| === radiation from the Sun
capable of producing heat, causing chemical reactions, or
generating electricity. The Sun is an extremely powerful
energy source, and solar radiation is by far the
largest source of energy received by the
Earth, but its intensity at the Earth's surface is actually
quite low. This is partly because the Earth's atmosphere and its
clouds absorb or scatter as much as 54 percent of all incoming
sunlight. Despite this, in the 20th century solar
energy became increasingly attractive as an energy
source owing to its inexhaustible supply and its nonpolluting
character, which are in stark contrast to such fossil-fuel
sources as coal, oil, and natural gas.
The
sunlight that reaches the ground consists of nearly 50
percent visible light, 45 percent infrared radiation, and
smaller amounts of ultraviolet light and other forms of
electromagnetic radiation. This radiation can be converted
either into
thermal energy (heat) or into electrical energy,
though the former is easier to accomplish. Two main types of
devices are used to capture solar energy and
convert it to thermal energy:
flat-plate collectors and
concentrating collectors. Because the intensity of solar
radiation at the Earth's surface is so low, both types of
collectors must be large in area. Even in sunny parts of the
world's temperate regions, for instance, a collector must have a
surface area of about 430 square feet (40 square m) to gather
enough energy to serve one person for one day.
The most widely used flat-plate collectors consist of a
blackened metal plate, covered with one or two sheets of glass,
that is heated by the sunlight falling on it. This heat is then
transferred to air or to water, called carrier fluids, that
flows past the back of the plate. The heat may be used directly
or it may be transferred to another medium for storage.
Flat-plate collectors are commonly used for hot-water heating
and house heating. The storage of heat for use at night or
during cloudy days is commonly accomplished by using insulated
tanks to store the water heated during sunny periods. Such a
system can supply a home with hot water drawn from the storage
tank or, with the warmed water flowing through tubes in floors
and ceilings, it can provide space heating. Flat-plate
collectors typically heat carrier fluids to temperatures ranging
from 66 to 93
C (150 to 200
F). The efficiency of such collectors (i.e., the
proportion of the energy received that they convert into
usable energy) ranges from 20 to 80 percent, depending on
the design of the collector. (See also
solar heating.)
When higher temperatures are needed, a concentrating, or
focusing, collector is used. These devices reflect sunlight from
a wide area and concentrate it onto a small blackened receiver,
thereby considerably increasing the light's intensity in order
to produce high temperatures. The arrays of carefully aligned
mirrors used in these so-called
solar furnaces can focus enough sunlight to heat a
target to temperatures of 2,000 C
(3,600 F) or more. This heat can be
used to study the properties of materials at high temperatures,
or it can be used to operate a boiler, which in turn generates
steam for a steam-turbine-electric-generator power plant. The
solar furnace has become an important tool in
high-temperature research. For producing steam, the movable
mirrors are so arranged as to concentrate large amounts of
solar radiation upon blackened pipes through which water is
circulated and thereby heated.
Solar radiation may be converted directly into
electricity by
photovoltaic cells. In such cells, a small electrical
voltage is generated when light strikes the junction between a
metal and a semiconductor (such as silicon) or a junction
between two different semiconductors. (See
photovoltaic effect.) The voltage generated from a single
photovoltaic cell is typically only a fraction of a volt. By
connecting large numbers of individual cells together, however,
as in modern
solar batteries, more than one kilowatt of electric
power can be generated. The energy efficiency of most
present-day photovoltaic cells is only about 7 to 11 percent;
i.e., only that fraction of the radiant energy
received is converted to electrical energy. And since the
intensity of solar radiation is low to begin with, huge
and costly assemblies of such cells are required to produce even
moderate amounts of power. Consequently, photovoltaic cells that
operate on solar light have so far been used mainly for
low-power applications--as power sources for calculators and
watches, for example. Larger units have been used to provide
power for weather and communications satellites.
Solar energy is also used on a small scale for
other purposes besides those described heretofore. In some
countries, for instance, specially designed solar ovens
are employed for cooking, and solar energy is used
to produce salt from seawater by evaporation.
The potential for solar energy is enormous,
since each day the Earth receives in the form of solar
energy about 200,000 times the total world
electrical-generating capacity. Unfortunately, though solar
energy itself is free, the high cost of its collection,
conversion, and storage has limited its exploitation.
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===Structure and principles of
operation
The basic structure of a typical solar cell, whether it is
used in a central power station, a satellite, or a calculator,
is shown in
Figure 1
Figure 1: A commonly used solar-cell structure. In
many such cells, the absorber layer and...
. As may be seen, light enters the device through a layer of
material called the antireflection layer. The function of this
layer is to trap the light falling on the solar cell and to
promote the transmission of this light into the
energy-conversion layers below. Such materials as silicon oxides
or titanium dioxide are employed as the antireflection layer in
solar cells. The
photovoltaic effect, which causes the cell to convert light
directly into electrical energy, occurs in the three
energy-conversion layers below the antireflection layer. The
first of these three layers necessary for energy conversion in a
solar cell is the top junction layer in
Figure 1 . The next layer in the structure is the core of
the device; this is the absorber layer. The last of the
energy-conversion layers is the back junction layer.
As may be seen from
Figure 1 , there are two additional layers that must be
present in a solar cell. These are the electrical contact
layers. There must obviously be two such layers to allow
electric current to flow out of and into the cell. The
electrical contact layer on the face of the cell where light
enters is generally present in some grid pattern and is composed
of a good conductor such as a metal. The grid pattern does not
cover the entire face of the cell since grid materials, though
good electrical conductors, are generally not transparent to
light. Hence, the grid pattern must be widely spaced to allow
light to enter the solar cell but not to the extent that the
electrical contact layer will have difficulty collecting the
current produced by the cell. The back electrical contact layer
has no such diametrically opposed restrictions. It need simply
function as an electrical contact and thus covers the entire
back surface of the cell structure. Because the back layer must
be a very good electrical conductor, it is always made of metal.
It is a fundamental fact of nature that, whenever different
materials are placed in contact, an electric field exists at the
interface, or junction, between these materials. The role of the
junction layers in
Figure 1 is to establish this electric field. The field
created in the solar cell by the different junction-forming
materials is termed the built-in electric field. An electric
field is needed in a solar cell because it exerts a force on
electrons. If electrons are not attached to specific atoms but
are free to roam about in a material, they always will move in a
direction dictated by the electric field. This movement
constitutes an electric current.
The electric field set up by the junction-forming layers of
the solar cell causes a current to flow when there are
free electrons present in the top junction-forming layer,
the absorber layer, and the back junction-forming layer. When
light falls on the cell, free electrons occur as a result of the
interaction of the light with the absorber layer. The special
attribute of this cell layer is that it absorbs light by
changing the energy and state (or condition) of some of the
electrons in the material. When light is absorbed in the
materials, the energy of an electron increases from the
so-called ground state energy to an excited energy state. In the
excited state, electrons are no longer associated with specific
atoms in the absorber, but they are, instead, free to move.
In summary, the absorption of light in the absorber material
of a solar cell results in energetic, free electrons that move
in the direction forced on them by the built-in electric field.
These energetic electrons of the induced current are then
collected by the electrical contact layers for use in an
external circuit where they can do useful work.
Since most of the energy in sunlight or indoor light is in
visible light, a solar-cell absorber should be a strong absorber
of electromagnetic radiation in that range of wavelengths.
Materials that absorb the visible light of sunlight or of indoor
light by producing excited free electrons belong to a class of
substances known as
semiconductor materials. Semiconductors can absorb all
incident visible light in thicknesses of about one-hundredth of
a centimetre or less; consequently, the thickness of a solar
cell can be of this size. Examples of semiconductor materials
employed in solar cells include silicon, gallium arsenide,
indium phosphide, and copper indium selenide.
The materials in a solar cell used for the junction-forming
layers need only be dissimilar, and, to carry the electric
current, they must be conductors. The two junction-forming
layers may be different semiconductors or they may be a metal
and a semiconductor. Thus, the materials used to construct the
various layers of solar cells are essentially the same materials
used to produce the diodes and transistors of solid-state
electronics and microelectronics (see also
electronics: Optoelectronics). Solar cells and
microelectronic devices share the same basic technology. In
solar-cell fabrication, however, one seeks to construct a
large-area device because the power produced is proportional to
the illuminated area. In microelectronics the goal is of course
to construct devices of very small area to increase the number
of circuit components on a single tiny semiconductor chip.
The
photovoltaic effect that causes the direct energy conversion
in a solar cell is summarized in the schematic of
Figure 2
Figure 2: Representation of an electron in a solar cell.
. An analogy between an electron in the solar cell and a
child at a slide is also presented in this figure. As shown,
initially both the electron and the child are in their
respective ground states. Next the electron is lifted up to its
excited state by consuming energy in the incoming light, just as
the child is lifted up to an excited state at the top of the
slide by consuming chemical energy stored in his body. In both
cases, there is now energy available in the excited state that
can be expended. The excited electron is free and moves to the
external circuit due to the built-in electric field. It is in
this external circuit that the electron will dissipate its
excess energy in some device, which in general can be termed a
load. The external load is shown here as a simple resistor, but
it can be any of a myriad of electrical or electronic devices
ranging from motors to radios. Correspondingly, the child moves
to the slide because of his desire for excitement. It is on the
slide that the child dissipates his excess energy. Finally, when
the excess energy is expended, both the electron and the child
are back in the ground state where they can, of course, begin
the whole process over again. As can be seen from the figure,
the motion of the electron, like that of the child, is in one
direction. In short, a solar cell produces a direct electric
current--namely, one that flows constantly in only a single
direction.
The photovoltaic process bears certain similarities to
photosynthesis in plants by which the energy in light is
converted into chemical energy. Since solar cells obviously
cannot produce electric power in the dark, part of the energy
they develop under light is stored, in many applications, for
use when light is not available. One common means of storing
this electrical energy is to charge chemical storage batteries.
This sequence of converting the energy in light into the energy
of excited electrons and then into stored chemical energy is
strikingly similar to the process of photosynthesis.
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