| Solar Enrgey Essay, Research Paper
THE SOLAR RESOURCE
SIMPLE APPLICATIONS OF SOLAR ENERGY
The Sun’s rays have long been used as the heat source for evaporating and distilling water. Solar evaporation has always been an important salt-production process. Salt water is pumped into shallow ponds that are open to the Sun. As the water evaporates, its salts form crystals that settle at the bottom and are eventually collected.
Producing drinkable water from brine is accomplished in a solar still, where salty water is evaporated. The salt becomes concentrated in the bottom of the still basin, while the water vapor rises, condenses on the still cover as fresh water, and is drawn off.
The solar cooking stove has become an important device in tropical countries where firewood is in short supply. The stove may be simply a hot box, an insulated container, perhaps with a mirrored cover that intensifies the Sun’s heat. More complex stoves have reflectors that focus sunlight directly on the cooking area, or they use separate heat-collector devices that transfer heat to water inside a steam cooker.
Solar heat is also used throughout the world to dry agricultural crops, fruits, and vegetables.
SOLAR ENERGY USE IN BUILDINGS
Both water and space heating are among the most successful small-scale applications of solar energy. Thousands of systems of these types have been installed throughout the world.
Solar Collectors
Solar collectors are devices that absorb solar energy and produce heat. They are mounted on the roofs of buildings or in other areas that are open to direct sunlight, and they are used for space heating and cooling and for heating water.
The flat-plate collector is made of a copper, aluminum, or steel heat-absorber plate, the surface of which is blackened to make the plate more efficient in absorbing solar heat. A heat-transfer liquid–usually a water-and-antifreeze solution–circulates through a set of tubes and removes heat from the plate. The tubes may be attached to the plate, or passageways for liquid or air may be incorporated into the plate itself, or the heat-transfer medium may simply flow across the surface of the plate.
To minimize convective and conductive heat losses into the atmosphere, two layers of glass or transparent plastic, separated by an air space, are placed above the plate. The cover layers also minimize reradiation from the collector. To reduce heat losses further, the back and sides of the collector are heavily insulated, as are all pipes and ducts leading to and from the heat-storage area.
Evacuated-tube collectors consist of two glass tubes, one within the other, with a vacuum between the tubes to minimize heat loss. Because the tubes are round and are backed with reflecting material, this type of collector can absorb more sunlight and has a significantly higher overall efficiency than the flat-plate collector. Evacuated-tube collectors are used in northern climates where light intensities are low.
Concentrating, or focusing, solar collectors focus the Sun’s rays on a tube (trough type), a point (dish type), or a concave mirror to provide higher temperatures for special purposes, such as industrial-process heat. Such collectors must be able to move both vertically and horizontally in order to track the Sun across the sky in all seasons.
Solar Water Heating
Solar water heating is an old and simple application of solar heat, and an inexpensive system for many buildings. The most common system consists of a collector located outside the building and tilted at an angle that favors uniform yearlong solar input. (The tilt angle is approximately equal to the local latitude.) In addition to the collector, there is a small fractional-horsepower pump for water circulation and a tank to store the heated water for later use. A simple controller that compares tank and collector temperature operates the pump. Whenever the collector is warmed to a temperature greater than that of the tank, the pump is turned on.
The size of collector needed can be approximately determined by the rule of thumb that states that the collection area, in sq ft, should be the same as the number of gallons of hot water needed per day. In the United States each resident in a home uses between 15 and 20 gal (57 to 76 liters) of hot water per day. A four-person family would therefore need 60 to 80 ft(2) (5.6 to 7.4 m(2)) of solar collector.
If poor weather reduces the amount of available sunlight, the solar system will produce no hot water. Then the conventional water-heating system will take over the task of providing domestic hot water.
An alternative to the pumped system is the “thermosiphon” system, where fluid circulation is produced by a density difference between hot fluid in the collector and cold fluid located above the collector in a tank. The lighter, warm fluid will tend to rise, causing cold fluid to replace it in the collector. The performance of these systems is good; the only problem is the need for the tank to be located in a position above the collector.
Active Solar Space Heating
Most of the heating energy used in residences is for space heating, that is, for providing the heat needed to maintain comfort within a building. Solar energy is a good match for this heating task because it is able to produce heat at temperatures close to those needed for heating buildings. In addition, the amount of solar energy falling on the roof of a properly oriented residence is roughly equivalent to what is needed to provide space heat.
A typical space heating system consists of a roof-mounted collector array whose tilt angle is equal to the local latitude plus 15 degrees, a heat-storage tank or bin, pumps or a fan, and a network of pipes or ducts through which the heat is sent.
Active systems use a liquid (again, the most common is a mixture of water and antifreeze) or air as the heat-transfer medium. Insulated channels carry the heat-transfer medium to the collector panels to absorb heat, and then to an insulated storage tank for water-based systems, or an insulated bin of a heat-retaining material such as rocks or pebbles for an air- based system. The heat is transferred to the storage medium, and the cooled heat-transfer medium is returned to the collectors.
An auxiliary heat source is used in periods when solar heat is not available. A control system operates the pumps or fans and the auxiliary heat source. Measures for reducing the need for conventional energy should include insulating and tightening a building before installing a solar system in it.
Passive Solar Space Heating
Passive systems avoid the use of mechanical components. The simplest passive system, the direct-gain system, involves larger-than-normal south-facing windows, with a massive floor slab that serves as the heat storage. This system is particularly effective in bringing up the temperature of a house quickly in the morning, but it can cause overheating problems in sunny climates if sufficient storage is not available.
The thermal storage wall avoids some of the shortcomings of the direct-gain system by interposing a thick concrete wall in the heated space next to the windows. The wall stores heat for night heating, while air circulation transports heat from the windows during the day.
In climates where heating is necessary, good architectural practice always includes some measure of passive solar heating, often with features of both of the approaches described here.
SOLAR ENERGY IN INDUSTRY
Temperatures sufficiently high to be of use in industry can be achieved using solar heat. In addition, the technology for converting solar radiation directly into electricity is proving more practical every year.
Solar Process Heat
In order to produce high temperatures, sunlight must be focused or concentrated via an optical focusing system. In one such system a parabolic “dish” focuses solar radiation onto an absorber, concentrating sunlight by a factor of over 100 to produce temperatures exceeding 538 degrees C (1,000 degrees F). If such high temperatures are not needed, smaller degrees of concentration can be obtained. Although not widely used, solar industrial-process heat systems have been effective in the food processing and other industries with moderate temperature needs.
Solar-Thermal Power Production
The high temperatures produced by concentrating solar collectors can be used to produce steam, which in turn can drive a turbine to produce electric power. A number of solar power plants have been built, although they are quite small relative to the normal fossil fuel or nuclear power plant. The technology has been shown to be reliable but more expensive, in most cases, than conventional technology.
A successful and economical 194-megawatt solar power plant has been constructed in the desert of southern California, however. Its collectors are curved, movable mirrors, each about 1.8 m (6 ft) high, that are mounted above the desert floor and motorized to track the Sun. Steel pipes circulate a special heat-transfer fluid that can reach temperatures of 391 degrees C (735 degrees F; considerably hotter than the temperature of the steam-producing water inside a nuclear plant). The fluid is used to boil water, and the steam that is made operates a conventional turbine. (See energy sources; power, generation and transmission of.)
Solar power plants are of particular interest to those utilities which have their maximum demand as a result of air- conditioning loads drawn off by homes and office buildings. Solar plants produce maximum output during sunny periods of the summer, just when this particular demand is greatest.
Photovoltaic Conversion
Photovoltaic cells, or solar cells, convert sunlight directly into electricity. The solar cell is a specially constructed semiconductor device fabricated from exceptionally pure silicon. Small portions of carefully selected impurities are added to produce a region where light energy breaks electron bonds, creating free charges. These charges migrate, producing current. The amount of current depends on the amount of solar radiation and the size of the cell. By connecting a number of cells in an array, any desired current and voltage level can be produced. (See photoelectric effect.)
The efficiency of solar cells is in the 12-15 percent range: that is, only 12-15 percent of the incident sunlight is actually utilized. However, cells with twice that efficiency have been tested, and significant gains in the efficiency of commercial cells are expected in the near future.
Single solar cells today power pocket calculators. Small arrays keep batteries charged, and they power irrigation pumps and refrigerators in areas where there is no commercial electricity. A solar power generating station near San Luis Obispo in California contains a solar-cell array that powers 2,300 homes.
OTHER FORMS OF SOLAR ENERGY
The energy applications discussed above all make direct use of solar energy as it strikes the Earth. There are also important indirect applications, where solar radiation is converted into other usable energy forms. For example, the Sun’s energy profoundly affects the world’s wind patterns, causes ocean water to evaporate as part of the hydrologic cycle, and is essential for plant growth. The hydrologic cycle makes hydroelectric power possible. Vegetation can be burned directly –for instance, as wood in a stove–or made into other forms of fuel in a process known as biomass conversion. The winds are used to turn windmills. Solar energy also makes it possible to harness ocean thermal energy, which uses the temperature difference between Sun-warmed surface water and cold water from the ocean depths to produce power. Geothermal energy taps the heat within the earth–either directly, by using naturally heated groundwater, as in Iceland, or by injecting water that is heated by hot interior rocks. The geothermal heat pump captures the solar heat retained by the earth. Water-filled coils of pipe are buried 1 m (3 ft) underground, and the heated water is passed to the pump, where the heat is compressed and distributed.
The utilization of the immense force of ocean tides to generate electricity is yet another form of solar energy (see tidal energy). By installing turbines that are powered by the inflow and outflow of tidal waters–within a narrow estuary, for example–this force can be made to produce large amounts of usable energy, at no environmental cost.
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