Solar Energy
Tran 1 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.
