21 Hydropower Generation-I

Rajesh Kumar Lohchab

Learning Objectives

1.      To understand the history and basics of hydropower

2.      To understand the role of solar power through water cycle in generation of hydropower

3.      To explain the components of Hydroelectric Power Plant

4.      To explain the advantages and disadvantages of Hydroelectric Power Plant

Introduction

Based on resources, power generation can be classified as coal and gas based thermal power plants (TPP), hydro power plants (HPP), nuclear power plants (NPP) and renewable energy based power generation plants. Power generation in India is unevenly distributed because hydro resources are available in Himalayan region, while fossil fuel resources are available in the central and western parts. For optimization of these resources, the power systems in our country were categorized into five power regions in the 1960s (Ramanathan and Abeygunawardena, 2007). That’s why regional power grids were developed. Later on in the 1980s a national grid was formed which strengthened the intraregional and inter-regional transmission systems. The Indian power system is also connected with the Bhutan and Nepal power systems.

Hydro Energy

Hydro power stations use the potential energy of water when it falls due to gravity. The fall and movement of water is part of water cycle. The force of moving water can be extremely powerful. Hydropower is a renewable source of energy. It is one of the cheapest sources of energy. Electricity production by hydropower is cheap because once a dam is built water is available free of cost.

History of Hydropower

From centuries hydropower has been used as source of energy. Greeks were using hydropower to produce flour from wheat 2,000 years ago. The force of falling water has been used to generate electricity since late 19th century and first hydroelectric power plant was built on the Fox River in 1882.

Hydropower Resource Potential of India

India is ranks fifth in terms of hydropower potential in the world. It is mainly spread on six major river systems. The Ganga, Indus and Brahmaputra account for about 80% of the total potential of Indian hydropower (Ramanathan and Abeygunawardena, 2007).

Rationale for hydropower

Hydropower is an established technology with cost effective renewable source of energy. Other benefits of hydropower plants are:

• Water supply
• Flood and drought control, and irrigation
• Navigation and recreational activities
• Electricity production without interruptions
• Safe operation with minimum risks
• Environmental and socially sustainable
• Large energy storage and operation flexibility for balancing the seasonal load

How Hydropower Works?

Hydroelectric power is a form of solar energy. The hydrological cycle is a sun driven process of water transport from the oceans to the atmosphere and from the atmosphere back to the earth surface and oceans. The hydrological cycle discoverer, Bernard Palissy (1580 CE), declare that rainfall itself is adequate for the maintenance of rivers. It explains the nonstop movement of water on, above and below the earth surface. The water travels from one source to another i.e. from river to ocean, or from the ocean to the atmosphere and back by evaporation, condensation, precipitation, infiltration, surface runoff and subsurface flow. During this it undergoes through liquid, solid (ice) and vapor (gas) phase. This cycle extend from an average depth of about one km in the lithosphere (the crust of the earth), to a height of about 15 km in the atmosphere. The water cycle maintain of life and ecosystems on the earth and used for households, industries, agriculture and production of power.

Water Reservoirs

A reservoir is an artificial lake constructing by making a dams across rivers to store water. It can also be formed on natural lake by constructing a dam at Lake outlet. They are used for power generation, downstream water supply, irrigation, flood control, canals and recreation. Reservoirs are highly managed structure used to balance the flow by taking in water during high flows and releasing it during low flows in controlled manner. Recreational uses of reservoir are fishing, boating bird watching, landscape painting, walking and hiking. Large reservoirs retain water for months or even years of average inflows basis and also provide flood protection and irrigation services. The design and provision of these services in a hydropower plant dependents on environment and social needs.

Catchment Area and Watershed

Catchment area is the area of land from which water is drain into river. It is also known as river basin, catchment basin, drainage basin, drainage area and watershed. It acts like a funnel and all water from this is channeled to a single point into a river. Catchment areas are topographically separated from each other by a ridge, hill or mountain and line which divide watershed or surface runoff between two adjacent river basins is called the topographic water divide, or the watershed divide or simply the divide. A network of rain gauges is placed to assess of water resources of a catchment. For each rain gauge catchment area should be small for accuracy and better results. Rain gauge density is expressed as area covered per gauge. According to IS: 4987-1968 the density of rain gauge network is one station per 520 km² in plains, one in 260 to 390km2 in moderately elevated area i.e. up to 1000m and one in 130 km² hilly area.

Hydrograph

It is a graphical representation of discharge variation with time thus it is the representation of rainfall input of a catchment. The discharge recorded in hydrograph is the combined result of surface runoff, interflow and base flow. Direct and indirect methods of flow measurements are used to calculate the discharge of a stream. Direct measurement of discharge in a stream is carried out velocity method, dilution techniques, moving boat method etc. whereas indirect measurement of discharge is done by using hydraulic structures like weirs and gated structures and slope area method.

Unit Hydrograph

When one cm of rainfall is applied at a uniform rate at a specified time period over the catchment area uniformly is referred as unit hydrograph. Unit hydrograph are used to predict the flood in a catchment by a storm.

Effective Rainfall Hydrograph

Effective rainfall hydrograph (ERH) is the subtraction of initial losses and infiltration losses from the rainfall hydrograph. It causes direct runoff which includes both surface runoff and interflow. Effective rainfall is slightly higher than the excess rainfall.

Hydropower Theory

The dam holds the water to create the height difference necessary to maintain potential energy. Water flow continues to the river downstream of the dam. The two vital factors necessary for hydropower generations are the flow and the head of the stream or river. The flow is the volume of water which can be captured and re-directed to turn the turbine generator, and the head is the distance of water fall on its way to the generator. The larger the flow more will be the water, and higher the head higher will be the distance the water falls, thus the more energy is available for conversion to electricity. Double the flow and double will be the power, and double the head, double will be the power again. A low head site i.e. head of ≤10 meters, you need to have a good volume of water flow to generate electricity. A high head site i.e. head of ≥20 meters gravity will give you an energy boost.

Kinetic energy of falling water is harnessed to provide electrical power. It depends on flow and height of the falling water. Hydroelectric Power is a Function of Height and Volume.

Power = Head x Flow x Gravity

The theoretical power from a site is calculated by equation given below (Gaiusobaseki, 2010):

P = ηρQgh

Where:-

P = Power (W)

η  = Dimensionless efficiency of the turbine (Approx 0.9) ρ = Density of Water (1000 kg/m³)

Q = Volumetric flow rate (m²/s)

G = Acceleration due to gravity (9.8m/s²)

h = Height difference between inlet and outlet (m)

Energy from Hydro-power

The potential theoretical energy in a volume of elevated water can be calculated by:

W = ρ V g h

Where:

W = energy (J)

V = volume of water (m3)

The Physics of Hydropower:

Based on the conservation of energy, hydropower energy transfer is as below:

Potential Energy → Kinetic Energy → Mechanical Energy → Electric Energy

Potential Energy:

Head level is the difference between the maximum heights of water to the minimum height of the water. It is directly proportional to the potential energy. A high head level would mean that the potential energy of the hydropower system is very high. The effective head is the difference between the energy head at the entrance to the turbine and the energy head at the exit of the draft tube. When the volume of waters moves from the maximum level to the minimum level for a height of h, work will be produced and defined by the equation;

Using the equation for work, it is possible to calculate the theoretical power output of the hydropower system. This is done by differentiating the work equation with respect to time.

Where Q is the volumetric flow rate through the turbine, Power is measured in units of Watts.

Kinetic Energy:

As the water hits the impulse vanes, a dynamic force will exist in order for the vanes or buckets to start rotating. The rotation of the vanes converts the potential energy to kinetic energy. The force on the moving vane or bucket by a jet of water is derived as the equation of force:

W = Weight of the water striking the vane

v = relative velocity of water with respect to moving vanes

m = coefficient for loss of velocity moving across vane

= angle of deflection of the jet from its original direction

The relative velocity can be found using the equation:

V = absolute velocity of the water

u = absolute linear velocity of the bucket

Dams

A dam increases the head or height of the water and controls its flow. Dams release water to generate electricity and excess water is released through special gates called spillway gates during heavy rain falls. Oldest known dam is Jawa Dam in Jordan constructed in 3000 BC. It was 9 meters high and 1 m wide stone wall supported by a 50 m wide earth rampart. Kallanai Dam is the fourth oldest dam in the world and it still serves the people of Tamil Nadu, India. The dam was constructed by King Karikala Chola of the Chola Dynasty in the 2nd century AD. A dam holds large amount of water in a lake or reservoir. The higher the level of water in a reservoir, the more will be available potential energy for electricity generation. Basin wise power generation capacity in India is shown in Table 1.

Hydropower Plants

The flowing water contains a huge amount of kinetic energy which can rotate the wheels for generating motion energy for generating electricity. Hydroelectric power plants use turbine generators to produce electricity, just as thermal (coal, natural gas, nuclear) power plants do, except they do not produce heat to spin the turbines.

Hydroelectric Power Plant

Hydropower plant consists of three parts (Figure 1).

1.        A power plant

2.        A dam

3.      A reservoir

To generate electricity, dam gates open and water from the reservoir allowed to flow through large tubes called penstocks. The fast-moving water spins the blades of turbines at the bottom of the penstocks. The turbines are connected to generators to produce electricity which is transported via huge transmission lines.

Head and Flow

The amount of electricity generation in a hydro power plant depends upon head and flow of water. Head is distance of water drops from highest level of the reservoir/dam to the point where turbine installed. A high head plant needs less water flow than a low-head plant to produce the same amount of electricity.

Storing Energy

Hydropower plant has ability to store energy as water can be stored in a reservoir and released when needed for electricity production. Storage also makes it possible to save water for high energy demand period and low rainfall such as summer.

Power station

In the power station, turbines and generators convert the kinetic energy of the water into electricity. A hydro power plant may have more than one power station.

Spillway

A spillway releases water from the power station back into a river, stream or lake. It is a channel designed to slow the water back to its normal speed.

Size of Hydropower Plant

Large Hydropower

According to U.S. Department of Energy (US DOE, 2004) large hydropower are those which have a capacity of more than 30 MW. Major dams of world and India are shown in Table 2 and 3, respectively.

Small Hydropower

According to U.S. Department of Energy (US DOE, 2004) small hydropower are those which have a capacity of 100 kilowatts to 30 MW. Some of some dams of India are Given in Table 4.

Micro Hydropower

Micro hydropower plants are those which have capacity of up to 100 kW. A micro hydropower plant can produce enough electricity for a home, farm, ranch, or village.

Hydropower plants are primarily classifies in three functional categories: run-of-river (RoR), reservoir (or storage) HPP, and pumped storage plants (PSP). The RoR hydropower plant harnesses energy for electricity production mainly from flow of the river.

Advantages of Hydro Energy

1.       It is a renewable form of electricity generation.

2.       It is a very effective method of converting mechanical energy into electricity.

3.       No greenhouse gas emissions.

4.       It does not pollute the air like thermal power plants that burn fossil fuels.

5.       It can produce electricity on demand by control flow of water.

6.       It provides clean electricity.

7.       It creates reservoirs for recreational opportunities like fishing, swimming and boating.

8.       Other benefits may include water supply and flood control.

Floods

Flood is a condition of river overflowing from its banks because of the abnormal meteorological conditions like heavy rainfall, melting of snow from the catchment, shifting of the river course, bank erosion, or blocking of river, or breaching of the river flood banks. Floods are very common in India, particularly in the rivers basins of Kosi, Brahmaputra, Godavari, Narmada and Tapti. Floods are responsible for loss of life and property, damage to crops, famine, epidemic diseases and other indirect losses.

Flood Control

The damages of floods can be minimized by adopting the following control measures.

1.      Construct reservoirs and detention basins to provide a temporary storage of the peak floods.

2.      Adopting soil conservation measures in the catchment area.

3.      Construct flood banks, dykes, or flood walls.

4.      Construct and improve channel by deepening river training works.

5.      Construct bypasses or flood ways to divert a part of the flood through these.

6.      Set up short term and long term warning systems of flood forecasting like rhythm signals and radar centers at vulnerable areas.

Disadvantages of Hydro Energy

Hydropower plant needs dams to create reservoirs at lakes or rivers, which flooded a large piece of land. Therefore there is a loss of farmland and residential areas which need shifting of people in new homes in new areas. Due to submergence of area there is loss of flora and fauna and disruption to animal, plant and aquatic ecosystems. Migration of fish is stopped by construction of dam, thereby their breeding and survival is adversely affected. Hydropower plant can impact water quality and flow by lowering the dissolved oxygen levels in the water. A minimum flow of water in downstream of a hydropower plant is required for the survival of riparian habitats. New hydropower plant affects the local environment and may compete with other uses for the land. Humans, flora, and fauna may lose their natural habitat. Local cultures and historical sites may be impinged upon.

• Scientists have traced the cause of over 100 earthquakes worldwide to dams. Filling of reservoirs of large dams has triggered seismic activity because it create extra water pressure in the micro-cracks and fissures in the ground under and near a reservoir. The water in the rocks acts as lubricant in faults which are already under tectonic strain.
• Sediments are the soil particles produced during erosion of soil and rocks by water and wind in the catchment and these are transported with flowing water in the river. By constructing a dam we retard the velocity of flow water which results in settling of sediments having density more than water at the bottom of reservoir under the force of gravity. It results in reduction of storage capacity and overall life of the reservoir. Sedimentation in a reservoir is a nonstop and complex process which affects the useful life of a reservoir. The monitoring of sediment and sedimentation process at bottom of reservoir is essential for efficient management of reservoir and river basin.
• Decreases in silt and nutrients in downstream of a river decrease soil fertility in riparian land, which harms the plants and animals that live and grow there. It causes animal habitat to drops and loss of biodiversity.

Summary

In this module we learnt about:

What is hydropower and how it works?

What is hydrological cycle? What its role in hydropower generation?

How the hydroelectric power plants works?

What are components hydroelectric power plants?

What are advantages and disadvantages of hydroelectric power plants?

References

• U.S. DOE (2004). Hydropower: setting a course for our energy future. United State Department of Energy, Washington D.C.
• Ramanathan K. and Abeygunawardena P. (2007). Hydropower development in India: a sector assessment. Asian Development Bank
• Gaiusobaseki T. (2010). Hydropower opportunities in the water industry. International Journal of Environmental Sciences 1(3):392-402.