Solar Energy

     About 47 percent of the energy that the sun releases to the earth actually
reaches the ground. About a third is reflected directly back into space by the
atmosphere. The time in which solar energy is available, is also the time we
least need it least - daytime. Because the sun's energy cannot be stored for use
another time, we need to convert the suns energy into an energy that can be
stored. One possible method of storing solar energy is by heating water that can
be insulated. The water is heated by passing it through hollow panels.

Black-coated steal plates are used because dark colors absorb heat more
efficiently. However, this method only supplies enough energy for activities
such as washing and bathing. The solar panels generate "low grade"
heat, that is, they generate low temperatures for the amount of heat needed in a
day. In order to generate "high grade" heat, intense enough to convert
water into high-pressure steam which can then be used to turn electric
generators there must be another method. The concentrated beams of sunlight are
collected in a device called a solar furnace, which acts on the same principles
as a large magnifying glass. The solar furnace takes the sunlight from a large
area and by the use of lenses and mirrors can focus the light into a very small
area. Very elaborate solar furnaces have machines that angle the mirrors and
lenses to the sun all day. This system can provide sizable amounts of
electricity and create extremely high temperatures of over 6000 degrees

Fahrenheit. Solar energy generators are very clean, little waste is emitted from
the generators into the environment. The use of coal, oil and gasoline is a
constant drain, economically and environmentally. Will solar energy be the wave
of the future? Could the worlds Tran 2 requirement of energy be fulfilled by the
"powerhouse" of our galaxy - the sun? Automobiles in the future will
probably run on solar energy, and houses will have solar heaters. Solar cells
today are mostly made of silicon, one of the most common elements on Earth. The
crystalline silicon solar cell was one of the first types to be developed and it
is still the most common type in use today. They do not pollute the atmosphere
and they leave behind no harmful waste products. Photovoltaic cells work
effectively even in cloudy weather and unlike solar heaters, are more efficient
at low temperatures. They do their job silently and there are no moving parts to
wear out. It is no wonder that one marvels on how such a device would function.

To understand how a solar cell works, it is necessary to go back to some basic
atomic concepts. In the simplest model of the atom, electrons orbit a central
nucleus, composed of protons and neutrons. Each electron carries one negative
charge and each proton one positive charge. Neutrons carry no charge. Every atom
has the same number of electrons as there are protons, so, on the whole, it is
electrically neutral. The electrons have discrete kinetic energy levels, which
increase with the orbital radius. When atoms bond together to form a solid, the
electron energy levels merge into bands. In electrical conductors, these bands
are continuous but in insulators and semiconductors there is an "energy
gap", in which no electron orbits can exist, between the inner valence band
and outer conduction band [Book 1]. Valence electrons help to bind together the
atoms in a solid by orbiting 2 adjacent nuclei, while conduction electrons,
being less closely bound to the nuclei, are free to move in response to an
applied voltage or electric field. The fewer conduction electrons there are, the
higher the electrical resistively of the material. Tran 3 In semiconductors, the
materials from which solar sells are made, the energy gap E.g. is fairly small.

Because of this, electrons in the valence band can easily be made to jump to the
conduction band by the injection of energy, either in the form of heat or light
[Book 4]. This explains why the high resistively of semiconductors decreases as
the temperature is raised or the material illuminated. The excitation of valence
electrons to the conduction band is best accomplished when the semiconductor is
in the crystalline state, i.e. when the atoms are arranged in a precise
geometrical formation or "lattice." At room temperature and low
illumination, pure or so-called "intrinsic" semiconductors have a high
resistively. But the resistively can be greatly reduced by "doping," i.e.
introducing a very small amount of impurity, of the order of one in a million
atoms. There are 2 kinds of doping. Those which have more valence electrons that
the semiconductor itself are called "donors" and those which have
fewer are termed "acceptors" [Book 2]. In a silicon crystal, each atom
has 4 valence electrons, which are shared with a neighboring atom to form a
stable tetrahedral structure. Phosphorus, which has 5 valence electrons, is a
donor and causes extra electrons to appear in the conduction band. Silicon so
doped is called "n-type" [Book 5]. On the other hand, boron, with a
valence of 3, is an acceptor, leaving so-called "holes" in the
lattice, which act like positive charges and render the silicon "p-type"[Book

5]. Holes, like electrons, will remove under the influence of an applied voltage
but, as the mechanism of their movement is valence electron substitution from
atom to atom, they are less mobile than the free conduction electrons [Book 2].

In a n-on-p crystalline silicon Tran 4 solar cell, a shadow junction is formed
by diffusing phosphorus into a boron-based base. At the junction, conduction
electrons from donor atoms in the n-region diffuse into the p-region and combine
with holes in acceptor atoms, producing a layer of negatively-charged impurity
atoms. The opposite action also takes place, holes from acceptor atoms in the
p-region crossing into the n-region, combining with electrons and producing
positively-charged impurity atoms [Book 4]. The net result of these movements is
the disappearance of conduction electrons and holes from the vicinity of the
junction and the establishment there of a reverse electric field, which is
positive on the n-side and negative on the p-side. This reverse field plays a
vital part in the functioning of the device. The area in which it is set up is
called the "depletion area" or "barrier layer"[Book 4]. When
light falls on the front surface, photons with energy in excess of the energy
gap interact with valence electrons and lift them to the conduction band. This
movement leaves behind holes, so each photon is said to generate an
"electron-hole pair" [Book 2]. In the crystalline silicon,
electron-hole generation takes place throughout the thickness of the cell, in
concentrations depending on the irradiance and the spectral composition of the
light. Photon energy is inversely proportional to wavelength. The highly
energetic photons in the ultra-violet and blue part of the spectrum are absorbed
very near the surface, while the less energetic longer wave photons in the red
and infrared are absorbed deeper in the crystal and further from the junction
[Book 4]. Most are absorbed within a thickness of 100 śm. The electrons and
holes diffuse through the crystal in an effort to produce an even distribution.

Some recombine after a lifetime of the order of one millisecond, neutralizing
their charges and giving up energy in the form of heat. Others reach the
junction before their lifetime has expired. There they are separated Tran 5 by
the reverse field, the electrons being accelerated towards the negative contact
and the holes towards the positive [Book 5]. If the cell is connected to a load,
electrons will be pushed from the negative contact through the load to the
positive contact, where they will recombine with holes. This constitutes an
electric current. In crystalline silicon cells, the current generated by
radiation of a particular spectral composition is directly proportional to the
irradiance [Book 2]. Some types of solar cell, however, do not exhibit this
linear relationship. The silicon solar cell has many advantages such as high
reliability, photovoltaic power plants can be put up easily and quickly,
photovoltaic power plants are quite modular and can respond to sudden changes in
solar input which occur when clouds pass by. However there are still some major
problems with them. They still cost too much for mass use and are relatively
inefficient with conversion efficiencies of 20% to 30%. With time, both of these
problems will be solved through mass production and new technological advances
in semiconductors.

Bibliography

1) Green, Martin Solar Cells, Operating Principles, Technology and System

Applications. New Jersey, Prentice-Hall, 1989. pg 104-106 2) Hovel, Howard Solar

Cells, Semiconductors and Semimetals. New York, Academic Press, 1990. pg 334-339

3) Newham, Michael ,"Photovoltaics, The Sunrise Industry", Solar

Energy, October 1, 1989, pp 253-256 4) Pulfrey, Donald Photovoltaic Power

Generation. Oxford, Van Norstrand Co., 1988. pg 56-61 5) Treble, Fredrick

Generating Electricity from the Sun. New York, Pergamon Press, 1991. pg 192-195