Hydro-Electric Power Plants

How does a Hydroelectric power plant work?

(Potential Energy)

(Kinetic Energy) (Mechanical Energy)

(Electrical Energy)

• Dam : Dams are structures built over rivers to stop the water flow and form a reservoir.

• Spillway : A spillway as the name suggests is a way for spilling of water from dams. It is  used to provide for the release of flood water from a dam. It is used to prevent over topping of the dams which could result in damage or failure of  dams.

• Penstock and Tunnel : Penstocks are pipes which carry water from the reservoir to the turbines inside power station. They are usually made of  steel. A tunnel serves the same purpose as a penstock. It is used when an obstruction is present between the dam and power station such as a mountain

• Surge Tank : The sudden surges of water in penstock is taken by the surge tank, and when the water requirements increase, it supplies the collected water thereby regulating water flow and pressure inside the penstock.

• Power Station : Power station contains a turbine coupled to a generator.

Components of a hydroelectric power plant

Introduction

• In hydroelectric power plants the energy of water is used to drive the turbine which, in turn, runs the generator to produce

electricity.

• Energy of water falling through an appreciable vertical height is converted to shaft work in the turbine.

• The hydraulic power is thus a naturally available renewable energy source given by the following equation:

Where,

P = Hydraulic power in watts

g = Acceleration due to gravity, 9.81 m/s2

Q = Water flow or discharge, m3/s

H = Height of fall of water, or head, m

QHgP

Introduction

• Substituting density of water (1000 kg/m3), the electrical energy produced over a period ‘t’ hours will then be:

or,

Here, η is the efficiency of the turbine-generator assembly and it varies between 0.5 and 0.9.

• The power developed thus depends on the quantity (Q) and head (H) of water.

wattstHQW 100081.9

kWhtQHW 81.9

Advantages of water power• Water source is perennially available. Water, after passing through turbines to do work, can continue downstream for irrigation and drinking water schemes. Its utility is therefore multifold.

• Its running costs are much less than thermal and nuclear plants because of absence of fuel and its transportation.

• No pollution or exhaust into atmosphere. No problem of effluent handling.

• The hydraulic turbine can be started and stopped quickly while thermal and nuclear plants have long start-up and shut-down time.

• Hydro electric plant is comparatively simple in concept and therefore more reliable.

• While effective life span of thermal and nuclear power plants is of the order of 30 years, hydroelectric plants can easily last more than 50 years.

• Hydro plants provide additional benefits like irrigation, flood control, afforestation and aqua culture.

• Because of its simplicity, hydro plants require less personnel for its operation.

Disadvantages of water power

• Hydro power plants are capital- intensive with low returns. The annual interest of the capital cost is a major portion of the power tariff.

• The gap between start and completion of hydro plants is large (10 to 15 years).

• Power generation is dependent upon quantity of water available which may change from season to season. Rain fall must be

adequate and timely for satisfactory operation of the plant.

• Hydro plants are generally far away from the load centre and therefore require long transmission lines to deliver power. This adds to cost and transmission loss.

• Large hydro plants disturb the ecology of the area by way of deforestation, destroying vegetation and displacement of people. Therefore the emphasis is on small, mini and micro hydel

stations.

Optimization of Hydro-Thermal mix

• Earlier, Hydroelectric power plant was an exclusive source of power. Because of seasonal variation of water availability for power

generation, it is no longer used as primary power source.

• To meet the variable demand for power, a combination of hydroelectric and thermal power plants is used for optimal power generation.

• Load sharing by hydroelectric plant is maximum when water availability is maximum and thermal plant is used for peak load demand.

• When water availability is low, thermal plant will be used as base load plant and peak load is met by hydroelectric plant.

Low flow day High flow day

Selection of site for Hydroelectric plant

Factors to be considered while selecting a site for hydroelectric power plant are:

• Availability of water

• Water storage capacity

• Available water head

• Accessibility of the site

• Distance from the load centre

• Type of land of site

Selection of site for Hydroelectric plant

Availability of water: Design and capacity of the hydel power plant depends on the amount of water available at the site. Data regarding run-off and precipitation with maximum and minimum quantity of water available should be made available to:

• Decide the capacity of the plant

• Decide the size of peak load plant (Steam, Diesel or Gas Turbine)

• Provide adequate spillways or gate relief during flood period

Water storage capacity: Since rain fall will not be uniform throughout the year, necessary storage of water has to be planned for continuous generation of power. The storage capacity can be estimated with the help of mass curve.

Selection of site for Hydroelectric plant

Availability water head: Since amount of power generated is a function of kinetic energy of water, sufficient head of water should be available for power generation. An increase in effective head, for a given output, reduces the quantity of water to be supplied to the turbines.

Accessibility of the site: The site should be easily accessible by rail and road. An inaccessible terrain will affect the movement of men and materials.

Distance from the load centre: In order to reduce transmission losses as well as transmission cost, the site should be as close as possible to the load centre.

Type of the land of the site: The dam constructed at the site should have large catchment area to store water at high head. The foundation rocks of the masonry dam should be strong enough to withstand earthquakes and the thrust of water when the reservoir is full.

The hydrological cycle

Runoff + Seepage + Evaporation + Transpiration = Precipitation ± Change in storage

Hydrographs

• Variation of river flow at a given site depends on the geographical, geological and topographical features of the drainage area.

• Hydrographs show the variation of discharge (river flow) with time.

Time

Hydrographs

Hydrographs

A storm hydrograph or flood hydrograph helps predict flooding events and hence implementation of flood prevention measures

0 12 24 36 48 30 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

• One way of representation is to plot flow duration curves which show the time when a stream flow is equaled or exceeded in any period – daily, weekly, monthly…

• The area under the flow duration curve represents the average yield from the stream.

• If we plot the power generated (instead of flow rate) vs time, the power duration curve is obtained and the area under the curve then represents the average yield of power from the hydel project.

• Figure in next slide :

• Area OABC represents primary yield and hence primary power

• Area BCDE represents secondary water power available (and hence available secondary water power)

• To meet the power demand under DEF, a thermal plant of capacity BF is required as a peak load station.

Flow duration curve

Primary yield

Primary yield (and power) shaded yellow (area under OABC)

Area under BCDE = Secondary power available from water

If power under DEF is to be met, then a thermal power plant of BF capacity needs to be established to take peak load. That is, beyond 50% of the time, thermal plant has to be operational.

Flow duration curve

Typical flow duration curves

Hydrograph & Flow duration curve – Worked example

MonthMean discharge

(millions of cu.m.)Month

Mean discharge (millions of cu.m.)

January 30 July 80

February 25 August 100

March 20 September 110

April 0 October 65

May 10 November 45

June 50 December 30

The run off data for a river at a particular site is tabulated below:

• Draw the hydrograph and find the mean flow.

• Draw the flow duration curve.

• Find the power developed if the head available is 90 m and the overall efficiency of generation is 86%. Assume each month has 30 days.

Hydrograph & Flow duration curve – Worked example

12

3045651101008050100202530arg

edischmeanTheThe mean discharge

sm /08.4712

565 3 million m3/month

Hydrograph

0

20

40

60

80

100

120

J F M A M J J A S O N D

Time, months

Dis

char

ge

(mill

ion

cu

.m./m

on

th)

Mean flow

Hydrograph & Flow duration curve – Worked example

Discharge / month (million cu.m)

Total no. of months during which flow is

availablePercantage time

0 12 100.00

10 11 91.70

20 10 83.30

25 9 75.00

30 8 66.70

40 6 50.00

50 5 41.70

60 4 33.30

70 3 25.00

80 3 25.00

90 3 25.00

100 2 16.70

110 1 8.30

To obtain the flow duration curve, first find the duration of time for which certain flows are available as shown in the table below:

Hydrograph & Flow duration curve – Worked example

P = (ρgHQη) 10-6 MW

(47.08x103x9.81x90x0.86)(30x24x3600)

=

= 13.79 MW

The mass curve

• The use of the mass curve is to compute the capacity of a hydel plant.

• Mass curve is a plot of Accumulated flow (in hectare-metre) against time, made from the records of mean monthly flows

of a stream.

• If the curve is horizontal, the flow is zero and if there is high rate of flow, the curve rises steeply. Dry periods are indicated as concave depressions. TIME

Acc

um

ula

te f

low

s (h

a-m

)

MASS CURVE

ST

OR

AG

E

Y

X

STORAGE VOLUME REQIRED FOR CONTINUOUS RELEASE OF WATER AT AVERAGE DISCHARGE RATE

• The vertical distance between tangents to high and low points on the curve is an indicator of the storage volume required for continuous release of water at average discharge rate

Classification of hydroelectric plants

Hydroelectric power plants can be classified as follows:

• According to availability of head

• High head power plants (100m and above)

• Medium head power plants (30 to 100m)

• Low head power plants (25 to 80m)

• According to nature of load

• Base load station

• Peak load station

• According to quantity of water available

• Run-of-river plant with out pondage

• Run-of-river plant with pondage

• Storage type plants

• Pump storage plants

• Mini and Micro plants

High head power plants

• These plants work with water heads in the range of 100 to 2000 metres.

• Penstock / Tunnel has surge chamber at exit of dam to absorb pressure fluctuations.

• Flow is controlled by head gates at the tunnel intake, butterfly valve at the top of the penstocks, and gate valve at the turbine.

• The Pelton wheel is the common prime mover for high head stations.

(Pelton wheel)

Medium head power plants• These plants work with water heads in the range of 30 to 100 metres.

• The forebay at the beginning of penstock serves as the reservoir.

• Forebay itself acts as surge tank in such plants. Surge tank prevents sudden pressure increase in penstock when the turbine inlet valve is suddenly closed when load reduces.

• The Francis turbine is the common prime mover for medium head stations.

Low head power plants

• These plants usually consist of a dam across a river.

• A sideway stream diverges from the river at the dam.

• Later this channel joins the river further downstream.

• Vertical shaft Francis turbine or Kaplan turbine is the generally used in low head stations.

(Vertical shaft Francis or Kaplan turbine}

Base load and Peak load power plants

• Base load power plant : 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. 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.

• Peak load power plant : These are 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%.

Run-of-river power plant Without pondage:

• This type of plant does not store water and uses the water as it comes.

• There is no control on the flow of water. During low loads or floods, water is wasted and during low flow, plant capacity is considerably reduced.

• During good flow conditions, such plant can cater to base load of the system and when the flow is low, it can function as a peak load station.

• The run-of-river plant may be made for load service with pondage though storage is seasonal.

With pondage:

• Pondage is collection of water behind a dam at the plant to increase the stream capacity for a short period, say a week.

• It is more reliable and its generating capacity is less dependent on the flow rates of water available.

Storage type power plant

• A storage type plant has a reservoir that supplies

more than the minimum natural flow on a continuous basis.

• Such plants can be used as base load plants as well

as peak load plants as water is available with control as required.

• Majority of hydroelectric plants are of this type.

Pumped storage type power plant

• Pumped storage plants are employed at places where quantity of water available for power generation is inadequate.

• Here, the water after passing through the turbine is stored in a tail race pond.

• During low load periods, the tail race water is pumped back to head reservoir using the extra energy available.

Advantages of Pumped storage type power plant

• Substantial increase in peak load capacity of the plant at comparatively low capital cost.

• Due to load comparable to rated load on the plant, the operating efficiency of the plant is high.

• There is an improvement in the load factor of the plant.

• The energy available during peak load periods is higher than that during off peak periods so that, in spite of losses incurred in pumping there is overall gain.

• Load on the hydroelectric plant remains uniform.

• The hydroelectric plant becomes partly independent of the stream flow conditions.

Some pumped storage plants use “Reversible Turbine Pump”. These units work as Turbines while generating power and as pump while pumping water to storage. The generator in this case works as motor during reverse operation. With the use of reversible turbine pump sets, additional capital investment on pump and its motor can be saved.

Mini and Micro hydroelectric power plants

• To tap the low head hydro potential scattered across our country, mini and micro hydel plants are advantageous. Estimated potential from such plants can be has high as 20,000 MW.

• Mini plants work in the water head range of 5 m to 20 m

• Micro plants work with water head less than 5 m

• Each plant can develop 100 kW to 1000 kW of power.

• It is possible to set up a small hydro generating plant of about 5 MW consisting of several mini / micro plants

within a short period of 12 to 18 months. Such plants are in existence in Himachal Pradesh, Arunachal Pradesh, West Bengal and Bhutan.

Dam

A dam performs the following two basic functions

• It develops a reservoir of the desired capacity to store

water.

• It builds up a head for power generation.

A dam can have moderate head and a large storage capacity like Aswan dam in Egypt (156 billion cu.m. and 111.5m head) or high head with moderate storage capacity like the Hoover dam in the USA (38 billion cu.m. and 222m head.

Classification of Dams

4 TYPES OF DAMS

• Gravity dams. • Arch dams. • Buttress dams. • Embankment dams

Gravity Dams

• Principle : Water pushes against the gravity dam, but the heavy weight of

the dam pushes down into the ground and prevents the structure from toppling.

• Gravity dams are solid concrete dams where the base of the dam is thicker

/ wider than the top.

• Gravity dams are usually built on solid rock

foundations.

The Grand Coulee dam, USA

Forces on a gravity dam

Arch Dams

• This type of dam is like an arch bridge.

• The arch curves towards the flow of the water.

• Arch dams are normally built in a narrow canyon or gorge.

• Due to water pressure, the arch squeezes together. The weight of the dam pushes the structure down to the ground.

The Hoover dam, USA

Forces on an Arch dam

Arch Dams

Buttress Dams

Forces on a buttress dam

Water pushes against the buttress dam, but the buttress pushes back and prevents the dam from toppling over. The weight of the buttress dam also pushes down into the ground.

Buttress Dams

• Embankment dams are very low in height compared to their length.

• The wall of the dam has a gently sloping curve.

• They are normally made of clay, stones and rock

• Water pressure acts on one side of the dam, but the wide base and

weight of the embankment dam provides stability to the dam.

Forces on an embankment dam

Tarbela dam, Pakistan

Embankment Dams

Embankment Dams

•Earth filled dams•Rock filled dams

DAMS based on materials

Concrete masonry dams

Nurek Dam, Tajikistan

Hoover Dam, USA

Nagarjunasagar dam (Stone masonry dam)

DAMS based on materials

7700 m long x 196 m high; 14 GW installed capacity

DAMS based on materials

Itaipú Dam (Brazil & Paraguay) – RCC dam

Guri Dam (Venezuela) (RCC dam, 690 m long)

DAMS based on materials

Grand Coulee Dam (USA); Concrete dam; 1592 m long x 168 m high; 6809 MW installed capacity

DAMS based on materials

HIRAKUD DAM (26KM LONG X 61 m HIGH) ACROSS MAHANADI – ORISSA (CONCRETE+EARTHEN)WORLD’S LARGEST EARTHEN DAM

DAMS based on materials

Selection of turbines for Hydel plants

Type of power plant Water headType of turbine

Turbine Type

Low head < 60 m Kaplan or Propeller

Reaction

Medium head >60 m and <300 m Francis Reaction

High head >300 m Pelton wheel

Impulse

Classification of hydraulic turbines

• Reaction Turbines

• Derive power from pressure drop across turbine

• Totally immersed in water

• Angular & linear motion converted to shaft power

• Francis and Kaplan turbines

• Impulse Turbines

• Convert kinetic energy of water jet hitting buckets

• No pressure drop across turbines

• Pelton turbines

Francis turbine

• A type of hydropower reaction turbine that contains a runner that has water passages through it formed by curved vanes or blades. The runner blades, typically 9 to 19 in number, cannot be adjusted.

• The Francis turbine has a wide range of applications and can be used for fall heights of 2–800 meters.

• The largest Francis turbines have an output of 750 MW.

Francis turbine

Francis turbine

Francis turbine – Grand Coulee Dam

• Kaplan turbines are well suited to situations in which there is a low head and a large amount of discharge.

• The adjustable runner blades enable high efficiency even in the range of partial load, and there is little drop in efficiency due to head variation or load.

Kaplan turbine

Kaplan turbine

Vertical Kaplan turbine

Horizontal Kaplan turbine

Pelton turbine

Pelton turbines are impulse type turbines and they are suited for high head, low flow applications

Pelton turbine

Penstock

Penstock is a pipe of adequate diameter and thickness that transports water from the storage point to the turbine in the power house.

• To avoid water hammer, a Surge Tank is installed between the dam and the powerhouse at the water entry of the penstock.

• When the flow to the turbine is reduced, water flows into the surge tank and conversely for increased load , the initial extra water required is from the

surge tank.

• The tank should not overflow when the turbine is suddenly shut down, nor allow air to be drawn into the system following a sudden increase in

demand.

Surge Tank

A simple surge Tank

Inclined surge tank

Inclined surge tank

Conduit

• Effective water surface area increases• Lesser height of surge tank can be used

Inclined surge Tank

Expansion chamber surge Tank

• Expansion chambers limit extreme water surges

Surge tank

Restricted orifice

• Throttled surge tank• Throttle creates appreciable friction loss when water rushes into the tank or flows out from the tank

Restricted orifice surge Tank

Water intake system

The intake system supplies water from the reservoir to the turbine through the penstock. The intake system consists of

• Gates

• Operating and hoisting system

• Trash screen

• Cleaning mechanism

Gates:

Gates have the following uses:

• The reservoir storage capacity can be increased by installation of movable gates.

• The water level in the reservoir can be varied by operating the gates.

Gates

There are different types of gates:

• Vertical lift gate

• Radial gate

• Rolling gate

• Drum gate

Vertical lift gates move vertically up and down on actuation to vary the effective opening available for flow of water. This type of gate is generally used for smaller capacity hydraulic power plants.

Gates

Gates

Radial gates are in the form of segment of a cylinder supported on a steel framework. The steel framework is supported on trunions. The gate is hoisted by means of winches.

Drum gates consist of a segment of a hollow cylinder which hinges in the recess provided on top of the spill way. The drum gate is actuated by the water pressure itself to maintain a constant level.

Gates

Penstock Valves

Penstock entry valves are generally sluice type gate valves that can admit required flow of water into the penstock. The gate is moved up or down to control the area available for flow of water.

Gate

Hand wheel for manual operation of small valves

Penstock interface-two sides

Butterfly ValvesUsed as shut-off or gross flow control functions on penstocks. Flapper opening within the housing determines the area available for flow of water.

View from inside penstock, valve in open position

Housing

Flapper

Shaft & Bearing

Butterfly Valves

Due to their large size, penstock butterfly valves are motor operated.

Flow control ValvesThey are needle type valves used for precise control of flow. In Pelton turbines, the entire pressure drop occurs across such valves and the resulting kinetic energy of water drives the Pelton turbine. Several such valves will be arranged along the periphery to facilitate partial admission of water to control power generation.

Butterfly valve (Shut-off)

Needle valves arranged around periphery

Flow control Valves

Pelton wheel bucket

High pressure water from penstock

Needle movement

Pressure drop across the needle valve results in high velocity water jet to strike the buckets of Pelton turbine to generate mechanical power. The needle can be moved axially to control the area of opening of the valve.