5. Towards Green Energy

Fuel: Coal is used as fuel in a thermal power plant.

Problems:

1. Air pollution: Burning coal releases carbon dioxide, sulphur oxides and nitrogen oxides, which are harmful to health. Carbon dioxide adds to global warming, and nitrogen oxides cause acid rain.
2. Soot particles: Soot released into the air causes respiratory problems like asthma.
3. Limited reserves: Coal reserves are limited. In future, coal will not be easily available.

Nuclear power plants, natural gas power plants and solar thermal power plants also use thermal energy.

1. Nuclear power plant: by nuclear fission of uranium-235 or plutonium in a reactor. The heat turns water into high-pressure steam.
2. Natural gas power plant: by burning natural gas with compressed air in a combustion chamber. The hot, high-pressure gases run the turbine.
3. Solar thermal power plant: by concentrating sunlight with reflectors onto absorbers. The heat produces steam.

Maximum steps: thermal (coal), nuclear and solar thermal power plants. Each has four steps of conversion. For example, in a nuclear plant: nuclear energy → thermal energy → kinetic energy of steam → kinetic energy of turbine → electrical energy.

Minimum steps: solar photovoltaic cells. They convert sunlight directly into electrical energy in a single step (photovoltaic effect), without a turbine or generator.

Green energy: Energy produced from sources that do not harm the environment is called green energy. Its generation causes no pollution, and its sources are never-ending.

Green energy sources: water reservoirs (hydroelectricity), wind, sunlight and biofuels.

Why:

• No fuel is burnt, so no harmful gases or soot are released.
• These sources are renewed by nature (rain, wind, sun), so they never get used up.
• They do not produce harmful nuclear waste.

Examples: solar photovoltaic power stations, wind farms, solar water heaters, and hydroelectric plants like Koyna.

• Burning fossil fuels like coal, petroleum and natural gas releases CO₂, CO, sulphur and nitrogen oxides and soot.
• CO₂ causes global warming, nitrogen oxides cause acid rain, and soot causes respiratory diseases like asthma.
• Fossil fuels took millions of years to form, and their reserves are limited. They cannot be renewed.

So energy from fossil fuels is not environment friendly, that is, it is not green energy.

• Our need for energy is increasing every day.
• Most of our electricity comes from coal, whose reserves are limited. Coal is expected to last only about 200 more years.
• Generating electricity from fossil fuels pollutes the environment.
• Energy saved is energy produced. Saving energy reduces fuel use and pollution, and keeps resources for future generations.

So we must switch off lights and fans when not needed and use energy only in the required amount.

• A single solar cell gives only about 0.5 V and a small current.
• When cells are connected in series, their potential differences add up, but the current stays the same.
• When cells are connected in parallel, their currents add up, but the potential difference stays the same.
• Many cells connected in series and parallel form a solar panel. For example, 36 cells of 100 cm² each in series give 18 V and 3 A.
• Panels connected in series form a string. Strings connected in parallel form a solar array.

In this way, any required potential difference and current, from a calculator to a power station of MW capacity, can be obtained.

Advantages:

• Solar energy is never-ending and free.
• No fuel is burnt, so there is no air pollution. It is green energy.
• Solar cells can give power from very small amounts (calculators) to large power stations.
• Solar cells can be used in remote places where there are no power lines.

Limitations:

• Electricity is generated only in the daytime, and less on cloudy days.
• Batteries are needed to store energy for use at night.
• Solar cells give DC; an inverter is needed to convert it into AC.
• Efficiency is low (about 15%), so large areas of panels are needed.

Coal burns in the boiler and its chemical energy becomes thermal energy. Water turns into high-pressure steam. The steam rotates the turbine, and the turbine drives the generator, which produces electricity.

Fission of uranium-235 in the reactor releases nuclear energy as heat. This heat produces high-pressure steam, which rotates the turbine. The turbine drives the generator.

Reflectors concentrate sunlight on absorbers, where solar energy becomes heat. The heat turns water into steam, which rotates the turbine. The turbine drives the generator.

Water stored in a dam has potential energy. When it flows down through the penstock, this becomes kinetic energy. The flowing water rotates the turbine at the bottom of the dam, and the turbine drives the generator.

• In every power plant, a turbine is rotated to drive the generator.
• But the energy source that rotates it is different: high-pressure steam (thermal, nuclear, solar thermal plants), very hot gases (natural gas plants), flowing water (hydroelectric plants) or wind (wind plants).
• Their pressure, temperature, speed and force are different.

So the turbine blades and design are made to suit each energy source.

• Fission of one uranium-235 nucleus releases three neutrons and about 200 MeV of energy.
• These neutrons cause fission of more nuclei, and the chain reaction grows very fast.
• If it is not controlled, a huge amount of energy is released at once. This can cause an explosion and the release of harmful radiation.

So the chain reaction must be controlled, so that heat is released steadily and safely for generating electricity.

• Water in dams is refilled by rain every year.
• Sunlight and wind are available continuously from nature.
• These sources are never-ending (perpetual) and do not get used up.

So they are called renewable energies.

• A silicon solar cell of 1 cm² gives about 0.5 V and 30 mA, that is, about 15 mW of power. This is enough for devices like calculators.
• Many cells connected in series and parallel form panels. Panels form strings, and strings form arrays.
• By connecting as many cells and panels as needed, power up to a few MW can be produced in solar power stations.

Reflectors concentrate sunlight on the absorber tube. Water in the tube turns into steam, which rotates the turbine and generator. The steam is cooled back into water in the condenser, and a pump sends it back to the absorber.

Points in favour:

• No fuel is burnt, so there is no air pollution.
• The water is refilled by rain every year, so it is a renewable source.
• With enough stored water, electricity can be generated whenever needed.

Points against:

• The backwater of the dam submerges villages, forests and fertile land.
• Large numbers of people are forced to migrate, which creates rehabilitation problems.
• Blocking the river’s flow harms fish and other living things in the river.

My opinion: Hydroelectric plants are much more environment friendly than coal or nuclear plants, because they cause no air pollution. But large dams do harm forests, rivers and people. So they are only partly environment friendly. Small dams, careful planning and proper rehabilitation of affected people can make them better.

• For 72 V: in series, potential differences add. Panels needed in series = 72 ÷ 18 = 4. This string gives 72 V and 3 A.
• For 9 A: in parallel, currents add. Strings needed in parallel = 9 ÷ 3 = 3.
• So connect 3 strings of 4 panels each in parallel. Total panels = 4 × 3 = 12.

• Generating electricity from fossil fuels (coal, natural gas) and nuclear fuels (uranium, plutonium) is not environment friendly.
• Air pollution: Burning fossil fuels releases CO₂, which causes global warming. Incomplete burning forms carbon monoxide, which harms health. Nitrogen dioxide causes acid rain, and soot causes respiratory problems like asthma.
• Limited reserves: Fossil fuels took millions of years to form. Coal may last about 200 more years and natural gas about 200–300 years.
• Nuclear energy: Safe disposal of radioactive nuclear waste is difficult, and accidents can release very harmful radiation.
• Solution: Electricity from water reservoirs, wind, sunlight and biofuels does not cause these problems. So the world is now moving towards green energy.

• Solar light: A solar panel charges a battery during the day. At night the battery lights an LED lamp. It is used for street lights and garden lights, and in villages without electricity.
• Solar water heating system: Water flows through black-painted pipes in a collector covered with glass, facing the sun. The pipes absorb heat and the hot water rises into an insulated tank on top. It is used in homes and hostels.
• Solar cooker: A box cooker has an insulated box painted black inside, a glass lid and a plane mirror reflector. Sunlight is trapped inside and food is cooked slowly. It saves fuel and causes no smoke.

Points for your report:

1. Name and location of the power plant.
2. Type of plant and energy source used.
3. Capacity in MW.
4. Main parts: boiler / dam / blades / panels, turbine, generator, transformer.
5. Step-by-step energy conversion (flow chart).
6. Effects on the environment and steps taken to reduce them.
7. Safety measures followed in the plant.