Hydroelectric power can be captured whenever a flow of water falls from a higher level to a lower level due to gravity. Hydraulic turbines convert the energy of flowing water into mechanical energy which in turn can be converted into electrical energy with the help of generators.
Ancient Greeks used wooden water wheels to convert kinetic energy of water into mechanical energy as far back as 2000 years ago. In Ancient Egypt, Persia and China, water wheels were used for irrigation, grinding of grains, etc.
In India the first hydroelectric power plant was built at Darjeeling in the year 1897. The second hydroelectric power plant was built at Shivasamudram in Karnataka in 1902.
There is a big gap between the demand and supply of electricity in our country. To me the challenge, small hydro power (SHP) upto 25 MW has been given top priority by the Ministry of New and Renewable Energy, Government of India as SHP has an estimated potential of more than 15000 MW in the country.
TERMS AND DEFINITIONS
1. Gestation period: Time required for completion of a project.
2. Flow: Quantity of water falling.
3. Hydraulic machine: A device which converts hydraulic energy of a fluid into mechanical energy or vice-versa.
4. Turbine: the hydraulic machine which converts hydraulic energy into mechanical energy. It is a prime mover for electric generators.
5. Pump: The hydraulic machine which converts mechanical energy into hydraulic energy.
6. Dam: A structure constructed across a river or a channel to store water.
7. Head race: Upper water storage.
8. Tail race: Channel carrying water away from the turbine.
9. Head: Vertical difference of water level between head race and tail race.
CONSTRUCTION OF SMALL HYDRO POWER PLANT
The main components of small hydro power plants are as described below:
1. Weir: The water from the river stream is diverted to a channel through a diversion structure called weir. This helps in maintaining constant flow in the channel for variable flow of water in the river.
2. Desilting tank: A desilting tank is provided to trap the pebbles and other coarse material in the water in order to prevent the damage to the turbine blades. The silt particles of size more than 0.5 mm deposited in the tank are flushed out periodically.
3. Power channel: Power channel is a water conductor system from weir to forebay through desilting tanks. It should be designed to minimize head loss and water seepage. The power channel has a trapezoidal cross-section and longitudinal slope of 1 : 500.
4. Forebay: At the end of the power channel, a large storage tank called forebay is constructed with reinforced cement concrete (RCC) and stone masonry. The water stored in the forebay is utilized for generation of electricity. The forebay provides the minimum water head required for the turbine. To filter out trash, ice, grasses, debris, a crash rack is provided before the penstock.
5. Penstock: Penstock is a water conduit joining the forebay and the turbine inlet. They are made of RCC concrete, mild steel or PVC depending on their diameter and length.
6. Surge tank: For a medium/high head plant, a surge tank is provided with the penstock near the turbine. This releases water from the penstock when excessive pressure develops in the penstock due to the water hammering effect. This happens whenever there is a sudden load throw on the generator causing the turbine inlet gates to close suddenly. Also water is taken from the surge tank into penstock at time of sudden opening of turbine inlet gates.
7. Spillway: It is an overflow arrangement provided in the forebay to discharge the surplus water into the downstream of the river.
8. Power house: It is a building constructed to house the turbine, generator, control panels, auxiliary equipment. The size of the building depends on the number of generating units.
9. Tail race: The water passing through the turbine is collected in the draft tube and discharged into a water channel called tail race. Tail race then drains out this discharged water into the river downstream.
POTENTIAL OF SMALL HYDROPOWER PLANTS
India has an estimated potential of small hydro power plants of 15,380 MW from 5,718 identified sites. Out of this about half lies in Himachal Pradesh, Uttarakhand, Jammu & Kashmir and Arunachal Pradesh. Plain regions in Maharashtra, Chattisgarh Karnataka, Punjab, Kerala, etc., have also sizable potential.
TURBINE
The selection of a suitable turbine for any particular hydro site depends on site characteristics mainly the head and the discharge available. The turbine should produce the expected power at particular speed, head and discharge.
Classification of Turbines
The turbines are classified as high head, medium head and low-head turbines on the basis of available head. They are also classified as impulse and reaction turbines based on the action of flowing water on turbine blades.
1. Impulse Turbine
The principle of operation of an impulse turbine is based on the conversion of pressure energy of water into kinetic energy when water passes through a nozzle and strikes the turbine blades with high impulse force causing rotation of the turbine. Main types of impulse turbine are as described below:
(a) Pelton wheel turbine: It consists of a wheel or runner with a series of split buckets set around its rim. The water from the penstock flows into the turbine through a jet nozzle. The high speed jet of water from the nozzle strikes the bucket attached to the wheel. This causes rotation of the turbine. The amount of water striking the vanes is controlled by forward and backward motion of a valve called a spear. A casing is provided to prevent splashing of water and to discharge the water into tail race. It is made of cast iron (CI) or fabricated steel plates.
The main feature of pelton wheel turbine are:
1. It is suitable for high water heads in the range of 60 to 700 m which can give an output power of 50 to 10,000 kW.
2. Turbine efficiency is 85-90%.
3. It is mostly used in a high head micro hydro system.
4. Its shaft is horizontally mounted.
5. Buckets have a minimum cavitation effect.
6. It is a radial flow turbine as water strikes tangentially at the wheel.
(b) Turgo Turbine: The turgo wheel is an upgraded version of pelton wheel. Here the water jet is designed to strike the plane of the runner at an angle about 20° so that the water enters the runner at one side and exits on the other. The speed of a turgo turbine is much higher than a pelton wheel. It can operate under low-flow conditions with medium or high head.
The main features of a turgo turbine are:
1. It is suitable for water heads in the range of 30-210 m.
2. It is suitable for horizontal shaft arrangement.
(c) Ossberger cross-flow turbine: It has a drum-shaped runner with a solid disk at each end connected together by a series of curved blades. The shaft of the turbine is always horizontal. A rectangular nozzle directs a jet of water over the full length of the runner. Water enters the top of the runner and impinges on the curved blades imparting its kinetic energy. Water before falling away strikes the runner blade again on its exit. Thus there is a cross-flow of water over the runner.
The main features of Ossberger crossflow turbine are:
- Its efficiency is about 85%.
- It is suitable for water heads in the range of 1 to 200 m.
- It is free from cavitation.
- The shaft of the turbine is horizontal.
2. Reaction Turbines
The runner of a reaction turbine is completely immersed in a water filled casing. The pressure difference across runner blades imposes reaction force which causes the rotor to rotate. All reaction turbines have a draft tube below the runner through which water discharges.
The main types of reaction turbines are described below:
(i) Francis Turbine: It consists of a spiral scrolled casing which distributes water around the perimeter of the runner. Guide vanes are fixed in the spiral casing which can be adjusted to control the flow of water into the runner. When guide vanes are opened, water is directed tangentially to the runner. When water passes over runner blades, it causes reaction force on it. This reaction force rotates the runner.
In this turbine water flows radially inwards into the runner and discharges axially out of the runner. The draft tube below the runner discharges the water into the tailrace. It is a vertically mounted type turbine.
(ii) Kaplan Turbine: Kaplan turbine is a reaction type axial flow turbine. It has adjustable blades inside a tube. The inlet guide vanes can be opened and closed to control the amount of water flow through the turbine. Fully closed guide vanes will stop the water flow completely and bring the turbine into rest. When guide vanes open water hits the turbine blades thereby causing rotation of the turbine. A draft tube is fitted below the turbine runner to extract water and discharge it into the tail race. Kaplan turbine is suitable for low head with high flow rate. Kaplan turbines are available in vertical axis, horizontal axis and inclined axis configurations. Horizontal axis turbines are slightly more efficient than vertical axis turbines. This turbine suffers cavitation of blades near the outlet edges due to silt water.
ENVIRONMENTAL IMPACT OF SMALL HYDRO POWER PLANTS
1. Construction of road, reservoir, power house switchyard, diversion tunnel in small hydro power plants adversely affect the environment.
2. SHP may adversely affect the flora and fauna of the area.
3. Small hydro power plants with reservoirs provide habitat for mosquitoes and snails which may cause diseases like malaria, yellow fever, dengue, encephalitis and schistosomiasis.
4. The operation of small hydropower equipment may increase the noise level of the surrounding area. This adversely affects the environment in hilly and mountainous regions.
5. Alteration of water flow such as broadening of stream and reduction of current may cause adverse environmental impact.
6. The other environmental impacts include sedimentation and deterioration of water quality.
7. The released water from SHP contains low dissolved oxygen, this may adversely affect fishery and cause mortality of sensitive species.
8. The construction activity in the SHP sites has an adverse impact on ecology (flora and fauna).
9. The small hydro power plants may cause loss of water falls and other recreational activities.
MERITS AND DEMERITS OF SHP
Mertis
1. It is a renewable and clean source of energy. It does not require any fuel, so the dependency on imported fuel also reduces.
2. It helps in reducing emission of greenhouse gasses.
3. Low operating and maintenance costs.
4. It benefits the people in remote areas where there is no grid connectivity.
5. It does not require large civil engineering construction.
6. The capital cost of the small hydro power plants is quite low as compared to conventional energy sources.
7. No large scale water storage is required, so there is no problem of submergence, deforestation and rehabilitation.
8. It is a more concentrated form of energy as compared to solar and wind energy which are dilute in nature.
9. Few operating manpower is required in small hydropower plants.
10. Energy can be tapped whenever water flows along small streams, small rivers, and even canals also.
11. The small hydro does not require much expertise to build and operate.
Demerits
1. SHP Plants have low installed capacity.
2. They are located in remote areas, so less chances of grid connectivity, so the surplus power cannot be utilized.
3. There are uncertainties about their potential as a reliable source of energy due to inadequate technical data.
4. SHP plants adversely affect migration of fish, fishing, boating, etc.
5. SHP technology is not fully developed.
6. Limited sites are available for SHP according to MNRE.
7. In many locations the water flow fluctuates seasonally. During the summer there will be less flow and therefore less power output.
8. Additional equipment is required for voltage and frequency control as they are mainly operated in off-grid mode.
9. The potential sites of SHP are worthy in hilly or mountainous regions where grid connectivity will probably never reach.
10. Although small hydro power is a clean, renewable energy source, the projects require approval from the Ministry of Environment and Forests (MOEF) and other departments.
SELECTION OF SITE
Selection of site depends on the following factors :
1. Topography and geography of the site.
2. Evaluation of the water source and its generating potential.
3. Site selection and layout.
4. Hydraulic turbines, generators and their control.
5. Environmental impact assessment and mitigation measures.
6. Economic evaluation and potential of finance.
7. Institutional framework.
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