Solar Energy

Made By-Archit TantiaAayush TainwalaChandrakant Sarda

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Introduction

Traditional Uses of Energy

Harnessing Solar Energy

Composition of Sunlight

The Extended Solar Spectrum

Solar Energy Devices

Solar Heating Devices

Differences between Solar Devices

Drawbacks of Solar Heating Devices

Acknowledgements

Exercises

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Solar 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. 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 space in the form of solar energy. Since the sun is very, very far away, so the earth and other planets receive only a very, very small fraction of this solar energy. The energy coming from the sun contains heat rays (called infra red rays), visible light, ultra-violet rays and some gamma rays. So, at the top of the atmosphere, the solar energy consists of heat rays (infra-red rays),visible light, ultra-violet rays, and gamma rays, in different proportions (gamma rays are electromagnetic rays having wavelengths even shorter than ultra-violet rays.

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Solar Energy

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 the earth. The atmosphere also absorb; a lot of solar energy as it comes down through it towards the surface of earth. For example. most of the ultra violet rays present in solar energy are absorbed and eliminated by the atmosphere. Similarly, the atmosphere also absorbs gamma rays. So, the solar energy, which reaches us on the earth’s surface, is mostly in the form of heat rays (infra-red rays) and visible-light (and very little ultra-violet rays). 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.

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Solar Energy

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.

We have just seen that :Solar constant = 1.4 kilojoules per second per square metre

OrSolar constant = 1.4 kJ/s/m2 (also written as 1.4 kJ/sm2)

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Solar Energy

We can also write the value of solar constant in terms of power by writing 'kilowatts per square metre' in place of `kilojoules per second per square metre'. Thus, we can also say that:

Solar constant = 1.4 kilowatts per square metreOr

Solar constant = 1.4 kW/m2

We can use the value of the solar constant in either of the two units given above depending on the requirement of the problem to be solved.

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Traditional Uses of Solar Energy

About one-third portion of the sunlight consists of infra-red rays. These infra-red rays have a property that they heat the objects on which they fall. So, when sunlight falls on an object it produces heat. And this heat can be used for various purposes. Traditionally the solar energy has been put to very limited uses. Some of the traditional uses of solar energy are given page.

 (i) Solar energy has been used for drying clothes.(ii) Solar energy has been used for obtaining salt from the sea-water. (iii) Solar energy has been used for reducing the moisture content in

food-grains after harvesting the crops.(iv) Solar energy has been used for the preservation of fruits,

vegetables and fish; etc., by the process of sun-drying.

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Harnessing Solar Energy

The known reserves of the fossil fuels like coal, petroleum and natural gas in the earth are very limited which will get exhausted in a few decades time. This will create an energy crisis on the earth. So, in order to overcome the impending energy crisis, man has accelerated the search for renewable sources of energy. In this context, the sun is one of the major sources of renewable energy. The sun is the most abundant and everlasting source of energy on the earth.

Solar energy an be harnessed (or utilized) in two ways: direct and indirect.

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Harnessing Solar Energy

(i) Direct Utilization of Solar EnergyDirect Utilization of Solar Energy - - The direct utilization of solar energy can be done either by collecting it as heat (as we do in a solar cooker or solar water heater) or by converting t directly into electricity (as we do in solar cells). In this way solar energy (or sun's energy) can be utilized directly.

(ii) Indirect Utilization of Solar EnergyIndirect Utilization of Solar Energy - - The indirect utilization of solar energy can be done by converting it into chemical energy like the bio-mass of plants. The other indirect ways of utilizing the solar energy are: Harnessing the energy of the wind and utilization of energy due to the temperature difference in the water in different levels in the seas: This is because the flow of wind; the formation of sea-waves and the temperature difference between water at different depths of the sea, all are due to solar energy.

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Composition of Sunlight

The sun-light consists of three- types of electromagnetic waves : ultra-violet rays, visible light and infra-red rays, all having different range of wavelengths.

Out of these, ultra-violet rays have the shortest wavelengths and the infra-red rays have the longest wavelengths. We can see only the visible part of the sun-light called "visible light" having the wavelength range from bout 400 nanometer to 700 nanometer. The ultra-violet rays and infra-red rays contained in sunlight are invisible, and cannot be seen with our eyes.

Infra-red rays produce a sensation of heat whereas ultra-violet rays produce fluorescence in certain substances.

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Composition of Sunlight

We receive the sun's energy in the form of three types of waves, all having different wavelength ranges. These are:

(i) Ultra-violet rays (Shortest wavelengths, invisible to eye)(ii) Visible- light (Intermediate wavelengths, visible to eye)(iii) Infra-red rays (Longest wavelengths, invisible to eye)

Ultra-violet rays are also called ultra-violet radiation and infra-red rays are also known as infra-red radiation

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The Extended Solar Spectrum

If we pass sunlight through a triangular glass prism, we get a band of seven colors: Red, Orange, Yellow, Green, Blue, Indigo and Violet. This is called visible spectrum because we can see it with our eyes. All the colors of the visible spectrum have different wavelengths, with red color having the longest wavelength, and violet color having the shortest wavelength. The formation of spectrum consisting of seven colors shows that the visible part of sunlight consists of seven different colors.

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The Extended Solar Spectrum

The sun's spectrum is not as small as we see it, but extends on both sides beyond the red and beyond the violet. The invisible part of the spectrum immediately beyond the red colour is known as infra-red and it is identical with heat radiations. And the invisible part of the spectrum, immediately beyond the violet colour is known as ultra-violet.

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Solar Energy Devices

The various devices which are commonly used for harnessing (or utilizing) sun's energy are : solar cooker, solar concentrator, solar water heater and solar cell. These are called solar energy devices. All the solar energy devices can be classified into two categories on the basis of their working. In one type of solar devices, the solar energy is collected in the form of heat. Solar cooker, solar concentrator and solar water heater are such devices in which solar energy is collected as heat and used as such.- In the other type of solar devices, the solar energy is converted directly into electricity. Solar cell is a device in which solar energy is directly converted into electricity.

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Solar Heating Devices

A device which gets heated by the sun's heat energy is called a solar heating device. All the solar heating devices are designed in such a way that they help in collecting as much sunlight energy as possible. This is done by using a black painted surface, a glass sheet cover and a reflector.

(i) The Use of Black Painted Surface in Solar Heating Devices The Use of Black Painted Surface in Solar Heating Devices -- We know that as compared to a white reflecting surface, a black coloured surface absorbs much more heat. Therefore, in the designing of solar heating devices, black painted surfaces are used so that they may absorb the maximum amount of heat from the sunlight or sunshine. For example, the inner walls and bottom of the box-type solar cooker are painted black to maximize absorption of heat and to minimize heat losses due to reflection.

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Solar Heating Devices

(ii) The Use of Glass Sheet Cover in Solar Heating DevicesThe Use of Glass Sheet Cover in Solar Heating Devices - -We know that a hot body can lose its heat by the processes

of conduction, convection and radiation. In order to prevent the loss, of absorbed heat in a solar heating device; the black painted metal box is placed inside an insulated wooden box and covered with a transparent glass sheet. When this solar heating device is kept in sunlight for some time then its glass cover allows both. visible light and short wavelength infra-red rays present in sunlight to pass through it and go inside the blackened box. The black inner surface of the solar heating device absorbs these infra red rays and becomes hot (because infra-red rays produce a heating effect). When the black inner surface of the solar heating device becomes quite hot, then it also starts radiating heat in the form of infra-red rays. But the infra-red rays emitted by the less hot inner surface of the solar heating device are of comparatively longer wavelengths.

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Solar Heating Devices

(iii) The Use of Reflector in Solar Heating DevicesThe Use of Reflector in Solar Heating Devices – –In order to increase the efficiency of a solar heating device; a reflector is also attached to it. Usually, a plane mirror is used as a reflector. The reflector is used to increase the area over which the solar energy is collected so that more and more heat rays of the sun may enter the solar heating device. In those solar heating devices where high temperature is to be produced, spherical reflectors are used. The two types of spherical reflectors which are used in solar heating devices are: concave reflector and parabolic reflector. A labelled diagram of a simple solar heating device is shown.

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Solar Cooker

A solar cooker is a device which is used to cook food by utilizing' the energy radiated by the sun. A solar cooker consists of an insulated metal box or wooden box B; which is painted all black from inside. A box made of a non-conducting material like plastic or fiberglass and painted black from inside can also be used for making the solar cooker: There is a thick glass sheet G as a cover over the box, and a plane mirror reflector R is also attached to the box as shown.

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Solar Cooker

The box-type solar cooker is used to cook those food materials which require slow heating. The box-type solar cooker cannot be used for baking and frying because they require stronger heating. For the purpose of baking and frying, spherical reflector type solar cooker is used. Before we describe spherical reflector type solar cooker, let us give the advantages and limitations of a box-type solar cooker.

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Diagram to show the principle of a spherical reflector type solar cooker. The spherical reflector concave or parabolic reflector) concentrates sun's energy in a small region F to enhance its heating effect.

Spherical Reflector Type Solar Cooker

Apart from the box-type solar cooker, there is a yet another design of solar cooker which is known as "solar cooker with spherical reflector" in which the sun-rays are concentrated to obtain higher temperatures. The principle of spherical reflector type solar cooker has been illustrated:

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(i) In the box-type solar cooker, a plane reflector (i) In the spherical reflector type solar cooker,

  is used. It does not concentrate solar energy   a concave reflector or a parabolic reflector is

  at a point.   used. It concentrates the solar energy in a

      small region called focus.

(ii)) In the box-type solar cooker, comparatively (ii) Quite high temperature is produced in the

  low temperature is, produced   spherical reflector type solar cooker.

(iii) Box-type solar cooker can be used for cooking (iii) Spherical reflector type solar cooker can be

  only those food materials which require slow   used for cooking: even those food materials

  heating.   which require strong heating:

(iv) Box-type solar cooker cannot be used for (iv) Spherical reflector type solar cooker can be

  baking and frying. For example, it is not   used for baking and frying also. For example,

  possible to bake chappaties and fry eggs in a   we can bake chappaties` and fry eggs in a

  Box-Type Solar Cooker   Spherical Reflector Type Solar Cooker

  box-type solar cooker.   spherical reflector type solar cooker,

Differences Between Box-Type Solar Cooker and Spherical Reflector Type Solar Cooker

The main differences between a box-type solar cooker and a spherical reflector type solar cooker are given below:

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Drawbacks of Solar Energy Heating Devices

Some of the major drawbacks of the solar energy heating devices are given below :

(i) One of the major drawbacks of the solar energy heating devices has been that their direction has to be changed after short intervals so that they keep on facing the sun and sunshine may fall on them directly during the whole day. Though it is very inconvenient but it is an essential requirement to collect the maximum amount of solar energy. These days, however, special arrangements are being made in solar heating devices that they keep on rotating themselves slowly. Because of this, the solar heating devices always keep facing the sun and sunshine falls an them the whole day:

(ii) Solar heating devices cannot be used during the night because- solar energy (or sunshine), is not available at that time.

(iii) If the day-sky is covered with clouds, even then the solar heating devices cannot be used.

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Exercises

1. What is solar energy?2. What type of reactions occurring inside the sun produce solar

energy ?3. Which component of solar energy is almost completely absorbed

and eliminated by the atmosphere ? 4. What is solar constant ? What is the value of solar constant ?5. State some of the traditional uses of solar energy.6. Solar energy can be harnessed directly as well as indirectly. Give

two examples of each type.7. If the energy received by 5m2 area in 10 minutes is 4200 kJ,

calculate the value of solar constant.

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Exercises

8. Name the radiations in the sunlight that give feeling of hotness. Explain the composition of sunlight.

9. Name the part of sunlight used in heating of land and water-bodies.10. What type of radiation is emitted by a kettle containing boiling

water ? '11. Which component of light received from the sun has the shortest

and longest wavelengths; respectively?12. Which -part of the solar radiation is responsible for maximum

heating ?13. Though a hat iron emits radiations, yet it is not visible in the dark.

Why ?14. What is the range of wavelengths in the visible region ?

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Exercises

How much solar energy will be received by 1 m2 area in one hour if the solar constant were 1.4 kilowatts per square metre ?

SolutionSolution::In this problem, the value of solar constant has been given in terms of power as 1.4 kW/m2 but to calculate the energy we have to use the value of solar constant in terms of energy which is 1.4 kJ/s/m2. Please note that 1 hour = 60 x 60 = 3600 seconds. Now,

In 1 s, 1 m2 area receives energy = 1.4 kJ So, In 3600 s, 1 m2 area will receive energy = 1.4 x 3600 kJ = 5040 kJ Thus, 504() kilojoules of energy will be received by 1 square metre area

in 1 hour

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Acknowledgements