Sources of Energy Notes - Conventional vs Non-Conventional, Solar, Nuclear and Environmental Impact

• In a physical or chemical process, total energy is conserved.
• One form of energy can be converted to another. For example:
  • When a plate hits the ground, potential energy is converted to sound energy.
  • In a burning candle, chemical energy is converted to heat energy and light energy.
• These energy forms cannot be reversed back to their previous forms.
• When hot water is kept at room temperature, the heat energy is lost to the environment. This lost energy cannot be re-collected to heat the water again.
• Dissipated usable energy cannot be used again.

WHAT IS A GOOD SOURCE OF ENERGY?

• Various sources of energy are used for doing work.
• Muscular energy is used to carry out physical work, electrical energy for running various appliances, and chemical energy for cooking food or running a vehicle, all of which come from some sources.
• Various fuels for cooking include electricity, LPG, wood, coal, solar cookers, etc.
• A good fuel is easily available, efficient, cheap, clean, and easy to use, producing no or minimal pollutants. For example, in homes, LPG and electricity are preferred as they are highly efficient, smokeless, easily available, and easy to use.
• Selection of fuels also depends on the work to be done and the amount of energy needed.

Advantages of a good source of energy:

• Does a large amount of work per unit volume or mass.
• Easily accessible.
• Easy to store and transport.
• Economical.

CONVENTIONAL (TRADITIONAL) SOURCES OF ENERGY

Fossil Fuels

• Ancient energy sources: Wood (most common source of heat energy), energy of flowing water, and wind.
• The exploitation of coal led to the industrial revolution. It increased the global demand for energy.
• The demand for energy was largely met by fossil fuels (coal and petroleum). But they are non-renewable sources because they were formed over millions of years ago and have only limited reserves. So we need to conserve them, and alternate sources must be explored.

• Disadvantages of burning fossil fuels:
  • Air pollution.
  • Release of acidic oxides of carbon, nitrogen, and sulphur. They cause acid rain, which affects water and soil.
  • Greenhouse effect of gases like CO₂.
• The pollution caused by fossil fuels can be reduced by increasing the efficiency of combustion and using various techniques to reduce the release of harmful gases and ashes.
• Fossil fuels are also used to generate electricity. This can be demonstrated by the following activity:
  • Make three slits in a table-tennis ball and put semi-circular fins cut out of a metal sheet into them.
  • Pivot the tennis ball on an axle through its center with a straight metal wire fixed to a rigid support. Ensure that the tennis ball rotates freely about the axle.
  • Connect a cycle dynamo and a bulb in series to this.
  • Direct a jet of water or steam produced in a pressure cooker at the fins. The bulb glows.

• This is the model of a turbine for generating electricity. The simplest turbines have a moving part called a rotor-blade (fan). The moving fluid spins the blades and imparts energy to the rotor. It turns the shaft of the dynamo and converts mechanical energy into electrical energy.

Thermal Power Plant

• It is the power station where fuel is burnt to produce heat energy, which is converted into electrical energy.
• Burning of fossil fuels → heats water → produces steam → runs the turbine → generates electricity.
• Many thermal power plants are set up near coal or oil fields because transmission of electricity is more efficient than transporting coal or petroleum.

Hydro Power Plants

• Here, kinetic energy of flowing water or potential energy of water at a height is used.
• Hydro power plants convert the potential energy of falling water into electricity.
• A quarter of the energy requirement in India is met by hydropower plants.
• There are only a few waterfalls as a source of potential energy. So, hydro power plants are associated with dams.
• To produce hydel electricity, dams are constructed on the river. They obstruct water flow and collect in larger reservoirs. The kinetic energy of flowing water is transformed into potential energy. Dam water is carried through pipes to the turbine at the bottom of the dam.
• Since the water in the reservoir is refilled by rainfall, hydroelectricity sources are not used up. So hydro power is a renewable source of energy.

• Problems of constructing big dams:
  • They can be constructed only in a few places, such as hilly terrains.
  • Agricultural land and human habitation will be submerged. Large ecosystems are destroyed when submerged under water in dams.
  • The submerged vegetation rots under anaerobic conditions to produce methane (a greenhouse gas).
• Opposition to the construction of Tehri Dam on the Ganga River and the Sardar Sarovar project on the Narmada River is due to such problems.

Improvements in the Technology for Using Conventional Sources of Energy

Biomass

• Biomass is plant or animal material used as fuel, e.g., firewood, cow-dung cakes, etc.
• It is a renewable source of energy.
• However, they do not produce much heat on burning and release a lot of smoke. So, technological inputs are used to improve their efficiency.
• When wood is burnt in a limited supply of oxygen, water and volatile materials present in it get removed to give charcoal. Charcoal burns without flames, is comparatively smokeless, and has higher heat generation.
• Cow-dung, crop residues, vegetable waste, sewage, etc., are decomposed in the absence of oxygen to give biogas. Here, the starting material is mainly cow-dung, so it is popularly known as gobar gas.

• The biogas plant has a dome-like structure (gas tank). A slurry of cow-dung and water is made in the mixing tank. It is fed into a digester (a sealed chamber without oxygen).
• Anaerobic microbes decompose complex compounds of the slurry and generate gases like methane, hydrogen, CO₂, and H₂S. These are the ingredients of biogas. It is stored in the gas tank and drawn through pipes.
• Biogas is an excellent fuel because:
  • It contains up to 75% methane.
  • It burns without smoke and leaves no residue (ash).
  • It has high heating capacity.
  • It is also used for lighting.
• Other advantages of biogas plant:
  • The slurry left behind is used as manure, rich in nitrogen and phosphorus.
  • Safe waste disposal of bio-waste and sewage.

Wind Energy

• Wind is the movement of air caused by unequal heating of the landmass and water bodies by solar radiation.
• The wind’s kinetic energy was harnessed by windmills to do mechanical work, e.g., in a water-lifting pump, the rotatory motion of the windmill is used to lift water from a well.
• Today, wind energy is also used to generate electricity.

• A windmill consists of a rotor with blades on a support. Its rotatory motion turns the turbine of an electric generator.
• The output of a single windmill is very small and cannot be used commercially. So, many windmills are erected over a large area. This is called a wind energy farm. The energy output of each windmill is coupled together to get electricity on a commercial scale.
• Wind energy is environment-friendly and renewable. It has no recurring expenses for the production of electricity.
• Limitations in harnessing wind energy:
  • Wind energy farms can be established only at places where wind blows for the greater part of a year.
  • Wind speed should be higher than 15 km/h to maintain the required speed of the turbine.
  • Needs backup facilities (like storage cells) to meet energy needs when there is no wind.
  • Wind energy farms need a large land area (2 hectares for a 1 MW generator). The initial cost is very high.
  • Towers and blades are exposed to rain, sun, storms, and cyclones, so they need a high level of maintenance.

ALTERNATIVE OR NON-CONVENTIONAL SOURCES OF ENERGY

Solar Energy

• The Sun has been radiating an enormous amount of energy for 5 billion years and will continue for the next 5 billion years.
• Only a small part of solar energy reaches the outer layer of the Earth’s atmosphere. Nearly half of it is absorbed in the atmosphere, and the rest reaches the Earth’s surface.
• It is estimated that India receives solar energy equivalent to more than 5,000 trillion kWh/year. Under clear sky conditions, the daily average varies from 4 to 7 kWh/m².
• Solar constant: The solar energy received per unit of time per unit of area on the outer edge of the Earth’s atmosphere, perpendicular to the Sun’s rays and at Earth’s average distance from the Sun. It is about 1.4 kJ per second/m² or 1.4 kW/m².
• A black surface absorbs more heat compared to a white or reflecting surface under identical conditions. This can be demonstrated by the following activity:
  • Take two conical flasks and paint one white and the other black. Fill both with water.
  • Place them in direct sunlight for ½–1 hour.
  • Touch the conical flasks. The black flask is hotter than the white one.
• Solar cookers and solar water heaters use this property. In some solar cookers, mirrors are used to focus the Sun’s rays to achieve high temperatures.
• Solar cookers are covered with a glass plate to trap sunlight easily and prevent heat from escaping (the greenhouse effect).

• Advantages of solar cooker:
  • Pollution-free and economical.
  • Easy to handle with no chance of accidents.
  • Nutrients in food are not destroyed.
• Limitations of solar cooker:
  • Cannot be used at night or during cloudy weather.
  • Takes more time to cook food.
  • The direction of the solar cooker must be continuously adjusted toward the Sun.
• These limitations are overcome by using solar cells that convert solar energy into electricity.
• A typical solar cell develops a voltage of 0.5–1 V and produces 0.7 W of electricity. Many solar cells are combined to deliver enough electricity for practical use, forming a solar cell panel.
• Principal advantages of solar cells:
  • No moving parts.
  • Require little maintenance.
  • Work efficiently without using any focusing device.
  • Can be set up in remote hamlets or rarely inhabited areas where laying power transmission lines is expensive.
• Silicon is used to make solar cells. It is abundant in nature, but special-grade silicon is limited. Also, silver is used to interconnect cells in the panel, making the manufacture of solar cells very expensive.
• Scientific and technological applications of solar cells:
  • In artificial satellites and space probes like Mars orbiters.
  • In radio or wireless transmission systems or TV relay stations in remote locations.
  • In traffic signals, calculators, and toys.
• Domestic use of solar cells is limited due to their high cost.

Energy from the Sea

Tidal Energy

• Due to the gravitational pull of the moon on the spinning Earth, the water level in the sea rises and falls, a phenomenon called high and low tides.
• The difference in sea levels provides tidal energy.
• Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening converts tidal energy to electricity.
• The locations where such dams can be built are limited.

Wave Energy

• Waves are generated by strong winds across the sea.
• Using devices, the kinetic energy of huge waves near the seashore is trapped. It rotates a turbine, producing electricity.

Ocean Thermal Energy

• The water at the sea surface is heated by the Sun, while deep water remains cold. This temperature difference is used to obtain energy in ocean-thermal-energy plants.
• They can operate if the temperature difference between surface water and deep water (up to 2 km) is 20°C or more.
• The warm surface water is used to boil a volatile liquid like ammonia. The vapors run the turbine of a generator. The cold deep water is pumped up to condense the vapor back to liquid.
• The energy potential from the sea (tidal energy, wave energy, and ocean thermal energy) is large, but efficient commercial exploitation is difficult.

Geothermal Energy

• Due to geological changes, molten rocks formed in the deeper regions of the Earth’s crust are pushed upward and trapped in certain regions called hot spots.
• When underground water meets the hot spot, steam is generated. Sometimes, hot water from that region comes to the surface, known as hot springs.
• Steam in rocks is piped to a turbine to generate electricity.
• The production cost is low, but commercially viable sites are very few.
• New Zealand and the USA have many geothermal power plants.

Nuclear Energy

• When the nucleus of a heavy atom (e.g., uranium, plutonium, or thorium) is bombarded with low-energy neutrons, it splits into lighter nuclei, a process called nuclear fission. This releases a tremendous amount of energy.
• The energy is released because the sum of the masses of the product nuclei is less than that of the original nucleus. The difference in mass (∆m) is converted to energy (E = ∆mc²), as derived by Albert Einstein, where c is the speed of light in a vacuum.
• The atoms that can release nuclear energy are called nuclear fuel. For example, the fission of a uranium atom produces 10 million times the energy produced by the combustion of a carbon atom from coal.
• Nuclear reactor: A device to generate energy from nuclear fuels, where a self-sustaining fission chain reaction occurs at a controlled rate. The released energy is used to produce steam and generate electricity.

Major hazards of nuclear power generation:

• Improper nuclear-waste storage and disposal cause environmental contamination. Uranium continues to decay into harmful subatomic particles (radiations).
• Accidental leakage of nuclear radiation.

High cost of installation, high risk of environmental contamination, and limited availability of uranium prohibit large-scale use of nuclear energy.

Nuclear energy was first used for destructive purposes. The fundamental physics of the fission chain reaction in a nuclear weapon (atom bomb) is similar to a controlled nuclear reactor, but they are engineered differently.

ENVIRONMENTAL CONSEQUENCES

• Exploiting sources of energy disturbs the environment.
• Selection of an energy source depends on factors such as:
  • Ease of extracting energy from the source.
  • Economics of extracting energy from the source.
  • Efficiency of the available technology.
  • Environmental damage caused by using the source.
• Some energy sources are cleaner than others, causing very little environmental damage, e.g., solar energy, CNG.
• Some devices like solar cells are pollution-free, but their assembly may cause environmental damage.

HOW LONG WILL AN ENERGY SOURCE LAST US?

• Energy sources are classified as follows:
  • Renewable source: They can be regenerated in nature, e.g., Sun, wind, moving water, biomass, etc.
  • Non-renewable source: They cannot be regenerated in nature, e.g., coal, petroleum, natural gas, etc.
  • Exhaustible sources: They will get depleted after a few hundred years, e.g., coal, petroleum, etc.
  • Inexhaustible resources: They are unlimited and will not be exhausted in the future, e.g., biomass, Sun, etc.
• Non-renewable resources are exhaustible.