Geothermal energy means energy stored as heat in the earth. You know that below the earth's crust lies a layer called the mantle. The temperature near the upper part of the mantle is around 1500°C. The material here is in a partially molten state and is called magma.
In some areas, hot magma swells up into the crust, but remains trapped below the surface of the earth. Such areas in the earth's crust are called hot spots. The rocks and groundwater above these hot spots get heated. At some places, the hot water comes to the surface and collects in pools called hot springs. In some cases, the water gets converted to steam. Steam or steam mixed with hot water pushes out of the surface of the earth with great force. The fountain of steam and water coming out from the surface of the earth is called a geyser. Steam as well as hot water from geothermal sources can be utilized by us. Some common uses include heating of buildings and generation of electricity.
Generating Electricity from Geothermal Energy
At places where dry steam comes out of the surface of the earth, it can be used directly to turn turbines connected to generators. The world's largest geothermal electric power plant, at Geysers Steam Field, California, uses this method to generate electricity. At places where steam does not come out on its own, arrangements are made to convert the hot underground water into steam. The steam is then used to turn turbines.
Advantages of geothermal energy
(a) Geothermal plants can operate round the clock, unlike those based on solar and tidal energy.
(b) Geothermal energy is almost pollution free.
(c) It is cheaper to run a geothermal plant than a coal-based plant.
(d) The source of energy is free and renewable.
The word 'fission' means to split or break up into parts. Nuclear fission is a reaction in which a heavy nucleus splits into two middleweight nuclei, releasing a lot of energy.
The process of a heavy nucleus splitting into two middleweight nuclei is called nuclear fission.
One of the isotopes of uranium is U-235 (235 is its mass number, i.e., the sum of the number of neutrons and protons). The nucleus of U-235 can be used to start a nuclear fission reaction by bombarding it with slow-moving neutrons. In this process, U-235 itself does not undergo fission. It converts to U-236, which is highly unstable.
As soon as it forms, U-236 splits in two parts, i.e., it fissions. Some neutrons, generally 2 or 3, are also emitted in this process.
The nuclei of the elements formed by fission are unstable. They disintegrate further to form stable nuclei. Gamma ray (electromagnetic radiation) and particles such as neutrons and electrons are emitted by the nuclei at different stages between the fission and the creation of stable nuclei.
In the fission of uranium, the combined mass of the fission products is less than the combined mass of the neutron and the U-235 nucleus, with which the reaction started. This loss in mass gets converted into energy. .
Energy released in fission reactions Einstein's famous equation, E = mc2. In this equation, E, m and c stand for energy, mass and the speed of light in vacuum respectively. From this equation we can get the amount of energy released when matter of a certain mass is destroyed. Taking c = 3 × 108 m/ s in vacuum, 1 kg of disappearing mass will give rise to 9 × 1016 J of energy.
When dealing with atoms and nuclei, scientists prefer to measure mass in a unit called atomic mass unit (u), which is defined as 1/12 of the mass of one atom of carbon-12. And they often use the unit electronvolt (eV) to measure energy.
1u → 1.67 × 10-27 kg.
1eV → 1.6 × 10-29 J.
It turns out that 1 u of mass, when converted into energy, releases about 931 MeV of energy.
How much energy do we get from the fission of one uranium nucleus? From the fission of one U-236 nucleus, we get about 200 × 106 eV (200 MeV) of energy. To get an idea of the amount of energy liberated by fission, let us do a comparison with energy released by burning coal. When 1 g of coal is burnt completely, 30 kJ of energy is produced. A fission reaction that consumes 1 g of U-235 produces 8.3 × 107 kJ of energy!
The energy released in nuclear fission can be used to generate electricity as well as to make atom bombs.
Suppose a slow neutron is absorbed by a nucleus of U-235 in a block of uranium. The resulting U-236 nucleus fissions, and in the process 2 or 3 neutrons are emitted. These neutrons can be absorbed by other U-235 nuclei, starting other fission reactions, from which neutrons are emitted. In this way, the fission reaction continues by itself, without requiring any further external neutrons. One fission triggers another fission, in a self-sustaining sequence of fission reactions.
A reaction that continues on its own as one occurrence of the reaction triggers the next occurrence is called a chain reaction.
A chain reaction in 1 kg of U-235 will cause the fission of all its nuclei in less than one minute.
This will release a tremendous amount of energy in a very short time, leading to an explosion. Fortunately, the rate of a chain reaction can be controlled with materials that absorb neutrons. This is done when fission is used in nuclear power plants.
Generating Electricity at Nuclear Power Plants
To generate electricity, nuclear fission is carried out in a setup called a nuclear reactor. The energy released is used to generate steam, which drives turbines connected to generators. The whole system, including the nuclear reactor, the turbine, etc., is called a nuclear power plant.
Advantages and Disadvantages of Nuclear Power Advantages
(a) Nuclear power plants consume very little fuel.
(b) If operated properly, nuclear power plants produce less atmospheric pollution than thermal power plants.
(c) A sizeable amount of fuel can be reclaimed by processing the spent fuel material. In contrast, fuels like coal cannot be reclaimed once they have been used.
(d) Some radioactive isotopes are produced as by-products in the process, and these are used in medicine and industry.
(e) Nuclear power is a viable option where fossil fuels like coal are not available, or where it is not possible to generate electricity from wind, water, etc.
(a) A lot of radioactive and toxic wastes are produced in the different stages of energy production from nuclear fission. They cannot be simply thrown away. So they are stored in long-term underground storage facilities, which are expensive to build.
(b) In case of accidental leakage of nuclear rediation. Nuclear radiation may affect those near the site. Radioactive materials that may leak out can contaminate vast areas of land, crops, water bodies, etc.
(c) Nuclear power plants cannot be located near populated areas.
(d) Nuclear power plants are expensive to build.
(e) Nuclear power plants also pose security problems, as the fuel and by-products can be used to build nuclear weapons.
You have seen that when a heavy nucleus breaks into two middleweight nuclei, a lot of energy is liberated. Energy is also liberated when two light nuclei combine to form a single nucleus. This process is called nuclear fusion.
The process of two or more light nuclei combining to form a heavier nucleus is called nuclear fusion.
An example of nuclear fusion is given below.
Deuteron is the nucleus of deuterium, an isotope of hydrogen. Two deuterons combine to form the nucleus of helium-3, an isotope of helium. In this reaction, the mass of the product nucleus is less than the combined mass of the starting nuclei. The difference of mass is converted to energy, given by E = mc2. (Einstein equation)
The energy of the sun comes from nuclear fusion in which, in a series of reactions, hydrogen gets converted into helium. This is accompanied by the release of a huge amount of energy.
We have plenty of hydrogen on the earth. Is it possible to use it to get energy from fusion?
Scientists are working hard to produce usable energy from fusion reactions. However, the high temperature (about 107 K) required for fusion causes problems. All materials vapourize at that temperature. So, we cannot even have a solid container to hold the particles for fusion.
Fusion has been used to make nuclear bombs that are more powerful than those based on fission. These are called hydrogen bombs or thermonuclear bombs. To start fusion, a small fission bomb is used as the first-stage of the thermonuclear bomb. Its blast creates the high temperature and pressure required for fusion.
USING ENERGY JUDICIOUSLY
The total coal reserves in our country are estimated to be about 80 billion tonnes, but we are consuming it at a rate of 250 million tonnes per year. At this rate the resource that nature produced over millions of years will be consumed in just a few hundred years! The situation is more alarming in the case of oil (petroleum) reserves. The known oil reserves of our country are about 500 million tonnes only, of which we are consuming 30 million tonnes per year. So, the oil reserves will last for a few decades only.
The demand for energy is increasing every day. The increasing demand cannot be met for a long time unless new resources are harnessed, since conventional, nonrenewable energy sources are depleting very fast. This situation is called energy crisis. Increased use of fossil fuels is also causing environmental problems such as air pollution which leads to health problems, acid rain and global warming.
To overcome the energy crisis and to save the environment, we have to use energy judiciously. This means using less of nonrenewable and more of renewable energy sources, not wasting energy and saving energy wherever possible.
We can stop the wastage of energy, for example, by switching off lights, fans, coolers, etc., when they are not in use. In large cities, sharing a car with other people going to work in the same area or using public transport can save a lot of fuel. Many such commonsense things can be done to save energy. Think about this: making sure that taps do not leak saves the energy used in pumping extra water.