European governments have established targets for renewable energy and have committed to reducing carbon emissions. A strong need is developing for new renewable energy technologies, with a focus on diversification of energy sources to provide an effective security of supply. Tidal currents throughout the UK and Europe represent a significant sustainable energy resource and much effort and resources have been allocated to tidal current energy technology R&D over the last few years.The technology is still at an early stage of development with companies and research institutions focused on economically and technically feasible solutions. While other energies sources, such as wind, biomass and small hydro, have established a market niche, tidal current energy technologies are still in the early development phase. However, with increased support from governments, private investors and, more recently, electric utilities, tidal current power is showing every promise of becoming an acceptable form of renewable energy which meets the environmental and commercial criteria of the market place.
Tidal Current Energy
The major benefits of tidal power are that it is non-polluting, reliable and predictable. A drawback of tidal power is that its peak availability often misses peak demand times because of the 12.5-hour cycle of the tides.
For an energy source to be viable and useful in a modern market, it does not necessarily need to be constant, but it must be reliable i.e. a supplier must be able to predict when the supply will be available, and in what quantities, so that it can be matched with other sources to meet the load demand. This poses a problem for many renewable technologies, such as solar, wind and wave, as their output depends on weather conditions which are difficult to accurately predict. Solar can be predicted only minutes ahead, wind for hours ahead and wave for days ahead, with decreasing precision as the forecast time is extended. Being dependent on astronomical forces rather than weather, tidal power is much more predictable than solar, wind or wave power. If the tidal current flow regime at a particular location has been properly studied, its variation over the tidal cycle, excluding the effects of weather, can be predicted with considerable accuracy years to decades in advance.
The tides are generated by the rotation of the earth within the gravitational fields of the moon and sun. The relative motion of these bodies causes the surface of the oceans to be raised and lowered periodically, according to the following interacting harmonic cycles:
- A half-day cycle: due to the rotation of the earth within the gravitational field of the moon, resulting in a period of 12 hours 25 minutes between successive high tides
- A 14-day cycle: resulting from superposition of the gravitational fields of the moon and sun. At new moon and full moon, the sun’s gravitational field reinforces that of the moon, resulting in maximum (spring) tides. At quarter phases of the moon, the sun’s attraction partially cancels that of the moon, resulting in minimum (neap) tides. The range of a spring tide is typically about twice that of a neap tide.
- Other cycles, of lesser significance, arise from the eccentric nature of the earth’s orbit around the sun and the moon’s orbit around the earth, and the tilt of the moon’s orbital plan relative to the earth’s axis of rotation.
The gravitational forces of the sun and the moon create two “bulges” in the earth’s oceans: one closest to the moon, and other on the opposite side of the globe. These “bulges” result in the two tides (high water to low water sequence) a day, the dominant tidal pattern in most of the world’s oceans.
The solar tidal bulge is only 46% as high as the lunar tidal bulge. While the lunar bulge migrates around the Earth once every 27 days; the solar bulge migrates around the Earth once every day. As the lunar bulge moves exactly into and then 90 degrees out of phase with the solar bulge, this gives rise to spring and neap tides, respectively.
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