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1
Photovoltaic Information:  
 
NOTE: for installations which must meet code requirements please make sure
that the modules you purchase are UL listed. Many people have gotten a deal
on modules only to find that they can't use them in their system. Photovoltaic
modules look great on your roof,
but not sitting in your garage or on some auction  site waiting for the next
victim to purchase them.
The "CE" or "CSA" labelling on modules are for European or Canadian
acceptance and are not recognized in the U.S.
Only FM (Factory Mutual) and UL (Underwriters Laboratory) listed
modules are NEC (National Electrical Code) compliant. Listed modules cost
more and that is a fact of life. Please consider that if you have a fire, and the
insurance company inspects the modules, rest assured that they will look at
labels and serial numbers.
 NOT ALL SYSTEMS REQUIRE LISTED MODULES
(i.e. stand-alone lighting, monitoring and communications systems as
well as small battery charging and water pumping).
copyright by John Drake Services, Inc.
 
Example of a stand-alone (not utility power) system which uses d.c. to
a.c. inverter and/or a generator to power all of the loads, which are ac.
This is how many people living "off grid" supply their power needs.
stand alone photovoltaic dc and ac layout
Example of a stand-alone system that powers 12 or 24 volt d.c. loads
only. This shows a low-volage direct current system which could be
made more versatile with the addition of a small inverter to power a.c.
loads.
stand alone photovoltaic dc only system layout
Photovoltaic Info.
Content links:
What are photovoltaic modules (solar panels)?  What are PV modules?
PV Cell Technologies.  PV Cell Technologies
Photovoltaic module construction. PV Module Construction
UL Listing. UL Listing
Diodes and their function. Diodes and their function
Diode types. Diode types
Diode uses in a photovoltaic system. Diode Uses in a PV System
Self-Regulating Photovotaic Modules. Self-Regulating PV Modules
Photovoltaic Info:
 

What are photovoltaic modules (solar panels)?
A photovoltaic module produces electricity (direct current) when it is exposed
to light. If you want the long and detailed story you can click onto this link
http://pvpower.com  for a detailed explanation of the photo (light) voltaic
(electricity) phenomenon. You can also click onto the Dept. of Energy
The short story is that photons (packets of energy) in light strike the surface of
the photovoltaic cell. These photons break loose electrons at the P/N junction
(positive / negative zone where the cell characteristics are different) and send
them through a wire.
You can also click onto this Department Of Energy link for an animated
explanation how a photovoltaic cell works http://www.eren.doe.gov/pv/
 

PV Cell Technologies - There are basically three types of photovoltaic cells:
single crystal, poly-crystalline (originally called semi-crystalline) and
amorphorous (also known as thin-film.) We will discuss each type along with
its characteristics.
Single Crystal (mono-crystalline) - pure silicon is doped (impurities added to
change its characteristics) and grown into an ingot. The cylindrical ingot (either
left with a round cross section or slabbed to give it a square cross section) is
sliced into extremely thin wafers. These wafers are etched (sometimes dyed
also) and conductors (wires) are imbedded into the suraces. The techniques
used are similar to those used in manufacturing micro chips in the electronics
industry. The cells are tested for output, batched into groups of the same output
and then are ready to be assembled into modules.
This is an oversimplification, but it is the basic idea. Single crystal cells are the
most expensive type to produce. Currently they have the highest sunlight to
electricity conversion efficiency and are the most stable (less power output loss
with long term exposure to sunlight) and produce the most power for a given
surface area. The downside is that they must be assembled into a rigid panel
as the cells do not bend well.
Poly-Crystal (a.k.a. multi or semi-crystalline) - These are produced in one of two
ways: either by using random wafer pieces of doped silicon or by string
ribbon technology. The poly-crystalline cells can be identified by the random
arrangement of chips (similar to osb [oriented stranded board] wood products
used in construction) of extremely thin silicon. They are usually dyed a beautiful
blue color.
The string ribbon technology (please click onto  http://www.evergreensolar.com
for more detailed information) is basically a continuous casting process that
forms the silicon into a continuous ribbon.
The advantage of these two technologies is that they are less expensive to
produce than the single-crystal and they can more easily fill the surface area of a photovoltaic module with less waste of materials. They are not as efficient as
single-crystal cells (very close) nor are they as stable (but close again). 
Amorphorous ( a.k.a. Thin-Film) - These are produced by vaporizing doped
silicon (or other materials) and spraying it onto a substrate (backing) of glass,
stainless steel, fiberglass or other materials. Thin Film Technology Drawing
This process is called "Thin Film Deposition" and uses such transfer
techniques as: plasma spray arc, thermal spray arc, ion deposition
and other proprietary methods. This is not new technology - did you ever
wonder how a layer of metal was applied to plastics?
Now you know. Thin film modules are cheaper and faster to produce
than the other technologies.
This technology allows for the manufacture of flexible  
modules when using a flexible substrate such as stainless steel. It also enables
the manufacturer to deposit layers of different materials which convert different
portions (wave bands) of sunlight to electricity. This increases the efficiency of
the module overall.  In the past, the methods were such a closely guarded
secret that when I attended the ARCO SOLAR school in 1988 we were
allowed to go through the entire plant - except for the thin film areas. thin-film
drawing Thin-film modules have a lower output for a given cell area than the
other technologies. A thin-film 30 watt module may be up to 25% larger than
a 30 watt single or poly-crystal module. They also do not have as stable a
power output with long term exposure to sunlight. An advantage of some
models of thin-film photovoltaic modules is that they are more resistant to
damage from shading than other technologies. This is due to the inherent low
electrical resistance in the modules. Some makers do not recommend the use
of bypass diodes for module protection.
As an example of the efficiency of thin-film products. We have two solar-
powered attic fans that use single crystal modules. Even in the winter, when
the sun is at its lowest in the sky, they continue to operate. We also have two
fans powered by thin-film modules and they do not run well in the winter due to
the oblique angle of the sunlight striking them.  
Most solar powered consumer products, such as watches and calculators,
utilize thin-film technology because of its ease of manufacture and the ability
to be rapidly sized to fit each application.  
 

Photovoltaic module construction - the finished cells (which produce about
0.45 volts direct current) are wired together in series to add up the voltage.
Please see   Series & Parallel  wiring. In some cases they are also wired into
series strings and then these strings of cells are paralleled with each other to add amperage without changing the output voltage. You will see this in large modules.
Since the cells produce 0.45 volts each, most modules will have a minimum of
36 cells wired in series. More cells (to increase voltage) are preferable in hot
climates for a higher voltage. Please see 100 Efficiency   for a more in depth
discussion on this matter. As the cell temperature increases, the voltage
decreases. For hot climates it is best to start with the highest output voltage to
begin with. The cells are mounted on a backing material such as Tedlar
(a white rubberlike product), glass or some other material which will seal
and protect the cells from the environment.
A tempered low-iron glass cover is placed over the cells. The iron in most glass  
will reduce the light transmission to the cell surfaces. Low iron? When you look
at the edge of a piece of glass, have you every wondered what caused that green
color? Well, now you know.
Some modules, flexible ones in particular, do not have a glass cover. Others may
have a plastic or rubber-like cover material.
These layers are then mounted in a frame (usually aluminum or plastic) and the
output cable or junction box is installed.
 

UL Listing - samples of a given photovoltaic module are been tested by
Underwriters Laboratories for electrical safety. They also test for output,
operations within a given temperature range, resistance to physical and
environmental damage. A UL listed module has the required bypass diode(s)
installed and is labelled with the electrical characteristics such as: maximum
system voltage and series fuse rating. The series fuse rating is the size of the
fuse placed on the output of a string of modules wired in series (to add up
voltage). The maximum system voltage is usually listed as 600 volts d.c. This
is the highest output voltage that a series string can be wired to produce.
Please see  Series & Parallel .
 

Diodes and their function - diodes are basically electrical one way gates.
A diode operates under one of two conditions. The first is called forward bias,
this is when electricity is travelling in the direction of the open gate. There is
some voltage loss due to the resistance of the diode itself. The second is
called reverse bias,  this is where the electricity is forced in the wrong direction
against the closed gate of the diode, if there is enough force the gate will be
overcome. Diodes produce some heat when in normal uses and some require
a heat sink to draw off and dissipate that heat.
 

Diode types - there are  two types of diodes used in a photovoltaic installation.
The first is the silicon diode. It has fairly high voltage losses when in normal use
but is very resistant to power surges and  voltage spikes such as lightning or
high voltage ground leakage.
The second type is the Schottky diode. It has less voltage loss than the silicon
diode but is not as resistant to transient spikes and surges. This is the most
commonly used blocking diode when using a charge controller which does
not disconnect the battery from the array at night.
 

Diode uses in a photovoltaic system:
Blocking diodes were originally used between the photovoltaic array and the
charge controller or battery. These diodes keep the array from draining power
from the batteries at night. Except when using a diversion only type of charge
controller, most systems do not require a blocking diode to protect the
batteries. The charge controller isolates batteries from the array at night. Our
main stand-alone system has three arrays. Each has its own incoming wiring
with a Schottky blocking diode. The heat sinks can become very warm when
the arrays are producing their maximum output.
Bypass diodes are installed in the photovoltaic modules. When a module is
wired into a series string, the output voltage is the sum of each individual
modules' voltage. If one module is shaded, its resistance goes up and power
from the other modules in the string can go into it. First off, this will
dramatically reduce the output of the series string. The shaded module can
also overheat which can destroy it at best , or cause a fire.  This is why UL
Listed modules have bypass diodes.
When a bypass diode is installed in a module (usually in the junction box),
it has a resistance to voltage flow which is higher than the cells in the module.
Under normal conditions, no power flows through this diode.  When the
module is shaded, it stops producing power and its electrical resistance
becomes greater than that of the diode. Power from the other modules in the
string bypass the cells in the shaded module and flow through the bypass
diode. This protects the shaded module while also somewhat reducing the  
power loss to the string of modules. The bypass diode is more for safety than
anything else as the shading of a single module in a series string can drop
the output voltage to the point where the system will not function. On some
large systems where series strings of modules are paralleled, a
blocking/bypass diode (you choose the term) is placed between the parallel
connections to prevent one shaded string from affecting the other strings.
Some modules have internal bypass diodes to reduce power loss when
the cells are wired in both series and parallel and there is partial shading of
the module. Remember that the module output voltage has to be greater than
the load, be it a battery or grid-tie inverter, to be of any use.
 

Self-Regulating Photovoltaic Modules - these panels have cells wired in
series in such a manner as to limit the output voltage.  In theory, as the battery
voltage reaches the full charge point, the voltage on the panel remains in the
realm of the battery full state of charge level.   
Where the two meet is supposed to be at the battery's full charge voltage.
These were advertised as not needing a charge controller in the system. If
you use a battery that is matched (in amp hours) to the panel and draw
power on a regular basis, these work ok. If you let the battery sit unused for
long periods of time (which is what these were promoted for) the battery
will overcharge. Also, since these panels are wired for a low output voltage,
in hot weather they do not work very well at all. They are still made today,
but not in the proliferation of the 1980's. We have used them for both
battery charging and in direct fan hook-ups. I can not recommend them.      
  copyright by John Drake Services, Inc.
 
Photvoltaic Module Maintenance:

Photovoltaic modules require almost no maintenance. When you see dirt on the
panels, just wash it off. Mornings are the best time as the panels may still have
dew on them which will make dirt removal much easier. We have many trees in
our area so I use a high pressure nozzle on a garden hose. Birds and dogs are
programmed by nature to do certain things. When a dog gets on your bed, it will
always lay cross ways to take up the most room. Birds like to chat with each other
while perched on the top of photovoltaic modules, especially steeply pitched ones.
Their programming requires them to face the back of the panels, or maybe it is
just the glare off of the glass, either way a high pressure nozzle comes in handy.     


 
Drawing copyright by Uni-Solar