Tidal power
Incoming water was contained in large storage ponds, and as the tide goes out, it turns waterwheels that use the mechanical power to mill grain.
Movement of tides causes a loss of mechanical energy in the Earth-Moon system: this results from pumping of water through natural restrictions around coastlines and consequent viscous dissipation at the seabed and in turbulence.
Some tidal generators can be built into the structures of existing bridges or are entirely submersed, thus avoiding concerns over aesthetics or visual impact.
Land constrictions such as straits or inlets can create high velocities at specific sites, which can be captured using turbines.
[12] Tidal barrages use potential energy in the difference in height (or hydraulic head) between high and low tides.
When using tidal barrages to generate power, the potential energy from a tide is seized through the strategic placement of specialized dams.
The pumping power could be provided by excess to grid demand renewable energy from for example wind turbines or solar photovoltaic arrays.
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.
If built, power plants would have been located in the northern border area of the US state of Maine and the southeastern border area of the Canadian province of New Brunswick, with various dams, powerhouses, and ship locks enclosing the Bay of Fundy and Passamaquoddy Bay (note: see map in reference).
[20] In April 2009 the PUD selected OpenHydro,[21] a company based in Ireland, to develop turbines and equipment for eventual installation.
The PUD proposed that the federal government provide an additional $10 million towards this increased cost, citing a gentlemen's agreement.
Its grid connected tidal test site is located at the Fall of Warness, off the island of Eday, in a narrow channel which concentrates the tide as it flows between the Atlantic Ocean and North Sea.
In terms of global warming potential (i.e. carbon footprint), the impact of tidal power generation technologies ranges between 15 and 37 gCO2-eq/kWhe, with a median value of 23.8 gCO2-eq/kWhe.
The Tethys database provides access to scientific literature and general information on the potential environmental effects of tidal energy.
As with all offshore renewable energies, there is also a concern about how the creation of electromagnetic fields and acoustic outputs may affect marine organisms.
Tidal energy removal can also cause environmental concerns such as degrading far-field water quality and disrupting sediment processes.
[57][58] Depending on the size of the project, these effects can range from small traces of sediment building up near the tidal device to severely affecting nearshore ecosystems and processes.
[59] Installing a barrage may change the shoreline within the bay or estuary, affecting a large ecosystem that depends on tidal flats.
[60] Environmentally, the main concerns are blade strike on fish attempting to enter the lagoon, the acoustic output from turbines, and changes in sedimentation processes.
One indication of cost-effectiveness is the Gibrat ratio, which is the length of the barrage in metres divided by the annual energy production in kilowatt hours.
[66] As tidal energy is reliable, it can reasonably be predicted how long it will take to pay off the high up-front cost of these generators.
Due to the success of a greatly simplified design, the orthogonal turbine offers considerable cost savings.
As a result, the production period of each generating unit is reduced, lower metal consumption is needed and technical efficiency is greater.
[67] A possible risk is rising sea levels due to climate change, which may alter the characteristics of the local tides reducing future power generation.
[68][69] The high load factors resulting from the fact that water is around 800 times denser than air, and the predictable and reliable nature of tides compared with the wind, make tidal energy particularly attractive for electric power generation.
