Solar Power
About 47 per cent 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 colours 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 sizeable 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 heaters.
Solar Power II
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 nucleii, while
conduction electrons, being less closely bound to the nucleii, are free to move
in response to an applied voltage or electric field. The fewer conduction
electrons there are, the higher the electrical resistivity of the material. In
semiconductors, the materials from which solar sells are made, the energy gap Eg
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 resistivity 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 resistivity. But the resistivity 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 dopant. 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 neighbouring 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]. The drawings in Figure 1.2 are
2-dimensional representations of n- and p-type silicon crystals, in which the
atomic nucleii in the lattice are indicated by circles and the bonding valence
electrons are shown as lines between the atoms. 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 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 (1.1 eV in crystalline
silicon) 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 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.