Hydropower Plant

Hydro Power Plant

Hydropower plants are renewable sources of generation of electricity. In a hydropower plant, falling water is used to drive a turbine which converts the kinetic energy into mechanical energy.

In the hydropower plant, the water is stored at a high elevation (upper level). And release it to lead downward (lower level). The difference between the upper level and the lower level is known as the Head.

The head is created artificially by constructing a dam. Generally, hydropower plants are located in hilly areas.

The potential power derived from the volume of water is directly proportional to the working head. Therefore, for the same power generation, the high head needs less volume of water compared to the lower head.

When it falls from the upper level, it releases kinetic energy and strikes the turbine blade. And it causes turbines to rotate.

The speed of the turbine can be controlled by the flow of water. An alternator is connected with the same shaft as the turbine.

So, the speed of the alternator is the same as the speed of the turbine. And by controlling the flow of water, the parameters of electrical energy are controlled. This is how the hydropower plant works.

Types of Hydro Power Plant

There are several types of hydropower plants. It is classified as shown in the below table.

Types of Hydro Power Plant
Types of Hydro Power Plant

According to Availability of Water Flow

The hydropower plants are classified into three categories;

  • Run-off River Plant with Pond
  • Run-off River Plant without Pond
  • Reservoir Plant

Run-off River Plant with Pond

The pond is used to store water. The size of the pond is very large and it can store a very large amount of water. The size of the plant depends on the size of the pond.

This type of plant is used to meet the fluctuating load demand. It is used to keep a load curve flat on certain limitations.

This type of plant can be used as a base load plant as well as a peak load plant. The hydro plant is a seasonal plant. Therefore, it depends on the availability of water.

This plant can be used as a base load plant during a high flow period. And this plant can be used as a peak load plant during the supply of water is limited.

Run-off River Plant without Pond

This type of plant is not used to operate constantly. Because the water is not available all time. There is no pond or other storage facility available in this type of plant.

Generally, this type of plant is placed in such an area where water is coming out directly from the river or pond. This type of plant is known as Run-off Hydro Power Plant.

The capacity of this plant is very low. During the high flow of water, if load demand is not enough then the flow of water wastes. And in such cases when the flow of water is not available, the plant may not capable to fulfill demand.

Therefore, this type of plant is not used to supply a constant load. This type of plant is only used to generate electricity during the high flood period. During this period, usage of this plant reduces the other natural fuels like coal, gas, etc.

Reservoir Power Plants

In this type of plant, water is stored behind the dam. And water may is available throughout the year. This type of plant is the most used and common plant in hydropower plants.

This type of plant is the most efficient plant. It can use for the base load as well as peak load conditions. The initial cost of this plant is very high. But the operating cost is almost zero and it can use for the entire year. Then also it depends on the size of the dam and the season of rain.

The water which is used to rotate the turbine, that water is again used for irrigation or supply to the city.

According to Availability of Water Head

The hydropower plants are classified into three types, according to the height of the water or water head.

  • Low head
  • Medium head
  • High head

Low head

There are no rules regarding water heads. But if the height of the water head is less than 30 meters that plant is considered as a low head plant.

For this type of plant, Francis, Kaplan, or Propeller turbines are used. No surge tank is required for this plant.

The dam is constructed on a river or pond to store the water. As we have discussed, if the water head is low, more flow of water is required. So, in this type of plant, a huge amount of water is required.

This type of plant uses low-speed and large-diameter type turbines.

Medium head

The hydropower plant which has a water head between 30 meters to 300 meters is known as a medium head plant. For this type of plant, a forebay is used as a surge tank.

Forebay tapped with the river and penstock is used to lead the water to the turbine.

High head

The hydropower plant has a water head greater than 300 meters and plants are known as high head hydropower plants.

A very big dam is constructed to store a huge amount of water. If the height of the water head is greater than 500 meters, the Pelton turbine is used. Below this head Francis turbine can use.

In this type of plant, water is coming from the reservoir to a valve house via a pressure tunnel. In case of the sudden closing of fixed gates of water, a surge tank is constructed to reduce the water hammering to the penstock.

The surge tanks also store some amount of water which is useful in pick load demand. The penstock is a connecting pipe that supplies water from the valve house to the turbine.

According to Types of Load Supply

According to types of load supply, hydropower plants are classified as;

  • Base Load Plant
  • Peak Load Plant

Base Load Plant

This type of plant has a large capacity. This type of plant is suitable to connect constantly with the load.

It works as a base portion of the load curve. Therefore, this plant is known as Base Load Plant. The load factor of this plant is very high.

Peak Load Plant

This type of plant is used when demand is high. It is suitable to maintain a load curve flat.

During peak demand, this power plant works and when demand is less, this plant shuts. This type of plant is also used in emergence conditions also.

Reservoir plants can be used as peak load plants. This type of plant can serve power throughout the year.

Parts of Hydro Power Plant

The major parts of the hydro plant are as follows;

  • Forebay
  • Trash Racks
  • Penstock
  • Surge Chamber
  • Turbine
  • Alternator
  • Tailrace

Forebay

It is a basin area where water is temporarily stored. The amount of storage of water in the forebay depends on water demand in that area.

Water is stored in the forebay before going into the intake chamber. In many hydro plants, reservoirs act as forebay.

Trash Racks

It is an intake structure that is placed in between the forebay and penstock. It consists of steel bars. These rods or bars are arranged with a gap of 10 to 30 cm.

These trash racks are used in cold seasons. In this season, there is a chance of the formation of ice. These trash racks are used to prevent ice to enter the penstock.

It is also used to clean water. If there is a solid impurity present in water, that will not enter into the penstock. If this impurity enters penstock, it will collide with the turbine blade and it may damage the blade.

Penstock

The penstock is used to collect water from a reservoir or forebay to the turbines. It is a long pipe with some slope which can carry water. So, water can run with some pressure.

These are designed to resist water hammering. Because at the time of opening and closing the gate, the pressure is very high. for short-length penstock, very heavy walls are provided to avoid hammering water. For a long penstock, a surge tank is used to store water and avoid hammering.

Steel or reinforced concrete is used to make a penstock. If the length is small, a separate penstock is used for each turbine. If the length is big, a large penstock is used and it separates into different branches at the end.

Surge Tank

It is a cylindrical tank that is used to control the pressure in the penstock. The water is coming from forebay to penstock via surge tank.

The water level in the surge tank increased, when the powerhouse rejected the water coming from the penstock. It will release the pressure of water.

When the powerhouse needs more water or it needs to accelerate the turbine at high speed, water from the surge tank is used to increase the pressure of water.

The size of the surge tank depends on the size of the plant.

Turbine

It is a device that converts the kinetic energy of water into mechanical energy. The water coming from the penstock is a strike by the blade of the turbine.

Due to the kinetic energy of water, the turbine rotor starts rotating. according to the type of plant and capacity of the plant, various types of turbines are available.

The turbine shaft is connected to the shaft of the alternator. Because of the same shaft, the speed of the turbine and the speed of an alternator are the same.

Alternator

The mechanical energy converted from the turbine is further given to the alternator. It is used to convert mechanical energy into electrical energy.

The alternator is also known as a generator or powerhouse. It is directly coupled with the turbine with a shaft. Generally, a synchronous generator is used to generate electricity.

In a hydropower plant, a salient pole synchronous generator is used. Because the speed of the turbine is less compared to a thermal power plant. And a number of poles are more in hydropower plants.

Tailrace

It is a channel that carries water away from the hydropower plant. It is constructed at a lower level than a reservoir.

The water coming after the rotating turbine, passes for other purposes. Proper design of the tailrace is compulsory otherwise it will damage the component of the plant or decrease the efficiency of the turbine.

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