# Tidal barrage

Last updated

A tidal barrage is a dam-like structure used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces. [1] [2]

## Contents

Instead of damming water on one side like a conventional dam, a tidal barrage allows water to flow into a bay or river during high tide, and releases the water during low tide. This is done by measuring the tidal flow and controlling the sluice gates at key times of the tidal cycle. Turbines are placed at these sluices to capture the energy as the water flows in and out. [1]

Tidal barrages are among the oldest methods of tidal power generation, with tide mills being developed as early as the sixth century. In the 1960s the 1.7 megawatt Kislaya Guba Tidal Power Station in Kislaya Guba, Russia was built.

## Generating methods

The barrage method of extracting tidal energy involves building a barrage across a bay or river that is subject to tidal flow. Turbines installed in the barrage wall generate power as water flows in and out of the estuary basin, bay, or river. These systems are similar to a hydro dam that produces static head or pressure head (a height of water pressure). When the water level outside of the basin or lagoon changes relative to the water level inside, the turbines are able to produce power.

The basic elements of a barrage are caissons, embankments, sluices, turbines, and ship locks. Sluices, turbines, and ship locks are housed in caissons (very large concrete blocks). Embankments seal a basin where it is not sealed by caissons. The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate, and rising sector.

Only a few such plants exist. The first was the Rance Tidal Power Station, on the Rance river, in France, which has been operating since 1966 and generates 240MW. A larger 254MW plant began operation at Sihwa Lake, Korea, in 2011. Smaller plants include the Annapolis Royal Generating Station on the Bay of Fundy, and another across a tiny inlet in Kislaya Guba, Russia. A number of proposals have been considered for a barrage across the River Severn, from Brean Down in England to Lavernock Point near Cardiff in Wales.

Barrage systems are dependent on high civil infrastructure costs associated with what is in effect a dam being placed across estuarine systems. As people have become more aware of environmental issues, they have opposed barrages because of the adverse effects associated with changing a large ecosystem that is habitat for many varieties of species.

### Ebb generation

The basin is filled through the sluices until high tide. Then the sluice gates are closed. (At this stage there may be "Pumping" to raise the level further). The turbine gates are kept closed until the sea level falls, in order to create sufficient head across the barrage. The gates are opened so that the turbines generate until the head is again low. Then the sluices are opened, turbines disconnected and the basin is filled again. The cycle repeats with the tides. Ebb generation (also known as outflow generation) takes its name because generation occurs as the tide changes tidal direction.

### Flood generation

The basin is filled through the turbines, which generate at tide flood. This is generally much less efficient than ebb generation, because the volume contained in the upper half of the basin (which is where ebb generation operates) is greater than the volume of the lower half (filled first during flood generation). Therefore, the available level difference – important for the turbine power produced – between the basin side and the sea side of the barrage, reduces more quickly than it would in ebb generation. Rivers flowing into the basin may further reduce the energy potential, instead of enhancing it as in ebb generation. Of course this is not a problem with the "lagoon" model, without river inflow.

### Pumping

Turbines are able to be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation). Much of this energy is returned during generation, because power output is strongly related to the head. If water is raised 2 ft (61 cm) by pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide.

### Two-basin schemes

Another form of energy barrage configuration is that of the dual basin type. With two basins, one is filled at high tide and the other is emptied at low tide. Turbines are placed between the basins. Two-basin schemes offer advantages over normal schemes in that generation time can be adjusted with high flexibility and it is also possible to generate almost continuously. In normal estuarine situations, however, two-basin schemes are very expensive to construct due to the cost of the extra length of barrage. There are some favourable geographies, however, which are well suited to this type of scheme.

### Tidal lagoon power

Tidal pools [3] are independent enclosing barrages built on high level tidal estuary land that trap the high water and release it to generate power, single pool, around 3.3W/m2. Two lagoons operating at different time intervals can guarantee continuous power output, around 4.5W/m2. Enhanced pumped storage [4] tidal series of lagoons raises the water level higher than the high tide, and uses intermittent renewables for pumping, around 7.5W/m2. i.e. 10 × 10 km2 delivers 750MW constant output 24/7. These independent barrages do not block the flow of the river.

## Environmental impact

The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the ecosystem. Many governments have been reluctant in recent times to grant approval for tidal barrages. Through research conducted on tidal plants, it has been found that tidal barrages constructed at the mouths of estuaries pose similar environmental threats as large dams. The construction of large tidal plants alters the flow of saltwater in and out of estuaries, which changes the hydrology and salinity and could possibly harm marine mammals that use the estuaries as their habitat. [5] The La Rance plant, off the Brittany coast of northern France, was the first and largest tidal barrage plant in the world. It is also the only site where a full-scale evaluation of the ecological impact of a tidal power system, operating for 20 years, has been made. [6]

French researchers found that the isolation of the estuary during the construction phases of the tidal barrage was detrimental to flora and fauna, however; after ten years, there has been a "variable degree of biological adjustment to the new environmental conditions" [6]

Some species lost their habitat due to La Rance's construction, but other species colonized the abandoned space, which caused a shift in diversity. Also as a result of the construction, sandbanks disappeared, the beach of St. Servan was badly damaged and high-speed currents have developed near sluices, which are water channels controlled by gates. [7]

### Turbidity

Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume of water being exchanged between the basin and the sea. This lets light from the Sun penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.

### Tidal fences and turbines

Tidal fences and turbines, if constructed properly, pose less environmental threats than tidal barrages. Tidal fences and turbines, like tidal stream generators, rely entirely on the kinetic motion of the tidal currents and do not use dams or barrages to block channels or estuarine mouths. Unlike barrages, tidal fences do not interrupt fish migration or alter hydrology, thus these options offer energy generating capacity without dire environmental impacts. Tidal fences and turbines can have varying environmental impacts depending on whether or not fences and turbines are constructed with regard to the environment. The main environmental impact of turbines is their impact on fish. If the turbines are moving slowly enough, such as low velocities of 25-50 rpm, fish kill is minimalized and silt and other nutrients are able to flow through the structures. For example, a 20 kW tidal turbine prototype built in the St. Lawrence Seaway in 1983 reported no fish kills. Tidal fences block off channels, which makes it difficult for fish and wildlife to migrate through those channels. In order to reduce fish kill, fences could be engineered so that the spaces between the caisson wall and the rotor foil are large enough to allow fish to pass through. Larger marine mammals such as seals or dolphins can be protected from the turbines by fences or a sonar sensor auto-braking system that automatically shuts the turbines down when marine mammals are detected. [5]

### Salinity

As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem.[ citation needed ] "Tidal Lagoons" do not suffer from this problem. [ citation needed ]

### Sediment movements

Estuaries often have high volume of sediments moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.

### Fish

Fish may move through sluices safely, but when these are closed, fish will seek out turbines and attempt to swim through them. Also, some fish will be unable to escape the water speed near a turbine and will be sucked through. Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15%[ citation needed ] (from pressure drop, contact with blades, cavitation, etc.). Alternative passage technologies (fish ladders, fish lifts, fish escalators etc.) have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only. Research in sonic guidance of fish is ongoing.[ citation needed ] The Open-Centre turbine reduces this problem allowing fish to pass through the open centre of the turbine.

Recently a run of the river type turbine has been developed in France. This is a very large slow rotating Kaplan-type turbine mounted on an angle. Testing for fish mortality has indicated fish mortality figures to be less than 5%. This concept also seems very suitable for adaption to marine current/tidal turbines. [8]

## Energy calculations

The energy available from a barrage is dependent on the volume of water. The potential energy contained in a volume of water is: [9]

${\displaystyle E\,=\,{\tfrac {1}{2}}\,A\,\rho \,g\,h^{2}}$

where:

• h is the vertical tidal range,
• A is the horizontal area of the barrage basin,
• ρ is the density of water = 1025 kg per cubic meter (seawater varies between 1021 and 1030 kg per cubic meter) and
• g is the acceleration due to the Earth's gravity = 9.81 meters per second squared.

The factor half is due to the fact, that as the basin flows empty through the turbines, the hydraulic head over the dam reduces. The maximum head is only available at the moment of low water, assuming the high water level is still present in the basin.

### Example calculation of tidal power generation

Assumptions:

• The tidal range of tide at a particular place is 32 feet = 10 m (approx)
• The surface of the tidal energy harnessing plant is 9 km² (3 km × 3 km)= 3000 m × 3000 m = 9 × 106 m2
• Density of sea water = 1025.18 kg/m3

Mass of the sea water = volume of sea water × density of sea water

= (area × tidal range) of water × mass density
= (9 × 106 m2 × 10 m) × 1025.18 kg/m3
= 92 × 109 kg (approx)

Potential energy content of the water in the basin at high tide = ½ × area × density × gravitational acceleration × tidal range squared

= ½ × 9 × 106 m2 × 1025 kg/m3 × 9.81 m/s2 × (10 m)2
=4.5 × 1012 J (approx)

Now we have 2 high tides and 2 low tides every day. At low tide the potential energy is zero.
Therefore, the total energy potential per day = Energy for a single high tide × 2

= 4.5 × 1012 J × 2
= 9 × 1012 J

Therefore, the mean power generation potential = Energy generation potential / time in 1 day

= 9 × 1012 J / 86400 s
= 104 MW

Assuming the power conversion efficiency to be 30%: The daily-average power generated = 104 MW * 30%

= 31 MW (approx)

Because the available power varies with the square of the tidal range, a barrage is best placed in a location with very high-amplitude tides. Suitable locations are found in Russia, USA, Canada, Australia, Korea, the UK. Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal range.

## Economics

Tidal barrage power schemes have a high capital cost and a very low running cost. As a result, a tidal power scheme may not produce returns for many years, and investors may be reluctant to participate in such projects.

Governments may be able to finance tidal barrage power, but many are unwilling to do so also due to the lag time before investment return and the high irreversible commitment. For example, the energy policy of the United Kingdom [10] recognizes the role of tidal energy and expresses the need for local councils to understand the broader national goals of renewable energy in approving tidal projects. The UK government itself appreciates the technical viability and siting options available, but has failed to provide meaningful incentives to move these goals forward.

## Related Research Articles

Hydropower or water power is power derived from the energy of falling or fast-running water, which may be harnessed for useful purposes. Since ancient times, hydropower from many kinds of watermills has been used as a renewable energy source for irrigation and the operation of various mechanical devices, such as gristmills, sawmills, textile mills, trip hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance.

Tidal power or tidal energy converts energy obtained from tides into useful forms of power, mainly electricity.

The Kaplan turbine is a propeller-type water turbine which has adjustable blades. It was developed in 1913 by Austrian professor Viktor Kaplan, who combined automatically adjusted propeller blades with automatically adjusted wicket gates to achieve efficiency over a wide range of flow and water level.

The Tees Barrage is a barrage across the River Tees just upriver of Blue House Point in the borough of Stockton-on-Tees in North East of England and is used to control the flow of the river, preventing flooding and the effects of tidal change. The Tees Barrage comprises a river barrage, road bridge, footbridge, barge lock, fish pass and white water course. The waters above the barrage are permanently held at the level of an average high tide and are used for watersports such as canoeing, jet skiing, dragonboat racing and incorporates a 1 km rowing course. The barrage is accessible by road only from Thornaby-on-Tees as there is very limited road access to the north bank of the Tees.

The Severn Estuary is the estuary of the River Severn, the longest river in Great Britain. It is the confluence of four major rivers, being the Severn, Wye, Usk and Avon, and other smaller rivers. Its high tidal range, approximately 50 feet (15 m), means that it has been at the centre of discussions in the UK regarding renewable energy.

The Gandhi Sagar Dam is one of the four major dams built on India's Chambal River. The dam is located in the Mandsaur, Neemuch districts of the state of Madhya Pradesh. It is a masonry gravity dam, standing 62.17 metres (204.0 ft) high, with a gross storage capacity of 7.322 billion cubic metres from a catchment area of 22,584 km2 (8,720 sq mi). The dam's foundation stone was laid by Prime Minister of India Pandit Jawaharlal Nehru on 7 March 1954, and construction of the main dam was done by leading contractor Dwarka Das Agrawal & Associates and was completed in 1960. Additional dam structures were completed downstream in the 1970s.

The Severn Barrage is any of a range of ideas for building a barrage from the English coast to the Welsh coast over the Severn tidal estuary. Ideas for damming or barraging the Severn estuary have existed since the 19th century. The building of such a barrage would constitute an engineering project comparable with some of the world's biggest. The purposes of such a project has typically been one, or several of: transport links, flood protection, harbour creation, or tidal power generation. In recent decades it is the latter that has grown to be the primary focus for barrage ideas, and the others are now seen as useful side-effects. Following the Severn Tidal Power Feasibility Study (2008–10), the British government concluded that there was no strategic case for building a barrage but to continue to investigate emerging technologies. In June 2013 the Energy and Climate Change Select Committee published its findings after an eight-month study of the arguments for and against the Barrage. MPs said the case for the barrage was unproven. They were not convinced the economic case was strong enough and said the developer, Hafren Power, had failed to answer serious environmental and economic concerns.

A tide mill is a water mill driven by tidal rise and fall. A dam with a sluice is created across a suitable tidal inlet, or a section of river estuary is made into a reservoir. As the tide comes in, it enters the mill pond through a one-way gate, and this gate closes automatically when the tide begins to fall. When the tide is low enough, the stored water can be released to turn a water wheel.

The Rance Tidal Power Station is a tidal power station located on the estuary of the Rance River in Brittany, France.

The Annapolis Royal Generating Station is a tidal power generating station in the Bay of Fundy in Nova Scotia, Canada. It is the only tidal generating station in North America and one of the few in the world. Located upstream of Annapolis Royal, Nova Scotia, it generates about 30 million kilowatt hours per year, enough for 4500 houses. Peak output is 20 megawatts.

Brilliant Dam is a hydroelectric dam on the Kootenay River near Castlegar, British Columbia, Canada. It was built during the Second World War, mostly by Doukhobour men exempt from military service, and its 129 MW twin turbines first came into operation in June, 1944. The Columbia Power Corporation purchased the dam from Teck Cominco in 1996.

Marine Current Turbines Ltd (MCT), a Siemens business, is a United Kingdom-based company which is developing tidal stream generators.

Severn Tidal Power Feasibility Study is the name of a UK Government feasibility study into a tidal power project looking at the possibility of using the huge tidal range in the Severn Estuary and Bristol Channel to generate electricity.

The shrouded tidal turbine is an emerging tidal stream technology that has a turbine enclosed in a venturi shaped shroud or duct (ventuduct), producing a sub atmosphere of low pressure behind the turbine. The venturi shrouded turbine is not subject to the Betz limit and allows the turbine to operate at higher efficiencies than the turbine alone by increasing the volume of the flow over the turbine. Claimed improvements vary, from 1.15 to 4 times higher power output than the same turbine minus the shroud. The Betz limit of 59.3% conversion efficiency for a turbine in an open flow still applies, but is applied to the much larger shroud cross-section rather than the small turbine cross-section.

New Zealand has large ocean energy resources but does not yet generate any power from them. TVNZ reported in 2007 that over 20 wave and tidal power projects are currently under development. However, not a lot of public information is available about these projects. The Aotearoa Wave and Tidal Energy Association was established in 2006 to "promote the uptake of marine energy in New Zealand". According to their 10 February 2008 newsletter, they have 59 members. However, the association doesn't list its members.

Low head hydropower refers to the development of hydroelectric power where the head is typically less than 20 metres, although precise definitions vary. Head is the vertical height measured between the hydro intake water level and the water level at the point of discharge. Using only a low head drop in a river or tidal flows to create electricity may provide a renewable energy source that will have a minimal impact on the environment. Since the generated power is a function of the head these systems are typically classed as small-scale hydropower, which have an installed capacity less than 5MW.

Sihwa Lake Tidal Power Station is the world's largest tidal power installation, with a total power output capacity of 254 MW. When completed in 2011, it surpassed the 240 MW Rance Tidal Power Station which was the world's largest for 45 years. It is operated by the Korea Water Resources Corporation.

A tidal farm is a group of multiple tidal stream generators assembled in the same location used for production of electric power, similar to that of a wind farm. The low-voltage powerlines from the individual units are then connected to a substation, where the voltage is stepped up with the use of a transformer for distribution through a high voltage transmission system.

Dynamic tidal power or DTP is an untried but promising technology for tidal power generation. It would involve creating a long dam-like structure perpendicular to the coast, with the option for a coast-parallel barrier at the far end, forming a large 'T' shape. This long T-dam would interfere with coast-parallel tidal wave hydrodynamics, creating water level differences on opposite sides of the barrier which drive a series of bi-directional turbines installed in the dam. Oscillating tidal waves which run along the coasts of continental shelves, containing powerful hydraulic currents, are common in e.g. China, Korea, and the UK.

A tidal stream generator, often referred to as a tidal energy converter (TEC), is a machine that extracts energy from moving masses of water, in particular tides, although the term is often used in reference to machines designed to extract energy from run of river or tidal estuarine sites. Certain types of these machines function very much like underwater wind turbines, and are thus often referred to as tidal turbines. They were first conceived in the 1970s during the oil crisis.

## References

1. "Tidal barrage" . Retrieved 2 November 2010.
2. "Tidal barrages and tidal turbines" . Retrieved 2 November 2010.
3. Pelc, Robin; Fujita, Rod M. (November 2002). "Renewable energy from the ocean". Marine Policy . 26 (6): 471–479. doi:10.1016/S0308-597X(02)00045-3.CS1 maint: ref=harv (link)
4. Retiere, C. (January 1994). "Tidal power and the aquatic environment of La Rance". Biological Journal of the Linnean Society . 51 (1–2): 25–36. doi:10.1111/j.1095-8312.1994.tb00941.x.CS1 maint: ref=harv (link)
5. Charlier, Roger H. (December 2007). "Forty candles for the Rance River TPP tides provide renewable and sustainable power generation". Renewable and Sustainable Energy Reviews . 11 (9): 2032–2057. doi:10.1016/j.rser.2006.03.015.CS1 maint: ref=harv (link)
6. "Vlh Turbine". Vlh Turbine. Retrieved 2013-07-19.
7. Lamb, H. (1994). Hydrodynamics (6th ed.). Cambridge University Press. ISBN   978-0-521-45868-9. §174, p. 260.