What is Hydroelectric energy?

Hydroelectric energy is also known as hydropower or hydroelectricity. The term hydropower refers to the power obtained from water motion. By the term 'power', primarily means the generation of electricity. A hydropower plant or hydroelectric power plant utilizes the kinetic energy or pressure energy of flowing water into electrical energy. Unlike conventional energy sources used for power generation, electricity obtained from hydraulic power contributes to zero greenhouse gas emissions.

With the growing demand for energy conservation and safeguarding the environment, people made a shift from conventional energy sources to non-conventional energy sources. These non-conventional energy sources are also known as renewable sources of energy. Renewable energy sources are those energy sources that do not get exhausted with time and are eco-friendly, meaning, they do not cause harm to the environment.

There are various renewable energy sources like solar energy, geothermal energy, nuclear energy, wind energy, energy from natural gases, and hydroelectric energy, etc. In this article, major emphasis has been made on hydroelectric energy.

Parts of a hydropower plant

A hydropower plant specializes in converting the pressure energy or kinetic energy of stored water in specially designed reservoirs into mechanical energy in a rotating turbine and finally converting the mechanical energy into electricity or power generation. To accomplish the task of electricity generation, a hydropower station incorporates various equipment. The present section outlines the different parts of a typical hydropower station.

Dam

A dam, for instance, the Itaipu Dam, is the primary unit of a typical hydropower plant. A dam can be referred to as a large reservoir that holds a large volume of water. Besides storing water, a dam also acts as a barrier that restricts the water flow thus adequately supplies water for human consumption and irrigation purposes. A dam performs the job of suppressing water which might cause floods into nearby areas.

In a hydropower plant, a dam holds back the water which can be released in a controlled way for power generation purposes by mechanically operating different gates provided at the base of the dam. The gates of the dam are made of high-strength materials to withstand huge hydrostatic forces induced by water.

In case of the absence of a dam, a method of energy harvesting from flowing water is practiced, to serve the purpose of electricity generation. This is also known as run-of-the-river or simply run-of-river.

Penstock

Penstock is a long-distance pipeline whose one end is connected near to the gates and the other end has a nozzle, which is directed towards the blades of a hydraulic turbine. The penstocks are so designed based on whether the hydraulic turbine functions with kinetic energy or potential energy of water as an input to the blades.

The length of the penstock will have a decreasing cross-sectional area as it approaches the turbine blades, in case the requirement is of kinetic energy. In case of pressure energy requirement, the gates of the dam are built at a higher elevation, or a secondary reservoir is built at an elevation, which is above the gates of the dam. The water is temporarily stored at this secondary reservoir and the penstock runs down to the turbine blades, creating pressure energy inlet to the turbine blades. The water stored at the secondary reservoir has stored potential energy which is converted to mechanical energy by the hydraulic turbines.

Turbine

A hydraulic turbine is one of the main components of a hydropower plant. A hydraulic turbine has numerous blades mounted on a rotating shaft, whose one of the ends is connected to an electric generator. Water with high kinetic energy or pressure energy impinges these blades through nozzles and generates forces. These forces cause an angular acceleration in the shaft. The distance, angular position, and the area of the nozzles decide the magnitude of angular acceleration. Based on the flow direction of water and the nature of input energy available at the turbine blades, the turbines are classified as Pelton turbine, Kaplan turbine, and Francis turbine.

Generators

The generators of a hydroelectric power plant are the energy conversion devices whose major role is to convert the mechanical energy of the shaft rotation into electricity. These generators have large magnets inside with numerous turns of copper coils. Due to the rotation of the generator shaft, the changing magnetic field induces electric current which is alternating in nature. The electricity thus generated is transmitted by the use of transmission lines.

Types of hydraulic turbines

The primary purpose of all the hydraulic turbines in a hydroelectric power plant is to convert the hydraulic energy into electricity. Various hydroelectric projects demand different turbines based on the available hydraulic head and hydroelectric power generation requirements whose data are sourced from U.S. Energy Information Administration.

The various kinds of hydraulic turbines are outlined below,

Impulse turbine

In impulse turbines, the available energy at the turbine blades is the kinetic energy of water. The velocity of water is used to induce rotational motion to the turbine blades. The pressure of the water at the turbine outlet is atmospheric. These kinds of turbines are suited for water having a high energy head and less flow.

These turbines are characterized by a series of blades in the form of buckets arranged around the periphery of the runner of the turbine. Impingement of water at these blades generated force components perpendicular to the central axis of the turbine, which causes rotation. Pelton turbine and cross-flow turbine are two of the popular turbines in this category.

Reaction turbine

Reaction turbines use both the pressure energy and kinetic energy of water as an input to the turbine blades. These turbines are useful for water having low heads but high flow. The turbines are directly placed into the flowing water that exerts forces on the blades causing them to rotate. Kaplan turbines and Francis are two of the popular turbines in this category.

Hydraulic head

Hydraulic head is the measure of the height of static volume of water from an arbitrary point present in the ground. The magnitude of the hydraulic head is usually measured in meters, m.

Hydraulic head refers to the amount of potential energy available in water in the static condition which can be converted to other forms of energy. The higher is the value of the hydraulic head, the more is energy possessed by water.

Context and Applications

This topic finds its existence in various undergraduate and postgraduate degree courses like

• Bachelors of Technology in Mechanical Engineering
• Masters of Technology in Mechanical Engineering
• Bachelors of Technology in Civil Engineering
• Masters of Technology in Civil Engineering

Practice Problems

1. Which of the following is a renewable energy source?

1. Hydroelectric energy
2. Energy from coal
3. Energy from petroleum
4. Energy from burning fossil fuels

Answer: Option a

Explanation: Hydroelectric energy is the energy obtained from water resources, which are considered to be a clear source of energy.

2. Which of the following is a reaction turbine?

1. Cross-flow turbine
2. Pelton turbine
3. Kaplan turbine
4. None of these

Answer: Option c

Explanation: A Kaplan turbine is an example of a reaction turbine.

3. Which of the following component converts mechanical energy into electric energy in hydroelectric power stations?

1. Dam
2. Hydraulic turbine
3. Generator
4. Penstock

Answer: Option c

Explanation: A generator is an energy conversion device that converts the mechanical energy from the hydraulic turbine into electrical energy.

4. Through which of the following component the flow of water from a dam can be controlled?

1. Gates
2. Penstock
3. Hydraulic motors
4. None of these

Answer: Option a

Explanation: Gates are provided to restrict the flow of water in a dam. These gates can be opened and closed or their precise control allowing the flow of water into the hydraulic turbines.

5. Which of the following is the unit of the hydraulic head?

1. Meters, m
2. Kilometers, km
3. Pascals, Pa
4. None of these

Answer: Option a

Explanation: The unit of the hydraulic head is "Meters, m".