The waste and dead parts of living organisms (plants and animals) such as wood, straw, cow dung, agricultural wastes and household rubbish are collectively called as bio mass. The bio energy produced by burning or processing of bio-mass is the main source of energy for millions of villagers in India. Bio-mass can be converted into energy by using different methods. One method is to bury the plant and animal wastes in the earth by digging wells. After some time, these waste materials decay to produce methane gas. The second method of obtaining bio energy from bio-mass is to directly burn the bio-mass such as dry cow dung and wood. But the burning of dry cow in the form of fuel is not advisable due to following reasons:

1. Burning of cow dung as a fuel produces a lot of smoke which is harmful for human health and cause environmental pollution.
2. Burning of cow dung as a fuel destroys useful nutrients present in it.
3. Very less amount of heat is produced during burning of cow dung.
4. A large amount of ash gets produced as residue after burning of cow dung.

Third method of obtaining energy from bio-mass is to prepare biodiesel. This is done by removing glycerine from vegetable oil. The biodiesel so produced from vegetable oils is then used as diesel fuel.

In spite of these methods, one more method to use bio-mass is to convert one type of fuel into another more useful fuel. For example, wood and cow dung can be converted into more useful fuels called charcoal and bio-gas respectively. The process of conversion of solid biomass into gaseous fuel is called bio mass gasification. Now, we will discuss in detail the methods to convert wood into charcoal and cow dung into biogas. But, before doing so we should know the meaning of destructive distillation. Destructive distillation can be defined as the process of conversion of complex carbon compounds into simple substances by heating strongly in a closed vessel in absence of air.

Destructive Distillation of Wood or Preparation of Charcoal
To prepare charcoal from wood, take two hard glass tubes. In first glass tube, put some pieces of wood and fix it in horizontal position with the help of a stand. In second test tube put some water and fix it in vertical position. Now, set the apparatus with the help of two delivery tubes as shown in fig. 9. Make it sure that the joints are air tight. Now, heat the glass tube containing pieces of wood first gently and then strongly with the help of a burner.

Destructive Distillation Of Wood

After sometime you will find a gas called wood gas is flowing out through second delivery tube. The wood gas is combustible and consists of many gases. You will also find that a black coloured residue called charcoal is left in first glass tube and a black liquid has settled at the bottom of second glass tube containing water.

It should be noted that charcoal burns easily without producing smoke and produces more heat as compared to wood on burning. Due to these reasons charcoal is considered better fuel than wood.

Test Your Understanding and Answer These Questions:

1. What is bio mass?
2. What is bio mass gasification?
3. What is destructive distillation?
4. Explain destructive distillation of wood.
5. Why burning of cow dung cake is not advisable?
6. Why charcoal is considered a better fuel than wood?

In those areas of the earth where the level of volcanic activities is quite high, the rocks present in the earth are usually very hot. This heat energy of rocks present in the earth is called geothermal energy and can be used to produce electricity. To produce electricity by using geothermal energy holes are drilled in the surface of earth and pipes are put into them down to hot rocks. Then water is run across these pipes. When this water comes in contact with the hot rocks, then it gets converted into steam and comes out of pipes with great pressure. This high pressure steam is then used to rotate the turbines which generate electricity.

Geothermal Power Plant

Advantages of Geothermal Energy

1. It is a renewable source of energy.
2. It does not produce any type of environmental pollution.
3. Geothermal power plants produce energy 24 hours a day.
4. It requires less area of land as compared to any other kind of power plant.

Test Your Understanding and Answer These Questions:

1. What is geothermal energy?
2. Is geothermal energy renewable?
3. How geothermal energy works?

Ocean Thermal Energy Different layers of ocean have different temperature. Usually, the water at the surface of oceans is hot due to sunlight. While the water present in the deeper layers of oceans is cold. The difference in temperature of hot water present at the surface and cold water present in deeper layers of oceans is used to produce electricity and is called ocean thermal energy (OTE). The device which is used to produce electricity from ocean thermal energy is called ocean thermal energy conversion (OTEC) power plant. It should be noted that ocean thermal energy conversion power plant requires a temperature difference of 20⁰C between the surface and deeper water of ocean to operate. This source of energy is also non-polluting and renewable. But unlike solar energy, ocean thermal energy is available 24 hours a day.

Sea Waves Energy
The waves produced on the surface of oceans due to blowing wind over it are called sea waves. The kinetic energy of ocean waves is used to produce electricity by designing special types of hydroelectric power plants. In these power plants, fast moving sea waves are allowed to move up and down inside covered chambers. The up and down movement of sea waves pushes and pulls the air present inside covered chambers and thus rotates the turbines fitted in them to produce electricity.

Test Your Understanding and Answer These Questions:

1. What is otec?
2. What is ocean thermal energy?
3. What is sea wave energy?

Oceans are major source of energy. From oceans following types of energies can be obtained.

1. Tidal energy
2. Sea waves energy and
3. Ocean thermal energy

Tidal Energy
Tidal energy may be defined as the energy possessed by water during high tides and low tides. The tidal energy of oceans can be used to produce electricity (tidal power). For this, usually a dam is constructed across river estuaries. During high tide, the water moves in the reservoir through small openings and rotates the turbines fixed inside them and thus produces electricity. The passed in water is collected in the reservoir by closing the gates of the openings. And during low tide the gates of openings are opened and the water collected in the reservoir is allowed to flow out into the sea in a controlled manner. This reverse flow of water through the openings again rotates the turbines and produces electricity.

Although tidal energy is also a non-polluting and renewable source of energy, it cannot be considered a reliable source. The reason for this is that tidal energy plants are suitable only at those places where the difference between high and low tide is 6m or more. Thus there are very few sites in the world which are suitable to construct tidal power plants. Also construction of tidal power plants across river estuaries may cause ecological imbalance by changing tidal levels.

Test Your Understanding and Answer These Questions:

1. What is tidal energy?
2. How tidal power plant works?

Flowing water is also an important renewable source of energy. The kinetic energy of flowing water has also been used for centuries by human beings for various works. For example, the energy of flowing water has been used for centuries by human beings to transport logs of wood in hilly areas. These days, the kinetic energy of flowing water is used to generate electricity. The electricity produced by flowing water is called hydroelectricity, and the stations where the energy of water is converted into electricity are called hydroelectric power plant. The hydroelectric power stations are generally built at natural waterfalls or by constructing a dam across a river.

How hydroelectric power plant works?
The basic principle of generation of electricity at hydroelectric power plant is very simple. First of all, dams are constructed across rivers to collect the water flowing in them. In this way, a large amount of potential energy gets stored in the water. Now, a turbine attached with armature of an electric generator, is placed at the bottom of the dam. Then the water stored in the dam is made to fall through pipes on the blades of turbine. During this, the potential energy of stored water gets converted into kinetic energy. This kinetic energy of falling water exerts a great amount of force on the blades of turbines due to which the turbine starts rotating. This rotating turbine rotates the armature of generator and produced electricity. The schematic representation of principle of working of a hydroelectric power station is shown in figure.

Production Of Electricity At Hydroelectric Power Station

Advantages of Generating Hydroelectricity
Following are the main advantages of generating hydroelectricity:

1. It is a renewable source of energy.
2. It does not produce any type of environmental pollution.
3. Construction of dams provides water for irrigation and prevents seasonal floods.
4. The running cost of a hydroelectric power station is very low.

Disadvantages of Generating Hydroelectricity
The various disadvantages associated with generating hydroelectricity are as follows:

1. The cost of construction of a hydroelectric power station is very high.
2. Also construction of dams on rivers may cause submerging of large areas of land, due to which the ecological balance may get upset.

Test Your Understanding and Answer These Questions:

1. What is hydroelectricity?
2. How hydroelectric power plant works?

Wind is a renewable source of energy. Blowing air is known as wind. Since wind is moving air, so it possesses kinetic energy. The kinetic energy of wind has been used for centuries by human beings for doing various types of works. For example,

1. Wind energy is used to propel sail boats.
2. Wind energy is used to remove husk from grains.
3. Wind energy is used for grinding grains.
4. Wind energy is used to pump out underground water.
5. Wind energy is used to generate electricity.
6. Wind energy is used also used in flying gliders and aeroplanes.

Wind Mills
A machine which works with the wind energy (moving air) is called wind mill. Wind mill is also known as wind turbine. A wind mill consists of large blades like that of an electric fan fixed at the top of a rigid support. In a wind mill, the kinetic energy of moving air is usually converted into electricity. The principle of working of a wind mill is very simple. The blades of a wind mill are of special shape. When high speed wind strikes against these blades, a force is exerted on them due to which the blades start rotating. To produce electricity, the wind mill is attached to the armature of an electric generator with the help of a shaft. When the blades of the wind mill rotate, the armature of electric generator also starts rotating due to which electricity is produced. The schematic representation of working of a wind mill is shown in diagram.

Diagram Of wind turbine or Wind Mill

Advantages of Wind Energy
Following are the advantages of wind energy:

1. It is renewable source of energy.
2. It does not produce any type of environmental pollution.
3. It is available free of cost.

Disadvantages of Wind Energy
Although the use of wind energy is pollution free and it is a renewable source of energy, yet it has a number of disadvantages also. These are:

1. Wind mills cannot be established at every place. The wind mills can be established only at those places where the wind blows with a minimum speed of 15 Km/h and for greater part of the year.
2. The wind mills require much space.
3. Wind mills can pose a threat to birds.
4. The cost of construction and installation of wind mills is very high.

Test Your Understanding and Answer These Questions:

1. What is wind mill?
2. What is wind energy?
3. How wind mill works?
4. What are advantages of wind energy?
5. What are disadvantages of wind energy?

 The devices which convert solar energy directly into electricity are called solar cells. Solar cells are made from thin layers of semi-conductor materials such as silicon and gallium. Semiconductors are those substances which have electrical conductivity more than insulators but less than conductors. When sun rays fall on a thin sheet of selenium, electricity is produced.

Solar Cell Efficiency
The efficiency of first solar cell (to convert solar energy into solar power) made in 1954 was about 1%. This efficiency of solar cell was so small that it could not be used for practical purposes. But, the modern solar cells have efficiencies of about 25% which is very high. Now days the semiconductor materials used for fabrication of solar cells are available in abundance and on cheap rates. The solar cells are also environment friendly as they cause no pollution. Due to all these reasons the use of solar cells has increased considerably in last few decades. The solar cells are used extensively as a main source of solar power in artificial satellites, water pumps, street lighting, traffic signals, research centers located in remote areas, toys, electronic instruments such as calculators and watches etc. the solar cells are also used by scientists to run cars on experimental basis.

Solar Cell Panel
A single solar cell can generate about 60 milliamperes of current which is quite low for working of various devices. So, in order to generate required amount of current a large number of solar cells are joined together in a definite manner. This group of large number of solar cells joined together in a definite manner to produce required amount of electric current is called solar cell panel. Solar cell panels are used in those areas which receive a lot of sunshine. The schematic arrangement of use of solar cell panel to provide electricity of a street light is shown in diagram.

Diagram Of Solar Panel Providing Electricity To A Street Light

During day the solar power (electric current) produced by a solar cell panel from solar energy is stored in a storage battery. The electric current stored in the battery can be used to run various devices whenever required.

In spite of various benefits of solar cells, it is not possible to make use of solar cells to meet all our energy needs. The main reason for this are:

1. The solar cells cannot be used during night and rainy days when the sky is covered with clouds.
2. The fabrication and installation of solar cells is still quite expensive.
3. The efficiency of solar cells is still low.

Test Your Understanding and Answer These Questions:

1. What is a solar cell?
2. What is a solar panel?
3. What is a semiconductor?
4. How do solar panels work?
5. What are drawbacks in solar cells?
6. What is efficiency of a solar panel?
7. Give two examples of semiconductors.

The device in which solar energy is directly used to heat the water is called solar water heater.

Construction of solar water heater
In solar water heaters, a pipe of copper with its outer surface painted black is fitted in the form of a coil in a box which is painted black from inside. Outer surface of copper tube and inner walls of box are pointed black to increase absorption of heat to produce high temperature. Like solar cooker, the box of solar water heater is also covered with a glass sheet to entrap the heat inside the box. One end of the copper pipe is joined to the bottom of water storage tank, while other end is joined near its middle. The schematic arrangement of a solar water heater is shown in figure.

Working of solar water heater
The cold water present in the water storage tank enters the copper pipe joined to it at the bottom and pass slowly through the coil of copper placed in box. When the cold water passes through the coil of copper it absorbs solar heat and gets heated. The hot water comes out from the other end of the copper pipe and enters the upper part of the water storage tank. This hot water is replaced by the cold water present in the tank. In this way, all the water present in the storage tank gradually heated up. In the tank, the hot water being lighter remains in the upper part. From the upper part of storage tank, the hot water is supplied to a tap for use.

Test Your Understanding and Answer These Questions:

1. What is solar water heater?
2. How solar water heater works?

A spherical reflector type solar cooker consists of a large spherical (concave) reflector which reflects the sun’s energy to a single focus point due to which a high temperature is produced in the focus area. Since, a spherical reflector type solar cooker reflects and concentrates the heat energy of sunlight into a small area; it is referred as solar concentrator. As much higher temperature is produced in spherical reflector type solar cooker as compared to box type solar cooker, so they are used to cook those food materials which require strong heating.

To cook food by this cooker the position of spherical reflector is adjusted in such a manner that it receives maximum sun rays. When sun rays fall on spherical reflector, they get reflected and concentrated at the focus point and produce very high temperature. Now, the cooking utensil containing raw food is placed at the focus of the cooker with the help of a stand. The temperature produced in solar concentrator is usually in the range of 180⁰C to 200⁰C depending on its size and the quality of reflecting surface.

Test Your Understanding and Answer These Questions:

1. What is a solar concentrator?
2. How solar concentrator works?

Construction of Box Type Solar Cooker
The main parts of a box type solar cooker are:

1. An insulated metal box painted black from inside.
2. A glass sheet.
3. A plane mirror as reflector.

Principle and Working of Box Type Solar Cooker
It is well known fact that a black coloured surface absorbs more heat than a white surface. So, the inner walls and bottom of metallic box to be used for making solar cooker are painted black to increase absorption of heat. This box is covered by a thick transparent sheet of glass. When the box with glass cover is placed in the sunlight, the glass cover allows the infra red rays present in sunlight to pass into the box. Most of these infra red rays are then absorbed by black surface of the box and the box becomes hot. But, after sometime when the black surface becomes hot it starts radiating out heat in the form of infra red rays. But, the glass sheet cover placed over the box does not allow the heat radiated by the black surface to go out from the box. In this way, the glass cover enables the cooker to entrap the heat inside it.

Usually, a plane mirror reflector is also attached on the top of the box. The plane mirror reflector increases the efficiency of solar cooker by reflecting more and more sunlight inside the box.

It should be noted that the temperature inside box type solar cooker can go from 100⁰C to 140⁰C within 2 to 3 hours when placed in sunlight. So, this type of solar cooker is used to cook only those food materials which require gentle heating.

Test Your Understanding and Answer These Questions:

1. What is box type solar cooker?
2. How solar cooker works?