Hydro Electric Power Plant

Components of Hydel Power Plant

 Catchment area – Whole area behind the dam, draining into a stream or river
across which the dam has been built.
 Reservoir –The purpose of the storing of water in the reservoir is to get a uniform
power output throughout the year. A reservoir can be either natural or artificial.
 Dam – A dam is any barrier that holds water; the water stored behind the dam is
used to drive turbines that are connected to electrical generators. It acts as an
artificial reservoir.

Based on structure and design, dams are classified as gravity dams, arch dams and
buttress dams.
Types of dams

1. Gravity dams –Gravity dams rely on their own weight to hold back large volumes
of water.
2. Arch dams – An arch dam is curved in plan, with its convexity towards the
upstream side. eg. Idukki dam.
3. Buttress dam – A buttress dam is a dam with a solid, water- tight upstream side
that is supported at intervals on the downstream side by a series of buttresses or
supports

 Trash rack – The function of trash rack is to prevent the flow of debris, sand
and fishes to the turbine.
 Surge tank –It is a storage reservoir used to absorb the sudden rises of water
pressure, as well as to provide extra water during a drop in water pressure.
  Turbine – The function of turbine is to act as a prime mover to convert the
potential energy of water in to mechanical energy. It is explained in a later
section in detail.
 Runner – The runner is a circular wheel on which a series of curved vanes are
mounted. Vanes are so designed that water enters and leaves the runner without
shock.
 Power house – The powerhouse accommodates prime mover, generator (generate
electrical power using mechanical power obtained from the turbine), accessories
and control room sometimes transformer also. Water after passing through the
turbine is discharged into a downstream called as tailrace, which carries it into the
river.

Classification hydro electric power plants

1. Classification with respect to quantity of water available

a) Run-off river plants – Run-of-the-river hydroelectric harvest the energy from
flowing water to generate electricity in the absence of a large dam and reservoir.
b) Reservoir plants – A reservoir plant is that which has a reservoir of such size as
to allow carrying over storage from wet season to the next dry season.

2. Classification according to availability of water head

a) High-head hydro-electric plants (head more than 250 m)
b) Medium-head hydro-electric plants (head ranges from 30 m – 250 m)
c) Low-head hydro-electric plants (head is less than 30 m)

3. Classification according to nature of load

a) Peak load plants – The peak load plants are used to supply power at the
peak demand phase.
 b) Base load plants – A base load power plant is one that provides a steady
flow of power regardless of total power demand.

Selection of site for a hydro power plant

1. Water available – The most important aspect of hydro-electric plant is the

availability of water at the site since all other designs are based on it. Therefore the run-
off data at the proposed site must be available.

2. Water-storage – The output of a hydropower plant is non-uniform due to

variations in rain fall. To have a uniform power output, storage is needed so that excess
flow at certain times may be stored to make it available at the times of low flow. To
select the site of the dam; careful study should be made of the geology and topography of
the catchment area to see if natural foundations could be found and put to the best use.

3. Head of water – In order to generate a requisite quantity of power it is necessary

that a large quantity of water at a sufficient head should be available. The level of water
in the reservoir for a proposed plant should always be within limits throughout the year.

4. Distance from load center – Most of the time the electric power generated in a
hydro- electric power plant has to be used some considerable distance from the site of
plant. For this reason, to be economical on transmission of electric power, the routes and
the distances should be carefully considered since the cost of erection of transmission
lines and their maintenance will depend upon the route selected.

5. Access to site – It is always a desirable factor to have a good access to the site of
the plant. This factor is very important if the electric power generated is to be utilized at
or near the plant site. The transport facilities must also be given due consideration.

Hydrologic cycle

The hydrologic cycle, also known as the water cycle describes the circulation of water in
the earth-atmosphere system.
1. Precipitation – It includes all the water that falls from atmosphere to earth surface.
Precipitation is of two types, viz., liquid precipitation (rain fall) and solid precipitation
(eg. snow).

2. Run-off – Run-off is the part of water cycle that flows over the land as surface water
instead of being infiltrated into soil or evaporating.

a) Surface runoff is that portion of rainfall which enters the stream immediately
after the rainfall.

b) Sub-surface runoff is that part of rainfall, which first reaches into the soil and
moves laterally without joining the water - table to the streams, rivers or oceans.

c) Base flow is that part of rainfall which after falling on the ground surface
which get infiltrated into the soil and meets the water table (level below the surface of
the ground where water can be found) and flow to the streams oceans, etc.

Runoff = Surface runoff + Base flow (Including sub - surface runoff)

4. Evaporation – Transfer of water from liquid to vapour state is called evaporation.

5. Transpiration – The process by which water is released to the atmosphere by the

plants is called transpiration.

6. Sublimation – Sublimation results from when pressure and humidity are low. It is
not only liquid water that can evaporate to become water vapor, but ice and snow, too.
Due to lower air pressure, less energy is required to sublimate the ice into vapour.

The hydrological cycle can be briefed as (hydrological equation). I – Q = ∆S;

where,

I = Inflow of water to a given area during any given time period,

Q = Outflow of water from the area during the selected time period,

ΔS = Change in storage of water in a given area during the time period.

This equation states that during a given period, the difference between the total inflow of
water and out flow of water must equal the change in storage of water
Factors affecting run-off

1. Topography of catchment area – Steep and impervious areas will produce large
percentage of run-off. The water will flow quickly and absorption losses will be
small. The size of catchment has a definite effect on the runoff. More the area,
more will be the runoff. So also, the shape will have a definite effect on the
runoff. In case of a fan-shaped catchment area, the period of the resulting
hydrograph will be less and thus more peak flow may be expected. In case of an
elongated catchment, the period of the resulting hydrograph (graph showing
discharge (runoff) of flowing water with respect to time for a specified time) will
be comparatively more and thus more will be the infiltration losses and less will
be the runoff
2. Nature of rainfall – Short and hard showers may produce relatively little run-off.
Rains lasting longer time results in larger run-off.
3. Geology of area – The run-off is very much affected by the types of surfaces soil
and sub-oil, types of rocks, etc. Rocky areas will give more run-off while pervious
soil and sandy soil will give less run-off.
4. Vegetation –Thick vegetation like forest consumes a portion of rain fall and also
acts as a obstruction for run-off.
5. Other climate factors – Other factors such as temperature wind velocity,
humidity, annual rainfall etc., affect the water losses from watershed (small
streams) area.

Disadvantages of hydro electric power plants.

1. Cost of transmission is high since most of the plants are in remote areas.

2. Hydro-power projects are capital-intensive with a low rate of return.

3. It takes considerable long time for the erection of such plants.

4. Power generation is dependent on the quantity of water available, which may vary
from season to season and year to year. If the rainfall is in time and adequate, then only
the satisfactory operation of the plant can be expected

5. Such plants are often far away from the load centre and require long transmission lines
to deliver power. Thus the cost of transmission lines and losses in them are more.

6. Large hydro-plants disturb the ecology of the area, by way of deforestation,

destroying vegetation and uprooting people. The emphasis is now more on small,
mini and micro hydel stations.

Nuclear Power Plant

 In nuclear power plant, heat energy available from nuclear fission is used for the
generation of steam.
 Nuclear fission can be defined as the process, in which a nucleus is split into two
divisions, more or less of equal mass releasing energy in the form of
electromagnetic radiation and kinetic energy.
 The heat produced by fission in the nuclear reactor is carried out of the reactor by
coolant. This heat is used to generate steam. This heat transfer takes place in a
heat exchanger such as boiler.
 The pressurized steam is then fed to a steam turbine which is connected to a
generator.
Components of Nuclear power plant

1. Nuclear reactor – It is an apparatus in which nuclear fuel is subjected to nuclear

fission.

2. Heat exchanger – The coolant gives up heat to the heat exchanger, which utilized
for generating steam. After giving up heat, the coolant is fed back to the reactor.

3. Steam turbine – The steam produced in the heat exchanger is fed to turbine for
doing useful work.

4. Generator – The steam turbine drives the generator which converts mechanical
energy in to electric power.
 Nuclear reactor is an apparatus in which nuclear fuel is subjected to nuclear
fission.

1. Fuel – Nuclear fuels usually used in the reactors are isotopes (atoms of the
same element having the same numbers of protons, but different numbers of neutrons) of
Uranium and Plutonium. Isotopes like U-233, U-235 and Pu-239 can be fissioned by
neutrons of all energies, whereas isotopes U-238, Th-232 (Thorium) and Pu-240 are
fissionable by high energy (14 MeV) only. Usually pellets of fissionable materials are
arranged in tubes to form fuel rods.

2. Moderator – Moderator is used to slow down the kinetic energy of fast

moving neutrons. This has to be done as only the slow neutrons maintain the fission chain
reaction. The neutrons collide directly with the moderator and thus slowed down.
Substances like light water, heavy water, carbon, beryllium are used as moderator.

3. Control rods – Control rods are used to control the nuclear chain reaction. It is
an essential part of a reactor and serves the following purposes.

a) For starting the reactor.

b) For maintaining at that level.

c) For shutting the reactor down under normal or emergency conditions.

Control rods are usually made up of cadmium and boron. Control rods control the
chain reaction by absorbing neutrons

4. Coolant – Purpose of coolant is to extract heat generated by the fission process.

The various fluids used as coolant are water (light water /heavy water), gas (Air, CO2,
Hydrogen), and liquid metal cooled reactors etc.

5. Reactor vessel – It is a strong walled container housing the reactor core, shield
and the reflector. It is strongly built so as to withstand high pressures developed.

6. Reflector – Reflector is used to reduce the loss of neutrons by reflecting back

into the core of the nuclear reactor. Reflector is generally made of the same material as
the moderator.

7. Shield – Shield prevents the transfer of radiation o the external world.

Advantages of nuclear power plant

1. No problem of fuel transportation, storage, etc.

2. Less man power is required.
3. It is more economical compared to thermal plant.
 4. Power capacity of plant is very high.
5. Capital cost except for reactor is very less.
6. It does not depend up on the condition of the weather.
7. By this process we can conserve the fuels like oil, coal gases and other by-
products.

Disadvantages of nuclear power plant

1. Nuclear radiation causes severe environmental problems.

2. Disposal of radioactive nuclear waste is menace.
3. Varying load conditions are not suitable.
4. Capital cost is very high for the reactor.

Pressurized Water Reactor (PWR)

  Pressurized Water Reactor (PWR) make use of two loops viz., primary and
secondary loops to convert the heat generated by the fuel into electric power.
 In the primary loop, the pressurizer maintains a high pressure in the water in the
range of 150 bar. The pressurized water (coolant) is circulated in the reactor. Due
to the high pressure of the water, the water does not boil.
 The coolant gets heated in the reactor and the hot water enters the boiler and
transfers heat to the feed water in the boiler in the secondary loop. The transfer of
heat is accomplished without mixing the two fluids, which is desirable since the
primary coolant might become radioactive.
 Feed water evaporates and runs the turbine.

Advantages of PWR

1. Because the water used in the high-pressure water loop is isolated from water in
the steam loop, no radioactive material is contained in the steam.
2. PWR has high power density and has compact size.

Disadvantages of PWR

1. Capital cost is high as high primary circuit requires strong pressure vessel.
2. In the secondary circuit, the thermodynamic efficiency of the plant is quite low.

Boiling Water Reactor (BWR)

  In Boiling Water Reactor (BWR), the coolant (water) used in the reactor absorbs
heat produced during the fission reaction in the reactor.
 The fuel used is enriched uranium oxide. Water evaporates and steam is generated
in the reactor itself. In this type of reactor, there is no need of separate boiler.
 In BWR, the coolant is in direct contact with turbines, so if a fuel rod had a leak,
radioactive material could be placed on the turbine.

Advantages of BWR

 A major advantage of the BWR is that the overall thermal efficiency is greater
than that of a pressurized water reactor because there is no separate heat
exchanger.
 The pressure inside the pressure vessel is not high so, a thicker vessel is not
required.

Disadvantages of BWR

 Possibility of radioactive contamination in the turbine mechanism.