sea level and wind pattern changes are expected, tidal energy is less likely to be affected. This industry also has the advantage of being predictable and quantifiable, both spatially and temporally.
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