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SUN – THE SOURCE OF ALL ENERGY
One of the major drivers of modern economic development is energy. The present development model based on fossil fuels has suffered a major set back due to resource crunch and climatic offences. Phasing out fossil fuels and increasing the share of renewable energy in the power mix is the need of the hour. The ecological balance is disturbed due to rising population, deforestation, modern transportation systems, use of chemical fertilizers to enhance agriculture produce etc.. Also, it is important to check the battle between man and nature. The attempt to overpower nature by the materialistic human should immediately be stopped. The indiscriminant use of fossil fuel in the name of development has driven mankind to a dead end where there is shortage of energy , reversal of the process of development and impacts of climate change and global warming. Of course, the per capita energy consumption is the index of development. The more we consume energy – the more developed we are. Hence, consumption of energy should not be inhibited. Rather we have to think of an unlimited-renewable resource of energy. Energy from abundant sources like Water, Wind, and Sun is considered as viable alternatives to conventional energy sources. Scientists are making untiring efforts to exploit these optimal sources. Our the then Prime minister late Smt. Indira Gandhi once told that “ The time has come when greater emphasis should be laid on deriving energy from the sun, wind and water… We must now make urgent and all out scientific effort to promote the development and utilization of solar and other forms of renewable energy to reduce our dependence on fossil fuel and to help safeguard our environment. ” Let us start exploiting “THE SUN – source of all Energy”
The bright yellow star Sun is about 15 crore kilometer away from us and supplying energy since more than 500 crore years. Energy like heat, light, tidal, chemical and fuel etc. are possible only due to sun. Due to the moderate distance (not too far & not too near) life is possible on earth. Hydrogen is the main constituent of the sun. Besides it consists of traces of 60 elements. The outer surface is about 6000 degree Celsius hot and the temperature at center ranges from 1.5 to 2.00 million degree. DUE TO EXCESS HEAT THESE ELEMENTS EXIST IN A STATE OF PLASMA. IN A IONISED GASIOUS STATE. The nucleus of the Hydrogen atom is continuously turned into Helium nucleus. Burning of 63 crore tons of Hydrogen per second 62.54 million tons of Helium is produced. The balance 46 lakh tons is spread away as different forms of energy. Only one part out of 220 million part of thus produced energy is received by our planet earth. It has been estimated that annual energy requirement of the earth can be meat out of 45 minutes solar radiation available to us. India is in the sunny belt of the earth. We receive solar energy equivalent to more than 5000 trillion KWh per year. Though the energy density is and availability is not continuous , it is now possible to harness reliably for many purposes. By converting it to usable heat and electricity.
The solar energy can be utilized in two ways. Solar Thermal & Solar Photovoltaic.
SOLAR THERMAL ENERGY
The solar thermal technology utilizes the heat energy of the solar radiation by the principle of BLACK BODY ABSORPTION. Black colour absorbs heat and retains if proper insulation is maintained. When the sun’s short wave radiation impinges on a blackened surface much of the incoming radiant energy can be absorbed and converted into heat. The temperature that results is determined by the intensity of the solar irradiance , the ability of the surface to absorb the incident radiation and the rate at which the resulting heat is emitted. By covering the absorbing surface with a transparent to sun’s short wave length radiation but opaque to long wave radiation emitted by the sun material (glass), the effectiveness can be enhanced. The energy thus collected can be accomplished distillation of sea water for salt production, heating of swimming pools, space heating, heating water for domestic - commercial and industrial use. Cooling by absorption or compressive refrigeration, cooking etc. is also possible.
Pool systems pump the water directly through the solar collectors. Because water in the pool is usually kept at about 80°F, collectors for pool heating systems may not have glazing or insulation. They also do not need to be protected against freezing because swimming pools are generally used only in warm weather or can be drained easily when it's cold.
Solar thermal energy can be collected at large scale and used to heat a transfer fluid that can then power a steam turbine to generate electricity. Sunlight may be collected using huge mirror arrays focused on to a receiver at the top of a tower or, at the other extreme, may be based on the temperature gradient produced when sunlight impinges on a "solar pond" (a large salty lake) and heats the water at its base.
Most commercially attractive is the solar thermal electricity generation system originally, which uses parabolic reflectors to warm a heat-transfer oil running through a pipe at the focus of the reflector. This heated oil is used to raise steam to power a turbine.
Flat plate collectors are the simplest and most common type. Copper pipes wind back and forth through the flat plate collector, which is painted black to absorb heat and covered with glass, or “glazing,” to prevent heat from escaping. Often the pipes are painted black and bonded to the material of the flat plate collector to maximize heat absorption.
The solar collector is usually mounted on the roof and is connected to a circuit containing water with propylene glycol anti-freeze added, if necessary. The heated liquid flows around the circuit, either under the action of a pump to warm the main hot water tank, or by a thermo-siphoning action to warm a solar water storage tank that then feeds the hot water tank.
SOLAR PHOTOVOLTAIC ENERGY
Photovoltaics = Photo (Light) + Voltaics (Electricity)
The energy transmitted from the sun is in the form of electromagnetic radiation. Conversion of solar radiation to electrical energy is made through solar cells. A solar cell or photovoltaic cell is a wide area electronic device that converts solar energy into electricity by the photovoltaic effect. This involves three major processes. Those are absorption of sun light in the semiconductor material, generation and separation of free positive and negative charges to different regions of the solar cell, creating voltage and transfer of separated charges for different applications in the form of electric current.
The absorption of sunlight by a solar cell depends on the intensity and quality of sunlight. When light is absorbed in the semiconductor, a negatively charged electron and positively charged holes are created. The heart of the solar cell is the electrical junction. This junction separates these electrons and hole from one another after they are created by the light. Pure silicon to which a trace of a fifth column element such as phosphorus has been added is n-type semiconductor. Each Phosphorous atom contributes one free electron, leaving behind the phosphorous atom bound to the crystal structure with a unit positive charge. Similarly pure silicon to which a trace amount of column three element such as Boron is added is a p-type semiconductor, when the electric current is carried by free holes. Each Boron atom contributes one hole, leaving behind the Boron atom with a unit negative charge.
The fixed charges at the interface due to the bound Boron and Phosphorous atoms create a permanent dipole charge layer with high electric field. When photons of light energy from the sun produce electron hole pairs near the junction, the build in electric field forces the hole to the p side and the electrons to the n side. This displacement of free charges result in a voltage difference between the two regions of the crystal, the p region being plus and n region minus. When load is connected at the terminals, an electron current flows. In short it can be stated that, Sun light striking on solar cells create electron – hole pairs in semiconductor materials. It induces an electric current and voltage across the solar cell junction. Enables direct conversion of sun light into electrical energy through semiconductors called SOLAR CELLS. About 0.5 volt and 0.6 watt energy is obtained out of a single silicon solar cell at 60 degree Celsius. Silicon solar cells assembled into panels and arrays to obtain higher voltage and power out put.
There are three types of solar panel available.
Originally, all panels were made from silicon slices cut from a large size crystal. A panel made in such way is termed as mono crystalline panel. Some manufacturers have switched to using silicon, which has been cast in blocks. These are called poly crystalline panels. These are cheaper to produce. Performance is a bit inferior in comparison to mono crystalline when they get hot.
The recent price breakthrough in solar panel manufacturing is amorphous silicon. These are less efficient than other panels. Production methods are less costly. For manufacturing silicon is spread over in thin layers on backing materials or directly on glass plates. The process also allow the amorphous material to be applied directly to stainless steel sheeting shaped like conventional roofing materials allowing solar panels to actually replace the roof – thus reducing the overall cost of the system.
Thin film technology
Thin film modules are made by depositing or vapor coating high purity semiconducting materials such as a-Si, CdTe, CIS, CIGS onto a substrate and then applying contacts. It takes much less energy and fewer raw materials to manufacture thin film modules, which makes them more environmentally friendly and cheaper to produce than crystalline modules. Other benefits of thin film technology are greater energy yields both at high temperatures and under diffuse light conditions. By exploiting potential efficiencies in the production process, continuing to improve the efficiency of existing technologies, and exploring new technological approaches, thin film technology is becoming increasingly competitive.
The newer thin film technology does not use technical silicon wafers; the functional layers are applied to a substrate - usually glass - via vacuum deposition. The processes involved are similar to those used in coating architectural glass or in the manufacture of flat screen televisions. The same basic design - front contact, absorber, and rear contact - is the same as the one used in crystalline technology, but the material and energy requirements of the manufacturing process are much lower. While the crystalline technology uses wafers of approximately 250µm, thin film technology absorbers are no more than a few micrometers thick. Thin film technology is gaining ground. Its market share is growing rapidly and currently stands at around 20 percent.
The cost of a solar power plant is not directly related to its surface area, but rather to the installed output in kilowatt-peak (kWp). The peak output of the installed solar module is expressed in kilowatt-peak. Peak output depends on the solar radiation and on the alignment and inclination angle of the system. In our latitudes, it is usually achieved at midday during the summer months and under clear skies. However, solar modules still produce electricity in the winter months or when the skies are cloudy. Because of the different efficiencies of the two technologies, thin film modules need a slightly larger surface area to produce the same output as crystalline modules. This is only of concern to owners or investors in cases when the available space is limited, or where space is a cost factor. Thin film modules require a comparatively smaller investment per kWp.
Since solar panels only work during daylight, and give best result in sunny periods some way is needed for providing electricity at night and cloudy days. Introduction of a storage device is essential. To cater this need storage batteries are utilized. For higher capacity of power production the mains electricity grid is used as a battery and the physical battery is eliminated.
Basing upon this principle today solar photovoltaic electricity is produced and met various requirements like light (Solar lanterns, Solar home lighting systems, Solar street lights., path finders, traffic lighting signal etc.) , water pumps, motive power, power for automobiles, management of telephone exchange, and energy requirement in space shuttle too. Recent trend in Solar PV is Building Integrated Photo-voltaic (BIPV) for residential and commercial buildings, which makes the building independent of grid-power.
Science and Technology are in the fickle wings of development. Efforts are on to reduce cost and enhance efficiency in respect of solar gadgets. As it can be seen technologies are evolving to harness the energy of sun in both thermal and photovoltaic routes. Because of lower efficiency levels the technologies are becoming material intensive and need more land for their installation. With the development of concentrating type PV cells and application of nano technology in PV cell manufacturing these shortcomings can soon be overcome. Similarly, development of high capacity storage batteries has also made significant strides in recent years. Solar PV cell manufacturing capacity in the country has also multiplied 5 times during last two years. Many more manufacturing companies with very high capacities are also entering the market.
Government aided projects like generation based incentive scheme for grid connected solar power plants, solar city program etc. are also in demand amongst developers and entrepreneurs.
Thus a total transformation to solar energy though still a little far away is not entirely un-achievable.
(*.Er.P.K.Mohanty is presenty working as Joint Director(Technical)
in Orissa Renewable Energy Development Agency, Under Science&TechnologyDepartment,Government of Orissa,INDIA. Tel.+919437138887 / +919040137898,