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| SCIENCE TRIBUNE | Thursday, January 13, 2000, Chandigarh, India |
Harnessing the power of the sun by Pravin Kumar Solar photo-voltaic cells are ideal for developing countries where many remote areas are yet to be electrified. They are a non-polluting source of power for a wide range of applications, from space satellites to oil-well platforms. With 75,000 villages yet to be electrified in India, the market and scope for solar power are immense. The photo-electric effect |
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Harnessing the power of the sun Solar photo-voltaic cells are ideal for developing countries where many remote areas are yet to be electrified. They are a non-polluting source of power for a wide range of applications, from space satellites to oil-well platforms. With 75,000 villages yet to be electrified in India, the market and scope for solar power are immense.
WE normally think of electricity as something that flows from a pair of wires or from a pencil cell. But, 93 million miles away from us, there is a boundless source of electricity that we are only now beginning to tap the sun. Ultimately, of course, the sun is the source of all energy except nuclear energy. Every square metre of the earth’s surface receives solar energy varying from four to seven kilowatt hours, depending on the location and the season. India receives solar energy equivalent to over 5,000 KW hours per year, which is many times the total energy consumed, in the country. Most parts of the country have 200 to 300 sunny days in the year. There are two direct ways of harnessing the sun’s energy. The first way is the thermal route, that is, using the heat of the sun for heating, cooling, water purification and power generation. The second method is the photo-voltaic route, using solar cells for converting sunlight into electricity to operate lights, pumps, refrigerators and telecommunication devices. Solar cells make use of the photo-voltaic effect. This effect can be observed in almost any junction of metals that have different electrical properties, but the best performance so far has been from elements like silicon and germanium; these conduct electricity indifferently and are, therefore called semiconductors. In the 1940s scientists at the Bell Telephone Laboratories in the USA found that, by adding the right amounts of boron and arsenic to wafers of selenium, an electric current could be induced in the presence of sunlight. The first silicon solar cell was made by G.L. Pearson and his group at the Bell Labs. It is the Pearson Cell that is the basis of modern solar photo-voltaic systems. Today, in countless sites in India, as well as abroad, silicon solar cells provide power for electrical gadgets far removed from electric power lines. For instance, on Bombay High in the Arabian Sea, the Oil and Natural Gas Commission’s unmanned platforms monitor the flow of oil by means of electronic equipment powered by arrays of solar photo-voltaic cells. The village of Kalyan in Uttar Pradesh uses a 100 kilowatt solar photo-voltaic plant to operate domestic lights, street lights and water pumps. Solar cells also power watches, television sets and space vehicles, besides operating power sources like roofing tiles and large portable panels. Writing in the journal Current Science, Ashok Parthasarathi of the Department of Science and Technology, says: “The solar photovoltaic market could now be truly said to have come of age, both in our country and the world over.” In India, solar photo-voltaics got off when Central Electronics Limited, in Sahibabad (Uttar Pradesh) began the development of solar cells and modules for terrestrial applications in 1975. In 1984, the first foldable module was supplied to a Mount Everest expedition. Foldable modules which can be carried as backpacks by soldiers, and used to power wireless sets, are now supplied to the Indian Army. Silicon for solar cells is obtained from silicon oxides in rocks or from silicon, the main constituent of sand. However, many stages of refinement are necessary. Single silicon crystals up to a few centimetres wide can be obtained by dipping into a crucible of molten silicon, a probe carrying a small “seed” crystal. When the probe is withdrawn, it bears a growing column of material with the perfectly regular structure of a single crystal. Very thin slices of this crystal are cut by sawing to obtain the wafers needed for photovoltaic cells. Growing single crystal silicon is a very slow and expensive process. An alternative is “polycrystalline silicon”. This is obtained by controlling the rate of cooling, so that a slice used for a photo-cell may contain a small number of boundaries between adjacent walls. Polycrystalline silicon converts solar energy into electricity with an efficiency about 4 per cent lower than the 12 per cent conversion efficiency of single-crystal silicon. However, production costs are lower by a factor of two. Also, the lower conversion efficiency matters little in space and communication applications. Even more economical is the “amorphous” or uncrystallized form of silicon, for the energy needed for its production is much less than for single-crystal silicon solar cells. However, amorphous silicon solar cells degrade when exposed to direct sunlight for long periods. Hence, such cells — almost all produced in Japan — are used in indoor applications like pocket calculators, electronic games and watches and in small nickel-cadmium battery chargers. Future solar cells may be based on a compound called gallium arsenide, which can withstand up to 1000 times full sunlight. Solar cells which can be fitted almost anywhere have been developed from a new material, copper indium disselenide, at the University of Florida (USA). Two or three microns thick—which is thinner than a human hair—these cells are made by placing an extremely thin film of the material on to base surface such as plastic or glass. This film can thus be moulded to fit the surface of any article during production. Overall, these copper indium disslenide cells are a hundred times lighter and thinner than ordinary silicon solar cells. Shang Li, a researcher working with the project, says that the efficiency of the cells in converting sunlight into electricity is eight to ten per cent, ‘which is not too bad’, as he puts it. If they get the manufacturing process right, they hope to approach 18 per cent, which will make the thin-film cells comparable with existing solar cells. Circular silicon solar cells have 11 to 14 per cent efficiency in converting solar energy into electricity. The theoretical maximum efficiency is about 30 per cent. High conversion efficiency will bring down the capital cost of solar photo-voltaic power. A single solar cell generates about 0.5 volt, which is too little to run most electrical appliances. Hence, a number of solar cells are connected in series to increase the generated voltage and power. The combinations, suitably connected in parallel and sealed hermetically with a transparent front glass for weather protection, constitute a solar photovoltaic module. An aggregation of such modules mounted on a metal frame forms a panel, and an assembly of panels forms an array. A module made up of 36 four-inch square cells will give a power output of 45 to 50 watts. The power and voltage of arrays may range from tens of watts to several thousands of watts, and from a few volts to thousands of volts. Electricity generated from solar photovoltaic systems is totally modular, that is, you can add to, or subtract from the total generating capacity without significantly affecting the cost per-unit installed capacity. Unlike other sources of electricity, solar photovoltaics can be tailored to individual needs. The output of a solar array may be enhanced by using a tracking system such that the array faces the sun continuously. Solar cell efficiency can be improved by employing low-cost optical concentrators which reduce the area of the solar cells required. Fresnel lenses have been advantageously used in concentrators; they are much thinner and lighter than conventional lenses of the same diameter and focal length. Unlike a thermal power plant, the output of a solar photo-voltaic system will vary according to the season, the time of day and the geographical location. However, the performance can be predicted. Using a FORTRAN-77 package and the weather data of the past 20 years, the Energy Research Centre, Thiruvananthapuram, has been able to predict the performance of solar photo-voltaic modules. Photo-voltaic systems are best used for those applications requiring electrical energy only during daytime, for example, farm operations in India. In a typical solar photovoltaic system, the solar panel generates DC electricity, which is used by day directly to operate a DC pump. The electricity generated during daytime can also be stored in a battery and drawn off the battery at night. The National Technology Missions now use 205 deep-well submersible pumps powered by photo-voltaic systems. The solar photo-voltaic pump is ideal for small power needs because it is easy to operate, needs little maintenance and has a long life. The pumps are supplied to villages under a soft loan or leasing scheme to offset the high initial cost of the equipment. Coming to the economics of sun-power, we have to remember that, unlike conventional energy sources, photo-voltaic uses sunshine which is available only about 30 per cent of the time in a day; this is converted into electricity at only about 12 per cent efficiency. Hence, solar cell power will have to be at least three times cheaper than conventional systems in order to compete with them. However, solar photo-voltaic modules have been becoming steadily cheaper, primarily due to technological refinements, large volume of production and government support in various countries. In 1957, it cost $ 2,000 to produce a watt of solar power; by late 1974, this had dropped to $ 20, and in 1985, to $ 8.15. A watt now costs about $ 5.8. Further drops are expected, due to higher conversion efficiencies and optimally engineered systems, leading to market expansion. The comparison with conventional energy sources is somewhat unfair to solar photo-voltaics. William W. Orr, an American solar energy expert, points out that fossil or nuclear energy systems have “hidden” costs, which include air pollution, nuclear waste disposal and the military costs of safeguarding oil outflows in areas like West Asia. One great advantage of solar power is that it is completely non-polluting. Even in advanced countries, solar — power has made great strides. In the USA, solar cells have proved an increasingly popular alternative to diesel generators to power traffic safety devices. In San Juan County (Utah, USA) solar power is projected to power an unmanned airport. Photo-voltaic modules can even be used in place of a conventional roof, resulting in savings in constructional material. In Oxford (U.K.) Dr Sue Roaf, an architect, has built a house with a 5 square-metre photo-voltaic roof which generates enough power for her family of three, (that is, about 3,500 KW of gas and electricity a year) and in addition exports about 1,000 KW to the National Grid. In areas like Rajasthan, with relatively few fuel resources and hydropower potential fully tapped, there is great scope for tapping the abundant solar energy. The Rajasthan Energy Development Agency is setting up two large solar power plants, which will be connected to the grid; In Udaipur, Jaisalmer and Ganganagar districts, 552 villages which are not likely to receive conventional grid power during the next five to 10 years are to be electrified with solar photovoltaic packs. There are now over 75
companies in India which produce solar cells, modules and
systems. Industrial production of photo-voltaic systems
is reported to have touched 7 megawatts in 1995-96,
compared government has been implementing a comprehensive
programme covering research and development,
demonstration and utilisation during the last 15 years.
India has now emerged as the second largest manufacturer
of photo-voltaic modules based on crystalline silicon
technology. With 75,000 villages in India yet to be
electrified, the scope for solar photo-voltaic technology
is vast. |
The photo-electric effect To understand the photo-voltaic effect, let us look at the atomic makeup of silicon and germanium. The crystals are formed by an ordered bonding of individual atoms, due to “valence” electrons which pair up with valence electrons of adjacent atoms to form “shared pair” bonds. Each atom contains four valence electrons, all of which are used to form crystal bonds. Now, if an atom with three valence electrons (like boron) is added to silicon (or germanium), it will bond with an adjacent atom of silicon (or germanium), but will result in an unsatisfied bond with one of the valence electrons of the host material. This results in electrical “holes”, and gives rise to a “p-type” semiconductor. On the other hand, if we add a material such as phosphorous (or arsenic), which has five valence electrons, this would result in an extra electron that is not required in the bond structure. We now have a ‘n-type’ semiconductor. If p- and n-type silicon wafers are joined in a manner called a p-n junction, and placed in sunlight, the absorbed energy causes electrons to move to the junction and an electric current to move in an external circuit connecting the two wafers. Both holes and electrons are free to move. Each individual solar cell will produce power at about 0.5 volt, with the current being directly proportional to the cell’s area. Circular silicon solar cells have ‘conversion efficiencies’ of 11 to 14 per cent in converting solar energy into electricity. In recent years,
hand-made cells in the laboratory, using hyper-pure
silicon wafers, have shown efficiencies as high as 24 per
cent for small-size cells (20mmx20mm). The theoretical
maximum conversion for a n-p junction cell is about 30
per cent. Higher conversion efficiencies will bring down
the cost of SPV power. Hence, researchers all over the
world are attempting to achieve these higher efficiencies
as average efficiencies of thousands of large-area cells
(100mmx100mm) per day in industrial plants. — P.K. |
Cybersurfing
with Amar Chandel Nearly all English dictionaries are currently online. So, one can search for a word in a jiffy. But if your queries are about style, grammar and usage, you get stuck at times. In such situations one website that comes in handy is www.dictionary.com . You can ask DoctorDictionary on any of these subjects. Then there are such lifesavers as Roget’s Thesaurus, Strunk’s Elements of Style giving usage and style in a nutshell, and also fun stuff like Word of the Day, Fun and Games, Crossword puzzles and more, which are updated daily. There is even a section on writing for the Internet. *** There was a time when the Internet did not boast of too many India-specific utilitarian websites. Things are changing, and fast. Given the unemployment problem, one channel which is attracting a lot of visits is www.naukri.com, because it offers a number of services under one roof. One can search resumes, find out vacancies, input a vacancy online and receive list of relevant jobs by e-mail. Students can know admission deadlines, career information and access the database on scholarships for Indians. There are also tips on GD and interviews. There is also a long list of agencies which help one go abroad. *** Which is the most popular column of a newspaper? There is a tossup between the weather forecast and the cartoon strip. And it is not only the young ones who rush to the cartoon page first. For all of them, one treasure trove is www.unitedmedia.com. There are a large number of toon characters to choose from. Among them are Adam, BC, Fat Cats, Garfield, Heathcliff, Wizard of Id, Ziggy and Peanuts. Once you have made your choice, you can go to their archives and spend hours there, provided you don’t mind the rather poor quality of the line drawings when seen on a monitor. Give me my daily newspaper any day. Cartoons can also be accessed at www.uexpress.com. *** Like cartoons, there are very many joke sites as well. But one private site which is replenished on a regular basis is www.rajiv.com. Rajiv Pant has a large number of jokes in his repertoire and posts them constantly. These are divided into various sections like film jokes, Ajit jokes, Gujju jokes etc. Some of them are indeed funny. Visuals are minimal but you don’t really miss them. Reading the address
www.boriyat.com, I had thought that it would be a website
on humour. But it is a sort of directory for people in
Delhi and tells them where they can watch which film,
learn music or dance, yoga or dine out. There is also
information on college campuses, plus astrological
predictions. |
Science
Quiz 1. This scientist demonstrated and explained the relationship between electricity and magnetism for the first time. A unit of magnetic field strength is named after him. Name this Danish physicist, who was also the first scientist to isolate pure metallic aluminium. 2. During the repair of Hubble Space Telescope recently, the US astronaut Steven Smith had to float freely in space to carry out the requisite repairs. What is such an activity called? 3. This disease, found among the elderly people, results in memory loss, difficulty in performing simple jobs, sudden changes in behaviour etc. Which disease are we talking about? Who discovered it? 4. Oil rises in the wick of a lamp, blotting paper soaks ink and fields are ploughed so that ground water can rise in the porous soil due to this action. Which action are we talking about? 5. This rare metal has the lowest ionisation potential of all the elements and hence used commercially in photoelectric cells. Name this silver-white metal, whose one isotope is used in treatment of cancer. 6. What name is given to activities like intentional introduction of viruses on a computer system, wrongly controlling, damaging, concealing or blocking access to any computer, data base software or computer network? 7. What is common between “Mahatma Gandhi”, “Blue Chip”, “Birbal Sahani”, “Basanti” and “Flirt”? 8. A telescope fixed on the earth has to be tilted slightly in order to observe a star continuously through the telescope. State two reasons due to which the telescope has to be inclined. 9. What name is given to a series of organisms, with each organism consuming the previous member of the series? 10. Where was the 87th Indian Science Congress held recently? What was the theme of this Congress? Answers |
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Better wireless
access Currently there is no solution to cope with all the issues involved while accessing a massively diverse amount of data over a wireless connection. This is particularly true for devices with limited processing capacity and resources such as cell phones, palm pilots, pagers or other hand-held devices. All Hurson and James B. Lim from the department of computer science and engineering, University of Pennsylvania, have proposed a new concept called mobile data access system (MDAS) that can tap heterogeneous data source—stock quotes, news, airline information, weather and the internet—through both wired and wireless connections. MDAS works by superimposing a multidatabase system on a wireless-mobile environment. Multidatabase gives users integrated access to multiple database with a single query. The MDAS acts like a super-search engine that not only locates information from multiple sources but also combines and integrates the information. For example, a single query to a multidatabase could not only produce a list of bed and breakfasts in a college, but also tell their rankings in two different on-line travel guides—a task that would require accessing at least three databases and integrating the resulting information. MDAS also provides an efficient way to manage the query traffic so that gridlock does not occur when more than one user wants to access the same database, according to a release from the university. Fertility after
menopause The procedure was performed in February on a 30-year-old woman in the hopes of reversing symptoms of early menopause brought on by the removal of her ovaries. Four months after the implant, she ovulated and had a normal menstrual cycle. Kutluk Oktay, who performed the operation at New York Methodist Hospital, says the surgery gives new hope for young infertile women, especially those who had cancer. He has stressed that the procedure should be reserved for women with premature menopause triggered by serious illnesses — and not be considered a panacea for otherwise healthy women who want to have children later in life. “It’s premature to expand the technology for use that way. It may come, like all these other developments in the fertility field. Gem-quality
diamonds The science of making gem-quality diamonds is known since the 1960s, but the machines were huge and the cost exceeded that of mining natural diamonds. About the size of a washing machine, the device starts with a carbon source and a tiny piece of a real diamond called a “seed”. The machine squeezes the seed with increasingly high pressure topping out at 850,000 pounds per square inch. Other equipment heat the core to 2,000 to 3,000 degrees Fahrenheit. The high pressure and temperatures transform the seed into a bigger diamond. The machines require very little electricity and are not expensive to build, but the Russian researchers were unable to make them consistently to produce diamonds of the same colour or quality. Assistance came from researchers of University of Florida in the U.S.A. who helped the Russians control the processing parameters like heat and pressure and modify them to get different results, according to a release from the university. Book
conservation using plasma A group of researchers comprising microbiologists and plasma chemists at Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB), Stuttgart, are collaborating with book restorers on the development of processes that would help prevent deterioration of precious books and remedy existing damage. The team intends to develop plasma technology for this purpose, reports Fraunhofer Gesellschaft Research News. |
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