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Green technologies

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Title: Green technologies


1
Green technologies
2
(No Transcript)
3
  • 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.

4
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.

5
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.

6
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.

7
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.

8
Mainstream forms of renewable energy
  • Wind power
  • Hydropower
  • Solar energy
  • Biomass
  • Biofuel

9
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

10
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.
11
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.

12
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.

13
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.

14
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.

15
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.

16
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.

17
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.

18
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

19
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.

20
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.

21
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.

22
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.

23
  • 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.

24
  • 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.

25
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.

26
  • 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.
  •  

27
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.
28
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

29
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.

30
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

31
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..

32
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33
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.

34
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.

35
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.

36
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.

37
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.

38
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.
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