Solar Energy for std 8 to 12 Chemistry

Solar energy
Concept
Natural arrangements that exist to transform solar energy to be useful to people
Solar devices
Solar cells
Solar heaters
Solar cookers.

Review
Electromagnetic radiations.
The energy that we get from the sun.
The nuclear reactions namely nuclear fission and nuclear fusion. 

INTRODUCTION
Solar energy is the radiant energy that we receive from the sun in the from of heat and light. Infact, it is an inexhaustible Source of energy for us. It is the ultimate Source of most of he sources of energy that we use today. The wind energy, the water energy, energy of fossil fuels and energy in food are all due to solar energy.
Around the year 1920, the technique of 'specturm analysis' helped to determining the composition of sun. It is established that the sun is made up mostly hydrogen. 

Note:
In the early days the sun was thought to be a huge mass of burning coal. Scientists have calculated that this coal would have been burnt in 5000 years whereas the sun has been shining for billions of years proving that the sun could not be burning coal.

SOURCE OF SOLAR ENERGY
The sun has been providing us with its heat and light. It was only recently that the real source of the sun's energy became known.
The discovery of radioactivity towards the end of the 19th century, led some scientists to suggest that the energy released during radioactive decay of radioactive materials in the sun, might be the source of solar energy. The liberation of energy in this process being extremely slow, the hypothesis had to be rejected.
Some scientists considering the temperatures existing in the solar interior, suggested by about 1929 that thermonuclear reactions could be the source of solar energy. This can explain the observed radiation from the sun. The most convincing explanation was provided by Hans Bethe and C.von Wiezsacker in 1939. Bethe suggested that the nuclear reactions in which hydrogen nuclei fuse to from helium nuclei is the source of solar energy. This is true for other stars also. In this process enormous amount of energy is released.

Note :
The rate of decrease in the mass of the sun is calculated to be about 4 million tonnes per second. Every square meter area of the earth’s upper atmosphere receives nearly 1.4kJ solar energy per second. Only 50 % of this reaches the earth’s surface. The total energy radiated per second by the sun is 4 * 1026 J. this corresponds to the fussion of a mass of 2 * 1019 kg of hydrogen per year.

Reactions in the sun:
There are two different cycles of processes. One is the proton-proton chain and the other is the carbon cycle. In p-p chain four hydrogen nuclei fuse into one helium nucleus with the liberation of energy. in the carbon cycle, the carbon atom acts as a nuclear catalyst. These two cycles account for sun's energy. 

NATURAL TRANSFORMATION OF SOLAR ENERGY.
Many of the useful sources of energy come from the natural transformation of solar energy.
Wind energy :
Water energy :
Energy in food :
Energy of fossil fuels :

Wind energy: 

Solar energy heats the air near the earth. The hot air becomes lighter and this air rises up. This results in a low pressure area. Cooler air flows into this region giving rise to wind. Thus wind energy is due to solar energy. Wind energy is harnessed for many useful purposes such as production of electricity, sailing of boats etc. 

Water energy:
Solar energy causes water to evaporate from water bodies. The water vapour rises to from clouds and the clouds bring rain. The rain water flowing downhill possesses kinetic energy. This energy can be used to generate electricity. Thus water energy is also due to solar energy.

Energy in food:
Plants use the solar energy for producing food for their sustenance. This food due to solar energy maintains every from of life on earth.

Energy of fossil fuels:
The fossil fuels are formed when living things get embedded in the earth's crust and undergo chemical change. Life forms are due to solar energy. Thus fossil fuels are also due to solar energy.

SOLAR DEVICES.
Introduction:
Nearly, one third of solar radiation consists of infrared radiation. Heat from the sun reaches us in the form of infrared radiation. Although oceans of energy pour to the earth from the sun, we use only a part of it.
Rapid depletion of fossil fuels has prompted the development of technology to harness solar energy. As a result a number of solar devices have come into use.

Solar devices:
The devices used to harness solar energy are called solar energy devices or solar devices. Solar devices can be broadly classified into two types based on the principle of working. In one type of devices, the solar energy is collected in the form of heat. Ex. solar cookers and solar water heaters. In the other type, solar energy is converted into electricity. Ex. solar cell.

(a) Solar cooker: 

solar cooker is a simple device for cooking food. It is make of a wooden box or heat resistant material. The inside of the box is painted black. The box is covered with a transparent plate of glass (Fig ). A Plane mirror is fitted to the inner surface of the box. Food to be cooked is taken in a metal container. It is placed inside the box. The cooker is placed such that sun's rays are incident directly on the mirror. The mirror reflects the solar radiation on to the container. This raises the temperature of the food and the food gets cooked. The black paint on the inside walls of the box and the outer surface of the container ensures better absorption of solar energy. The glass plate does not allow heat energy to flow out of the box. Thus heat energy, trapped inside the box, is utilised in cooking food. 

Solar Cooker with food making Process

Note:
Glass has a peculiar property. Plain glass is transparent to light and infrared radiations of short wave lengths. But it is opaque to infrared radiations of longer wavelengths are absorbed and the inside of the box emits infrared radiations of longer wavelengths, which will be retained inside the box. 

(b) Solar water heaters: 

Solar water heaters are designed on the same principle as the solar cookers. In solar water heaters, a copper pipe with its outer surface painted black, is fixed in the form of a coil in a box similar to that of a cooker [ Fig ]. 

Solar Water  heaters Pipe line arrangements.

Bending copper pipe as a coil, helps to increase the area for absorption of solar energy. A reservoir kept at a higher level stores water. This is connected to a small cylindrical tank, kept at a slightly higher level above the heater. One end of the copper pipe is connected to the bottom of the small tank with the other end connected near its middle. Schematic arrangement of the heater is shown in fig.
Water from the tank flows continuously through the pipe. The water absorbs heat and gets heated up. Hot water remains in the upper portion of the tank and this an be drawn through a tap. Cold water from the reservoir tank replaces the hot water drawn.

Note:
Some solar heaters are designed to reflect and concentrate solar radiation over a small area using large mirrors (concave or parabolic). A much higher temperature is obtained with this type of heater.

(c) Solar cells: 

The devices that covert solar energy directly into electrical energy, are called solar cells. The first practical solar cell was fabricated in 1954.
A Typical solar consists of a small piece (2 sq cm area) of light absorbing material like silicon. Solar energy entering releases electrons from silicon atoms to produce electricity. The voltage produced is just about 0.5 V and power produced is 0.7 Watt. To obtain higher voltage and power, a large number of cells are connected in series. This arrangement of cells is known as solar panel.
In spite of low efficiency and high cost (Pure silicone is required for efficiency), solar cells are begin used for many purposes. They are the main source of energy i artificial satellites. They are used as a source of electricity for radio or wireless transmission, at TV relay stations, traffic lights and research centres in remote areas. They may be viewed as the future source of electricity for domestic purposes.

POINTS TO REMEMBER
(1)  Solar energy is the radiant energy that we receive from the sun in the form of heat and light.
(2)  Bethe suggested that, nuclear reactions in which hydrogen nuclei fuse to form helium nuclei is the source of solar energy.
(3)  Proton-proton chain and carbon cycles are the two processes by which solar energy is produced.
(4) Solar energy is the source of many useful form of energy.
(5) The different forms of energy are wind energy, water energy, energy in food, energy of fossil fuels.
(6) Solar radiation consists of nearly 1/3 of infra-red radiations.
(7) Commonly used solar devices are solar water heater, solar cells, solar cooker, solar lighting.