SOLAR FACTS
from
STRAIGHT CREEK VALLEY FARM
I have always wondered how things work, and even simple things, like hand cranked can
openers, fascinate me. When we began to build our lives at Straight Creek Valley Farm,
we knew that we would be doing things we had never done before. I learned how to plow
a field, easier said than done. I learned how to help Greg change out the hydraulic lines
on our old backhoe, also easier said than done, and we both learned how to set up and
live with solar energy. As we put the small, two panel system together, I decided that it
was time to learn how it is that solar cells work. I wanted to understand the why and the
how behind the solar generated electricity that burned brightly in the cabin’s light bulbs
and baked potatoes in the microwave oven.
I found several sites on the internet that share volumes of solar energy information. One
of the best is Solar Panel Info. This site can guide anyone interested in installing a solar
system, either in a new or existing home, through the research and design of appropriate
systems. Another site, Solar Panel Faqs answers many basic questions about solar
energy and sets forth seemingly complicated, technical information in a very
understandable format. Finally, Solar Energy Solutions sets forth the same technical
information in a very reader friendly format. And of course, there are many more solar
related sites on the internet, but I found these to be among the most useful.
No doubt about it, solar panels are a bit more complicated than can openers, but in
today's world, they are really just about as common. We see them every day on
calculators, garden lights and wrist watches. The proper term for a solar panel is
photovoltaic cell, or module. Photovoltaic is a compound word made up of photo,
meaning light, and voltaic, which means electricity. Photovoltaic cells thus convert
sunlight into electricity.
I learned that photovoltaic (PV) cells are typically made out of silicon. When light strikes
the cell, much of it is reflected away, but some of the light’s energy is absorbed by the
silicon. This absorbed energy actually knocks electrons off of the silicon atoms, allowing
them to flow freely about the silicon. Free flowing silicon electrons, however, would not be
of much use, so PV cells are also designed with a built in electric field that forces the
freed electrons to flow in a particular direction. It is this orderly flow of electrons that forms
a current, and by placing metal contacts on the top and bottom of the PV cell, that current
can be drawn off and put to use. As we well know, the current can power calculators, wrist
watches, garden lights, houses, towns and maybe even the world. It is this current,
together with the cell's voltage, which is the result of its built-in electric field, that defines
the power, or wattage, that the solar cell can produce.
So what does all of that mean? How does a silicon solar cell really work? It all starts with
an understanding of the basic chemical properties of silicon. It is these properties,
especially in silicon’s crystalline form, that make it a perfect PV cell material. To start with,
an atom of silicon has 14 electrons, arranged in three different shell-like layers. The first
two layers, closest to the center, contain their full quota of electrons, and do not have any
room for any more electrons. The third, or outer shell, however, is not full and has room
for four more electrons. A silicon atom is thus always looking for ways to fill up its outer
shell, which would be complete with a total of eight electrons. In order to fill that outer
shell, the atom bonds with, or shares, four electrons with four of its neighboring silicon
atoms. It is this sharing of neighborly electrons that forms silicon’s crystalline structure,
and it is this crystalline structure that is important in the creation of photovoltaic energy.
Pure silicon, however, is not a good conductor of electricity, because its electrons are all
locked into the neighborly crystalline sharing and are thus not free to move about, unlike
the electrons in good electricity conductors such as copper. The crystalline structure
basically locks the electrons in place. When energy, such as heat from sunlight, is added
to pure silicon, it can cause a few electrons to break free of their crystalline bonds and
leave their atoms. An electron vacancy is then left behind in these atoms. The broken
away, or free electrons, called free carriers, wander randomly around the crystalline
lattice looking for another vacancy to fall into. It is these free carriers that can carry
electrical current, but there are so few of them in pure silicon, that they really aren't very
useful. The silicon in a solar cell is thus modified slightly so that more electrons are able
to break free and carry electricity. Impurities are actually added to the silicon. In short,
other atoms are mixed in with the silicon atoms.
When phosporous is added to silicon, the crystalline structure looses some of its tight
bonding properties. Phosphorous atoms have five electrons in their outer shells, not four,
like silicon. A phosphorous atom can still bond with its neighboring silicon atoms, but
because the phosphorous atom has one extra electron, that isn‘t needed by a
neighboring silicon atom, that one electron remains unbonded, not forming part of the
tight crystalline structure. It thus takes a lot less energy to knock that unneeded, extra
phosphorous electron loose, so that it can become a free carrier and transport an electric
current.
This process of purposefully adding impurities to a substance is called doping. When
pure silicon is doped with phosphorous, the resulting silicon is called N-type, “N" standing
for negative because of the prevalence of free floating electrons. N-type doped silicon, as
discussed above, is thus a much better conductor of electricity than pure silicon.
But only part of a PV cell is made up of N-type silicon. The other part is doped with
boron, which has only three electrons in its outer shell instead of silicon’s four. This
boron doped silicon is called P-type silicon. The “P” stands for positive. Instead of having
an extra electron, like phosphorous, P-type silicon atoms have a vacancy that positively
attracts freely wandering electrons. Because of this positive attraction of electrons, the
boron atoms are thus positively charged.
Now remember at the outset that I wrote that every PV cell must have at least one electric
field in order for there to be an orderly flow of electrical current. What is it that makes up
this electric field? An electric field forms when the N-type and P-type silicon are in contact
with each other. The free electrons on the N side, which have been wandering in search
of vacant holes to fall into, are attracted to the more numerous vacancies on the P side,
and so rush to the P side to fill in those more numerous holes. It is this rush, from the N
side to the P side, that forms an electric current.
Right at the junction of the N-type and P-type silicon, however, the activity of the free
phosphorous electrons and the boron holes they fall into, forms a sort of energy barrier,
making it harder and harder for electrons on the N side to cross over to the P side.
Eventually, as the barrier gets stronger, the flow of electrons stops, and equilibrium
between the two sides is reached. It is at this point of equilibrium that the barrier itself
becomes the electric field, separating the two sides
The electric field barrier however, does not completely stop the flow of electrons. It
actually reverses the flow of electrons and even pushes, electrons to flow from the P side
back over to the to the N side. It will not, however, allow electrons to flow from the N side
to the P side.
I have already described how electrons are freed from the crystalline lattice when light, in
the form of photons, strikes the solar cell. Each photon of light will normally free exactly
one electron, and will thus result in one free hole. If this happens close to the electric
field, or if a free electron and free hole happen to wander into the electric field’s range of
influence, the field will send the free electron to the N side and the hole to the P side.
When an external path is provided, the electrons will flow along that path back over to
the P side where they really want to go and are attracted by the more numerous holes.
As the electrons flow along that external path, they can be put to work, powering watches,
lights and even houses. It is this flow through the external path that provides us with an
actual useable electric current. It is the strength of the solar cell’s electric field, or the
barrier between the P and N sides, that defines the cell’s voltage or power. With both
current and voltage, we have electric power, which is the product of the two.
As I noted at the outset, most of the sunlight that strikes a PV cell not absorbed. Simple
solar cells typically absorb between fifteen and twenty percent of the sunlight that strikes
them. Why so little?
Light is actually made up of a range of different wavelengths, and therefore different
energy levels. The light that we see, or visible light, is only part of the light spectrum.
Since the light that hits a PV cell is made up of photons with a wide range of energies, it
turns out that some of them simply won't have enough energy to knock loose an electron.
These weaker photons will simply pass through the cell as if it were transparent. Still other
photons have too much energy and that extra energy is also lost and simply passes
through the cell. There is actually a specific amount of energy, measured in electron
volts (eV) that is required to knock loose an electron, and the specific amount required is
determined by the material from which the PV cell is made. About 1.1 eV’s are required
to knock a crystalline silicon electron loose. Thus, either too much photon energy or too
little photon energy account for the loss of approximately 80 percent of the light energy
that strikes a solar cell.
Why then, can't PV cells be made out of materials that require low amounts of energy to
knock loose electrons? If this could be done, then more of the photons that struck the
cell could be absorbed and put to use. Unfortunately, the strength, or voltage, of the PV
panel’s electric field, is also determined by the material used, and the lower the energy
required to knock loose an electron, the lower the cell’s voltage. Thus, what could be
made up in extra current, by absorbing more photons and freeing more electrons, is lost
by having a lower voltage cell. Power, once again, is voltage times current. Thus the
lower the voltage, or the weaker the electric field, the less the power. A solar cell’s optimal
power is thus a balance between these two properties within the PV material.
You may have noticed that solar cells are typically covered with a metallic grid. This is
because silicon is a merely a “semiconductor” and is not nearly as good as metal for
transporting electric current. To minimize this lack of conductivity, PV cells are covered by
a metallic contact grid that shortens the distance that the freed electrons have to travel.
The gird obviously does not cover the entire PV cell surface, so that it can still allow
photons to strike the silicon and knock electrons loose, but even so, some photons are
blocked by the grid. The grid wires also cannot be too small, or they would not allow
enough electrons to pass along them and resistance would be too high, allowing the grid
to overheat and short out with too much electron travel. Once again, a proper balance is
required.
You may have also noticed that silicon is a very shiny material, which also means that it is
very reflective. Obviously, photons that are reflected by the cell are not absorbed and put
to use freeing electrons. An antireflective coating is therefore applied to the top of the PV
cell to help reduce reflection losses. Finally, a glass cover plate is placed on top of the PV
cell to protect it from the weather.
When several PV cells are connected together, the combined unit is referred to as a
module. As many as 36 PV cells are often linked together in a parallel series in order to
achieve useful levels of voltage and current. The modules are supported by a sturdy
frame, and sometimes the entire structure is covered by protective glass. Positive and
negative terminals are placed on the back of the frame, functioning just like the battery in
a car and allowing current to be drawn off of the module.
Crystal silicon isn't the only material used in PV cells. A similar substance called
polycrystalline silicon is also used in an attempt to cut manufacturing costs, although the
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.
These different materials all have different properties and each requires photons of
different energies to knock electrons loose. Some solar cells are even manufactured
using two or more layers of different materials with different photon energy requirements.
The material that can absorb the higher energy photons is on the surface. The lower
energy photons that pass through that first layer are then absorbed by the material with
the lower photon energy requirements that lies below. This layering technique can result
in far higher absorption efficiencies and these cells, called multi-junction cells, can have
more than one electric field, allowing for more energy to be drawn off of the cell.
How then does one power a cabin, a house, or a city with solar energy? First of all, the
solar panels have to have the correct orientation to take advantage of the sun's energy.
Non-tracking PV systems in the Northern Hemisphere should point toward true south and
should be inclined at an angle equal to the area's latitude in order to absorb the maximum
amount of energy year-round. We manually adjust our PV panels at Straight Creek each
season. We aim them towards the south at a steeper angle during the winter so that the
sun continues to strike the panels at a ninety degree angle as it passes farther to the
south. This ninety degree angle maximizes the amount of photons that are absorbed by
the panel.
PV modules should also never be shaded by nearby trees or buildings, no matter what
the time of day or season of year. Even if just one of a module’s 36 cells is shaded,
power production can be reduced by more than half. It is thus very important to place PV
modules out in the open, away from any sources of potential shade. Greg and I recently
tested this phenomenon by placing a block of wood over a corner of one of our PC cells.
We could hear the faint hum of our charge controller drastically cut back as less energy
was generated by the partially blocked panel.
It is also important to decide what size PV system is needed. Meteorological data can be
found in the Farmer’s Almanac, giving the average hours of monthly sunlight for each
geographical zone. PV systems should be designed to produce enough energy to meet
household requirements during the least sunny month. Average household energy
demand can be determined by simply reading the monthly utility bill. For example, our
electric bill to run our Cincinnati city house for the month of March cost us $75.76 for 715
kilowatts of electricity.
Household voltage requirements will also need to be determined. With the boom of
recreational vehicle and pleasure boating, there are now many great twelve volt
appliances on the market that are easy to obtain. We actually wired our cabin at Straight
Creek with both a 12 volt system and a 110 system that runs through an inverter. We
simply plug our twelve volt appliances into circular twelve volt outlets that run right off of
our battery bank. When using the 110 outlets, we simply turn on the inverter and then
plug in the appliance to a typical household outlet.
But how does a solar home operate when the sun is not shining? Certainly, no one would
accept only having electricity during the day, and then only on clear sunny days. To get
around the obvious lack of sunshine issue, all solar systems have some form of energy
storage. If the system is not connected to the main energy grid, and is thus off-grid, the
only storage option is batteries. Unfortunately, batteries add a substantial cost to the PV
system and also require regular maintenance. However, if a home is connected to the
main grid, power can be bought from the utility when needed and then sold back to the
utility when more is produced than is needed. This way, the utility acts as a maintenance
free storage system. The utility has to agree, however, to the terms of use and sell back,
and in most cases will buy excess power at a much lower rate than their own selling
price. Special equipment is also needed to make sure that the power sold back to the
utility is compatible with the utility power. Safety is an issue as well. The utility has to be
certain that if there is a power outage, the PV system will not try to feed electricity into
lines on which a lineman might be working.
The typical lifetime of a PV module is now about 20 years or more, but batteries, even if
well maintained, will only last four to ten years. Batteries can also be very dangerous,
both because of the energy they store and the acidic electrolytes they contain. They also
emit hydrogen, which is quite explosive. Remember the Hindenberg? PV system
batteries will thus need to be kept in a well ventilated enclosure, preferably removed from
the main living space. We keep our four batteries at Straight Creek Farm in a well
ventilated, easily accessible box that Greg built and set right outside the cabin.
Several different kinds of batteries are commonly used in PV systems, but they are all
what is known as deep-cycle batteries. Unlike a car battery, which is a shallow-cycle
battery, deep-cycle batteries can discharge more of their stored energy without causing
them any damage. Car batteries are designed to discharge a large current for a very
short time, as when engaging the ignition and starting the engine, and then are
immediately recharged once the car is driven. PV batteries, on the other hand, generally
discharge a smaller current for a longer period of time and are recharged at a far slower,
more prolonged rate. The most commonly used deep-cycle batteries are lead-acid
batteries and nickel-cadmium batteries. Nickel-cadmium batteries are more expensive, but
last longer and can be discharged more completely without harm, but even deep-cycle
batteries can not be discharged 100 percent without seriously damaging the battery and
shortening battery life. Most PV systems are accordingly designed with a charge
controller that will not allow the batteries to discharge more than 40 percent to 50 percent.
The charge controller is a very integral part of the PV system. It not only prohibits the
batteries from discharging too much, it also makes certain that the batteries are not
excessively overcharged. Once the batteries have reached a full charge, the charge
controller shuts off the flow of current from the PV module. Similarly, once the batteries
have been drained to a certain predetermined level, the charge controller will not allow
any more current to be drained from the batteries until the there has been a sufficient
recharge. The charge controller simply measures the batteries’ voltage and thus
determines when to shut down the inflow or outflow of electric current
One final piece of integral PV system equipment is an inverter. The electricity generated
by the PV modules, and stored in the batteries, is direct current (DC), while the electricity
supplied by the utility company that runs every household appliance, is alternating
current (AC). The inverter is a device that converts DC to AC. Some PV modules, called
AC modules, actually have an inverter already built into them, eliminating the need for a
large, central inverter, and simplifying wiring issues. At Straight Creek Farm, however, we
double wired the cabin, so that we can run 12 volt appliances right off of our batteries.
We thus only need to use the inverter when we run our 110 volt appliances.
The only remaining equipment necessary to get a PV system up and running is wiring,
junction boxes, fuses and grounding equipment. Electrical wiring codes should be
followed and there is now a section in the National Electrical Code specifically for PV.
Once the system is up and running, there are not only no more utility bills to pay, but
there is the serene satisfaction of knowing that your daily life runs on clean energy.
The cost of installing a PV system is still rather high. A system large enough to power a
small typical American household could currently cost as much as $25,000. That's why
PV electricity is typically used in remote areas, far from conventional sources of
electricity. Our PV system cost us only just over $7,000, well less than the $30,000 it
would have cost us to run electricity and phone lines down the gravel road to Straight
Creek Valley Farm. Thus, we were certainly far ahead by going off grid, but remember,
Greg has done all of the design and installation at substantial savings. Still, the costs of
PV systems are continually decreasing as ongoing research refines the technology and
as the government offers tax rebates and incentives. Some day, hopefully soon, it will be
as cost effective to install a PV system in an urban area as it now is in more remote
locations like Straight Creek Farm. As demand slowly increases, manufacturing also
increases, and as more systems are produced, the costs of the individual component
parts goes down. In short, photovoltaics is fast becoming a technology with a very bright
future.
And if we think of the source of solar energy, the sun, which has been burning for the
past four and a half billion years and will likely burn for the next ten billion, we should not
have to worry about running out of solar energy any time soon.