hydro power presentation

American International University-Bangladesh

(AIUB)

Hydro Electric Power Station

1

Hydro Power

Hydro-electric power is the power obtained from the energy of falling water where

as hydro-electric power plant is the power plant utilizing the potential energy of water at

a high level for the generation of electrical energy.

Hydro-electric power plants, however, can not be located everywhere. Firstly there

must be an ample quantity of water at sufficient head and secondly a suitable site must

be available. The amount of power that can be developed depends on the quantity of

water available, the rate at which it is available, the head etc. The electrical power, P

developed is given by the expression:

P = w Q H 9.81 watts

where w = specific weight of water in kg/m3

Q = rate of flow of water in m3/s

H = height of fall or head in metres

and = efficiency of generation.

In a hydro-electric power station, water head is created by constructing a dam across a

river or lake. The pressure head of water or kinetic energy of water is utilized to drive

the water turbines coupled to alternators and, therefore, generation of electrical power.

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Ten of the largest hydroelectric producers

Major schemes under construction

Worlds 5 largest dams

Three Gorges Dam, China

Merits of Hydro-electric Power Plants Hydro-electric power plants offer many distinct advantages over other power plants. These advantages can be summarized as under: No fuel is required by such plants as water is the source of energy. Hence operating costs are low and there are no problems of handling and storage of fuels and disposal of ash. The plant is highly reliable and it is cheapest in operation and maintenance. The plant can be run up and synchronized in a few minutes. The load can be varied quickly and the rapidly changing load demands can be met without any difficulty. Very acute governing is possible with water turbines so such power plants have constant speed and hence constant frequency. There are no standby losses in such plants. Such plants are robust and have got longer life (around 50 years). The efficiency of such plants does not fall with the age. It is very neat and clean plant because no smoke or ash is produced. Highly skilled engineers are required only at the time of construction but later on only a few experienced persons will be required. Such plants in addition to generation of electric power also serve other purposes such as irrigation and flood control.

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Demerits of Hydro-electric Power Plants

However, the hydro-electric power plants have the following demerits also: It requires large area. Its construction cost is enormously high and takes long time for erection

(owing to involvement of huge civil engineering works). Long transmission lines are required as the plants are located in hilly areas

which are quite away from the load centre. The output of such plants is never constant owing to vagaries of monsoons

and there dependence on the rate of water flows in a river. Long dry seasons may affect the power supply.

Hydro-electric power plant reservoir submerges huge areas, uproots large population and creates social and other problems.

16

Site Selection of Hydro-electric Power Plants

The hydro-electric power plant is only a small part of the whole project. The

power station should be near the dam and storage reservoir. Such a location reduces the length of the penstock and the loss of head in the penstock. In view of this, several structures such as dam, intake, surge tank, power house are involved in the site selection.

The essential requirements for hydro-schemes are: large catchment areas, high rainfall, step gradients, favorable site for reservoir, solid sub-soil etc. Many factors have to be considered in the selection but the following are the most important:

Availability of water Water storage Head of water Geological investigation Water pollution Sedimentation Environmental effects Access to site

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Hydrograph:

Hydrograph shows the variation of stream flow in m3/sec with time for a particular river site. The time may be hour, week, month or year. It is similar to the chronological load curve.

A hydrograph provides the following information: The discharge at any time during the period under

consideration. The maximum and minimum run off during the period. The mean run off during the period. Total volume of flow up to any time is given by the area under

the curve up to that point.

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Hydrograph:

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

2500

2000

1500

1000

500

0

MONTHS

RU

N O

FF

in

m3/s

ec


2500

2000

1500

1000

500

0

MONTHS

RU

N O

FF

in

m3/s

ec


2500

2000

1500

1000

500

0

MONTHSR

UN

OF

F i

n m

3/s

ec


2500

2000

1500

1000

500

0

MONTHSR

UN

OF

F i

n m

3/s

ec

Figure: Hydrograph of a flashy river Figure: Hydrograph of a river with steady flow

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Flow Duration Curve

Flow duration curve is a re-arrangement of all the stream flow elements of a hydrograph in a descending order. It is similar to the load duration curve. Each point on a flow duration curve shows the percentage time during the period when the flow was equal or greater than the given value. The area under a flow duration curve gives the total quantity of run off during that period.

0 20 40 60 80 100

2500

2000

1500

1000

500

0

PERCENT OF TIME

RU

N O

FF

in

m3/s

ec

0 20 40 60 80 100

2500

2000

1500

1000

500

0

PERCENT OF TIME

RU

N O

FF

in

m3/s

ec

Figure: Flow duration curve

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Mass Curve A mass curve indicates the total volume of

run off in cubic metre up to a certain time. The abscissa can be a day, month or year. The slope of the curve at any point shows the rate of flow at that time. If the rainfall is uniform throughout the year, the mass curve will be straight line having a uniform slope. Mass curves are used in estimating the capacity of storage reservoir in hydro-projects.

The ordinate of mass curve can also be plotted in terms of second-metre-day or second-metre-month which means the flow collected at a rate of one cubic metre per second for one day or one month respectively.

1 second-metre-month = 1 30 24 60 60

= 25,92,000 cubic metre.

Figure: Mass curve


12000

10000

8000

6000

4000

2000

0

MONTHS

Se

co

nd

-me

ter-

mo

nth

s


12000

10000

8000

6000

4000

2000

0

MONTHS

Se

co

nd

-me

ter-

mo

nth

s

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PROBLEMS TO SOLVE

1. A hydro-electric power plant operates under an effective head of 50 m and a discharge of 94 m3/sec. Determine the power developed. [46.107 MW] 2. The mean monthly discharge at a particular site is given below:

Month

Discharge

in m3/sec

Month Discharge

in m3/sec

January

February

March

April

May

June

200

400

600

2400

1200

1800

July

August

September

October

November

December

1600

1200

2000

1200

800

400

Draw the hydrograph, flow duration curve and mass curve. Determine the average inflow and the power that can be developed at an effective head of 90 m. Determine the capacity of the storage reservoir based on the above one year data neglecting the losses due to seepage, evaporation etc. Assume overall generation efficiency to be 80%. [1150 m3/sec, 812.268 MW, 3350 second-metre-months]

PROBLEMS TO SOLVE

3. A hydro-electric power station is supplied from a reservoir of capacity 3 107 m3 at a head of 150 m. Determine the total energy available if the overall efficiency of the

plant is 70%. [8.58375 106 KWH] 4. A hydro-electric power station is supplied from a reservoir having an area of 50 km2

and a head of 50 m. If the overall efficiency of the plant be 60%, find the rate at which

the water level will fall when the station is generating 30,000 KW. [7.337 mm/hour] 5. A hydro-electric scheme has a catchment area of 120 sq. km. The available run off is

50% with annual rainfall of 100 cm. A head of 250 m is available on the average.

Efficiency of the power plant is 70%. Find (i) average power produced and (ii) capacity

of the plant assuming the load factor to be 0.6. [(i) 3,266 KW, (ii) 5,443 KW] 6. A hydro-electric power station is supplied from a catchment area of 150 km2 with an

annual rainfall of 200 cm and effective head of 300 metres. Assuming a yield factor of

60%, calculate (i) the available continuous power, (ii) the rating of the generator

installed [assume the load factor as 0.6] and (iii) net energy available in kwh.

[(i) [12598.46 KW, (ii) 21 MW, (iii) 110.3625 106 KWH] 7. A hydro-electric power station is supplied from a catchment area of 480 km2 with an

annual rainfall of 1100 mm and effective head of 40 metres. 20% of the rainfall is lost

due to evaporation etc. The loss of head in the penstock is estimated to be 10%. The

turbine efficiency is 85% and the generator efficiency is 92%. Find capacity of the

plant assuming the load factor to be 0.6. [6.17 MW]

PROBLEMS TO SOLVE

5. A hydro-electric scheme has a catchment area of 120 sq. km. The available run off is

50% with annual rainfall of 100 cm. A head of 250 m is available on the average.

Efficiency of the power plant is 70%. Find (i) average power produced and (ii) capacity

of the plant assuming the load factor to be 0.6.

6. A hydro-electric power station is supplied from a catchment area of 150 km2 with an

annual rainfall of 200 cm and effective head of 300 metres. Assuming a yield factor of

60%, calculate (i) the available continuous power, (ii) the rating of the generator

installed [assume the load factor as 0.6] and (iii) net energy available in kwh.

7. A hydro-electric power station is supplied from a catchment area of 480 km2 with an

annual rainfall of 1100 mm and effective head of 40 metres. 20% of the rainfall is lost

due to evaporation etc. The loss of head in the penstock is estimated to be 10%. The

turbine efficiency is 85% and the generator efficiency is 92%. Find capacity of the

plant assuming the load factor to be 0.6.

PROBLEMS TO SOLVE

2. The mean monthly discharge at a particular site is given below:

Month

Discharge

in m3/sec

Month Discharge

in m3/sec

January

February

March

April

May

June

200

400

600

2400

1200

1800

July

August

September

October

November

December

1600

1200

2000

1200

800

400

Draw the hydrograph, flow duration curve and mass curve. Determine the average inflow and the power that can be developed at an effective head of 90 m. Determine the capacity of the storage reservoir based on the above one year data neglecting the losses due to seepage, evaporation etc. Assume overall generation efficiency to be 80%.

Schematic Arrangement of a Hydro-

Electric Power Plant

The chief requirement for hydro-electric power plant is the availability of water in huge quantity at sufficient head and this requirement can be met by constructing a dam across a river or lake. The schematic arrangement is shown in the following figure.

RESERVOIR

DAM

PRESSURE TUNNEL

SURGE TANK

TAIL

RACE

VALVE

HOUSE

PENSTOCK

POWER HOUSE

RESERVOIR

DAM

PRESSURE TUNNEL

SURGE TANK

TAIL

RACE

VALVE

HOUSE

PENSTOCK

POWER HOUSE

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Schematic Arrangement of a Hydro-

Electric Power Plant

RESERVOIR

DAM

PRESSURE TUNNEL

SURGE TANK

TAIL

RACE

VALVE

HOUSE

PENSTOCK

POWER HOUSE

RESERVOIR

DAM

PRESSURE TUNNEL

SURGE TANK

TAIL

RACE

VALVE

HOUSE

PENSTOCK

POWER HOUSE

An artificial storage reservoir is formed by constructing a dam across a river (or lake) and a pressure tunnel is taken off from the reservoir to the valve house at the start of the penstock. The valve house contains main sluice valves for controlling water flow to the power station and automatic isolating valves for cutting off water supply in case the penstock bursts. A surge tank is also provided just before the valve house for better regulation of water pressure in the system. From the reservoir the water is carried to valve house through pressure tunnel and from valve house to the water turbine through pipes of large diameter made of steel or reinforced concrete, called the penstock. The water turbine converts hydraulic energy into mechanical energy and the alternator coupled to the water turbine converts mechanical energy into electrical energy. Water after doing useful work is discharged to the tail race.

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Important Elements of a Hydro-Electric

Power Plant A hydro-electric plant consists of a reservoir for storage of water, a diversion dam, an intake structure for controlling and regulating the flow of water, a conduit system to carry the water from the intake to the water wheel, the turbines coupled with generators, the draft tube for conveying water from water wheel to the tailrace, the tailrace and a power house i.e. is the building to contain the turbines, generators, the accessories and other miscellaneous items. Few of these elements are discussed below:

Storage Reservoir Dam Forebay Spillway Intake Surge Tank Penstock Tail Race

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Classification of Hydro-electric Power Plants The hydro-electric power plants may be classified according to (i) the extent of

water flow regulation available (ii) the availability of water head and (iii) the type of load they supply.

(i) According to the extent of water flow regulation available the hydro-electric power plants may be classified into:

Run-off River Power Plants without Pondage Run-off River Power Plants with Pondage

Reservoir Power Plants (ii) According to availability of water head the hydro-electric plants may be

classified into: Low Head Hydro-electric Power Plants Medium Head Hydro-electric Power Plants High Head Hydro-electric Power Plants

(iii) According to the load supplied the hydro-electric power plants may be

classified into: Base Load Plants Peak Load Plants Pumped Storage Power Plants for Peak Load

41

Classification of Hydro-electric Power Plants

Some hydro-power plants are so located that the water is taken from the river directly, and no pondage or storage is possible. Such plants are called the run-off river power plants without pondage. Such plants can use water only as and when available; these can not be used at any time at will or fit any desired portion of the load curve. In such plants there is no control on flow of water. During high flow and low load periods, water is wasted and during the lean flow periods the plant capacity is very low. Such plants can be built at a considerably low cost. During the high flow periods such plants can be employed to supply a substantial portion of base load.

Run-off River Power Plants without Pondage

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Pondage refers to storage at the plant to take care of hour to hour fluctuations in load on the station. Pondage increases the firm capacity of the of the station provided that the floods do not raise the tail race water level thus reducing the effective water head and plant output. Such plants can serve as base load or peak load plants depending on the stream flow. When plenty of water is available, these plants can be used as base load plants. When stream flow decreases, these plants can be made to work as peak load plants.

Run-off River Power Plants with Pondage

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When water is stored in a big reservoir behind a dam, it is possible to control the flow of water and use it most effectively. Storage increases the firm capacity of the plant and it can be used efficiently throughout the year. Such a plant can be used as a base load plant or as a peak load plant as per requirement depending the water stored in the reservoir, the rate of inflow and the system load. Most of the hydro-electric power plants, everywhere in the world, belong to this category.

Reservoir Power Plants

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When water head is less than 30 m, the plant is called a low head plant. A dam or barrage across the river creates the necessary head. The power plant is located near the dam and, therefore, no surge tank is needed. Either one half of the barrage has regulating gates for discharge of surplus water while the plant is in front of the second half or the plant is constructed by the side of the river. Francis or Kaplan turbines are used.

Low Head Hydro-electric Power Plants:

45


Medium head plants operate at heads between 30 & 100 metres. An open channel brings water from main reservoir to the forebay from where penstocks carry water to the turbines. Francis or Kaplan turbines are used.

Medium Head Hydro-electric Power Plants:

46


The plants operating at heads above 100 m are generally classified as high head plants. The civil works for these plants include dam, reservoir, tunnel, surge tank and penstock. Generally Francis turbines are used for heads below 200 m and Pelton turbines for still higher heads.

High Head Hydro-electric Power Plants:

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They feed the base load of the system. Thus they supply almost constant load throughout and operate on a high load factor. Base plants are usually of large capacity. Run-off river power plants without pondage and reservoir power plants are used as base load plants. For a plant to be used as base load plant, the unit cost of energy generated by the plant should be low.

Base Load Plants:

48


They are meant to supply the peak load of the system. Run-off river power plants with pondage can be used as peak load plants during lean flow periods. Reservoir power plants can, of course, be used as peak load plants also. Peak load plants have large seasonal storage. They store water during off-peak periods and are run during peak load periods. They operate at a low load factor.

Peak Load Plants:

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This is a unique design of peak load plant in which the plant pumps back all or a portion of its water supply during low load period. The usual construction is a tail water pond and a head water pond connected through a penstock. The plant utilizes some of the surplus energy generated by the base load plant to pump the water from the tail water pond into the head water pond during off peak hours. During peak load period this water is used to generate power by allowing it to flow from the head water pond through the water turbine to the tail water. The capacity of the plant should be such that the plant can supply the peak load for 4 to 10 hours. The plants can be used in conjunction with hydro, steam and I.C. engine plants. This plant is also called a hydraulic accumulator system.

Pumped Storage Power Plants for Peak Load:

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DAM

HEAD

WATER

LEVEL

TAIL

WATER

POND

POWER

HOUSE,

TURBINES

AND PUMPS

PENSTOCK

DAM

HEAD

WATER

POND

DAM

HEAD

WATER

LEVEL

TAIL

WATER

POND

POWER

HOUSE,

TURBINES

AND PUMPS

PENSTOCK

DAM

HEAD

WATER

POND

Figure: Pumped Storage Power Plant

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In the older plants separate motor driven pumps and turbine driven generators were used. A recent development is a reversible turbine pump. Francis turbine, which is just the reverse of Centrifugal pump, is normally used. During peak loads, the turbine drives the alternator and the plant generates electrical energy. During low loads, the alternator runs as a motor and drives the turbine which now works as a pump for pumping the water into the head water pond. This arrangement reduces the capital cost of the plant. The power for driving the motor is taken from the system.

The efficiency of the plant is around 60 to 70 percent. Some water may evaporate from the head water pond resulting in the reduction in the stored energy or there might be run off through the soils. Also there will be some energy loss in generating and pumping equipment and in power transmission. Such plant can be operated only in inter-connected systems where other types of generating plants, such as steam, nuclear, hydro, diesel plants, are available. In carrying the peak loads of the system, such plants reduces the operating costs of the steam or nuclear plants working in combination with them by improving the load factor of the steam or nuclear plant and added capacity to meet peak loads


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Advantages of Pumped Storage Power Plants

Pumped storage power plants have some very important advantages. Some of these are given below:

Peak loads can be supplied at lower cost than that when supplied by steam and nuclear power plants.

The steam and nuclear power plants can be operated at almost unity load factor which ensures their most efficient and economic operation.

Because of their ability to take up loads in a very short time (pumped storage plants need a starting time of only 2-3 seconds and can be loaded fully in about 15 seconds), the spinning reserve requirement of the system is reduced.

In the event of extra demand coming up suddenly on the system, such plants can be immediately switched on to meet this extra demand.

They can be used for load frequency control.

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hydro power presentation

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