Chandigarh, Thursday, February 18, 1999
Chandigarh, Thursday, February 18, 1999 |
Photo-voltaics has come of age by Pravin Kumar The fading whimper by Satya Prakash Gupta |
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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 is immense. Photo-voltaics
has come of age IN countless sites in India, as well as abroad, electrical gadgets are working without being connected to electric power lines. On Bombay High in the Arabian Sea, for example, the ONGC’s unmanned platforms monitor the flow of oil by means of electronic equipment, though they are not connected to any power grid or generator. The electricity comes from arrays of solar photo-voltaic (SPV) cells. Kalyanpur village in Uttar Pradesh has 450 domestic lights, 50 street lights and 15 five HP water pumps-all powered by a 100 kilowatt SPV plant. A solar-powered boat, capable of carrying six persons and claimed to be the world’s largest such vessel, now ferries guests daily between the Old City Palace and the floating Lake Palace Hotel in Udaipur (Rajasthan). A 50 megawatt SPV plant is being set up in Rajasthan’s Thar desert, which will be the world’s biggest such plant. Today, SPV cells power watches, TVs and battery chargers, besides power sources like roofing tiles and large portable panels. Writing in Current Science journal, Ashok Parthasarathi of the Department of Science and Technology says: "The solar photo-voltaic market could now be truly said to have come of age, both in our country and the world over". 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 been from elements like silicon and germanium, which 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, an electric current could be induced in wafers of selenium 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 (SPV) systems. (Fig 1). Each individual solar cell will produce power at about 0.5 volt. Circular silicon solar cells have "conversion efficiencies", of 11 to 14 per cent in converting solar energy into electricity. The theoretical maximum efficiency is about 30 per cent. Higher conversion efficiencies will bring down the capital 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 (100 mm X 100 mm) per day in industrial plants. Modules A single solar generates too little power 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 an SPV module. An aggregation of SPV 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 levels 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 SPV systems is totally modular, that is, one can add to, or subtract from, the total generating capacity without significantly affecting the cost per unit installed capacity. Unlike any other source of electricity, SPV system can be tailored to individual needs. Solar PV systems are best used for those applications where electrical energy in needed only during daytime. Most farm operations in India — irrigation, harvesting, etc — are done in the daytime; hence, a solar PV system can conveniently be used. In a typical SPV system, the photo-voltaic panel generates DC electricity, which is used by day directly to operate a DC pump (Fig 2). However, the electricity generated during daytime can also be stored in a battery and drawn off the battery at night. If the load is an AC device, the DC power generated by the panels (or from the battery pack) is converted into AC power and used to power the load. The National Technology "missions now use 205 deep-well submersible pumps powered by SPV systems. Each pump can pumps 30,000 to 40,000 litres of water through a head of 5 to 6 metres on a bright day. The SPV pump is ideal for small power needs, because it is easy to operate, needs little maintenance and has a long life. The pump are supplied to villages under a soft loan/leasing scheme to offset the initial of the equipment. Manufacture Silicon for the solar cells is obtained from silicon oxides in igneous rock or from silica, the main constituent of sand. Many stages of refinement are necessary. Single crystals up to a few centimetres wide can be obtained by very gradual solidification of material on a small "seed" crystal mounted on a probe dipping into a crucible of molten silicon. The probe is very slowly withdrawn, bearing with it 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 needs for photo-voltaic cells. Growing single silicon crystals 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 cells. The conversion efficiency of polycrystalline silicon is about 4 per cent lower; however, production costs are cheaper by a factors of two. Also, the lower efficiency matters little in space and communications applications. The "amorphous" or uncrystallised form of silicon is even more economical for the production of solar cells, the energy needed for production being much less than for single-crystal silicon solar cells. However, amorphous silicon solar cells degrade when exposed to direct sunlight for long periods. Hence, they cannot be used for outdoor applications. Such cells, almost all produced in Japan, go into consumer devices like pocket calculators, electronic games and watches and for small nickel-cadmium battery charging. Other materials Future SPV cells may be based on gallium arsenide, which can withstand up to 1,000 times full sunlight. Large SPV devices may be based on copper indium diselenide, which has an efficiency of at least 8 per cent; smaller cells manufactured under extremely clean laboratory conditions have 14 per cent efficiency. Global Solar Energy, an Arizona-based company is reported to have started manufacturing 255,000 square feet of copper indium selenide modules annually — enough to produce 1.5 megawatts of electricity. In India, Central Electronics Limited (CEL) Sahibabad (UP) started developing solar cells and modules for terrestrial applications in 1975. Over 35,000 SPV power sources have been supplied to the Department of Tele-communications’ Single Channel VHF Radio Links as part of the National Mission for Rural Communications. Lightweight folding modules, which can be carried as backpacks by soldiers and used to power wireless radio sets, are now commercially manufactured by CEL and supplied to the Army. Bharat Heavy Electricals Ltd, Bangalore, has also entered the SPV scene. In Rajasthan, a state with abundant solar energy and relatively few fossil fuel resources, the Rajasthan Energy Development Agency has embarked upon a scheme to provide PV lighting systems. In Udaipur, Jaisalmer and Ganganagar districts, 552 villages which are not likely to receive conventional grid power during the next decade are to be electrified with SPV power packs. Two large SPV power plants are being set up, which will be later connected to the power grid to supply power. Economics Unlike conventional energy sources, like coal or oil, photo-voltaics uses sunshine, which is on tap only about 30 per cent of the time, and 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 then conventional systems in order to compete with them. However, SPV module costs have been steadily falling, primarily due to technological innovation, larger volumes of production and government support in various countries. In 1957, it cost $ 2,000 to produce a watt of solar PV power; by late 1974, this had dropped to $ 20; and in 1985, a watt peak of solar power cost $ 8.15. In industrialised countries, solar power has made great strides. In the US, solar cells have proved an increasingly popular alternative to diesel generators to power traffic safety devices. Mass-transit providers from Florida to California have saved money by combining solar lighting with advertising on shelters. Many water and natural gas companies protect their underground pipes with solar-powered cathodic anti-corrosion systems, which work on the principle that a small electric current, if properly applied, can slow down metal corrosion. There are now over 75
companies producing solar cells, modules and systems in
India. According to Drs S.P. Sukatme and J.K. Nayak of
the Indian Institute of Technology, Powai (Mumbai),
industrial production of SPV systems touched 7 megawatts
during 1995-96, compared to 0.12 magawatts in 1983. The
Government has been implementing a comprehensive
programme of research and development, demonstration,
commercialisation and utilisation over the last 15 years.
The Indian market and programme are the largest among the
developing countries and India has emerged as the second
largest manufacturer of PV modules based on crystalline
silicon technology. With 75,000 villages in India yet to
be electrified, the scope for solar photo-voltaics is
vast. |
The fading whimper THE universe is a dark and cold place, but it is not quite as dark and cold as one think. Quite apart from the light of stars and other celestial objects, the sky shines with a faint glow that appears to extend across all wavelengths. If one assumes that the glow is emitted by some thing, the temperature of some that thing would be 3 degree Kelvin (3 degree C about absolute zero). What is the glow’s origin? One of the most startling discoveries of this century, made after world War I by E.P. Hubble, is that the universe is expanding, that on the average the galaxies are rushing away from one another. This being the case, if one could run the cosmic movies backward, the galaxies would be rushing toward one another. Between 10 and 20 billion years ago they would coalesce; instead all the matter in the universe would coalesce. Not only all the matter would coalesce but also all the energy. The primordial universe would, therefore, be fantastically hot, to run the film forward again, the expansion of the primordial universe is the big bang. As the universe expanded the density of the energy in it was diluted, another way of saying that the temperature dropped. The 3-degree glow as the remnant of the primordial energy is the big bang’s fading whimper. If the diluted energy of the 3-degree glow were reconcentrated, the universe would be so hot that the leptons and quarks would in effect melt. What is more, gravity, the electroweak force and the strong force would blend into one another. At that time everything may have been in a unified state, and if one could mathematically describe that state, one might succeed in describing all matter and energy in a unified theory. As the primordial universe expands, first the three forces part company. Then the quarks and leptons freeze out, like so many snowflakes freezing out of water vapour. The time required for these processes is impressively short; gravity separates from the forces at 10.44 second. The electroweak force and the strong force separate at 10-36 second. The quarks freeze out at about the same time. Then the quarks combine in to heavier particles at 10-11 second. The leptons freeze out at 10-4 second. The term "freeze" is almost fanciful. At the time the quarks freeze out the temperature is 10-14 degrees Kelvin, and at the time the leptons freeze out it is 10-12 degrees. The universe is still far too hot the formation of atoms; that doesn’t happen until it cools to 3000 degrees after 300000 years. Then the universe is filled with a gas, and it becomes possible for stars and galaxies to materialise. The writer is from
the physics department, Kurukshetra University. |
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THE house occupied by Dr Susan Road, an architect with Oxford Brookes University (UK) generates enough power for her family of three and also exports about 1000 watts a year to the National Grid. The energy is mainly generated by the 5 sq metres photo-voltaic roof which is a pioneering example of solar power that is being increasingly used in homes and offices throughout the UK. The entire back-roof is made up of glass solar panels, some of which generate electricity, some heat water and some let in sunlight to heat the thermal mass of the house (Fig3). The 48 solar PV modules provide 4 kilowatt (peak) of electricity and are linked via an AC/DC inverted to the domestic warring load, producing electricity, both for the house and to charge the family car which is plugged in overnight. Any shortfall from the solar array is made up automatically from the supply connection. During the first nine months of its operation in 1995, the house has exported electricity at an average rate of 141 kwh per month. One of the most attractive features of roof-integrated PV systems is that PV modules can be used in place of a conventional roof, resulting in savings in material. The saving on electricity per annum is estimated to be £ 146.4. The cost of the PV system was £ 21,150. At today’s prices, the returns on the investment are poor. However, the costs of installation are falling at about 20 per cent per year. On-going improvements in materials, efficiency and scale of production should see further decline in costs. The Oxford Solar House
is used as a working example of photo-voltaics and other
technologies in a truly domestic situation and is used
for practical instruction of architecture students at
Oxford Brookes University. |
Walls made of solar cells Solar cells are also doing duty as building materials. Instead of having bulky rooftop arrays for tapping the Sun’s energy, the photo-voltaic cells can be directly embedded into walls and roofs. The idea had not many converts when Steven Strong, a power station engineer, started Solar Design Associates, a company for making energy-efficient houses, in Boston in the early 1970s. Twenty years on, contractors and house-owners are taking to Strong’s pioneering concept because, for one thing, solar cells cost just one-third of what they did in 1980. In Texas city of Austin, an electric utility foots the bill for rooftop systems on dozens of houses, because when the buildings start generating surplus electricity, the company can draw on the pool.
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Digital phone for the deaf British researchers have unveiled a videotelephone system that can interpret sign language of deaf people and help them communicate easily. Using the system, developed by researchers at the Royal National Institute for the Deaf (RNID), a person at the other end can be seen and the deaf user can be connected to an interpreter, reports Spectrum. Even though videotelephony has been available for quite some time, it is only now that digital technology driven by powerful computers has made it possible to compress enormous amount of picture and sound data down Integrated Services Digital Network (ISDN) telephone lines to provide clearer conversation in real time. Called ‘Typetalk’, the system helps deaf people to communicate through 250 operators whom they contact. Quick detection of salmonella In a discovery that could save lives, scientists at the US Agriculture Department have developed a technique to rapidly detect a deadly strain of Salmonella bacteria. "Quickly identifying outbreaks of this bacteria is vital to protect public health," Reuter quoted US agriculture secretary Dan Glickman statement. This important research cuts identification time from six weeks to two hours". The strain of Salmonella, DT104, is especially dangerous because it is resistant to many antibiotics. Researchers were able to find the strain’s gene sequence allowing scientists to quickly identify the bacteria. Glickman said, the US government hopes to develop test kits that could detect the strain in blood samples or in run off water. Salmonella has been linked to as many as 3.8 million illnesses annually in the United States and was the second-most common bacteria found in food poisoning cases in 1997, according to a Centres for Diseases Control (CDC), report. Solmonella is usually transmitted in raw or under-cooked eggs, poultry and meat and in raw milk and dairy products. Symptoms include stomach pain, diarrhoea, nausea, chills, fever and headaches. The bacteria is estimated to cause losses worth billions of dollars in productivity and due to increased medical expenses in the United States every year. Explosives detector It’s more sensitive than an X-ray scanner, has a better nose than most sniffer dogs and its inventors say, it can detect a pinhead of explosives in a football stadium. Physicists at the University of Leeds in northern England said the newly discovered prismscope, a device that uses lasers to detect hidden explosives, could revolutionise air safety and anti-terrorism measures and take the fear out of flying, reports Reuter. The suitcase-sized instrument uses lasers which scatter light at different wavelengths. The change in wavelength gives a frequency for the vibration of a particular molecule. "These frequencies give you a fingerprint by which you can identify the molecule," David Batchelder, a member of the team that developed the device, said. The developers said, it is cheaper and more powerful than any explosives detector now available and is so sensitive that it should even be able to detect Semtex, the explosive preferred by many extremists. "The system is so sensitive that, under the right conditions, you could detect a single molecule," said Batchelder. The researchers have been working on the project for two years. The device is designed to be used in enclosed areas such as large shipping containers that are inaccessible to sniffer dogs and impossible to X-ray. A gas sample, or vapour,
is sucked into the instrument and the distinctive type of
explosive is picked out by using beams of laser light. |
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