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Solar Energy - Standing eight light minutes away to power up the Earth every solar day !
The are two different main technology streams that have been developed to trap solar energy. One is called solar themal, which uses reflectors and piped water to collect the heat energy. The other is using materials that show photo-electric properties such as silicon and other semiconductors which absorb solar energy as light and generate electricity. These are called photovoltaic and photo-electrochemical devices.
In general the panels convert the light to electricity by
a simple mechanism of electron excitation by the absorbed photon. The
overall mechanism is as illustrated generation of free conduction electron at
p-n junction, i.e. at the junction of p-type and n-type semiconductor. A
In places where the Sun light is abundant and is available all the year around, a simple trapping of the Solar energy as heat using simple heat transfer technology is used to generate steam which in turn is used to generate electricity by turning steam turbines. For instance, Siemens is providing steam turbines for Nevada Solar One, a solar-thermal plant whose collectors cover an area of one square kilometer. The collectors focus sunlight and reflect the concentrated energy onto steel pipes filled with heat-transfer oil, which is then used to heat water in a heat exchanger. The resulting steam drives a 64-MW Siemens turbine. When it goes on line this summer, the facility will supply about 40,000 households with electricity. This is control at molecular level. This is whole new field of science where the semiconductors such as silicon are grown into nano sized rods or crystals. The promise of low cost photovoltaics with no sacrifice in efficiency in comparison to traditional single crystal semiconductors, is the main motivation for development of these materials. The efficiency of absorption is enormous when compared to polished wafers. This is mainly because of great reduction in loss due to reflection and the significant increase in surface area. The ability to specifically attach molecules to the dangling bonds is a further advantage. With nano-fiber semiconductors, it is possible to architect the materials into flexible thin sheets and even integrate it to clothing. The status at present it is possible to have solar powered jackets with enough power for portable devices such as ipods. Even tents that power themselves. For the latest publications check out the news section. Another way to trap sunlight is to use it in Photo-electrochemical devices where the sunlight excited Si electrode can effect important chemical reactions such as splitting of water to yield Hydrogen. These are yet to be commercialized in any significant way. Main limitations being low efficiency. The natural biochemical traps such as chlorophylls have been subject of much research. Algae have been effectively used in the manufacture of many useful products. These are discussed in the Bio-Reactor section. Of more recent interest are certain synthesized novel materials such as semiconductor polymers which are also effective traps of visible spectrum photons. i.e. they exhibit interesting electrical and chromatic activities upon interaction with the visible light. Composite materials such as polymer-fullerene blends are of great interest in achieving high efficiency low cost solar panels.
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The technology to harness this energy in a form that can be
adapted for the current day requirements will require further development before
it becomes cost effective. The current day installations of the solar panels for
buildings has an average life time of 25 years. So it can be cost effective in
the regions of regular and abundant sunshine to have roof top installation of
solar panels. As it stands there are relatively cheap solar panels that can
produce electricity for limited and special requirements.It is not uncommon to
see the Solar panel decorated homes with part dependence on Solar power for
heating and lighting. These energy efficient homes are possible only in the
developed countries where sufficient affordability is there for the initial cost
of installation. However with greater public awareness and participation this
can in fact be made more cost effective. In places of abundant Sunlight even
simple traps such as solar light inlets and wind operated ventilators reduce the
heating consumption from regular power supplies. These being cost effective, are
increasingly used in modern buildings. 
semiconductor such as Si having a band-gap which corresponds to the available
energy level of the visible light spectrum can be used effectively. It would be
most efficient to use single crystal Si. However it is more cost effective to
use polycrystalline Si which are cheaper to manufacture. At the p-n junction due
charge migration i.e., electrons from n migrate to fill the holes on the p side
of the junction. Because of this a region of charge depletion is created at the
junction. This causes shift in energy levels causing an overall effect called
schottky barrier. The shift in the energy of the top of the valence band and the
bottom of the conduction band due to p and n type impurities is called the fermi
level. This is seen to be approximately at the middle of the bang-gap in the
middle of the depletion region. Electron excitation from the fermi level to the
conduction band happens at this junction. These electrons result in the observed
photovoltaic current.
Although
solar powered cars have been regularly showcased, for
it to be practicable, the energy efficiency of the panels have to be much more
to reduce cost and surface area outlay. The current day reported efficiency of
photovoltaic panels does not warrant its use to totally power motor vehicles.
However modern day innovators have not lost the enthusiasm to try the panels for
not just road travel, but also for air travel. Panels on extended wings of
airplane have been showcased for fuel efficiency and improvement in reduction of
emissions. The most effective use of solar panels is on artificial satellites for
powering sub-orbital flight.