Download How Solar Cells Work

How Solar Cells Work
by
Hiroaki Okamoto
Two of the most common questions about photovoltaic technology are "what is it?" and "why
should I use it?" Photovoltaic (PV) technology is basically "solar electricity" that results from
converting sunlight into electricity. The sun provides enough clean energy in one day to
provide a year's supply of FREE FUEL for powering buildings. Why we should consider
using PV is simple: it helps preserve the Earth's finite fossil-fuel resources such as coal, oil,
and natural gas. It also helps reduce air and water pollution associated with these energy
sources. When faced with power outages and future electricity price increases, we can also
benefit from the utilization of a PV system.
You've probably seen calculators that have solar cells -- calculators that never need
batteries, and in some cases don't even have an off button. As long as you have enough light,
they seem to work forever. You may have seen larger solar panels -- on emergency road signs
or call boxes, on buoys, even in parking lots to power lights. Although these larger panels
aren't as common as solar powered calculators, they're out there, and not that hard to spot if
you know where to look. There are solar cell arrays on satellites, where they are used to power
the electrical systems.
You have probably also been hearing
about the "solar revolution" for the last 20
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.
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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.
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).
Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight
directly into electricity. Once used almost exclusively in space, photovoltaics 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
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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.
Silicon
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.
Silicon in Solar Cells
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.
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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. So where has all this gotten us?
N-type Plus P-type Silicon
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.
Before now, our silicon was all electrically neutral. Our extra electrons were balanced out
by the extra protons in the phosphorous. Our missing electrons (holes) were balanced out by
the missing protons in the boron. When the holes and electrons mix at the junction between
N-type and P-type silicon, however, that neutrality is disrupted. Do all the free electrons fill
all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right
at the junction, however, they do mix and form a barrier, making it harder and harder for
electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and we have
an electric field separating the two sides.
This electric field acts as a diode,
allowing (and even pushing) electrons
to flow from the P side to the N side,
but not the other way around. It's like a
hill -- electrons can easily go down the
hill (to the N side), but can't climb it (to the P side). So we've got an electric field acting as a
diode in which electrons can only move in one direction. Let's see what happens when light
hits the cell.
When Light Hits the Cell
When light, in the form of photons, hits our solar cell, its energy frees electron-hole pairs.
Each photon with enough energy will normally free exactly one electron, and result in a free
hole as well. If this happens close enough to the electric field, or if free electron and free hole
happen to wander into its range of influence, the field will send the electron to the N side and
the hole to the P side. This causes further disruption of electrical neutrality, and if we provide
an external current path, electrons will flow through the path to their original side (the P side)
to unite with holes that the electric field sent there, doing work for us along the way. The
electron flow provides the current, and the cell's electric field causes a voltage. With both
current and voltage, we have power, which is the product of the two.
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How much sun- light energy does our PV cell absorb? Unfortunately, the most that our
simple cell could absorb is around 25 percent, and more likely is 15 percent or less.
Why so little?
Energy Loss
Why does our solar cell absorb only about 15 percents of the sunlight's energy? 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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Single crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also
used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as
single crystal silicon. Amorphous silicon, which has no crystalline structure, is also used,
again in an attempt to reduce production costs. Other materials used include gallium arsenide,
copper indium diselenide and cadmium telluride. Since different materials have different band
gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. One
way efficiency has been improved is to use two or more layers of different materials with
different band gaps. The higher band gap material is on the surface, absorbing high-energy
photons while allowing lower-energy photons to be absorbed by the lower band gap material
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beneath. This technique can result in much higher efficiencies. Such cells, called multijunction cells, can have more than one electric field.
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.
Finishing the Cell
There are a few more steps left before we can really use our cell. Silicon happens to be a
very shiny material, which means that it is very reflective. Photons that are reflected can't be
used by the cell. For that reason, an antireflective coating is applied to the top of the cell to
reduce reflection losses to less than 5 percent.
The final step is the glass cover plate that protects the cell from the elements. PV modules
are made by connecting several cells (usually 36) in series and parallel to achieve useful levels
of voltage and current, and putting them in a sturdy frame complete with a glass cover and
positive and negative terminals on the back.
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Powering a House
Now that we have our PV module, what do we do with it? What would you have to do to
power your house with solar energy? Although it's not as simple as just slapping some
modules on your roof, it's not extremely difficult to do, either.
First of all, not every roof has the correct orientation or angle of inclination to take
advantage of the sun's energy. Non-tracking PV systems in the Northern Hemisphere should
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point toward true south (this is the orientation). They should be inclined at an angle equal to
the area's latitude to absorb the maximum amount of energy year-round. A different
orientation and/or inclination could be used if you want to maximize energy production for
the morning or afternoon, and/or the summer or winter. Of course, the modules should never
be shaded by nearby trees or buildings, no matter the time of day or the time of year. In a PV
module, even if just one of its 36 cells is shaded, power production will be reduced by more
than half.
If you have a house with an unshaded, south-facing roof, you need to decide what size
system you need. This is complicated by the facts that your electricity production depends on
the weather, which is never completely predictable, and that your electricity demand will also
vary. These hurdles are fairly easy to clear. Meteorological data gives average monthly
sunlight levels for different geographical areas. This takes into account rainfall and cloudy
days, as well as altitude, humidity, and other more subtle factors. You should design for the
worst month, so that you'll have enough electricity all year. With that data, and knowing your
average household demand (your utility bill conveniently lets you know how much energy you
use every month),there are simple methods you can use to determine just how many PV
modules you'll need. You'll also need to decide on a system voltage, which you can control by
deciding how many modules to wire in series.
Obstacles
You may have already guessed a couple of problems that we'll have to solve. First, what do
we do when the sun isn't shining? Certainly, no one would accept only having electricity
during the day, and then only on clear days, if they have a choice. We need energy storage -batteries. Unfortunately, batteries add a lot of cost and maintenance to the PV system.
Currently, however, it's a necessity if you want to be completely independent. One way around
the problem is to connect your house to the utility grid, buying power when you need it and
selling to them when you produce more than you need. This way, the utility acts as a
practically infinite storage system. The utility has to agree, of course, and in most cases will
buy power from you at a much lower price than their own selling price. You will also need
special equipment to make sure that the power you sell to your utility is synchronous with
theirs -- that it shares the same sinusoidal waveform and frequency. Safety is an issue as well.
The utility has to make sure that if there's a power outage in your neighborhood, your PV
system won't try to feed electricity into lines that a lineman may think is dead. This is called
islanding.
If you decide to use batteries, keep in mind that they will have to be maintained, and then
replaced after a certain number of years. The PV modules should last 20 years or more, but
batteries just don't have that kind of useful life. Batteries in PV systems can also be very
dangerous because of the energy they store and the acidic electrolytes they contain, so you'll
need a well-ventilated, non-metallic enclosure for them.
Deep-cycle Batteries
What kind of batteries are used in PV systems? Although several different kinds are
commonly used, the one characteristic that they should all have in common is that they are
deep-cycle batteries. Unlike your car battery, which is a shallow-cycle battery, deep-cycle
batteries can discharge more of their stored energy while still maintaining long life. Car
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batteries discharge a large current for a very short time -- to start your car -- and are then
immediately recharged as you drive. PV batteries generally have to discharge a smaller current
for a longer period (such as all night), while being charged during the day.
The most commonly used deep-cycle batteries are lead-acid batteries (both sealed and
vented) and nickel-cadmium batteries. Nickel-cadmium batteries are more expensive, but last
longer and can be discharged more completely without harm. Even deep-cycle lead-acid
batteries can't be discharged 100 percent without seriously shortening battery life, and
generally, PV systems are designed to discharge lead-acid batteries no more than 40 percent or
50 percent.
Also, the use of batteries requires the installation of another component called a charge
controller. Batteries last a lot longer if care is taken so that they aren't overcharged or drained
too much. That's what a charge controller does. Once the batteries are fully charged, the
charge controller doesn't let current from the PV modules continue to flow into them.
Similarly, once the batteries have been drained to a certain predetermined level, controlled by
measuring battery voltage, many charge controllers will not allow more current to be drained
from the batteries until they have been recharged. The use of a charge controller is essential
for long battery life.
DC to AC
The other problem is that the electricity generated by your PV modules, and extracted
from your batteries if you choose to use them, is direct current, while the electricity supplied
by your utility (and the kind that every appliance in your house uses) is alternating current.
You will need an inverter, a device that converts DC to AC. Most large inverters will also
allow you to automatically control how your system works. Some PV modules, called AC
modules, actually have an inverter already built into each module, eliminating the need for a
large, central inverter, and simplifying wiring issues.
Throw
in
the
mounting
hardware,
wiring, junction boxes,
grounding equipment,
overcurrent protection,
DC
and
AC
disconnects and other
accessories and you
have yourself a system.
Electrical codes must
be followed (there's a
section in the National
Electrical Code just for
PV), and it's highly
recommended that the
installation be done by
a licensed electrician
who has experience with PV systems. Once installed, a PV system requires very little
maintenance (especially if no batteries are used), and will provide electricity cleanly and
quietly for 20 years or more.
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If photovoltaics are such a wonderful source of free energy, then why doesn't the whole
world run on solar power? Some people have a flawed concept of solar energy. While it's true
that sunlight is free, the electricity generated by PV systems is not. As you can see from our
discussion of a household PV system, quite a bit of hardware is needed. Currently, an installed
PV system will cost somewhere around $9 per peak Watt. To give you an idea of how much
a house system would cost, let's consider the Solar House -- a model residential home in
Raleigh, North Carolina, with a PV system set up by the North Carolina Solar Center to
demonstrate the technology. It's a fairly small home, and it is estimated that its 3.6-kW PV
system covers about half of the total electricity needs (this system doesn't use batteries -- it's
connected to the grid). Even so, at $9 per Watt, this installed system would cost you around
$32,000.
That's why PV is usually used in remote areas, far from a conventional source of electricity.
Right now, it simply can't compete with the utilities. Costs are coming down as research is
being done, however. Researchers are confident that PV will one day be cost effective in
urban areas as well as remote ones. Part of the problem is that manufacturing needs to be done
on a large scale to reduce costs as much as possible. That kind of demand for PV, however,
won't exist until prices fall to competitive levels. It's a Catch-22 situation. Even so, demand
and module efficiencies are constantly rising, prices are falling, and the world is becoming
increasingly aware of environmental concerns associated with conventional power sources,
making photovoltaics a technology with a bright future.
Appendix I: Light and the PV Cell
We've looked at how to construct a solar cell using crystalline silicon. And we've used this
basic type of cell to explain the photoelectric effect, which is the phenomenon operating at the
heart of a solar cell. Here, we want to take a look at sunlight, the energy source actually used
by solar cells. A brief discussion of several terms will help us better understand aspects of
light's interaction with solar cells.
Wavelength, Frequency, and Energy
The energy from the sun is vital to life
on Earth. It determines the Earth's surface
temperature and supplies virtually all the
energy that drives natural global systems
and cycles. Some other stars are enormous
sources of energy in the form of X-rays
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and radio signals, but our sun releases the
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However, visible light represents only a
fraction of the total spectrum of radiation.
Specifically, infrared and ultraviolet rays
are also significant parts of the solar
spectrum.
The sun emits almost all of its energy
in a range of wavelengths from about 2x10-7 to 4x10-6 meters. Most of this energy is in the
visible light region. Each wavelength corresponds to a frequency and an energy: the shorter
the wavelength, the higher the frequency and the greater the energy (which is expressed in
8
electron-volts, or eV). Red light is at the low-energy end of the visible spectrum and violet
light is at the high-energy end, where it has half again as much energy as red light. In the
invisible portions of the spectrum, radiation in the ultraviolet region, which causes the skin to
tan, has more energy than that in the visible region. Likewise, radiation in the infrared region,
which we feel as heat, has less energy than the radiation in the visible region.
Solar cells respond differently to the different wavelengths, or colors, of light. For example,
crystalline silicon can use the entire visible spectrum, plus some part of the infrared spectrum.
But energy in part of the infrared spectrum, as well as longer-wavelength radiation, is too low
to produce current flow. Higher-energy radiation can produce current flow, but much of this
energy is likewise not usable. In summary, light that is too high or low in energy is not usable
by a cell to produce electricity. Rather, it is transformed into heat.
Air Mass
The sun is continually releasing an enormous amount of radiant energy into the solar
system. The Earth receives a tiny fraction of this energy; yet, an average of 1367 watts (W)
reaches each square meter (m2) of the outer edge of the Earth's atmosphere. The atmosphere
absorbs and reflects some of this radiation, including most X-rays and ultraviolet rays. Still,
the amount of the sun's energy that reaches the surface of the Earth every hour is greater than
the total amount of energy that the world's human population uses in a year.
How much energy does
light lose in traveling from
the edge of the atmosphere
to the surface of the Earth?
This energy loss depends
on the thickness of the
atmosphere that the sun's
energy must pass through.
The radiation that reaches
sea level at high noon in a
clear sky is 1000 W/m2
and is described as "air
mass 1" (or AM1)
radiation. As the sun
moves lower in the sky, the light passes through a greater thickness (or longer path) of air,
losing more energy. Because the sun is overhead for only a short time, the air mass is
normally greater than one—that is, the available energy is less than 1000 W/m2.
Scientists have given a name to the standard spectrum of sunlight at the Earth's surface:
AM1.5G (where G stands for "global" and includes both direct and diffuse radiation,
described next) or AM1.5D (which includes direct radiation only). The number "1.5" indicates
that the length of the path of light through the atmosphere is 1.5 times that of the shorter path
when the sun is directly overhead.
The standard spectrum outside the Earth's atmosphere is called AM0, with no light passing
through the atmosphere. AM0 is typically used to predict the expected performance of PV
cells in space. The intensity of AM1.5D radiation is approximated by reducing the AM0
spectrum by 28%, where 18% is absorbed and 10% is scattered. The global spectrum is 10%
greater than the direct spectrum. These calculations give about 970 W/m2 for AM1.5G.
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However, the standard AM1.5G spectrum is "normalized" to give 1000 W/m2, because of
inherent variations in incident solar radiation.
Direct and Diffuse Light
As we have noted, the Earth's atmosphere and cloud cover absorb, reflect, and scatter some
of the solar radiation entering the atmosphere. Nonetheless, an enormous amount of the sun's
energy reaches the Earth's surface and can therefore be used to produce PV electricity. Some
of this radiation is direct and some is diffuse, and the distinction is important because some
PV systems (flat-pate systems) can use both forms of light, but concentrator systems can only
use direct light.
• Direct light consists of radiation that comes straight from the sun, without reflecting off of
clouds, dust, the ground, or other objects. Scientists also talk about direct-normal radiation,
referring to the portion of sunlight that comes directly from the sun and strikes the plane of
a PV module at a 90-degree angle.
• Diffuse light is sunlight that is reflected off of clouds, the ground, or other objects. It
obviously takes a longer path than a direct light ray to reach a module. Diffuse light cannot
be focused by the optics of a concentrator PV system.
• Global radiation refers to the total radiation that strikes a horizontal surface. Global sunlight
is composed of direct-normal and diffuse components of sunlight. Additionally, diffuse and
direct-normal sunlight generally
have different energy spectra or
distributions of color.
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Insolation
The actual amount of sunlight
falling on a specific geographical
location is known as insolation—or
"incident
solar
radiation."
Insolation values for a specific site
are sometimes difficult to obtain.
Weather stations that measure solar radiation components are located far apart and may not
carry specific insolation data for a given site. Furthermore, the information most generally
available is the average daily total—or global—radiation on a horizontal surface.
When sunlight reaches the Earth, it is distributed unevenly in different regions. Not
surprisingly, the areas near the Equator receive more solar radiation than anywhere else on the
Earth. Sunlight varies with the seasons, as the rotational axis of the Earth shifts to lengthen
and shorten days with the changing seasons. For example, the amount of solar energy falling
per square meter on Yuma, Arizona, in June is typically about nine times greater than that
falling on Caribou, Maine, in December. The quantity of sunlight reaching any region is also
affected by the time of day, the climate (especially the cloud cover, which scatters the sun's
rays), and the air pollution in that region. Likewise, these climatic factors all affect the amount
of solar energy that is available to PV systems.
10
Appendix II: Various PV Systems
A photovoltaic (PV) or solar cell is the basic building block of a PV (or solar electric)
system. An individual PV cell is usually quite small, typically producing about 1 or 2 watts of
power. To boost the power output of PV cells, we connect them together to form larger units
called modules. Modules, in turn, can be connected to form even larger units called arrays,
which can be interconnected to produce more power, and so on. In this way, we can build PV
systems able to meet almost any electric power need, whether small or large.
PV systems can be classified into two general categories: flat-plate systems or concentrator
systems. We will talk about the differences between these two types of systems later on.
By themselves, modules or arrays do not represent an entire PV system. We also need
structures to put them on that point them toward the sun, and components that take the directcurrent electricity produced by modules and "condition" that electricity, usually by converting
it to alternate-current electricity. We might also want to store some electricity, usually in
batteries, for later use. All these items are referred to as the "balance of system" (BOS)
components.
Combining modules with the BOS components creates an entire PV system. This system
is usually everything we need to meet a particular energy demand, such as powering a water
pump, or the appliances and lights in a home, or, if the PV system is large enough, all the
electrical requirements of a whole community.
PV in Simple, "Stand-Alone" Systems
Stand-alone PV systems are often best in places where utility-generated power is either
unavailable (because the area is so remote from power plants), undesirable (because of a
possible utility power outage in an emergency), or too costly to hook up to (because of the
price of extending power lines). Stand-alone systems are also excellent for uses that don't
require a lot of power.
The same sunny days that dry out plants, make animals thirsty, and heat up buildings and
cars just happen to be very good days for generating electricity with photovoltaics. And that's
exactly what stand-alone PV systems (those not connected to a utility power grid) do every
sunny day, and on some cloudy days, too, all over the world. The electricity is then used to
power water pumps for irrigation and drinking wells, for example, or ventilation fans for
cooling. For this reason, the most simple PV systems are those that generate direct-current
(dc) electricity so it can be used right away to run water pumps, fans, and many other
appliances that use dc electricity.
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These basic PV systems have several
advantages that make them suitable for
these jobs. First, they produce energy where
and when it's needed, so complex wiring,
storage, and control systems aren't needed.
Second, small systems that produce less
than 500 watts and weigh less than 68
kilograms (150 pounds) are easy to
transport and install. Most installations take
only a few hours. And, although pumps and fans require regular maintenance, PV modules
require only an occasional inspection and cleaning. See the case study on PV-powered water
for cattle for an example of a stand-alone system in action.
PV for Utility Power Production
When should utilities consider PV
power?
Actually,
large-scale
photovoltaic power plants, which
consist of many PV arrays working
together, can be very useful to utilities
in a variety of ways.
For example, utilities can build PV
plants much more quickly than they
can build conventional fossil or
nuclear power plants, because PV
arrays are fairly easy to install and
connect. Also, utilities can build PV
power plants where they're most
needed in the grid, because siting PV
arrays is usually much easier than siting a conventional power plant. And, unlike conventional
power plants, modular PV plants can be expanded incrementally as demand increases. Finally,
PV power plants consume no fuel and produce no air or water pollution while they silently
generate electricity. This makes PV power an attractive option for utilities that want or need to
cut fuel costs while meeting local environmental regulations.
So, why aren't more utilities building PV plants? There are actually several reasons. For
example, using current utility accounting practices, PV-generated electricity still costs more
than electricity generated by conventional plants in most places, and regulatory agencies
require most utilities to supply the lowest-cost electricity. Furthermore, photovoltaic systems
produce power only during daylight hours, and their output thus can vary with the weather.
Utility planners must therefore treat a PV power plant differently than they would treat a
conventional plant.
Despite these obstacles, more utilities are becoming more involved in PV power. For
example, DOE, the Electric Power Research Institute, and several utilities have formed a joint
venture called Photovoltaics for Utility-Scale Applications (PVUSA). This project operates
three pilot test stations in different parts of the country for utility-scale PV systems. The pilot
projects allow utilities to experiment with newly developing PV technologies with little
financial risk.
In another experiment, utilities are exploring connecting PV systems to the utility grid in
places where they have a higher value. For example, adding PV generation near the places
where the electricity is used prevents the energy losses associated with sending an electric
current long distances through conventional power lines. This means the PV system is worth
more to the utility when it can be placed near the customer.
PV systems could also be installed at places in utility distribution system service areas
where the population is increasing rapidly. In these places, using PV systems could eliminate
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a utility's need to increase the size of power lines as well as entire servicing areas. Installing
PV systems near other utility distribution equipment, such as substations, can also prevent
overloading of the equipment in the substation. For an example of a utility-scale PV
application, see the case study in this section.
PV Connected to the Utility Grid
Using grid-connected PV power can have
economic as well as environmental
advantages. Where utility power is available,
consumers can use a grid-connected PV
system to supply some of the power they
need and use utility-generated power at night
and on very cloudy days. When the PV
system supplies power to the grid as well as
to a specific building or piece of equipment,
the utility becomes a kind of storage device
or battery for PV-generated power.
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Several homeowners, considered pioneers
屋根瓦型システム
in the use of renewable energy, have sizable PV systems connected to the utility grid. They
like that the system reduces the amount of electricity they purchase from the utility each
month. They also like the fact that PV consumes no fuel and generates no pollution.
The owner of a grid-connected PV system can often sell as well as buy electricity each
month! This is because electricity generated by the PV system can be used on site or fed
through a meter into the utility grid. When a home or business requires more electricity than
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the PV array is generating (for example, in the evening), the need is automatically met by
utility power. When that home or business requires less electricity than the PV array is
generating, the excess can often be fed (or sold) back to the utility through net metering,
which is becoming more and more common throughout the nation. At the end of the month, a
credit for electricity sold is deducted from charges for electricity purchased. See also the case
study of an energy-efficient home for an example of this.
PV Systems with Battery Storage
PV systems with batteries for storage are
excellent for supplying electricity when and
where you need it. These systems are
especially suitable in areas where utility
power is unavailable or utility line
extensions would be too expensive. The
ability to store PV-generated electrical
energy makes the PV system a reliable
source of electric power both day and night,
rain or shine. PV systems with battery
storage are used all over the world to
provide electricity for lights, sensors,
recording equipment, switches, appliances,
telephones, televisions, and even power
tools!
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Speaking about the "Solar Independence"
PV system, John Thornton, engineer at the
National Center for Photovoltaics, says, "The objective is to raise people's awareness about
the value of these technologies. And the only way to do that is to show them the technology."
PV systems with batteries can be designed to power equipment that requires dc or ac
electricity. People running conventional ac equipment will add a power conditioning device
called an inverter between the batteries and the load. Although a small amount of energy is
lost in converting dc electricity to ac, an inverter makes PV-generated electricity behave like
utility power so it can operate everyday ac appliances, lights, and even computers.
We operate PV/battery systems by connecting the photovoltaic modules to a battery, and
the battery, in turn, to the load. During the day, the PV modules charge the battery, and then
the battery supplies power to the load as needed. A simple electrical device called a charge
controller keeps the batteries charged properly and helps prolong their life by protecting them
from overcharging or from being completely drained.
Batteries make PV systems useful in more situations, but also require some maintenance.
The batteries used in PV systems are similar to car batteries, but they're built somewhat
differently to allow more of their stored energy to be used each day. (They're said to be "deep
cycling," like the batteries used on golf carts.) Batteries designed for PV projects pose the
same risks and demand the same caution in handling and storage as automotive batteries. We
need to check the fluid in unsealed batteries periodically, and protect them from extremely
cold weather.
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The amount of electricity that can be used after sunset or on cloudy days is determined by
the output of the PV modules and the nature of the battery bank. Including more modules and
batteries increases system costs, so energy usage needs to be studied carefully to determine the
best system size for the load. A well-designed PV-battery system balances cost and
convenience with meeting the user's needs, and it can be expanded if those needs change. See
an example of a PV-battery system used in a modern residence for more information.
Appendix III: Solar Cell Performance
We measure the performance of a PV device to predict the power the cell will produce.
Current-voltage (I-V) relationships that measure the electrical characteristics of PV devices
are depicted by what we call "I-V curves." These I-V curves are obtained by exposing the cell
to a constant level of light, while maintaining a constant cell temperature, varying the
resistance of the load, and measuring the current that is produced.
On an I-V plot, the vertical axis refers to current and the horizontal axis refers to voltage.
The actual I-V curve typically passes through two significant points:
•
The short-circuit current (Isc) is the current produced when the positive and negative
terminals of the cell are short-circuited, and the voltage between the terminals is zero, which
corresponds to a load resistance of zero.
•
The open-circuit voltage (Voc) is the voltage across the positive and negative
terminals under open-circuit conditions, and the current is zero, which corresponds to a load
resistance of infinity.
The cell may be operated over a wide range of voltages and currents. By varying the load
resistance from zero (a short circuit) to infinity (an open circuit), we can determine the highest
efficiency as the point where the cell delivers maximum power. Remember that power is the
product of voltage times current. Therefore, on the I-V curve, the maximum-power point
(Pm) occurs where the product of current times voltage is a maximum. No power is produced
at the short-circuit current with no voltage, or at open-circuit voltage with no current. So we
expect to find maximum power generated somewhere between these two points. Maximum
power is generated at only one place on the power curve, at about the "knee" of the curve.
This point represents the maximum efficiency of the solar device in converting sunlight into
electricity.
A parameter known as fill factor measures the "squareness" of the I-V curve and describes
the degree to which the voltage at the maximum power point (Vmp) matches Voc and that the
current at the maximum power point (Imp) matches Isc. The higher the fill factor's percentage
or match, the "squarer" the curve.
The conversion efficiency of a solar cell is the percentage of the solar energy shining on a
PV device that is converted into electrical energy, or electricity. Improving this conversion
efficiency is a key goal of much research and helps to make PV technologies cost competitive
with more traditional sources of energy. The efficiency of solar cells is affected by a variety of
factors. For an example, solar cells work best at low temperatures, as determined by their
material properties. All cell materials lose efficiency as the operating temperature rises. Much
of the light energy shining on cells becomes heat, so it is good to either match the cell material
to the operation temperature or continually cool the cell.
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Appendix IV: Why Solar Photovoltaics ?
The world population is now rapidly increasing mainly in developing countries toward the
mid 21st century. According to the prediction by the United Nations, it will reach 10 billion
by the year 2050. To support this ever-increasing population, and to provide overall
improvements in the living standard, the world economy will have to continue to grow at an
annual rate of some 3 percents over this period. Along with this economic growth, the
global energy demand around the year 2050 will increase to two or three times that in 2000 in
terms of oil consumption, which is anticipated to be completely exhausted within these 100
years. An efficient utilization of alterenative & renewable energy resources including solar
energy is then the most important technological evolution to resolve the energy issue as well
an the environmental issue. The wide-spreaded application of solar cells is indispensable not
only to accelerate the acceptance of renewable energy technologies but also to meet the worldwide demands for environmentally allowable electricity power generation.
Scientists estimate that the solar radiation falling on the Earth is only 1/22-billionths of the
energy that the sun emits. Yet just one hour of the energy that the Earth receives is equal to
the total amount of energy consumed by humans in one year. Solar radiation can be utilized
in various forms. Examples of direct utilization on a comparatively large scale include
photovoltaic and solar heat power generation. Wind, hydroelectric and wave power generation
can be considered examples of energy derived from solar radiation.
Clean: The combustion of fossil fuels results in the production of gases such as carbon
dioxide and nitrogen dioxide. These gases have a significant, negative influence on the Earth's
environment. Solar energy utilization, however, produces very little exhaust gas and is
extremely clean.
Infinite: Many resources on Earth exist in limited amounts. For instance, it is estimated that
petroleum will be exploitable for another 43 years, coal for 174 years, natural gas for 56 years,
and uranium for 66 years. Solar energy, however, will exist virtually forever. To achieve
practical utilization of solar energy, the following characteristics present a significant
challenge to technological development and must be taken into account.
Wide fluctuation: Insolation varies greatly according to climate and day-to-day weather
conditions.
Low density: Although the amount of energy falling on the Earth is vast, its density is 1
kW/m2, lower than would be expected.
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