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About Solar Energy

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:

Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2).[1] 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).[4] 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


Types of technologies:

Many technologies have been developed to make use of solar radiation. Some of these technologies make direct use of the solar energy (e.g. to provide light, heat, etc.), while others produce electricity.



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, for space heating or to heat swimming pools. These systems are composed of solar thermal collectors, a storage tank and a circulation loop. The three basic classifications of solar water heaters are:
Batch systems which consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs, however; the exposed tank can be vulnerable to cooldown.
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.

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:

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 directly offsets energy use in electric lighting systems and indirectly offsets energy use through a reduction in cooling load.Although difficult to quantify, the use of natural light also offers physiological and psychological benefits.
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 MWh) 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 1,700 MW as of the end of 2005. 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.

In 2003, worldwide production of solar cells increased by 32%. Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%.2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.Analysts have predicted similar supply problems for 2006 and 2007


Solar thermal electric power plants:

Solar thermal energy can be focused on a heat exchanger, and converted in a heat engine to produce electric power or applied to other industrial processes.


Solar pond:

A solar pond is simply a pool of water which collects and stores solar energy. It contains layers of salt solutions with increasing concentration (and therefore density) to a certain depth, below which the solution has a uniform high salt concentration. It is a relatively low-tech, low-cost approach to harvesting solar energy. The principle is to fill a pond with 3 layers of water:
A top layer with a low salt content.
An intermediate insulating layer with a salt gradient, which sets up a density gradient that prevents heat exchange by natural convection in the water.
A bottom layer with a high salt content which reaches a temperature approaching 90 degrees Celsius.
The layers have different densities due to their different salt content, and this prevents the development of convection currents which would otherwise transfer the heat to the surface and then to the air above. The heat trapped in the salty bottom layer can be used for heating of buildings, industrial processes, generating electricity or other purposes. One such system is in use at Bhuj, Gujarat, India and another at the University of Texas El Paso.


Solar chemical:

Solar chemical is any process that harnesses solar energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis in plants but without using living organisms. No practical process has yet emerged.

A promising approach is to use focused sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.

While metals, such as zinc, have been shown to drive photoelectrolysis of water, more research has focused on semiconductors. Further research has examined transition metal compounds, in particular titanium, niobium and tantalum oxides.Unfortunately, these materials exhibit very low efficiencies, because they require ultraviolet light to drive the photoelectrolysis of water. Current materials also require an electrical voltage bias for the hydrogen and oxygen gas to evolve from the surface, another disadvantage. Current research is focusing on the development of materials capable of the same water splitting reaction using lower energy visible light.

It is also possible to use solar energy to drive industrial chemical processes without a requirement for fossil fuel.


Advantages and disadvantages of solar power:

The 89 petawatts of sunlight reaching the earth's surface is plentiful compared to the 15 terawatts of average power consumed by humans.] Additionally, solar electric generation has the highest power density (global mean of 170 W/m2) among renewable energies.
Solar power is pollution free during use. Production end wastes and emissions are manageable using existing pollution controls. End-of-use recycling technologies are under development.
Facilities can operate with little maintenance or intervention after initial setup.
Solar electric generation is economically competitive where grid connection or fuel transport is difficult, costly or impossible. Examples include satellites, island communities, remote locations and ocean vessels.
When grid connected, solar electric generation can displace the highest cost electricity during times of peak demand (in most climatic regions), can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand; such application is encouraged by net metering. Time-of-use net metering can be highly favorable to small photovoltaic systems.
Grid connected solar electricity can be used locally thus minimizing transmission/distribution losses (approximately 7.2%).
Once the initial capital cost of building a solar power plant has been spent, operating costs are low when compared to existing power technologies


Disadvantages:

Polysilicon Solar cells are costly, requiring a large initial capital investment, and silicon shortages raise prices. Costs are expected to come down, however, due to increased manufacturing, economies of scale and Balance of System planning. Thin film technology uses less silicon; and Lease/Rental options* are currently being introduced.
Limited power density: Average daily insolation in the contiguous U.S. is 3-9 kW·h/m2 usable by 7-17.7% efficient solar panels.
To get enough energy for larger applications, a large number of photovoltaic cells is needed. This increases the cost of the technology and requires a large plot of land.
Like electricity from nuclear or fossil fuel plants, it can only realistically be used to power transport vehicles by converting light energy into another form of stored energy (e.g. battery stored electricity or by electrolysing water to produce hydrogen) suitable for transport.
Solar cells produce DC which must be converted to AC when used in currently existing distribution grids. This incurs an energy loss of 4-12%