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
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
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.
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