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.
In nuclear science, energy is expressed in units of electron volts (eV). 1 eV = 1.602 × 10–19 joules. One atomic mass unit (u) is equivalent to 931 mega electron volts (MeV) of energy.
Nuclear power reactors at Tarapur (Maharashtra), Rana Pratap Sagar (Rajasthan), Kalpakkam (Tamil Nadu), Narora (UP), Kakrapar (Gujarat), and Kaiga (Karnataka) have a capacity of less than 3% of India’s total electricity generation capacity. In many industrialized countries, it is above 30%.
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.
Nuclear fusion:
- The joining of lighter nuclei into a heavier nucleus, e.g., hydrogen or hydrogen isotopes creating helium: 2H + 2H → 3He (+ n).
- It releases tremendous energy, as the mass of the product is less than the sum of the masses of the original nuclei, per Einstein’s equation.
- It is the source of energy in the Sun and other stars, requiring considerable energy, extreme temperature, and pressure.
- The hydrogen bomb is based on a thermonuclear fusion reaction. A nuclear bomb based on the fission of uranium or plutonium is placed at the core, embedded in a substance containing deuterium and lithium. When detonated, the temperature rises to 107 K in microseconds, generating energy for nuclear fusion and releasing a devastating amount of energy.
Ultimate sources of energy:
- The ultimate source of energy for biomass, wind, and ocean thermal energy is the Sun.
- In geothermal energy and nuclear energy, the ultimate source is nuclear material.
- The ultimate source of energy for hydroelectricity is the potential energy of water. For wave energy, it is the Sun, as waves are formed due to wind, and wind is formed due to unequal heating of air by the Sun.