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 due the tides and potential energy from the difference in height (or head) between high and low tides.
The extraction of potential energy involves building a barrage. The barrage traps a water level inside a basin. Head is created when the water level outside of the basin 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, 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 orbit of the moon around the Earth, and the deceleration of the Earth's rotation. 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 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 fiords. Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal waves.
Like 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 earth's gravitational force. 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).
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.
The turbines in use are modifications of the Kaplan turbine, which, unlike other hydropower turbines, work at very low heads by virtue of adjustable blade angles. The three main types are tubular, bulb and rim (Straflo) turbines.
The total rated capacity (in MW) of the turbines is optimised for each barrage. At a low relative installed capacity, a small amount of energy will be produced, but production will last for a long period of time. At maximum installed capacity, the basin will drain very quickly in every cycle, producing the maximum possible energy, at very high rate, but only for a short period of time, making it difficult to absorb into the grid. A fast decrease in water level could also have negative effects on the environment.
The addition of capacity to a turbine follows the law of diminishing returns: for each additional MW, less extra energy will be produced.
Modes of operation
The basin is 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.
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).
Generation occurs both as the tide ebbs and floods. This mode is only comparable to ebb generation at spring tides, and in general is less efficient. Turbines designed to operate in both directions are less efficient.
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, and a surplus of up to 10% of energy is gained through pumping.
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.
Marine current turbines
Various designs are proposed to extract kinetic energy from tidal currents. A generic current turbine will be rotated by water currents in the same way as wind power generators are rotated by air currents. The scheme would convert the currents' energy into electricity, slowing the currents down.
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.
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.)
- Power output
- Sediment movements
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.
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.
Turbidity (the amount of matter in suspense 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.
Again as a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem.
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.
Once again, as a result of reduced cubature, 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 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. The Severn Barrage, when built, is projected to save a 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.
Tidal power schemes have a very high capital cost and a very low running cost. As a result, a tidal power scheme, will not produce returns for decades after it is built. Clearly, investors will not participate in tidal power projects. Only governments may be able to finance tidal power, but many are unwilling to do so also due to the extremely long time before returns and the huge irreversible commitment.
Resource around the world
Operating tidal power schemes
The first tidal power station was built over a period of 6 years from 1960 to 1966 at La Rance. It has 240MW installed capacity. There was a tidal power scheme built at Annapolis Royal in the Bay of Fundy as a pilot project for further developments in the Bay. Another pilot project was built by the Soviet Union on Kislaya Guba, with 400kW installed capacity. China has developed a number of small tidal power projects and one large, in Jiangxia.
Baker, A. C. 1991, Tidal power, Peter Peregrinus Ltd., London.
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Hammons, T. J. 1993, 'Tidal power', Proceedings of the IEEE, [Online], v81, n3, pp 419-433. Available from: IEEE/IEEE Xplore. [26 July 2004].
Lecomber, R. 1979, 'The evaluation of tidal power projects', in Tidal Power and Estuary Management, eds. Severn, R. T., Dineley, D. L. & Hawker, L. E., Henry Ling Ltd., Dorchester, pp 31-39.