Power station or power plant and classification

Power Station or Power Plant and classification

Power Station or Power Plant :

A power station or power plant is a facility for the generation of electric

power. 'Power plant' is also used to refer to the engine in ships, aircraft and

other large vehicles. Some prefer to use the term energy center because it

more accurately describes what the plants do, which is the conversion of

other forms of energy, like chemical energy, gravitational potential energy or

heat energy into electrical energy. However, power plant is the most

common term in the U.S., while elsewhere power station and power plant

are both widely used, power station prevailing in many Commonwealth

countries and especially in the United Kingdom.

At the center of nearly all power stations is a generator, a rotating machine

that converts mechanical energy into electrical energy by creating relative

motion between a magnetic field and a conductor. The energy source

harnessed to turn the generator varies widely. It depends chiefly on what

fuels are easily available and the types of technology that the power

company has access to.

Classification of Power plants :

Power plants are classified by the type of fuel and the type of prime mover

installed.

By fuel

• In Thermal power stations, mechanical power is produced by a heat

engine, which transforms thermal energy, often from combustion of a

fuel, into rotational energy

• Nuclear power plants use a nuclear reactor's heat to operate a steam

turbine generator.

• Fossil fuel powered plants may also use a steam turbine generator or

in the case of Natural gas fired plants may use a combustion turbine.

• Geothermal power plants use steam extracted from hot underground

rocks.

• Renewable energy plants may be fuelled by waste from sugar cane,

municipal solid waste, landfill methane, or other forms of biomass.

• In integrated steel mills, blast furnace exhaust gas is a low-cost,

although low-energy-density, fuel.

• Waste heat from industrial processes is occasionally concentrated

enough to use for power generation, usually in a steam boiler and

turbine.

By prime mover

• Steam turbine plants use the pressure generated by expanding steam

to turn the blades of a turbine.

• Gas turbine plants use the heat from gases to directly operate the

turbine. Natural-gas fuelled turbine plants can start rapidly and so are

used to supply "peak" energy during periods of high demand, though

at higher cost than base-loaded plants.

• Combined cycle plants have both a gas turbine fired by natural gas,

and a steam boiler and steam turbine which use the exhaust gas from

the gas turbine to produce electricity. This greatly increases the

overall efficiency of the plant, and most new baseload power plants

are combined cycle plants fired by natural gas.

• Internal combustion Reciprocating engines are used to provide power

for isolated communities and are frequently used for small

cogeneration plants. Hospitals, office buildings, industrial plants, and

other critical facilities also use them to provide backup power in case

of a power outage. These are usually fuelled by diesel oil, heavy oil,

natural gas and landfill gas.

• Microturbines, Stirling engine and internal combustion reciprocating

engines are low cost solutions for using opportunity fuels, such as

landfill gas, digester gas from water treatment plants and waste gas

from oil production.

Other sources of energy :

Other power stations use the energy from wave or tidal motion, wind,

sunlight or the energy of falling water, hydroelectricity. These types of

energy sources are called renewable energy.

Thermal power plant,Advantages and Disadvantages

Thermal power plant or Steam power plant :

A generating station which converts heat energy of coal combustion in to

electrical energy is known as Thermal power plant or Steam power plant.

Some of its advantages and disadvantages are given below.

Advantages

1. The fuel used is quite cheap.

2. Less initial cost as compared to other generating plants.

3. It can beinstalled at any place iirespective of the existence of

coal. The coal can be transported to the site of the plant by rail or

road.

4. It require less space as compared to Hydro power plants.

5. Cost of generation is less than that of diesel power plants.

Disadvantages

1. It pollutes the atmosphere due to production of large amount of

smoke and fumes.

2. It is costlier in running cost as compared to Hydro electric plants.

Electric Power Systems and its components

Electric Power Systems :

Electric Power Systems, components that transform other types of energy

into electrical energy and transmit this energy to a consumer. The

production and transmission of electricity is relatively efficient and

inexpensive, although unlike other forms of energy, electricity is not easily

stored and thus must generally be used as it is being produced.

Components of an Electric Power System

A modern electric power system consists of six main components:

1. The power station

2. A set of transformers to raise the generated power to the high

voltages used on the transmission lines

3. The transmission lines

4. The substations at which the power is stepped down to the

voltage on the distribution lines

5. The distribution lines

6. the transformers that lower the distribution voltage to the level

used by the consumer's equipment.

Power Station

The power station of a power system consists of a prime mover, such as a

turbine driven by water, steam, or combustion gases that operate a system

of electric motors and generators. Most of the world's electric power is

generated in steam plants driven by coal, oil, nuclear energy, or gas. A

smaller percentage of the world’s electric power is generated by

hydroelectric (waterpower), diesel, and internal-combustion plants.

Transformers

Modern electric power systems use transformers to convert electricity into

different voltages. With transformers, each stage of the system can be

operated at an appropriate voltage. In a typical system, the generators at

the power station deliver a voltage of from 1,000 to 26,000 volts (V).

Transformers step this voltage up to values ranging from 138,000 to

765,000 V for the long-distance primary transmission line because higher

voltages can be transmitted more efficiently over long distances. At the

substation the voltage may be transformed down to levels of 69,000 to

138,000 V for further transfer on the distribution system. Another set of

transformers step the voltage down again to a distribution level such as

2,400 or 4,160 V or 15, 27, or 33 kilovolts (kV). Finally the voltage is

transformed once again at the distribution transformer near the point of use

to 240 or 120 V.

Transmission Lines

The lines of high-voltage transmission systems are usually composed of

wires of copper, aluminum, or copper-clad or aluminum-clad steel, which are

suspended from tall latticework towers of steel by strings of porcelain

insulators. By the use of clad steel wires and high towers, the distance

between towers can be increased, and the cost of the transmission line thus

reduced. In modern installations with essentially straight paths, high-voltage

lines may be built with as few as six towers to the kilometer. In some areas

high-voltage lines are suspended from tall wooden poles spaced more closely

together. For lower voltage distribution lines, wooden poles are generally

used rather than steel towers. In cities and other areas where open lines

create a safety hazard or are considered unattractive, insulated underground

cables are used for distribution. Some of these cables have a hollow core

through which oil circulates under low pressure. The oil provides temporary

protection from water damage to the enclosed wires should the cable

develop a leak. Pipe-type cables in which three cables are enclosed in a pipe

filled with oil under high pressure (14 kg per sq cm/200 psi) are frequently

used. These cables are used for transmission of current at voltages as high

as 345,000 V (or 345 kV).

Supplementary Equipment

Any electric-distribution system involves a large amount of supplementary

equipment to protect the generators, transformers, and the transmission

lines themselves. The system often includes devices designed to regulate the

voltage or other characteristics of power delivered to consumers.

To protect all elements of a power system from short circuits and overloads,

and for normal switching operations, circuit breakers are employed. These

breakers are large switches that are activated automatically in the event of a

short circuit or other condition that produces a sudden rise of current.

Because a current forms across the terminals of the circuit breaker at the

moment when the current is interrupted, some large breakers (such as those

used to protect a generator or a section of primary transmission line) are

immersed in a liquid that is a poor conductor of electricity, such as oil, to

quench the current. In large air-type circuit breakers, as well as in oil

breakers, magnetic fields are used to break up the current. Small air-circuit

breakers are used for protection in shops, factories, and in modern home

installations. In residential electric wiring, fuses were once commonly

employed for the same purpose. A fuse consists of a piece of alloy with a low

melting point, inserted in the circuit, which melts, breaking the circuit if the

current rises above a certain value. Most residences now use air-circuit

breakers.

Power Failures,Protection from outages and Restoration

Power Failures :

A power outage (Also power cut, power failure or power loss) is the loss of

the electricity supply to an area.

The reasons for a power failure can for instance be a defect in a power

station, damage to a power line or other part of the distribution system, a

short circuit, or the overloading of electricity mains. While the developed

countries enjoy a highly uninterrupted supply of electric power all the time,

many developing countries have acute power shortage as compared to the

demand. Countries such as Pakistan have several hours of daily power-cuts

in almost all cities and villages except the metropolitan cities and the state

capitals. Wealthier people in these countries may use a power-inverter or a

diesel-run electric generator at their homes during the power-cut.

A power outage may be referred to as a blackout if power is lost completely,

or as a brownout if the voltage level is below the normal minimum level

specified for the system, or sometimes referred to as a short circuit when

the loss of power occurs over a short time (usually seconds). Systems

supplied with three-phase electric power also suffer brownouts if one or

more phases are absent, at reduced voltage, or incorrectly phased. Such

malfunctions are particularly damaging to electric motors. Some brownouts,

called voltage reductions, are made intentionally to prevent a full power

outage. 'Load shedding' is a common term for a controlled way of rotating

available generation capacity between various districts or customers, thus

avoiding total wide area blackouts.

Power failures are particularly critical for hospitals, since many life-critical

medical devices and tasks require power. For this reason hospitals, just like

many enterprises (notably colocation facilities and other datacenters), have

emergency power generators which are typically powered by diesel fuel and

configured to start automatically, as soon as a power failure occurs. In most

third world countries, power cuts go unnoticed by most citizens of upscale

means, as maintaining an uninterruptible power supply is often considered

an essential facility of a home.

Power outage may also be the cause of sanitary sewer overflow, a condition

of discharging raw sewage into the environment. Other life-critical systems

such as telecommunications are also required to have emergency power.

Telephone exchange rooms usually have arrays of lead-acid batteries for

backup and also a socket for connecting a diesel generator during extended

periods of outage.

Power outages may also be caused by terrorism (attacking power plants or

electricity pylons) in developing countries. The Shining Path movement was

the first to copy this tactic from Mao Zedong.

Live Examples of breakdown in interconnected grid system

In most parts of the world, local or national electric utilities have joined in

grid systems. The linking grids allow electricity generated in one area to be

shared with others. Each utility that agrees to share gains an increased

reserve capacity, use of larger, more efficient generators, and the ability to

respond to local power failures by obtaining energy from a linking grid.

These interconnected grids are large, complex systems that contain

elements operated by different groups. These systems offer the opportunity

for economic savings and improve overall reliability but can create a risk of

widespread failure. For example, a major grid-system breakdown occurred

on November 9, 1965, in eastern North America, when an automatic control

device that regulates and directs current flow failed in Queenston, Ontario,

causing a circuit breaker to remain open. A surge of excess current was

transmitted through the northeastern United States. Generator safety

switches from Rochester, New York, to Boston, Massachusetts, were

automatically tripped, cutting generators out of the system to protect them

from damage. Power generated by more southerly plants rushed to fill the

vacuum and overloaded these plants, which automatically shut themselves

off. The power failure enveloped an area of more than 200,000 sq km

(80,000 sq mi), including the cities of Boston; Buffalo, New York; Rochester,

New York; and New York City.

Similar grid failures, usually on a smaller scale, have troubled systems in

North America and elsewhere. On July 13, 1977, about 9 million people in

the New York City area were once again without power when major

transmission lines failed. In some areas the outage lasted 25 hours as

restored high voltage burned out equipment. These major failures are

termed blackouts.

The worst blackout in the history of the United States and Canada occurred

August 14, 2003, when 61,800 megawatts of electrical power was lost in an

area covering 50 million people. (One megawatt of electricity is roughly the

amount needed to power 750 residential homes.) The blackout affected such

major cities as Cleveland, Detroit, New York, Ottawa, and Toronto. Parts of

eight states—Connecticut, Massachusetts, Michigan, New Jersey, New York,

Ohio, Pennsylvania, and Vermont—and the Canadian provinces of Ontario

and Québec were affected. The blackout prompted calls to replace aging

equipment and raised questions about the reliability of the national power

grid.

The term brownout is often used for partial shutdowns of power, usually

deliberate, either to save electricity or as a wartime security measure. From

November 2000 through May 2001 California experienced a series of

planned brownouts to groups of customers, for a limited duration, in order to

reduce total system load and avoid a blackout due to alleged electrical

shortages. However, an investigation by the California Public Utilities

Commission into the alleged shortages later revealed that five energy

companies withheld electricity they could have produced. In 2002 the

commission concluded that the withholding of electricity contributed to an

“unconscionable, unjust, and unreasonable electricity price spike.” California

state utilities paid $20 billion more for energy in 2000 than in 1999 as a

result, the head of the commission found.

The commission also cited the role of the Enron Corporation in the California

brownouts. In June 2003 the Federal Energy Regulatory Commission (FERC)

barred Enron from selling electricity and natural gas in the United States

after conducting a probe into charges that Enron manipulated electricity

prices during California’s energy crisis. In the same month the Federal

Bureau of Investigation arrested an Enron executive on charges of

manipulating the price of electricity in California. Two other Enron

employees, known as traders because they sold electricity, had pleaded

guilty to similar charges. See also Enron Scandal.

Despite the potential for rare widespread problems, the interconnected grid

system provides necessary backup and alternate paths for power flow,

resulting in much higher overall reliability than is possible with isolated

systems. National or regional grids can also cope with unexpected outages

such as those caused by storms, earthquakes, landslides, and forest fires, or

due to human error or deliberate acts of sabotage.

Protecting the power system from outages

In power supply networks, the power generation and the electrical load

(demand) must be very close to equal every second to avoid overloading of

network components, which can severely damage them. In order to prevent

this, parts of the system will automatically disconnect themselves from the

rest of the system, or shut themselves down to avoid damage. This is

analogous to the role of relays and fuses in households.

Under certain conditions, a network component shutting down can cause

current fluctuations in neighboring segments of the network, though this is

unlikely, leading to a cascading failure of a larger section of the network.

This may range from a building, to a block, to an entire city, to the entire

electrical grid.

Modern power systems are designed to be resistant to this sort of cascading

failure, but it may be unavoidable (see below). Moreover, since there is no

short-term economic benefit to preventing rare large-scale failures, some

observers have expressed concern that there is a tendency to erode the

resilience of the network over time, which is only corrected after a major

failure occurs. It has been claimed that reducing the likelihood of small

outages only increases the likelihood of larger ones. In that case, the short-

term economic benefit of keeping the individual customer happy increases

the likelihood of large-scale blackouts.

Power Analytics

Power Analytics is the term used to describe the management of electrical

power distribution, consumption, and preventative maintenance throughout

a large organization’s facilities, particularly organizations with high electrical

power requirements. For such facilities, electrical power problems – including

the worst-case scenario, a full power outage – could have a devastating

serious impact. Additionally, it could jeopardize the health and safety of

individuals within the facility or in the surrounding community.

Power Analytics use complex mathematical algorithms to detect variations

within an organization’s power infrastructure (measurements such as

voltage, current, power factor, etc.). Such variations could be early

indications of longer-term power problems; when a Power Analytics system

detects such variations, it will begin to diagnose the source of the variation,

surrounding components, and then the complete electrical power

infrastructure. Such systems will – after fully assessing the location and

potential magnitude of the problem – predict when and where the potential

problem will occur, as well as recommend the preventative maintenance

required preempting the problem from occurring.

Restoring power after a wide-area outage

Restoring power after a wide-area outage can be difficult, as power stations

need to be brought back on-line. Normally, this is done with the help of

power from the rest of the grid. In the absence of grid power, a so-called

black start needs to be performed to bootstrap the power grid into

operation.

Latest Power Outages,Causes and factors contributing to it

Latest Power Outages :

Electricity Blackout in Germany on November 4th 2006 -even France, Italy,

Spain and other countries were affected.

One of the worst and most dramatic power failures in three decades plunged

millions of Europeans into darkness over the weekend, halting trains,

trapping dozens in lifts and prompting calls for a central European power

authority. The blackout, which originated in north-western Germany, also

struck Paris and 15 French regions, and its effects were felt in Austria,

Belgium, Italy and Spain. In Germany, around 100 trains were delayed.

Additional Power Outages

09/24/2006 On September 24th afternoon 1.30pm Pakistan was hit by a

nationwide blackout. Millions of homes across Pakistan were left without

power for several hours. Power has been restored in capital Islamabad after

over a two-hour breakdown. The outage was caused due to a fault that

occurred during maintenance of a high-tension transmission line.

07/12/2006 Electricity Blackout in Auckland (New Zealand) - 700,000 people

without electricity for up to 10 hours. An earth wire, which snapped in high

winds, fell into Transpower's Otahuhu substation, damaging 110 kilovolt

supply lines. The cause - a simple metal shackle.

11/25/2005 Electricity Blackout in Münsterland - 250,000 people without

electricity for up to six days. Ice and storm had caused serious damage to

the network , leading to the blackout.

10/24/2005 -11/11/2005 Hurricane Wilma caused loss of power for most of

South Florida and Southwest Florida, with hundreds of thousands of

customers still powerless a week later, and full restoration not complete.

09/12/2005 A blackout in Los Angeles affected millions in California.

08/29/2005 Millions of Louisiana, Mississippi and Alabama residents lost

power after a stronger Hurricane Katrina badly damaged the power grid.

08/26/2005 On 1.3 Million People in South Florida lost power due to downed

trees and power lines caused by the then minimal Hurricane Katrina. Most

customers affected were without power for four days, and some customers

had no power for up to one week.

08/22/2005 All of southern and central Iraq, including parts of the capital

Baghdad, all of the second largest city Basra and the only port Umm Qasr

went out of power for more than 7 hours after a feeder line was sabotaged

by insurgents, causing a cascading effect shutting down multiple power

plants.

08/18/2005 Almost 100 million people on Java Island, the main island of

Indonesia which the capital Jakarta is on, and the isle of Bali, lost power for

7 hours. In terms of population affected, the 2005 Java-Bali Blackout was

the biggest in history.

05/25/2005 On most part of Moscow was without power from 11:00 MSK

(+0300 UTC). Approximately ten million people were affected. Power was

restored within 24 hours.

09/04/2004 On five million people in Florida were without power at one point

due to Hurricane Frances, one of the most widespread outages ever due to a

hurricane.

12/20/2003 Apower failure hit San Francisco, affecting 120,000 people.

09/27/2003- 09/28/2003 Italy blackout - a power failure affected all of Italy

except Sardinia, cutting service to more than 56 million people.

09/23/2003 A power failure affected 5 million people in Denmark and

southern Sweden.

09/02/2003 A power failure affected 5 states (out of 13) in Malaysia

(including the capital Kuala Lumpur) for 5 hours starting at 10 am local time.

08/28/2003 There was a 2003 London blackout on which won worldwide

headlines such as "Power cut cripples London" but in fact only affected

500,000 people.

Direct Causes and Contributing Factors to power outage:

• Failure to maintain adequate reactive power support

• Failure to ensure operation within secure limits

• Inadequate vegetation management

• Inadequate operator training

• Failure to identify emergency conditions and communicate that status

to neighboring systems

• Inadequate regional-scale visibility over the bulk power system.

Conclusions and Recommendations:

• Conductors contacting trees

• Ineffective visualization of power system conditions and lack of

situational awareness

• Ineffective communications

• Lack of training in recognizing and responding to emergencies

System Enhancement & Elimination of Bottlenecks

• Insufficient static and dynamic reactive power supply: FACTS

• Need to improve relay protection schemes and coordination

• On-Line Monitoring and Real-Time Security Assessment

• Increase of Reserve Capacity : HVDC / Generation

Electricity Power Blackout and Outage tips

Electricity Power Blackout and Outage tips :

• Assemble an emergency kit with:

(i) plenty of water (in general a minimum of 4 litres per person per

day is needed);Water can be partially supplemented with canned or

tetra pak juices.

(ii) ready-to-eat foods that do not need refridgeration.. Don't forget

the manually operated can opener;

(iii) flashlights;

(iv) portable radio;

(v) alkaline batteries, stored separately from electronic equipment

(such as radios) in case of battery leakage."Heavy duty batteries" are

not recommended for emergency use, as they have much less power

capability, a shorter shelf life and are much more prone to leaking.

(vi) money. Remember bank machines will not operate during a

blackout. You may want to keep a small amount of cash ready for this

situation.

• Place the emergency kit in a pre-designated location so that you can

find it in the dark.

• Do not use candles for lighting. Candles are in the top three causes of

household fires.

• Turn off all but one light or a radio so that you'll know when the power

returns.

• Check that the stove, ovens, electric kettles, irons, air conditioners

and (non-wall or ceiling mounted) lights are off. This can be serious

safety issues if you forget you have left some of these devices on.

Also by keeping them turned off will prevent heavy start-up loads

which could cause a second blackout when the utilities restart the

power.

• Turn off or unplug home electronics and computers to protect them

from damage when the electricity returns, in case of power surges.

• Listen to local radio and television for updated information. (The

reason for having a battery powered (ie. portable) radio.)

• Keep refrigerator and freezer doors closed. A full modern freezer will

stay frozen for up to 48 hours; partially full freezers for 24 hours.

Most food in the fridge will last 24 hours except dairy products, which

should be discarded after six hours. These estimates decrease each

time the refrigerator door is opened.

• Do not ration water (or juice). If you are thirsty you need the fluids. If

it is hot you need to drink plenty of fluids even if you do not feel

thirsty.

• Remember to provide plenty of fresh, cool water for your pets.

• Keep off the telephone unless it is an emergency, or for short periods

if it is for an important purpose such as checking up on your loved

ones, particularly people who have disabilities or infirmaties.

• In summer: open windows at opposing ends of a room to create a

cross breeze in the absence of air conditioning and electric fans.

• In summer: close blinds, curtains, drapes, windows and doors on the

sunny side of your home to block out the heat from the sun.

• In winter: open blinds, curtains and drapes during the day on the

sunny side of your home to let sunlight and its heat during the sunny

days, and close during the night. Otherwise keep them closed to keep

the heat in. You may also want to use window insulation kits or plastic

sheeting to add extra insulation to keep the heat in.

• In winter: make sure you have extra blankets. Also make sure you

have a bucket and a wet mop to soak up any water from frozen and

burst water pipes.

• While generally unnecessary and expensive, if you are using a gas-

powered generator, run it in a well-ventilated area and not in a closed

areas such as a room or garage. They can give off deadly carbon

monoxide fumes. And do not hook up the generator to your local

wiring, instead plug in the items you want or need into the generator.

For short-term use a much safer and cheaper alternative is an

Inverter with built-in battery.

• Do not use propane or other combustion-type heaters indoors due to

the probability of toxic carbon monoxide buildup.

Other notes:

• Water pressure may drop and even stop above a certain height in

high-rise buildings due to their water pumps losing power.

• Remember that electrical devices such as elevator will not work. You

can not predict when a blackout will strike to make a choice about

using elevators, but if a blackout does strike, check the elevators of

any of the building you are in to hear if there are people stuck; in

which case call up the fire department to get the people out.

• Electrically operated garage doors will not work. While landlords may

be able to hoist the heavy door up manually, some may not want to

do so for security purposes or because it volates the conditions of

their insurance policies.

Thermal Power Plant Layout and Operation

Thermal Power Plant Lay out :

The above diagram is the lay out of a simplified thermal power plant and the

below is also diagram of a thermal power plant.

The above diagram shows the simplest arrangement of Coal fired (Thermal)

power plant.

Main parts of the plant are

1. Coal conveyor 2. Stoker

preheater 7. Electrostatic precipitator

Condenser 11. Transformers

13. Generator 14. High

Basic Operation :A thermal power plant basically works on

Coal conveyor : This is a belt type of arrangement.With this coal is

transported from coal storage place in power plant to the place near by

boiler.

Stoker : The coal which is brought near by boiler has to put in boiler

furnance for combustion.This stoker is a mechanical device for feeding coal

to a furnace.


Main parts of the plant are

. Stoker 3. Pulverizer 4. Boiler 5. Coal ash

. Electrostatic precipitator 8. Smoke stack 9. Turbine

. Transformers 12. Cooling towers

. High - votge power lines

A thermal power plant basically works on

This is a belt type of arrangement.With this coal is


The coal which is brought near by boiler has to put in boiler

combustion.This stoker is a mechanical device for feeding coal


. Coal ash 6. Air

. Turbine 10.

A thermal power plant basically works on Rankine cycle.

This is a belt type of arrangement.With this coal is


The coal which is brought near by boiler has to put in boiler

combustion.This stoker is a mechanical device for feeding coal

Pulverizer : The coal is put in the boiler after pulverization.For this

pulverizer is used.A pulverizer is a device for grinding coal for combustion in

a furnace in a power plant.

Types of Pulverizers

Ball and Tube Mill

Ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to

three diameters in length, containing a charge of tumbling or cascading steel

balls, pebbles, or rods.

Tube mill is a revolving cylinder of up to five diameters in length used for

fine pulverization of ore, rock, and other such materials; the material, mixed

with water, is fed into the chamber from one end, and passes out the other

end as slime.

Ring and Ball

This type consists of two rings separated by a series of large balls. The lower

ring rotates, while the upper ring presses down on the balls via a set of

spring and adjuster assemblies. Coal is introduced into the center or side of

the pulverizer (depending on the design) and is ground as the lower ring

rotates causing the balls to orbit between the upper and lower rings. The

coal is carried out of the mill by the flow of air moving through it. The size of

the coal particals released from the grinding section of the mill is determined

by a classifer separator. These mills are typically produced by B&W (Babcock

and Wilcox).

Boiler : Now that pulverized coal is put in boiler furnance.Boiler is an

enclosed vessel in which water is heated and circulated until the water is

turned in to steam at the required pressure.

Coal is burned inside the combustion chamber of boiler.The products of

combustion are nothing but gases.These gases which are at high

temperature vaporize the water inside the boiler to steam.Some times this

steam is further heated in a superheater as higher the steam pressure and

temperature the greater efficiency the engine will have in converting the

heat in steam in to mechanical work. This steam at high pressure and

tempeture is used directly as a heating medium, or as the working fluid in a

prime mover to convert thermal energy to mechanical work, which in turn

may be converted to electrical energy. Although other fluids are sometimes

used for these purposes, water is by far the most common because of its

economy and suitable thermodynamic characteristics.

Classification of Boilers

Bolilers are classified as

Fire tube boilers : In fire tube boilers hot gases are passed through the

tubes and water surrounds these tubes. These are simple,compact and

rugged in construction.Depending on whether the tubes are vertical or

horizontal these are further classified as vertical and horizontal tube

boilers.In this since the water volume is more,circulation will be poor.So

they can't meet quickly the changes in steam dema

steam are not possible,maximum pressure that can be attained is about

17.5kg/sq cm.Due to large quantity of water in the drain it requires more

time for steam raising.The steam attained is generally wet,economical for

low pressures.The outut of the boiler is also limited.

Water tube boilers : In these boilers water is inside the tubes and hot gases



y and suitable thermodynamic characteristics.

Classification of Boilers

Bolilers are classified as

In fire tube boilers hot gases are passed through the


construction.Depending on whether the tubes are vertical or



they can't meet quickly the changes in steam demand.High pressures of




outut of the boiler is also limited.

In these boilers water is inside the tubes and hot gases



In fire tube boilers hot gases are passed through the


construction.Depending on whether the tubes are vertical or



nd.High pressures of




In these boilers water is inside the tubes and hot gases

are outside the tubes.They consists of drums and

tubes.They may contain any number of drums (you can see 2 drums in

fig).Feed water enters the boiler to one drum (here it is drum below the

boiler).This water circulates through the tubes connected external to

drums.Hot gases which surrounds these tubes wil convert the water in tubes

in to steam.This steam is passed through tubes and collecte

the drum since it is of light weight.So the drums store steam and water

(upper drum).The entire steam is collected in one drum and it is taken out

from there (see in laout fig).As the movement of water in the water tubes is

high, so rate of heat transfer also becomes high resulting in greater

efficiency.They produce high pressure , easily accessible and can respond

quickly to changes in steam demand.These are also classified as

vertical,horizontal and inclined tube depending on the arrangeme

tubes.These are of less weight and less liable to explosion.Large heating

surfaces can be obtained by use of large number of tubes.We can attain

pressure as high as 125 kg/sq cm and temperatures from 315 to 575

centigrade.

Superheater : Most of the modern boliers are having superheater and

reheater arrangement. Superheater is a component of a steam

unit in which steam, after it has left the boiler drum, is heated above its

saturation temperature. The amount of superheat added to the s

influenced by the location, arrangement, and amount of superheater surface

are outside the tubes.They consists of drums and


s the boiler to one drum (here it is drum below the



in to steam.This steam is passed through tubes and collecte




heat transfer also becomes high resulting in greater



vertical,horizontal and inclined tube depending on the arrangeme




the modern boliers are having superheater and

reheater arrangement. Superheater is a component of a steam


saturation temperature. The amount of superheat added to the s



s the boiler to one drum (here it is drum below the



in to steam.This steam is passed through tubes and collected at the top of




heat transfer also becomes high resulting in greater



vertical,horizontal and inclined tube depending on the arrangement of the




the modern boliers are having superheater and

reheater arrangement. Superheater is a component of a steam-generating


saturation temperature. The amount of superheat added to the steam is


installed, as well as the rating of the boiler. The superheater may consist of

one or more stages of tube banks arranged to effectively transfer heat from

the products of combustion.Superheaters are classified as convection ,

radiant or combination of these.

Reheater : Some of the heat of superheated steam is used to rotate the

turbine where it loses some of its energy.Reheater is also steam boiler

component in which heat is added to this intermediate-pressure steam,

which has given up some of its energy in expansion through the high-

pressure turbine. The steam after reheating is used to rotate the second

steam turbine (see Layout fig) where the heat is converted to mechanical

energy.This mechanical energy is used to run the alternator, which is

coupled to turbine , there by generating elecrical energy.

Condenser : Steam after rotating staem turbine comes to

condenser.Condenser refers here to the shell and tube heat exchanger (or

surface condenser) installed at the outlet of every steam turbine in Thermal

power stations of utility companies generally. These condensers are heat

exchangers which convert steam from its gaseous to its liquid state, also

known as phase transition. In so doing, the latent heat of steam is given out

inside the condenser. Where water is in short supply an air cooled condenser

is often used. An air cooled condenser is however significantly more

expensive and cannot achieve as low a steam turbine backpressure (and

therefore less efficient) as a surface condenser.

The purpose is to condense the outlet (or exhaust) steam from steam

turbine to obtain maximum efficiency and also to get the condensed steam

in the form of pure water, otherwise known as condensate, back to steam

generator or (boiler) as boiler feed water.

Why it is required ?

The steam turbine itself is a device to convert the heat in steam to

mechanical power. The difference between the heat of steam per unit weight

at the inlet to turbine and the heat of steam per unit weight at the outlet to

turbine represents the heat given out (or heat drop) in the steam turbine

which is converted to mechanical power. The heat drop per unit weight of

steam is also measured by the word enthalpy drop. Therefore the more the

conversion of heat per pound (or kilogram) of steam to mechanical power in

the turbine, the better is its performance or otherwise known as efficiency.

By condensing the exhaust steam of turbine, the exhaust pressure is

brought down below atmospheric pressure from above atmospheric

pressure, increasing the steam pressure drop between inlet and exhaust of

steam turbine. This further reduction in exhaust pressure gives out more

heat per unit weight of steam input to the steam turbine, for conversion to

mechanical power. Most of the heat liberated due to condensing, i.e., latent

heat of steam, is carried away by the cooling medium. (water inside tubes in

a surface condenser, or droplets in a spray condenser (Heller system) or air

around tubes in an air-cooled condenser).

Condensers are classified as (i) Jet condensers or contact condensers (ii)

Surface condensers.

In jet condensers the steam to be condensed mixes with the cooling water

and the temperature of the condensate and the cooling water is same when

leaving the condenser; and the condensate can't be recovered for use as

feed water to the boiler; heat transfer is by direct conduction.

In surface condensers there is no direct contact between the steam to be

condensed and the circulating cooling water. There is a wall interposed

between them through heat must be convectively transferred.The

temperature of the condensate may be higher than the temperature of the

cooling water at outlet and the condnsate is recovered as feed water to the

boiler.Both the cooling water and the condensate are separetely with

drawn.Because of this advantage surface condensers are used in thermal

power plants.Final output of condenser is water at low temperature is passed

to high pressure feed water heater,it is heated and again passed as feed

water to the boiler.Since we are passing water at high temperature as feed

water the temperature inside the boiler does not dcrease and boiler efficincy

also maintained.

Cooling Towers :The condensate (water) formed in the condeser after

condensation is initially at high temperature.This hot water is passed to

cooling towers.It is a tower- or building-like device in which atmospheric air

(the heat receiver) circulates in direct or indirect contact with warmer water

(the heat source) and the water is thereby cooled (see illustration). A cooling

tower may serve as the heat sink in a conventional thermodynamic process,

such as refrigeration or steam power generation, and when it is convenient

or desirable to make final heat rejection to atmospheric air. Water, acting as

the heat-transfer fluid, gives up heat to atmospheric air, and thus cooled, is

recirculated through the system, affording economical operation of the

process.

Two basic types of cooling towers are commonly used. One transfers the

heat from warmer water to cooler air mainly by an evaporation heat-transfer

process and is known as the evaporative or wet cooling tower.

Evaporative cooling towers are classified according to the means employed

for producing air circulation through them: atmospheric, natural draft, and

mechanical draft. The other transfers the heat from warmer water to cooler

air by a sensible heat-transfer process and is known as the nonevaporative

or dry cooling tower.

Nonevaporative cooling towers are classified as air-cooled condensers and as

air-cooled heat exchangers, and are further classified by the means used for

producing air circulation through them. These two basic types are sometimes

combined, with the two cooling processes generally used in parallel or

separately, and are then known as wet-dry cooling towers.

Evaluation of cooling tower performance is based on cooling of a specified

quantity of water through a given range and to a specified temperature

approach to the wet-bulb or dry-bulb temperature for which the tower is

designed. Because exact design conditions are rarely experienced in

operation, estimated performance curves are frequently prepared for a

specific installation, and provide a means for comparing the measured

performance with design conditions.

Economiser : Flue gases coming out of the boiler carry lot of heat.Function

of economiser is to recover some of the heat from the heat carried away in

the flue gases up the chimney and utilize for heating the feed water to the

boiler.It is placed in the passage of flue gases in between the exit from the

boiler and the entry to the chimney.The use of economiser results in saving

in coal consumption , increase in steaming rate and high boiler efficiency but

needs extra investment and increase in maintenance costs and floor area

required for the plant.This is used in all modern plants.In this a large

number of small diameter thin walled tubes are placed between two

headers.Feed water enters the tube through one header and leaves through

the other.The flue gases flow out side the tubes usually in counter flow.

Air preheater : The remaining heat of flue gases is utilised by air

preheater.It is a device used in steam boilers to transfer heat from the flue

gases to the combustion air before the air enters the furnace. Also known as

air heater; air-heating system. It is not shown in the lay out.But it is kept at

a place near by where the air enters in to the boiler.

The purpose of the air preheater is to recover the heat from the flue gas

from the boiler to improve boiler efficiency by burning warm air which

increases combustion efficiency, and reducing useful heat lost from the flue.

As a consequence, the gases are also sent to the chimney or stack at a lower

temperature, allowing simplified design of the ducting and stack. It also

allows control over the temperature of gases leaving the stack (to meet

emissions regulations, for example).After extracting heat flue gases are

passed to elctrostatic precipitator.

Electrostatic precipitator : It is a device which removes dust or other

finely divided particles from flue gases by charging the particles inductively

with an electric field, then attracting them to highly charged collector plates.

Also known as precipitator. The process depends on two steps. In the first

step the suspension passes through an electric discharge (corona discharge)

area where ionization of the gas occurs. The ions produced collide with the

suspended particles and confer on them an electric charge. The charged

particles drift toward an electrode of opposite sign and are deposited on the

electrode where their electric charge is neutralized. The phenomenon would

be more correctly designated as electrodeposition from the gas phase.

The use of electrostatic precipitators has become common in numerous

industrial applications. Among the advantages of the electrostatic

precipitator are its ability to handle large volumes of gas, at elevated

temperatures if necessary, with a reasonably small pressure drop, and the

removal of particles in the micrometer range. Some of the usual applications

are: (1) removal of dirt from flue gases in steam plants; (2) cleaning of air

to remove fungi and bacteria in establishments producing antibiotics and

other drugs, and in operating rooms; (3) cleaning of air in ventilation and air

conditioning systems; (4) removal of oil mists in machine shops and acid

mists in chemical process plants; (5) cleaning of blast furnace gases; (6)

recovery of valuable materials such as oxides of copper, lead, and tin; and

(7) separation of rutile from zirconium sand.

Smoke stack :A chimney is a system for venting hot flue gases or smoke

from a boiler, stove, furnace or fireplace to the outside atmosphere. They

are typically almost vertical to ensure that the hot gases flow smoothly,

drawing air into the combustion through the chimney effect (also known as

the stack effect). The space inside a chimney is called a flue. Chimneys may

be found in buildings, steam locomotives and ships. In the US, the term

smokestack (colloquially, stack) is also used when referring to locomotive

chimneys. The term funnel is generally used for ship chimneys and

sometimes used to refer to locomotive chimneys.Chimneys are tall to

increase their draw of air for combustion and to disperse pollutants in the

flue gases over a greater area so as to reduce the pollutant concentrations in

compliance with regulatory or other limits.

Generator : An alternator is an electromechanical device that converts

mechanical energy to alternating current electrical energy. Most alternators

use a rotating magnetic field. Different geometries - such as a linear

alternator for use with stirling engines - are also occasionally used. In

principle, any AC generator can be called an alternator, but usually the word

refers to small rotating machines driven by automotive and other internal

combustion engines.

Transformers :It is a device that transfers electric energy from one

alternating-current circuit to one or more other circuits, either increasing

(stepping up) or reducing (stepping down) the voltage. Uses for

transformers include reducing the line voltage to operate low-voltage

devices (doorbells or toy electric trains) and raising the voltage from electric

generators so that electric power can be transmitted over long distances.

Transformers act through electromagnetic induction; current in the primary

coil induces current in the secondary coil. The secondary voltage is

calculated by multiplying the primary voltage by the ratio of the number of

turns in the secondary coil to that in the primary.

Boiling Water Reactor (BWR) - Advantages and Disadvantages

Boiling Water Reactor (BWR) A boiling water reactor (BWR) is a type of light-water nuclear reactor

developed by the General Electric Company in the mid 1950s.

1.Reactor pressure vessel 2.Fuel rods 3. Control rod 4.Circulating pump

5.Control rod drive 6.Fresh steam 7. Feedwater 8.High pressure turbine

9.Low pressure turbine 10.Generator 11.Exciter 12.Condenser 13.Cooling

water 14.Preheater 15.Feedwater pump 16. Cooling water pump

17.Concrete shield

The above diagram shows BWR and its main parts.The BWR is characterized

by two-phase fluid flow (water and steam) in the upper part of the reactor

core. Light water (i.e., common distilled water) is the working fluid used to

conduct heat away from the nuclear fuel. The water around the fuel

elements also "thermalizes" neutrons, i.e., reduces their kinetic energy,

which is necessary to improve the probability of fission of fissile fuel. Fissile

fuel material, such as the U-235 and Pu-239 isotopes, have large capture

cross sections for thermal neutrons.

In a boling water reactor, light water (H2O) plays the role of moderator and

coolant, as well. In this case the steam is generted in the reactor it self.As

you can see in the diagrm feed water enters the reactor pressure vessel at

the bottom and takes up the heat generated due to fission of fuel (fuel rods)

and gets converted in to steam.

Part of the water boils away in the reactor pressure vessel, thus a mixture of

water and steam leaves the reactor core. The so generated steam directly

goes to the turbine, therefore steam and moisture must be separated (water

drops in steam can damage the turbine blades). Steam leaving the turbine is

condensed in the condenser and then fed back to the reactor after

preheating. Water that has not evaporated in the reactor vessel accumulates

at the bottom of the vessel and mixes with the pumped back feedwater.

Since boiling in the reactor is allowed, the pressure is lower than that of the

PWRs: it is about 60 to 70 bars. The fuel is usually uranium dioxide.

Enrichment of the fresh fuel is normally somewhat lower than that in a PWR.

The advantage of this type is that - since this type has the simplest

construction - the building costs are comparatively low. 22.5% of the total

power of presently operating nuclear power plants is given by BWRs.

Feedwater

Inside of a BWR reactor pressure vessel (RPV), feedwater enters through

nozzles high on the vessel, well above the top of the nuclear fuel assemblies

(these nuclear fuel assemblies constitute the "core") but below the water

level. The feedwater is pumped into the RPV from the condensers located

underneath the low pressure turbines and after going through feedwater

heaters that raise its temperature using extraction steam from various

turbine stages.

The feedwater enters into the downcomer region and combines with water

exiting the water separators. The feedwater subcools the saturated water

from the steam separators. This water now flows down the downcomer

region, which is separated from the core by a tall shroud. The water then

goes through either jet pumps or internal recirculation pumps that provide

additional pumping power (hydraulic head). The water now makes a 180

degree turn and moves up through the lower core plate into the nuclear core

where the fuel elements heat the water. When the flow moves out of the

core through the upper core plate, about 12 to 15% of the flow by volume is

saturated steam.

The heating from the core creates a thermal head that assists the

recirculation pumps in recirculating the water inside of the RPV. A BWR can

be designed with no recirculation pumps and rely entirely on the thermal

head to recirculate the water inside of the RPV. The forced recirculation head

from the recirculation pumps is very useful in controlling power, however.

The thermal power level is easily varied by simply increasing or decreasing

the speed of the recirculation pumps.

The two phase fluid (water and steam) above the core enters the riser area,

which is the upper region contained inside of the shroud. The height of this

region may be increased to increase the thermal natural recirculation

pumping head. At the top of the riser area is the water separator. By

swirling the two phase flow in cyclone separators, the steam is separated

and rises upwards towards the steam dryer while the water remains behind

and flows horizontally out into the downcomer region. In the downcomer

region, it combines with the feedwater flow and the cycle repeats.

The saturated steam that rises above the separator is dried by a chevron

dryer structure. The steam then exists the RPV through four main steam

lines and goes to the turbine.

Control systems

Reactor power is controlled via two methods: by inserting or withdrawing

control rods and by changing the water flow through the reactor core.

Positioning (withdrawing or inserting) control rods is the normal method for

controlling power when starting up a BWR. As control rods are withdrawn,

neutron absorption decreases in the control material and increases in the

fuel, so reactor power increases. As control rods are inserted, neutron

absorption increases in the control material and decreases in the fuel, so

reactor power decreases. Some early BWRs and the proposed ESBWR

designs use only natural ciculation with control rod positioning to control

power from zero to 100% because they do not have reactor recirculation

systems.

Changing (increasing or decreasing) the flow of water through the core is

the normal and convenient method for controlling power. When operating on

the so-called "100% rod line," power may be varied from approximately

70% to 100% of rated power by changing the reactor recirculation system

flow by varying the speed of the recirculation pumps. As flow of water

through the core is increased, steam bubbles ("voids") are more quickly

removed from the core, the amount of liquid water in the core increases,

neutron moderation increases, more neutrons are slowed down to be

absorbed by the fuel, and reactor power increases. As flow of water through

the core is decreased, steam voids remain longer in the core, the amount of

liquid water in the core decreases, neutron moderation decreases, fewer

neutrons are slowed down to be absorbed by the fuel, and reactor power

decreases.

Steam Turbines

Steam produced in the reactor core passes through steam separators and

dryer plates above the core and then directly to the turbine, which is part of

the reactor circuit. Because the water around the core of a reactor is always

contaminated with traces of radionuclides, the turbine must be shielded

during normal operation, and radiological protection must be provided during

maintenance. The increased cost related to operation and maintenance of a

BWR tends to balance the savings due to the simpler design and greater

thermal efficiency of a BWR when compared with a PWR. Most of the

radioactivity in the water is very short-lived (mostly N-16, with a 7 second

half life), so the turbine hall can be entered soon after the reactor is shut

down.

Safety

Like the pressurized water reactor, the BWR reactor core continues to

produce heat from radioactive decay after the fission reactions have

stopped, making nuclear meltdown possible in the event that all safety

systems have failed and the core does not receive coolant. Also like the

pressurized water reactor, a boiling-water reactor has a negative void

coefficient, that is, the thermal output decreases as the proportion of steam

to liquid water increases inside the reactor. However, unlike a pressurized

water reactor which contains no steam in the reactor core, a sudden

increase in BWR steam pressure (caused, for example, by a blockage of

steam flow from the reactor) will result in a sudden decrease in the

proportion of steam to liquid water inside the reactor. The increased ratio of

water to steam will lead to increased neutron moderation, which in turn will

cause an increase in the power output of the reactor. Because of this effect

in BWRs, operating components and safety systems are designed to ensure

that no credible, postulated failure can cause a pressure and power increase

that exceeds the safety systems' capability to quickly shutdown the reactor

before damage to the fuel or to components containing the reactor coolant

can occur.

In the event of an emergency that disables all of the safety systems, each

reactor is surrounded by a containment building designed to seal off the

reactor from the environment.

Comparison with other reactors

Light water is ordinary water. In comparison, some other water

reactor types use heavy water. In heavy water, the deuterium isotope of

hydrogen replaces the common hydrogen atoms in the water molecules

(D2O instead of H2O, molecular weight 20 instea

The Pressurized Water Reactor (PWR) was the first type of light

reactor developed because of its application to submarine propulsion. The

civilian motivation for the BWR is reducing costs for commercial applications

through design simplification and lower pressure components. In naval

reactors, BWR designs are used when natural circulation is specified for its

quietness. The description of BWRs below describes civilian reactor plants in

which the same water used for reactor cooling is also

cycle turbine generators. A Naval BWR is designed like a PWR that has both

primary and secondary loops.

In contrast to the pressurized water reactors that utilize a primary and

secondary loop, in civilian BWRs the steam going to the tu

the electrical generator is produced in the reactor core rather than in steam

generators or heat exchangers. There is just a single circuit in a civilian BWR

in which the water is at lower pressure (about 75 times atmospheric

pressure) compared to a PWR so that it boils in the core at about 285°C. The

reactor is designed to operate with steam comprising 12

Comparison with other reactors

Light water is ordinary water. In comparison, some other water



(D2O instead of H2O, molecular weight 20 instead of 18).

The Pressurized Water Reactor (PWR) was the first type of light



cation and lower pressure components. In naval



which the same water used for reactor cooling is also used in the Rankine


primary and secondary loops.


secondary loop, in civilian BWRs the steam going to the turbine that powers




mpared to a PWR so that it boils in the core at about 285°C. The

reactor is designed to operate with steam comprising 12–15% of the volume

Light water is ordinary water. In comparison, some other water-cooled



The Pressurized Water Reactor (PWR) was the first type of light-water



cation and lower pressure components. In naval



used in the Rankine



rbine that powers




mpared to a PWR so that it boils in the core at about 285°C. The

15% of the volume

of the two-phase coolant flow (the "void fraction") in the top part of the

core, resulting in less moderation, lower neutron efficiency and lower power

density than in the bottom part of the core. In comparison, there is no

significant boiling allowed in a PWR because of the high pressure maintained

in its primary loop (about 158 times atmospheric pressure).

Advantages

• The reactor vessel and associated components operate at a

substantially lower pressure (about 75 times atmospheric pressure)

compared to a PWR (about 158 times atmospheric pressure).

• Pressure vessel is subject to significantly less irradiation compared to a

PWR, and so does not become as brittle with age.

• Operates at a lower nuclear fuel temperature.

• Fewer components due to no steam generators and no pressurizer

vessel. (Older BWRs have external recirculation loops, but even this

piping is eliminated in modern BWRs, such as the ABWR.)

• Lower risk (probability) of a rupture causing loss of coolant compared

to a PWR, and lower risk of a severe accident should such a rupture

occur. This is due to fewer pipes, fewer large diameter pipes, fewer

welds and no steam generator tubes.

• Measuring the water level in the pressure vessel is the same for both

normal and emergency operations, which results in easy and intuitive

assessment of emergency conditions.

• Can operate at lower core power density levels using natural

circulation without forced flow.

• A BWR may be designed to operate using only natural circulation so

that recirculation pumps are eliminated entirely. (The new ESBWR

design uses natural circulation.)

Disadvantages

• Complex operational calculations for managing the utilization of the

nuclear fuel in the fuel elements during power production due to "two

phase fluid flow" (water and steam) in the upper part of the core (less

of a factor with modern computers). More incore nuclear

instrumentation is required.

• Much larger pressure vessel than for a PWR of similar power, with

correspondingly higher cost. (However, the overall cost is reduced

because a modern BWR has no main steam generators and associated

piping.)

• Contamination of the turbine by fission products.

• Shielding and access control around the steam turbine are required

during normal operations due to the radiation levels arising from the

steam entering directly from the reactor core. Additional precautions

are required during turbine maintenance activities compared to a

PWR.

• Control rods are inserted from below for current BWR designs. There

are two available hydraulic power sources that can drive the control

rods into the core for a BWR under emergency conditions. There is a

dedicated high pressure hydraulic accumulator and also the pressure

inside of the reactor pressure vessel available to each control rod.

Either the dedicated accumulator (one per rod) or reactor pressure is

capable of fully inserting each rod. Most other reactor types use top

entry control rods that are held up in the withdrawn position by

electromagnets, causing them to fall into the reactor by gravity if

power is lost.

Classification of Nuclear Reactors

Classification of Nuclear Reactors Nuclear Reactors, specifically fission reacors, are classified by several

methods, a brief outline of these classification schemes is given below.

Classification by use

Research reactors : Typically reactors used for research and training,

materials testing, or the production of radioisotopes for medicine and

industry. These are much smaller than power reactors or those propelling

ships, and many are on university campuses. There are about 280 such

reactors operating, in 56 countries. Some operate with high-enriched

uranium fuel, and international efforts are underway to substitute low-

enriched fuel.

Production reactors

Power reactors

Propulsion reactors

Classification by moderator material

Graphite moderated reactors

water moderated reactors

• Light water moderated reactors (LWRs)

• Heavy Water moderated reactors

Classification by coolant

Gas cooled reactor

Liquid metal cooled reactor

Water cooled reactor

• Pressure water reactor

• Boiling water reactor

Classification by type of nuclear reaction

Fast Reactors

Thermal reactors

Classification by role in the fuel cycle

Breeder reactors

burner reactors

Classification by Generation

Generation II reactor

Generation III reactor

Generation IV reactor

Classification by phase of fuel

Solid fueled

Fluid fueled

Gas Fueled

The Nuclear Fuel Cycle

The Nuclear Fuel Cycle

• The nuclear fuel cycle is the series of industrial processes which

involve the production of electricity from uranium in nuclear power

reactors.

• Uranium is a relatively common element that is found throughout the

world. It is mined in a number of countries and must be processed

before it can be used as fuel for a nuclear reactor.

• Electricity is created by using the heat generated in a nuclear reactor

to produce steam and drive a turbine connected to a generator.

• Fuel removed from a reactor, after it has reached the end of its useful

life, can be reprocessed to produce new fuel.

The various activities associated with the production of electricity from

nuclear reactions are referred to collectively as the nuclear fuel cycle. The

nuclear fuel cycle starts with the mining of uranium and ends with the

disposal of nuclear waste. With the reprocessing of used fuel as an option for

nuclear energy, the stages form a true cycle.

Uranium

Uranium is a slightly radioactive metal that occurs throughout the earth's

crust. It is about 500 times more abundant than gold and about as common

as tin. It is present in most rocks and soils as well as in many rivers and in

sea water. It is, for example, found in concentrations of about four parts per

million (ppm) in granite, which makes up 60% of the earth's crust. In

fertilisers, uranium concentration can be as high as 400 ppm (0.04%), and

some coal deposits contain uranium at concentrations greater than 100 ppm

(0.01%). Most of the radioactivity associated with uranium in nature is in

fact due to other minerals derived from it by radioactive decay processes,

and which are left behind in mining and milling.

There are a number of areas around the world where the concentration of

uranium in the ground is sufficiently high that extraction of it for use as

nuclear fuel is economically feasible. Such concentrations are called ore.The

below figure represents various stages in

Uranium Mining

Both excavation and in situ techn

Excavation may be underground and open pit mining.

In general, open pit mining is used where deposits are close to the surface

and underground mining is used for deep deposits, typically greater than

120 m deep. Open pit mines require large holes on the surface, larger than

the size of the ore deposit, since the walls of the pit must be sloped to

prevent collapse. As a result, the quantity of material that must be removed

in order to access the ore may be large. Under

small surface disturbance and the quantity of material that must be removed

to access the ore is considerably less than in the case of an open pit mine.

An increasing proportion of the world's uranium now comes from in situ

leaching (ISL), where oxygenated groundwater is circulated through a very

porous orebody to dissolve the uranium and bring it to the surface. ISL may


the ground is sufficiently high that extraction of it for use as


below figure represents various stages in Nuclear Fuel cycle

Both excavation and in situ techniques are used to recover uranium ore.

Excavation may be underground and open pit mining.



pit mines require large holes on the surface, larger than



in order to access the ore may be large. Underground mines have relatively




aching (ISL), where oxygenated groundwater is circulated through a very



the ground is sufficiently high that extraction of it for use as


Nuclear Fuel cycle

iques are used to recover uranium ore.



pit mines require large holes on the surface, larger than



ground mines have relatively




aching (ISL), where oxygenated groundwater is circulated through a very


be with slightly acid or with alkaline solutions to keep the uranium in

solution. The uranium is then recovered from the solution as in a

conventional mill.

The decision as to which mining method to use for a particular deposit is

governed by the nature of the orebody, safety and economic considerations.

In the case of underground uranium mines, special precautions, consisting

primarily of increased ventilation, are required to protect against airborne

radiation exposure.

Uranium Milling

Milling, which is generally carried out close to a uranium mine, extracts the

uranium from the ore. Most mining facilities include a mill, although where

mines are close together, one mill may process the ore from several mines.

Milling produces a uranium oxide concentrate which is shipped from the mill.

It is sometimes referred to as 'yellowcake' and generally contains more than

80% uranium. The original ore may contains as little as 0.1% uranium.

In a mill, uranium is extracted from the crushed and ground-up ore by

leaching, in which either a strong acid or a strong alkaline solution is used to

dissolve the uranium. The uranium is then removed from this solution and

precipitated. After drying and usually heating it is packed in 200-litre drums

as a concentrate.

The remainder of the ore, containing most of the radioactivity and nearly all

the rock material, becomes tailings, which are emplaced in engineered

facilities near the mine (often in mined out pit). Tailings contain long-lived

radioactive materials in low concentrations and toxic materials such as heavy

metals; however, the total quantity of radioactive elements is less than in

the original ore, and their collective radioactivity will be much shorter-lived.

These materials need to be isolated from the environment.

Conversion

The product of a uranium mill is not directly usable as a fuel for a nuclear

reactor. Additional processing, generally referred to as enrichment, is

required for most kinds of reactors. This process requires uranium to be in

gaseous form and the way this is achieved is to convert it to uranium

hexafluoride, which is a gas at relatively low temperatures.

At a conversion facility, uranium is first refined to uranium dioxide, which

can be used as the fuel for those types of reactors that do not require

enriched uranium. Most is then converted into uranium hexafluoride, ready

for the enrichment plant. It is shipped in strong metal containers. The main

hazard of this stage of the fuel cycle is the use of hydrogen fluoride.

Enrichment

Natural uranium consists, primarily, of a mixture of two isotopes (atomic

forms) of uranium. Only 0.7% of natural uranium is "fissile", or capable of

undergoing fission, the process by which energy is produced in a nuclear

reactor. The fissile isotope of uranium is uranium 235 (U-235). The

remainder is uranium 238 (U-238).

In the most common types of nuclear reactors, a higher than natural

concentration of U-235 is required. The enrichment process produces this

higher concentration, typically between 3.5% and 5% U-235, by removing

over 85% of the U-238. This is done by separating gaseous uranium

hexafluoride into two streams, one being enriched to the required level and

known as low-enriched uranium. The other stream is progressively depleted

in U-235 and is called 'tails'.

There are two enrichment processes in large scale commercial use, each of

which uses uranium hexafluoride as feed: gaseous diffusion and gas

centrifuge. They both use the physical properties of molecules, specifically

the 1% mass difference, to separate the isotopes. The product of this stage

of the nuclear fuel cycle is enriched uranium hexafluoride, which is

reconverted to produce enriched uranium oxide.

Fuel fabrication

Reactor fuel is generally in the form of ceramic pellets. These are formed

from pressed uranium oxide which is sintered (baked) at a high temperature

(over 1400°C). The pellets are then encased in metal tubes to form fuel

rods, which are arranged into a fuel assembly ready for introduction into a

reactor. The dimensions of the fuel pellets and other components of the fuel

assembly are precisely controlled to ensure consistency in the characteristics

of fuel bundles.

In a fuel fabrication plant great care is taken with the size and shape of

processing vessels to avoid criticality (a limited chain reaction releasing

radiation). With low-enriched fuel criticality is most unlikely, but in plants

handling special fuels for research reactors this is a vital consideration.

Power generation

Inside a nuclear reactor the nuclei of U-235 atoms split (fission) and, in the

process, release energy. This energy is used to heat water and turn it into

steam. The steam is used to drive a turbine connected to a generator which

produces electricity. Some of the U-238 in the fuel is turned into plutonium

in the reactor core. The main plutonium isotope is also fissile and it yields

about one third of the energy in a typical nuclear reactor. The fissioning of

uranium is used as a source of heat in a nuclear power station in the same

way that the burning of coal, gas or oil is used as a source of heat in a fossil

fuel power plant.

As with as a coal-fired power station about two thirds of the heat is dumped,

either to a large volume of water (from the sea or large river, heating it a

few degrees) or to a relatively smaller volume of water in cooling towers,

using evaporative cooling (latent heat of vapourisation).

Used fuel

With time, the concentration of fission fragments and heavy elements

formed in the same way as plutonium in a fuel bundle will increase to the

point where it is no longer practical to continue to use the fuel. So after 12-

24 months the 'spent fuel' is removed from the reactor. The amount of

energy that is produced from a fuel bundle varies with the type of reactor

and the policy of the reactor operator.

Typically, some 36 million kilowatt-hours of electricity are produced from

one tonne of natural uranium. The production of this amount of electrical

power from fossil fuels would require the burning of over 20,000 tonnes of

black coal or 8.5 million cubic metres of gas.

Used fuel storage

When removed from a reactor, a fuel bundle will be emitting both radiation,

principally from the fission fragments, and heat. Used fuel is unloaded into a

storage pond immediately adjacent to the reactor to allow the radiation

levels to decrease. In the ponds the water shields the radiation and absorbs

the heat. Used fuel is held in such pools for several months to several years.

Depending on policies in particular countries, some used fuel may be

transferred to central storage facilities. Ultimately, used fuel must either be

reprocessed or prepared for permanent disposal.

Reprocessing

Used fuel is about 95% U-238 but it also contains about 1% U-235 that has

not fissioned, about 1% plutonium and 3% fission products, which are highly

radioactive, with other transuranic elements formed in the reactor. In a

reprocessing facility the used fuel is separated into its three components:

uranium, plutonium and waste, containing fission products. Reprocessing

enables recycling of the uranium and plutonium into fresh fuel, and produces

a significantly reduced amount of waste (compared with treating all used

fuel as waste).

Uranium and Plutonium Recycling

The uranium from reprocessing, which typically contains a slightly higher

concentration of U-235 than occurs in nature, can be reused as fuel after

conversion and enrichment, if necessary. The plutonium can be directly

made into mixed oxide (MOX) fuel, in which uranium and plutonium oxides

are combined.

In reactors that use MOX fuel, plutonium substitutes for the U-235 in normal

uranium oxide fuel.

Used fuel disposal

At the present time, there are no disposal facilities (as opposed to storage

facilities) in operation in which used fuel, not destined for reprocessing, and

the waste from reprocessing can be placed. Although technical issues related

to disposal have been addressed, there is currently no pressing technical

need to establish such facilities, as the total volume of such wastes is

relatively small. Further, the longer it is stored the easier it is to handle, due

to the progressive diminution of radioactivity. There is also a reluctance to

dispose of used fuel because it represents a significant energy resource

which could be reprocessed at a later date to allow recycling of the uranium

and plutonium. (There is a proposal to use it in Candu reactors directly as

fuel.)

A number of countries are carrying out studies to determine the optimum

approach to the disposal of spent fuel and wastes from reprocessing. The

general consensus favours its placement into deep geological repositories,

initially recoverable.

Wastes

Wastes from the nuclear fuel cycle are categorised as high-, medium- or

low-level wastes by the amount of radiation that they emit. These wastes

come from a number of sources and include:

• low-level waste produced at all stages of the fuel cycle;

• intermediate-level waste produced during reactor operation and by

reprocessing;

• high-level waste, which is waste containing fission products from

reprocessing, and in many countries, the used fuel itself.

The enrichment process leads to the production of much 'depleted' uranium,

in which the concentration of U-235 is significantly less than the 0.7% found

in nature. Small quantities of this material, which is primarily U-238, are

used in applications where high density material is required, including

radiation shielding and some is used in the production of MOX fuel. While U-

238 is not fissile it is a low specific activity radioactive material and some

precautions must, therefore, be taken in its storage or disposal.

Nuclear Energy,Nuclear Fuels

Nuclear Energy Nuclei are made up of protons and neutron, but the mass of a nucleus is

always less than the sum of the individual masses of the protons and

neutrons which constitute it. The difference is a measure of the nuclear

binding energy which holds the nucleus together.

Nuclear energy is energy released from the atomic nucleus. Atoms are tiny

particles that make up every object in the universe. There is enormous

energy in the bonds that hold atoms together.This binding energy can be

calculated from the Einstein relationship: mass-energy equivalence formula

E = mc², in which E = energy, m = mass, and c = the speed of light in a

vacuum (a physical constant).The alpha particle gives binding energy of 28.3

MeV

Nuclear energy is released by several processes:

• Radioactive decay, where a radioactive nucleus decays spontaneously

into a lighter nucleus by emitting a particle;

• Endothermic nuclear reactions where two nuclei merge to produce two

different nuclei. The following two processes are particular examples:

• Fusion, two atomic nuclei fuse together to form a heavier nucleus;

• Fission, the breaking of a heavy nucleus into two nearly equal parts.

Nuclear Fuels

Nuclear fuel is any material that can be consumed to derive nuclear energy,

by analogy to chemical fuel that is burned to derive energy. By far the most

common type of nuclear fuel is heavy fissile elements that can be made to

undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear

fuel can refer to the material or to physical objects (for example fuel bundles

composed of fuel rods) composed of the fuel material, perhaps mixed with

structural, neutron moderating, or neutron reflecting materials.

Not all nuclear fuels are used in fission chain reactions. For example, 238Pu

and some other elements are used to produce small amounts of nuclear

power by radioactive decay in radiothermal generators, and other atomic

batteries. Light isotopes such as 3H (tritium) are used as fuel for nuclear

fusion. If one looks at binding energy of specific isotopes, there can be an

energy gain from fusing most elements with a lower atomic number than

iron, and fissioning isotopes with a higher atomic number than iron.

The most common fissile nuclear fuels are natural urnium,enriched

uranium,plutonium and 233U.Natural uranium is the parent material.The

materials 235U,233U and 239Pu are called fissionable materials.The only

fissionable nuclear fuel occuring in nature is uraium of which 99.3% is 238U

and 0.7% is 235U and 234U is only a trace.Out of these isotopes only 235U

will fission in a chain reaction.The other two fissionable materials can be

produced artificially from 238U and 232Th which occur in nature are called

fertile materials.Out of the three fissionable materials 235U has some

advantages over the other two due to its higher fission

percentage.Fissionable materials 239Pu and 233U are formed in the nuclear

reactors during fission process from 238U and 232Th respectively due to

absorption of neutrons with out fission.Getting 239Pu process is called

conversion and getting 233U is called breeding.

Nuclear Fission

Nuclear Fission

Nuclear fission—also known as atomic fission—is a process in nuclear physics

and nuclear chemistry in which the nucleus of an atom splits into two or

more smaller nuclei as fission products, and usually some by-product

particles, Hence, fission is a form of elemental transmutation. The by-

products include free neutrons, photons usually in the form gamma rays,

and other nuclear fragments such as beta particles and alpha particles.

Fission of heavy elements is an exothermic reaction and can release

substantial amounts of useful energy both as gamma rays and as kinetic

energy of the fragments (heating the bulk material where fission takes

place).

Nuclear fission produces energy for nuclear power and to drive explosion of

nuclear weapons. Fission is useful as a power source because some

materials, called nuclear fuels, generate neutrons as part of the fission

process and undergo triggered fission when impacted by a free neutron.

Nuclear fuels can be part of a self-sustaining chain reaction that releases

energy at a controlled rate in a nuclear reactor or at a very rapid

uncontrolled rate in a nuclear weapon.

The amount of free energy contained in nuclear fuel is millions of times the

amount of free energy contained in a similar mass of chemical fuel such as

gasoline, making nuclear fission a very tempting source of energy; however,

the byproducts of nuclear fission are highly radioactive and remain so for

millennia, giving rise to a nuclear waste problem.

Splitting the Uranium Atom:

Uranium is the principle element used in nuclear reactors and in certain

types of atomic bombs. The specific isotope used is 235U. When a stray

neutron strikes a 235U nucleus, it is at first absorbed into it. This creates

236U. 236U is unstable and this causes the atom to fission. The fissioning of

236U can produce over twenty different products. However, the products'

masses always add up to 236. The following two equations are examples of

the different products that can be produced when 235U fissions:

235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY

235U + 1 neutron 2 neutrons + 92Sr + 140Xe + ENERGY

Let's discuss those reactions. In each of the

above reactions, 1 neutron splits the atom. When the atom is split, 1

additional neutron is released. This is how a chain reaction works. If more

235U is present, those 2 neutrons can cause 2 more atoms to split. Each of

those atoms releases 1 more neutron bringing the total neutrons to 4. Those

4 neutrons can strike 4 more 235U atoms, releasing even more neutrons.

The chain reaction will continue until all the 235U fuel is spent. This is

roughly what happens in an atomic bomb. It is called a runaway nuclear

reaction.

Where Does the Energy Come From?

In the section above we described what happens when an 235U atom

fissions. We gave the following equation as an example:

235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY

You might have been wondering, "Where does the energy come from?". The

mass seems to be the same on both sides of the reaction:

235 + 1 = 2 + 92 + 142 = 236

Thus, it seems that no mass is converted into energy. However, this is not

entirely correct. The mass of an atom is more than the sum of the individual

masses of its protons and neutrons, which is what those numbers represent.

Extra mass is a result of the binding energy that holds the protons and

neutrons of the nucleus together. Thus, when the uranium atom is split,

some of the energy that held it together is released as radiation in the form

of heat. Because energy and mass are one and the same, the energy

released is also mass released. Therefore, the total mass does decrease a

tiny bit during the reaction.

Fission in Nuclear Reactors

To make large-scale use of the energy released in fission, one fission event

must trigger another, so that the process spreads thoughout the nuclear fuel

as in a set of dominos. The fact that more neutrons are produced in fission

than are consumed raises the possibility of a chain reaction. Such a reaction

can be either rapid (as in an atomic bomb) or controlled (as in a reactor).

In a nuclear reactor, control rods made of cadmium or graphite or some

other neutron-absorbing material are used to regulate the number of

neutrons. The more exposed control rods, the less neutrons and vice versa.

This also controls the multiplication factor k which is the ratio of the number

of neutrons present at the beginning of a particular generation to the

number present at the beginning of the next generation. For k=1, the

operation of the reactor is said to be exactly critical, which is what we wish it

to be for steady-power operation. Reactors are designed so that they are

inherently supercritical (k>1); the multiplication factor is then adjusted to

the critical operation by inserting the control rods.

An unavoidable feature of reactor operation is the accumulation of

radioactive wastes, including both fission products and heavy "transuranic"

nuclides such as plutonium and americium.

Nuclear Power

Nuclear Power Nuclear power is the controlled use of nuclear reactions to release energy for

work including propulsion, heat, and the generation of electricity. Use of

nuclear power to do significant useful work is currently limited to nuclear

fission and radioactive decay. Nuclear energy is produced when a fissile

material, such as uranium-235 (235U), is concentrated such that nuclear

fission takes place in a controlled chain reaction and creates heat — which is

used to boil water, produce steam, and drive a steam turbine. The turbine

can be used for mechanical work and also to generate electricity. Nuclear

power provides 7% of the world's energy and 15.7% of the world's

electricity and is used to power most military submarines and aircraft

carriers.

The United States produces the most nuclear energy, with nuclear power

providing 20% of the electricity it consumes, while France produces the

highest percentage of its electrical energy from nuclear reactors—80% as of

2006. In the European Union as a whole, nuclear energy provides 30% of

the electricity.Nuclear energy policy differs between countries, and some

countries such as Austria, Australia and Ireland have no nuclear power

stations.

Concerns about nuclear power

The use of nuclear power is controversial because of the problem of storing

radioactive waste for indefinite periods, the potential for possibly severe

radioactive contamination by accident or sabotage, and the possibility that

its use in some countries could lead to the proliferation of nuclear weapons.

Proponents believe that these risks are small and can be further reduced by

the technology in the new reactors. They further claim that the safety record

is already good when compared to other fossil-fuel plants, that it releases

much less radioactive waste than coal power, and that nuclear power is a

sustainable energy source. Critics, including most major environmental

groups, claim nuclear power is an uneconomic and potentially dangerous

energy source with a limited fuel supply, especially compared to renewable

energy, and dispute whether the costs and risks can be reduced through

new technology.

There is concern in some countries over North Korea and Iran operating

research reactors and fuel enrichment plants, since those countries refuse

adequate IAEA oversight and are believed to be trying to develop nuclear

weapons. North Korea admits that it is developing nuclear weapons, while

the Iranian government vehemently denies the claims against Iran.

Several concerns about nuclear power have been expressed, and these

include:

• Concerns about nuclear reactor accidents, such as the Chernobyl

disaster

• Vulnerability of plants to attack or sabotage

• Use of nuclear waste as a weapon

• Health effects of nuclear power plants

• Nuclear proliferation

Nuclear Power Plant,Types, Advantages and Disadvantages

Nuclear Power Plant

Nuclear power is generated using Uranium, which is a metal mined in

various parts of the world.

The structure of a nuclear power plant in many aspects resembles to that of

a conventional thermal power station, since in both cases the heat produced

in the boiler (or reactor) is transported by some coolant and used to

generate steam. The steam then goes to the blades of a turbine and by

rotating it, the connected generator will produce electric energy. The steam

goes to the condenser, where it condenses, i.e. becomes liquid again. The

cooled down water afterwards gets back to the boiler or reactor, or in the

case of PWRs to the steam generator.

The great difference between a conventional and a nuclear power plant is

how heat is produced. In a fossile

which means that the chemical energy of the fuel is converted into heat. In a

nuclear power plant, however, energy that comes from fission reactions is

utilized.

How it works

• Nuclear power stations work in pretty much the same way as fossil

fuel-burning stations, except that a "chain reaction" inside a nuclear

reactor makes the heat instead.

• The reactor uses Uranium rods as fuel, and the heat is generated by

nuclear fission. N

atoms, which split roughly in half and release energy in the form of

heat.

• Carbon dioxide gas is pumped through the reactor to take the heat

away, and the hot gas then heats water to make steam.

• The steam drives turbines which drive generators. Modern nuclear

power stations use the same type of turbines and generators as

conventional power stations.


how heat is produced. In a fossile plant, oil, gas or coal is fired in the boiler,



Nuclear power stations work in pretty much the same way as fossil

burning stations, except that a "chain reaction" inside a nuclear

reactor makes the heat instead.

The reactor uses Uranium rods as fuel, and the heat is generated by

eutrons smash into the nucleus of the uranium


Carbon dioxide gas is pumped through the reactor to take the heat


ves turbines which drive generators. Modern nuclear


conventional power stations.


plant, oil, gas or coal is fired in the boiler,



Nuclear power stations work in pretty much the same way as fossil

burning stations, except that a "chain reaction" inside a nuclear

The reactor uses Uranium rods as fuel, and the heat is generated by

eutrons smash into the nucleus of the uranium


Carbon dioxide gas is pumped through the reactor to take the heat


ves turbines which drive generators. Modern nuclear


In Britain, nuclear power stations are built on the coast, and use sea water

for cooling the steam ready to be pumped round again. This means that they

don't have the huge "cooling towers" seen at other power stations.

The reactor is controlled with "control rods", made of boron, which absorb

neutrons. When the rods are lowered into the reactor, they absorb more

neutrons and the fission process slows down. To generate more power, the

rods are raised and more neutrons can crash into uranium atoms.

Nuclear Power Plant Types

Several nuclear power plant (NPP) types are used for energy generation in

the world. The different types are usually classified based on the main

features of the reactor applied in them. The most widespread power plant

reactor types are:

• Light water reactors: both the moderator and coolant are light water

(H2O). To this category belong the pressurized water reactors (PWR)

and boiling water reactors (BWR).

• Heavy water reactors (CANDU): both the coolant and moderator are

heavy water (D2O).

• Graphite moderated reactors: in this category there are gas cooled

reactors (GCR) and light water cooled reactors (RBMK).

• Exotic reactors (fast breeder reactors and other experimental

installations).

• New generation reactors: reactors of the future.

Advantages

• Nuclear power costs about the same as coal, so it's not expensive to

make.

• The amount of fuel required is quite small ,therfore there is no

problem of transportation, storage etc.

• Does not produce smoke or carbon dioxide, so it does not contribute to

the greenhouse effect.

• Produces huge amounts of energy from small amounts of fuel.

• Produces small amounts of waste.

• The output control is most flexible.

• Nuclear power is reliable.

Disadvantages

• The fuel used is expensive and is difficult to recover.

• The fission by-products are generally radio active and may cause a

dangerous amount of radio active pollution.

• Although not much waste is produced, it is very, very dangerous. It

must be sealed up and buried for many years to allow the radioactivity

to die away.

• The initial capital cost is very high as compared to other power plants.

• Nuclear power is reliable, but a lot of money has to be spent on safety

- if it does go wrong, a nuclear accident can be a major disaster.

People are increasingly concerned about this - in the 1990's nuclear

power was the fastest-growing source of power in much of the world.

In 2005 it was the second slowest-growing.

• The cooling water requirements of a nuclear power plant are very

heavy.

Pelton Wheel

Pelton Wheel

A Pelton wheel, also called a Pelton turbine, is one of the most efficient types

of water turbines. It was invented by Lester Allan Pelton (1829-1908) in the

1870s, and is an impulse machine, meaning that it uses Newton's second

law to extract energy from a jet of fluid.

The pelton wheel turbine is a

along the tangent to the path of the runner. Nozzles direct forceful streams

of water against a series of spoon

of a wheel. Each bucket reverses the flow of wa

diminished energy. The resulting impulse spins the turbine. The buckets are

mounted in pairs, to keep the forces on the wheel balanced, as well as to

ensure smooth, efficient momentum transfer of the fluid jet to the wheel.

The Pelton wheel is most efficient in high head

Since water is not a compressible fluid, almost all of the available energy is

extracted in the first stage of the turbine. Therefore, Pelton wheels have

only one wheel, unlike turbines that operate with

Applications

Peltons are the turbine of choice for high head, low flow sites. However,

Pelton wheels are made in all sizes. There are multi

mounted on vertical oil pad bearings in the generator houses of hydroelectric

plants. The largest units can be up to 200 megawatts. The smallest Pelton

wheels, only a few inches across, are used with household plumbing fixtures

to tap power from mountain streams with a few gallons per minute of flow,

but these small units must have thirty meters or more of head. Depending

on water flow and design, Pelton wheels can operate with heads as small as

15 meters and as high as 1,800 meters.

In general, as the height of fall increases, less volume of water can generate

a bit more power. Energy is force times distance, in the instance of fluid flow

power is expressed as P = Constant x Pressure x Volume/t. The power P

turbine is a tangential flow impulse turbine, water flows


of water against a series of spoon-shaped buckets mounted around the edge

of a wheel. Each bucket reverses the flow of water, leaving it with




most efficient in high head applications.



only one wheel, unlike turbines that operate with compressible fluids.


Pelton wheels are made in all sizes. There are multi-ton Pelton




untain streams with a few gallons per minute of flow,



15 meters and as high as 1,800 meters.

the height of fall increases, less volume of water can generate



turbine, water flows


shaped buckets mounted around the edge

ter, leaving it with




applications.



compressible fluids.


ton Pelton wheels




untain streams with a few gallons per minute of flow,



the height of fall increases, less volume of water can generate



grows linearly with flow rate and grows with f(Pressure^3/2.) Thus it is

usually best to seek as much head or pressure as possible in hydro designs

then go for flow rate.

Kaplan Turbine

Kaplan Turbine The Kaplan turbine is a propeller-type water turbine that has adjustable

blades. It was developed in 1913 by the Austrian professor Viktor Kaplan.

The Kaplan turbine was an evolution of the Francis turbine. Its invention

allowed efficient power production in low head applications that was not

possible with Francis turbines.

Kaplan turbines are now widely used throughout the world in high-flow, low-

head power production.

The Kaplan turbine is an inward flow reaction turbine, which means that the

working fluid changes pressure as it moves through the turbine and gives up

its energy. The design combines radial and axial features.

The above figures shows flow in a Kaplan turbine. In the picture, pressure on

runner blades and hub surface is shown using colormapping (red = high,

blue = low).

The diameter of the runner of such machines is typically 5 to 8 meters.

The inlet is a scroll-shaped tube that wraps around the turbine's wicket gate.

Water is directed tangentially, through the wicket gate, and spirals on to a

propeller shaped runner, causing it to spin.

The outlet is a specially shaped draft tube that helps decelerate the water

and recover kinetic energy.

The turbine does not need to be at the lowest point of water flow, as long as

the draft tube remains full of water. A higher turbine location, however,

increases the suction that is imparted on the turbine blades by the draft

tube. The resulting pressure drop may lead to cavitation.

Variable geometry of the wicket gate and turbine blades allow efficient

operation for a range of flow conditions. Kaplan turbine efficiencies are

typically over 90%, but may be lower in very low head applications.

Applications

Kaplan turbines are widely used throughout the world for electrical power

production. They cover the lowest head hydro sites and are especially suited

for high flow conditions.

Inexpensive micro turbines are manufactured for individual power production

with as little as two feet of head.

Large Kaplan turbines are individually designed for each site to operate at

the highest possible efficiency, typically over 90%. They are very expensive

to design, manufacture and install, but operate for decades.

Variations

The Kaplan turbine is the most widely used of the propeller-type turbines,

but several other variations exist:

Propeller turbines have non-adjustable propeller vanes. They are used in low

cost, small installations. Commercial products exist for producing several

hundred

watts from only a few feet of head.

Bulb or Tubular turbines are designed into the water delivery tube. A large

bulb is centered in the water pipe which holds the generator, wicket gate

and runner. Tubular turbines are a fully axial design, whereas Kaplan

turbines have a radial wicket gate. Pit turbines are bulb turbines with a gear

box. This allows for a smaller generator and bulb.

Straflo turbines are axial turbines with the generator outside of the water

channel, connected to the periphery of the runner.

S- turbines eliminate the need for a bulb housing by placing the generator

outside of the water channel. This is accomplished with a jog in the water

channel and a shaft connecting the runner and generator.

Tyson turbines are a fixed propeller turbine designed to be immersed in a

fast flowing river, either permanently anchored in the river bed, or attached

to a boat or barge.

Francis Turbine

Francis Turbine The Francis turbine is a type of water turbine that was developed by James

B. Francis. It is an inward flow reaction turbine that combines radial and

axial flow concepts.

Francis turbines are the most common water turbine in use today. They

operate in a head range of ten meters to several hundred meters and are

primarily used for electrical power production.

The Francis turbine is a reaction

turbine, which means that the working fluid changes pressure as it moves

through the turbine, giving up its energy. A casement is needed to contain

the water flow. The turbine is located between the high pressure water

source and the low pressure water exit, usually at the base of a dam.

The inlet is spiral shaped. Guide vanes direct the water tangentially to the

runner. This radial flow acts on the runner vanes, causing the runner to spin.

The guide vanes (or wicket gate) may be adjustable to allow efficient turbine

operation for a range of water flow conditions.

As the water moves through the runner its spinning radius decreases,

further acting on the runner. Imagine swinging a ball on a string around in a

circle. If the string is pulled short, the ball spins faster. This property, in

addition to the water's pressure, helps inward flow turbines harness water

energy.At the exit, water acts on cup shaped runner features, leaving with

no swirl and very little kinetic or potential energy. The turbine's exit tube is

shaped to help decelerate the water flow and recover the pressure.

Application

Large Francis turbines are individually designed for each site to operate at

the highest possible efficiency, typically over 90%.

Francis type units cover a wide head range, from 20 meters to 700 meters

and their output varies from a few kilowatt to 1000 megawatt. Their size

varies from a few hundred millimeters to about 10 meters.

In addition to electrical production, they may also be used for pumped

storage; where a reservoir is filled by the turbine (acting as a pump) during

low power demand, and then reversed and used to generate power during

peak demand.

Francis turbines may be designed for a wide range of heads and flows. This,

along with their high efficiency, has made them the most widely used

turbine in the world.

Water Turbines and its Classification

Water Turbine

Water turbine is a device that convert the energy in a stream of fluid into

mechanical energy by passing the stream through a system of fixed and

moving fan like blades and causing the latter to rotate. A turbine looks like a

large wheel with many small radiating blades around its rim.

Classification of Water turbines

According to the type of flow of water : The water turbines used as prime

movers in hydro electric power stations are of four types.They are

• axial flow : having flow along shaft axis

• inward radial flow : having flow along the radius

• tangential or peripheral : having flow along tangential direction

• mixed flow : having radial inlet axial outlet

If the runner blades of axial flow turbines are fixed,those are called propeller

turbines.

According to the action of water on moving blades water turbines are of 2

types namely impulse ad reaction type turbines.

Impulse Turbines :These turbines change the direction of flow of a high

velocity fluid jet. The resulting impulse spins the turbine and leaves the fluid

flow with diminished kinetic energy. There is no pressure change of the fluid

in the turbine rotor blades. Before reaching the turbine the fluid's Pressure

head is changed to velocity head by accelerating the fluid with a nozzle.

Pelton wheels and de Laval turbines use this process exclusively. Impulse

turbines do not require a pressure casement around the runner since the

fluid jet is prepared by a nozzle prior to reaching turbine. Newton's second

law describes the transfer of energy for impulse turbines.

Reaction Turbines : These turbines develop torque by reacting to the

fluid's pressure or weight. The pressure of the fluid changes as it passes

through the turbine rotor blades. A pressure casement is needed to contain

the working fluid as it acts on the turbine stage(s) or the turbine must be

fully immersed in the fluid flow (wind turbines). The casing contains and

directs the working fluid and, for wat

imparted by the draft tube.

this concept. For compressible working fluids, multiple turbine stages may

be used to harness the expanding gas efficiently. Newton's third law

describes the transfer of energy for reaction turbines.

According to the Head and quantity of water available

of 2 types.Those are high head

to medium discharge turbines.

According to the name of the originator

Pelton Wheel,Francis tubine and Kaplan turbine.

Hydro Power Plant Working

How HydroPower Plant works

A hydroelectric power plant harnesses the energy found in moving or still

water and converts it into electricity.



directs the working fluid and, for water turbines, maintains the suction

imparted by the draft tube. Francis turbines and most steam turbines



ribes the transfer of energy for reaction turbines.

According to the Head and quantity of water available the water turbines are

high head - low flow and low to medium head and high

turbines.

of the originator water turbines are of 3 types namely

Pelton Wheel,Francis tubine and Kaplan turbine.

Hydro Power Plant Working

How HydroPower Plant works


water and converts it into electricity.



er turbines, maintains the suction

Francis turbines and most steam turbines use



the water turbines are

low to medium head and high

water turbines are of 3 types namely


Moving water, such as a river or a waterfall, has mechanical energy.

‘Mechanical energy is the energy that is possessed by an object due to its

motion or stored energy of position.’ This means that an object has

mechanical energy if it’s in motion or has the potential to do work (the

movement of matter from one location to another,) based on its position.

The energy of motion is called kinetic energy and the stored energy of

position is called potential energy. Water has both the ability and the

potential to do work. Therefore, water contains mechanical energy (the

ability to do work), kinetic energy (in moving water, the energy based on

movement), and potential energy (the potential to do work.)

The potential and kinetic/mechanical energy in water is harnessed by

creating a system to efficiently process the water and create electricity from

it. A hydroelectric power plant has eleven main components. The first

component is a dam.

The dam is usually built on a large river that has a drop in elevation, so as to

use the forces of gravity to aid in the process of creating electricity. A dam is

built to trap water, usually in a valley where there is an existing lake. An

artificial storage reservoir is formed by constructing a dam across a

river.Notice that the dam is much thicker at the bottom than at the top,

because the pressure of the water increases with depth.

The area behind the dam where water is stored is called the reservoir. The

water there is called gravitational potential energy. The water is in a stored

position above the rest of the dam facility so as to allow gravity to carry the

water down to the turbines. Because this higher altitude is different than

where the water would naturally be, the water is considered to be at an

altered equilibrium. This results in gravitational potential energy, or, “the

stored energy of position possessed by an object.” The water has the

potential to do work because of the position it is in (above the turbines, in

this case.)

Gravity will force the water to fall to a lower position through the intake and

the control gate. They are built on the inside of the dam. When the gate is

opened, the water from the reservoir goes through the intake and becomes

translational kinetic energy as it falls through the next main part of the

system: the penstock. Translational kinetic energy is the energy due to

motion from one location to another. The water is falling (moving) from the

reservoir towards the turbines through the penstock.

The intake shown in figure includes the head works which are the structures

at the intake of conduits,tunnels or flumes.These structures include

blooms,screens or trash - racks, sluices to divert and prevent entry of debris

and ice in to the turbines.Booms prevent the ice and floating logs from going

in to the intake by diverting them to a bypass chute.Screens or trash-

racks(shown in fig) are fitted irectly at the intake to prevent the debris from

going in to the take.Debris cleaning devices should also be fitted on the

trash-racks.Intake structures can be classified in to high pressure intakes

used in case of large storage reservoirs and low pressure intakes used in

case of small ponds.The use of providing these structures at the intake

is,water only enters and flows through the penstock which strikes the

turbine.

Control gates arrangement is provided with Spillways.Spillway is constructed

to act as a safety valve.It dischargs the overflow water to the down stream

side when the reservoir is full.These are generally constructed of concrete

and provided with water discharge opening,shut off by metal control

gates.By changing the degree to which the gates are opened,the discharge

of the head water to the tail race can be regulated inorder to maintain water

level in reservoir.

The penstock is a long shaft that carries the water towards the turbines

where the kinetic energy becomes mechanical energy. The force of the water

is used to turn the turbines that turn the generator shaft. The turning of this

shaft is known as rotational kinetic energy because the energy of the moving

water is used to rotate the generator shaft. The work that is done by the

water to turn the turbines is mechanical energy. This energy powers the

generators, which are very important parts of the hydroelectric power plant;

they convert the energy of water into electricity. Most plants contain several

generators to maximize electricity production.

The generators are comprised of four basic components: the shaft, the

excitor, the rotor, and the stator. The turning of the turbines powers the

excitor to send an electrical current to the rotor. The rotor is a series of large

electromagnets that spins inside a tightly wound coil of copper wire, called

the stator. “A voltage is induced in the moving conductors by an effect called

electromagnetic induction.” The electromagnetic induction caused by the

spinning electromagnets inside the wires causes electrons to move, creating

electricity. The kinetic/mechanical energy in the spinning turbines turns into

electrical energy as the generators function.

The transformer, another component, takes the alternating current and

converts it into higher-voltage current. The electrical current generated in

the generators is sent to a wire coil in the transformer. This is electrical

energy. Another coil is located very close to first one and the fluctuating

magnetic field in the first coil will cut through the air to the second coil

without the current. The amount of turns in the second coil is proportional to

the amount of voltage that is created. If there are twice as many turns on

the second coil as there are on the first one, the voltage produced will be

twice as much as that on the first coil. This transference of electrical current

is electrical energy. It goes from the generators to one coil, and then is

transferred through an electromagnetic field onto the second coil. That

current is then sent by means of power lines to the public as electricity

Now, the water that turned the turbines flows through the pipelines

(translational kinetic energy, because the energy in the water is being

moved,) called tailraces and enters the river through the outflow. The water

is back to being kinetic/mechanical/potential energy as it is in the river and

has to potential to have the energy harnessed for use as it flows along

(movement.)

Pumped Storage Plants

Pumped Storage

"Pumped Storage" is another form of hydro

facilities use excess electrical system capacity, generally available at night,

to pump water from one reservoir to another reservoir at a higher elevation.

During periods of peak electrical demand, water from the higher reservoir is

released through turbines to the lower reservoir, and electricity is produced .

Although pumped storage sites are not net producers of electricity

actually takes more electricity to pump the wa

it is released - they are a valuable addition to electricity supply systems.

Their value is in their ability to store electricity for use at a later time when

peak demands are occurring. Storage is even more valuable if interm

sources of electricity such as solar or wind are hooked into a system.

Pumped storage plant is a unique design of peak load plant in which the

plant pumps back all or portion of its water supply during lo load period.The

usual construction is a lowand high elevation reservoirs connected through a

penstock.The generating pumping plant is at the lower end.The plant utilises

some of the surplus energy generated by the base load plant to pump water

from low elevation to highelevation reservoir during

peak load period this water is used to generate power by allowing it to flow

from high elevation reservoir through reversible hydraulic turbine of this

plan to low elevation reservoir.Thus same water is used again and again and

extra water is required only to take care of evaporation and seepage.

The main important point in this plant is reversible turbine/generator



"Pumped Storage" is another form of hydro-electric power. Pumped storage



eak electrical demand, water from the higher reservoir is


Although pumped storage sites are not net producers of electricity

actually takes more electricity to pump the water up than is recovered when

they are a valuable addition to electricity supply systems.


peak demands are occurring. Storage is even more valuable if interm




owand high elevation reservoirs connected through a



from low elevation to highelevation reservoir during off peak hours.During




water is required only to take care of evaporation and seepage.


electric power. Pumped storage



eak electrical demand, water from the higher reservoir is


Although pumped storage sites are not net producers of electricity - it

ter up than is recovered when

they are a valuable addition to electricity supply systems.


peak demands are occurring. Storage is even more valuable if intermittent




owand high elevation reservoirs connected through a



off peak hours.During




water is required only to take care of evaporation and seepage.


assemblies act as pump and turbine (usually a Francis turbine

design).During low load periods it acts as pump and pumps water from low

to high elevation reservoir.During peak load periods it acts as turbine when

water flows from high to low elevation reservoir.

To see the flash animation of pumped storage plant working Click here

Advantages

• Without some means of storing energy for quick release, we'd be in

trouble.

• Little effect on the landscape.

• No pollution or waste

Disadvantages

• Expensive to build.

• Once it's used, you can't use it again until you've pumped the water

back up. Good planning can get around this problem.

Hydro Power - Introduction and Types

Hydro Power :

Hydro power has played an important historical role in the industrialization

of society from grinding flour to powering industry. Hydro energy originates

from the sun, and hence, is renewable and its fuel is free.

“Hydro” means “water” in Latin – so “hydro power” is made from

water.Hydropower is the capture of the energy of moving water for some

useful purpose.The analysis of hydroelectric generation begins with the

potential energy of the water. The gravitational potential energy (PE) is

defined based on a material’s mass (m) and height (H) from a reference

point.

PE = m.g.H

where g is gravitational constant. The power generation (P) depends upon

the period (T) over which the water is discharged through that height, often

times referred to as the head.

The water mass may be expressed in terms of its density (ρ) and volume

(V), i.e., m=ρV.

Often,the volume of water is measured in acre

occupied by a foot of water covering an acre of area; one acre

equivalent to 43,560 ft³. The standard density of water is 1,000 kg/m³ or

62.4 lbm/ft³. The power can then be represented i

rate or volumetric flow rate

The electric power output is reduced by the hydraulic

turbine-generator efficiency.

There are many forms of water power:

• Waterwheels , used for hundreds of years to power mills and

machinery

• Hydroelectric energy

• Tidal power, which captures energy from the tides in horizontal

direction

• Tidal stream power

• Wave power, which uses the energy in waves

Hydroelectric power

Hydroelectric power now supplies about 715,000 MW or 19% of world

electricity (16% in 2003). Large dams are still being designed. Apart from a

few countries with an abundance of it, hydro power is normally applied to

peak load demand because it is readily

hydroelectric power is probably not a major option for the future of energy

production in the developed nations because most major sites within these

nations are either already being exploited or are unavailable for other

reasons, such as environmental considerations.

Hydropower produces essentially no carbon dioxide or other harmful

emissions, in contrast to burning fossil fuels, and is not a significant

contributor to global warming through CO2.

Hydroelectric power can b

as the head.


volume of water is measured in acre-feet which is the volume

occupied by a foot of water covering an acre of area; one acre


62.4 lbm/ft³. The power can then be represented in terms of the mass flow

rate or volumetric flow rate


generator efficiency.

There are many forms of water power:

, used for hundreds of years to power mills and

tric energy, a term usually reserved for hydroelectric dams.

, which captures energy from the tides in horizontal

Tidal stream power, which does the same vertically

, which uses the energy in waves




peak load demand because it is readily stopped and started. Nevertheless,




reasons, such as environmental considerations.



contributor to global warming through CO2.

Hydroelectric power can be far less expensive than electricity generated


feet which is the volume

occupied by a foot of water covering an acre of area; one acre-foot is


n terms of the mass flow


, used for hundreds of years to power mills and

, a term usually reserved for hydroelectric dams.

, which captures energy from the tides in horizontal




stopped and started. Nevertheless,






e far less expensive than electricity generated

from fossil fuel or nuclear energy. Areas with abundant hydroelectric power

attract industry. Environmental concerns about the effects of reservoirs may

prohibit development of economic hydropower sources.

The chief advantage of hydroelectric dams is their ability to handle seasonal

(as well as daily) high peak loads. When the electricity demands drop, the

dam simply stores more water (which provides more flow when it releases).

Some electricity generators use water dams to store excess energy (often

during the night), by using the electricity to pump water up into a basin.

Electricity can be generated when demand increases. In practice the

utilization of stored water in river dams is sometimes complicated by

demands for irrigation which may occur out of phase with peak electrical

demands.

Tidal power

Harnessing the tides in a bay or estuary has been achieved in France (since

1966), Canada and Russia, and could be achieved in other areas with a large

tidal range. The trapped water turns turbines as it is released through the

tidal barrage in either direction. Another possible fault is that the system

would generate electricity most efficiently in bursts every six hours (once

every tide). This limits the applications of tidal energy.

Tidal stream power

A relatively new technology, tidal stream generators draw energy from

currents in much the same way that wind generators do. The higher density

of water means that a single generator can provide significant power. This

technology is at the early stages of development and will require more

research before it becomes a significant contributor.

Several prototypes have shown promise. In the UK in 2003, a 300 kW

Periodflow marine current propeller type turbine was tested off the coast of

Devon, and a 150 kW oscillating hydroplane device, the Stingray, was tested

off the Scottish coast. Another British device, the Hydro Venturi, is to be

tested in San Francisco Bay.

The Canadian company Blue Energy has plans for installing very large arrays

tidal current devices mounted in what they call a 'tidal fence' in various

locations around the world, based on a vertical axis turbine design.

Wave power

Harnessing power from ocean surface wave motion might yield much more

energy than tides. The feasibility of this has been investigated, particularly in

Scotland in the UK. Generators either coupled to floating devices or turned

by air displaced by waves in a hollow concrete structure would produce

electricity. Numerous technical problems have frustrated progress.

A prototype shore based wave power generator is being constructed at Port

Kembla in Australia and is expected to generate up to 500 MWh annually.

The Wave Energy Converter has been constructed (as of July 2005) and

initial results have exceeded expectations of energy production during times

of low wave energy. Wave energy is captured by an air driven generator and

converted to electricity. For countries with large coastlines and rough sea

conditions, the energy of waves offers the possibility of generating electricity

in utility volumes. Excess power during rough seas could be used to produce

hydrogen.

Hydro Electric Plants - Classification, Advantages and Disadvantages

Classification of Hydro Electric Plants

The classification of hydro electric plants based upon :

(a) Quantity of water available (b) Available head (c) Nature of load

The classification acording to Quantity of water available is

(i) Run-off river plants with out pondage : These plants does not store

water; the plant uses water as it comes.The plant can use water as and

when available.Since these plants depend for their generting capacity

primarly on the rate of flow of water, during rainy season high flow rate may

mean some quantity of water to go as waste while during low run-off

periods, due to low flow rates,the generating capacity will be low.

(ii) Run-off river plants with pondage : In these plants pondage permits

storage of water during off peak periods and use of this water during peak

periods.Depending on the size of pondage provided it may be possible to

cope with hour to hour fluctuations.This type of plant can be used on parts

of the load curve as required,and is more useful than a plant with out

storage or pondage.

When providing pondage tail race conditions should be such that floods do

not raise tail-race water level,thus reducing the head on the plant and

impairing its effectiveness.This type of plant is comparitively more reliable

and its generating capacity is less dependent on avilable rate of flow of

water.

(iii) Reservoir Plants :A reservoir plant is that which has a reservoir of such

size as to permit carrying over storage from wet season to the next dry

season.Water is stored behind the dam and is available to the plant with

control as required.Such a plant has better capacity and can be used

efficiently through out the year.Its firm capacity can be increased and can be

used either as a base load plant or as a peak load plant as required.It can

also be used on any portion of the load curve as required.Majority of the

hydroelectric plants are of this type.

The classification according to availability of water head is

(i) Low-Head (less than 30 meters) Hydro electric plants :"Low head" hydro-

electric plants are power plants which generally utilize heads of only a few

meters or less. Power plants of this type may utilize a low dam or weir to

channel water, or no dam and simply use the "run of the river". Run of the

river generating stations cannot store water, thus their electric output varies

with seasonal flows of water in a river. A large volume of water must pass

through a low head hydro plant's turbines in order to produce a useful

amount of power. Hydro-electric facilities with a capacity of less than about

25 MW (1 MW = 1,000,000 Watts) are generally referred to as "small

hydro", although hydro-electric technology is basically the same regardless

of generating capacity.

(ii) Medum-head(30 meters - 300 meters) hydro electric plants :These

plants consist of a large dam in a mountainous area which creates a huge

reservoir. The Grand Coulee Dam on the Columbia River in Washington (108

meters high, 1270 meters wide, 9450 MW) and the Hoover Dam on the

Colorado River in Arizona/Nevada (220 meters high, 380 meters wide, 2000

MW) are good examples. These dams are true engineering marvels. In fact,

the American Society of Civil Engineers as designated Hoover Dam as one of

the seven civil engineering wonders of the modern world, but the massive

lakes created by these dams are a graphic example of our ability to

manipulate the environment - for better or worse. Dams are also used for

flood control, irrigation, recreation, and often are the main source of potable

water for many communities. Hydroelectric development is also possible in

areas such as Niagra Falls where natural elevation changes can be used.

(iii) High-head hydro electric plants :"High head" power plants are the most

common and generally utilize a dam to store water at an increased

elevation. The use of a dam to impound water also provides the capability of

storing water during rainy periods and releasing it during dry periods. This

results in the consistent and reliable production of electricity, able to meet

demand. Heads for this type of power plant may be greater than 1000 m.

Most large hydro-electric facilities are of the high head variety. High head

plants with storage are very valuable to electric utilities because they can be

quickly adjusted to meet the electrical demand on a distribution system.

The classification according to nature of load is

(i) Base load plants :A base load power plant is one that provides a steady

flow of power regardless of total power demand by the grid. These plants

run at all times through the year except in the case of repairs or scheduled

maintenance.

Power plants are designated base load based on their low cost generation,

efficiency and safety at set outputs. Baseload power plants do not change

production to match power consumption demands since it is always cheaper

to run them rather than running high cost combined cycle plants or

combustion turbines. Typically these plants are large enough to provide a

majority of the power used by a grid, making them slow to fire up and cool

down. Thus, they are more effective when used continuously to cover the

power baseload required by the grid.

Each base load power plant on a grid is allotted a specific amount of the

baseload power demand to handle. The base load power is determined by

the load duration curve of the system. For a typical power system, rule of

thumb states that the base load power is usually 35-40% of the maximum

load during the year.Load factor of such plants is high.

Fluctuations, peaks or spikes in customer power demand are handled by

smaller and more responsive types of power plants.

(ii) Peak load plants :Power plants for electricity generation which, due to

their operational and economic properties, are used to cover the peak load.

Gas turbines and storage and pumped storage power plants are used as

peak load power plants.The efficiency of such plants is around 60 -70%.

Advantages of hydroelectric plants

• operation , running and maintenance costs are low.

• Once the dam is built, the energy is virtually free.

• No fuel is burnt and the plant is quite neat & clean.

• No waste or pollution produced.

• generating plants have a long lifetime.

• Much more reliable than wind, solar or wave power.

• Water can be stored above the dam ready to cope with peaks in

demand.

• unscheduled breakdowns are relatively infrequent and short in

duration since the equipment is relatively simple.

• hydroelectric turbine-generators can be started and put "on-line" very

rapidly.

• Electricity can be generated constantly

Disadvantages of hydroelectric plants

• very land-use oriented and may flood large regions.

• The dams are very expensive to build.However, many dams are also

used for flood control or irrigation, so building costs can be shared.

• Capital cost of generators, civil engineering works and cost of

transmission lines is very high.

• Water quality and quantity downstream can be affected, which can

have an impact on plant life.

• Finding a suitable site can be difficult - the impact on residents and the

environment may be unacceptable.

• fish migration is restricted.

• fish health affected by water temperature change and insertion of

excess nitrogen into water at spillways

• available water and its temperature may be affected

• reservoirs alter silt-flow patterns

Top 10 Rules for Saving Energy

Top 10 Rules for Saving Energy

1. DO shut off the lights when you’re done using them,

and turn off the TV, computer, video games and other electrical stuff when

you leave the room.

2. DO lower the thermostat during the winter. To keep warm without

wasting energy, put on a sweatshirt or snuggle under a blanket.

3. DON'T leave the refrigerator door open. Every time you

open the door, up to one-third of the cold air can escape.

4. DO replace a burnt-out light bulb with a new compact

fluorescent bulb. Fluorescent bulbs use 75 percent less energy, and they

last 10 times longer.

5. DO remind grown-ups to use cold water in the washing

machine. Hot water won’t get the clothes any cleaner, and it wastes a lot of

energy.

6. DO turn off dripping faucets. One drop per second can

add up to 165 gallons of hot water a month - that's more than one person

uses in two weeks!

7. DON’T take a long bath – take a short shower instead.

It might take 25 gallons of hot water to fill the bathtub, compared to only

seven gallons for a quick shower.

8. DO close the curtains during hot summer days to block

the sun. During the winter, keep the curtains open.

9. Help a grown-up put plastic sheeting on windows.

Blocking cold drafts is called “weatherizing” and it can save a lot of energy.

10. DO help your mom or dad plant a tree to help shade

your house on hot summer days.

What Is Renewable Energy?

What Is Renewable Energy?

All the energy we use comes from the earth. The electricity we use every

day doesn't come directly from the earth, but we make electricity using the

earth's resources, like coal or natural gas.

Both coal and natural gas are called “fossil fuels” because they were formed

deep under the earth during dinosaur times.

The problem is that fossil fuels can't be replaced - once we use them up,

they're gone forever. Another problem is that fossil fuels can cause pollution.

Renewable energy is made from resources that Mother Nature will replace,

like wind, water and sunshine.

Renewable energy is also called “clean energy” or “green power” because it

doesn’t pollute the air or the water.

Why don’t we use renewable energy all the time?

Unlike natural gas and coal, we can’t store up wind and sunshine to use

whenever we need to make more electricity. If the wind doesn’t blow or the

sun hides behind clouds, there wouldn’t be enough power for everyone.

Another reason we use fossil fuels like coal and natural gas is because

they’re cheaper. It costs more money to make electricity from wind, and

most people aren’t willing to pay more on their monthly utility bills.

How can we use renewable energy?

You might be using renewable energy today without knowing it! Iowa is

home to more than 600 wind turbines, creating enough electricity to power

140,000 homes. Wisconsin and Minnesota also have lots of wind farms – and

the number is growing every day.

Nuclear Power Plant Operation

Nuclear Power Plant Operation

The below diagram shows the schematic of nuclear power plant.Nuclear

power generation is much similar to that of conventional steam power

generation.The difference lies only in the steam generation part i.e coal or

oil boiling furnance and boiler are replaced by nuclear reactor.

Thus a nuclear power plant consists of a nuclear reactor,steam

generator,turbine, generator, condenser etc. as shown in the above

figure.As in a conventional steam plant, water for raising steam forms a

closed feed system.However, the reactor and the cooling circuit have to be

heavily shielded to eliminate radiation hazards.

A nuclear power plant uses the heat generated by a nuclear fission process

to drive a steam turbine which generates usable electricity.Fission is the

splitting of atoms into smaller parts. Some atoms, themselves tiny, split

when they are struck by even smaller particles, called neutrons. Each time

this happens more neutrons come out of the split atom and strike other

atoms. This process of energy release is called a chain reaction. The plant

controls the chain reaction to keep it from releasing too much energy too

fast. In this way, the chain reaction can go on for a long time.

Few natural elements have atoms that will split in a chain reaction. Iron,

copper, silver and many other common metals will not split, or fission. There

are isotopes of iron, copper, etc. that are radioactive. This means that they

have an unstable nucleus and they emit radioactivity. However, just being

radioactive does not mean that they will fission, or split. But uranium will. So

uranium is suitable to fuel a nuclear power plant.

As atoms split and collide, they heat up. The plant uses this heat to create

steam.The heat is transfered to the water through heat exchanging tubes in

steam generator in the primary loop.After extractig this heat, water is

converted in to steam and collected at the top of steam generator.The

pressure of the expanding steam turns a turbine which is connected to a

generator in the secondary loop.After rotating turbine - generator set steam

passes to the condenser.After that the function of condenser and coling

towers is same as that of thermal plant.

After the steam is made, a nuclear plant operates much like a fossil fuel fired

plant: the turbine spins a generator. The whirling magnetic field of the

generator produces electricity. The electricity then goes through wires strung

on tall towers you might see along a highway to an electrical substation in

your neighborhood where the power is regulated to the proper strength.

Then it goes to your home.

In the case of nuclear power plant operation the following factors must be

considered

• Control -- Keeping the nuclear reaction from dying out or exploding.

• Safety -- If something goes wrong it can be contained.

• Refueling -- Adding more nuclear fuel without stoping the reactor.

• Waste production -- The byproducts of the reaction must be

manageable.

• Efficiency -- Capture as much of the heat as possible.

Control is the most important aspect to a design. When an atom of nuclear

fuel (uranium) absorbs a neutron, the uranium will fission into two smaller

atoms (waste) and release one to three neutrons. The kinetic energy of the

waste is used to heat the water for the steam turbine. The neutrons are

used to fission the next lot of uranium atoms and the process continues. If

none of these neutrons are absorbed by another uranium atom then the

reaction dies out. If too many neutrons are absorbed then the reaction

grows extremely quickly and could explode. Current reactor designs are

most usefully classified by how they ensure this nuclear reaction is kept at a

level which produces power without getting out of hand.

The Nuclear Regulatory Commission (NRC), part of our government, makes

sure nuclear power plants in the United States protect public health and

safety and the environment. The NRC licenses the use of nuclear material

and inspects users to make sure they follow the rules for safety.

Since radioactive materials are potentially harmful, nuclear power plants

have many safety systems to protect workers, the public, and the

environment. These safety systems include shutting the reactor down

quickly and stopping the fission process, systems to cool the reactor down

and carry heat away from it and barriers to contain any radioactivity and

prevent it from escaping into the environment.

One of the greatest benefits of nuclear plants is that they have no smoke

stacks! The big towers many people associate with nuclear plants are

actually for cooling water used to make steam. (Some other kinds of plants

have these towers, too.) The towers spread the water out so as much air as

possible can reach it and cool it down. Most water is then recycled into the

plant.

Nuclear power plants are very clean and efficient to operate. However,

nuclear power plants have some major environmental risks. Nuclear power

plants produce radioactive gases. These gases are to be contained in the

operation of the plant. If these gases are released into the air, major health

risks can occur. Nuclear plants use uranium as a fuel to produce power. The

mining and handling of uranium is very risky and radiation leaks can occur.

The third concern of nuclear power is the permanent storage of spent

radioactive fuel. This fuel is toxic for centuries, handling and disposal is an

ongoing environmental issue.

CANDU Reactor

CANDU Reactor

The CANDU reactor is a Pressurized Heavy Water Reactor developed initially

in the late 1950s and 1960s by a partnership between Atomic Energy of

Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario

(now known as Ontario Power Generation), Canadian General Electric (now

known as GE Canada), as well as several private industry participants. The

acronym "CANDU", a registered trademark of Atomic Energy of Canada

Limited, stands for "CANada Deuterium Uranium". This is a reference to its

deuterium-oxide (heavy water) moderator and its use of natural uranium

fuel. This type of reactor is meant for those countries which do not prodce

enriched uranium.Enrichment of uranium is costly and this reactor uses

natural uranium as fuel and heavy water as moderator.

In heavy water reactors both the modeartor and coolant are heavy water

(D2O). A great disadvantage of this type comes from this fact: heavy water

is one of the most expensive liquids. However, it is worth its price: this is

the best moderator. Therefore, the fuel of HWRs can be slightly (1% to 2%)

enriched or even natural uranium. Heavy water is not allowed to boil, so in

the primary circuit very high pressure, similar to that of PWRs, exists.

CANDU fuel is made from uranium that is naturally radioactive. Small

amounts of uranium can generate large amounts of energy in the form of

heat. The uranium is mined, refined and made into solid ceramic pellets (two

pellets are the size of an AA battery). The pellets are put in metal tubes,

which are welded together to form a fuel bundle that weighs around 23

kg.The bundle is about the size of a fireplace log and can provide enough

energy for an average home for 100 years. The figure below shows the

CANDU reactor and its main parts.

In CANDU reactors, the moderator and coolant are spatially separated: the

moderator is in a large tank (calandria), in which there are pressure tubes

surrounding the fuel assemblies. The coolant flows in these tubes only.

The advantage of this construction is that the whole tank need not be kept

under high pressure, it is sufficient to pressurize the coolant flowing in the

tubes. This arrangement is called pressurized tube reactor. Warming up of

the moderator is much less than that of the coolant; its is simply lost for

heat generation or steam production. The high temperature and high

pressure coolant, similarly to PWRs, goes to the steam generator where it

boils the secondary side light water. Another advantage of this type is that

fuel can be replaced during operation and thus there is no need for outages.

Fission reactions in the reactor core heat a fluid, in this case heavy water

(see below), which is kept under high pressure to raise its boiling point and

avoid significant steam formation in the core. The hot heavy water

generated in this primary cooling loop is passed into a heat exchanger

heating light (ordinary) water in the less-pressurized secondary cooling loop.

This water turns to steam and powers a conventional turbine with a

generator attached to it. Any excess heat energy in the steam after flowing

through the turbine is rejected into the environment in a variety of ways,

most typically into a large body of cool water (lake, river, or ocean). More

recently-built CANDU plants (such as the Darlington station near Toronto,

Ontario) use a discharge-diffuser system that limits the thermal effects in

the environment to within natural variations.

CANDU reactors employ two independent, fast-acting safety shutdown

systems. Control rods penetrate the calandria vertically and lower into the

core in the case of a safety-system trip.A second shutdown system is via

gadolinium nitrate liquid "neutron poison" injection directly in to the low

pressure moderator. Both systems operate via separate and independent

trip logic.

CANDU-specific features and advantages

Use of natural uranium as a fuel

• CANDU is the most efficient of all reactors in using uranium: it uses

about 15% less uranium than a pressurized water reactor for each

megawatt of electricity produced.

• Use of natural uranium widens the source of supply and makes fuel

fabrication easier. Most countries can manufacture the relatively

inexpensive fuel .

• There is no need for uranium enrichment facility.

• Fuel reprocessing is not needed, so costs, facilities and waste disposal

associated with reprocessing are avoided.

• CANDU reactors can be fuelled with a number of other low-fissile

content fuels, including spent fuel from light water reactors. This

reduces dependency on uranium in the event of future supply

shortages and price increases .

Use of heavy water as a moderator

• Heavy water (deuterium oxide) is highly efficient because of its low

neutron absorption and affords the highest neutron economy of all

commercial reactor systems. As a result chain reaction in the reactor

is possible with natural uranium fuel.

• Heavy water used in CANDU reactors is readily available. It can be

produced locally, using proven technology. Heavy water lasts beyond

the life of the plant and can be re-used .

CANDU reactor core design

• Reactor core comprising small diameter fuel channels rather that one

large pressure vessel

• Allows on-power refueling - extremely high capability factors are

possible .

• The moveable fuel bundles in the pressure tubes allow maximum burn-

up of all the fuel in the reactor core.

• Extends life expectancy of the reactor because major core components

like fuel channels are accessible for repairs when needed.

Pressurized Water Reactor (PWR)

Pressurized Water Reactor (PWR)

Pressurized water reactors (PWRs) (also VVER if of Russian design) are

generation II nuclear power reactors that use ordinary water under high

pressure as coolant and neutron moderator. The primary coolant loop is kept

under high pressure to prevent the water from boiling, hence the name.

PWRs are one of the most common types of reactors and are widely used all

over the world. More than 230 of them are in use to generate electric power,

and several hundred more for naval propulsion. They were originally

designed by the Bettis Atomic Power Laboratory as a nuclear submarine

power plant.The below diagram shows the PWR and its main parts.

1.Reactor vessel 2.Fuel

elements 3.Control rods 4.Control rod drive 5.Pressurizer 6.Steam generator

7.Main circulating pump 8.Fresh steam 9.Feedwater 10.High pressure

turbine 11.Low pressure turbine 12.Generator 13.Exciter 14.Condenser

15.Cooling water 16.Feedwater pump 17.Feedwater pre-heater 18.Concrete

shield 19.Cooling water pump

The pressurized water reactor belongs to the light water type: the moderator

and coolant are both light water (H2O). It can be seen in the figure that the

cooling water circulates in two loops, which are fully seperated from one

another.

The primary circuit water (dark blue) is continuously kept at a very high

pressure and therefore it does not boil even at the high operating

temperature. (Hence the name of the type.) Constant pressure is ensured

with the aid of the pressurizer (expansion tank). (If pressure falls in the

primary circuit, water in the pressurizers is heated up by electric heaters,

thus raising the pressure. If pressure increases, colder cooling water is

injected to the pressurizer. Since the upper part is steam, pressure will

drop.) The primary circuit water transferes its heat to the secondary circuit

water in the small tubes of the steam generator, it cooles down and returns

to the reactor vessel at a lower temperature.

Since the secondary circuit pressure is much lower than that of the primary

circuit, the secondary circuit water

(red). The steam goes from here to the turbine, which has high and low

pressure stages. When steam leaves the turbine, it becomes liquid again in

the condenser, from where it is pumped back to the steam generator after

pre-heating.

Normally, primary and secondary circuit waters cannot mix. In this way it

can be achieved that any potentially radioactive material that gets into the

primary water should stay in the primary loop and cannot get into the

turbine and condenser. This is a barrier to prevent radioactive contamination

from getting out.

In pressurized water reactors the fuel is usually low (3 to 4 percent)

enriched uranium oxide, sometimes uranium and plutonium oxide mixture

(MOX). In today's PWRs the primary pres

while the outlet temperature of coolant is 300 to 320 °C. PWR is the most

widespread reactor type in the world: they give about 64% of the total

power of the presently operating nuclear power plants.

Two things are characteristic for the pressurized water reactor (PWR) when

compared with other reactor types:


circuit, the secondary circuit water in the steam generator starts to boil







. This is a barrier to prevent radioactive contamination



(MOX). In today's PWRs the primary pressure is usually 120 to 160 bars,



power of the presently operating nuclear power plants.

teristic for the pressurized water reactor (PWR) when

compared with other reactor types:


in the steam generator starts to boil







. This is a barrier to prevent radioactive contamination



sure is usually 120 to 160 bars,



teristic for the pressurized water reactor (PWR) when

• In a PWR, there are two separate coolant loops (primary and

secondary), which are both filled with ordinary water (also called light

water). A boiling water reactor, by contrast, has only one coolant

loop, while more exotic designs such as breeder reactors use

substances other than water (i.e., liquid metal as sodium) for the

task.

• The pressure in the primary coolant loop is at typically 15-16

Megapascal, notably higher than in other nuclear reactors. As an

effect of this, the gas laws guarantee that only sub-cooled boiling will

occur in the primary loop. By contrast, in a boiling water reactor the

primary coolant is allowed to boil and it feeds the turbine directly

without the use of a secondary loop.

Coolant

Ordinary water is used as primary coolant in a PWR and flows through the

reactor at a temperature of roughly 315 °C (600 °F). The water remains

liquid despite the high temperature due to the high pressure in the primary

coolant loop (usually around 2200 psig [15 MPa, 150 atm]). The primary

coolant loop is used to heat water in a secondary circuit that becomes

saturated steam (in most designs 900 psia [6.2 MPa, 60 atm], 275 °C [530

°F]) for use in the steam turbine.

Moderator

Pressurized water reactors, like thermal reactor designs, require the fast

fission neutrons in the reactor to be slowed down (a process called

moderation) in order to sustain its chain reaction. In PWRs the coolant water

is used as a moderator by letting the neutrons undergo multiple collisions

with light hydrogen atoms in the water, losing speed in the process. This

"moderating" of neutrons will happen more often when the water is more

dense (more collisions will occur). The use of water as a moderator is an

important safety feature of PWRs, as any increase in temperature causes the

water to expand and become less dense; thereby reducing the extent to

which neutrons are slowed down and hence reducing the reactivity in the

reactor. Therefore, if reactor activity increases beyond normal, the reduced

moderation of neutrons will cause the chain reaction to slow down,

producing less heat. This property, known as the negative temperature

coefficient of reactivity, makes PWR reactors very stable.

Fuel

The uranium used in PWR fuel is usually enriched several percent in 235U.

After enrichment the uranium dioxide (UO2) powder is fired in a high-

temperature, sintering furnace to create hard, ceramic pellets of enriched

uranium dioxide. The cylindrical pellets are then put into tubes of a

corrosion-resistant zirconium metal alloy (Zircaloy) which are backfilled with

helium to aid heat conduction and detect leakages. The finished fuel rods are

grouped in fuel assemblies, called fuel bundles, that are then used to build

the core of the reactor. As a safety measure PWR designs do not contain

enough fissile uranium to sustain a prompt critical chain reaction (i.e,

substained only by prompt neutron). Avoiding prompt criticality is important

as a prompt critical chain reaction could very rapidly produce enough energy

to damage or even melt the reactor (as is suspected to have occurred during

the accident at the Chernobyl plant). A typical PWR has fuel assemblies of

200 to 300 rods each, and a large reactor would have about 150-250 such

assemblies with 80-100 tonnes of uranium in all. Generally, the fuel bundles

consist of fuel rods bundled 14x14 to 17x17. A PWR produces on the order

of 900 to 1500 MWe. PWR fuel bundles are about 4 meters in

length.Refuelings for most commercial PWRs is on an 18-24 month cycle.

Approximately one third of the core is replaced each refueling.

Control

Generally, reactor power can be viewed as following steam (turbine) demand

due to the reactivity feedback of the temperature change caused by

increased or decreased steam flow. Boron and control rods are used to

maintain primary system temperature at the desired point. In order to

decrease power, the operator throttles shut turbine inlet valves. This would

result in less steam being drawn from the steam generators. This results in

the primary loop increasing in temperature. The higher temperature causes

the reactor to fission less and decrease in power. The operator could then

add boric acid and/or insert control rods to decrease temperature to the

desired point.

Reactivity adjustments to maintain 100% power as the fuel is burned up in

most commercial PWR's is normally controlled by varying the concentration

of boric acid dissolved in the primary reactor coolant. The boron readily

absorbs neutrons and increasing or decreasing its concentration in the

reactor coolant will therefore affect the neutron activity correspondingly. An

entire control system involving high pressure pumps (usually called the

charging and letdown system) is required to remove water from the high

pressure primary loop and re-inject the water back in with differing

concentrations of boric acid. The reactor control rods, inserted through the

top directly into the fuel bundles, are normally only used for power changes.

In contrast, BWRs have no boron in the reactor coolant and control the

reactor power by adjusting the reactor coolant flow rate.Due to design and

fuel enrichment differences, naval nuclear reactors do not use boric acid.

Advantages

• PWR reactors are very stable due to their tendency to produce less

power as temperatures increase, this makes the reactor easier to

operate from a stability standpoint.

• PWR reactors can be operated with a core containing less fissile

material than is required for them to go prompt critical. This

significantly reduces the chance that the reactor will run out of control

and makes PWR designs relatively safe from criticality accidents.

• Because PWR reactors use enriched uranium as fuel they can use

ordinary water as a moderator rather than the much more expensive

heavy water.

• PWR turbine cycle loop is separate from the primary loop, so the water

in the secondary loop is not contaminated by radioactive materials.

• The reactor has high power density.

• The reactor responds to supply more power when the load increases.

Disadvantages

• The coolant water must be heavily pressurized to remain liquid at high

temperatures. This requires high strength piping and a heavy pressure

vessel and hence increases construction costs. The higher pressure

can increase the consequences of a Loss of Coolant Accident.

• Most pressurized water reactors cannot be refueled while operating.

This decreases the availability of the reactor- it has to go offline for

comparably long periods of time (some weeks).

• The high temperature water coolant with boric acid dissolved in it is

corrosive to carbon steel (but not stainless steel), this can result in

radioactive corrosion products to circulate in the primary coolant loop.

This not only limits the lifetime of the reactor, but the systems that

filter out the corrosion products and adjust the boric acid

concentration add significantly to the overall cost of the reactor and

radiation exposure.

• Water absorbs neutrons making it necessary to enrich the uranium

fuel, which increases the costs of fuel production. If heavy water is

used it is possible to operate the reactor with natural uranium, but the

production of heavy water requires large amounts of energy and is

hence expensive.

• Because water acts as a neutron moderator it is not possible to build a

fast neutron reactor with a PWR design. For this reason it is not

possible to build a fast breeder reactor with water coolant.

• Because the reactor produces energy more slowly at higher

temperatures, a sudden cooling of the reactor coolant could increase

power production until safety systems shut down the reactor.

Date post:	20-May-2015
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