Tidal power, also called tidal energy, is a form of hydro power that converts the energy of tides into electricity or other useful forms of power. Tidal energy is energy that could be obtained from the changing sea levels. It is considered to be a renewable source of energy since it only uses the energy from the changing of the tides instead of burning or consuming any form of energy source. It is also considered to be inexhaustible.
Gravitational forces between the moon, the sun and the earth cause the rhythmic rising and lowering of ocean waters around the world that results in Tide Waves. The moon exerts more than twice as great a force on the tides as the sun due to its much closer position to the earth.
As a result, the tide closely follows the moon during its rotation around the earth, creating diurnal tide and ebb cycles at any particular ocean surface.
The energy of the tide wave contains two components, namely, potential and kinetic. The potential energy is the work done in lifting the mass of water above the ocean surface. The kinetic energy of the water mass is its capacity to do work by virtue of its velocity.
Knowledge of the potential energy of the tide is important for designing conventional tidal power plants.
Tidal power utilization
Water is trapped or held within specially made structures during high tide. There is a water containment reservoir called a tidal lagoon. The lagoon is filled with water as the tide goes up the reservoir.
When the tide shifts from high to low level, there exists a difference in pressure between the water (in the higher structure and in the open water source). This forms a head pressure (hydrostatic pressure) on the containment structure. That difference results to potential energy which could be utilized when the contained water is released.
Main 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 has the lower cost and lower ecological impact compared to tidal barrages.
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.
Migratory fish could get through, mud flats could still be exposed at low tide, and it would be able to generate power for more hours in the tidal cycle.
Dynamic tidal power (DTP)
It is a theoretical generation technology that would exploit 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 coast-parallel oscillating tidal currents. Each dam would generate power at a scale of 6 – 15 GW.
Benefits and limitations
Benefit of tidal power is that once constructed, it’s free. It does not generate any unsafe greenhouse gases or hazardous waste. It functions without any fuel requirement, only with natural tidal energy. It is a consistent source of electricity with very little maintenance cost. Offshore turbines and vertical-axis turbines are economical to construct and do not have a big ecological effect.
Tidal energy could only be harnessed in places with significant water level changes. The conversion of the potential energy that the tides will hold about 80% efficiency. This means that 20% of total potential energy is usually lost and only 80% of potential energy is typically utilized for electricity generation.
Tide energy and environment
Each specific site is different and the impacts depend greatly upon local geography. Local tides changed only slightly due to the barrage range, and the environmental impact has been negligible, but this may not be the case for all other sites.
The change in water level and possible flooding would affect the vegetation around the coast, having an impact on the aquatic and shoreline ecosystems. The quality of the water in the basin or estuary would also be affected, the sediment levels would change, affecting the turbidity of the water and therefore affecting the animals that live in it and depend upon it such as fish and birds. Fish would undoubtedly be affected unless provision was made for them to pass through the barrage without being killed by turbines. All these changes would affect the types of birds that are in the area, as they will migrate to other areas with more favourable conditions for them.
The environmental effects of utilising tidal streams are in no way as severe as those for a tidal barrage. They will obviously affect the seabed where they are positioned and this might have an effect on the aquatic life in the area. This is again site specific and hard to predict; as long as proper environmental impact assessments are done then this can be avoided or minimised.