Generating Stations | Power plant | Coal Power Plant | Steam power plant | Hydro electric power plant | Nuclear power plant | Diesel power plant | Thermal Power plant

Power Plants

Generating Stations or Power Plants.

A power plant or generating station is an industrial location where electrical power is generated in a large scale. A power plant consists of prime mover coupled with alternator for production of electricity. Prime mover( Turbine) energy from some other form (e.g. water, steam etc) into mechanical energy and electric generators convert mechanical energy into electrical. Electric generators used in power plants to produce AC electric power are popularly called as alternators. There are various types of energy sources which are used to generate electrical power. Most of the power stations use fossil fuels such as coal, oil or natural gas to generate electricity. There are others sources too, such as nuclear power, hydro power, renewable energy sources etc. 

The different type of generating station are 
1. Hydro Electric generating Station
2. Steam / Thermal Generating Station.
3. Nuclear Generating Station
4. Diesel Generating Station.

Hydro Electric Generating Station

    Hydroelectric power generation involves the storage of a  water, conversion of the potential energy of the water into mechanical (kinetic) energy through turbine, and conversion of the mechanical energy to electrical energy through electric generator.

Hydro-electric Generating stations are generally located in hilly areas where dams can be built conveniently and large water reservoirs can be obtained. In a hydro-electric generating station, water head is created by constructing a dam across a river or lake. Water stored in a dam possesses potential energy. From the dam, water is led to fall on turbine blades, turbine starts to rotate.in this way water turbine converts the potential energy of water into mechanical energy. The turbine in turn drives the alternator (generator) which converts mechanical energy into electrical energy. The total energy produced in hydro – electric power plant is 


P = 9.81Q \rhoh Kw       

or 
P = 9.81Qh\eta  Kw
where
        P = Power produced
        Q = discharge in m3/sec

h = Head in Meters        

\rho = density of water = 1000Kg/m3           

       \eta = efficiency 

Simple Layout Hydro-Power Plant

Classification of Hydro-electric power plants on basis of Head

On the basis of availability of head hydro – electric power plants are classified into three categories 
1. High head Hydro – Electric generating station.
2. Medium head Hydro – Electric generating station
3. Low head hydro – Electric generating station

High Head hydro – Electric Power Plant:

These type of power stations have head ranging from 100 m to 500 m. in these plants water is carried from main reservoir by a tunnel up-to the surge tank and then from the surge tank to power house through pen stock. The pelton wheel is the common turbine used in these power stations.

High Head Power Station

Medium Head hydro – Electric Power Plant:

These type of power stations have head ranging from 30m to 100m. In thse type of power stations uses Francis turbines. The forebay provided at the beginning of the pen stock serves as water reservoir. The water is carried out from main reservoir to forebay through open channels and then to power house through pen stock

Medium Head Power Plant

Low Head hydro – Electric Power Plant:

In such power plant the head of dam is below 30m. No surge tank is required. Excess of water is allowed to fall over the dam. In these plants Francis or Kaplan Turbine is used.

Low Head Power Plant

Advantages 

1. it requires no fuel as water is used for generating electric power which is naturally available. 
2.  Hydro- Electric generating station uses water as fuel, so it’s a clean fuel source, meaning it won’t pollute the air like power plants that burn fossil fuels, such as coal or natural gas
3. It requires very small running charges because water is the source of energy which is available free of cost.
4. In addition to a sustainable fuel source, hydro-electric generating stations efforts produce a number of benefits, such as flood control, irrigation, and water supply.

5. It is comparatively simple in construction and requires less maintenance
6. Hydroelectric power is a domestic source of energy, allowing each state to produce their own energy without being reliant on international fuel sources.
7. Hydro-electric power is completely renewable, which means it will never run out unless the water stops flowing. As a result,  hydro plants are built to last. In some cases, equipment that was built to last 25 years is still operational after double the amount of time has passed.

Disadvantages

1. The capital cost of the hydro – electric generating station is high due to construction of Dam.
2. There is uncertainty about the availability of huge amount of water due to dependence on weather conditions
3. It is challenging to find a suitable spot that has a large year-round water supply, with the right amount of water and is close enough to existing power lines. It is also a delicate balancing act to keeping enough river water wild (meaning without dams), versus damming up many rivers for power.   
4. It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.
5. Flood Risk: When dams are built at higher elevations, they pose a serious risk to any town nearby that is below it. While these dams are built very strong, there are still risks. The biggest dam failure in history is the Banqiao Dam failure. Due to excess rainfall from a typhoon, the dam collapsed. This resulted in the deaths of 171,000 people.

Steam / Thermal Generating Station (or thermal power plant)

Steam Power plant is  generating station which generates electrical energy by burning of fossil fuels mainly coal. It is sometimes called thermal power plant. The steam power station works on the Rankine cycle In Steam generating station a fossil fuel such as coal is burned to produce heat. This heat is then used to boil the water and convert it into the super-heated steam. The super-heated steam is passed into a steam turbine. The Steam turbine are rotated due to the pressure of the steam. The Steam is mechanically coupled with the rotor of the alternator/ generator. with the rotation of steam turbine, the rotor of the alternator also rotates, Electrical energy is produced. 

The Modern steam power plant consists four circuits 
1. Coal and Ash circuit
2. Air and gas Circuit
3. Feed water and Steam flow circuit
4. Cooling water circuit

Coal and Ash Circuit : 

Coal from coal yard is supplied to the boiler furnace by means of conveyors, elevators etc. where its combustion takes place and ash is produced which is taken out form back of the boiler. 

Air and Gas Circuit :

The air from atmosphere is supplied to combustion chamber of boiler by forced draught fan ( FD fan) and induced draught fan ( ID Fan). The air is heated by flues gases in air pre-heater before entering the combustion chamber to aid in the combustion of fuel. 

Feed Water and Steam Flow :

The Steam from the boiler is supplied to the turbine, from turbine it is given to condenser for condensation. The condensate is preheated by the flue gases in the economizer. the preheated water is then supplied to boiler.

Cooling Water Circuit:

The exhaust steam from the turbine is condensed in condenser. In the condenser, cold water is circulated to condense the steam into water. The steam is condensed by losing its latent heat to the circulating cold water. Thus the circulating water is heated. This hot water is then taken to a cooling tower, In cooling tower, the water is sprayed in the form of droplets through nozzles. The atmospheric air enters the cooling tower from the openings provided at the bottom of the tower. This air removes heat from water. Cooled water is collected in a pond (known as cooling pond). This cold water is again circulated through the pump, condenser and cooling tower. Thus the cycle is repeated again and again. Some amount of water may be lost during the circulation due to vaporization etc. Hence, make up water is added to the pond by means of a pump. This water is obtained from a river or lake.

Components of Steam / Thermal Power Plant

1.Coal mill: Coal mills grind the coal into fine powder, and the air entering the mill dries and drives the coal into the combustion chamber. 
2. Boiler: It converts the water into steam at high temperature and pressure. The mixture of pulverized coal and air (usually preheated air) is taken into boiler and then burnt in the combustion zone. The heat produced due to combustion of coal converts the water into the tubes of boiler into steam at high pressure and temperature. The flue gases from the boiler make their way through super-heater, economizer, air pre-heater and finally get exhausted to the atmosphere from the chimney.

Super-heater: Super-heater converts the steam produced in a boiler into super-heated saturated steam. The super-heated saturated steam is then fed to high prssure turbine. For converting steam into super-heated steam super – heater takes heat from flue gases. 

Economizer:  it is feed water heater which heats the water before entering into boiler. The heater required for heating the water is taken from flue gases.


Air Pre-heater: The air taken from atmosphere by Forced draught fan is heated by air pre-heater by taking heat from flue gases before entering the combustion chamber of boiler. 

3. Steam Turbine:  The super-heated steam at high pressure and temperature is fed to the high pressure steam turbine. in The turbine steam cause the turbine to rotate, thus converting heat energy of steam into mechanical energy. The steam after leaving the high pressure steam turbine losses temperature and pressure. The steam at low pressure is fed to low pressure turbine. The steam before entering the low pressure turbine is heat again by Re-heater. Re-heater takes heat from the flue gases. 
4. Condenser: The exhausted steam after leaving the low pressure turbine is condensed in the condenser by means of cold water circulation. The steam loses it’s pressure and temperature and it is converted back into water. Condensing is essential because, compressing a fluid which is in gaseous state requires a huge amount of energy with respect to the energy required in compressing liquid. Thus, condensing increases efficiency of the cycle.

5. Alternator: The steam turbine is coupled to rotor of an alternator / Generator. When the turbine rotates the rotor of the alternator, electrical energy is generated. This generated electrical voltage is then stepped up with the help of a transformer and then transmitted where it is to be utilized.


6. Feed water Pump: The condensate from the condenser is used as feed water to the boiler. Some water may be lost in the cycle which is suitably made up from external source. The feed water on its way to the boiler is heated by water heaters and economiser. This helps in raising the overall efficiency of the plant.


7. Electrostatic Precipitator: An electrostatic precipitator (ESP) is a filter device that removes fine particles, like dust and smoke, from a flue gases using the force of an induced electrostatic charge. Electrostatic precipitators are used for air pollution control, particularly for removing harmful particulate matter from flue gases. Such particulates reduce visibility, can contribute to climate change, and lead to serious health problems in humans, including lung damage and bronchitis.

8.Chimney: It is used to release the flue gases or smoke from the furnace to the environment at appropriate height. The height of the tower is very high such that it can easily throw the smoke and exhaust flue gases at the appropriate height. And it cannot affect the population living near the steam power plant.


9. Cooling Towers: It is a tower which contains cold water. Cold water is circulates to the condenser for condensing the steam leaving from steam turbine.

Steam Power Plant

Salient Features of Steam Power Plant

  1. Generally located near the load center but other factor like transportation of fuel, enough water for cooling the condensate, cost of land are kept in mind.
  2. initial cost is lower than other thermal plants (e.g. Solar etc)
  3. Cost of fuel is high as compared to nuclear and hydro-electric power plant
  4. Maintenance cost is also higher than hydro-electric and diesel but comparable as compared to nuclear power plant.
  5. Running cost is high as compared to hydro and nuclear
  6. Generally used to supply base load.

Advantage of Steam Power plant

  1. Less initial cost as compared to other power plants
  2. It can be installed at any place irrespective of the existence of coal. The coal can be transported to the site of the plant by rail or road.
  3. It requires less space as compared to the hydroelectric power station.
  4. The cost of generation is lesser than that of the diesel power station.

Disadvantages

  1. It pollutes the atmosphere due to the production of large amount of smoke and fumes.
  2. It is costlier in running cost as compared to hydroelectric plant.
  3. Starting up the plant and bringing into service takes more time.
  4.  Ash handling is a big problem.

Nuclear Power Plant

In these power plants nuclear energy is converted into electrical energy. Nuclear energy is obtained by nuclear fission of heavy elements such as Uranium or Thorium in a special reactor called nuclear reactor. in these power plants the heat generated by the reactor, converts water into steam, which spins a turbine, which in turn rotates the generator, thus electrical energy is produced. 

Nuclear Power Plant

Components of Nuclear Power Plant

The various components of nuclear power plant are :-

  1.  Nuclear Reactor.
  2. Heat Exchanger ( or Steam generator)
  3. Steam Turbine.
  4. Condenser
  5. Electric Generator ( or Alternator)

Nuclear Reactor:-  Nuclear reactor is the main component of nuclear power plant. The main function of the nuclear reactor is control nuclear chain reactions that produces heat by nuclear fission reaction- the process in which a heavy atomic nucleus splits into two smaller fragments. The fuel in the nuclear reactor are Uranium-235, Uranium-233 and Plutonium-239. Uranium-233 and Plutonium-239 can be produced artificially from Uranium-238 and Thronium-232 which occur in nature and are called fertile materials.  

U^{238}+n^{1}\rightarrow U^{239}+\gamma\\\\U^{239}\rightarrow Np^{239}+e\\\\Np^{239}\rightarrow Pu^{239}+e

In reactor chain reaction is controlled by control rods. The material used for the control rods have high absorption capacity for neutrons. The common material used for control rods are cadmium, boron or hafnium. 
Reactor is enclosed in a shield. Material for Shield is concrete. Addition of Boron compounds in concrete helps in arresting neutrons and secondary radiations. 


Heat Exchanger (or Steam Generator): The coolant after receiving heat in the Nuclear reactor, temperature of the coolant increases. The hot coolant after leaving the Nuclear reactor flows through the tubes of Heat Exchanger, and passes its heat to water and its water gets converted into steam. After giving up heat, the coolant is again fed to the reactor. The coolant commonly used are gas ( CO2, air, hydrogen, helium ), Water, heavy water, liquid metals (Sodium or Sodium potassium ) etc. 


Steam Turbine: The steam produced in the heat exchanger is fed to the steam turbine, where it rotates the turbine. Thus heat energy of steam is converted into mechanical energy. The steam turbine in-turn rotates the alternator and electrical energy is produced. 


Condenser: After doing a useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump.

Salient features of Nuclear Power Plant
1. Site should be located near load center and nuclear fuel should be easily transported. There should be availability of water and space for burial of radio-active material.
2. Initial cost of material is moderate.
3. Cost of fuel is low
4. Skilled and well trained staff is required to handle the equpments
5. Running cost is low as compared to thermal power plant.
6. used for base Load


Advantages of Nuclear Power Plant 
1. it is pollution free. Produces no polluting gases.
2. it Most Reliable Energy Source. Nuclear power plants run 24 hours a day, 7 days a week. They are designed to operate for longer stretches and refuel every 1.5 – 2 years. In 2019, nuclear plants operated at full power more than 92% of the time, making it the most reliable energy source on the grid today.
3. it has Very low fuel costs as compared to thermal power palnt.
4. Power station has very long lifetime.


Disadvantages of Nuclear Power Plant
1. Safe disposal of radioactive material is difficult and expensive. 
2. Local thermal pollution from wastewater affects marine life.
3. Costs of building and safely decommissioning are very high.
4. Cannot react quickly to changes in electricity demand.
5. The efficiency of nuclear power Plant is less than thermal power plant because of low operating temperature and pressure conditions. 

Diesel Power Plant

A generating station in which diesel engine is used as the prime mover for the generation of electrical energy is known as diesel power station. For generating electrical power, it is essential to rotate the rotor of an alternator by means of a prime mover. The prime mover can be driven by different methods. Using diesel engine as prime mover is one of the popular methods of generating power. When prime mover of the alternators is diesel engine, the power station is called diesel power station. The mechanical power required for driving alternator comes from combustion of diesel. As the diesel costs high, this type of power station is not suitable for producing power in large scale in our country. But for small scale production of electric power, and where, there is no other easily available alternatives of producing electric power, diesel power station are used. 
Diesel power plant is used in as central station, standby unit,  peak load power plant, Emergency power Plant, Supply unit for cinemas , hospitals etc

Components of Diesel Power Plant

The various components of Diesel power Plant are

1. Fuel supply system: It consists of storage tank, strainers, fuel transfer pump and all day fuel tank. The fuel oil is supplied at the plant site by rail or road. This oil is stored in the storage tank. From the storage tank, oil is pumped to smaller all day tank at daily or short intervals. From this tank, fuel oil is passed through strainers to remove suspended impurities. The clean oil is injected into the engine by fuel injection pump.

2.Air intake system: This system supplies necessary air to the engine for fuel combustion. It consists of pipes for the supply of fresh air to the engine manifold. Filters are provided to remove dust particles from air which may act as abrasive in the engine cylinder.

3. Exhaust system: This system leads the engine exhaust gas outside the building and discharges it into atmosphere. A silencer is usually incorporated in the system to reduce the noise level.

4. Cooling system: The heat released by the burning of fuel in the engine cylinder is partially converted into work. The remainder part of the heat passes through the cylinder walls, piston, rings etc. and may cause damage to the system. In order to keep the temperature of the engine parts within the safe operating limits, cooling is provided. The cooling system consists of a water source, pump and cooling towers. The pump circulates water through cylinder and head jacket. The water takes away heat form the engine and itself becomes hot. The hot water is cooled by cooling towers and is recirculated for cooling.

5. Lubricating system: This system minimizes the wear of rubbing surfaces of the engine. It comprises of lubricating oil tank, pump, filter and oil cooler. The lubricating oil is drawn from the lubricating oil tank by the pump and is passed through filters to remove impurities. The clean lubricating oil is delivered to the points which require lubrication. The oil coolers incorporated in the system keep the temperature of the oil low.

6. Engine starting system: This is an arrangement to rotate the engine initially, while starting, until firing starts and the unit runs with its own power. Small sets are started manually by handles but for larger units, compressed air is used for starting. In the latter case, air at high pressure is admitted to a few of the cylinders, making them to act as reciprocating air motors to turn over the engine shaft. The fuel is admitted to the remaining cylinders which makes the engine to start under its own power.

Diesel Power Plant

Advantages

  1.  The design and layout of the plant are quite simple. 
  2. It occupies less space as the number and size of the auxiliaries is small. 
  3. It can be located at any place.
  4. (iv) It can be started quickly and can pick up load in a short time.
  5. There are no standby losses. 
  6. It requires less quantity of water for cooling.
  7. The overall cost is much less than that of steam power station of the same capacity.
  8. The thermal efficiency of the plant is higher than that of a steam power station.
  9. It requires less operating staff.

Disadvantages  

  1. The plant has high running charges as the fuel (i.e., diesel) used is costly.
  2. The plant does not work satisfactorily under overload conditions for a longer period.
  3. The plant can only generate small power. 
  4. The cost of lubrication is generally high.
  5. The maintenance charges are generally high.

Read Also

  1. Introduction to Electrical Power System

Author: AJAZ UL HAQ

AJAZ UL HAQ has above 8 years of Experience in Electrical Power Transmission, Distribution and Substation. Presently He is working with KEI Industries Limited as Engineer-EPC/EHV.

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