Information sheet 1) Solar Panels - Basics

Information sheet  1) Solar Panels - Basics
Information sheet
1) Solar Panels - Basics
A solar cell, sometimes called a photovoltaic cell, is a
device that converts light energy into electrical energy.
A single solar cell creates a very small amount of energy
so solar cells are usually grouped together in an
integrated electrical panel called a solar panel. Sunlight
is a somewhat diffuse form of energy and only a portion
of the light captured by a solar cell is converted into
electricity. The current generation of solar cells convert
only 12% to 15% of the sun's light into
electricity. However in recent years there have been
significant improvements in their design. Some new cells on the market now have
efficiencies around 20% while some laboratory prototypes even reach as much
as 30%. Given this it is likely that solar cell efficiency will continue to improve over
time.
The output of a solar panel is usually stated in watts. The amount of watts of
electricity generated by a panel is determined by multiplying the rated voltage by the
rated amperage. The formula for wattage is:
VOLTS x AMPS = WATTS
Let's use as an example a large solar panel measuring about 1 x 1,5m that might be
used in a typical home energy system. The solar panel has a rated voltage of 26V
and rated amperage of 7A. The wattage calculation would look like this:
26V x 7A = 182W
If a particular location has an average of 6 hours of peak sun per day, then the solar
panel in this example can produce an average of 1092Wh (6 x 182) power per day or
a little over 1kWh per day. Most homes use between 10-25kWh per day. Given this it
is going to take a lot more than one solar panel to generate enough electricity to
completely power a home. For a household needing 20kWh per day it would take
approximately 19 panels to provide 100% of the electricity. Most houses do not have
enough space on their south facing roof for this amount of panels.
Consequently, in most home applications where a connection to the grid is available,
a solar panel system should only provide part, but not all of the necessary energy.
2) Solar Panels – Functionality
A solar cell is based upon the "photovoltaic effect" (PV-effect) discovered in 1839 by
Edmund Becquerel, a French physicist. In his experiments he found that certain
materials would produce small amounts of electric current when exposed to
sunlight. Sunlight is made up of packets of energy called photons. When the photons
strike the semi-conductor layer (usually silicon) of a solar cell a portion of the photons
are absorbed by the material rather than bouncing off of it or going through the
material. When a photon is absorbed the energy of that photon is transferred to an
electron in an atom of the cell causing the electron to escape from its normal
position. This creates, in essence, a hole in the atom. This hole will attract another
electron from a nearby atom now creating yet another hole, which in turn is again
filled by an electron from another atom. This hole filling process is repeated a few
zillion times, thus creating an electric current.
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3) Solar Panel – Structure
A typical solar cell is a multi-layered material. The layers are:
•
•
•
•
•
•
•
Cover Glass - this is a clear glass layer that provides outer protection from the
(weather) elements.
Transparent Adhesive – to stick the glass to the lower layers of the solar cell.
Anti-reflective Coating - this substance prevents light that strikes the cell from
bouncing off so that the maximum energy is absorbed into the cell.
Front Contact - transmits the electric current.
N-Type Semiconductor Layer - This is a thin layer of silicon which has been doped
with phosphorous.
P-Type Semiconductor Layer - This is a thin layer of silicon which has been doped
with boron.
Back Contact - transmits the electric current.
4) Solar Panels - Types
There are a number of different types of solar panels manufactured today. Briefly,
they are:
Mono-crystalline -This type of solar cell uses a
single layer of silicon for the semi-conductor.
In order to produce this type of cell, the used
silicon must be extremely pure which means it
is the most expensive type of solar cell.
However, they are the most efficient type of
solar panels. Their performance is somewhat
better in low light conditions (but not as good
as some advertising hype would have you
believe). Overall efficiency on average is
about 12-15%. Most panels of this type are
warranted for 20-25 years. They are usually
blue-grey in colour and have a fairly uniform
consistency.
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Poly-crystalline
To
make
polycrystalline silicon cells, liquid silicon
is
poured
into
blocks
that
are
subsequently sawed into plates. This
type of approach produces some degree
of degradation of the silicon crystals
which
makes
them
less
efficient.
However,
this
type
of
approach is easier and cheaper to
manufacture. Currently, poly-crystalline
solar
panels
are
the
most
common. They are slightly less efficient
than single crystal, but once set into a
frame with 35 or so other cells, the
actual difference in W/m² is not that
high.
Poly-crystalline
cells
look
somewhat like shattered glass and have
a dark blue to almost black colour.
Overall efficiency on average is about
11-13%.
Amorphous - Amorphous solar panels are
also referred to as "thin film" solar
panels. This type of solar cell uses layers
of semiconductor that are only a few
micrometers thick (about 1/100th the
thickness of a human hair). This lowers the
material cost but makes it even less
efficient than the other types of silicon.
However, because it is so thin this type of
cell has the advantage that it can be placed
on a wide variety of flexible materials in
order to make things like solar shingles or
roof tiles. Because they can be put on to
flexible backings they have proven very
valuable in certain types of applications
where flexibility is more critical than
power. For example, these types of solar
panels are often used in portable products
such as solar backpacks and solar bags.
Overall efficiency on average is about 56%.
Another way of defining solar cells is in terms of the types of materials they are made
of. While silicon is the most commonly used crystal a number of other materials and
substances can be used as well. Different types of substances perform better under
certain light conditions. Some cells perform better outdoors (e.g. optimized for
sunlight), while others perform better indoors (optimized for fluorescent light).
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5) Solar System - Overview
A complete solar system consists of a solar panel, a battery and a charge controller.
In some cases, a power inverter is also required.
Composed of multiple solar cells in series or in parallel, a solar panel produces direct
current (DC) power; when a solar panel is connected to a battery, this power is stored
in the battery. A charge controller connected between the solar panel and the battery
monitors the battery and prevents the solar panel from overcharging the battery
while assuring a complete charge.
A solar system requires an inverter when the DC power needs to be converted into
alternating current (AC) power to operate appliances or supply power to the utility
grid.
6) Solar panels – series or parallel
To increase voltage or amperage of a solar system, the solar panels can be placed in
series (higher voltage) or parallel (higher amperage) or a combination of both.
Series wiring: connect the positive terminal of one panel to the negative terminal of
the next. The resulting voltage between the outer positive and negative
terminals is the sum of the panel voltages, but the amperage stays the same
as for one panel.
E.g.
2x 12V/3A panels in series produce 24V at 3A.
4x 12V/3A panels in series produce 48V at 3A.
Parallel wiring: connect the positive terminals to positive terminals and negative to
negative. The resulting voltage stays the same, but amperage becomes the
sum of the panel amperages.
E.g.
2x 12V/3A panels in parallel produce 12V at 6A
4x 12V/3A panels in parallel produce 12V at 14A
Note: in most cases it is not profitable to provide a high input voltage to a charge
(shunt) regulator as part of it will probably be dissipated as heat.
E.g.
When placing 5x 12V/3A in series, 60V is produced at 3A
a 24V regulator might ‘waste’ 36V (heating of the regulator)
Consider placing the panels in parallel.
Remark: when in doubt contact an experienced solar panel professional.
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7) Solar System - Why a charge controller is necessary
The brighter the sunlight, the more voltage the solar cells produce. This excessive
voltage could damage the batteries. A charge controller is used to maintain the
proper charging voltage on the batteries. As the input voltage from the Solar panels
array rises, the charge controller regulates the charge to the batteries preventing
overcharging.
Most quality charge controller units have what is known as a 3 stage charge cycle:
1) BULK phase: the voltage gradually rises to the bulk level (usually 14,4V 14,6V) while the batteries draw maximum current. When bulk level voltage is
reached the next phase begins.
2) ABSORPTION phase: the voltage is maintained at bulk voltage level for a
specified time period (usually an hour) whilst the current gradually tapers off
as the batteries charge up.
3) FLOAT phase: after the absorption time passes the voltage is lowered to float
level (usually 13,4V – 13,7V) and the batteries draw a small maintenance
current until the next cycle
When using for example 4 x 80W/12V solar panels, the charge controller should be
rated up to 40A. For 8 x 80W solar panels, a 2 x 40A charge controllers is needed to
handle the power or the system voltage could be increased to 24V to use just one
40A charge controller.
Notice that the charge controller in the above example seems heavily overdimensioned. But even though the solar panels don't normally produce that much
current, there is an 'edge of cloud effect' one has to take into consideration.
Clouds affect solar panels. The amount of power your solar panels can produce is
directly dependent on the level of light they receive. In full, bright sunlight, solar
panels receive maximum levels of light. During those "peak" sunlight hours, your
solar panels will produce power at their maximum capacity.
When clouds cover the sun, light levels are reduced. This does not shut down power
production, however. If there is enough light to cast a shadow, in spite of the clouds,
your solar panels should operate at about half of their full capacity. Thicker cloud
cover will reduce operations further. Eventually, with heavy cloud cover, solar panels
will produce very little useful power.
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The effects of clouds on a solar panel can however turn out positive; your solar panels
will deliver their ultimate amount of peak power during cloudy weather!
As the sun moves into a hole between the clouds, your solar panels will see full direct
sunlight combined with reflected light from the clouds! They will absorb more energy
than they could on a cloudless day!
The effects of clouds on a solar panel could produce peaks at or above 50% more
than its direct-sun output. Due to this phenomenon it has been seen that 4 x
80W/12V panels (4 x 6,66A=26,66A) pump out over 32A. This is well over their rated
maximum.
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