Lesson 5: Towards Green Energy

Rule 1: How electricity is generated — Most power plants work on electromagnetic induction (Michael Faraday): when the magnetic field around a conductor changes, a potential difference is produced across it. The machine based on this is the electric generator. The field changes if the magnet rotates near a stationary conductor, or the conductor moves near a stationary magnet. A turbine (with blades) is rotated by a flow of liquid or gas and rotates the magnet of the generator. Sequence: energy source → turbine → generator → electrical energy. Turbine design differs from plant to plant.

In the figure : In the figure the common steps in every power plant.

• Easy : Easy: a bicycle dynamo lights a lamp because the magnet turns near a coil; this is electromagnetic induction.
• Medium : Medium: in a thermal plant, steam turns the turbine; in a hydro plant, falling water turns it. The generator part is the same.
• Exam-type : Exam: write the chain: energy source → turbine → generator → electrical energy.

Rule 2: Sources of electricity — World: coal 41%, natural gas 22%, hydroelectric 16%, nuclear 11%, petroleum 4%, renewable 6%. India: coal 60%, natural gas 8%, hydroelectric 14%, nuclear 2%, petroleum 0.3%, renewable 15.7%.

In the figure : In the figure electricity sources in the world and India.

• Easy : Easy: India makes 60 units of electricity out of every 100 from coal.
• Medium : Medium: in India, hydro + renewable = 14 + 15.7 = 29.7%, nearly a third of electricity.
• Exam-type : Exam: compare world and India – India depends more on coal (60% against 41%).

Spot the difference

Rule 1: Thermal power plant — Coal burns in a boiler and makes high-pressure steam that rotates the turbine. The steam is cooled in the condenser (water from the cooling tower) and returned to the boiler. Energy: chemical energy of coal → thermal energy → kinetic energy of steam → kinetic energy of turbine → electrical energy. Problems: gases (carbon dioxide, sulphur oxide, nitrogen oxide) and soot cause air pollution and respiratory problems; coal reserves are limited.

In the figure : In the figure steps in a coal power plant.

• Easy : Easy: in the boiler, water turns to steam; in the condenser, steam turns back to water.
• Medium : Medium: the white cloud from the cooling tower is water vapour, not smoke.
• Exam-type : Exam: write the energy changes in a thermal plant: chemical → thermal → kinetic → electrical.

Rule 2: Nuclear power plant — Heat from the fission (विखंडन) of Uranium or Plutonium nuclei makes the steam. A neutron striking Uranium-235 forms unstable Uranium-236, which splits into Barium and Krypton, releasing three neutrons and 200 MeV of energy. These neutrons cause more fissions: a chain reaction, which is controlled in the plant. There is no air pollution from fuel burning, but nuclear waste is radioactive and hard to dispose of, and an accident can release harmful radiation.

In the figure : In the figure one fission frees three more neutrons: a chain reaction.

• Easy : Easy: one fission frees 3 neutrons; if each causes another fission, the next step has 3 fissions, then 9, then 27.
• Medium : Medium: after 4 steps there would be 3 × 3 × 3 × 3 = 81 fissions, which is why the reaction has to be controlled.
• Exam-type : Exam: two disadvantages of nuclear power – radioactive waste and risk of accident.

Spot the difference

Rule 1: Natural gas power plant — A compressor sends pressurised air into the combustion chamber, where the gas burns; the hot high-pressure gas runs the gas turbine, which drives the generator. Its efficiency is higher than that of a coal plant and it pollutes less (no sulphur).

• Easy : Easy: a gas plant is cleaner than a coal plant since natural gas has no sulphur.
• Medium : Medium: in a gas plant, hot gas itself turns the turbine; there is no boiler of water.
• Exam-type : Exam: name two advantages of a gas plant over a coal plant: higher efficiency and less pollution.

Rule 2: Pollution from fossil fuels — Burning coal, natural gas and similar fuels gives carbon dioxide (global warming), carbon monoxide (incomplete combustion), nitrogen dioxide (acid rain) and soot (asthma). Fossil fuels took millions of years to form; coal may last about 200 years and natural gas about 200–300 years. Water-reservoir, wind, sunlight and biofuel energy sources are perpetual and do not cause these problems: they are environment friendly, also called green energy.

In the figure : In the figure pollution from burning fossil fuels.

• Easy : Easy: CO₂ causes global warming; NO₂ causes acid rain; soot causes breathing trouble such as asthma.
• Medium : Medium: if coal lasts about 200 years and use keeps growing, it will finish sooner, so we need green energy.
• Exam-type : Exam: why is green energy called perpetual? Water, wind and sunlight are renewed by nature and never finish.

Spot the difference

Rule 1: Hydroelectric plant — Potential energy of water in the dam → kinetic energy of flowing water → kinetic energy of the turbine (at the bottom of the dam) → electrical energy. No fuel is burnt and the reservoir is refilled in the rainy season. Problems: back-water may submerge villages, forests and fertile land (rehabilitation), and river life is affected. Examples: Tehari (Uttarakhand), Koyana (Maharashtra).

In the figure : In the figure from dam water to electricity.

• Easy : Easy: the higher the water level behind the dam, the more potential energy it has to give.
• Medium : Medium: Koyana in Maharashtra and Tehari in Uttarakhand are hydroelectric projects.
• Exam-type : Exam: one advantage and one disadvantage of a dam: no fuel burnt; submerged villages and forests.

Rule 2: Wind energy — In a wind turbine, wind rotates the blades; a gear-box (गिअर-बॉक्स) increases the rotations per unit time and drives the generator. The capacity ranges from under 1 kW to about 7 MW. Wind velocity is high on sea shores. Wind energy is clean but not available everywhere.

In the figure : In the figure how a wind turbine makes electricity.

• Easy : Easy: 7 MW = 7 × 1000 = 7000 kW, enough for thousands of homes using 1 kW each.
• Medium : Medium: a turbine rated 2 MW, running at full power for 5 hours, gives 2 × 5 = 10 MWh = 10,000 kWh.
• Exam-type : Exam: why does a wind turbine need a gear-box? The blades turn slowly; the gear-box speeds up the rotation for the generator.

Spot the difference

Rule 1: The solar cell — The solar photovoltaic cell (a semiconductor, for example silicon) converts sunlight directly into DC electricity; no generator is needed. A 1 cm² silicon cell gives about 30 mA at about 0.5 V; the potential difference does not depend on area, so a 100 cm² cell gives about 3 A.

• Easy : Easy: a 200 cm² cell gives about 30 mA × 200 = 6000 mA = 6 A at 0.5 V.
• Medium : Medium: area does not change the voltage; a big cell and a small cell both give about 0.5 V.
• Exam-type : Exam: power of a 100 cm² cell = V × I = 0.5 × 3 = 1.5 W.

Rule 2: Series, parallel and storage — In series (श्रेणी), the potential differences add and the current stays the same. In parallel (समांतर), the currents add and the potential difference stays the same. 36 cells of 100 cm² in series give 18 V and 3 A. Cells → panel → string → array. Efficiency is about 15%. The output is DC; an inverter converts it to AC, and batteries store it for use at night. Cells work only in daytime. A solar thermal plant uses reflectors to concentrate sunlight on absorbers; the heat makes steam to run the turbine and generator.

In the figure : In the figure series raises voltage; parallel raises current.

• Easy : Easy: 10 cells of 0.5 V in series give 10 × 0.5 = 5 V.
• Medium : Medium: 4 cells of 3 A each in parallel give 4 × 3 = 12 A at 0.5 V.
• Exam-type : Exam: how to get 12 V at 3 A from 100 cm² cells? Series of 12 ÷ 0.5 = 24 cells.

In the figure : In the figure from sunlight to household electricity.

• Easy : Easy: the panel gives DC, the inverter changes it to AC, and the home appliances use AC.
• Medium : Medium: batteries store extra energy in the day so that lights can work at night.
• Exam-type : Exam: a panel with 15% efficiency receives 1000 W of sunlight and gives 1000 × 15 ÷ 100 = 150 W.

Spot the difference