Tidal power

Tidal power is a means of electricity generation achieved by capturing the energy contained in moving water mass due to tides. Two types of tidal energy can be extracted: kinetic energy of currents between ebbing and surging tides and potential energy from the difference in height (or head) between high and low tides. The former method - generating energy from tidal currents - is considered much more feasible today than building ocean-based dams or barrages, and many coastal sites worldwide are being examined for their suitability to produce tidal (current) energy.

One method of extracting tidal energy involves building a barrage and creating a tidal lagoon. The barrage traps a water level inside a basin. Head is created when the water level outside of the basin or lagoon changes relative to the water level inside. The head is used to drive turbines. In any design this leads to a decrease of tidal range inside the basin or lagoon, implying a reduced transfer of water between the basin and the sea. This reduced transfer of water accounts for the energy produced by the scheme.

Tidal power is classified as a renewable energy source, because tides are caused by the orbital mechanics of the solar system and are considered inexhaustible within a human timeframe. The root source of the energy comes from the slow deceleration of the Earth's rotation. The Moon gains energy from this interaction and is slowly receding from the Earth. Tidal power has great potential for future power and electricity generation because of the total amount of energy contained in this rotation. Tidal power is reliably predictable (unlike wind energy and solar power).

The efficiency of tidal power generation in ocean dams largely depends on the amplitude of the tidal swell, which can be up to 10 m (33 ft) where the periodic tidal waves funnel into rivers and fjords. Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal waves.

As with wind power, selection of location is critical for a tidal power generator. The potential energy contained in a volume of water is

E = xMg

where x is the height of the tide, M is the mass of water and g is the acceleration due to gravity. Therefore, a tidal energy generator must be placed in a location with very high-amplitude tides. Suitable locations are found in the former USSR, USA, Canada, Australia, Korea, the UK and other countries (see below).

Several smaller tidal power plants have recently started generating electricity in Norway. They all exploit the strong periodic tidal currents in narrow fjords using sub-surface water turbines.

Barrages

Barrages are used to close off a basin for trapping a water level inside them. The basic elements of a barrage are caissons, embankments, sluices, turbines and ship locks. Sluices, turbines and ship locks are housed in caisson (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.

Modes of operation

Ebb generation

The basin is filled through the sluices and freewheeling turbines until high tide. Then the sluice gates and turbine gates are closed. They are kept closed until the sea level falls to create sufficient head across the barrage and 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 emptied through the sluices and turbines 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 (the domain of flood generation). This is compounded by the fact that there is usually a river flowing into the basin, filling the basin as the tide rises 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.

Ebb and Flood generation in one basin

To have enough water at the turbines the stream of water is 'collected'. Flood energy is created after the tide has risen above the (choosen) level of water. The water goes downward. Water power comes for the largest part from eccellarated water (speed x mass). The same system can be used for different levels of the water. Ebb energy excists when it is used for the waterflow from the basin to the sea.

When the basin has filled it can be used as the named Ebb basin above. Pressure is highest when the opening is at the bottom. This means most energy can be created if the basin can be completely emptied in the time between tides (after Ebb tide). This proportion can be assembled by the diameter of the turbine.

A waterlock can be used to empty the basin at the end of Ebb. See picture below.

--Dezen 07:39, 8 July 2006 (UTC)

Pumping

Turbines can be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation and two-way generation). This energy is returned during generation.

Two-basin schemes

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 "wind farms"

A new scheme plans to use turbines simailar to those found in wind farms to generate electricity via large current areas such as Cook Strait in New Zealand

Intermittent nature of power output

Tidal power schemes do not produce energy 24 hours a day. A conventional design, in any mode of operation, would produce power for 6 to 12 hours in every 24 and will not produce power at other times. As the tidal cycle is based on the period of revolution of the Moon (24.8 hours) and the demand for electricity is based on the period of revolution of the Sun (24 hours), the energy production cycle will not always be in phase with the demand cycle. This causes problems for the electric power transmission grid, as capacity with short starting and stopping times (such as hydropower or gas fired power plants) will have to be available to alternate power production with the tidal power scheme.

Mathematical modelling

In mathematical modelling of a scheme design, the basin is broken into segments, each maintaining its own set of variables. Time is advanced in steps. Every step, neighbouring segments influence each other and variables are updated.

The simplest type of model is the flat estuary model, in which the whole basin is represented by one segment. The surface of the basin is assumed to be flat, hence the name. This model gives rough results and is used to compare many designs at the start of the design process.

In these models, the basin is broken into large segments (1D), squares (2D) or cubes (3D). The complexity and accuracy increases with dimension.

Mathematical modelling produces quantitative information for a range of parameters, including:

Water levels (during operation, construction, extreme conditions, etc.)
Currents
Waves
Power output
Turbidity
Salinity
Sediment movements

Physical modelling

Small-scale physical representations of a tidal power scheme can be built. These have to be large to be accurate. Physical models are very expensive and are used only in critical projects.

Environmental impact

Local environmental impact

The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the fish. Lagoons, on the other hand, could be used for fish or lobster farming, adding to their economic viability.

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

Again as a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. Again, 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.

Pollutants

Once again, as a result of reduced volume, the pollutants accumulating in the basin will be less efficiently dispersed. Their concentrations will increase. For biodegradable pollutants, such as sewage, an increase in concentration is likely to lead to increased bacteria growth in the basin, having impacts on the health of the human community and the ecosystem.

The concentrations of conservative pollutants will also increase.

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.). This can be acceptable for a spawning run, but is devastating for local fish who pass in and out of the basin on a daily basis. 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.

Global environmental impact

A tidal power scheme is a long-term source of electricity. A proposal for the Severn Barrage, if built, has been projected to save 18 million tons of coal per year of operation. This decreases the output of greenhouse gases into the atmosphere. More importantly, as the fossil fuel resource is likely to be eliminated by the end of the twenty-first century, tidal power is one of the alternative source of energy that will need to be developed to satisfy the human demand for energy.

Economic considerations

Tidal 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 years, and investors are thus reluctant to participate in such projects. Governments may be able to finance tidal 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 ^* (see for example key principles 4 and 6 within Planning Policy Statement 22) 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

Gourt Home::Gourt :: Tidal power