SOLAR ENERGY
Alternative or non-conventional sources of energy
With technological progress, our demand for energy increases day by day. Our lifestyles are also changing, we use machines to do more and more of our tasks. Our basic requirements are also increasing as industrialisation improves our living standards.
Activity 14.4:
* Find out from your grandparents or other elders –
a) how did they go to school?
b) how did they get water for their daily needs when they were young?
a) what means of entertainment did they use?
* Compare the above answers with how you do these tasks now.
* Is there a difference? If yes, in which case more energy from external sources is consumed?
As our demand for energy increases, we need to look for more and more sources of energy. We could develop the technology to use the available or known sources of energy more efficiently and also look to new sources of energy. Any new source of energy we seek to exploit would need specific devices developed with that source in mind. We shall now look at some of the latest sources of energy that we seek to tap, and the technology designed to capture and store energy from that source.
Solar energy
The Sun has been radiating an enormous amount of energy at the present rate for nearly 5 billion years and will continue radiating at that rate for about 5 billion years more. Only a small part of solar energy reaches the outer layer of the earth’s atmosphere. Nearly half of it is absorbed while passing through the atmosphere and the rest reaches the earth’s surface.
Activity 14.5:
* Take two conical flasks and paint one white and the other black. Fill both with water.
* Place the conical flasks in direct sunlight for half an hour to one hour.
* Touch the conical flasks. Which one is hotter? You could also measure the temperature of the water in the two conical flasks with a thermometer.
* Can you think of ways in which this finding could be used in your daily life?
A black surface absorbs more heat as compared to a white or a reflecting surface under identical conditions. Solar cookers (Fig. 14.6) and solar water heaters use this property in their working. Some solar cookers achieve a higher temperature by using mirrors to focus the rays of the Sun. Solar cookers are covered with a glass plate. Recall what we have learnt about the green-house effect. Does this explain why a glass plate is used.
Figure 14.6: A solar cooker
Activity 14.6:
* Study the structure and working of a solar cooker and/or a solar water heater, particularly with regard to how it is insulated and maximum heat absorption is ensured.
* Design and build a solar cooker or water-heater using low-cost material available and check what temperatures are achieved in your system.
* Discuss what would be the advantages and limitations of using the solar cooker or water heater.
It is easy to see that these devices are useful only at certain times during the day. This limitation of using solar energy is overcome by using solar cells that convert solar energy into electricity. A typical cell develops a voltage of 0.5–1 V and can produce about 0.7 W of electricity when exposed to the Sun. A large number of solar cells are, combined in an arrangement called solar cell panel (Fig. 14.7) that can deliver enough electricity for practical use.
Figure 14.7: A solar cell panel
The principal advantages associated with solar cells are that they have no moving parts, require little maintenance and work quite satisfactorily without the use of any focussing device. Another advantage is that they can be set up in remote and inaccessible hamlets or very sparsely inhabited areas in which laying of a power transmission line may be expensive and not commercially viable.
Silicon, which is used for making solar cells, is abundant in nature but the availability of the special grade silicon for making solar cells is limited. The entire process of manufacture is still very expensive, silver used for interconnection of the cells in the panel further adds to the cost. In spite of the high cost and low efficiency, solar cells are used for many scientific and technological applications. Artificial satellites and space probes like Mars orbiters use solar cells as the main source of energy. Radio or wireless transmission systems or TV relay stations in remote locations use solar cell panels. Traffic signals, calculators and many toys are fitted with solar cells. The solar cell panels are mounted on specially designed inclined rooftops so that more solar energy is incident over it. The domestic use of solar cells is, however, limited due to its high cost.
More to know
India is lucky to receive solar energy for the greater part of the year. It is estimated that during a year India receives energy equivalent to more than 5,000 trillion kWh. Under clear (cloudless) sky conditions, the daily average varies from 4 to 7 kWh/m2. The solar energy reaching unit area at the outer edge of the earth’s atmosphere exposed perpendicularly to the rays of the Sun at the average distance between the Sun and Earth is known as the solar constant. It is estimated to be approximately 1.4 kJ per second per square metre or 1.4 kW/m2.
Think it over
Some people say that if we start living as our ancestors, this would conserve energy and our ecosystem. Do you think this idea is feasible?
Questions
1. What kind of mirror – concave, convex or plain – would be best suited for use in a solar cooker? Why?
Source: This topic is taken from NCERT TEXTBOOK
ENERGY FROM THE SEA – TIDAL ENERGY, WAVE ENERGY, OCEAN THERMAL ENERGY
Tidal Energy
Due to the gravitational pull of mainly the moon on the spinning earth, the level of water in the sea rises and falls. If you live near the sea or ever travel to someplace near the sea, try and observe how the sea-level changes during the day. This phenomenon is called high and low tides and the difference in sea-levels gives us tidal energy. Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening of the dam converts tidal energy to electricity. As you can guess, the locations where such dams can be built are limited.
Wave Energy
Similarly, the kinetic energy possessed by huge waves near the seashore can be trapped in a similar manner to generate electricity. The waves are generated by strong winds blowing across the sea. Wave energy would be a viable proposition only where waves are very strong. A wide variety of devices have been developed to trap wave energy for rotation of the turbine and the production of electricity.
Ocean Thermal Energy
The water at the surface of the sea or ocean is heated by the Sun while the water in deeper sections is relatively cold. This difference in temperature is exploited to obtain energy in ocean thermal energy conversion plants. These plants can operate if the temperature difference between the water at the surface and water at depths up to 2 km is 20 K (20°C) or more. The warm surface water is used to boil a volatile liquid like ammonia. The vapours of the liquid are then used to run the turbine of a generator. The cold water from the depth of the ocean is pumped up and condense vapour again to liquid.
The energy potential from the sea (tidal energy, wave energy, and ocean thermal energy) is quite large, but efficient commercial exploitation is difficult
Questions
1. What are the limitations of the energy that can be obtained from the oceans?
Source: This topic is taken from NCERT TEXTBOOK
GEOTHERMAL ENERGY
Due to geological changes, molten rocks formed in the deeper hot regions of the earth’s crust are pushed upward and trapped in certain region called ‘hotspot’. When underground water comes in contact with the hotspot, steam is generated. Sometimes hot water from that region finds outlets at the surface. Such outlets are known as hot springs. The steam trapped in rocks is routed through a pipe to a turbine and used to generate electricity. The cost of production would not be much, but there are very few commercially viable sites where such energy can be exploited. There are a number of power plants based on geothermal energy operational in New Zealand and the United States of America.
Questions
1. What is geothermal energy?
Source: This topic is taken from NCERT TEXTBOOK
NUCLEAR ENERGY
How is nuclear energy generated? In a process called nuclear fission, the nucleus of a heavy atom (such as uranium, plutonium or thorium), when bombarded with low-energy neutrons, can be split apart into lighter nuclei. When this is done, a tremendous amount of energy is released if the mass of the original nucleus is just a little more than the sum of the masses of the individual products. The fission of an atom of uranium, for example, produces 10 million times the energy produced by the combustion of an atom of carbon from coal. In a nuclear reactor designed for electric power generation, such nuclear ‘fuel’ can be part of a self-sustaining fission chain reaction that releases energy at a controlled rate. The released energy can be used to produce steam and further generate electricity.
The major hazard of nuclear power generation is the storage and disposal of spent or used fuels – the uranium still decaying into harmful subatomic particles (radiations). Improper nuclear-waste storage and disposal result in environmental contamination. Further, there is a risk of accidental leakage of nuclear radiation. The high cost of installation of a nuclear power plant, high risk of environmental contamination and limited availability of uranium makes large-scale use of nuclear energy prohibitive.
Nuclear energy was first used for destructive purposes before nuclear power stations were designed. The fundamental physics of the fission chain reaction in a nuclear weapon is similar to the physics of a controlled nuclear reactor, but the two types of devices are engineered quite differently.
More to know
1. In nuclear fission, the difference in mass, \(\Delta\)m, between the original nucleus and the product nuclei gets converted to energy E at a rate governed by the famous equation,
E = \(\Delta\)m c2,
first derived by Albert Einstein in 1905, where c is the speed of light in a vacuum. In nuclear science, energy is often expressed in units of electron volts (eV): 1 eV = 1.602 \(\times\)10–19 joules. It is easy to check from the above equation that 1 atomic mass unit (u) is equivalent to about 931 mega electron volts (MeV) of energy.
2. Nuclear power reactors located at Tarapur (Maharashtra), Rana Pratap Sagar (Rajasthan), Kalpakkam (Tamil Nadu), Narora (UP), Kakrapar (Gujarat) and Kaiga (Karnataka) have an installed capacity of less than 3% of the total electricity generation capacity of our country. However, many industrialised countries are meeting more than 30% of their electrical power needs from nuclear reactors.
3. Nuclear fusion: Currently all commercial nuclear reactors are based on nuclear fission. But there is another possibility of nuclear energy generation by a safer process called nuclear fusion. Fusion means joining lighter nuclei to make a heavier nucleus, most commonly hydrogen or hydrogen isotopes to create helium, such as
2H + 2H → 3He (+ n)
It releases a tremendous amount of energy, according to the Einstein equation, as the mass of the product is little less than the sum of the masses of the original individual nuclei.
Such nuclear fusion reactions are the source of energy in the Sun and other stars. It takes considerable energy to force the nuclei to fuse. The conditions needed for this process are extreme – millions of degrees of temperature and millions of pascals of pressure.
The hydrogen bomb is based on thermonuclear fusion reaction. A nuclear bomb based on the fission of uranium or plutonium is placed at the core of the hydrogen bomb. This nuclear bomb is embedded in a substance which contains deuterium and lithium. When the nuclear bomb (based on fission) is detonated, the temperature of this substance is raised to 107 K in a few microseconds. The high temperature generates sufficient energy for the light nuclei to fuse and a devastating amount of energy is released.
Activity 14.7:
* Discuss in class the question of what is the ultimate source of energy for bio-mass, wind and ocean thermal energy.
* Is geothermal energy and nuclear energy different in this respect? Why?
* Where would you place hydroelectricity and wave energy?
Questions
1. What are the advantages of nuclear energy?
Source: This topic is taken from NCERT TEXTBOOK