Solar Power
The Basics
Solar power is the technology of obtaining usable energy from the light of the Sun. Solar energy has been used in many traditional technologies for centuries and has come into widespread use where other power supplies are absent, such as in remote locations and in space.
Solar energy is currently used in a number of applications:
- Heat (hot water, building heat, cooking)
- Electricity generation (photovoltaics, heat engines)
- Desalination of seawater.
Energy from the Sun
Theoretical annual mean insolation, at the top of Earth's atmosphere (top) and at the surface on a horizontal square meter.
Map of global solar energy resources. The colors show the average available solar energy on the surface during 1991 to 1993. The scale is in watts per square meter. For comparison, the dark disks represent the land area required to supply the primary energy demand in the year 2010, using currently available technology.Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2). The first map shows how the solar energy varies in different latitudes.
While traveling through the atmosphere 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed resulting in a peak irradiance at the equator of 1,020 W/m2. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. Atmospheric conditions not only reduce the quantity of insolation reaching the earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum.
The second map shows the average global irradiance calculated from satellite data collected from 1991 to 1993. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m2 (3 to 9 kWh/m2/day). This represents the available power, and not the delivered power. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m2 or 0.45 - 1.35 (kW·h/m2)/day.
The dark disks in the third map on the right are an example of the land areas that, if covered with 8% efficient solar panels, would produce slightly more energy in the form of electricity than the total world primary energy supply in 2003. While average insolation and power offer insight into solar power's potential on a regional scale, locally relevant conditions are also important to the potential of a specific site.
After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and Infrared radiations. Plants use solar energy to create chemical energy through photosynthesis. Humans regularly use this energy burning wood or fossil fuels, or when simply eating the plants.
A recent concern is global dimming, an effect of pollution that is allowing less sunlight to reach the Earth's surface. It is intricately linked with pollution particles and global warming, and it is mostly of concern for issues of global climate change, but is also of concern to proponents of solar power because of the existing and potential future decreases in available solar energy. The order of magnitude is about 4% less solar energy available at sea level over the timeframe 1961-90, mostly from increased reflection from clouds back into outer space.
Solar design in architecture
Solar design in architecture involves the use of appropriate solar technologies to maintain a building's environment at a comfortable temperature through the sun's daily and annual cycles. It may do this by storing solar energy as heat in the walls of a building, which then acts to heat the building at night. Another approach is to keep the interior cool during a hot day by designing in natural convection through the building's interior.
Solar heating systems
Solar hot water systems use sunlight to heat water. They may be used to heat domestic hot water or for space heating. These systems are basically composed of solar thermal collectors and a storage tank. The three basic classifications of solar water heaters are:
Active systems which use pumps to circulate water or a heat transfer fluid.
Passive systems which circulate water or a heat transfer fluid by natural circulation. These are also called thermosiphon systems.
Batch systems using a tank directly heated by sunlight.
A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing wall. Sunlight heats the air space during the day causing natural circulation through vents at the top and bottom of the wall and storing heat in the thermal mass. During the evening the Trombe wall radiates stored heat.
A transpired collector is an active solar heating and ventilation system consisting of a perforated sun-facing wall which acts as a solar thermal collector. The collector pre-heats air as it is drawn into the building's ventilation system through the perforations. These systems are inexpensive and commercial models have achieved efficiencies above 70%. Most systems pay for themselves within 4-8 years.
Solar cooking
Solar Cookers use sunshine as an alternative to fire for cooking.A solar box cooker traps the sun's energy in an insulated box; such boxes have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner breathing environment for the cooks.
The first known western solar oven is attributed to Horace de Saussure in 1767, which impressed Sir John Herschel enough to build one for cooking meals on his astronomical expedition to the Cape of Good Hope in Africa in 1830. Today, there are many different designs in use around the world.
Solar lighting
Solar lighting or daylighting is the use of natural light to provide illumination. Daylighting offsets energy use in electric lighting systems and reduces the cooling load on HVAC systems (this assumes that daylighting is replacing incandescent lighting, which produces more heat than light). The use of natural light also offers physiological and psychological benefits, although this is difficult to quantify.
Daylighting features include building orientation, window orientation, exterior shading, sawtooth roofs, clerestory windows, light shelves, skylights and light tubes. These features may be incorporated in existing structures but are most effective when integrated in a solar design package which accounts for factors such as glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of sustainable design.
Daylight saving time (DST) can be seen as a method of utilising solar energy by matching available sunlight to the hours of the day in which it is most useful. DST energy savings have been estimated to reduce total electricity use in California by 0.5% (3400 MW·h) and peak electricity use by 3% (1000 MW).
Photovoltaics
Solar cells, also referred to as photovoltaic cells, are devices or banks of devices that use the photovoltaic effect of semiconductors to generate electricity directly from sunlight. Until recently, their use has been limited because of high manufacturing costs. One cost effective use has been in very low-power devices such as calculators with LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third use has been in powering orbiting satellites and spacecraft.
Total peak power of installed PV is around 1700 MW as of the end of 2005.[citation needed] This is only one part of solar-generated electric power.
Declining manufacturing costs (dropping at 3 to 5% a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array declined from $7.50 to $4 per watt between 1990 and 2005. With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - those systems that use an inverter to connect to the utility grid instead of relying on batteries - now make up the largest part of the market.
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This Information was taken from Wikipedia under the terms of the GNU Free Documentation Licence.
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