Tidal-power is the power 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.
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). In Europe, Tide Mills have been used for nearly 1,000 years, mainly for grinding grains.
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 = x**
where x is the height of the tide, M is the mass of water and g is the acceleration due to gravity at the Earth's surface. 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.
Tidal Energy Efficiency
Tidal energy has an efficiency of 80% in converting the potential energy of the water into electricity, which is efficient compared to other energy resources such as solar power.
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. A tidal current turbine will have a much lower impact.
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.
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.
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.
With turbine generation, taking its power from the flow of the tidal stream, there will likely be a swirl of water down stream of the turbine. If this horizontal vortex touches the bottom, it will cause erosion. While the amount of sediment added to the tidal stream will likely be insignificant, this could, over time, erode the foundation of the turbine. Turbines held down with pilings would be largely immune to this problem but turbines held by heavy weights sitting on the bottom could eventually tip over.
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 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.
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 provide meaningful incentives to move its goals forward