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The availability of a particular thing is not important on its own but also depends to a great extent on the external circumstances. Let me make myself clearer by taking a simple example from everyday life. You may be passing by one of the best restaurants in the town but unless you have a desire or urge to eat it will not be of much interest to you. Similarly you might be starving without adequate source of food available in the immediate vicinity.
Nature loves to play this game of requirement and availability (we also call it the demand supply game in business parlance) and get better of the other party. You can well imagine this fact that though nearly 2/3rds of the earth's surface is covered with water, not every nation, region or location is suited to generate power using tidal energy of the oceans, but there are specific requirements to set up a tidal power plant.
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Factors Affecting Suitability of Tidal Power Plants
Water is a free natural resource, yet everywhere it is in chains (with due respect to Karl Marx). What I mean to say by that is several chains of bindings are necessary to utilize the freely available water of the oceans for tidal power generation. Tidal power generation plants are costly to build in the first place, so should not be even imagined unless these conditions are met at the locations where they are intended to be set up.
The presence of tidal pattern is not alone sufficient to qualify it as a place for tidal power plant set up. The difference between flood tide and ebb tide must be minimally of the order of 4.6 meters or above that.
The volume of water moving around during the tide or the cubature of the tidal flow is an important factor which determines the suitability of a site. Just imagine a place where tide may go up and down by say around 10 meters (just a hypothetical example) but with minuscule volume of water associated with it. It would serve no useful purpose as it may not possess the requisite energy to drive the turbine blades. Cubature in turn depends on factors such as tidal range and width of the estuary mouth.
Wave actions as well as storms are very destructive in nature and hence the site for the tidal power plant should be well protected from these natural agents, otherwise the cost of embankment would be unreasonably high and make the project financially unfeasible.
The site should not interrupt the flow of normal shipping traffic which passes through the estuary otherwise it will interrupt the economic cycle of the region.
It should be possible to construct a barrage which stores the maximum quantity of water with minimum cost of construction.
There are several other factors as well such as suitability to marine environment, silt index of water and so on which require discussion in more detail.
Hence we see that not all regions are suitable to set up tidal power plants which are financially and ecologically feasible. Its something similar to "water water everywhere, but not a drop to drink". Well actually the situation is not so bad in case of tidal power plants and we have quite a few successful ventures all across the globe.