While
photovoltaic panels are based on a similar structure
of cells and enabling components, there are many
variations on the standard solar panel, differing
primarily in the types of photovoltaic cell that
they use. Each panel type is manufactured in a
different way and has its own advantages and
disadvantages.
The vast majority of solar panels produced today
depend on the use of crystalline silicon as the
material in their cells. The properties of
crystalline silicon are described in The Science
Behind Photovoltaics. It is used in moncrystalline
(or single-crystalline), polycrystalline (or
multicrystalline), and ribbon (or thin-layer)
silicon panels.
Other panels, like thin-film technologies, depend on
amorphous silicon, and still others use completely
different semiconductors known as Group III-IV
materials. Panels can also be enhanced in a number
of ways to increase their efficiency or improve
their versatility through the use of multijunction
devices, concentrator systems, or building
integrated systems.
Here we will look at the panel types in use today.
Panel Types
Monocrystalline Silicon Panels
Polycrystalline Silicon Panels
String Ribbon or Thin-Layer Silicon Panels
Amorphous Silicon panels or Thin
Film
Group III-V Technologies
Enhanced Panels
Building-integrated PV panels
Concentrator systems
High-Efficiency Multijunction Devices
Monocrystalline Silicon Panels
15-18% efficiency
Monocrystalline panels use crystalline silicon
produced in large sheets which can be cut to the
size of a panel and integrated into the panel as a
single large cell. Conducting metal strips are laid
over the entire cell to capture electrons in an
electrical current.
These panels are more expensive to produce than
other crystalline panels but have higher efficiency
levels and, as a result, are sometimes more
cost-effective in the long run.
Polycrystalline Silicon Panels
12-14% efficiency
Polycrystalline, or multicrystalline,
photovoltaics use a series of cells instead of one
large cell. These panels are one of the most
inexpensive forms of photovoltaics available today,
though the costs of sawing and producing wafers can
be high. At the same time, they have lower
conversion efficiencies than monocrystalline panels.
For this technology, several techniques can be
used:
Cast Polysilicon:
In this process, molten silicon is first cast in
a large block which, when cooled, is in the form of
crystalline silicon and can be sawn across its width
to create thin wafers to be used in photovoltaic
cells. These cells are then assembled in a panel.
Conducting metal strips are then laid over the
cells, connecting them to each other and forming a
continuous electrical current throughout the panel.
String Ribbon Silicon
String ribbon photovoltaics use a variation on
the polycrystalline production process, using the
same molten silicon but slowly drawing a thin strip
of crystalline silicon out of the molten form. These
strips of photovoltaic material are then assembled
in a panel with the same metal conductor strips
attaching each strip to the electrical current.
This technology saves on costs over standard
polycrystalline panels as it eliminates the sawing
process for producing wafers. Some string ribbon
technologies also have higher efficiency levels than
other polycrystalline technologies.
EFG (RWE Schott):
Figure out what type this is
Amorphous Silicon or Thin Film Panels
5-6% efficiency
Thin-film panels are produced very differently
from crystalline panels. Instead of molding, drawing
or slicing crystalline silicon, the silicon material
in these panels has no crystalline structure and can
be applied as a film directly on different
materials. Variations on this technology use other
semiconductor materials like copper indium
diselenide (CIS) and cadmium telluride (CdTe). These
materials are then connected to the same metal
conductor strips used in other technologies, but do
not necessarily use the other components typical in
photovoltaic panels as they do not require the same
level of protection needed for more fragile
crystalline cells.
The primary advantages of thin-film panels lie in
their low manufacturing costs and versatility.
Because amorphous silicon and similar semiconductors
do not depend on the long, expensive process of
creating silicon crystals, they can be produced much
more quickly and efficiently. As they do not need
the additional components used in crystalline cells,
costs can be reduced further. Because they can be
applied in thin layers to different materials, it is
also possible to make flexible solar cells.
However, thin-film panels have several
significant drawbacks. What they gain in cost
savings, they lose in efficiency, resulting in the
lowest efficiency of any current photovoltaic
technology. Thin-film technologies also depend on
silicon with high levels of impurities. This can
cause a drop in efficiency within a short period of
use.
Thin-film panels have the potential to grow in
use, and already figure in some of the most exciting
enhanced photovoltaic systems, including
high-efficiency multijunction devices and building
integrated photovoltaics.
Group III-V Technologies
25% efficiency
These technologies use a variety of materials
with very high conversion efficiencies. These
materials are categorized as Group III and Group V
elements in the Periodic Table. A typical material
used in this technology is gallium arsenide, which
can be combined with other materials to create
semiconductors that can respond to different types
of solar energy.
Though these technologies are very effective,
their current use is limited due to their costs.
They are currently employed in space applications
and continue to be researched for new applications.
Enhanced Systems
Building-Integrated Photovoltaics (BIPV)
BIPV technologies are designed to serve the dual
purpose of producing electricity and acting as a
construction material. There are many forms that
this technology can take. One common structure is
the integration of a semi-translucent layer of
amorphous silicon into glass, which can then be used
as window panes that let controlled amounts of light
into a building while producing electricity. Another
common structure is the use of shingle-sized panel
of amorphous silicon as a roofing material.
Currently, BIPV technologies have very low
efficiency levels due to their use of amorphous
silicon, but present the advantage of replacing
other construction materials and offering a wide
variety of aesthetic choices for the integration of
photovoltaics into buildings.
Concentrator Systems
Concentrator systems are designed to increase the
efficiency of solar photovoltaics. These systems
cover a standard photovoltaic panel with
concentrating optics, or lenses that gather sunlight
and increase its intensity in hitting the
photovoltaic panel. These systems reduce the amount
of photovoltaics needed to produce electricity, and
also reduce the amount of space needed for a
photovoltaic installation.
Their main disadvantage is that they depend
solely on direct light to produce electricity, while
stand-alone photovoltaic panels can use both direct
and diffuse light. Many regions do not receive
enough direct light throughout the year for these
systems to be practical. Another disadvantage is the
complexity of their construction, which makes these
systems more difficult to build and install than
photovoltaic panels on their own.
High-Efficiency Multijunction Devices
Multijunction devices receive their name from
their use of multiple layers of cells, each layer
acting as a junction where certain amounts of solar
energy are absorbed. Each layer in a multijunction
device is made from a different material with its
own receptivity to certain types of solar energy.
In a typical device, the top photovoltaic layer
responds to solar waves that travel in short
wavelengths and carry the highest energy, absorbing
this energy and creating an electrical charge. As
other solar waves pass through this layer, they are
absorbed and translated into electricity by the
lower layers. Typical materials used in this device
include gallium arsenide and amorphous silicon.
Though some two-junction devices have
successfully been built, these devices are still
largely in the research and development stage, with
most research focused on three- and four-junction
devices.
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