Transcript Document 7139799

Green technologies
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Plan by the Energy Market Authority (EMA) to transform
Pulau Ubin into a high-tech test site for renewable energies.
Pulau Ubin, an island located at the Northeast of Singapore, will
be made into a model ‘green island‘ powered entirely by the
energies generated on the island.
Referring to the picture illustration from the report, the possible
sources of clean and renewable energies will come from
wind, solar, hydrogen fuel cell, biomass waste and/or sea
current.
Currently, Pulau Ubin does not draw electricity from Singapore’s
main power grid because it has been too expensive to lay
transmission cables for such low demand. Instead, about 100
villagers use diesel generators, which are not environmentally
friendly.
The Ubin project will be a great move to provide the inhibitants
and the island with self-sufficient, renewable and clean energy.
What is technology and green
technology?
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The term "technology" refers to the application of
knowledge for practical purposes.
The field of "green technology" encompasses a
continuously evolving group of methods and
materials, from techniques for generating energy
to non-toxic cleaning products.
The present expectation is that this field will bring
innovation and changes in daily life of similar
magnitude to the "information technology"
explosion over the last two decades. In these
early stages, it is impossible to predict what
"green technology" may eventually encompass.
The goals that inform developments in
this rapidly growing field include:
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Sustainability - meeting the needs of society in ways that
can continue indefinitely into the future without damaging or
depleting natural resources. In short, meeting present needs
without compromising the ability of future generations to meet
their own needs.
"Cradle to cradle" design - ending the "cradle to grave"
cycle of manufactured products, by creating products that can
be fully reclaimed or re-used.
Source reduction - reducing waste and pollution by changing
patterns of production and consumption.
Innovation - developing alternatives to technologies whether fossil fuel or chemical intensive agriculture - that have
been demonstrated to damage health and the environment.
Viability - creating a center of economic activity around
technologies and products that benefit the environment,
speeding their implementation and creating new careers that
truly protect the planet.
Examples of green technology
subject areas
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Energy
Perhaps the most urgent issue for green technology, this includes the
development of alternative fuels, new means of generating energy and
energy efficiency.
Green building
Green building encompasses everything from the choice of building materials
to where a building is located.
Environmentally preferred purchasing
This government innovation involves the search for products whose contents
and methods of production have the smallest possible impact on the
environment, and mandates that these be the preferred products for
government purchasing.
Green chemistry
The invention, design and application of chemical products and processes to
reduce or to eliminate the use and generation of hazardous substances.
Green nanotechnology
Nanotechnology involves the manipulation of materials at the scale of the
nanometer, one billionth of a meter. Some scientists believe that mastery of
this subject is forthcoming that will transform the way that everything in the
world is manufactured. "Green nanotechnology" is the application of green
chemistry and green engineering principles to this field.
Renewable energy
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Renewable energy flows involve natural
phenomena such as sunlight, wind, tides and
geothermal heat
International Energy Agency explains:
Renewable energy is derived from natural
processes that are replenished constantly. In its
various forms, it derives directly from the sun,
or from heat generated deep within the earth.
Included in the definition is electricity and heat
generated from solar, wind, ocean, hydropower,
biomass, geothermal resources, and biofuels
and hydrogen derived from renewable
resources.
Mainstream forms of renewable energy
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Wind power
Hydropower
Solar energy
Biomass
Biofuel
Wind power
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Airflows can be used to run wind turbines.
Turbines with rated output of 1.5–3 MW have become the
most common for commercial use; the power output of a
turbine is a function of the cube of the wind speed, so as
wind speed increases, power output increases
dramatically.
Areas where winds are stronger and more constant, such
as offshore and high altitude sites, are preferred locations
for wind farms.
Offshore resources experience mean wind speeds of
~90% greater than that of land, so offshore resources
could contribute substantially more energy.
Wind power is renewable and produces no greenhouse
gases during operation, such as carbon dioxide and
methane
Hydro-power
Hydroelectricity is the term referring to
electricity generated by hydropower; the
production of electrical power through the
use of the gravitational force of falling or
flowing water.
It is the most widely used form of
renewable energy. Once a hydroelectric
complex is constructed, the project
produces no direct waste, and has a
considerably lower output level of the
greenhouse gas carbon dioxide (CO2)
than fossil fuel powered energy plants.
The Gordon Dam in Tasmania is a large conventional
dammed-hydro facility, with an installed capacity of up to
430 MW.
Conventional
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Most hydroelectric power comes from the
potential energy of dammed water driving a water
turbine and generator. The power extracted from
the water depends on the volume and on the
difference in height between the source and the
water's outflow. This height difference is called
the head. The amount of potential energy in
water is proportional to the head. To deliver water
to a turbine while maintaining pressure arising
from the head, a large pipe called a penstock may
be used.
Other ways
Pumped-storage
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This method produces electricity to supply high peak demands by
moving water between reservoirs at different elevations.
At times of low electrical demand, excess generation capacity is
used to pump water into the higher reservoir.
When there is higher demand, water is released back into the
lower reservoir through a turbine.
Pumped-storage schemes currently provide the most
commercially important means of large-scale grid energy storage
and improve the daily capacity factor of the generation system.
Tide
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A tidal power plant makes use of the daily rise and fall of water
due to tides
Such sources are highly predictable, and if conditions permit
construction of reservoirs, can also be dispatchable to generate
power during high demand periods.
Advantages of hydroelectricity
Economics
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The major advantage of hydroelectricity is elimination of the cost of fuel.
The cost of operating a hydroelectric plant is nearly immune to
increases in the cost of fossil fuels such as oil, natural gas or coal, and
no imports are needed.
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Hydroelectric plants also tend to have longer economic lives than fuelfired generation, with some plants now in service which were built 50 to
100 years ago.
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Operating labor cost is also usually low, as plants are automated and
have few personnel on site during normal operation.
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Where a dam serves multiple purposes, a hydroelectric plant may be
added with relatively low construction cost, providing a useful revenue
stream to offset the costs of dam operation. It has been calculated that
the sale of electricity from the Three Gorges Dam will cover the
construction costs after 5 to 8 years of full generation.
Advantages of hydroelectricity
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CO2 emissions
Since hydroelectric dams do not burn fossil fuels, they do not
directly produce carbon dioxide. While some carbon dioxide is
produced during manufacture and construction of the project, this
is a tiny fraction of the operating emissions of equivalent fossilfuel electricity generation.
Hydroelectricity produces the least amount of greenhouse gases
and externality of any energy source. Coming in second place was
wind, third was nuclear energy, and fourth was solar photovoltaic.
Other uses of the reservoir
Reservoirs created by hydroelectric schemes often provide
facilities for water sports, and become tourist attractions
themselves. In some countries, aquaculture in reservoirs is
common. Multi-use dams installed for irrigation support
agriculture with a relatively constant water supply. Large hydro
dams can control floods, which would otherwise affect people
living downstream of the project.
Disadvantages of hydroelectricity
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Ecosystem damage and loss of land
Large reservoirs required for the operation of hydroelectric power
stations result in submersion of extensive areas upstream of the
dams, destroying biologically rich and productive lowland and
riverine valley forests, marshland and grasslands. The loss of land
is often exacerbated by the fact that reservoirs cause habitat
fragmentation of surrounding areas.
Hydroelectric projects can be disruptive to surrounding aquatic
ecosystems both upstream and downstream of the plant site.]
Turbine and power-plant designs that are easier on aquatic life are
an active area of research. Mitigation measures such as fish
ladders may be required at new projects or as a condition of relicensing of existing projects.
Generation of hydroelectric power changes the downstream river
environment. Water exiting a turbine usually contains very little
suspended sediment, which can lead to scouring of river beds and
loss of riverbanks. Since turbine gates are often opened
intermittently, rapid or even daily fluctuations in river flow are
observed.
Comparison with other methods of
power generation
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Hydroelectricity eliminates the fuel gas emissions from fossil fuel
combustion, including pollutants such as sulfur dioxide, nitric oxide,
carbon monoxide, dust, and mercury in the coal.
Hydroelectricity also avoids the hazards of coal mining and the indirect
health effects of coal emissions.
Compared to wind farms, hydroelectricity power plants have a more
predictable load factor. If the project has a storage reservoir, it can be
dispatched to generate power when needed. Hydroelectric plants can be
easily regulated to follow variations in power demand.
Unlike fossil-fuelled combustion turbines, construction of a hydroelectric
plant requires a long lead-time for site studies, hydrological studies, and
environmental impact assessment.
Hydrological data up to 50 years or more is usually required to
determine the best sites and operating regimes for a large hydroelectric
plant.
Unlike plants operated by fuel, such as fossil or nuclear energy, the
number of sites that can be economically developed for hydroelectric
production is limited; in many areas the most cost effective sites have
already been exploited. New hydro sites tend to be far from population
centers and require extensive transmission lines. Hydroelectric
generation depends on rainfall in the watershed, and may be significantly
reduced in years of low rainfall or snowmelt. Long-term energy yield may
be affected by climate change. Utilities that primarily use hydroelectric
power may spend additional capital to build extra capacity to ensure
sufficient power is available in low water years.
Biomass
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Biomass (plant material) is a renewable energy source
because the energy it contains comes from the sun.
Through the process of photosynthesis, plants capture
the sun's energy. When the plants are burned, they
release the sun's energy they contain. In this way,
biomass functions as a sort of natural battery for
storing solar energy. As long as biomass is produced
sustainably, with only as much used as is grown, the
battery will last indefinitely.
In general there are two main approaches to using
plants for energy production: growing plants
specifically for energy use, and using the residues from
plants that are used for other things. The best
approaches vary from region to region according to
climate, soils and geography.
Biofuel
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Liquid biofuel is usually either bioalcohol such as bioethanol or an oil
such as biodiesel.
Bioethanol is an alcohol made by fermenting the sugar components of
plant materials and it is made mostly from sugar and starch crops.
With advanced technology being developed, cellulosic biomass, such
as trees and grasses, are also used as feedstocks for ethanol
production. Ethanol can be used as a fuel for vehicles in its pure form,
but it is usually used as a gasoline additive to increase octane and
improve vehicle emissions. Bioethanol is widely used in the USA and
in Brazil.
Biodiesel is made from vegetable oils, animal fats or recycled greases.
Biodiesel can be used as a fuel for vehicles in its pure form, but it is
usually used as a diesel additive to reduce levels of particulates,
carbon monoxide, and hydrocarbons from diesel-powered vehicles.
Biodiesel is produced from oils or fats using transesterification and is
the most common biofuel in Europe.
Biofuels provided 1.8% of the world's transport fuel in 2008
Solar energy
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Solar energy is the energy derived from the sun through the form
of solar radiation. Solar powered electrical generation relies on
photovoltaics and heat engines.
A partial list of other solar applications includes space heating
and cooling through solar architecture, daylighting, solar hot
water, solar cooking, and high temperature process heat for
industrial purposes.
Solar technologies are broadly characterized as either passive
solar or active solar depending on the way they capture, convert
and distribute solar energy.
Active solar techniques include the use of photovoltaic panels and
solar thermal collectors to harness the energy.
Passive solar techniques include orienting a building to the Sun,
selecting materials with favorable thermal mass or light
dispersing properties, and designing spaces that naturally
circulate air.
Nanotechnology thin-film solar panels
Solar power panels that use nanotechnology, which can create
circuits out of individual silicon molecules, may cost half as much
as traditional photovoltaic cells, according to executives and
investors involved in developing the products. Nanosolar has
secured more than $100 million from investors to build a factory
for nanotechnology thin-film solar panels.
Energy storage methods
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Solar energy is not available at night, and energy storage is an important issue
because modern energy systems usually assume continuous availability of energy.
Thermal mass systems can store solar energy in the form of heat at domestically
useful temperatures for daily or seasonal durations. Thermal storage systems
generally use readily available materials with high specific heat capacities such as
water, earth and stone. Well-designed systems can lower peak demand, shift timeof-use to off-peak hours and reduce overall heating and cooling requirements.
Phase change materials such as paraffin wax and Glauber's salt are another
thermal storage media. These materials are inexpensive, readily available, and can
deliver domestically useful temperatures (approximately 64 °C). The "Dover
House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating
system, in 1948.
Solar energy can be stored at high temperatures using molten salts. Salts are an
effective storage medium because they are low-cost, have a high specific heat
capacity and can deliver heat at temperatures compatible with conventional power
systems. The Solar Two used this method of energy storage, allowing it to store
1.44 TJ in its 68 m³ storage tank with an annual storage efficiency of about 99%.
Off-grid PV systems have traditionally used rechargeable batteries to store excess
electricity. With grid-tied systems, excess electricity can be sent to the
transmission grid. Net metering programs give these systems a credit for the
electricity they deliver to the grid. This credit offsets electricity provided from the
grid when the system cannot meet demand, effectively using the grid as a storage
mechanism.
Pumped-storage hydroelectricity stores energy in the form of water pumped when
energy is available from a lower elevation reservoir to a higher elevation one. The
energy is recovered when demand is high by releasing the water to run through a
hydroelectric power generator.
Solar cells
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Solar Cells are designed to convert (at least a portion of)
available light into electrical energy. They do this without the
use of either chemical reactions or moving parts.
Solar cells are often electrically connected and encapsulated
as a module. Photovoltaic modules often have a sheet of
glass on the front (sun up) side, allowing light to pass while
protecting the semiconductor wafers from the elements
(rain, hail, etc.).
Solar cells can also be applied to other electronics devices to
make it self-power sustainable in the sun. There are solar
cell phone chargers, solar bike light and solar camping
lanterns that people can adopt for daily use.
Theory
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Photons in sunlight hit the solar panel and are
absorbed by semiconducting materials, such as
silicon.
Electrons (negatively charged) are knocked loose
from their atoms, allowing them to flow through
the material to produce electricity. Due to the
special composition of solar cells, the electrons
are only allowed to move in a single direction.
An array of solar cells converts solar energy into a
usable amount of direct current (DC) electricity.
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Structure
Modern solar cells are based on semiconductor physics -they are basically just P-N junction photodiodes with a very
large light-sensitive area. The photovoltaic effect, which
causes the cell to convert light directly into electrical energy,
occurs in the three energy-conversion layers.
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The first of these three layers necessary for energy
conversion in a solar cell is the top junction layer (made of
N-type semiconductor ). The next layer in the structure is
the core of the device; this is the absorber layer (the P-N
junction). The last of the energy-conversion layers is the
back junction layer (made of P-type semiconductor).
As may be seen in the above diagram, there are two
additional layers that must be present in a solar cell.
-electrical contact 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.
Solar cells: Thin end of the wedge
Tiny silver nanoparticles boost the efficiency of thinfilm solar cells
Published online 06 January 2010
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In the quest to reduce the costs of solar cells to increase the use
of solar energy, scientists are focusing on the use of cheap thin
films rather than thick wafers of silicon. However, light absorption
in thin films is often poor, which limits the minimum thickness of
a film.
Researchers from the Institute of High Performance Computing of
A*STAR, Singapore, in collaboration with co-workers from CSIRO
Materials Science and Engineering, Australia, have now revealed
how metallic nanostructures can enhance light absorption—even
in very thin silicon films—and thus increase the performance of
thin-film solar cells.
Silicon thin films are particularly poor at absorbing infrared light,
which means a broad range of incoming solar light is
squandered. New methods are required to overcome this
fundamental problem, points out Yuriy Akimov, who led the
research team.
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In the past few years, adding small metallic nanostructures to the films,
such as silver nanoparticles, has been proposed as a means to enhance
their efficiency. The nanoparticles act like tiny mirrors, but they
concentrate light much more strongly than conventional mirrors. The
effect is based on surface plasmons—the collective motions of electrons
at the nanoparticle surface—that intensify the incoming light and focus it
into the silicon layer (Fig. 1), which significantly improves light
absorption.
Although other researchers observed this effect previously, what has
been lacking is a detailed understanding of the influence of parameters
such as nanoparticle diameter and surface coverage. Akimov and his coworkers therefore simulated solar cell performance for a broad range of a
number of nanoparticle parameters. Although it proved difficult to
optimize all parameters simultaneously, a clear range of suitable
nanoparticle properties emerged. For example, they found that the
nanoparticle surface coverage required for sufficient enhancement of a
thin film can be as small as a few percent of the total area. Overall,
projected enhancements in light absorption can reach about 30%
compared to the same solar cell without nanoparticles.
“Nanoparticle-enhanced solar cells use quite complex phenomena and
require optimization studies for many parameters,” says Akimov.
Plasmonic enhancements are very sensitive to nanoparticle shape, so
structures other than spheres could enhance absorption even further.
Similarly, the combined use of different metals could also lead to
enhancements over a broad range of wavelengths.
Improved solar cells are therefore expected from the further optimization
of metallic nanostructures. Indeed, we may soon be able to buy solar
cells based on enhanced light emission facilitated by surface plasmons.
Fig. 1: Schematic diagram depicting a way
to boost solar cell performance. Silver
nanoparticles (Ag) are placed on a silicon
solar cell (a-Si:H), separated by a thin
transparent conductive oxide (ITO).
Incoming light (yellow arrow) is focused
onto the silicon layer, which increases the
photocurrent (I) in the solar cell.
Solar cell efficiency factors
Energy conversion efficiency
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Dust often accumulates on the glass of solar panels seen
here as black dots.
A solar cell's energy conversion efficiency (η, "eta"), is
the percentage of power converted (from absorbed light
to electrical energy) and collected, when a solar cell is
connected to an electrical circuit. This term is calculated
using the ratio of the maximum power point, Pm, divided
by the input light irradiance (E, in W/m2) under
standard test conditions (STC) and the surface area of
the solar cell (Ac in m2).
STC specifies a temperature of 25 °C and an irradiance
of 1000 W/m2
Solar cell efficiencies
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Solar cell efficiencies vary from 6% for amorphous
silicon-based solar cells to 40.7% with multiplejunction research lab cells and 42.8% with multiple
dies assembled into a hybrid package
Solar cell energy conversion efficiencies for
commercially available multicrystalline Si solar cells are
around 14-19%.
The highest efficiency cells have not always been the
most economical — for example a 30% efficient
multijunction cell based on exotic materials such as
gallium arsenide or indium selenide and produced in
low volume might well cost one hundred times as much
as an 8% efficient amorphous silicon cell in mass
production, while only delivering about four times the
electrical power.
Lifespan
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Most commercially available solar
cells are capable of producing
electricity for at least twenty years
without a significant decrease in
efficiency. The typical warranty
given by panel manufacturers is for
a period of 25 – 30 years, wherein
the output shall not fall below 85%
of the rated capacity
High-efficiency solar cells
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are a class of solar cell that can generate more electricity per
incident solar power unit (watt/watt).
Much of the industry is focused on the most cost efficient
technologies in terms of
cost per generated power. The two main strategies to bring down
the cost of photovoltaic electricity are increasing the efficiency of
the cells and decreasing their cost per unit area.
However, increasing the efficiency of a solar cell without
decreasing the total cost per kilowatt-hour is not more
economical, since sunlight is free. Thus, whether or not
"efficiency" matters depends on whether "cost" is defined as cost
per unit of sunlight falling on the cell, per unit area, per unit
weight of the cell, or per unit energy produced by the cell. In
situations where much of the cost of a solar system scales with its
area (so that one is effectively "paying" for sunlight), the
challenge of increasing the photovoltaic efficiency is thus of great
interest, both from the academic and economic points of view..
Multiple-junction solar cells
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The record for multiple junction solar cells
is disputed. Teams led by the University
of Delaware, the Fraunhofer Institute for
Solar Energy Systems, and NREL all claim
the world record title at 42.8, 41.1, and
40.8%, respectively
Spectrolab also claims commercial
availability of cells at nearly 42%
efficiency in a triple junction design.
Thin-film solar cells
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In 2002, the highest reported efficiency for thin film solar
cells based on CdTe is 18%, which was achieved by research
at Sheffield Hallam University, although this has not been
confirmed by an external test laboratory.
The US national renewable energy research facility NREL
achieved an efficiency of 19.9% for the solar cells based on
copper indium gallium selenide thin films, also known as CIGS
(also see CIGS solar cells).
These CIGS films have been grown by physical vapour
deposition in a three-stage co-evaporation process. In this
process In, Ga and Se are evaporated in the first step; in the
second step it is followed by Cu and Se co-evaporation and in
the last step terminated by In, Ga and Se evaporation again.
Thin film solar has approximately 15% marketshare; the
other 85% is crystalline silicon. Most of the commercial
production of thin film solar is CdTe with an efficiency of 11%.
Crystalline Silicon
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The highest efficiencies on silicon have been achieved
on monocrystalline cells. The highest commercial
efficiency (22%) is produced by SunPower, which uses
expensive, high-quality silicon wafers.
The University of New South Wales has achieved 25%
efficiency on monocrystalline silicon in the lab,
technology that has been commercialized through its
partnership with Suntech Power. Suniva, a U.S.
manufacturer of solar cells and modules using low-cost
techniques, has units with efficiencies of 18% currently
in commercial production, with a goal of putting 20%
cells currently in the laboratory into high-volume
production by 2011.
Crystalline silicon devices are approaching the
theoretical limiting efficiency of 29% and achieve an
energy payback period of 1–2 years.
Light-absorbing materials
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All solar cells require a light absorbing material contained within
the cell structure to absorb photons and generate electrons via the
photovoltaic effect. The materials used in solar cells tend to have
the property of preferentially absorbing the wavelengths of solar
light that reach the Earth surface. However, some solar cells are
optimized for light absorption beyond Earth's atmosphere as well.
Light absorbing materials can often be used in multiple physical
configurations to take advantage of different light absorption and
charge separation mechanisms.
Photovoltaic panels are normally made of either silicon or thin-film
cells:
Many currently available solar cells are configured as bulk materials
that are subsequently cut into wafers and treated in a "top-down"
method of synthesis (silicon being the most prevalent bulk
material).
Other materials are configured as thin-films (inorganic layers,
organic dyes, and organic polymers) that are deposited on
supporting substrates, while a third group are configured as
nanocrystals and used as quantum dots (electron-confined
nanoparticles) embedded in a supporting matrix in a "bottom-up"
approach. Silicon remains the only material that is well-researched
in both bulk (also called wafer-based) and thin-film configurations.
Low-cost solar cell
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Dye-sensitized solar cell, and luminescent solar
concentrators are considered low-cost solar cells.
This cell is extremely promising because it is
made of low-cost materials and does not need
elaborate apparatus to manufacture, so it can be
made in a DIY way allowing more players to
produce it than any other type of solar cell. In
bulk it should be significantly less expensive than
older solid-state cell designs. It can be
engineered into flexible sheets. Although its
conversion efficiency is less than the best thin film
cells, its price/performance ratio should be high
enough to allow it to compete with fossil fuel
electrical generation.
Silicon processing
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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 one to 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 are not 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 °C. 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 °C).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.
Silicon processing
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Another approach is also to reduce the amount of silicon used and
thus cost, is by micromachining wafers into very thin, virtually
transparent layers that could be used as transparent architectural
coverings.
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 cm2 per side into about 1000 slivers having
dimensions of 100 mm × 2 mm × 0.1 mm, yielding a total exposed
silicon surface area of about 2000 cm2 per side. As a result of this
rotation, the electrical doping and contacts that were on the face of
the wafer are located at the edges of the sliver, rather than at the
front and rear as in the case of 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).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.