See also:

• Tidal Force and Acceleration


• Tidal Stream Generators

Categories of Tidal Power

Tidal power can be classified into two main types:

• Tidal stream systems make use of the kinetic energy of moving water to power turbines, in a similar way to windmills that use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact compared to barrages.

• Barrages make use of the potential energy in the difference in height (or head) between high and low tides. Barrages suffer from very high civil infrastructure costs, a worldwide shortage of viable sites, and environmental issues.

Modern advances in turbine technology may eventually see large amounts of power generated from the ocean, especially tidal currents using the tidal stream designs. Tidal stream turbines may be arrayed in high-velocity areas where natural tidal current flows are concentrated such as the west and east coasts of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in south east Asia and Australia. Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated.

Barrage tidal power

Rance tidal power plant

An artistic impression of a tidal barrage, including embankments, a ship lock and caissons housing a sluice and two turbines.

Artist's impression of the Severn Barrage and road link proposed in 1989. The scheme would have generated 6% of the UK's electricity supply

With only three operating plants globally Rance River, Bay of Fundy and Kislaya Guba the barrage method of extracting tidal energy involves building a barrage as in the case of the Rance River in France. The barrage turbines generate as water flows in and out of the estuary, 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 largest such installation has been working on the Rance river, France, since 1966 with an installed (peak) power of 240 MW, and an annual production of 600 GWh (about 68 MW average 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.

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 ebbs.

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 (and making the difference in levels between the basin side and the sea side of the barrage), (and therefore the available potential energy) less than it would otherwise be. This is not a problem with the "lagoon" model; the reason being that there is no current from a river to slow the flooding current from the sea.

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.

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.

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.

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 to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.

Salinity

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

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% (from pressure drop, contact with blades, cavitation, etc.). Alternative passage technologies (fish ladders, fish lifts, 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. 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 basically is a very large slow rotating Kaplan type turbine mounted on an angle. Testing for fish mortality has indicated much lower mortality figures, less than 5%. This concept seems very suitable for adaption to marine current/tidal turbines also VLH TURBINE

Energy calculations

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

E = Mgh

where:
h is the height of the tide
M is the mass of water = 1025 kg per cubic meter (seawater varies between 1021 and 1030 kg per cubic meter)
g is the acceleration due to gravity = 9.81 meters per second squared at the Earth's surface.

Mathematical demonstration of a sample Tidal power generation

Assumptions:

• Let us assume that the height of tide at a particular place is 32 feet = 10 m (approx)

• The surface of the tidal energy harnessing plant is 9 sq km (3 km * 3 km)= 3000 m * 3000 m = 9 * 10⁶ m²

• Specific gravity of Sea water = 1025.18 kg/m³

Mass of the water = volume of water * specific gravity

               = (area * height) of water * specific gravity
               = (9 * 106 m2 * 10 m) * 1025.18 kg/m3
               = 92266 * 106 kg (approx)

Energy content of the water mass = Mass of water * g * height

               = 92266 * 106 kg * 9.81 m/s2 * 10 m
               = 9051 * 109 J (approx)

Now we have 2 high tides and 2 low tides every day.

Therefore the total energy generation potential per day = Energy for a single tide * 4

               = 9051 * 109 J
               = 36 * 1012 J

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

               = 36 * 1012 J / 86400 s
               = 419 MW

Since we have assumed the power conversion efficiency to be 30%, The power generated = 419 MW * 30%

               = 126 MW (approx)

A barrage is therefore 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.

• Simple Approximation: P=hrk, where P is power in watts, h is height in meters, r is rate in cubic meters per second, and k is 7,500 watts (assuming an efficiency factor of about 75 percent).

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 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.