Electric Power Generation : Concepts -- 3 ( sai saikumar jn)

Electric Power Generation : Concepts

Hydroelectric Power Generation :

These convert energy of falling water (hydraulic) into Electrical energy. The entire arrangements can be divided into the following stages for the sake of simplicity :
1. Water reservoir.
2. Dam.
3. Valve house.
4. Penstock
5. Water turbine.
6. Alternator.


(Hydro Electric Power Station)

Hydro-electric plants may be classified according to the available head as:

(i) Hydroelectric Power Generation > Low head plants:

For such plants a small dam is constructed across the river to provide necessary head. The excess water is allowed to flow over the dam itself. No surge tank is required for such plants. In figure shows the arrangements of such a plant.


(Low Head Plant)

(ii) Hydroelectric Power Generation > Medium Head Plants:

The forebay provided at the beginning of penstock serves as water reservoir for such plants. In such plants water is generally carried in open channels from main reservoir to the forebay and then to the power house through the penstock. The forbay itself work as surge tank in this case. The common types of prime movers used in these plants are Francis, Propeller and Kaplan. General arrangement of such a plant is shown in the figure.


(Medium Head Plants)

(iii) Hydroelectric Power Generation > High Head Plants :

A typical arrangements for this type of plant is shown in the figure in such plants first of all water is carried from the main reservoir by a tunnel up to the surge tank and then from the surge tank to the power house in penstocks. For heads above 500 meters pelton wheels are used while for lower heads Francis turbine is used.


(High Head Plants)

Hydroelectric plants may also be classified as :

1. Hydroelectric Power Generation > Peak load plants :

A plant used only when the demand of power exceeds the limits of other power plants in the connected system.

2. Hydroelectric Power Generation > Base load plants :

Such plants have high load factors and continue to run for longer durations. Such plants must have enough availability of water. Peak load of such system may be shared by a diesel engine plant, gas turbine plant or even steam power plant

3. Hydroelectric Power Generation > Pumped storage plant :

Such plants are used when the quantity of water available for power generation is sufficient During peak period such plants generate power and the discharged water is collected in tail water pond. During off-peak hours, this water is pumped back to head water pond. Reversible turbines are used in these units so that the same unit can run as turbine for power generation and also as pump for pumping water.

Classification of Turbines:

(a) According to the action of water:

(i) Impulse turbines
(ii) Reaction turbines.

(b) According to the direction of flow:

(i) Radial outwards
(ii) Radial inwards
(iii) Axial flow
(iv) Mixed flow.

(c) Position of shaft:

(i) Horizontal
(ii) Vertical.

Following are the impulse turbines:
(i) Pelton wheel
(ii) Girard wheel
(iii) Jonal turbine
(iv) Banki turbine
(v) Turgo turbine.

Following are the reaction turbines:
(i) Fourneyron turbine
(ii) Francis turbine
(iii)Thomson turbine
(iv) Kaplan turbine
(v) Propeller turbine.

Performance of hydro turbine Turbines commonly used in hydro-power stations are Pelton wheel, Francis turbine and Kaplan turbine. The relative performance of these turbines is shown in the figure.



(Performance of Hydro Turbine)


Some terms associated with hydro-electric power plants are given below:

(a) Spillways , Hydro Spillways of hydroelectric power plant :

A spill way provides discharge of surplus water from storage reservoir into the river down stream side of the dam. It is a sort of safety valve for the dam.

(b) Run off of hydroelectric power plant :

A run off is that portion of precipitation which makes its way towards stream, lakes or oceans. Run off is possible only when the rate of precipitation is more than the rate of water infiltration into the sub-oil.

(c) Hydrograph of hydroelectric power plant :

Hydrograph is a graphical representation between discharge and time. It shows the variation of flow with time.

(d) Unit hydrograph of hydroelectric power plant :

Unit hydrograph is a hydrograph with a volume of 1 cm (or meter) of run-off resulting from a rainfall of specified duration and a real pattern.

(e) Flow duration curve of hydroelectric power plant :

It is a plot of flow available during a period against percentage of time.

(f) Mass curve of hydroelectric power plant :

Mass curve is a plot of cumulative volume of water that can be stored from stream flow versus time in days, weeks or months.

(g) Run of the river plant without pondage :

These plants do not have facilities for storage of water. Flowing water of the stream is used for generating power.

(h) Run of the river plant with pondage :

The utility of a run off the river plant is enhanced by providing storage facilities for water. Such plants can be operated even during off season when the flow of water in the river declines.

Trash rack of hydroelectric power plant :

Trash rack is provided at the inlet end of penstock on the dam to prevent entry of debris which might damage the wicket gates and turbine runners. These are made of steel bars.

Forebay of hydroelectric power plant:

This is a regulating reservoir used to store water temporarily when the load on the plant is reduced. The water from forebay can be used when the load on the turbine increases.

Surge tank of hydroelectric power plant :

Surge tank is provided usually on medium and high head plants. As the load on the turbine decreases the surge tanks acts as a reservoir for holding water. Also pressure variations due to rapid changes of flow of water are taken care of by the surge tanks.

Penstock of hydroelectric power plant :

Penstock is a conduit for conveying water from storage to the turbines. Generally steel pipes are used for this purpose.

Runway speed of hydroelectric power plant :

It is the maximum speed at which a turbine wheel would run under the worst conditions of operation (with all gates open allowing maximum flow of water under maximum head).

Governor of hydroelectric power plant:

Governor is a device used on turbines to regulate the flow of water in proportion to the load on the plant so that constant turbine speed may be maintained.

Draft tube of hydroelectric power plant:

Draft tube a pipe work installed at the turbine discharge to permit a negative or suction head to be established at the runner exit so that turbine can be installed above the tail race. It also converts a large proportion of the velocity energy rejected from the runner into useful pressure head i.e., it acts as recuperator of pressure energy.

Cavitation of hydroelectric power plant:

Cavitation is the formation of bubbles or cavities when the pressure in any part of the turbine drops below the vapour pressure cavitation causes pitting of metal surfaces.

Gas turbine power plant :

A gas turbine power plant consists of a rotary multistage compressor, generally of axial flow type, in which air or working substance is compressed. Compressed air flows to the combustion chamber where fuel is burnt, thereby raising the temperature of the working substance. The high pressure, high temperature working substance expands in a turbine producing mechanical power. Turbine in turn drives a generator for producing electrical energy. A gas turbine works on Brayton cycle.

Simple Gas Turbine Cycle:

(Gas Turbine cycle modified for higher efficiency)

Improving efficiency of simple Brayton cycle

The efficiency of simple Brayton cycle can be improved by

(i) Use of heat exchanger between compressor delivery and combustion chamber, utilizing heat of exhaust gases;
(ii) Use of two stage expansion with re-heating;
(iii) Use of multistage compression with inter-cooling.

A gas turbine using atmospheric air as working medium is known as open cycle gas turbine. In closed cycle gas turbine the working substance is recirculated and it does not come into direct contact with atmospheric air. A fluid with better thermodynamic properties can be used as working substance in such turbines.

Gas turbines can run on gaseous, liquid as well as solid fuels. As compared to a steam turbine, it does not require condenser and associated bulky cooling arrangements. Gas turbines operate on lower pressures as compared to steam turbines hence stress on various parts is less.

For starting a gas turbine enough power is required to drive the compressor which is nearly 30 to 40% of the normal output.

Sometimes gas turbines are used in combination with steam cycle where exhaust heat of the gas turbine is used in boiler for steam cycle.

Nuclear Power Stations : These convert nuclear energy into electrical energy.

Nuclear power reactor :

Nuclear power reactor is basically that part of nuclear power plant where energy released as a result of nuclear fission of radioactive material is utilized to heat the coolant which may in turn generate steam or be used in a gas turbine. The nuclear reactor may thus be regarded as a substitute for the boiler fire box of steam plant or combustion chamber or a gas turbine plane. The steam or the gas may be used as working fluid in nuclear power plant. The nuclear power plant may be of steam driven turbine or gas driven turbine as per the choice of the fluid.

The following functions are associated with the working of nuclear reactor:
(i) Producing a chain reacting or critical system,
(ii) Controlling the level of power release from the system,
(iii) Using spare neutrons to convert fertile into fissile material,
(iv) Protecting personnel from harmful radiations emanating from the core.



(Nuclear Power Station)

The purpose of the moderator in the reactor core is to moderate or reduce the neutron speeds to a value that increased the probability of Fission occurring. The moderator is a material having low atomic number and small neutron absorption cross-section which slows down the fast neutron by elastic collision. Three commonly used moderators are:

(i) Graphite
(ii) Heavy water
(iii) Beryllium.



A control rod in a nuclear power reactor is used to initiate the nuclear chain reaction when starting the reactor from cold; and to maintain the chain reaction at a steady value during the operation of the reactor. Also control rod shuts down the reactor automatically under emergency conditions. All this is being done without melting of fuel rods disintegration of coolant and destructive of reactor as the amount of energy released is enormous.

The materials used for control rods must have very high absorption capacity for, neutrons. The commonly used materials for control rod are:
Cadmium, boron

Three different types of fuel used in nuclear reactors are:

(i) Uranium ₉₂ U ²³⁵
(ii) Secondary Uranium ₉₂ U ²³⁵
(iii) Plutonium ₉₂ U ²³⁵

 

Classification of nuclear reactors :

Nuclear reactors may be classified on the basis of neutron energies as follows:

1. Fast Nuclear reactors :

Here fusion occurs with high energy neutrons, in the absence of moderator.

2. Intermediate reactors :

With the use of some moderator, the reactors are known as intermediate reactors.

3. Thermal Reactors :

When the energy of neutrons is reduced to low electron voltages i.e. thermal range, the reactors are known as thermal reactors.

On the basis of fuel reproduction characteristics, the nuclear reactors may be classified as

A. Non regenerative type Nuclear Reactors :

These reactors do not create an appreciable amount of replacement fuel as the fuel is burned. Reactors using highly enriched fuel containing 90% or more of U²³⁵ in the fuel are of non-regenerative type.

B. Regenerative type :

In these reactors fuel is slightly enriched. These reactors do not replace the used fuel fully.

C. Heterogeneous reactors :

Here fuel is in the form of rods, plates and is placed in the matrix of moderator.

D. Fast Breeder reactor :

These reactors use highly enriched uranium or plutonium as fuel. A small core of few cubic meters is sufficient to give a heat output of about 100 MW. The core is surrounded by a blanket or ordinary fertile Uranium 238 or thorium which can be converted to fissile plutonium or Uranium 233, It is possible to get more new atoms of fissile material in the blanket than are destroyed by fission in the core. Such a reactor is known as breeders type. The major problem in such reactors is the removal of from the core. Liquid sodium is used as the coolant. A primary and secondary coolant circuit is used to transfer heat. Foolproof arrangements are incorporated to guard against the failure of coolant circuit. Even a momentary stoppage of the blow of coolant will result in disastrous rise in temperature.



(Fast Breeder Reactor)

Terms associated with nuclear reactions are given below :

Reflector in nuclear reactor:

In order to keep the size of the reactor small, and hence the amount of the fashionable material, it is necessary to conserve the neutrons. For this purpose the reactor core is surrounded by a material which reflects the escaping neutrons back into the core. This material is known as reflector.

Shielding in nuclear reactor:

The intensity of gamma and neutron radiation coming from the reactor core is far greater than the human body can tolerate. Hence it is necessary to surround the reactor with a shielding material to reduce the radiation intensity to the levels which are not harmful to personnel.

from ur's -- Bellapuri saikumar
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