Tidal Power

By Charlotte Helston

I. Overview

Tidal Power by the Numbers

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. Though tidal energy is carbon free, it is not environmentally benign. Concerns over the health of shoreline and aquatic ecosystems mar this otherwise clean source of energy. Older tidal barrage technology can devastate fish populations.

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. However, the technology is quickly evolving with numerous test plants popping up around the globe.

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. The smaller Race Rocks facility in British Columbia, installed in 2006, uses tidal current technology to generate 65 kW of power. Studies have estimated a potential 4,000 MW of untapped energy flowing along the coasts of BC. Canada, and the shores of British Columbia, are home to some of the world's most attractive locations for tidal power development.

The Race Rocks tidal current turbine prior to installation just off the coast of Victoria. This is the only operational tidal barrage in British Columbia as of 2012.
The Race Rocks tidal current turbine prior to installation just off the coast of Victoria. This is the only operational tidal barrage in British Columbia as of 2012.

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. The tidal mills were mainly used for grain grinding and were of similar design to the conventional water mills with the exception of the addition of a dam and reservoir. The industrial revolution increased demand for power but tidal energy never got off the ground, undercut by cheap fossil fuels and other developments which offered easier access to power generation. Existing tidal mills became as obsolescent as pre-industrial water-mills. The first large scale modern tidal electric plant started to operate in La Rance Estuary, St. Malo, France in the 1960s and has been operating ever since. In recent years the search for renewable, non-polluting energy sources and the increase in fossil fuel prices has encouraged renewed interest in tidal power.

II. How Tidal Power Works

The energy potential of tidal power 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.

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. Tidal energy can be harnessed both in the sea, and in tidal rivers and estuaries. On some shorelines, water levels can vary up to 12 metres. It is this drastic change in water level that makes the first type of tidal energy — tidal barrages — possible. Tides can occur once or twice a day depending on location. Due to the upward gravitational rotation of the moon, the water level rises gradually until it reaches its highest point and then gradually falls back to its lowest point. Also, the tide does not occur at the same time every day, rather it fluctuates over a period of two weeks or so.

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 hydrostatic head.

The La Rance tidal power plant in France is the world's secpmd largest tidal barrage structure.
The La Rance tidal power plant in France is the world's second largest tidal barrage structure.

As the tide ebbs, a gradually increasing head differential is created between receding water levels and the fixed level within the barrier. When the head differential has reached the desired value, the potential energy created can be converted into mechanical energy or electrical energy simply by opening the gates and allowing the water to flow through the turbine. A proper site for this type of technology should have sufficient tidal range in addition to the location of a natural bay. It is also important to locate the facility in such a way that it will not reduce the tidal range.

Tidal Barrage Scheme.
Tidal Barrage Scheme.

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 flood and ebb tides. Tidal turbines are essentially submersible wind turbines that use water instead of air to turn the blades. Tidal turbines are sunk 20-30 meters, and can be situated anywhere that possesses a strong tidal flow. Because water is about 800 times denser than air tidal turbines must be built much sturdier than their terrestrial counterparts. Shrunken diameters help to reduce the structural strain. The advantage of greater density of water is that relatively large amounts of power can be produced with relatively small current and rotor diameter. For example: a rotor with diameter of 10-15 meters can generate 200-700 kW of power, whereas a 600 kW wind turbine requires a rotor diameter of 45 meters. Tidal turbines function best at flow rates of 7-11 km/hr.. An irrefutable advantage to tidal turbines, in contrast with wind turbines, is their predictability. Tides flow in and out every day, promising daily energy.

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. Therefore, it is essential to determine the optimum number of turbines, as well as their optimum location, in each distinct site. Fence installations are presumed to be less expensive to develop than tidal barrages, as well as less impacting on the environment. No tidal fences, sometimes also called tidal farms, are currently in operation. Their main component after all — tidal current turbines — are still only in the demonstration phase.

III. Geography of Tidal Power

Bay of Fundy at High Tide.
Bay of Fundy at High Tide, an ideal location for a tidal barrage.

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:

  1. The tide has to rise to unusual height. At least 7 meter tidal differences are required.
  2. Stable conditions for a barrier or turbine to be built into.
  3. 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.

Bay of Fundy at Low Tide.
Bay of Fundy at low tide.

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. In the Far East, useful currents are found near Taiwan and the Kurile Islands.

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. Ungava Bay and numerous estuaries along British Columbia's coast also feature large tidal ranges. The coasts of Argentina, NW Australia, Brazil, France, India, Korea, the UK, Russia and California, Maine and Alaska display strong potential for tidal barrages as well.

IV. Tidal Power Around the World

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 rise and fall of the tides dissipates about 3,000 GW of energy in shallow seas worldwide. The potential capacity worldwide is about 239 GW.

Where have tidal barrages been constructed?

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. Currently, the Koreans are looking into the possibility of building and expanding seven more facilities, including the second largest tidal barrage, the Icheron, with potential of 700-1000MW. They are also looking into expanding the Uldolmock plant from 1 MW to 90 MW by 2013.

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. Built in 1984 as a pilot project to test the effects of such a plant, Annapolis Royal will be not be alone in the bay for much longer. Recent test programs and government incentives have boosted development, and proposals for tidal current turbines have been announced.

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. As of 2012 this is the only tidal plant in Russia.

Annapolis Royal Tidal Power Plant in Nova Scotia, Canada.
Annapolis Royal Tidal Power Plant in Nova Scotia, Canada.

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.

Operational Tidal Turbines and Fences

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. Since SeaGen's success, numerous plans for development and pilot projects have emerged, with the United Kingdom leading the development effort, followed by the United States, Canada and Norway.

Development of all tidal power has been slow and haphazard since La Rance was built nearly 50 years ago.

V. Tidal Power in Canada

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. The bay already hosts a tidal power plant — one of only three major plants in the world. What began as a government pilot project, the Annapolis tidal power station now contributes 20 MW to the electrical grid. Compared to France's La Rance power plant, which has a capacity of 240 MW, the Annapolis plant seems small — especially as it's nestled in the world's most attractive tidal waters. It is thought that the bay could provide up to 8,000 MW of installed capacity. It isn't surprising, then, that numerous projects are being considered to tap more of Fundy's tidal energy. Both the Cumberland Basin and the Minas Basin have been assessed for development. In 2008, the Nova Scotia government launched its Fundy Ocean Research Centre for Energy (FORCE) program, which is aimed at developing a local test centre. In 2009, developers were chosen by FORCE to begin work on a series of test beds located in the Minas Passage. It is hoped that the provincial government's active participation and support of tidal power will motivate expansion.

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. That's only enough power to produce electricity for 10 houses if put into commercial service, but it's a start.

Canada Potential Tidal Resource sites.
Canada's potential tidal resource sites.

VI. Tidal Power in B.C.

B.C. is believed to have tidal potential 4,000 MW. The challenge is assessing that bountiful sea of energy, and determining realistic sites for development.

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. The project has been mired in the permitting procedure for several years, but with an announced $2 million offered up by the BC government in 2009, the project (which has a total cost of $6,375,000) looks promising. Currently the project is still in the permitting phase waiting to be approved by the province. Assessments of the potential power at Haida Gwaii and site identification are also being made but so far no site development has started.

British Columbia's Tidal Resource sites.
British Columbia's potential tidal resource sites.

VII. Politics of Tidal Power

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 Feed-in Tariff (ComFIT) program proposes tidal tariffs of 78 cents/kWh, which is almost as high as Ontario's microFIT tariff for rooftop solar PV systems, at 80 cents/kWh. If the proposed figures withstand the approval process, Nova Scotia will boast the first feed-in tariff for tidal power in North America, as well as the first feed-in tariff dedicated to a community-owned renewable. Hearings on the proposed tariffs occurred in April of 2011, and final decisions are expected to be reached before 2012.

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.

VIII. Economics of Tidal Power

How Much Does it Cost to Construct a Tidal Barrage Power Plant?

Artist's rendition of a tidal fence.
Artist's rendition of a tidal fence.

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. Tidal plants, however, do benefit from long life spans and a relatively low cost of operation compared to other types of power plants. For example, France's La Rance tidal barrage had an initial cost of about $66 million. Despite the high initial costs, the La Rance power station has been working for almost 45 years to generate enough electricity for around 300,000 homes and the plant's costs have now been recovered. As with any tidal barrage, it has seen low operational costs, no fuel costs, and minimal maintenance. Studies say operation and maintenance costs are typically less than 0.5% of initial capital costs.

How Much Do Tidal Turbines and Tidal Fences Cost?

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. This figure was met with $3,000,000 investment from project partner EnCana's Environmental Innovation Fund, and a grant of just under $1 million awarded to Pearson College and their partners in the project.

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.

What Do Consumers of Tidal Generated Electricity Pay?

SeaGen, the world's first commercial current turbine generator, located in Strangford Lough, Northern Ireland.
SeaGen, the world's first commercial current turbine generator, located in Strangford Lough, Northern Ireland.

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. The cost is even lower than that of France's nuclear power, which is 3.8 cents/kWh. Only hydroelectric plants, at 3.2 cents, are more efficient.

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.

Economic Effects on Tourism and Fishing

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. Sites have a potential to double as information centers, employing individuals in a range of tourism positions, in addition to the general operation jobs created by the power plant itself. Temporary construction jobs are opened up as well during the installation of the facilities.

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. Additionally, sedimentation caused by tidal barrages could kill clams, while also damaging local shellfish fisheries. Studies on fisheries impacts caused by tidal development are hard to come by, and comparison with the effects of existing facilities only offers a possible prediction for new power plants. The La Rance facility displayed no major effects on the immediate fish community or local fisheries. The area, however, had a minute fishing industry to begin with and no professional fisherman after 1960. Impacts are expected to be much more apparent in locations where fish are abundant and fish passage is repeated by the same populations multiple times over the year, such as Canada's Bay of Fundy site.

IX. Environmental Considerations of Tidal Power

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.

How do Tidal Barrages Affect the Environment

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.

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.64 The intertidal area provides a key feeding ground for birds. When the condition of this area is compromised, birds are likely to starve, or else forage for food in new ecosystems, potentially offsetting the natural balance there.

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. The study did indicate some environmental recovery about 20 years after the initial construction, though it is still not the intact ecosystem it once was.

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. The intertidal area provides a key feeding ground for birds. When the condition of this area is compromised, birds are likely to starve, or else forage for food in new ecosystems, potentially offsetting the natural balance there. The trapping of salt waters, where they would naturally flow into delicate salt marshes, can cause these areas to become diluted with fresh water, destroying a formerly intact ecosystem. Some estuaries may have formerly provided nurseries for breeding fish that would be jeopardized by tidal power development. It is also possible for fish and marine mammals to suffer damage or death by collision with the barrage or turbines, though fish passages can be used with varying degrees of success.

'Introduction of tidal turbines into open ocean current systems will cause widespread impact on marine populations resulting in significant declines in abundance.'

- 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. Injury or mortality of fish can occur in several ways during turbine passage, including mechanical strike, shear (the fish is caught between two streams with different velocities), pressure changes and cavitation (implosion of air bubbles which produces shock waves). The study of Annapolis estuary concluded "that introduction of tidal turbines into open ocean current systems will cause widespread impact on marine populations resulting in significant declines in abundance."

How do Tidal Current Turbines Affect the Environment?

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. Screens placed in front of the blades can provide a further deterrent to injuries and deaths. Units are designed to extract only a small portion of the tidal energy flowing through a given area, thus, the total effects on tidal activities is minimal when small numbers of turbines are installed.

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. Reduced tidal ranges can diminish feeding areas for birds in the intertidal zone, and possibly affect the ecology of salt marshes. A model of tidal turbines in the Bristol Channel suggests that tidal turbines might reduce the tidal velocity and hence the sediment transport and shoreline erosion. Another laboratory experiment points out that the sound generated by the turbines causes changes in pressure in the water. This ultimately results in tissue damage among fish.

Tidal Power and Climate Change

The operation of a tidal power plant is mostly emissions-free. As a general trend, as the capacity of tidal generation increases, it displaces conventional generation in the area and reduces green house gas emissions. But the installation process produces emissions.

Case Study: The Severn Barrage Tidal Power Project, UK

Artist's conception of concrete cassions being moved into position during the proposed construction of tidal barrage on the Severn.
Artist's conception of concrete cassions being moved into position during the proposed construction of tidal barrage on the Severn.

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 carbon cost of construction in the accompanying chart.

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.

The nine years of construction anticipated for the project is predicted to have a carbon pay-back time of approximately 5.5 months.

Tidal Generation Emissions Savings.
Tidal Generation Emissions Savings.

Future Outlook

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. It is also hoped that with future development of tidal current turbine technology, the impact upon marine life can be reduced. In case of malfunction these type of facilities do not impose any major catastrophic damage to the surroundings, compared to,say, nuclear or hydroelectric dam failure.

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To ensure continuity of material, all of the external web pages linked and presented on our site were cached in May 2012. Readers are recommended to explore the current links for any changes.

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XI. References