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LW1DSE > TECH 08.02.08 00:01l 268 Lines 15847 Bytes #999 (0) @ WW
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Subj: Solar Cells (3 of 3)
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ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
º °±² Solar Cells ²±° º
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To be viwed in CP 437
(From 3 of 2)
6) Concentrating photovoltaics (CPV)
Concentrating photovoltaic systems use a large area of lenses or mirrors
to focus sunlight on a small area of photovoltaic cells.[22] If these systems
use single or dual-axis tracking to improve performance, they may be referred
to as Heliostat Concentrator Photovoltaics (HCPV). The primary attraction of
CPV systems is their reduced usage of semiconducting material which is
expensive and currently in short supply. Additionally, increasing the
concentration ratio improves the performance of general photovoltaic materials
and also allows for the use of high-performance materials such as gallium
arsenide. Despite the advantages of CPV technologies their application has
been limited by the costs of focusing, tracking and cooling equipment. On
October 25, 2006, the Australian federal government and the Victorian state
government together with photovoltaic technology company Solar Systems
announced a project using this technology, Solar power station in Victoria,
planned to come online in 2008 and be completed by 2013. This plant, at 154
MW, would be ten times larger than the largest current photovoltaic plant in
the world.
7) Silicon solar cell device manufacture
Because solar cells are semiconductor devices, they share many of the
same processing and manufacturing techniques as other semiconductor devices
such as computer and memory chips. However, the stringent requirements for
cleanliness and quality control of semiconductor fabrication are a little more
relaxed for solar cells. Most large-scale commercial solar cell factories
today make screen printed poly-crystalline silicon solar cells. Single
crystalline wafers which are used in the semiconductor industry can be made
into excellent high efficiency solar cells, but they are generally considered
to be too expensive for large-scale mass production.
Poly-crystalline silicon wafers are made by wire-sawing block-cast
silicon ingots into very thin (180 to 350 micrometer) slices or wafers. The
wafers are usually lightly p-type doped. To make a solar cell from the wafer,
a surface diffusion of n-type dopants is performed on the front side of the
wafer. This forms a p-n junction a few hundred nanometers below the surface.
Antireflection coatings, which increase the amount of light coupled into the
solar cell, are typically applied next. Over the past decade, silicon nitride
has gradually replaced titanium dioxide as the antireflection coating of
choice because of its excellent surface passivation qualities (i.e., it
prevents carrier recombination at the surface of the solar cell). It is
typically applied in a layer several hundred nanometers thick using
plasma-enhanced chemical vapor deposition (PECVD). Some solar cells have
textured front surfaces that, like antireflection coatings, serve to increase
the amount of light coupled into the cell. Such surfaces can usually only be
formed on single-crystal silicon, though in recent years methods of forming
them on multicrystalline silicon have been developed.
The wafer is then metallized, whereby a full area metal contact is made
on the back surface, and a grid-like metal contact made up of fine "fingers"
and larger "busbars" is screen-printed onto the front surface using a silver
paste. The rear contact is also formed by screen-printing a metal paste,
typically aluminium. Usually this contact covers the entire rear side of the
cell, though in some cell designs it is printed in a grid pattern. The paste
is then fired at several hundred degrees Celsius to form metal electrodes in
ohmic contact with the silicon. After the metal contacts are made, the solar
cells are interconnected in series (and/or parallel) by flat wires or metal
ribbons, and assembled into modules or "solar panels". Solar panels have a
sheet of tempered glass on the front, and a polymer encapsulation on the back.
Tempered glass can't be used with amorphous silicon cells because of the high
temperatures during the deposition process.
8) Current research on materials and devices
There are currently many research groups active in the field of
photovoltaics in universities and research institutions around the world.
This research can be divided into three areas: making current technology
solar cells cheaper and/or more efficient to effectively compete with other
energy sources; developing new technologies based on new solar cell
architectural designs; and developing new materials to serve as light
absorbers and charge carriers.
8.1) Silicon processing
One way of reducing the cost is to develop cheaper methods of obtaining
silicon that is sufficiently pure. Silicon is a very common element, but is
normally bound in silica, or silica sand. Processing silica (SiO2) to produce
silicon is a very high energy process - at current efficiencies, it takes over
two years for a conventional solar cell to generate as much energy as was
used to make the silicon it contains. More energy efficient methods of
synthesis aren't only beneficial to the solar industry, but also to industries
surrounding silicon technology as a whole.
The current industrial production of silicon is via the reaction
between carbon (charcoal) and silica at a temperature around 1700 degrees
Celsius. In this process, known as carbothermic reduction, each tonne of
silicon (metallurgical grade, about 98% pure) is produced with the emission
of about 1.5 tonnes of carbon dioxide.
Solid silica can be directly converted (reduced) to pure silicon by
electrolysis in a molten salt bath at a fairly mild temperature (800 to 900
degrees Celsius). While this new process is in principle the same as the FFC
Cambridge Process which was first discovered in late 1996, the interesting
laboratory finding is that such electrolytic silicon is in the form of porous
silicon which turns readily into a fine powder, (with a particle size of a
few micrometres), and may therefore offer new opportunities for development
of solar cell technologies.
Another approach is also to reduce the amount of silicon used and thus
cost, as done by Professor Andrew Blakers at the Australian National
University with their "Sliver" cells, by micromachining wafers into very
thin, virtually transparent layers that could be used as transparent
architectural coverings. Using this technique, one silicon wafer is enough to
build a 140 watt panel, compared to about 60 wafers needed for conventional
modules of same power output.
Yet another way to achieve cost improvements is to reduce wastes during
the crystal formation by improved modelisation of the process, as done by
FemagSoft, spin-off of the Universit‚ Catholique de Louvain.
Another novel approach employed by Evergreen Solar is to grow silicon
ribbons from specialized "string puller" furnaces. They claim to be able to
produce thinner cells without machining waste plus the resulting cells are
naturally rectangular in shape.
8.2) Thin-film processing
Thin-film solar cells use less than 1% of the raw material (silicon or
other light absorbers) compared to wafer based solar cells, leading to a
significant price drop per kWh. There are many research groups around the
world actively researching different thin-film approaches and/or materials,
however it remains to be seen if these solutions can generate the same
space-efficiency as traditional silicon processing.
One particularly promising technology is crystalline silicon thin films
on glass substrates. This technology makes use of the advantages of
crystalline silicon as a solar cell material, with the cost savings of using
a thin-film approach.
Another interesting aspect of thin-film solar cells is the possibility
to deposit the cells on all kind of materials, including flexible substrates
(PET for example), which opens a new dimension for new applications.
8.3) Polymer processing
The invention of conductive polymers (for which Alan Heeger, Alan G.
MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the
development of much cheaper cells that are based on inexpensive plastics.
However, all organic solar cells made to date suffer from degradation upon
exposure to UV light, and hence have lifetimes which are far too short to be
viable. The conjugated double bond systems in the polymers, which carry the
charge, are always susceptible to breaking up when radiated with shorter
wavelengths. Additionally, most conductive polymers, being highly unsaturated
and reactive, are highly sensitive to atmospheric moisture and oxidation,
making commercial applications difficult.
8.4) Nanoparticle processing
Experimental non-silicon solar panels can be made of quantum
heterostructures, eg. carbon nanotubes or quantum dots, embedded in
conductive polymers or mesoporous metal oxides. In addition, thin films of
many of these materials on conventional silicon solar cells can increase the
optical coupling efficiency into the silicon cell, thus boosting the overall
efficiency. By varying the size of the quantum dots, the cells can be tuned
to absorb different wavelengths. Although the research is still in its
infancy, quantum dot-modified photovoltaics may be able to achieve up to 42
percent energy conversion efficiency due to multiple exciton generation(MEG).
8.5) Transparent conductors
Many new solar cells use transparent thin films that are also
conductors of electrical charge. The dominant conductive thin films used in
research now are transparent conductive oxides (abbreviated "TCO"), and
include fluorine-doped tin oxide (SnO2:F, or "FTO"), doped zinc oxide (e.g.:
ZnO:Al), and indium tin oxide (abbreviated "ITO"). These conductive films are
also used in the LCD industry for flat panel displays. The dual function of a
TCO allows light to pass through a substrate window to the active light
absorbing material beneath, and also serves as an ohmic contact to transport
photogenerated charge carriers away from that light absorbing material. The
present TCO materials are effective for research, but perhaps aren't yet
optimized for large-scale photovoltaic production. They require very special
deposition conditions at high vacuum, they can sometimes suffer from poor
mechanical strength, and most have poor transmittance in the infrared portion
of the spectrum (e.g.: ITO thin films can also be used as infrared filters in
airplane windows). These factors make large-scale manufacturing more costly.
A relatively new area has emerged using carbon nanotube networks as a
transparent conductor for organic solar cells. Nanotube networks are flexible
and can be deposited on surfaces a variety of ways. With some treatment,
nanotube films can be highly transparent in the infrared, possibly enabling
efficient low bandgap solar cells. Nanotube networks are p-type conductors,
whereas traditional transparent conductors are exclusively n-type. The
availability of a p-type transparent conductor could lead to new cell designs
that simplify manufacturing and improve efficiency.
8.6) Silicon wafer based solar cells
Despite the numerous attempts at making better solar cells by using new
and exotic materials, the reality is that the photovoltaics market is still
dominated by silicon wafer-based solar cells (first-generation solar cells).
This means that most solar cell manufacturers are equipped to produce these
type of solar cells. Therefore, a large body of research is currently being
done all over the world to create silicon wafer-based solar cells that can
achieve higher conversion efficiency without an exorbitant increase in
production cost. The aim of the research is to achieve the lowest $/watt
solar cell design that is suitable for commercial production.
8.7) Sliver cells
Professor Andrew Blakers and Dr Klaus Weber, working at Australian
National University and Origin Energy have developed a technique for slicing
a single silicon wafer, which allows a significantly larger collector surface
area from each wafer, compared to usual solar cells. The technique involves
taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of
parallel, transverse slices across the wafer, creating a large number of
slivers that have a thickness of 50 micrometres and a width equal to the
thickness of the original wafer. These slices are rotated 90 degrees, so that
the surfaces corresponding to the faces of the original wafer become the
edges of the slivers. The result is to convert, for example, a 150 mm
diameter, 2 mm-thick wafer having an exposed silicon surface area of about
175 cmý per side into about 1000 slivers having dimensions of 100 mm x 2 mm x
0.1 mm, yielding a total exposed silicon surface area of about 2000 cmý per
side. As a result of this rotation, the electrical doping and contacts that
were on the face of the wafer are located the edges of the sliver, rather
than the front and rear as is the case with conventional wafer cells. This
has the interesting effect of making the cell sensitive from both the front
and rear of the cell (a property known as bifaciality).
9) Makers
Solar cells are manufactured primarily in Japan, Germany, USA, and
China, though numerous other nations have or are acquiring significant solar
cell production capacity. While technologies are constantly evolving toward
higher efficiencies, the most effective cells for low cost electrical
production aren't necessarily those with the highest efficiency, but those
with a balance between low-cost production and efficiency high enough to
minimize area-related balance of systems cost. Those companies with large
scale manufacturing technology for coating inexpensive substrates may, in
fact, ultimately be the lowest cost net electricity producers, even with cell
efficiencies that are lower than those of single-crystal technologies.
10) My own experience
I placed 3 panels of 25 cm by 40 cm rated at 10 W each. One of them is
placed at an angle of about 45ø looking to NE (to capture morning sun), a
second placed at same angle to NW (to catch the evening sunlight) and the
third at 30ø to the North, for the midday. They are "ored" with Shottky diodes
STPS15-45 and they are charging a 12V 70AH lead/acid car battery, used to
power a Yaesu FT2400 and the packet modem, a Yaesu FT7800 UHF & VHF (fone) and
6 emergency lights Osram Neolux 12V 11W (I use one or two of them when mains
faults). Reazon for diodes is the following: if a module or the cables
shortcicuits, the diode prevents the discharge of the battery. A 1mA ammeter
shunted to expand the scale up to 2 Amper and a 3 Amps. fuse, all mounted in
a metallic box.
During a cluodless day, at the midday, the 2 Amps ammeter goes to the
end of the scale. If the sun is partially covered, no more that 1.5 A goes
into the battery. In a dark day (very cloudly), less than 1 Amp. Clear
mornings and evenings, about of 500 to 800 mA is normal. Battery's voltage
oscillates between 13.5 to 14.6V at full charge, and 12.5 to 13V at darkness.
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º Compilled from Wikipedia.com in the Intenet. Translatted to ASCII by º
º LW1DSE Osvaldo F. Zappacosta. Banfield (1832), Buenos Aires, Argentina. º
º Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486. º
º January 28, 2008 º
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