Green technologies

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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 Singapores 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.

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What is technology and green technology?

• 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.

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The goals that inform developments in this
rapidly growing field include

• 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.

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Examples of green technology subject areas

• 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 buildingGreen building encompasses
  everything from the choice of building materials
  to where a building is located.
• Environmentally preferred purchasingThis
  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 chemistryThe invention, design and
  application of chemical products and processes to
  reduce or to eliminate the use and generation of
  hazardous substances.
• Green nanotechnologyNanotechnology 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.

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Renewable energy

• 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.

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Mainstream forms of renewable energy

• Wind power
• Hydropower
• Solar energy
• Biomass
• Biofuel

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Wind power

• Airflows can be used to run wind turbines.
• Turbines with rated output of 1.53 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

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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.
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Conventional

• 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.

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Other ways

• Pumped-storage
• 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
• 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.

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Advantages of hydroelectricity

• Economics
• 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.
• Hydroelectric plants also tend to have longer
  economic lives than fuel-fired generation, with
  some plants now in service which were built 50 to
  100 years ago.
• Operating labor cost is also usually low, as
  plants are automated and have few personnel on
  site during normal operation.
• 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.

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Advantages of hydroelectricity

• 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 fossil-fuel 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.

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Disadvantages of hydroelectricity

• 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 re-licensing 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.

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Comparison with other methods of power generation

• 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.

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Biomass

• 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.

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Biofuel

• 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

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Solar energy

• 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.

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Energy storage methods

• 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 time-of-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.

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Solar cells

• 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.

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Theory

• 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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• StructureModern 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.

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Solar cells Thin end of the wedgeTiny silver
nanoparticles boost the efficiency of thin-film
solar cells Published online 06 January 2010

• 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 ASTAR, Singapore, in
  collaboration with co-workers from CSIRO
  Materials Science and Engineering, Australia,
  have now revealed how metallic nanostructures can
  enhance light absorptioneven in very thin
  silicon filmsand 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 plasmonsthe
  collective motions of electrons at the
  nanoparticle surfacethat 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 co-workers 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.
•  

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Fig. 1 Schematic diagram depicting a way to
boost solar cell performance. Silver
nanoparticles (Ag) are placed on a silicon solar
cell (a-SiH), 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.
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Solar cell efficiency factorsEnergy conversion
efficiency

• Dust often accumulates on the glass of solar
  panels seen here as black dots.
• A solar cell's energy conversion efficiency (?,
  "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

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Solar cell efficiencies

• Solar cell efficiencies vary from 6 for
  amorphous silicon-based solar cells to 40.7 with
  multiple-junction 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.

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Lifespan

• 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

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High-efficiency solar cells

• 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..

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Multiple-junction solar cells

• 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.

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Thin-film solar cells

• 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.

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Crystalline Silicon

• 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 12 years.

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Light-absorbing materials

• 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.

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Low-cost solar cell

• 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.

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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 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.

39
Silicon processing

• 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.