Hydroelectric Power Plant Working Principle

Hydroelectric Power Plant Working Principle

Hydropower or hydroelectricity is a renewable source of energy that utilizes the energy of fast-flowing water to generates electricity. The use of hydropower for various purposes is not a modern concept; its application can be seen even a thousand years ago. The ancient people used to run the wheels with the application of waterpower to grind grains and wheat into flour. Nowadays, modern hydropower turbines are used to utilize waterpower. The hydroelectric power plants generate electricity from the potential and kinetic energy of the water. It is one of the most cost-effective methods of electricity generation, which is why it is the most preferred and widely used as compared to other methods of electricity generation. As natural sources of fuels like oil, coal, and petroleum are exhaustible, hydroelectric power plants are very useful to meet the high demands of electricity. In this article, we’ll discuss the various components, types, and working principles of the hydroelectric power plant.

Components of Hydroelectric Power Plant

Hydropower plants are generally constructed in the hilly areas across the rivers, oceans, or other water bodies, where the dams and large water reservoirs can be easily built. Different types of hydroelectric power plants are constructed as per the requirements, but every hydroelectric plant consists of some basic components, which are explained below.

Components of Hydroelectric Power Plant

Reservoir

A reservoir is the most essential part of the hydropower plant. It stores the water and supplied it down to the hydro turbine for electricity generation. The reservoir can be natural lakes in the hilly areas, or it can be made artificially by establishing a dam across the water bodies. The reservoirs of the hydropower plants are also used for flood control, irrigation purposes, industrial, and aquaculture.

Reservoir at Hydroelectric Power Plant

Forebay

A Forebay is an area to temporarily store the water before flowing it down to the turbine. It stores the excess water in the case of rainy seasons and supplies it during the dry seasons, i.e., it maintains the amount of water to be needed as per the requirement at the load area. The forebay is constructed when the hydroelectric plants are situated far from the reservoir, else the reservoir itself acts as the forebay when it is located near the plant.

Forebay

Dam

A dam is the most expensive element of the hydroelectric power plant. It is a barrier constructed across the water bodies to restrict the flow of the naturally flowing water and to raise the water level in the reservoirs. They are usually made of concrete, rocks, earth, or stonemasonry. The type of material to be used for their construction depends upon the geography of the area, transportation availability, and the occurrence of any natural disasters such as earthquakes or floods in that particular area. For example, in the case of the area having a narrow canyon, a stonemasonry dam is suitable, while for a wide valley, the earth dam is preferred.

Spillways

In case of heavy rainfall or flood situations, the water level in the reservoir may rise beyond its storage capacity that may affect the proper functioning of the hydropower plant. To prevent this situation, a hydraulic structure called a spillway is built at the site of the dam. The spillway safely diverts the extra water from the reservoir to a downstream area. Spillways are constructed either as a part of the dam or just beyond them. They are usually made of concrete having metal control gates to stop or discharge the water from the reservoir.

Tailrace

The water left at the hydroelectric plant after being generated electricity by the hydro turbine is carried away from that area through a channel called tailrace. The tailrace is present behind the dams at a lower level than that of the reservoir. As the potential energy of the water due to the elevated reservoir is being used up by the hydro turbine, the water through the tailrace flows at the natural speed of the water and joins the same or another water stream.

Hydroelectric Power Plant

Penstocks

Penstocks are the channels or large pipes at the hydroelectric station that carries the water down to the turbines at the power station from the reservoir. The penstocks are generally made of steel or reinforced cement concrete (RCC). The material to be used for constructing penstocks depends upon the water-head of the dam. The water-head is the vertical height between the source of the water and the turbine, i.e., the vertical distance travelled by the water from the elevation to the turbine; it is usually measured in meters or feet. The steel penstocks can be used for any head or working pressure of water, whereas the RCC penstocks are used for the low water heads, usually less than 30 meters. As a large amount of water flows through the penstocks hence the abrupt opening and closing of the gates at the ends of the penstock can cause a water hammer effect (pressure surges). To withstand the water hammer effect, the penstocks are carefully designed, such as short-length penstocks are provided with thick walls, and surge tanks are installed in the long-length penstocks.

Penstocks

Water Intakes

The water intake includes the structures that collect the water stored in the reservoir or forebay and direct it towards the turbines through the penstocks. Water intakes consist of several gates, screens, filters, booms, sluices, and trash racks that control the amount of water that reaches the turbines and also block any kind of debris such as trunks, waste products, or branches from entering the channel by diverting it to the bypass chute. The screens and trash racks are installed at the entrance of the penstock to forbid the debris from entering inside as debris can cause damage to the various important hydraulic parts such as turbine blades, nozzles, and turbine runners. Trash racks are usually made of rod-shaped steel material having a gap of about 10-30 cm in between the rods. Generally, intake structures are categorized as high-pressure intakes (in case of large storage) and low-pressure intakes (in case of small storage). There may also be the chances of ice formation on the water surfaces during the cold seasons. To block the ice from entering the penstock, the trash racks are heated, which eventually melts the ice when it comes in contact with them.

Trash Racks

Trash Racks

Sluice

Sluices are also part of water intake structures. The flow of water through the penstocks is controlled by the sluice; the sluice is the gate that is installed at the ends of the penstocks, which can be raised or lowered according to the requirement of the water at the turbine. The water freely flows through the penstocks when the sluice is open completely, but less water flows through the penstock when the sluice is partially closed. They are generally kept open in the dry seasons to allow water to pass through the penstocks, but in rainy seasons they are slightly kept closed to avoid flooding. A small containment water pond is constructed before the upper opening of the penstock to store the water that can not be entered through the penstock in case of the closed or partially closed sluice and also to avoid pressure on the closed sluice. The instalment of the sluice in the penstocks prevents the overall dam failures, and they ensure that penstocks can be easily cleaned, inspected, and repaired in case of any damages such as holes, or cracks in the penstocks.

Sluice Gate

Surge Tank

The sudden water surges due to the changes in the water flow may result in variations in the pressure that can damage the components of the hydropower plant. To control the pressure changes, small cylindrical water storage tanks called surge tanks are used. Surge tanks are open from their top to reduce or neutralize the pressure changes in the reservoir and are used to regulate the turbines. They protect the conduit (channel) from excess internal pressure and are also capable of storing water to raise the internal pressure in case of pressure drop. Surge tanks are usually located at the centre of the penstock (steep-sloped) before the water turbine. The type of surge tanks to be used in the plant depends upon the length of the penstock or other requirements of the hydroelectric plant.

Powerhouse

A powerhouse is a separate room or building at the hydroelectric power stations, which consist of various electrical and hydraulic components. The powerhouse is responsible for controlling the various inlet and outlet gates and stopping the flow of water in the equipment areas in case of repairing or changing various pieces of equipment. Like other power plants, hydroelectric power plants also consists of general auxiliaries like control panels, service area, testing rooms, generators, and transformers.

Powerhouse of Hydroelectric Power Plant

Turbines

The water turbines are used to convert the potential energy and kinetic energy of the water into mechanical energy. The fast-flowing water running down from the elevated surface is directed towards the blades of the turbine, and the turbine blades start spinning due to the force on the blades by the water. As the blades are spinning, and the force is acting through a distance hence, the work is said to be done (W=Fs, where W=work done, F=force applied, s=distance covered), i.e., the water-energy is transferred to the mechanical energy of the turbine shaft. The type of turbine to be used in the hydroelectric power plant depends upon the water-head, the volume of the water, and the depth at which the turbine is to be placed. Hydroelectric turbines are generally divided into two categories according to the way of energy exchanges between the turbine and the water. These two types are the impulse turbine and the reaction turbine.

Impulse Turbine

The impulsive turbine consists of a wheel fitted at the centre (runner) with buckets (blades) along its edges. The pressure inside the runner is kept constant, while the runner is at atmospheric pressure. It uses the velocity of the flowing water to rotate the runner. The water streams strike every bucket of the turbine runner. The pressure generated by the water is converted to kinetic energy in a nozzle placed at the junction of the penstock and the turbine. The velocity of the jet nozzle directs the energy to the buckets that rotate the runner. This turbine is preferred for the high water-heads and low water flow. The two widely used impulse turbines include the cross-flow turbine and the Pelton turbine.

Impulse Turbine

Reaction Turbine

The reaction turbines convert the water-energy into mechanical energy by using the pressure and force of the flowing water. The turbine is kept in the direct water stream that allows the water to flow over all the blades instead of individually striking each blade of the turbine. Reaction turbines are suitable for low water heads and a high quantity of water flow. The widely used reaction turbines involve Kaplan turbines and Francis turbines.

Diagrammatic Representation of Reaction Turbine

Draft Tube

The draft tube is a conduit or the pipes of appropriate diameter that are connected from the turbine runner to the trail race. These tubes discharge the left water (after rotating through the turbine) from the turbine to the trail race by maintaining the atmospheric pressure of the discharged water. As we know that most of the water pressure (potential and kinetic energy) is converted into mechanical energy by the hydraulic turbine, the discharged water at the end is left with the pressure less than the atmospheric pressure at the exit of the turbine. This may result in the backflow of water from the tailrace to the turbine. To prevent this situation of backflow of water, draft tubes are used as they increase the water pressure above the atmospheric pressure that eventually prevents the water from backflowing.

Draft Tube

 Generator

The electric generator used at the hydroelectric power plant converts the mechanical energy of the water turbine into electrical energy. The working of the generator is based on the principle of Faraday’s law; it states that the voltage induced in the electric circuit is directly proportional to the rate of change of the magnetic flux in the circuit. The generator consists of two main components, i.e., the stator and the rotor. The mechanical force of the turbine shaft is applied at the rotating structure of the generator, i.e., the rotor, while the stator is the stationary part of the generator at which the voltage is induced upon exciting or magnetizing the rotor. The rotor is fixed with the field poles (electromagnets) at the inside of its edges, and when the rotor rotates, the field poles rotate around the conductors of the stator. This results in the induced voltage and the flow of electricity at the output terminal.

Water Turbine Generator

Transformer

The electricity generated by the hydroelectric power plant is not of the appropriate voltage that can be used at the homes or the other general purposes, which is why transformers are used at the hydroelectric power stations. The transformers convert the alternating current (A.C) produced at the plant into the required voltage while maintaining the constant electric power. This power supply is connected to the national grid, which is further distributed for industrial or domestic use.

Transformer at Hydroelectric Power Plant

Electricity Transmission Lines

The next and final step after the electricity is converted into the required voltage is supplying it to the required areas. Dams are often established in remote areas, hence the electricity generated at the hydroelectric station is transmitted to the domestic or the industrial area through long cables or transmission lines.

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Working Principle of Hydroelectric Power Plant

To understand the working principle of the hydroelectric power plant, let’s first understand the potential energy and the kinetic energy.

Potential energy: It is the energy possessed by the body due to its position relative to the other objects. When the objects are displaced from their equilibrium, they gained some energy, which gets stored in the objects in the form of potential energy. For example, when the spring is stretched or compressed, it gains potential energy, and when you throw the ball to another person, the ball has more potential energy when it was in the air than it falls on the ground.

Kinetic Energy: It is the energy possessed by the body due to its motion, i.e., the higher the speed of the body, the higher will be the kinetic energy.

Working principle of Hydroelectric Power Plant

The working principle of the hydroelectric power plant is that it converts the potential energy (due to the elevation from the channel) and the kinetic energy (due to fast-flowing water) of the water into mechanical energy with the help of turbines. The water that is stored in the reservoir or forebay behind the dam falls through the penstock, and it strikes the blades of the turbine with high pressure, and the turbine runner starts rotating. The runner is attached to the central shaft that is connected to the generator, which eventually generates electricity, i.e., the turbine’s mechanical energy is converted into electricity through electric generators. The electrical energy obtained is then supplied for domestic or industrial uses through the transmission lines after the voltage regulation by the transformers. The electrical energy obtained through the hydroelectric plants is proportional to the rate of flow of water and the elevation drop.

Types of Hydroelectric Power Plant

The type of Hydropower plant to be built depends upon the requirements, such as the immediate or delayed requirement of the electricity, load, or storage purposes. Based on that, the hydropower plants are generally categorized into the following types.

Run-of-River Hydroelectric Power Plant (RoR)

These types of hydroelectric power plants are built on rivers that maintain a constant water flow throughout the year. They pull out the water from the river through a canal or a penstock and direct it to the turbines that eventually generate electricity. Due to the diversion of the water, the construction of the dam is not required for these types of hydroelectric power plants. The constant flow of water is required in the working of these power plants. Electricity generation by these power plants is largely dependent upon the natural elevation drop of the river, which eventually depends upon the rain, and the environmental conditions. Apart from Run-of-River, similar types of power plants, i.e., In-stream power plants are also used sometimes, their working is almost the same as that of the Run-of-the-river power plants, however, in the case of in-stream power plants the turbine is present within the dam across the riverbed, hence it does not requires to divert the river flow.

Run-of-River Hydropower Plant

Hydroelectric Power Plant with a Reservoir

In these types of power plants, the water is stored in a reservoir to raise the water level for any future use, which is also called an impoundment. The reservoir helps to maintain the water flow through the turbine that generates the electricity. It is better than the run of the river power plants because they can produce energy throughout the year as the water level of the river can be controlled with the help of the reservoirs in case low elevation drop of the river due to low rainfall in a particular year. The major disadvantage of these dams is that they cause harmful social and environmental impacts.

Hydroelectric Power Plant with a Reservoir

Pumped-Storage Hydroelectric Power Plant (PSH)

Pumped storage hydroelectric power plants consist of reservoirs at different heights, i.e., the upper reservoir and the lower reservoir. These reservoirs are used to fulfil the extra demand for water to produce electricity. The water stored in the upper reservoir runs through the turbine and enters the lower reservoir, where the electricity generates. The water in the lower reservoir is again pumped back to the upper reservoir when there is no requirement for electricity. These hydroelectric plants have the capacity to store large-scale energy; however, they consume a lot of extra energy due to the pumping process. PSH is of two types, i.e., Open-loop PSH and Closed-loop PSH. Open-loop PSH is connected (hydrologic connection) to the natural water bodies, while the closed-loop PSH does not have any hydrologic connection to the outside water bodies. Hydrologic connection is the exchange of water between the reservoirs, impoundments, and groundwater through a pathway or corridor present underground.

Offshore Hydropower

These types of hydroelectric power plants utilize the power of waves and tidal currents of the sea to generate electricity. The concept of offshore hydropower is not much established yet, but this technology is expected to show significant growth in the coming years.

Offshore Hydropower Plant

Advantages of Hydroelectric Power Plant

  • Although the initial instalments of the various components of hydroelectric plants require high investment, once’s it’s installed, it is the cheapest method of electricity generation in a long run. The dams, once constructed, usually last for an average of 50-100 years and only require some maintenance cost after its establishment.
  • Hydroelectric power plants are a renewable source of energy as they only require the energy of fast-flowing water and not the water itself, i.e., the water is not consumed in the process. Moreover, they do not release harmful emissions into the environment as compared to the other fossil fuels power plants.
  • Hydropower stations are very agile, reliable, and adjustable. The production of electricity can be easily maintained by controlling the flow of water through the penstocks. One need not wait for the sunlight or the wind as in the case of other electricity generation methods as water is always available in the reservoirs for the constant supply of electricity.
  • Hydroelectric power plants provide enormous energy as the water at the high altitudes has a remarkable amount of potential energy, and even the smaller part of the fast-flowing water is enough to provide abundant energy that can serve the need for electricity for millions of people.
  • As the water stored in the reservoirs is clean and usable, hence reservoirs are also used as water reserves that can be used for various purposes in case of draughts situations. The flood can also be prevented by establishing flood control dams as they prevent the extra water from running down in the villages or the cities by storing them in the reservoirs.
  • As the dams are often located in remote rural areas that are not much developed. The construction of hydropower plants requires the transport of various equipment and materials from the cities, and to improve the connectivity, proper highways, and roads need to be built, which eventually results in the development of the rural towns. The lakes (reservoirs) can also be used for recreational purposes, which may attract mass tourism and eventually results in an economic boost for the nearby towns.

Disadvantages of Hydroelectric Power Plant

  • As the hydropower plant can only be established in a suitable location. These locations are very limited in number and are usually found away from the cities and the villages. Hence, it requires a large number of components and long transmission lines to supply the generated electricity to the required areas.
  • The electricity generation process from the hydroelectric power plant may not produce emissions, but the plants that are grown at the bottom of the reservoirs releases harmful greenhouse gases like methane and carbon dioxide upon decomposition that can impact the environment.
  • The construction of dams restricts the natural flow of water that can cause a negative impact on the breeding cycle of the fishes and other aquatic animals, which may result in the extinction of some species.
  • Although flood control dams store the extra water and prevent floods, heavy rainfall, or any kind of malfunctioning in the dams may result in the collapse of the dams, which can cause the lives of people residing in the nearby areas at risk. The ‘Banqiao Dam’ collapse due to the typhoon Nina is considered the worst dam failure, in history which resulted in many casualties.

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