Hydro-electric power plant

HYDRO-ELECTRIC POWER PLANT

Working Principle

Hydro means water. Hydro-Electric Power Plant (Hydel Plant) utilizes the Potential Energy of water stored in a dam built across the river.

Potential Energy is the energy which a substance as due to its position or state. The potential energy of the stored water is converted into kinetic energy by first passing it through the penstock pipe. The kinetic energy of water is then converted into mechanical energy in a water turbine. That is, the kinetic energy of water is used to drive the turbine.

The turbine is coupled to the electric generator. The mechanical energy available at the shaft of the turbine is converted into electrical energy by means of the generator.

Classification of Hydro-Electric Power Plants

Hydel plants are classified according to the Head of Water under which they work.

1. High Head Power Plant: When the operating head of water exceeds 70 meters, the plant is known as High Head Power Plant. Pelton turbine is the prime mover used.

2. Medium Head Plant: When the head of water ranges from 15 to 70 meters, then the power plant is known as Medium Head Plant. It uses Francis turbine.

3. Low Head Plant: When the head is less than 15 meters, the plant is named as Low Head Plant. It uses Francis or Kaplan turbine as prime mover.

 

1. LAYOUT OF A HYDRO-ELECTRIC POWER PLANT

Fig. 3 shows the schematic representation of a Hydro-Electric Power Plant. Its elements are:

1. Catchment Area

2. Water Reservoir

3. Head Race Level:

4. Dam

5. Gate Valve

6. Penstock Pipe

7. Surge Tank

8. Waterway

9. Water Turbine

10. Draft Tube

11. Tail Race and Tail Race Level

12. Power House

1. Catchment Area

Catchment Area of a hydro-plant is the whole area behind the dam, draining into a stream or river across which the dam is built at a suitable location.

2. Water Reservoir

Continuous availability of water is the basic necessity for a hydro-electric plant. Whole of the water collected from catchment area during rainy season is stored in the reservoir. The main purpose of the reservoir is to store the water during rainy season and supply the same during dry season. Capacity of the reservoir depends upon the catchment area and rainfall at that place. The water head available for power generation depends on the reservoir height.

3. Head Race Level

Water surface in the storage reservoir is known as Head Race Level.

4. Dam

A Dam is a structure of masonry or some other material built at a suitable location across the river. The function of a Dam is to increase the height of water level behind it (called Water Head), which ultimately increases the reservoir capacity. In order to generate the required quantity of power, it is necessary that a sufficient water head is available.

Spillway: Excess accumulation of water endangers the stability of dam construction. During rainy season, water after a certain safe level in the reservoir overflows through spillway without allowing an increase in water level in the reservoir.

5. Gate Valve

A Gate Valve is used to regulate or control the flow of water from the dam.

6. Penstock Pipe

Penstock Pipe is used to bring water from the dam to the hydraulic turbine. Penstock pipes are made up of steel or reinforced concrete. The turbine is installed at a lower level from the dam. Penstock is provided with a Gate Valve at the inlet and a Control Valve to control the water flow rate into the turbine.

7. Surge Tank

Water Hammer: There may be sudden increase of pressure in the penstock pipe due to sudden backflow of water, as load on the turbine is reduced. The sudden rise of pressure in the penstock pipe is known as Water Hammer.

Surge tank is introduced between the dam and the turbine to reduce the sudden rise of pressure in the penstock. Otherwise, penstock pipe will be damaged by water hammer effect.

Surge Tank is a small tank into which water flows in or from which water flows out due to suuden valtations of pressure. sudden variations of pressure.

Surge tank also serves as a Supply Tank, delivering additional water when the water in the pipe is accelerated due to the increased load on the turbine and as a Storage Tank collecting water, when the water is decelerated due to the reduced load on the turbine.

8. Waterway

Waterway through penstock pipe carries water from the dam to Power House.

9. Water Turbine (Hydraulic Turbine) - Prime Mover

Water Turbine is also known as Hydraulic Turbine. Water through the penstock pipe enters into the turbine through the Control Valve. Prime movers which are in common use are Pelton Turbine, Francis Turbine and Kaplan Turbine. The potential energy of water entering the turbine is converted into mechanical energy. The mechanical energy available at the turbine shaft is used to run the electric generator. The water is then discharged to the river through the Draft Tube.

10. Draft Tube

Turbine outset is connected to the Draft Tube by the Tail Race. Water after doing work in the turbine passes to the river through the draft tube.

Draft tube is a metallic pipe or concrete tunnel having gradually increasing cross-sectional area towards the outlet. Its function is to ensure that little or no energy is left in water as it discharges into the river.

The water, after transferring the major part of its energy, enters the draft tube with remaining kinetic energy. This kinetic energy is quite considerable. All this energy will be lost, if this water is allowed to discharge freely. So, by passing the water at the turbine outlet through the draft tube, the velocity is very much reduced with a corresponding increase in pressure. Therefore, the net head on the turbine increases and hence the output of the turbine.

11. Tail Race and Tail Race Level

Tail Race is a waterway (passage) for discharging the water from the turbine to the river or canal. The water held in the tail race is called Tail Race Water Level. Water from the tail race is released for irrigation purposes.

12. Power House

Power House accommodates water turbine, generator, control room and transformer. The function of the step-up transformer is to raise the voltage generated at the generator terminal before transmitting power to the consumers through transmission lines.

 

2. ADVANTAGES OF HYDRO-ELECTRIC POWER PLANT

1. Renewable Source of Energy: Water is a renewable source of energy. Water which is the operating fluid, is neither consumed nor converted into something else.

2. Cheapest Source of Energy: Water is the cheapest source of energy, because it exists as a free gift of Nature. The fuels needed for the thermal, diesel and nuclear plants are exhaustible and expensive.

3. No Fuel Transportation: No fuel transportation problem unlike other power stations, since water from rivers and rain is directly received in the catchment area behind the dam.

4. No Ash Disposal Problem: There is no ash disposal problem as in the case of thermal power plant.

5. No Air Pollution: Hydro-plant does not pose the problem of air pollution as in the case of thermal plant or radiation hazards as in the case of nuclear plant.

6. Variable Loads: Variable loads do not affect the efficiency in the case of a hydro-plant.

7. Life of the Plant: Life of hydro-plant is very long (100 – 125 years), compared with thermal plant (30 – 40 years). This is because the hydro-plants operate at atmospheric temperature, whereas thermal plants operate at very high temperatures (about 500 to 800°C).

8. Running Speed: Hydel plant has a low running speed of 300 to 400 rpm. But, the turbines in thermal plant run at a speed of 3000 to 4000 rpm and they require special alloy steel materials and rigid construction.

9. Additional Benefits: Hydro-plant provides additional benefits like irrigation, flood control, fishery and serve as a center of tourist attraction.

10. Domestic Water Supply: The water storage of hydro-plant can also be used for domestic water supply

11. Easy Operation: Hydel plant takes a few minutes to start and run the plant for power generation. The hydraulic turbines can be put off and on in a matter of minutes. Thermal power plants and nuclear power plants lack this facility.

12. Auxiliaries: Auxiliaries needed for hydro-plant are less compared to thermal plant of equal capacity. The former requires dam, penstock pipe, surge tank, water turbine and draft tube. The latter requires boiler, steam turbine, condenser, cooling tower, L.P. heater, feed pump, H.P. heater, economizer, air pre-heater and chimney.

13. Less Staff: Hydel plant requires less supervising staff.

14. Low Maintenance Cost: Maintenance cost is low as compared to thermal plant or nuclear plant of same capacity.

 

3. DISADVANTAGES OF HYDRO-ELECTRIC POWER PLANT

1. Transmission Losses: Hydro-plants are situated away from the load centers. Hence, long transmission lines are required for delivery of power. This increases the cost of transmission lines and also transmission losses. But, a thermal plant can be located near the load center, thereby the transmission cost and transmission losses are considerably reduced.

2. Failure of Monsoon: The power produced by hydro-plant depends upon the quantity of water which in turn is dependent upon the rainfall. The dry year affects the hydro-power generation considerably or power production may be even stopped due to insufficient water head in the reservoir.

3. Evaporation Loss: Water in the reservoir is lost due to evaporation.

4. High Initial Cost: Initial cost or capital investment of the plant is high.

5. Erection of the Plant: Erection of hydro-plant (construction of dam, etc.) usually takes long period of time.