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 æm. 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