In the long term,
solar energy is the best answer to the greenhouse effect. Solar energy is CO2
free and virtually inexhaustible. According to the International Energy Agency
(IEA), at present, solar energy constitutes a mere 0.04% of worldwide energy
consumption whereas fossil fuels feed over 80% of the world energy supply.
However, some key facts on solar energy highlight its significant potential:
- Two billion
people in the world have no access to electricity. For most of them, solar
energy would be their cheapest electricity source, but they cannot afford
it.
- Unlike fossil
fuels, which produce significant amounts of pollution and enormous amounts
of greenhouse gases, the sun's energy is clean and its supply virtually
limitless.
- In just one hour,
the Earth receives more energy from the sun than human beings consume
during an entire year.
- According to America's Department of Energy, solar
panels could, if placed on about 0.5% of USA’s mainland landmass,
provide for all of its current electricity needs.
- The Sun has
sufficient helium mass to provide the Earth with energy for another five
billion years and, every 15 minutes, it emits more energy than humankind
uses in an entire year.
- The Earth
receives only one half of one billionth of the Sun's radiant energy, but
in just a few days it gets as much heat and light as could be produced by
burning all the oil, coal and wood on the planet.
In addition to the dramatic benefits to reducing global
warming, harnessing solar power can reap significant benefits for individuals
and businesses alike. For example, businesses can use solar energy to:
- Reduce the risk
of volatile and rising fossil fuel prices, thereby reducing or stabilizing
operating costs, particularly as governments move to tax carbon emissions.
- Take advantage of
government incentives and rebates which are designed to increase the use
of renewable energy sources.
- Reduce the risk
and cost of power outages and backup power systems.
- Strengthen
relationships with key stakeholders such as customers and the community,
by demonstrating sensitivity to climate change issues.
Solar panels absorb sunlight through their silicon
membrane and turn the energy absorbed into useable power.
You have probably also been
hearing about the "solar revolution" for the last 22 years -- the
idea that one day we will all use free electricity from the sun. This is a
seductive promise: On a bright, sunny day, the sun shines approximately 1,000
watts of energy per square meter of the planet's surface, and if we could
collect all of that energy we could easily power our homes and offices for
free.
In this article, we will examine solar cells to learn how
they convert the sun's energy directly into electricity. In the process, you
will learn why we are getting closer to using the sun's energy on a daily
basis, and why we still have more research to do before the process becomes
cost effective.
Photovoltaic Cells: Converting
Photons to Electrons
The solar cells that you see on calculators and
satellites are photovoltaic cells or modules (modules are simply a group of
cells electrically connected and packaged in one frame). Photovoltaic’s, as the
word implies (photo = light, voltaic = electricity), convert sunlight directly
into electricity. Once used almost exclusively in space, Photovoltaic’s are
used more and more in less exotic ways. They could even power your house.
How do these devices work?
Photovoltaic (PV) cells are
made of special materials called semiconductors such as silicon, which is
currently the most commonly used. Basically, when light strikes the cell, a
certain portion of it is absorbed within the semiconductor material. This means
that the energy of the absorbed light is transferred to the semiconductor. The
energy knocks electrons loose, allowing them to flow freely.
PV cells also all have one or more electric fields that act to force electrons
freed by light absorption to flow in a certain direction. This flow of
electrons is a current, and by placing metal contacts on the top and bottom of
the PV cell, we can draw that current off to use externally. For example, the
current can power a calculator. This current, together with the cell's voltage
(which is a result of its built-in electric field or fields), defines the power
(or wattage) that the solar cell can produce.
That's
the basic process, but there's really much more to it. Let's take a deeper look
into one example of a PV cell: the single-crystal silicon cell.
How Silicon Makes a Solar
Cell
Silicon has
some special chemical properties, especially in its crystalline form. An atom
of silicon has 14 electrons, arranged in three different shells. The first two
shells, those closest to the center, are completely full. The outer shell,
however, is only half full, having only four electrons. A silicon atom will
always look for ways to fill up its last shell (which would like to have eight
electrons). To do this, it will share electrons with four of its neighbor
silicon atoms. It's like every atom holds hands with its neighbors, except that
in this case, each atom has four hands joined to four neighbors. That's what
forms the crystalline structure, and that structure turns out to be important
to this type of PV cell.
We've now described pure, crystalline silicon. Pure silicon is a poor conductor
of electricity because none of its electrons are free to move about, as
electrons are in good conductors such as copper. Instead, the electrons are all
locked in the crystalline structure. The silicon in a solar cell is modified
slightly so that it will work as a solar cell.
A solar cell has silicon with impurities -- other atoms mixed in with the
silicon atoms, changing the way things work a bit. We usually think of
impurities as something undesirable, but in our case, our cell wouldn't work
without them. These impurities are actually put there on purpose. Consider
silicon with an atom of phosphorous here and there, maybe one for every million
silicon atoms.
Phosphorous has five electrons in its outer shell, not four. It still bonds
with its silicon neighbor atoms, but in a sense, the phosphorous has one
electron that doesn't have anyone to hold hands with. It doesn't form part of a
bond, but there is a positive proton in the phosphorous nucleus holding it in
place.
When energy is added to pure silicon, for example in the form of heat, it can
cause a few electrons to break free of their bonds and leave their atoms. A
hole is left behind in each case. These electrons then wander randomly around
the crystalline lattice looking for another hole to fall into. These electrons
are called free carriers, and can carry electrical current.
There are so few of them in pure silicon,
however, that they aren't very useful. Our impure silicon with phosphorous
atoms mixed in is a different story. It turns out that it takes a lot less
energy to knock loose one of our "extra" phosphorous electrons
because they aren't tied up in a bond -- their neighbors aren't holding them
back. As a result, most of these electrons do break free, and we have a lot
more free carriers than we would have in pure silicon. The process of adding
impurities on purpose is called doping, and when doped with phosphorous, the
resulting silicon is called N-type ("n" for negative) because of the
prevalence of free electrons. N-type doped silicon is a much better conductor
than pure silicon is.
Actually, only part of our solar cell is N-type. The other part is doped with
boron, which has only three electrons in its outer shell instead of four, to
become P-type silicon. Instead of having free electrons, P-type silicon
("p" for positive) has free holes. Holes really are just the absence
of electrons, so they carry the opposite (positive) charge. They move around
just like electrons do.
The interesting part starts when you put N-type silicon together with P-type
silicon. Remember that every PV cell has at least one electric field. Without
an electric field, the cell wouldn't work, and this field forms when the N-type
and P-type silicon are in contact. Suddenly, the free electrons in the N side,
which have been looking all over for holes to fall into, see all the free holes
on the P side, and there's a mad rush to fill them in.
Anatomy of a Solar Cell
Solar energy is renewable energy for humans. It's also clean
energy, do not generate any environmental pollution. Solar photovoltaic (PV)
was the most watched item in the researching of solar energy utilize.
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| Solar PV Panel |
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The production of solar cells
based on semiconductor materials, and its working principle is photovoltaic
materials photoelectron conversion reaction after absorb light energy,
according to different materials, solar cells can be divided into:
1, silicon
solar cells;
2 multi-material cells using inorganic salts such as gallium
arsenide III-V compounds, cadmium sulfide, copper indium selenium compounds;
3,
polymer materials solar cells;
4, nano-crystalline solar cells.
etc.
1.Silicon solar cells
Silicon solar cell's structure and working principle,
Solar cells' elements is the photoelectric effect of semiconductors, normally
simiconductors have below structure:
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| Solar Cell |
As shown in the picture,
positive charge(+) means silicon atom,
negtive charge(-) means electron around
the silicon atom.
A hole will exist in the crystalline silicon when the
cyrstalline silicon mixed
with boron,
it's shape as below picture:
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| Silicon Crystalline |
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In the picture, Positive
charge (+) means silicon atom, Negetive charge(-) means electron around the
silicon atom. and the yellow means mixed boron atom, as only 3 electron around
the boron atom, it's bring the hole as in blue, this hole is unstable as it's
without electron, easily absorb electron to neutralize to be a P(positive) type
semiconductor.
Sameness, when mixed with phosphor atom, it's become highly active as the
phosphor atom have 5 electron, it's comes the N(negative) type semiconductor.
as shown in below picture, the yellow means Phosphor atom, the red means
superfluous electron.
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yellow means Phosphor atom,
the red means superfluous electron.
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N type semiconductor contains
more hole, while the P type semiconductor contains more electron, in this way,
the electric potential difference will be formed when the P and N type
semiconductor combine, that comes the PN junction.
When the P and N-type semiconductor combine, the two types of semiconductors at
the interface region will form a special thin-layer, the P side contains
negative electron, N side contains positive electron. This is because P-type
semiconductor have many hole, N-type semiconductor have many free electrons.
Electron from N-zone will be spread to the P-zone, hole from the P-zone will
spread to the N-zone.
When the lights reach the
crystalline silicon, the hole from N-type semiconductor move to P zone, and
electron from P-zone move to N-zone, that formed the electric current from
N-zone to P-zone, then formed the electric potential difference, that comes the
electricity source. (shown in below picture)
Because the semiconductor
is not a good conductor of electricity, the electron will waste very much when
passed the P-N junction and flow in semiconductor as it's large resistance.
However, if painted a metal upper, sunlight can not going through, electric
current will not be able to produce, so in general with a metal mesh covering
the p-n junction, in order to increase the size of the incident light.
In addition, the silicon surface is very bright, will reflect many of the sun lights,
could not be used by the solar cells. Therefore, scientists painted it with a
very small reflectance film, to decrease the sunlight reflection loss below 5%
or eve less. A single solar cell can provide only a limited current and
voltage, so people join many pieces of solar cells (usually 36) in parallel or
series to become the solar modules.
2. Crystalline silicon solar cell manufacturing process.
Usual
crystalline silicon solar cells are made up from the high-quality silicon at
thickness of 350 ~ 450μm, such silicon wafers are cutted from Czochralski or
casted silicon ingot
The above method consum more silicon material. In
order to save materials, the current preparation of polycrystalline silicon
thin-film solar cells using chemical vapor deposition method, including low
pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor
deposition (PECVD) process. In addition, liquid phase epitaxy (LPPE) and
sputtering deposition method can also be used to prepare poly-silicon thin-film
battery.
Chemical vapor deposition mainly the SiH2Cl2, SiHCl3, SiCl4 or SiH4, as the
reaction gas, It's react at a certain protection atmosphere and deposit silicon
atoms at the heated substrate, the general substrate materials are Si, SiO2,
Si3N4, etc.. But the researching found that it's difficult to form the large
crystal on the amorphous silicon (a-si) substrates and easy to cause interspace
between crystal. Solutions for this problem is to deposited a thin layer of
amorphous silicon on the substrate by LPCVD, then annealing this layer of
amorphous silicon, to get larger crystal, and then deposited a more thick
poly-crystalline silicon film at this layer, therefore, re-crystallization
technology is no doubt a very important aspect, the current technology used is
solid-phase crystallization and recrystallization in the FZ method. Poly-silicon
thin-film solar cells not onlyi use the re-crystallization process, also used almost
all of the mono-crystalline silicon solar cells preparation technology, the
solar cells made by this way have a remarkably increased it's conversion
efficiency.
3. Nanocrystalline chemistry solar cell
Silicon solar cells are undoubtedly the most sophisticated atom all solar
cells, but because of its high cost, can not meet the requirements of
large-scale application. Therefore, Peoples always explore in process, new
material and thin film solar cells etc, among this, the newly developed nano
TiO2 crystalline chemistry solar cells get a great importance from home and
abroad scientists.
For example, the dye-sensitized nano-crystalline solar cell (DSSCs), such solar
cells mainly includes a glass substrate deposited with trasparent conductive
film, dye-sensitized semiconductor materials, electrode and electrolyte etc.
As shown in below picture, the white ball means TiO2, red ball means dye
molecules. Dye molecules transite to excited state after absorb solar energy,
excited state unstable, the electron rapidly injected into the nearby TiO2
conduction band, Dye lost the electron is quickly be compensated from the
electrolyte, electron enter the conduction band of TiO2 and eventually enter
the electric conductive film, and then through the outer loop photo-current
generated.
Nano-crystallineTiO2 solar
cells have it's advantages of cheap cost, simple production process and a
stable performance. Photoelectric efficiency stability at 10%, and the
production costs is only 1 / 5 ~ 1 / 10 of silicon solar cells. Life expectancy
can achieve more than 20 years. However, because of such a solar cell
researching and development still in its infancy, it is estimated to be in the
market gradually.
Anode: dye-sensitized semi-conductive thin film ( TiO2
film)
Cathode: TCO glass deposted with platinic
Electrolyte: I3-/I-
How do solar panels
work?
Solar panels collect solar
radiation from the sun and actively convert that energy to electricity. From a
solar-powered calculator to an international space station, solar panels
generate electricity using the same principles of electronics as chemical
batteries or standard electrical outlets. With solar panels, it's all about the
free flow of electrons through a circuit.
To understand how solar panels generate electrical power, it might help to take
a quick trip back to high school chemistry class. The basic element of solar
panels is the same element that helped create the computer revolution -- pure
silicon. When silicon is stripped of all impurities, it makes a ideal neutral
platform for the transmission of electrons. Silicon also has some atomic-level
properties which make it even more attractive for the creation of solar panels.
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| Solar CCTV System |
Silicon atoms have room for eight electrons in their outer bands, but only
carry four in their natural state. This means there is room for four more
electrons. If one silicon atom contacts another silicon atom, each receives the
other atom's four electrons. This creates a strong bond, but there is no
positive or negative charge because the eight electrons satisfy the atoms'
needs. Silicon atoms can combine for years to result in a large piece of pure
silicon. This material is used to form the plates of solar panels.
Here's where science enters the picture. Two plates of pure silicon would not
generate electricity in solar panels, because they have no positive or negative
charge. Solar panels are created by combining silicon with other elements that
do have positive or negative charges.
Phosphorus, for example, has five electrons to offer to other atoms. If silicon
and phosphorus are combined chemically, the result is a stable eight electrons
with an additional free electron along for the ride. It can\'t leave, because
it is bonded to the other phosphorus atoms, but it isn't needed by the silicon.
Therefore, this new silicon/phosphorus plate is considered to be negatively
charged.
In order for electricity to flow, a positive charge must also be created. This
is achieved in solar panels by combining silicon with an element such as boron,
which only has three electrons to offer. A silicon/boron plate still has one
spot left for another electron. This means the plate has a positive charge. The
two plates are sandwiched together in solar panels, with conductive wires
running between them.
With the two plates in place, it's now time to bring in the 'solar' aspect of
solar panels. Natural sunlight sends out many different particles of energy,
but the one we're most interested in is called a photon. A photon essentially
acts like a moving hammer. When the negative plates of solar cells are pointed
at a proper angle to the sun, photons bombard the silicon/phosphorus atoms.
Eventually, the 9th electron, which wants to be free anyway, is knocked off the
outer ring. This electron doesn't remain free for long, since the positive
silicon/boron plate draws it into the open spot on its own outer band. As the
sun's photons break off more electrons, electricity is generated. The
electricity generated by one solar cell is not very impressive, but when all of
the conductive wires draw the free electrons away from the plates, there is
enough electricity to power low amperage motors or other electronics. Whatever
electrons are not used or lost to the air are returned to the negative plate
and the entire process begins again.
One of the main problems with using solar panels is the small amount of
electricity they generate compared to their size. A calculator might only
require a single solar cell, but a solar-powered car would require several
thousand. If the angle of the solar panels is changed even slightly, the
efficiency can drop 50 percent.
Some power from solar panels can be stored in chemical batteries, but there
usually isn't much excess power in the first place. The same sunlight that
provides photons also provides more destructive ultraviolet and infrared waves,
which eventually cause the panels to degrade physically. The panels must also
be exposed to destructive weather elements, which can also seriously affect
efficiency.
Many sources also refer to solar panels as photovoltaic cells, which references
the importance of light (photos) in the generation of electrical voltage। The challenge for future scientists will be to create
more efficient solar panels are small enough for practical applications and
powerful enough to create excess energy for times when sunlight is not
available.
Energy Loss in a Solar Cell
Visible light is only part of the electromagnetic spectrum. Electromagnetic
radiation is not monochromatic -- it is made up of a range of different
wavelengths, and therefore energy levels. Light can be separated into different
wavelengths, and we can see them in the form of a rainbow. Since the light that
hits our cell has photons of a wide range of energies, it turns out that some
of them won't have enough energy to form an electron-hole pair. They'll simply
pass through the cell as if it were transparent. Still other photons have too
much energy. Only a certain amount of energy, measured in electron volts (eV)
and defined by our cell material (about 1.1 eV for crystalline silicon), is
required to knock an electron loose. We call this the band gap energy of a material.
If a photon has more energy than the required amount, then the extra energy is
lost (unless a photon has twice the required energy, and can create more than
one electron-hole pair, but this effect is not significant). These two effects
alone account for the loss of around 70 percent of the radiation energy
incident on our cell.
Why can't we choose a material with a really low band gap, so we can use more
of the photons? Unfortunately, our band gap also determines the strength
(voltage) of our electric field, and if it's too low, then what we make up in
extra current (by absorbing more photons), we lose by having a small voltage.
Remember that power is voltage times current. The optimal band gap, balancing
these two effects, is around 1.4 eV for a cell made from a single material.
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| Speed Dome with Solar |
We have other losses as
well. Our electrons have to flow from one side of the cell to the other through
an external circuit. We can cover the bottom with a metal, allowing for good
conduction, but if we completely cover the top, then photons can't get through
the opaque conductor and we lose all of our current (in some cells, transparent
conductors are used on the top surface, but not in all). If we put our contacts
only at the sides of our cell, then the electrons have to travel an extremely
long distance (for an electron) to reach the contacts.
Remember, silicon is a semiconductor -- it's not nearly as good as a metal for
transporting current. Its internal resistance (called series resistance) is
fairly high, and high resistance means high losses. To minimize these losses,
our cell is covered by a metallic contact grid that shortens the distance that
electrons have to travel while covering only a small part of the cell surface.
Even so, some photons are blocked by the grid, which can't be too small or else
its own resistance will be too high.
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