Tidal Power Plants: Components, Classification and Operation

In this article we will discuss about:- 1. Components of Tidal Power Plants 2. Classification of Tidal Power Plants 3. Operation.

Components of Tidal Power Plants:

For utilization of tidal energy, water must be trapped at high tide behind a dam or barrage and then made to drive turbine coupled to an electric generator as it returns to sea during low tides. The available energy is proportional to the square of the amplitude.

The main components of tidal power plants are:

(i) Dam

(ii) Sluice ways from basin to sea and vice versa, and

(iii) Power house.

The function of a dam is to form a barrier between the sea and the basin or between one basin to the other basin in case of multiple basins. The most suitable word for tidal power plant is barrage. Barrages have to resist waves whose shock can be severe and where pressure changes sides continuously.

The sluice ways are gate controlled devices. They are employed to fill the basin during the high tide or empty the basin during low tide. In existing plants, vertical lift gates have been employed. Flap gates are also used. The flap gates allow only in the direction of sea to basin. Therefore, the level of the basin rises.

Auxiliary equipments, turbines and generators are the main components of the power house. Large sized turbines are used because of low head available. Bulb types and rim type turbines are commonly used. Shaft turbines are also under steady.

Classification of Tidal Power Plants:

Tidal power plants can be classified on the basis of basins used in power generation.

There are two types of basin systems viz.:

1. Single Basin System, and

2. Double Basin System.

1. Single Basin System:

Single basin system can generate power only intermittently. This is the simplest system of generating tidal power. The single basin scheme has only one basin. The basin is separated from the sea by a dam (barrage, Dyke). The sluice way is opened during high tide to fill the basin. The turbine-generator units are mounted within the ducts inside the barrage.

The single basin system can be further classified as:

(i) Single Ebb-Cycle System,

(ii) Single Tide-Cycle System,

(iii) Double Cycle System.

(i) Single Ebb-Cycle System:

In single ebb-cycle system, when the high tides (flood side) are falling, sluices are opened to permit the sea water to enter the basin, while the turbine sets are shut. The level of the basin begins increasing. The energy is stored in the form of tidal range. Tidal range provides water head during low tides. The generation of power takes place, when the water from the basin flows over the turbine into the low level sea water. The turbines are designed for single way operation. The power output from such system is intermittent in nature and highly variable.

(ii) Single Tide-Cycle System:

In single tide cycle system, the generation is affected when the sea is at flood tide. The sea water is admitted into the basin over the turbines. As the flood tide period is over and the sea level begins falling again, the generation is stopped. The basin is drained into the sea through the sluice ways. In this system also the power output is intermittent.

(iii) Double Cycle System:

In double cycle system, the reversible turbines are installed and power is generated during filling and emptying of basin. Filling process occurs when the ocean is at high tide while the water in basin at low tide level, the emptying occurs when the ocean is at low tide and basin at high tide level.

The flow of water in both directions is used to drive the reversible turbines. Each turbine drives the generator. In this system also continuous generation of power is not possible because of short duration. Electric power is generated during two short periods, during each tidal period of 12 hours 25 minutes or once every 6 hours and 12.5 minutes.

2. Double Basin System:

There are two basins at different levels. A dam is provided between two basins. The turbines are located in the dam. The sluice gates are provided in the dam. One basin is called the upper basin; the water level is maintained above that in the other, the low basin. The high level basin gates are called the inlet gates and low level gates as outlet gates. The upper basin is filled with water.

When the water level in upper basin A provides a sufficient difference of head between the two basins, the turbines are started. The water flows from basin A to basin B through the turbines and the power is generated. The power generation thus continues simultaneously along with filling up of water in basin A. When the tide attains its peak value, the water level in basin A is maximum; the inlet sluices are then closed. The water flows from the upper basin to the lower basin through the turbines.

Thus, the water level in the upper basin falls and that in the lower basin rises. When the rising level in lower basin B becomes equal to the level of the falling tide, the outlet sluices are opened. When the tide reaches its lower most level, the outlet gates are closed. After some time the tide rises. When its level becomes equal to low level of the upper basin, the inlet gates are opened. Consequently the level of water in basin A starts rising. Thus, the cycle is repeated.

Two basin schemes have the advantages over normal schemes is that generation time can be adjusted with high flexibility and it is also possible to generate almost continuously. In normal estuarine situations, however, two basin schemes are very expensive to construct due to the cost of extra length of barrage. There are some favourable geographies, however, which are well suited to this type of scheme.

Operation of Tidal Power Plants:

The tidal power scheme may be designed to operate in any one of following modes:

1. Ebb Generation:

The basin is filled through the sluices until high tide. Then, the sluice gates are closed. At this stage, there may be pumping to raise the level further. The turbine gates are closed until the sea level falls to develop sufficient head across the barrage, and then are opened so that the turbines generate power until the head is again low. Then the sluices are opened, turbines disconnected and the basin is filled again. The cycle repeats. Ebb generation, also known as outflow generation, takes its name because generation occurs as the tide changes tidal direction.

2. Flood Generation:

The basin is filled through the turbines, which produce at tide flood. This is usually much less efficient than Ebb generation, because the volume contained in the upper half of the basin (which is where Ebb generation operates) is greater than the volume of the lower half (filled first during flood generation).

Thus, the available level difference between the basin side and the sea side of the barrage reduces more quickly than it would in Ebb generation. Rivers flowing into the basin may further reduce the energy potential, instead of increasing it as in case of Ebb generation. Of course, this is not a problem with the “lagoon” model, without river in flow.

3. Pumping:

Turbines are operated as pumps by excess energy in the grid to enhance the water level in the basin at high tide (Ebb generation). This energy is more than returned energy during generation, because power output is strongly related to the head. If water is raised 0.61 m by pumping on a high tide of 3 m, this will have been raised by 3.7 m at low tide. The cost of a 0.61 m rise is returned by the benefits of a 3.7 m rise. This is since the correlation between the potential energy is not a linear relationship, rather is related by the square of the tidal high variation.