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Tidal Power Generating Methods of electricity
Tidal Power Generating Methods of electricity
6:18 PM
Posted by Energetic
Tidal power, also called tidal energy, is a form of hydropower that converts the energy of tides into electricity or other useful forms of power. The first largescale tidal power plant is Rance Tidal Power Station, started operation in 1966.
Although not yet widely used, tidal power has potential for future electricity generation. Tides are more predictable than wind energy and solar power. Among sources of renewable energy, tidal power has traditionally suffered from relatively high cost and limited availability of sites with sufficiently high tidal ranges or flow velocities, thus constricting its total availability. However, many recent technological developments and improvements, both in design (e.g. dynamic tidal power, tidal lagoons) and turbine technology (e.g. new axial turbines, crossflow turbines), indicate that the total availability of tidal power may be much higher than previously assumed, and that economic and environmental costs may be brought down to competitive levels.
Historically, tide mills have been used, both in Europe and on the Atlantic coast of North America. The earliest occurrences date from the Middle Ages, or even from Roman times.Tidal power can be classified into three generating methods:
Tidal stream generator
Tidal stream generators (TSGs) make use of the kinetic energy of moving water to power turbines, in a similar way to wind turbines that use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact compared to tidal barrages.
A tidal stream generator (TSG) is a machine that extracts energy from moving masses of water, or tides. These machines function very much like underwater wind turbines, hence are also sometimes referred to as tidal turbines.
TSGs are the cheapest and the least ecologically damaging among the three main forms of tidal power generation.Turbine power
Various turbine designs have varying efficiencies and therefore varying power output. If the efficiency of the turbine "ξ" is known the equation below can be used to determine the power output of a turbine.
The energy available from these kinetic systems can be expressed as:^{}
where:
 ξ = the turbine efficiency
 P = the power generated (in watts)
 ρ = the density of the water (seawater is 1025 kg/m³)
 A = the sweep area of the turbine (in m²)
 V = the velocity of the flow
Relative to an open turbine in free stream, depending on the geometry of the shroud shrouded turbines are capable of as much as 3 to 4 times the power of the same turbine rotor in open flow. ^{}
Resource assessment
While initial assessments of the available energy in a channel have focus on calculations using the kinetic energy flux model, the limitations of tidal power generation are significantly more complicated. For example, the maximum physical possible energy extraction from a strait connecting two large basins is given to within 10% by:^{}^{}
where
 ρ = the density of the water (seawater is 1025 kg/m³)
 g = gravitational acceleration (9.81 m/s^{2})
 ΔH_{max} = maximum differential water surface elevation across the channel
 Q_{max}= maximum volumetric flow rate though the channel.
Tidal Barrage
Tidal barrages make use of the potential energy in the difference in height (or head) between high and low tides. Barrages are essentially dams across the full width of a tidal estuary, and suffer from very high civil infrastructure costs, a worldwide shortage of viable sites and environmental issues.
A Tidal barrage is a damlike structure used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces.^{}^{}
Instead of damming water on one side like a conventional dam, a tidal barrage first allows water to flow into the bay or river during high tide, and releasing the water back 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 then placed at these sluices to capture the energy as the water flows in and out.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 largest is the Rance Tidal Power Station, on the Rance river, in France, which has been operating since 1966, and generates 240MW. Smaller plants include one on the Bay of Fundy, and another across a tiny inlet in Kislaya Guba, Russia). A number of proposals have been considered for a Severn barrage across the River Severn, from Brean Down in England to Lavernock Point near Cardiff in Wales.
Barrage systems are affected by problems of high civil infrastructure costs associated with what is in effect a dam being placed across estuarine systems, and the environmental problems associated with changing a large ecosystem.
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 to create sufficient head across the barrage, and then 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 itself. 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). This energy is more than 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. The cost of a 2 ft rise is returned by the benefits of a 12 ft rise. This is since the correlation between the potential energy is not a linear relationship, rather, is related by the square of the tidal height variation.
Twobasin 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. Twobasin 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, twobasin 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 ^{} 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 ^{} tidal series of lagoons raises the water level higher than the high tide, and uses intermittant renewables for pumping, around 7.5W/m2. i.e. 10 x 10 km delivers 750MW constant output 24/7. These independent barages do not block the flow of the river and are a viable alternative to the Severn Barrage.
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:^{}
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:
 Let us assume that 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 × 10^{6} m^{2}
 Density of sea water = 1025.18 kg/m^{3}
Mass of the sea water = volume of sea water × density of sea water

 = (area × tidal range) of water × mass density
 = (9 × 10^{6} m^{2} × 10 m) × 1025.18 kg/m^{3}
 = 92 × 10^{9} kg (approx)
Potential energy content of the water in the basin at high tide = ½ × area × density × gravitational acceleration × tidal range squared

 = ½ × 9 × 10^{6} m^{2} × 1025 kg/m^{3} × 9.81 m/s^{2} × (10 m)^{2}
 =4.5 × 10^{12} 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 × 10^{12} J × 2
 = 9 × 10^{12} J
Therefore, the mean power generation potential = Energy generation potential / time in 1 day

 = 9 × 10^{12} J / 86400 s
 = 104 MW
Assuming the power conversion efficiency to be 30%: The dailyaverage power generated = 104 MW * 30% / 100%

 = 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 highamplitude 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.
Dynamic tidal power
Dynamic tidal power (or DTP) exploits an interaction between potential and kinetic energies in tidal flows. It proposes that very long dams (for example: 30–50 km length) be built from coasts straight out into the sea or ocean, without enclosing an area. Tidal phase differences are introduced by the dam, leading to a significant water level differential (at least 2–3 meters) in shallow coastal seas featuring strong coastparallel oscillating tidal currents such as found in the UK, China and Korea. Each dam would generate power at a scale of 6  15 GW.
Dynamic tidal power or DTP is the newest technique of tidal power generation. It involves creating large damlike structure extending from the coast straight to the ocean, with a perpendicular barrier at the far end, forming a large 'T' shape.
This long Tdam interferes with coastparallel oscillating tidal waves which run along the coasts of continental shelves, containing powerful hydraulic currents (common in e.g. China, Korea, and the UK).^{}^{}^{}^{}
This entry was posted on October 4, 2009 at 12:14 pm, and is filed under
Hydroelectricity Technology
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