What is Hydroelectric Power?Hydroelectricity is a term referring to the electricity

generated by hydro power

A hydropower development is essentially to utilize thehydraulic power possessed by the water flowing in astream and to develop from it electric power throughhydraulic turbines coupled to electric generators

The total amount of hydroelectric power that would bemade available from a stream depends on continuousflow rate of the stream and the head possessed by theflowing water

Lay out of typical hydroelectric power scheme

Comparison of Hydroelectric and Thermal Power Plants

Criterion Hydroelectric Power Plant Thermal Power Plant

1. Initial cost and life expectancy

Higher initial cost due to costlyinfrastructure such as dam,spillway, et., and its location ata place far off from theconsumer or load centre

Lower initial cost due to itsproximity to consumer or loadcentre

Higher life expectancy, may beabout 50 years

Lower life expectancy

2. operation, maintenance and repair (OMR) cost

Low OMR cost due to no fuelcost

High OMR cost due to costlyfuel and the cost of fueltransport

Less maintenance and repaircost and staff needed foroperation is less

High maintenance and repaircost and staff needed foroperation is more

Water is replenished every yearan put to non-consumable use.However, supply of water maybe erratic year after year

Non-replenishable source

Low taxes due to its location inremote areas

High taxes due to its locationnear cities

Criterion Hydroelectric power plant Thermal power plant

3. Transmission lossesMore due to long transmissionlines

Less due to short transmissionlines

4. Pollution No problem of pollution orecological unbalance later

Causes air pollution as well asstream pollution resulting inecological imbalance unlesscooling towers or cooling pondsare provided

No waste product and hence noproblem of its disposal

Problem of haulage and disposalof waste product viz., smoke,ash etc.

5. Actuating timeA few seconds to 3 to 4 minutesdepending on the length ofconduit

About 30 minutes

Quite suitable to take up peakload

Not suitable for use as peakload plants

Selection of Suitable Types of Turbines

S.No Head in meters Type of turbine Specific speed

1 300 or morePelton wheel,

single or multiple jet

8.5 to 47

2 150 to 300Pelton or Francis

30 to 85

3 60 to 150Francis or

Deriaz85 to 188

4 Less than 60Kaplan or

Deriaz188 to 860

Advantages of Hydroelectric Power

Fuel is not burned so there is minimal pollution

Water to run the power plant is provided free bynature

Hydropower plays a major role in reducinggreenhouse gas emissions

Relatively low operations and maintenance costs

The technology is reliable and proven over time

It's renewable - rainfall renews the water in thereservoir, so the fuel is almost always there

Disadvantages of Hydroelectric Power

High investment costs

Hydrology dependent (precipitation)

In some cases, inundation of land and wildlifehabitat

In some cases, loss or modification of fish habitat

Fish entrainment or passage restriction

In some cases, changes in reservoir and streamwater quality

In some cases, displacement of local populations

Classification of Hydel Plants1. Classification based on storage characteristics

Run-off river plants

- Utilize the minimum flow of a river

- Do not have appreciable storage on the u/s side

- Suitable only on perennial rivers

Storage plants

- It has an upstream storage reservoir

- Continuous firm power generation is possible

Pumped storage plants

- Power is generated during peak hours, but during off-peak hours water ispumped back from the tail water pool to head water pool for future use

- Primarily meant for assisting an existing thermal or other hydel plant

Tidal plants

- Works on the principle that there is a rise in sea waterduring high tide period and fall during the low ebbperiod

- The tidal range, i.e. the difference between high andlow tide levels is utilized to generate power

2. Classification according to functional basis

Base load plants

- These are the plants which are capable of substantiallycontinuous operation in the base of the load curve throughoutthe year

Peak load plants

- It is a hydel plant designed and constructed primarily for takingcare of the peak load of a power system

3. Classification based on the basis of head

Micro head plants – H = 10 m to 20 m

Low head plants – H <= 30 m

Medium head plants – H = 30 m to 250 m

High head plants – H > 250 m

Important Terms Connected With Hydropower

Water Power Potential

- It is the amount of power generated when Q cumecs of wateris allowed to fall through a head difference of H meters and isgiven by:

P = η γ Q H

Normal Water Level (NWL)

- It is the highest elevation of water level that can bemaintained in the reservoir without any spillway discharge

Minimum Water Level (MWL)

- It is the elevation of the water level which produces minimumnet head on the power units (i.e. 60% of the design head)

Rated Head – The head at which the turbine functioning atfull gate opening will produce a power output, equal to thatspecified on the name plate of the turbine.

Gross Head – It is the difference in water level elevation at thepoint of diversion of water for the hydel scheme and the pointof return of water back to the river.

Operating Head – It is the difference between the elevation ofwater surface in the fore-bay and the tailrace after makingdue allowance for approach and exit velocity heads

Net Head or Effective Head – It is the difference of head at thepoint of entry and exit of turbine and includes the respectivevelocity and pressure heads at both places

Installed Capacity – It is the total capacity in kilowatts ormillion kilowatts of all the turbine generator units installed ina power house.

Dependable Capacity – It is the load carrying capability of thepower house with respect to the load characteristics during aspecified time interval

Load Factor (LF) – It is defined as the ratio of the average loadover a certain period of time to the peak load during the sameperiod

Utilization Factor (UF) or Plant Use Factor

Average load over a certain period

Peak load during that periodLoad Factor

Water actually utilized for power production

Water available in the riverUF

Capacity Factor or Plant Factor – It is the ratio of average outputof the plant for a given period of time to the plant capacity or, itcan also be defined as:

Firm Power – The net amount of power which is continuouslyavailable from a plant without any break on firm or guaranteedbasis.

Secondary Power – The excess power available over the firmpower during the off peak hours or during monsoon season

Power Factor – It is the ratio of actual power in kilowatts to theapparent power in kilo volt amperes (KVA)

Energy actually produced in a given time

Max. energy that can be produced by the plant during the same timeCF

Principal Components of a Hydroelectric Scheme

The fore-bay

Intake structure

Penstocks

Surge tank or surge chambers

Turbines

Power house

Draft tube

The tailrace

Components of a Hydroelectric Power Plant

1. Forebay

An enlarged body of water provided just in front of the penstock

It is provided in the case of run-of-river plants and storage plantswhen the power house is located far away from the dam

The main function of forebay is to provide a small balancingstorage upstream of the power house

2. Intake structures

The water from the reservoir or forebay is let into the penstocksthrough intake structure

The main components of an intake structure are trash racks andgates

Trash rack is provided to prevent the entry of debris into the waterpassage of hydropower plant

The gates are provided to control the entry of water into thepenstocks

3. Penstocks Penstocks are the pipes of large diameter used for conveying

water from the reservoir to the turbines

A sufficient water depth should be provided above the penstockentrance to avoid formation of vortices which may carry air intothe penstock and result in lowered turbine efficiency andundesirable pressure surges

4. Surge tanks

A surge tank is a cylindrical open-topped storage tank which isconnected to the penstock at a suitable point.

These are provided to relieve the penstocks of excess pressurecaused by water hammer

They also provide additional supply of water when the turbinesare in need of more water on account of increased load

A surge tank should be provided as close to the power house as ispossible

4. Power house

It is a structure which houses the various hydraulic and electricequipment

The various hydraulic equipments are turbines, gates or gatevalves, governors, etc.

The various electrical equipments are, generators, transformers,switching equipments, transmission lines and transmissionstructures

5. Turbines

The machine which converts hydraulic energy to mechanicalenergy, and finally to electrical energy

6. Tail race

A channel into which the water is discharged after passingthrough the turbines

7. Draft tube: - An elongated pipe connecting turbine to tail race

Example ProblemsExample 1: Three turbo-generators each of capacity 10000 kW havebeen installed at a hydel power station. During a certain period ofload, the load on the plant varies from 12000 kW to 26000 kW.Calculate (i) total installed capacity, (ii) load factor, (iii) plant factorand (iv) utilization factor.

Example 2 : During a low water week a river has an average dailyflow of 32 m3/sec with a fluctuation during the day requiring apondage capacity of approximately 15% of the daily discharge. a HEPis to be located on the river which will operate 5 days a week, 24hours a day, but will supply power at varying rate such that the dailyload factor is 55% corresponding to which the pondage required isequal to 0.2 times the mean flow to the turbine. On Saturday andSunday all the flow is ponded for use on rest of the days.

Contd.

If the effective head on the turbines when the pond is full is to be 25m and the maximum allowable fluctuation in pond level is 1 m find(a) the surface area of the pond to satisfy all the operatingconditions, (b) the weekly output at the switch board in kwh.

Assume turbine efficiency 55% and generator efficiency 92%