4.2.2 PHOTOVOLTAIC CELLS: -
Photovoltaic (PV) cells take advantage of the photovoltaic effect that occurs when a junction of two suitable materials, such as metal and a semiconductor or two opposite polarity semiconductors, is exposed to electromagnetic radiation. As a PV cell absorbs solar radiation, electrons are mobilized at the negative contact, and if a suitable circuit is made to the positive contact, an electrical current is generated. Several cells connected together and encapsulated for protection form a PV module. These are available in a variety of power outputs, typically rated from 20 to 100 W, though the exact power output varies directly with the intensity of solar radiation. PV modules produce electricity in DC format, so if alternating current (AC) electricity is required, the output must be converted with an inverter.
The large majority of modules used at present are based on cells made from wafers of high-grade silicon. These are comparatively efficient, typically converting between 10 and 15 per cent of the sunlight to electricity, but are labour and energy intensive to manufacture. The alternative to crystalline silicon is thin-film technology, so called because a very thin film of PV material is deposited on to a suitable substrate. This promises easier and less energy intensive mass production, which should in the longer-term help to reduce the cost of PV technology; but thin films are slightly less efficient solar energy converters than crystalline versions.
Arrays of PV modules have been routinely used as the main power source for space satellites since the late 1950s. PV systems are now being used increasingly in areas of the world where there is no mains electricity to provide power for lights, refrigerators, water-pumping equipment, and communications devices. Several countries, particularly Japan, Germany, and the United States, are now promoting the incorporation of PV into homes and offices as a means of generating electricity at the point of use in an environmentally acceptable manner.
Table 2. Photovoltaic Technology Installed in India as of July 1996
Technology Total Number Installed Average Estimated Cost*
Solar Lanterns 73,526 $113
Domestic Lighting Systems 36,517 $311
Street Lights 30,917 $621
Photovoltaic Pumps for Agriculture 1,318 NA
Community Television/Lighting Systems 921 NA
Village Power Plants 166 $11,300
*Average cost to install 1 kW. The aggregate capacity of the 166 installed plants is 909 kilowatts.NA = not available.
4.2.3 SOLAR FURNACES: -
One important high-temperature application of concentrators is in solar furnaces. The largest of these, located at Odeillo in the Pyrenees in France, uses 9,600 reflectors with a total area of approximately 1,860 sq m (20,000 sq ft) to produce temperatures as high as 4,000 C (7,200 F). Such furnaces are ideal for research requiring high temperatures and contaminant-free environmentsfor example, materials research.
4.2.4 Other Passive Applications: -
People have devised a number of ways to make direct use of the sun's energy. These uses include heating water, heating and cooling buildings, generating electricity, and cooking food.
Solar Heaters: Many people in warm climates heat water with simple, inexpensive batch heaters. A batch heater consists mainly of an insulated tank with several layers of clear glass covering the side of the tank that faces the sun. Manufacturers blacken the outside of the tank because black absorbs more sunlight than any other colour. The black surfaces convert the sunlight to heat and thus warm the water. The glass prevents most of the heat from escaping from the tank. The hot water rises to the top of the tank and flows from there directly to a tap.
Devices called flat-plate collectors are used to heat water and the air inside buildings. A flat-plate collector consists chiefly of an insulated box covered by one or more layers of clear glass or plastic. Inside the box is a plate of black metal or black plastic. The plate absorbs sunlight and converts it to heat, which becomes trapped under the glass. Air, water, or some other fluid circulates through tubes welded to the plate and absorbs heat from the plate. The heated fluid then flows to a heat exchanger, where it transfers its heat to water. The heated water is stored in a tank and is pumped from the tank to taps in the house.
Many buildings use passive solar energy systems for heating air. In most cases, these buildings have large windows facing the sun to trap heat. During the day, sunlight passes through the windows and heats walls and floors of stone or brick. At night, the walls and floors release heat. Additional heat may be stored by placing water or special phase-change materials inside the walls. These phase-change materials melt at about room temperature. As they melt, the materials store large amounts of heat. The materials later release the heat as they become solid again. In buildings with passive solar energy systems, special insulating shades or shutters help keep heat from escaping through the windows at night.
Solar air conditioning: Most solar air conditioning systems use solar collectors and special materials called desiccants that can absorb large amounts of water. The air conditioning process begins when fans force air from outdoors through a desiccant, which removes moisture from the air. The air then flows through a revolving wheel that acts as a heat exchanger and removes heat. Next, the air passes over a surface soaked with water. As the water comes into contact with the dry air, it evaporates, and absorbs more heat from the air. The cooled air then passes through the building. After the air leaves the building, the solar collectors reheat it. Blowing the reheated air through it dries out the desiccant, and the process begins again.