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Utilization and Types of Alternative Sources of Energy
Those sources of energy which are not based on the burning of fossil fuels or the splitting of atoms of nuclear fuels, are called alternative sources of energy (or non-conventional sources of energy). We know that both fossil fuels as well as nuclear fuels are dug from the earth, so we can also say that: Those sources of energy which do not use fuels dug from the earth are called alternative sources of energy.
The examples of alternative sources of energy are : Hydroelectric energy, Wind energy, Solar energy, Biomass energy, Energy from the sea (Tidal energy, Sea-wave energy, Ocean thermal energy), and Geothermal energy.
The scientists have started showing renewed interest in exploring and using the alternative sources of energy due to two reasons :
(i) because the fossil fuels and nuclear fuels in the earth are limited which may not last for long, and
(ii) because of the undesirable effects of pollution both from the burning of fossil fuels and from the radioactive nuclear wastes of nuclear power plants. We will now describe all the alternative sources of energy, one by one. Let us start with hydroelectric energy.
Flowing water possesses kinetic energy. The energy of flowing water has been traditionally used for rotating the water-wheels and drive water-mills to grind wheat to make flour. The traditional use of energy of flowing water has been modified by improvements in technology and used to generate electricity.
This is done by establishing hydro-power plants. At hydro-power plants, the energy of falling water (or flowing water) is tapped by using a ‘water turbine’ and then made to drive generators. We will now describe a hydro-power plant in detail.
Hydro-Power Plant (or Hydroelectric Power Plant)
A power plant that produces electricity by using flowing water to rotate a turbine (which drives the generator), is called hydro-power plant (or hydroelectric power plant). The electricity produced by using the energy of falling water (or flowing water) is called hydroelectricity. A hydro-power plant produces electricity as follows.
Rain falling on the high ground in hilly areas flows down as rivers. In order to produce electricity, a high-rise dam D is built to stop the flowing river water (see Figure). Due to this, a large lake or reservoir R builds up behind the dam. As more and more water collects in the reservoir, the level of water behind the dam rises to a large height.
In this way, the kinetic energy of the flowing river water is converted into the potential energy of water stored behind the dam. Thus, the water stored behind a tall dam has a lot of potential energy (due to its great height).
The sluice gates (sliding gates) at half the height of dam are opened to allow some of the stored water to escape. This water is taken through pipe P to the turbine T installed at the bottom of the dam (see Figure). Since the water falls down through a large height from the dam, it flows very fast (its potential energy is changed into kinetic energy).
A high pressure jet of fast flowing water pushes on the blades of turbine with a great force and makes the turbine rotate rapidly. The turbine is connected to generator through its shaft S. When the turbine rotates, its shaft also rotates and drives the generator. The generator produces electricity.
When the hydroelectric generator works and produces electricity, water flows out of the dam continuously and the level of water in the reservoir falls slowly. The reservoir is filled up again by the rain water brought in by the rivers. Please note that since water in the dam reservoir needed for generating electricity is refilled each time it rains, therefore, hydroelectric power is a renewable source of energy. It will never get exhausted.
Another point to be noted is that water to the turbine is taken from a point midway up the dam (see Figure), so that the generators continue to work even if the water level in the reservoir falls below normal.
A hydro-power plant converts the potential energy of water stored in the reservoir of a tall dam into electric energy. The dams for generating hydroelectricity can be built only in a limited number of places, usually in the hilly areas or at the foothill (where the water can fall from a considerable height). At present, of the total electric power generated in our country, almost one-fourth is contributed by hydroelectricity.
Advantages of Generating Hydroelectricity
The advantages of using the energy of flowing water for the generation of electricity are the following :
- The generation of electricity from flowing water does not produce any environmental pollution.
- Flowing water is a renewable source of electric energy which will never get exhausted.
- The construction of dams on rivers helps in controlling floods, and in irrigation.
Disadvantages of Generating Hydroelectricity
The production of hydroelectric power by constructing high-rise dams on rivers has certain problems associated with it. Some of these are given below.
1. Large areas of agricultural land, a vast variety of flora and fauna (plants and animals) as well as human settlements (or villages) get submerged in the water of reservoir formed by the dam. Due to this many plants and trees are destroyed, animals get killed and many people are rendered homeless. This creates the problem of satisfactory rehabilitation of the people displaced from the dam site.
2. Large eco-systems are destroyed when land is submerged under the water of reservoir of a dam. The construction of dam on a river also disturbs the ecological balance in the downstream area of the river. For example, due to the construction of dam there are no annual floods in the river. And because of this the soil of downstream region does not get nutrient-rich “silt”. This decreases the fertility of soil in the downstream area and finally the crop yields also decrease.
3. Due to the construction of dam on the river, the fish in the downstream area do not get sufficient nutrient materials due to which the production of fish decreases rapidly (because the fish nutrients remain trapped in the reservoir formed by the dam).
4. The vegetation which is submerged under water at the dam site rots under anaerobic conditions and produces a large amount of methane which is a greenhouse gas (and hence harmful for the environment).
The opposition to the construction of Tehri Dam on the river Ganga and Sardar Sarovar Project on the river Narmada are due to such problems. So, before taking a decision to generate hydroelectricity by constructing high-rise dams on rivers, it is necessary to consider its long-term effects on the environment and social life carefully.
The energy of flowing water is called hydroenergy. The energy of flowing water (or hydroenergy) is in fact an indirect source of solar energy. This is because it is the solar energy which recirculates water in nature in the form of water-cycle. It is this water which then flows in the rivers and makes water-energy available to us.
Moving air is called wind. Wind has energy. The energy possessed by wind is due to its high speed (or motion). So, the wind possesses kinetic energy. It is this kinetic energy of wind which is utilised for doing work. Solar energy (or sun’s energy) is responsible for the blowing of wind. Wind blows due to the uneven heating of earth by the sun in different regions. So, wind energy comes from the sun in an indirect way.
The energy of wind is harnessed by using a windmill. A windmill consists of big sized, table fan like blades which are fixed over the top of a tall pole in such a way that they are free to rotate. When the fast moving wind strikes on the blades of windmill, it makes them rotate continuously. The rotatory motion of the windmill is then used to do mechanical work through a shaft connected to the rotating blades.
In the past, wind energy was used through windmills to pump water (or lift water) from a well and to grind wheat into flour. This happened as follows : When the blades of windmill rotate by the force of wind, a shaft connected to them also rotates.
This rotating shaft was made to run a water pump to lift water from a well by a suitable mechanism. It could also turn the mill-stones of a flour mill. The traditional use of wind energy has now been modified by the improvement in technology to generate electricity through wind-powered generators. This is explained below.
The windmill used for generating electricity is called a wind turbine, and the complete set-up of generating electricity by using wind energy is called ‘wind generator’. We will now describe a wind powered electric generator in somewhat detail.
A wind generator which is used to generate electricity by using wind energy is shown in Figure. When the fast moving wind strikes the blades of wind turbine, then the wind turbine starts rotating continuously. The shaft of wind turbine is connected to a small generator. When the wind turbine rotates, its shaft also rotates and drives the generator. The generator produces electricity.
The electricity generated by a single wind turbine is quite small (because a single wind turbine can run only a small generator). So, in order to generate a large amount of electricity, a large number of wind turbines are erected over a big area of land.
Such a set-up of having a large number of wind turbines working at a place to generate electrical energy on a large scale is called a wind energy farm (see Figure). The small electricity generated by each wind generator is combined together to obtain electricity on a large scale.
An important advantage of using wind energy for generating electricity is that its use does not cause any pollution. Another advantage is that wind energy is a renewable source of energy which will never get exhausted. As long as the sun keeps shining, the wind will keep blowing and provide us energy. The increased use of wind energy for generating electricity will help us conserve fossil fuels (like coal, petroleum and natural gas) so that they may last longer. Moreover, wind energy is available free of cost !
Some of the limitations of harnessing wind energy for generating electricity are as follows :
- Wind energy farms cannot be established everywhere. The wind energy farms can be established only at those places where wind blows for most part of the year.
- The wind required for generating electricity should be strong and steady to maintain the desired level of generation. The minimum wind speed necessary for satisfactory working of a wind generator is about 15 km/h. This is not always so.
- The wind energy farms require a large area of land.
- The setting up of wind energy farms is very expensive.
The wind-power potential of our country is estimated to be about 45,000 MW. This means that if India’s wind energy is fully harnessed, then 45,000 megawatt of electrical power can be generated. At present we are generating more than 1025 MW of electricity from wind energy. This is going to increase soon when some more wind energy farms start working.
The largest wind energy farm established near Kanyakumari in Tamil Nadu can generate 380 MW electricity. India is ranked fifth in the world for harnessing wind energy for the production of electricity. The first position is occupied by Germany. Denmark is, however, called the country of ‘winds’ because more than 25% of the electricity needs of Denmark are met by utilising wind energy.
The sun is the source of all energy. The sun provides us heat and light energy free of cost! The energy obtained from the sun is called solar energy (see Figure). The nuclear fusion reactions taking place inside the sun keep on liberating enormous amounts of heat and light energy.
This heat and light energy is radiated by the sun in all directions in the form of solar energy. The sun has been radiating an enormous amount of energy at the present rate for nearly 5 billion years (5 × 109 years) and will continue radiating energy at that rate for about 5 billion years more. Since the sun is very, very far away, only a small fraction of the solar energy radiated by the sun reaches the outer layer of the earth’s atmosphere.
A little less than half (about 47 per cent) of solar energy which falls on the periphery (top surface) of the atmosphere actually reaches the surface of earth (the rest of solar energy is reflected back into space’ by the atmosphere and also absorbed by the atmosphere as it comes down through it towards the surface of earth).
The solar energy which reaches the earth is absorbed by land and water-bodies (like rivers, lakes and oceans), and plants. The solar energy trapped by land and water-bodies causes many phenomena in nature like winds, storms, rain, snowfall, and sea-waves, etc. And the plants utilise solar energy to prepare food by the process of photosynthesis.
India is fortunate to receive solar energy for greater part of the year. It has been estimated that India receives solar energy equivalent to more than 5000 trillion kWh (5000 × 1018 kWh) during a year. Under clear cloudless sky conditions, the daily average of solar energy varies from 4 to 7 kWh/m2 in our country.
The energy that the near earth space receives from the sun is about 1.4 kilojoules per second per square metre, and this quantity is called the solar constant. We can define the solar constant as follows : The amount of solar energy received per second by one square metre area of the near earth space (exposed perpendicularly to the rays of the sun) at an average distance between the sun and the earth, is called solar constant.
Thus, the solar constant tells us the amount of energy which falls in 1 second on a 1 square metre area of the near earth space at an average distance between the sun and the earth. The value of solar constant is 1.4 kJ/s/m2 or 1.4 kW/m2 (because : 1 kJ/s = 1 kW).
Solar Energy Devices
The sun is one of the major sources of renewable energy. Solar energy consists of heat and light. Scientists have made a number of devices which help in utilising the solar energy directly in our everyday life. The devices which work by using solar energy (or sun’s energy) are : Solar cooker, Solar water heater, and Solar cell.
A device which gets heated by using sun’s heat energy is called a solar heating device. A solar cooker and solar water heater are solar heating devices. Though a large amount of sun’s heat energy falls on the earth but it is very much diffused (spread over a large area) and not concentrated. So, in order to use sun’s heat energy for heating purposes, we have to collect and concentrate it.
All the solar heating devices are designed in such a way that they help in collecting as much sun’s heat rays as possible. This is done by using a black painted surface, a glass sheet cover, and in some cases also a plane mirror reflector. A black painted surface is used because black surface absorbs more heat rays of the sun than a white surface.
A glass sheet is used as a cover in solar heating devices because a glass sheet traps more and more of sun’s heat rays by producing greenhouse effect. A plane mirror reflector is used (in solar cooker) to increase the area over which solar energy is collected so that more heat rays of the sun may enter the solar cooker box. Keeping these points in mind we will now describe the construction and working of a solar cooker in detail.
The solar cooker is a device which is used to cook food by utilising the heat energy radiated by the sun. A solar cooker consists of an insulated metal box or wooden box which is painted all black from inside (see Figure). There is a thick glass sheet cover over the box and a plane mirror reflector is also attached to the box as shown in Figure.
The food to be cooked is put in metal containers which are painted black from outside. These metal containers are then placed inside the solar cooker box and covered with the glass sheet. We will now describe how the solar cooker works.
In order to cook food, the solar cooker is kept in sunshine, outside the house. The reflector of solar cooker is adjusted in such a way that it faces the sun. When the sun’s rays fall on the reflector, the reflector sends them to the top of solar cooker box in the form of a strong beam of sunlight. The sun’s heat rays pass through the glass sheet cover and get absorbed by the black inside surface of the cooker box.
Once the sun’s heat rays enter the cooker box, then the glass sheet cover does not allow them to go back. In this way, more and more heat rays of the sun get trapped in the box due to which the temperature in the solar cooker box rises to about 100°C to 140°C in two to three hours. This heat cooks the food materials kept in the black containers. For example, the food materials like rice, pulses (dal) and vegetables can be cooked in a box-type solar cooker.
The box-type solar cooker can be used to cook only those food materials which require slow heating. The box-type solar cooker cannot be used to cook those foods where strong heating is required. For example, the box-type solar cooker cannot be used for baking (chapattis, etc.) and for frying because they require stronger heating.
In order to achieve higher temperatures required for baking and frying, spherical reflector-type solar cooker (having a concave reflector) is used. When a concave mirror reflector is attached to a solar cooker, it converges a large amount of sun’s heat rays at its focus due to which a high temperature is produced in the focus area (which is suitable for baking and frying).
Thus, a concave mirror reflector is best suited for use in a solar cooker. We will now give the advantages and limitations of a solar cooker.
The important advantages of a solar cooker for cooking food are the following :
- The use of solar cooker for cooking food saves precious fuels like coal, kerosene and LPG.
- The use of solar cooker does not produce smoke due to which it does not pollute air.
- When food is cooked in solar cooker, its nutrients do not get destroyed. This is because in a solar cooker, food is cooked at a comparatively lower temperature.
- In a solar cooker, up to four food items can be cooked at the same time.
Some of the important limitations (or disadvantages) of a solar cooker are given below :
- The solar cooker cannot be used to cook food during
night time (because sunshine is not available at that time).
- If the day-sky is covered with clouds, even then solar cooker cannot be used to cook food.
- The direction of reflector of solar cooker has to be changed from time to time to keep it facing the sun.
- The box-type solar cooker cannot be used for baking (making chapattis, etc.) or for frying.
If a coil of copper tube painted black from outside is placed in a box similar to that of solar cooker, it will work as a solar water heater. This is because when water is passed through the copper coil, it will absorb sun’s heat rays and become hot.
Solar water heaters are used to supply hot water in big buildings like hotels and hospitals. A solar water heater cannot be used to get hot water during the night or on a cloudy Figure. Solar water heater fitted on the roof of a day. Let us solve one problem now. house.
A student constructed a model of box-type solar cooker. He used a transparent plastic sheet to cover the open face of the box. He found that this cooker does not function well. What modification should he make to enhance its efficiency ? Give reason.
Instead of using a plastic sheet, the student should use a transparent glass sheet to cover the open face of the box. This is because a glass sheet can trap the heat rays of the sun very effectively by producing greenhouse effect. A transparent plastic sheet cannot do that.
Solar cells use the energy in sunlight to produce electricity. Thus, solar cell is a device which converts solar energy (or sun’s energy) directly into electricity. Since solar energy is also called sunlight energy, so we can also say that a solar cell converts sunlight energy into electrical energy. We will now discuss the solar cell in somewhat detail.
A solar cell is usually made from silicon. A simple solar cell consists of sandwich of a ‘silicon-boron layer’ and a ‘silicon-arsenic layer’ (see Figure). The amount of boron and arsenic present in the two silicon layers is, however, very small. A small piece of wire is soldered into the top of upper layer of cell and another piece of wire is soldered at the bottom of the lower layer (to tap the current).
The solar cell is covered with a glass cover or a transparent plastic cover for protection. When sunlight falls on the surface of solar cell, it makes the loosely held electrons in the silicon atoms move due to which a current begins to flow in the wires connected to the top and bottom of the solar cell.
The strength of current produced depends on the brightness of the light and the efficiency of solar cell. A potential difference (or voltage) of about 0.5 V is generated between the top and bottom surface of a solar cell. At present, the best designed solar cells can generate 240 W/m2 in bright sunlight, with a maximum efficiency of about 25%.
A single solar cell can produce only a small amount of electricity. In those cases where more electrical power is needed, a large number of solar cells are joined together in series. This group of solar cells is called a ‘solar cell panel’. Thus, a solar cell panel consists of a large number of solar cells joined together in a definite pattern (see Figure). A solar cell panel can provide much more electric power than a single solar cell.
The various solar cells in a solar cell panel are joined together by using connecting wires made of silver metal. This is because silver is the best conductor of electricity. The use of silver for connecting solar cells makes it more expensive but it increases the efficiency of solar cell panel. At many places, the solar cell panels are mounted on specially designed inclined roofs so that more solar energy (or sunlight) is incident on them.
Solar cell panels can provide electricity at all those places where the usual electricity is not available. For example, solar cell panels are used to provide electricity in remote and inaccessible rural areas. This electricity is used there for lighting purposes, to run radio and TV sets and to operate water pumps for irrigation of fields.
In Figure, a solar cell panel is being used for running a water pump for irrigation in a distant village. The solar cell panel S has hundreds of solar cells joined together. The electricity produced by this solar cell panel is stored in battery B (by charging it). This battery runs an electric motor M. And finally, this motor drives a pump P which pumps out the underground water.
The solar cells cannot work at night (when sunlight is not available). So, the electricity produced by a solar cell panel during the day time can be stored by charging a battery (like a car battery) and then used at night to power lights.
The main advantages of solar cells are that they have no moving parts, they require almost no maintenance, and work quite satisfactorily without the use of any light focussing device. Another advantage of solar cells is that they can be set up in remote, inaccessible and very sparsely inhabited areas where the laying of usual power transmission lines is difficult and expensive.
The main disadvantage of solar cells is that they are very expensive. This is due to the following reasons :
- the special grade silicon needed for making solar cells is expensive,
- silver wire used to interconnect solar cells for making solar cell panels is expensive, and
- the entire process of making solar cells is still very expensive.
So, the extensive use of solar cells for generating electric power is limited due to their high cost. Another disadvantage of solar cells is their low efficiency. They can convert only about 25 per cent of the light energy falling on them into electricity.
Uses of Solar Cells
- Solar cells are used for providing electricity in artificial satellites and space probes.
- Solar cells are used for providing electricity to remote, inaccessible and isolated places where normal electricity transmission lines do not exist.
- Solar cells are used for the transmission of radio and television programmes in remote areas.
- Solar cells are used for providing electricity to ‘light houses’ situated in the sea and to off-shore oil drilling rig platforms.
- Solar cells are used for operating traffic signals, watches, calculators and toys.
Before we end this discussion on solar energy, we would like to compare the fossil fuels and the sun as sources of energy.
- The sun is a renewable source of energy but fossil fuels are a non-renewable source of energy.
- The sun’s energy does not cause any pollution but burning of fossil fuels causes a lot of pollution.
- The sun’s energy is available in a diffused form (scattered form) but fossil fuels provide energy in concentrated form.
- A special device (like solar cooker or solar cell) is always needed to utilise sun’s energy but this is not so in the case of fossil fuels.
- The sun’s energy is available only during the day time when the sun shines but energy of fossil fuels can be used all the time.