Renewable Sources Of Energy

RENEWABLE SOURCES OF ENERGY SOLAR ENERGY (PHOTOVOLTAIC CELLS) Presented by T.RAKESH III YEAR Mech Kamala Institute Of Tech. & Science Email id: [email protected]

ABSTRACT: With the increasing technological and scientific growth the solar energy is proving to be one of the formidable Renewable sources of Energy. Solar power s a synthesis of the established mechanical, chemical and electrical engineering design principles with a good induction of the theory of optics and engineering economics. To meet the ever-increasing load demands, the growth of power generation is a continuous process. The need of the hour is to evolve sustainable growth pattern in generation, transmission and distribution so as to have economical, efficient and ecofriendly power. Expert system approach for emergency control of power system is fast emerging as a vital tool. Photovoltaic cell is a way to tap this abundant source of solar energy. These devices convert sunlight directly into electricity using a method that differs fundamentally from the heat engines used in all other modes of electricity generation. The basic PV building block is the Photovoltaic cell, Referred to as a "cell", because it produces direct current (DC) electricity like a battery, In practical applications of Photovoltaics, groups of cells are joined together to form a module and modules may be connected into an array. These cells, modules, and arrays can provide electricity in any quantity, ranging from a few milli watts (mW) powering a calculator to several megawatts (MW), the size of a large power plant. Sunlight the fuel for photovoltaic; so the cost of manufacturing the cells is the main cost of producing electricity.

This paper mainly deals with the following topics photovoltaic:  Principle  Construction  Power generation by photovoltaic  Applications  Novel designs for improving efficiency of P.V. System

INTRODUCTION: The electricity requirements of world including India are increasing at a alarming rate and the power demand has been running ahead of supply. It’s also widely recognized that fossil fuels and other conventional resources presently being used for electricity generation may not be sufficient to keep pace with the everincreasing demand. In such an advent of energy crises, the world is forced to develop new and alternative methods of power generation. The Photo Voltaic power generation is one of the examples of unique methods of' power generation. The invention of the ‘solar car’ is one of the prominent examples for the development in the field of solar power generation. The P. V effect was first recorded by French physicist Edmund Becquerel, in 1839, when he noted the appearance of a voltage when illuminating two identical electrodes in a weak conducting solution. The first practical PV cells were made out of crystalline silicon in 1954 by Bell Laboratories in the United States.

PRINCIPLE: The Photovoltaic effect ,is a generation of emf as a result of the absorption of ionizing radiations. Energy conversion devices used to convert sunlight into electricity by Photo Voltaic effect are known as ‘Photo Voltaic cells or Solar cells’. Thus a solar cell is a transducer which converts the sun’s radiant energy directly to electricity and is basically a semiconductor capable of developing a voltage of 0.5 to 1

V and current densitv of 20- 40mA per cm square depending on the materials used and sunlight conditions. When the photons of sunlight arc absorbed in a semiconductor, they create free electrons with higher energies than electrons, which provide the bonding in the base crystal. Once these free electron hole pairs are created, there must be an electric field to induce these higher energy elections and holes to flow out of the semi-conductor to do useful work. It is know that an electric field exists across a p-n junction and this field sweeps the electrons in one direction and holes in other. Many types of solar cells have been proposed. The two types of' Solar cells available commercially are: Single crystal silicon cells  Cadmium sulphide / cuprous sulphide cells.

CONSTRUCTION: The solar cell is a silicon photo diode designed to produce electrical energy from sun light. A large area diode is formed by diffusion of boron from boric acid trichloride vapour into a slab of n-type silicon. The process is regulated to form a layer of p-type silicon on the surface with the p-n junction about 10exp( -4) cm below the surface. Because of thinness of p-type layer, sunlight falling on the surface is largely absorbed in or near the depletion layer at p-n junction, and the Photovoltaic effect is produced. The open circuit emf is said to be O.5V. When the external load is optimized for maximum power output. The photo emf is about O.3V and the energy conversion efficiency is of the order of 10%. The two semi-conductors are selenium and cuprous oxide, CU20, the later being p-type because of an excess of oxygen. With both selenium and cuprous oxide cells, when the external resistance is low the current is proportional to the rate at which energy is received i.c., the characteristic is linear. Increase in resistance not only reduces the current but also causes a departure from linearity. Two basic strategies are pursued for reducing system costs The first is to reduce the cost of photovoltaic modules by producing large areas of active materials at low cost. These are called "flat-plate systems".  The second strategy avoids using large areas of photosensitive materials and instead uses lenses or other optical devices to focus sunlight on a small area or active material. These are called "concentrator systems".

THE MATERIALS APPROACH: The most suitable materials for the solar cells are the Semi conductors; mainly, silicon. Considerable effort and energy has to be expended in growing this material with lowest defect concentration. The crystalline silicon solar cell was one of the first type to be developed and it is still the most common type in use.

Crystalline silicon is made by growing large cylindrical single crystals. called hollies. The boules are sliced into thin wafers, from which photovoltaic devices are made. Slicing is an expensive and material wasteful process. A less expensive material, polycrystalline silicon, bypasses the expensive and energy intensive crystal growth process. The molten silicon is instead cast directly into either cylindrical or rectangular ingots. Another approach to produce less expensive materials is to avoid most of the sawing all together. The first commercial success was the Edge Define Film fed growth (EFG) ribbon process in which polycrystalline silicon is grown by extracting the crystallizing silicon melt through a graphite die. By this technique thin ribbons of polycrystalline silicon can be grown either as multiple separate ribbons or as polygons of materials that can be separated into silicon blanks for lubrication into finished solar cells with minimal loss of malarial. The lowest cost approach would be to minimize the required amount of semi conducting material. Thin rums have been developed that are only a few micrometers thick. Such films are produced by a number of physical and chemical methods suited for automation. To day amorphous silicon (a-Si) has received the most attention. In recent years interest in thin film based on copper indiumdiselenide (CIS), cadmium telluride (CdTe) and other materials are growing. The CIS & CdTe thin films are not yet commercially available in large volumes but offer strong competition to amorphous silicon. Single junction a-Si cells of

11-12% efficiency were developed

at the Indian Association for the Cultivation of Science (lACS), Calcutta. High efficiency amorphous devices rely on multi junction cell designs. These cells are made by building several cells on top of each other, The light not absorbed by upper layers is captured by deeper layers. Typically the cells are designed so that each layer is sensitive to a different color of incident light. The layers can be made by combining silicon cells with cells made from other materials. Combined a-Si/CIS prototype has been constructed by mechanically stacking a-Si cells on top the CIS cells and has demonstrated an efficiency or 15.6%.

P.V. SYSTEM IN POWER GENERATION: With the increasing technological and scientific growth the solar energy is proving to be one of the formidable Renewable sources of Energy and photovoltaic cell is a way to tap this abundant source of free energy. This conversion of solar energy into electrical energy is done by the application of photovoltaic cells in the solar power plants on a large scale. According to estimations, the maximum possible output of a solar array is about 250 W/m sq. Thus 250 MW plant needs an array of one square km size. In spite of high costs studies, photovoltaic power generation schemes has been undertaken. The photovoltaic array produces dc power and this must be converted to ac power and feeding into the grid. The power conditioner consists of inverter, a power tracking device in order that the solar array can deliver maximum energy over a given, operating period and control circuitry. The output of the power conditioner can be fed to the local load or to the grid depending on the requirement. In case of low power availability from the photovoltaic generation the local load can be fed from the grid. Some from of energy storage is invariably used so that at times of excess generation the energy may be stored to be used at times of low generation. The regulation and dispatch unit regulates the flow of photovoltaic power systems into the grid and vice-versa. A solar power battery charger recently been developed in India. This charger can charge 12, 18& 24V nickel cadmium and lead acid batteries for operating mobile radio sets. The parts of a photovoltaic power generating system are as shown below:

The solar cells can be connected in series and parallel and incorporated in a module. Several modules may be interconnected to compromise a solar array. In principle array sizes at a few mega watt level are possible, the only limitation being the land requirement. The power output of a cell module or array is expressed in "watt peak" (wp), which is the power output at midday at clear sky sun. Practically this power output is defined when the sun light intensity is 0.1 w/cm2. The actual power output caries during the day depending on intensity of sunlight. It starts from zero at the time of rise reaches a peak value at mid day and decreases to zero at, sun set, When the direction of array is fixed the variation of power output during the day is considerable. The use of two-axis tracking enables almost constant output during the day. The efficiency of the P. V system can be improved by making the panels self orienting according to the position of the sun. The area under this curve is the energy output of the array during the day while 'W' indicates the constant load during the 24-hour period. The area under the W curve is the energy required by the load during the 24-hour period.

ENERGY CONVERSION SYSTEM: The energy conversion system ,converts heat energy to electrical energy. The fig. shows the basic elements of a distributed collector system solar power plant,

The solar heat from the receiver or storage converts feed water into steam which is fed to the turbine. The turbine drives the generator. Steam is condensed in the condenser and water returns to the boiler for reuse as feed water. The heat of the cooling water of the condenser may be used for subsidiary purposes.

SOLAR POWER TOWER: The solar power tower is used for conversion of solar energy to electrical energy on a large scale. This is done by a central receiver situated at the top of the tower which receives concentrated solar energy from a field of mirrors which stimulates a very large parabolic on ground. The solar energy collected is converted into electrical energy using the energy conversion system.

The complete system consists of: (1) Heliostat or collection sub-system (2) Receiver/Tower subsystem (3) Steam Turbine and Generator (4) Cooling tower for cooling the water used in condenser (5) Master control for controlling the whole system (6) Storage subsystem The use of more efficient Heliostats confirms better collection of the sun's rays. Further solar heat can be converted directly into electricity using thermionic and thermoelectric converters.

APPLICATIONS OF PHOTOVOLTAICS: Photovoltaic systems and power plants have emerged as viable power sources for applications such as lighting, water pumping and telecommunications and arc bring increasingly used for meeting electrical needs in remote village, hamlets etc. Solar photovoltaic (SPV), water pumping systems are technically proven and have potential of replacing diesel pumping systems, commonly used in un electrified locations for lilting water from shallow depths.

SOLAR LIGHTING: The solar lantern is one of the most popular applications. It is very effective in replacing the kerosene/hurricane lanterns being used in the remote and rural homes. A typical solar lantern has a 10 watt P. V. module, and a 7AH-12V sealed maintenance free battery and is designed to work about 3-4 hours a day. A solar lantern can save 100 liters of kerosene every year.

WATER PUMPING: Water pumping was one of the earliest applications of PV technology developed in India. Small pumping systems with a PV array) capacity of 300-360 W was developed. Later, motor pump sets with higher capacity using PV array upto 3000W were introduced.

GRID-CONNECTED DISTRIBUTED: PV systems are a relatively recent applicatiol1 where a PV system is installed to supply power to a building or other load that is also connected to the utility grid. These systems are increasingly integrated into the built environment and are likely to become commonplace. They are used to supply electricity to residential dwellings, commercial and industrial buildings, and are typically between 0.4 k Wand 100 k W in size.

NOVEL DESIGNS FOR IMPROVING EFFICIENCY OF PV SYSTEM Most of P. V research focuses on crystalline and film silicon, but efficiency of the P.V systems can be improved by undertaking the following strategies:

DYE SENSITIVE FILMS: Semiconductor material (titanium dioxide) can be covered within thin layer di sensitive film, which is an efficient light absorber and passes an energetic to the semiconductor material. The overall light to electric energy yield can be 7.1 % to 7.9% under stimulated solar light conditions.

SILICON SPHERES: Large sheets of photosensitive materials can be fabricated from arrays of small, low cost crystalline spheres. Col1centric spheres built up from relatively in expensive silicon raw materials can be used. The outer layer of the sphere is removed on one side of the sheet, exposing the interior of the spheres so that contact can be made. These measures can further reduce the effective cost of the cells.

CONCLUSION: Photovoltaic is the most promising form of renewable energy. It is pollution free and abundantly available even in space, and can be operated by diffused light. These traits give the P. V cells a promising future for the application in space power stations. The high price of solar cell modules is a barrier, causing the delay of large scale power application or P. V systems. A great effort has been exerted to achieve breakthroughs in developing and improving solar cells materials, cell structure and mass production processes of P. V Systems during the last ten years, Within the next 20 years, we shall sec P. V. systems employed everywhere in our daily life from power generation, to home lighting, to signaling, to transport and in flight.

REFERENCES: 1. Ken Zweibel, Harnessing Solar Power: The Photovoltaics Challenge, New York, Plenum Publishing, 1990. 2. Michal Brower, Cool Energy: The Renewable Solution to Global Warming. Union of Concerned Scientists, 1990. 3. Carl .J. Weinberg & Robert H. Williams, "Energy from the Sun" Scientific American, September 1990. 4. Kreider, J.F. Medium and High Temperature solar processes, Academic press Inc., Orlando, Florida, USA, 1979. 5. Garg, H.P. Treatse on Solar Energy , Vol. 1, John Wiley and Sons Ltd., Chischester, Sussex. PO 191 VD, 1982.