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The French scientist Alexandre
Edmond Becquerel discovered around 1840 that
some materials produce a current (electricity) when light shines on them. PV cells,
as we know them now, were first developed in
1954 by Bell Telephone researchers and
first applied to power satellites in space. Over the past three decades,
cost has been decreasing continuously, while efficiencies have been increasing.
PV cells are now widely used to power:
- the utility grid (in modest amounts)
- Satellites (the first application to use PV
- Highway signs
- Remote transmitters, pumps, and other equipment
- Construction equipment lighting
- outdoor lights over doorways and along sidewalks
Project: See how many applications you can find
in your own neighborhood!
Types of PV Cells
Most PV cells are made from purified silicon, which
is doped with other elements to achieve the desired photoelectric properties.
There are several basic kinds of cells:
- Crystalline: Each cell is a single
silicon crystal: These have pretty high efficiency (10-30%), and very long
lifetimes because the crystal structure is very stable.
- Polycrystalline: Each cell is a
collection of many small silicon crystals, as one can see in the photo
below. These also have pretty high
efficiency (10-30%), and very long lifetimes because the crystal structure
is very stable.
- Thin film or amorphous silicon: These are
the type most commonly found in calculators. Each cell is made from
non-crystalline thin films of silicon atoms, and typically have a uniform
gray appearance. These tend to have lower efficiencies (5-10%), and more
limited lifetimes because the silicon atoms have some freedom to move around
over time. On the other hand, thin film cells can be much cheaper and
require much less energy to
produce, and promising efforts are being made to extend their lifetimes.
Typical commercially available PV panels have an
efficiency of about 15%, which means that they can deliver about 150 watts of
power per square meter.
are mostly made of silicon. Each silicon atom has four electrons in its
outermost (valence) shell. To complete the shell and achieve the most stable
configuration, the atom would like to have eight instead (this is due to the
quantum mechanical properties of electron orbitals). To achieve this, each
silicon atom shares each of its four electrons with four other silicon atoms.
This sharing of atoms binds the atoms to each other, and these bonds are called
"covalent" bonds. These covalent bonds cause the silicon atoms to form
a very stable silicon crystal. Because each of these other four atoms also each share one of
their electrons with the original atom, our original atom gets to use eight
electrons, and so achieves the stable configuration it likes.
all the valence electrons are involved in the covalent bonds, they can't move
from one atom to another, and therefore a pure silicon crystal is a very bad
conductor of electricity.
we can make the silicon crystal conduct electricity with a sneaky trick: we add
a small number of phosphorous atoms to the silicon crystal. Each phosphorous
atom has five electrons in its valence shell, instead of four. But only four of
these electrons are needed to bond with four nearby silicon atoms, so the fifth
one is left over. Because it is not involved in a bond, it is can move much more
freely through the silicon.
process of adding another element is called "doping". As we have seen,
when phosphorous is the dopant, extra electrons are added. Because electrons
have a negative charge, we call the doped material "n-material", where
the n stands for the negative charge of the electrons. Its important to keep in
mind that n-material doesn't have a net negative charge, because the
nucleus of the phosphorous atoms have an extra proton as well (relative to
silicon), and this balances out the extra electrons. What the n-material does
have that the silicon doesn't have is charge carriers that can move, and so can
way that charge carriers can be added to the silicon is to add an element such
as boron, which has only three instead of four electrons in its valence shell.
The doped silicon crystal that results will then have electron vacancies in its
structure, called "holes". These holes can actually move, because
nearby electrons can fill these holes, leaving behind a new hole nearby. This
kind of material is called p-material, where p stands for positive, because we
may think of the holes as having a positive charge.
electron-hole concept may seem a little tricky at first. The simplest way to
think of it is simply that in the p-material, the electrons can't move unless
other electrons move out of their way. A hole is simply the space created by an
electron moving out of the way.
any case, for either p or n type material, electrons can move, so that
electricity can be conducted.
the two types of material are brought together, say, with the n-material on the
top, a very interesting thing happens. Some of the extra, mobile electrons in
n-material migrate over into the p-material and
fill some of
the holes there. This makes the upper layer of the p-material negatively charged, while the
n-material now lacks electrons and becomes positively
charged. In the diagram below, these charges are symbolized with minus signs
(for the negative charges), and plus signs (for the positive charges).
These charges create an electric field, or voltage, across
of the two wafers, called a p-n junction, balances (stops) further
(net) migration. This electric field remains permanently "built-in".
there is no sunlight shining on the material, there is no net movement of electrons in the
material, despite the fact that there is an electric field inside the material.
of light strikes the material, however, some normally non-mobile
the material absorb the photons, and become mobile by
virtue of their increased energy. This creates new holes too - which are just
the vacancies created by the newly created mobile electrons. Because of the "built in" electric field,
the new mobile electrons in the n-material cannot cross over into the
p-material. In fact, if they are created near or in the junction where the
electric field exists, they are pushed by the field towards the upper surface of
the n-material (such an event is shown in the diagram below). If a wire is connected from the n-material to the p-material,
however, they can flow through the wire, and deliver their energy to a
the other hand, the holes created in the n-material, which are positively
charged, are pushed over into the p-material. In fact, what is really happening
here is that an electron from the p-material, which was also made mobile by the
adsorption of a photon, is pushed by the electric field across the junction and
into the n-material to fill the newly created hole. This completes the circuit -
we now see that there are electrons flowing all the way around the circuit,
dropping the energy they acquired from photons at a load.
crucial step in the whole process is that just described - the pushing of mobile
electrons across the p-n junction. This suggests a nice way to think of the PV
process - like a tennis player making an overhead serve:
an electron absorbs a photon and become mobile. This is like the first step
in a tennis players serve, when they throw the ball upwards into the air.
the built in electric field pushes the electron into the n-material. This is
like the tennis racket crashing into the ball, and accelerating across the
is a diagram showing the whole process:
PV cells are packaged
individual pv cell is about 1/2 inch
to 4 inches
in size, and can produce from 1 to 2 watts of power. To
power, many cells are electricity wired together into a
weather-tight modules, which usually have an aluminum frame.
modules can be further connected to form an array. In the field
of photovoltaics, the term array refers to the entire set of modules an
uses, whether it is made up of one or several thousand
PV solar cells can be used singly in small
applications such as calculators, or they can be bundled together into "PV
solar panels", which can be used for generating arbitrary large amounts of
electricity. A typical house in the sunbelt can be powered with about 200
square feet of solar panels, less than the surface area of a typical bedroom!
The basic components of a PV system are:
- PV panels
- Batteries: Typically about 12 deep-cycle lead
- Charge controller: To regulate the charging of
- Inverter (pictured below): Converts the low
voltage DC (direct current) power from the batteries into 110 volt
alternating current for use by appliances .
The following diagram shows they are connected
First, the Sun shines on the panels to produce
electrical power. That power is routed through a charge controller to the
batteries. The charge controller regulates the charging of the batteries - the
voltage on the batteries needs to be increased slowly, because charging them too
fast or routinely overcharging the batteries quickly degrades them. Next, the
inverter converts the dc (direct current) electrical power from the batteries
into ac (alternating current) electrical power at 110 volts. This can then be
fed to household appliances via a wall socket.
The costs of typical PV system range from anywhere
between a few thousand for a hunting cabin system, to about ten thousand for a
small home, and upwards of $35,000 for a large home (for more details on home PV
system costs, see
our curriculum project "Calculate
the cost of Photovoltaic Systems"). Component costs break down roughly
- About $5-7 per watt for the panels, so for a
typical 2 kilowatt system the panels cost about $10,000-14,000.
- Several hundred dollars for the charge
- About $1 per watt for the inverter: a typical 2
kilowatt system would therefore need a $2000 inverter
- About $100 per kilowatt-hour of energy storage:
a typical 2 kilowatt system might require 20 kwh of storage (only 10 kwh in
active use to extend battery life) and therefore about $2000 worth of
Today's crystalline PV panels have a very long
lifetime, at least 25 years, and possibly much longer. This is because
crystalline silicon is very stable (silicon crystals can remain intact on
geological time scales). The primary cause of failure is due to degradation of
the transparent laminates that protect the cells from the elements, and from
problems such as broken contacts.
typically last 3-10 years before they need to be replaced. Fortunately, US law
requires that the batteries be recycled. Many solar enthusiasts are hopeful that
energy storage systems using hydrogen fuel cells will become available in coming
decades to replace the need for short-lived batteries. You may want to see our
materials on electrolysis and fuel
cells to see how this might be accomplished.
A common myth, probably promoted by those who
oppose the development of renewable energy for competitive reasons, is that PV panels take more energy
to manufacture than they produce. In fact, PV panels typically pay back their
energy in 2 to 5 years, depending on the available sunlight.
Here are some Quick Facts About PV from the US
Department of Energy Photovoltaics Program (http://www.nrel.gov/ncpv/):
- PV modules covering 0.3% of the land in the
United States, one-fourth the land occupied by roadways, could supply all
the electricity consumed here.
- In 1995, PV systems generated more than 800
million kilowatt-hours of electricity.
- The PV systems installed since 1988 provide
enough electricity to power 150,000 homes in the United States or 8
million homes in the developing world.
- PV-generated power correlates well with
utilities' daily load patterns, because the power is available when it is
needed most--during daylight hours.
- The combined efforts of industry and the
Department of Energy have reduced PV system costs by more than 300% since
1982. The PV market is estimated to be growing at 20% per year today. The
number of U.S. companies producing PV panels has doubled since the late
1970s to about 20 today.
- The most frequently seen application of PV is
in consumer products, which use tiny amounts of direct current (dc) power,
less than 1 watt (W). More than 1 billion hand-held calculators, several
million watches, and a couple of million portable lights and battery
chargers are all powered by PV cells.
- PV is rapidly becoming the power supply of
choice for remote and small-power, dc applications of 100 W or less.
- More than 200,000 homes worldwide depend on PV
to supply all of their electricity. Most of these systems are rated at
about 1 kW and often supply alternating current (ac) power.
- PV module production for terrestrial use has
increased 500-fold in the past 20 years. Worldwide PV module shipments in
1993 were 60 megawatts (MW). The United States now shares more than 1/3 of
- Worldwide production of PV modules includes
48% single-crystal silicon, 30% polycrystalline silicon, and 20% amorphous
silicon, mostly used in consumer products. Modules based on cadmium
telluride are expected to enter the consumer market by the end of 1996.
- The cost of larger PV systems (greater than 1
kW) is measured in "levelized" costs per kWh--the costs are
spread out over the system lifetime and divided by kWh output. The
levelized cost is now about $0.25 to $0.50/kWh. At this price, PV is cost
effective for residential customers located farther than a quarter of a
mile from the nearest utility line. Reliability and lifetime are steadily
improving; PV manufacturers guarantee their products for up to 20 years.
- The worldwide PV industry has grown from sales
of less than $2 million in 1975 to greater than $750 million in 1993. The
companies with the largest increase in sales in the 1990s have been U.S.
companies, reflecting their strong, competitive position. In 1994, the
United States regained the lead over Japan in gross annual sales of PV
- In 1994, more than 75% of the PV modules
produced in the United States were exported, mostly to developing
countries where 2 billion people still live without electricity.
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