The tidal power plants are generally classified on the basis of the number of basins used for the power generation.
They are further subdivided as one-way or two-way system as per the cycle of operation for power generation.
A dam is constructed to create basin to store the water. Hence power is generated during both flood & ebb tide.
By using reversible water turbine the turbine can run continuously both during high tide and low tide.
A Tidal power plant mainly consists of the following
A barrage with gates and sluices
One or more basins
A power house
A barrage is a barrier constructed across the sea to create a basin for storing water.
The barrage has to withstand the pressure exerted by the water head and also should resist the shock of the waves.
A basin is the area where water is retained by the barrage. Low head reversible water turbines are installed in the barrage separating the sea from the basin.
Principle
Tide or wave is periodic rise and fall of water level of the sea. Tides occur due to the attraction of sea water by the moon. Tides contain large amount of potential energy which is used for power generation. When the water is above the mean sea level, it is called flood tide. When the water level is below the mean level it is called ebb tide.
Working
The ocean tides rise and fall and water can be stored during the rise period and it can be discharged during fall.
A dam is constructed separating the tidal basin from the sea and a difference in water level is obtained between the basin and sea.
During high tide period, water flows from the sea into the tidal basin through the water turbine.
The height of tide is above that of tidal basin. Hence the turbine unit operates and generates power, as it is directly coupled to a generator. During low tide period, water flows from tidal basin to sea, as the water level in the basin is more than that of the tide in the sea.
During this period also, the flowing water rotates the turbine and generator power.
The generation of power stops only when the sea level and the tidal basin level are equal. For the generation of power economically using this source of energy requires some minimum tide height and suitable site.
Working of different tidal power plants
Single basin-one-way cycle
This is the simplest form of tidal power plant. In this system a basin is allowed to get filled during flood tide and during the ebb tide, the water flows from the basin to the sea passing through the turbine and generates power. The power is available for a short duration ebb tide.
Single-basin two-way cycle
In this arrangement, power is generated both during flood tide as well as ebb tide also.
The power generation is also intermittent but generation period is increased compared with one-way cycle. However, the peak obtained is less than the one-way cycle.
Single – basin two-way cycle with pump storage
In this system, power is generated both during flood and ebb tides. Complex machines capable of generating power and pumping the water in either direction are used.
A part of the energy produced is used for introducing the difference in the water levels between the basin and sea at any time of the tide and this is done by pumping water into the basin up or down.
Double basin type
In this arrangement, the turbine is set up between the basins as shown in figure. One basin is intermittently filled tide and other is intermittently drained by the ebb tide.
Therefore, a small capacity but continuous power is made available with this system as shown in figure.
The main disadvantages of this system are that 50% of the potential energy is sacrificed in introducing the variation in the water levels of the two basins turbine generating head.
Double basin with pumping
In this case, off peak power from the base load plant in a interconnected transmission system is used either to pump the water up the high basin.
Net energy gain is possible with such a system if the pumping head is lower than the basin-to basin turbine generating head.
Advantages of tidal power plants.
It is free from pollution as it does not use any fuel.
It is superior to hydro-power plant as it is totally independent of rain.
Disadvantages
Tidal power plants can be developed only if natural sites are available on the bay.
As the sites are available on the bays which are always far away from load centers, the power generated has to be transmitted to long distances. This increases the transmission cost and transmission losses.
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