Towards Green Energy Class 10 Science Notes Maharashtra State Board
Energy and Use of Energy
In modern civilization, energy has become a primary need along with food, cloth, and shelter. We need energy in different forms for diverse types of work. The energy that we need may be in the form of mechanical energy, chemical energy, sound energy, light energy, or heat energy. How do we get these different forms of energy?
We know that energy can be converted from one form to another. Different sources of energy are used for the different forms of energy necessary for us. In previous standards, we have learned about energy, sources of energy, and various concepts related to them. Here we will learn about various sources that are now used for the generation of electrical energy, the methods that are used for this, the scientific principles that are used there, the advantages and disadvantages of these methods, and also what is meant by green energy.
Generation of Electrical Energy
Most of the electric power plants are based on the principle of electromagnetic induction invented by Michael Faraday. According to this principle, whenever the magnetic field around a conductor changes, a potential difference is generated across the conductor. The field around a conductor can be changed in two ways.
If a conductor is stationary and the magnet is rotating, the field around the conductor changes or if a magnet is stationary, but the conductor is moving then also the field around the conductor will change. Thus, in both these cases, a potential difference is created across the conductor. The electrical power-generating machine based on this principle is called an electric generator.
Such large generators are used in commercial power generation plants. A turbine is used to rotate the magnet in the generator. A turbine has blades. When a flow of liquid or gas is directed on the blades of the turbine, it rotates. because of the kinetic energy of the flow. This turbine is connected to an electric generator. Thus the magnet in the electric generator starts rotating and electric energy is produced. This method of electric energy generation can be represented as below.
Thus, to generate electricity based on the principle of electromagnetic induction, we need a generator. To rotate the generator we need a turbine and to drive the turbine, we need an energy source. Based on which type of energy source is used to rotate the turbine, there are different types of power generating stations. The design of the turbine used in different types of power stations is also different.
Thermal Energy-Based Electric Power Station
In this, the turbine is rotated using steam. Water is heated in a boiler. Using the thermal energy released due to the burning of coal. Steam of very high temperature and pressure is generated. The energy in the steam drives the turbine. Thus, the generator connected to the turbine rotates and electrical energy is produced. The steam is converted back into water and re-circulated to the boiler. This is shown in the flow chart in the figure.
Since thermal energy is used here to generate electrical energy, such power plants are called thermal power plants. In thermal power plants, the chemical energy in the coal is converted into electrical energy through several steps which are shown in the figure.
If you see a thermal power station, you will observe two types of towers there. What are they? If you observe the schematic of the thermal power station in Figure, you will get an answer to this question. Compare the schematic of the thermal power station with the block diagram above and you will understand how the boiler, turbine, generator, and condenser are arranged in the power station.
After the combustion of fuel (here, coal) in the boiler, the emitted gases are released into the atmosphere through a very high tower. Once the turbine is rotated using the steam at high temperature and high pressure, steam temperature and pressure decrease. This steam is converted back to water by taking out heat from it (i.e. by cooling it). This is done in the condenser using water in the cooling tower. The water in the cooling tower is circulated through the condenser. Heat energy in the steam is given to the water and the steam condenses back to water. The heat absorbed by the water is then released into the atmosphere through vapour and heated air through the cooling tower. Although thermal power generation is a major way of electricity generation today, it suffers from certain problems.
Problems
- Air pollution due to the burning of coal: Burning of coal results in the emission of gases like carbon dioxide, sulphur oxide, and nitrogen oxide which are harmful to health.
- Along with the emission of gases due to the burning of coal, soot particles are also released into the environment. This may cause serious health problems related to the respiratory system.
- The reserves of fuel used in this method i.e. coal are limited. Therefore, in the future, there will be limitations on the availability of coal.
Power Plant Based on Nuclear Energy
In a power plant based on nuclear energy also, a steam turbine is used to rotate the generator. However, here, the energy released by the fission of nuclei of atoms like Uranium or Plutonium is used to generate the steam of high temperature and high pressure. The energy in the steam rotates the turbine, which in turn drives the generator producing electricity. The flow chart of the nuclear power plant is shown in the figure.
Thus, here nuclear energy is converted into thermal energy, thermal energy is converted into the kinetic energy of steam, the kinetic energy of steam is converted into the kinetic energy of the turbine, and finally, the kinetic energy of the turbine is converted into electrical energy. The step-by-step transformation of energy is shown in the figure.
How does nuclear fission take place?
When a neutron is bombarded on an atom of Uranium – 235, it absorbs the neutron and converts it into its isotope Uranium – 236. Uranium – 236 being extremely unstable converts into atoms of Barium and Krypton through a process of fission releasing three neutrons and 200 MeV energy. The three neutrons generated in this process cause the fission of three other Uranium – 235 atoms releasing more energy.
The neutrons released in this reaction release more energy through the fission of more uranium nuclei. This process of fission of Uranium -235 atoms continues and is called the chain reaction. In nuclear power plants, a controlled chain reaction results in the release of thermal energy, which is used for electric energy generation.
A nuclear power plant does not use fossil fuels like coal. Therefore, problems like air pollution do not arise. Also, if sufficient nuclear fuel is available, this can be a good source of electrical energy. However, there are a few problems associated with nuclear power generation.
Problems:
- The products after the fission of nuclear fuel are also radioactive and emit harmful radiation. The products are called nuclear waste. How to dispose of the nuclear waste safely is a big challenge for the scientists.
- An accident in a nuclear power plant can be very fatal. This is because the accident may result in the release of very harmful radiation.
Power Generation Plant Based on Energy of Natural Gas
In this plant, the turbine is run by a gas at a very high temperature and pressure generated by the combustion of natural gas. A flow chart showing various stages in the power generation plant based on natural gas energy is shown in the figure.
There are three main sections in this type of plant. Pressurized air is introduced into the combustion chamber using a compressor. In the combustion chamber the natural gas burns in the presence of the air. The gas at a very high temperature and pressure generated in this chamber runs the turbine. The turbine then drives the generator to produce electricity. The step-by-step transformation of energy in this plant is shown in the figure.
The efficiency of this type of power generation plant is higher than that of a power generation plant based on coal. Moreover, since natural gas does not contain sulfur, burning natural gas results in less pollution. The schematic of the power plant based on natural gas is given in the figure.
Towards Environment-Friendly Energy…..Towards Green Energy:
There are other ways of electricity production which avoid the above problems. Electricity generation from water reservoirs, wind, Sunlight, biofuels, etc are examples of such methods. The energy sources used in such options i.e. water reservoir, wind, sunlight, biofuel are never-ending i.e. perpetual. Moreover, the use of these sources does not lead to the environmental problems discussed above. Therefore, electricity generation through these sources can be called environment-friendly. We can also call the energy generated by these processes as green energy. Looking at the problems in electricity generation using fuels like coal, natural gas, and nuclear fuels, the world is now heading towards environment-friendly energy i.e. green energy.
Electric Energy Generation and Environment
Electricity generation based on fossil fuels like coal, and natural gas, and nuclear fuels like uranium and plutonium are not environment friendly. It means, that if electrical energy is generated using these fuels, it can lead to environmental degradation.
- We have seen that the burning of fossil fuels like coal, and natural gas leads to the emission of certain gases and soot particles. This results in air pollution. The incomplete combustion of fuels leads to the formation of carbon monoxide. It adversely affects our health. An increase in the percentage of carbon dioxide in the air due to the burning of fuels affects the environment severely. The phenomenon of global warming is an example of this. Nitrogen dioxide generated due to the burning of fuels like coal, diesel, petrol, etc. leads to problems like acid rain. Soot particles generated due to incomplete burning of fossil fuel cause air pollution. It can lead to problems related to the respiratory system, like asthma.
- It took millions of years for the formation of fossil fuels like coal, crude oils, and natural gases (LPG and CNG). Also, the reserves of these fuels are limited. They are going to deplete in the future. It is said that with the current speed of their use, the coal reserves in the world would last for another about 200 years or so, and the natural gas reserves for about 200-300 years.
- We have also discussed above the problems in the use of nuclear energy like the disposal of nuclear waste and the possibility of disaster due to accidents in nuclear power plants.
Considering all these points, it can be said that the energy generation from fossil fuels and nuclear fuels is not environmentally friendly.
Hydroelectric Energy
The kinetic energy in flowing water or the potential energy in a water reservoir is a conventional source of energy. In a hydroelectric power plant, the potential energy in water stored in the dam is converted into kinetic energy of water. Fast flowing is brought from the dam to the turbine at the bottom of the dam. The kinetic energy of the flowing water drives the turbine. The turbine in turn drives the generator to generate electricity. The block diagram showing different components of the hydroelectric power plant is shown in the figure.
Electricity Generation Using Wind-Energy
The schematic of the hydroelectric plant is shown in Figure. Water from about the middle of the total height of the dam is taken to the turbine, as shown by point B in the diagram.
Since no fuel is burnt in the hydroelectric plant, no air pollution due to combustion of fuel results. However, considering issues like forced migration of large communities, submerging of forests and fertile land, and adverse effects on living creatures in the river, it has always been a point of debate whether hydroelectricity is environment-friendly or not.
Advantages of Hydroelectric Power Generation:
- Since no fuel is burnt in hydroelectric power generation, there is no pollution resulting from the combustion of fuels.
- If there is sufficient water storage in the dam, it is possible to generate electricity as and when necessary.
- Although a water reservoir is used for power generation, it can be replenished during the rainy season leading to uninterrupted power generation.
Problems Associated with Hydroelectric Power Plant:
- The backwater due to the storage of water in a dam may submerge villages or towns in that area. This leads to the problems of re-habitation of the displaced population. Moreover, this can also submerge forests as well as fertile land.
- The obstruction of the flow of river water may hurt the living world in the river.
The kinetic energy in wind has been used for a long for the lifting of water, for driving floor mills, etc. The wind energy can also be used for electricity generation. The machine which converts the kinetic energy of wind to electrical energy is called a wind turbine. As the wind strikes the blades of the turbine, the blades rotate. The axle of the turbine is connected to an electric generator through a gearbox. The function of the gearbox is to increase the rotations per unit time. Thus, the rotating blades drive the turbine and the turbine in turn drives the generator to generate electricity. Various stages in the wind-energy generation system can be shown in the figure and schematics of a windmill are shown in the figure.
The energy conversion process is shown in the figure. Wind turbines with capacities right from less than 1 kW to about 7 MW (7000 kW) are commercially available. Depending on the wind velocity available at the site of installation, a wind turbine with a specific capacity is selected. The wind velocity at a specific location depends on many geographical factors.
Wind velocity is usually high on sea shores and that environment is appropriate for the installation of wind turbines. Wind energy is a clean energy source. However, the wind velocity necessary for wind energy generation is not available everywhere. In that sense, the use of wind energy is limited.
Electric Energy Generation Using Solar Energy
Using the energy in the Sunlight, electric energy can be generated in two ways:
- In all the above methods of electricity generation we have studied, the electric generator is driven by using some source of energy, and electricity is generated by making use of the principle of electromagnetic induction. However, electrical energy can be generated directly from solar radiation without using a generator and without using the principle of electromagnetic induction. This happens in solar photovoltaic cells. Solar photovoltaic cells convert the solar energy directly into electrical energy.
- In the second method, the energy in solar radiation is converted into thermal energy first. Then a turbine-generator system is driven using that thermal energy to generate electricity.
Solar Photovoltaic Cell
A solar photovoltaic cell converts solar radiation energy directly into electrical energy. This is called the solar photovoltaic effect. The electrical energy generated through this energy transformation process is DC in nature. These solar cells are made of a special type of material called semiconductors (e.g. silicon). A silicon solar cell of dimension 1 cm2 generates a current of about 30 mA and a potential difference of about 0.5 V. Thus, a silicon solar cell of dimension 100 cm2 will generate about 3 A (30 mA/cm2 × 100 cm2 = 3000 mA = 3A) current and 0.5 V. Remember that the potential difference available from a solar cell is independent of its area.
If two solar cells are connected in series as shown in Figure, the potential difference obtained from this combination is the addition of the potential differences of individual solar cells. However, the current generated from this combination is equal to the current from an individual cell. It means that when solar cells are connected in series, currents from the individual cells are not added.
Similarly as shown in the figure, if two solar cells are connected in parallel, the current generated from this combination is the summation of the currents from individual solar cells. However, the potential difference obtained from this combination is the same as the potential difference obtained from individual cells. Thus, if two solar cells are connected in parallel, the potential differences between the two cells are not added.
In this way, by connecting many solar cells in series and parallel solar panels generate the required current, and potential differences are made. See Figure. For example, if 36 solar cells, each of size 100 cm2 are connected in series in a solar panel, it will give a potential difference of 18 V and a current of 3 A. Many such panels are connected to generate electricity on a larger scale. A good solar cell can have an efficiency of around 15%. It means that if a solar panel receives power of 100 watts from solar radiation, the electrical power output from the panel will be 15 watts.
Many solar panels are connected in series and in parallel to generate a required current and potential difference. As shown in Figure, the solar cell is the basic unit in the solar electric plant. Many solar cells come together to form a solar panel. Many solar panels connected in series form a solar string, and, many solar strings connected in parallel form a solar array. As we can obtain as much electrical power as needed, they are used in applications that need marginal power (e.g. calculators that run on solar energy) to power stations of MW capacity.
The power available from the solar cells is DC. So, in applications that need DC power, e.g. electric lights based on Light Emitting Diodes, the energy can be directly used. However, since the energy from the solar cells is available only in the presence of sunlight, the energy has to be stored in batteries for use at a later time. However, most of the equipment for domestic as well as industrial use runs on AC power. In such cases, the DC solar power must be converted to AC power using an electronic device called an inverter.
We have seen that many solar panels can be connected to generate whatever energy we need. As shown in Figure, the DC power generated from these panels is first converted into AC power. A transformer transforms the voltage and current levels of the generated power and then it is fed into the electricity distribution network. Figure is a schematic diagram of the solar photovoltaic power station. In this way, electricity is generated without any fuel combustion and so without any air pollution. However, since the energy is generated using solar radiation, solar cells can generate electricity during the daytime only.
Solar Thermal Power Plant
We have seen that thermal energy generated from coal and nuclear fuel can be used to generate electricity. Thermal energy can also be generated from solar radiation and can be used for electricity production. Different stages in such solar thermal power plants are shown in the figure.
As shown in Figure, many reflectors reflect and concentrate solar radiation on absorbers. Their solar energy is converted into heat energy. Using this heat energy steam is generated to drive the turbine and generator.
Maharashtra State Board Class 10 Science Notes Towards Green Energy can be used for revisiting and reinforcing previously learned content.