In this article, you learn how does hydro power plant work? It’s importance, components, advantages and types of hydro power plant.
Hydro Power Plant and Types
In hydro power plant, the energy of water is used to move the turbines which in turn run the electric generators. The energy of the water used for power generation may be kinetic or potential. The kinetic energy of water is its energy in movement and is a function of mass and velocity, while the potential energy is a function of the difference in level per head of water between two points.
In either case, continuous availability of water is a basic necessity, to ensure this, water collected in natural lakes and reservoirs at high altitudes may be used or water may be stored by constructing dams across flowing streams.
Hydro-power is a conventional renewable source of energy which is clean, free from pollution and generally has a good environmental effect, but it requires a large investment and involves increased cost of power transmission.
Hydro power plants are developed for the following purposes :
- Generation of electricity at low cost.
- To control the floods of the rivers.
- Is to store the water for drinking and irrigation.
- To develop the surrounding area.
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Importance of Hydro-electric Power Plant
Water is the naturally available renewable source of energy. The power generation from a hydro-electric power plant is clean and free from pollution, generally, it has a good environmental impact.
The main aim of a hydro-electric power plant is to harnessing power from water flowing under pressure. Nearly 30 to 35% of the total power generation of the world is met by a hydro-electric power plant.
Hydro-power plants are also developed for their following advantages:
- To control the floods of the rivers.
- Is to develop the irrigated lands.
- To have storage of drinking water.
- The running cost of these plants is very low compared to other power plants.
- Greater control over the turbines,
- High reliability compared to other plants.
- Absolutely no fuel charges.
- The load can be varied quickly as per the changing demand.
- These plants have no disposal problem
- These plants have no environmental problems.
Storage and Pondage:
During the rainy season, when the stream is in floods it carries a huge quantity of water as compared to the stream in other times of the year i.e. the quantity of water carried by it is very less. However, the demand for power normally does not match such variation of the natural flow of the stream.
Therefore, some arrangement in the form of storage and pondage of water is required for the proper handling of the flow of water so as to make it available in important quantity to meet the power demand at a given time.
The storage may be defined as the impounding of a considerable amount of excess runoff during seasons of surplus flow for use in dry seasons. This is achieved by constructing a dam across the stream at a suitable site and building a storage reservoir on the upstream side of the dam.
The pondage may be defined as a regulating body of water in the form of a relatively small pond or reservoir provided at the plant. The pondage is used to regulate the variable water flow to meet power demand. It helps in short term variations which occur due to:
- Sudden rise or drop in load on the turbine.
- Sudden changes in the inflow of water.
- Change of water demand by turbines and the natural flow of water from time to time.
The turbines are required to meet the power demand higher than the average load when the pondage supplies the excess quantity of water required during that period.
Pondage increases the capacity of a river over a short time, such as a week.
Storage, however, increases the capacity of a river over an extended period of 6 months to as much as 2 years.
Factors to be considered for Selection of Site for Hydro-electric Power Plant:
Following factors should be considered while selecting the site for hydro-power plant :
- Availability of water: Large quantity of water should be available throughout the year at the proposed site.
- A requirement of head flow availability and storage capacity.
- The character of foundation, particularly for the dams.
- The land should be cheap and rocky.
- The topography of the surface at the proposed location.
- Accessibility of the site i.e. the site should have transportation facilities like road and rail.
- Nearness to the load centre.
- Availability of the materials for the construction.
- Arrangement and type of dam, intakes, conduits, surge tank and powerhouse.
- Cost of project and period required for completion.
- Impacts of water pollution.
General Layout and Essential Elements of Hydro-electric Power Plant:
A simple layout of the hydro-electric power plant as shown in fig. 2.3. It consists of the catchment area, reservoir, dam, slice gate or valve, surge tank, penstock, inlet valve, turbine, draft tube, powerhouse equipment, tailrace etc.
The collected water from the reservoir is supplied from the dam through slice gate, penstock, inlet valve to the turbine. The turbine converts the potential energy of the water into mechanical energy to run the generator. The generator produces electric power. After doing the work water flows into the tailrace through draft tube.
Essential Elements or Components of Hydro Power Plant:
Following are the essential components of the hydro-power plant :
- Catchment area
- Surge tanks
- Prime movers
- Draft tubes
- Powerhouse and equipment.
1. Catchment Area
The whole area behind the dam draining into a stream or river across which the dam has been built at a suitable place is called catchment area.
It is the area where the water is stored and utilized for power generation. A reservoir may be natural or artificial.
A natural reservoir is a lake in high mountains. An artificial tank is built by erecting a dam across the river.
A dam is a barrier built across the river to store the water for power generation. Dams are built of concrete or stone masonry, earth or rockfill. The dam stores the water one side and on the other side, it is having a powerhouse to generate the power.
4. Spill Ways
It is a safety valve for a dam. It is provided to discharge the excess water from the dam to safeguard the dam against floods.
It is a pipe connected between surge tank and prime mover, usually, these are of steel-reinforced concrete pipes.
6. Surge Tank
There is a sudden increase in pressure in the penstock due to the sudden decrease in the rate of water flow to the turbine when the gates admitting water to the turbines are suddenly closed owing to the action of the governor.
This happens when the load on the generator decreases. This sudden rise of pressure in the penstock above normal due to reduced load on the generator is known as “water hammer”.
A surge tank is a small reservoir employed between dam and powerhouse nearer to the powerhouse to reduce the pressure swings in the penstock by allowing the excess water to enter into the surge tank during low load periods and the stored water can be supplied to the penstock during high load periods.
7. Prime Mover
These are the turbines used to convert the kinetic energy of the water into mechanical energy to produce electric energy.
8. Draft Tube
It is a diverging discharge passage connected to the tailrace. It supports the runner for utilizing the remaining kinetic energy of the water at the discharge end of the runner.
A powerhouse consists of two main parts, a substructure to support the hydraulic and electric equipment such as turbines, generators, valves, pumps, governors, etc., and superstructure to house and protects these types of equipment.
Types of Hydro-electric Power Plant
Different types of hydro-electric power plant can be classified as follows:
- According to the availability of head
- High head power plants
- Medium head power plants
- Low head power plants.
- According to the nature of the load
- Base load plants
- Peak load plants.
- According to the availability of water
- Runoff river plant without pondage
- Runoff river plant with pondage
- Storage type plants
- Pump storage plants
- Mini and micro-hydel plants.
Base Load Plants
This types of power plant work independently and supply the power to the whole load. This plant takes the load on the base portion of the load curve. The load on the plant remains more or less uniform during the operation period. It works for the whole time i.e. it supplies the power when there is a requirement.
Baseload plants are generally large in capacity. The run-off-river and storage type plants are used as baseload plants. The load factor for such plants is considerably high.
Peak Load Plants
The peak load plants are designed for taking care of peak loads of the demand curve. Run-off river plant with pondage and pumped storage plants are generally used as peak load plants. These plants supply the power to the load premises when there is a peak load period only. Rest of the time the power is supplied by the main plant.
In this type of plants, the main power plant is always required and hydro power plant works as secondary plant and shares the load of two to three hours. In case of runoff river hydro plants with poundage, a large pound is essential and extensive seasonal storage is usually provided.
These power plants have large seasonal storage and relatively high heads and are likely to be located on small watersheds. They store the water during off-peak period and supply during peak periods on the top of the load curve. The load factor of peak load plants is considerably low compared with baseload plants.
Runoff River Plants
A runoff river may be classified into two types:
- Runoff river plant without pondage
- Runoff river plant with pondage.
A runoff river plant without pondage as shown in the figure. This plant does not store the water and uses the water as it comes. There is no control on the flow of water so that during high floods or low loads water is wasted while during low run-off the plant capacity is considerably reduced.
Due to non-uniformity of supply and lack of assistance from a firm capacity the utility of these plants is much less than those of other types. The head-on which these plants work varies considerably. During good flow conditions, these plants may cater to the baseload of the system, when flow reduces they may supply the peak demands.
Runoff river plant with pondage uses storage of water behind a dam at the plant and increases the stream capacity for a short period, say a week. Storage means a collection of water in upstream reservoirs and this increases the capacity of the stream over an extended period of several months.
The storage plants may work suitably as baseload and peak load plants. This type of plant compared to without pondage, is more reliable.
Storage Type Plant (Reservoir Type)
A storage-type plant is one with a reservoir of sufficiently large size to permit carry-over storage from the wet region to the dry region, and therefore water supply is substantially constant and more than the minimum natural flow of the water.
This plant can be used as baseload plant as well as peak load plant as water is available with control when required. A simple storage plant is shown in the figure.
It consists of a reservoir, a dam with penstock, powerhouse arrangements. The powerhouse is placed at the toe of the dam. The water is allowed to store in a reservoir from the river or lakes in sufficient quantity.
The water flows from the dam through the penstock when cresh gate is opened to the powerhouse. In powerhouse water with high pressure enters into the turbine to generate power. After doing the work water is allowed to flow to the tailrace. A Pelton wheel is the common prime mover used in such power plants.
Pumped Storage Plants
The pumped storage plants are used at the places where the quantity of water available for power generation is low. Here the water passing through the turbine is stored in “tailrace pond”. During the low load periods, this water is drawn back to the head reservoir applying the extra energy available.
This water can be reused for generating power during peak load periods. The pumping of water may be done seasonally or regular depending upon the conditions of the site and the nature of the load on the plant.
The simple construction of the stored hydro-power plant is shown in the figure. It consists of headwater pond and dam, penstock connected power house with pumps and turbines and trail race pond with the dam. The water from head water pond is supplied to the power house through the penstock, where turbines are rotated for power generation.
From the turbine, the water is discharged into the tailrace pond. The water stored in the tailrace pond is pumped back to the head reservoir with the help of the pump during low load periods. This water is again used for power generation during peak load periods.
Such plants are usually interconnected with steam or diesel power plants so that off-peak capacity of interconnecting stations is used in pumping water and the same is used during peak load periods.
Advantages of Pumped Storage Power Plants
- There is a substantial increase in peak load capacity.
- Increased operating efficiency.
- It can be used as both base loads plant and peak load plant.
- Load the plant remain uniform.
- Improved load factor.
Mini and Microhydel Plants
The hydro power plants working with 5 m to 20 m head are known as mini hydel plants and the hydel power plants working with the heads less than 5 m head are known as micro hydel plants. These plants can generate power ranging from 100 KW to 5 MW with a period of one and half year.
These plants having a small reservoir with the dam and small capacity power house using bulb turbines with straight diverging tube acts as a draft tube. The water flows from a small reservoir through the small penstock into the turbine in power house and generates the power. After generating the power the water is discharged into the tailrace through draft tube.
Micro-hydel plants make use of standardised bulb sets with unit output ranging from 100 to 1000 KW working under heads between 1.5 to 5 m.
Advantages of Hydro-electric Power Plant
Following are the advantages of a hydro-electric power plant:
- Low operating cost compared to a thermal power plant.
- The cost of generation is unaffected by the load factor.
- No fuel charges.
- High useful life of about 100 – 125 years.
- Low maintenance cost compared to the thermal power plant
- Highly reliable.
- It can be started quickly and synchronize the plant.
- There is no problem with fuel and ash handling.
- No nuisance of smoke exhaust gases and soots.
- No health hazards due to air pollution.
- It has no standby losses.
- The machines used in hydel plants are robust and no problem of high temperature and pressure.
- The efficiency of the hydel plant does not change with age.
- The number of operations required is considerably small.
- It can serve the purpose of flood control and stored water can be used for drinking and irrigation work.
- Less labour is required to operate the plant.
Disadvantages of Hydro-electric Power Plant
Following are the disadvantages of a hydro-electric power plant:
- High capital cost.
- Power generation only dependent on the quantity of water availability.
- It takes a considerably long time for the construction
- Site of the hydro-electric power station is always away from the load centre, therefore transmission cost becomes high.
- Sometimes isolated sites are difficult to access.
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