Tidal Energy - Power from the Moon 'n Sun
Tidal energy devices exploit the natural rise and fall of coastal tidal waters caused principally by the interaction of the gravitational fields of the Sun and Moon. The tide moves a huge amount of water twice each day. Additional monthly and annual lunar cycles vary the strength of these currents, and narrow and shallow constrictions produce the fastest and most powerful movements of current. Different coastlines experiences different fluctuations in tides, with some estuaries accentuating the effect creating tidal ranges of up to 11 metres.
Like many other alternative energies, tidal energy is not really anything new. Tide mills were in use on the coasts of Spain, France and the UK before 1100 AD and in tidal estuaries around 1800 AD (the Schelde River in Belgium). The difference between the tide mill and today's tidal power plant is the size of the unit and the amount of energy generated.
There are basically two methodologies for creating tidal power:
- by building semi-permeable barrages across estuaries with a high tidal range to dam the water - barrages allow tidal waters to fill an estuary via sluices and to empty through turbines
- by harnessing offshore tidal streams - tidal streams can be harnessed using offshore underwater devices similar to wind turbines.
Dams are based on using a barrage at a bay or estuary with a large tidal range. Power is generated primarily at ebb tides as the barrage creates a significant head of water, much like a hydroelectric dam, only smaller. The ebb and flow of the tides can be used to turn a turbine, or it can be used to push air through a pipe, which then turns a turbine. Large barrage facilities can have large lock gates, like the ones used on canals to allow ship s to pass.
Barrage technology is very well established at La Rance, France where a 240 MW plant has operated since 1966. Tidal dams have a serious drawback. Estuaries are amongst the world's most productive and sensitive ecosystems, and the flooding by these barrages causes a great disruption to their natural processes. For this reason this type of tidal energy capture is not considered sustainable.
Harnessing offshore tidal streams has the advantage of being much cheaper to build, and does not have the environmental problems that a tidal barrage would bring. The range of suitable sites is far greater also.
Disadvantages:
- barrage systems are costly to build, and affect a very wide area - the environment is changed for many kilometres both up- and down-stream
- the tide needs to actually be moving, therefore they only provide power for approximately 10 hours each day
- reduced flushing, winter icing and erosion considerably change the ecosystem
- is only available in a small number of regions - it requires a basin or gulf that has a mean tidal amplitude of 7 metres or above. Also need semi-diurnal tides where there are two high and low tides everyday.
Advantages:
- is a renewable energy source
- needs no fossil fuel
- offers free power - once the system is built
- is a reliable source of electricity
- produces no greenhouse gases or other pollutants
- is a predictable system
- offshore tidal stream turbines and vertical-axis turbines are not very expensive to build and do not have a large environmental impact
- may protect a large stretch of coastline against damage from high storm tides
- can provide readymade road bridges
- the economic life of a tidal power plant is expected long - 75-100years, compared to 35 years for a conventional fossil fuel generating plant.
Tidal energy is currently more expensive to generate than conventional energy or that from many other renewable sources.